Lectures M2-Final Flashcards
Optimal fat reserves
for animals
- Animals must maintain short-term and long-term fat reserves
Benefits of fat reserves: insurance, insulation
Costs of fat reserves: weight, increased energy expenditure, reduced mobility → increased predation risk
- Animals show adaptive fat storage (must optimally balance fat reserves, which is a cost)
- Winter in higher latitudes = longer nights, lower temperature, higher variance + uncertainty
Fat reserves in humans
- Sufficient energy for over 50 days!
> 60% of the brain (myelin is mostly fat)
10-20% in men
15-25% in women
When do animals store more fat
- animals in higher latitudes would store more fat in the winter than in either fall or spring
- animals that experience more interruptions would store more fat
4 considerations when determining what an animal should eat
- encounter rate
- energy content
- handling time
- predation risk
Encounter rate
of items encountered per unit time
Handling time
time from capture to complete ingestion
e.g. takes time to feed, access, sit in the flower, etc.
Should the grizzly eat the whole fish or just brain and eggs (richest in calories)?
Answer: it depends:
- Assume that prey 1 (brain) and prey 2 (fish body) have the same encounter rate
- Prey 1 has about twice the energy content than prey 2
- If the encounter rate with prey 1 is sufficiently high, avoid prey 2
The optimal diet model
depends on (3 things)
a simple mathematical model that allows us to predict what prey types a forager should eat based on the energy content, encounter rate, and handling time of each type
2 key predictions of the optimal diet model
- If the encounter rate with the most profitable prey (prey 1) is above a threshold level, only this prey should be eaten
- The encounter rate with prey 2 should not affect the decision whether to feed on prey 2
A test of the optimal diet model with great tits
mealworm density
Presented mealworms at varying density and encounter rate
Prediction:
- At low prey densities, both types should be eaten
- At high prey densities, even when the large prey is less common than the small prey (0.3:0.7), only the large prey should be eaten
Result:
- pretty much that except some small prey were still taken
The optimal patch (of food) residence time
- A forager should stay in the current patch until its rate of intake is equal to the average rate of intake in other patches
- A forager should stay longer in a patch if the travel time between patches is longer
Travel time great tits experiments
- became more difficult to find more mealworms the more they caught them
- greater travel time increased duration in the patch
same as predictions
Simple mathematical models predict… (3 things)
all related to food
- what food types animals should choose,
- where they should forage, and
- how long they should feed in a patch before moving on to another patch
The moose dilemma + solution
dilemma: balancing (i) energy-rich, sodium-poor terrestrial plants and (ii) sodium-rich, energy-poor aquatic plants
The moose solution: such a model predicts that a moose should spend ~18% of its foraging time on the sodium-rich, energy-poor aquatic plants (approximately what moose do)
The central challenge for most animals is staying alive
predation
Predators to humans
Few large predators: bear, mountain lions
Large animals: bison, elk
Venomous animals: snapes, etc.
3 adaptations by humans that reduce their probability of being hit by cars
like predator avoidance
- avoidance in space
- avoidance in time
- inspection/vigilance
Analogy is imperfect because, unlike predators, car drivers do not attempt to hit pedestrians
Predator-prey arms race: vervet monkeys and crown eagles
- monkeys stay away from exposed tree tops
→ eagles fly among trees
→ monkeys have a special eagle alarm call and attempt to drop to dense bushes
→ eagle has short wings, which helps if it has to plunge through the canopy
Prey has evolved good counter adaptations
Balancing courtship and antipredatory behaviour: tree frogs
tree frog males use their call to attract females, but this might instead attract a bat predator and biting flies that transmit disease
Balancing feeding and antipredatory behaviour - 2 key questions
Food and patch choice: should an animal prefer lesser quality food at a safer place?
What is the optimal amount of fat?
- feed or scan
- keep feeding or flee
- stay in shelter or resume feeding
Balancing feeding and anti predatory behaviour - ants example
Ants could choose between two patches containing liquid ant diet of different concentrations. The high concentration patch was either safe or had a predator
Ants preferred the safer, less rewarding patch as long as:
- it was not much worse (as low as ~ 8 times worse concentration)
- the predator remained active
Horse sensitivity to weight
Horse racing: must calculate the exact weight they must put on a horse to equalise horses
0.3% of horse weight (1.5 kg) would cause a ½ body length speed reduction
Great tits optimal body fat
predation
- added weight decreases ascent angle
- can’t evade predation
- aviaries with more protection = fatter birds
- more fat in years with no predators
- More fat when food is less certain
- Less fat when predation risk is higher
Optimal egg hatching choice in red-eyed tree frogs
the over water hatching thing
- respond to vibrations signalling predation by dropping into the water
The leaves are associated with terrestrial predators (snakes and wasps), the water has aquatic predators (shrimp and fish)
strongly prefer to hatch @ 7 days night
if certain of terrestrial predation, hatch days 5/6
otherwise hatch days 7–11
Parasitism
- Major cause of mortality in all animals including humans
- For historic reasons, we term tiny parasites such as bacteria and viruses disease
Death from ‘parasites’
~37 million people in the world have AIDS, 2.1 million are infected annually, 1.1 million died in 2015
- More than 35 million people have died of AIDS since 1981
~2 million people, mostly children die from malaria each year
- Flu kills about 30,000 people a year in the USA
- SARS, avian flu, COVID-19
Malaria
- Malaria may have killed half of all the people that ever lived
- More people are now infected than at any point in history
- Up to half a billion cases & 2 million deaths every year, half of those are children in sub-Saharan Africa
Parasite avoidance in tree frogs
- Snails are intermediate hosts of a trematode parasite (Protozoa)
- Many parasites have a complex lifestyle where they move from one host to another
= tree frogs use them as a cue - Low density + low parasite infections rates
Compared egg laying sites with:
- No snails (control)
- Low density + high parasite infections rates
- High density + high parasite infections rates
- Low density + low parasite infections rates
- Low density + low parasite infections rates
No snails: happy tree frogs laid thousands of eggs
Infected snails: no eggs laid
Uninfected snails: laid some eggs, but more when few snails
Dramatic sensitivity to presence of parasites and host that transmits this parasite
Lion’s sex roles
Females raise young and hunt (risky)
Males eat, have sex and protect their female mates and their offspring
If the male of the family dies, a male will invade and eat her cubs because he won’t protect cubs that aren’t his, then they make new babies together
roles provide greatest fitness to both sexes
Cinderella effect (step-parents)
Step-parents are more likely to use violence against, and withdraw resources from, their children than are genetic parents
Why do males and females look different?
Who is usually more ornamented?
Because of sexual selection (which is a type of natural selection)
Males → tend to be more colourful and ornamented than the females in many species
Natural selection
differential survival or reproduction of individuals differing in one or more heritable traits
Sexual selection
differential reproduction owing to heritable variation in the ability to obtain mates
Optimal gamete size
result of?
result of divergent selection
Females: large gametes (egg)
Human egg is 100μm
Males: small gametes (sperm)
Sperm is 3μm
(bimodal distribution graph)
Investment per gamete
Female:
- Gametes: large, immobile
- Limited by: resources to gametes
- Can invest in relatively few gametes
Male:
- Gametes: small, mobile
- Limited by: access to females (this underlies sexual selection
- Can produce many gametes
Operational sex ratio
of males ready to mate / # of females ready to mate
Two typical outcomes of operational sex ratio + connection to male ornamentation
- Females are choosy, males are typically more promiscuous
- Male ornaments serve in female choice - Males compete with other males for access to females
- Male weapons serve in male-male competition
Sexual selection stronger in males or females
- Typically stronger in males
- Males can gain more than females from traits that increase their mating success
- Typically, males have more ornaments, so females would choose them, and weapons for male-male competition
Why are human sex roles confusing
- not clear which sex is more ornamented
- males larger and stronger
due to…
- biparental care + mutual mate choice
Grebe mating and parental care (video)
Mutual assessment of partners through coordinated movement and food exchange:
- Dancing, repeating each other’s movements
- Each gives a gift: sample of what each plants to contribute when they make their nest together
Shared parental care
Mate choice - who chooses who?
- The focus is on females choosing males (fundamental aspect of sexual selection)
- Do males choose females? If the investment is more equal like in biparental care, it makes sense for males to choose a female as well
Mate choice experiment - flies multistep process
How do female flies choose?
1. Male orients when discovering a female
- Taps her
- Sings a song to convince the female he is right for her
- Licking: instincts rely on smell and taste, so male gets both olfactory and taste info about the female from a variety of chemicals
- Male attempts to copulate, but female has full control over the mating
- If she accepts, she typically changes her posture so he can copulate
What do female flies consider while choosing males? (4 things)
- Smell/taste = body odour/flavour composition: compatibility and quality
- Song: compatibility and quality
- Body size: larger is better (below a species threshold)
- Recent matings: discriminate against more than 2 recent matings
Mate choice in pied flycatchers
2 vague things they need
female pied flycatchers prefer males who:
1. sing
2. have contrasting colours
Why do birds sing
means come mate + establishing territories
- each bird listens to its ones range of sound among the other
When do birds sing
Dawn:
- Light intensities are too low for finding food, no insects out
- Air is calm, which is good for sound transmission
- This makes dawn the best time for singing
calm air + cannot find food + female fertility is highest at dawn
What birds sing (m/f) + why
Typically, only the males (suggests that sexual selection is involved)
- Attract females
- In male-male interactions (territorial defense)
Song repertoire size and pairing date in sedge warblers
Large repertoire was preferred by females, so they paired with females early = earlier pairing date
increases fitness
- offspring older by migrating time
- those who nest earlier have a higher chance of successfully producing offspring (due to possible nesting failures)
Muting red-winged blackbirds experiment
Territorial red-winged blackbirds were trapped and their syrinx nerves were removed
vs. control males
Results
Muted males had:
- more intrusions
- more fights
- more territory losses
Shows: important function of bird song in male mating
So, why do females choose?
4 reasons
- Direct benefits
- Good genes (indirect benefits)
- Runaway selection
- Sensory exploitation
Direct benefits - mate choice
Females prefer males with the highest positive effect on females’ fitness (survival and / or reproduction)
This also exists in humans (e.g. gifts)
Nuptial gifts in scorpionflies = example of …
example of a direct benefit
- Females reject males that do not offer them prey (dead insects) + prefer males that offer large prey
- Direct fitness benefit of nuptial gifts: females that receive larger nuptial gifts produce more eggs and perhaps have longer lifespan
- females mate longer with those who bring them large prey (up to 20mm^2)
Female choice in barn swallows = example of …
relates to parasitism
example of a direct benefit
- Males have longer tails than females
- Males with longer tails have fewer parasitic mites → females prefer longer-tailed males
- Longer tail = fewer parasitic mites + more parental care of young + greater reproductive success + fledglings with larger body mass
- Females that choose males with longer tails likely have a higher reproductive success
Why can’t we just look at direct benefits
- In many animals, the males contribute only sperm (genes) to the “family”
- That is, the males provide neither food nor help in raising the offspring
- So direct benefit is less likely
Sage grouse
leks
- Only 66 males on leks in spring 2009
- Canada’s most endangered species: 400 individuals (2019)
used to be 10 million - Sagebrush plant is necessary for nesting and wintering habitat
- High quality cover is essential for nesting and brood rearing
- The leks are at traditional sites, used for many generations (indicates social learning)
Leks
a specific location where males come in spring every year, and when females are ready, they come and choose one of the males
SARA: Species At Risk Act
- A new Canadian law from 2003
- Scientists and NGOs have worked with the government on a bill that can truly protect animals and plants
Environmental organisations have won in 2012 a legal battle that required the federal government to take further action for protecting the sage grouse in Alberta
Ecojustice Canada
Ecojustice goes to court and uses the power of the law to defend nature, combat climate change, and fight for a healthy environment for all
Cock of the rock and leks video = example of…
mate choice copying
The male’s contribution: sperm transferred during a brief mating
- Males assemble in groups of dozens
- Female is dull, males are bright orange (sexual selection)
- Each male owns a particular patch of ground on which he displays alone, each tries to persuade the female to land beside them.
- Males compete and females raise the family on their own
Two females made the same male choice (remember fish colour copying)
Larval hymenoptera predator defense
secrete a viscous compound that glues together the appendages of predators, sometimes killing them
Annelid predator defense
Bioluminescent bombs: small sacs of green fluorescent light that annelid worms secrete when encountering predators, so the worm can escape the danger
Natural selection on antipredator tactics
If an individual makes an error with respect to the antipredator tactics it uses, its future reproductive success may be zero
- So natural selection operates very strongly
Two types of antipredator behaviours
- Help prey avoid detection by predators
- Function once prey encounters a predator
Benefit of avoiding predators
Avoiding detection helps as they decrease not only the probability of being captured and eaten but also the costs associated with fleeing or fighting back
Three ways to avoid predators
- Blending into the environment
- Being quiet
- Choosing safe habitats
Blending into the background - 3 examples
- Beach mouse
- Cephalopods
- Australian cuttlefish
Beach mouse camouflage
what mutation
- Fur coloration matches the background of the beaches on which they live
- Further from the beach = darker, closer to the beach = lighter (light sand)
- Mechanism: single mutation in the melanocortin-1 receptor (Mc1r)
Cephalopods (octopuses, squids, cuttlefish) camouflage
- Behaviorally change colour to blend into their environment, decreasing their chances of being attacked by a predator
- Have sharp night vision to also do this at night
Australian cuttlefish different camouflage types (3)
- used to hide from predators with good night vision
- Uniform camouflage (rare):
pattern: single skin colour that matched their background
used when: mimicking the rocks around them - Mottled camouflage (common):
pattern: small dark/light splotches all over skin, the size and colour mimicking its background
used when: background is composed of small rocks and dark algae - Disruptive camouflage:
pattern: changed colour + pattern, taking on large light and dark stripes to visually break up it’s body to not look like a cuttlefish
used when: sometimes used to mimic background, or just in general so it doesn’t look like a cuttlefish
Gulf toadfish and being quiet
- Gulf toadfish are preyed on by adult bottlenose dolphins
- Dolphins orient toward the “boat-whistle” sound produced by male toadfish during breeding season
toadfish listen for pop sounds from dolphins, then reduce their call rates + showed higher cortisol
Choice of Nesting Sites in Parrots
TC nesters: bird taxa that contain both species that nest in tree cavities
OC “other” cavity nesters: species that nest in other sorts of cavities (ex. Termite mounds, the sides of cliffs)
- ancestral state was tree cavity nesting
- multiple independent evolutions of other cavity nesting
- likely due to predation pressures
bc OC nesters have longer nesting periods but not larger clutch sizes (=not due to competition)
Coevolution, Naive Prey, and Introduction Programs - environmental conservation
what type of cues
Olfactory cues from predators that were similar to predators in Europe that rabbits have evolved with: (foxes, cats, ferrets) Australian rabbits responded with adaptive antipredator behaviours
Olfactory cues from quoll: (an endemic Australian predator), rabbits did not display antipredator behaviours, leaving them susceptible to quoll predation.
Neuroendocrine changes
regulated by … cortex
Frontal cortex regulates the effect of stressors on behaviour, alters neurological and endocrinological responses to stressors
Mice with stressors (behaviour)
Expose mice to two different stressors: the odour of a predator OR physical stress via immobilisation using the universal mouse restrainer
- increased the circulation of the neurotransmitters acetylcholine, serotonin, and dopamine within the frontal cortex
predators caused more anxiety than being restrained
chlordiazepoxide (reduces anxiety in humans) = no more increase in these neurotransmitters
5 behaviours upon predator encounter
- Fleeing
- Approaching to obtain info
- Feigning death
- Signalling to the predator
- Fighting back
Examples of fleeing behaviours
most common antipredator response
- Bird flying into the trees for safety
- Fish heading for cover in a coral reef
- Embryonic tadpoles falling from a branch into the relative safety of the water
Meta-analysis: what determines if an animal will flee
The meta-analysis found that many variables, including the degree of crypsis, the distance to a refuge, morphological traits relating to defence, and the prey’s experience with a predator influenced decisions about when to flee
Flight initiation distance
distance between prey and predator at which point they decide to flee
The Genetics of Schooling Behaviour in Fish
Variation in the Eda gene (chromo 4) in part explains variation in stickleback schooling behaviour
Offspring from benthic (shore) population parents with the inserted promoter schooled in a manner more similar to pelagic (open water) than wild-type benthic fish.
Heritability of Conditioned Fear Responses - rodents
Hypothesis: if natural selection has shaped conditioned fear responses, then this suite of behaviours should be heritable
Fear conditioning = increased amygdala activity = increased heart rate, defecation, freezing behavior, etc = suggests it plays a key role in the conditioned fear response
Approaching predators benefit + definition + result
- Allow prey to gather important information to reduce chance of mortality
Characterized by: a series of moves toward the predator interrupted by stationary pauses and sometimes alternating with moves away from the predator
Result in: prey retreating, prey rejoining a social group of conspecifics nearby, or an escalation in which prey actually attack the predator
The Costs and Benefits of Thomson’s Gazelles Approaching a Predator
Pros:
- Decrease current risk of predation
- Allow gazelles to gather information about a potential threat
- Serve to warn other group members of the potential danger associated with predators
Cons:
- opportunity cost of lost time
Larger gazelle groups = inspection behaviour is most common and most pronounced = cheetahs move farther away = decreased prey mortality
Death feigning in insects
in response to a predator, an insect falls and then remind frozen, absolutely still (tonic immobility)
Death Feigning in the Adzuki Bean Beetle
Negative genetic correlation exist between the length of death feigning and the ability to fly
Long-duration = poor flyers
Short-duration = better flyers
and v.v.
beetles in populations selected for long bouts of death feigning had higher brain concentrations of dopamine than beetles from populations selected for short bouts of death feigning
die longer = happy
Signalling to predators purpose
prey transmit information to a predator to deter an attack, warning the predator of the dangers of contact, or that it has been sighted and may not succeed in capturing a prey
Warning Coloration in Monarch Butterflies
Monarch butterflies: ingest milkweed plants, which contain cardiac glycosides: chemicals that are toxic to birds, but do not harm the monarchs
If a bird predator eats a monarch: toxins in the monarch make the predator violently ill + birds learn to associate monarch colour with illness, so they avoid feeding on monarchs
predators learn from watching others get sick, also sense toxin upon only touching
Tail Flagging as a Signal in Ungulates
individuals flag their tails after a predator has been sighted, most often when the predator is at a safe distance
Many possible functions…
- Entice predator to attach from further = unsuccessful
- Warn group members
= Can make them close ranks or engage in other antipredator behaviours
- Signal to the predator that is has been spotted and that the deer will be too fast to catch
Chemical Defense in Bombardier Beetles - current
Predation threat = they mix chemicals from the reservoir and reaction chamber to spray a hot, acidic, noxious chemical of p-benzoquinones that injures predators
selectively aim
Chemical Defense in Bombardier Beetles - ancestors
M. contractus: oldest extant bombardier beetle species has a similar mechanism, suggesting that it is an ancestral characteristic
Rear attack = froth secretion, which builds up on the body of the beetle and wards off predators.
Front attack = forces the chemical secretion forward, along tracks on its forewings
Why? Lowers the temperature of the chemicals from 100°C to 55°C
Suggests that: spraying an extremely hot chemical secretion may be a derived trait, but that frothing and using the forewing tracks to disseminate a somewhat reduced heat spray is the ancestral version of the trait
Social Learning and Mobbing in Blackbirds
- Noisy friarbird: novel bird that neither had seen before and it looked nothing like their known predators
- Naive blackbird saw the friarbird alone. Model blackbird saw friarbird + a little owl (known predator)
- From the viewpoint of the naive subject, the little owl was out of sight, so that when the model mobbed the little owl, the naive individual saw it mobbing a friarbird
Results:
- Cultural transmission of information about what constitutes a danger: when naive blackbirds saw a model mobbing a friarbird, they were more likely to mob this odd new creature than if they had not been exposed to the mode
- Found the blackbird cultural transmission chain to be six birds long: naive birds who learned from models were able to become new models, etc.
Predation and Foraging Trade-Offs
Time engaged in anti predator activity could be spent on: foraging, mating, resting, playing, etc.
Antipredator behaviour can create pressure to perform other behaviours differently: forage in the vicinity of a refuge, mate at times when predation is minimal, etc
Predation pressure affects virtually every aspect of foraging
Predation and Foraging in the Grey Squirrel
Squirrels alter foraging as a result of predation pressure from red tailed hawks:
- Squirrels were more likely to carry food to an area of safe cover, particularly as the distance to safe cover decreased, instead of eating them where they found them
- More likely to carry larger (rather than smaller) items to safe areas
- Profitability of small food items is greater than the profitability of large food items, so in the absence of predation, we would expect that squirrels would always take any small food item that they encountered
- Total handling time for larger food items is great enough that optimal foraging models predicted that larger items should be brought to cover, where it is safe, before being eaten
Mate choice and female pronghorns → Good Genes
- In the 2 weeks preceding estrus, each female visits several potential mates that hold widely spaced harems
- Males cannot coerce or force copulation and do not offer resources
- Female sampling causes a small subset of males to gain most copulations
Good gene prediction: Male harem-holding ability would be positively correlated with offspring quality
Offspring benefits of male attractiveness
- positive correlation between male attractiveness (more offspring) and offspring survival to weening
- also higher survivorship over 5 years
- also higher growth rates
Mate choice for good genes vs. compatible genes
Only weak evidence compared to very convincing evidence for direct benefits
- More evidence for compatible genes
Problem: there is no clear distinction between direct benefits and good genes
Mate choice for compatible genes
- Major Histocompatibility Complex (MHC): a set of genes involved in immune response
- Large individual variation in MHC (heterozygosity is higher than expected)
- Evidence from mice/rats that females prefer males with MHC different than their own
- Large variation in MHC = it provides better immune response
- Preference for mates with different MHC insures inbreeding avoidance
Mate choice for compatibility in humans
- Subjects agreed about intensity but not about pleasantness (some found some shirts stinky, others didn’t)
- Subjects did not correctly identify the sex of the shirt wearer
- Negative correlation between MHC similarity and pleasantness (smellers found shirts of different MHC from their own more pleasant)
MHC analysis of couples
Results:
- Fewer shared MHC than expected
- Observed line was significantly, below 0 (expected avg)
Indicates that:
- Smell unconsciously plays a role in mate choice
- Some mate choice in humans is unconscious but relies on body odour that is related to MHC
Men (and women not on the birth control pill) show a small preference for people with a different MHC than their own
Models of runaway sexual selection
Assume that there are 2 gene loci in all individuals (males and females):
- Gene 1 codes for a certain male trait (e.g. bright colour)
- Gene 2 codes for female preference (e.g. preference for bright male colour)
- All individuals have these loci but there is heritable individual variation
The runaway:
- Females with stronger preferences for brighter males have sons that are bright and daughters that prefer bright males
- The colour and preference alleles may become linked over time
Runaway selection in stalk-eyed flies
- Stalk eyes are long on males and short on females, so we expect sexual selection
Field observations: Females prefer to sleep with males with longer eye span distance
- Individual variation in eye span distance
- Weak positive correlation between eye span distance and female preference
Experiment:
Artificially selected for long and short eye stalk lengths and had females mate with males at random (selection pressure was only on the male trait)
Conclusion:
- After a few generations, when females were given the choice between mating with males with short and long eye stalks, females from the short eye-stalk line preferred males with short eye stalks (predicted by the runaway selection model)
- In the long eye stalk line, females preferred males with long eye stalks, but no more so than control females (not predicted by the model)
- Predict it would have been detected with more time
- Male ornament is linked with female preference for the ornament
Learning and sexual behavior
- Until relatively recently, scientists assumed that sexual behaviour in most animals is innate
- Research in the past few decades has indicated effects of individual and social learning on sexual behaviour in a variety of animals
Assortative mating: +/-
Assortative mating: non-random mating based on phenotype
Positive assortative mating: the tendency to mate with others of similar phenotype
Negative assortative mating: the tendency to mate with others of distinct phenotype
Assortative mating in snow geese
Do snow geese prefer mates with the same colour (white or blue) as their parents?
Cross-fostering experiment
- Actual results indicated more like-foster parent pairs than expected by chance
= imprinting on the foster parent colour
Assortative mating can cause…
A major factor causing speciation (the formation of new species)
Humans positive/negative assortative mating
Do humans have positive assortative mating? Yes: age, culture, religion, class, etc.
- Meta-analysis shows positive correlations between partners in all 22 traits examined
Do humans have negative assortative mating? Yes: MHC alleles
Assortative mating for visual appearance in humans
Assortative mating for visual appearance in humans results from phenotype matching or imprinting on opposite-sex parents
- Judges found significant resemblance of facial traits between a woman’s husband and her adoptive father
- Girls who received more emotional support from their adoptive father were more likely to subsequently choose mates similar to the adoptive father
Mate-choice copying
a female observer is more likely to mate with the male that a female demonstrator chooses
Y (% chance of mating if he has recently mated) - X (% chance of mating if he has not recently mated) > 0
Mate-choice copying in guppies
- Female guppies typically prefer to mate with more orange males
Observer females preferred the male that demonstrator females “preferred” as long as the male was only moderately less orange (up to 25%)
Mate-choice copying increases the ________
variance in male reproductive success
Mate choice copying in humans
Women and men observers rated the attractiveness of individual men’s faces
Observers then saw pairs of images of a female smiling or neutrally looking at the man, and rated it for attractiveness
Women: weak mate choice copying where the observer was influenced by smiling women
Men: rated males who had someone smile at them less attractive (opposite)
Male-male competition
- Direct interactions
- Extra-pair copulations
- Sneaking etc.
Bison (buffalo) video
Rut: the annually recurring sexual excitement in (some) male mammals
- Males smell female genitals to assess receptivity before rivals approach
- Most male-male interactions involve only mutual assessment
- High testosterone = roar, pee on ground and roll in scent of their hormones, display
- Few result in fighting
- Combination of rolling and roaring is a clear sign there will be a fight
- One ton charging at ~50 km/h
Red deer (elk)
- Only the strongest males hold harems (which they have to fight to get)
- male red deer use roaring as an honest indicator of fighting ability
Males advertise their strength to males and females through roaring
If one male does not withdraw from roaring, parallel walk (mutually assessing)
If one male does not withdraw from parallel walking, they fight
- Most male-male interactions are settled with one withdrawing
- Roaring rates is correlated with a males’ harem holding ability
- Fighting is the last resort
Suggests that male-male competition for females has led to a social system involving fine-tuned communication designed to maximize access to females while minimizing injuries that result from serious fights
Common loons
25% of all territorial evictions are fatal
Duel
a formal armed combat between 2 people in the presence of witnesses, to settle differences or a point of honour
Dunnock extra-pair copulation video
Extra pair copulation: probably occurs in all animals that have social pairing
Dunnocks have bi-parental care
Female gains from cheating on ɑ male: both think they are the father, so she gets more parental care
(peck out cum)
Sneaking
- Occurs in many fish (with external fertilization)
- Sneaker male is smaller than the parental male and sneaks to fertilize eggs during spawning
- Midshipman fish (sneaker males, female, parental male)
Satellite males
Mimic females, confuse parental males who attempt to spawn with both apparent females, at which point they release their own sperm
Why is fighting necessary?
- Not “otherwise, many animals would be injured”
- Because it is not based on a proper evolutionary analysis of individual fitness; it implicitly assumes group selection
- Fighting would be common if it is the strategy that maximizes fitness
Evolutionary stable strategy (ESS)
a strategy that can not be invaded by alternative strategies
- Modified from game theory
- ESS was initially developed to explain the rarity of animal fighting, but is used widely to understand social interactions
The hawk-dove game (general)
- Suppose that 2 individuals contest a resource with a fitness value V and that they can adopt 1 of 2 strategies:
Hawk: escalate & continue until injured or until opponent retreats
Dove: Display but retreat if opponent escalates
- Injury cost is C
The hawk-dove game (ESS)
V>C
- dove not an ESS
- hawk an ESS
V<C
- mixed strategy ESS
- ESS when 1/2 hawks, 1/2 doves
- hawk advantageous when doves common, dove advantageous when hawks common
- advantageous to be rare
= negative frequency dependence
The hawk-dove game (specific)
V>C
frequency of hawks = 1
V<C
frequency of hawk = V/C
The two most common causes of fighting
- Access to better quality or safer food sources,
- Access to females
Winner effects
winning an aggressive interaction increases the probability of future winds
Proximate: high testosterone
Loser effects
losing an aggressive interaction increases the probability of future losses
Proximate: increases levels of glucocorticoid stress hormones + lowers testosterone
Winners typically
- Are larger or with better fighting ability
- Have better endurance
bc smaller might not escalate
Establishment of dominance hierarchy
- Winners spent more time on the food cup
- Over the course of successive encounters, winners attacked more than losers
and
losers retreated more than winners (Img. C) - Dominance relationships quickly develop with experience
Mating Ritual in Great Bowerbirds
special nests
- Build elaborate nests to attract mates (teepee-like nest or avenue nest)
- Nests are surrounded by every trinket males can find (leaves, bones, shells, acorns, berries, fruits, shiny rocks, bottle caps, glass, etc)
use a forced-perspective optical illusions to enhance their mating ritual
= reproductive success
The Descent of Man and Selection in Relation to Sex - Darwin
First evolutionary theory of natural selection saying it “depends on the advantage which certain individuals have over other individuals of the same sex and species in exclusive relation to reproduction”
Bateman’s Principle
choosiness
- Females should be the choosier sex because eggs are expensive to produce + a female’s potential reproductive success is limited compared with that of a male
- Female’s greater choosiness in mate selection should translate into greater variance in the reproductive success of males
Intersexual selection
individuals of one sex choose which individuals of the other sex to take as mates
Why is female choice of a male more prevalent?
- Females have more to lose by choosing a bad mate
- Females invest much more energy in each gamete they produce, so they should be choosier than males in terms of who has access to their gametes
- Females with internal gestation: invest lots of energy in babies, so they should be under strong selection pressure to choose mates that make healthy offspring
Male traits that play a role in attracting mates (secondary/epigametic sexual characteristics = beyond what’s necessary for mating)
- Ornamental plumage
- Bright colours
- Courtship displays
E.g. in male fruit fly pulse song, there is heritable variation in interpulse interval (IPI), which affects females mate choice
Intrasexual selection
members of one sex compete with each other for access to the other sex
Any male trait that ______________ will increase in frequency because _____________
Any male trait that confers mating and fertilization advantages + is heritable will increase in frequency in a population because males with such traits will produce more offspring than their competitors
Sexual selection more in mono or poly
Greater variation in reproductive success = stronger selection pressure: thus, sexual selection is stronger in polygamous/polyandrous systems than monogamous systems = more variation in poly success
A. burtoni cichlid fish observation of aggression and mate choice
- impact on females
- Females prefer dominant, aggressive males as mates
- Females saw the preferred male either win or lose a fight contest
- Looked at c-fos and egr-1 expression in females brain
Results:
Observing a preferred male win:
- Increased gene expression in both c-fos + egr-1 in the preoptic area (POA) and ventromedial hypothalamus (Vm) area of the brain, both linked to reproductive physiology and reproductive behavior
Observing a preferred male lose:
- Increased gene expression in both c-fos + egr-1 in the lateral septum (LS) of the brain, linked to anxiety-like behaviours
What do these results mean?
- Watching a preferred male win a fight primes a female for reproduction
- Watching a preferred male lose a fight triggers anxiety-like responses, which might reduce the likelihood of mating and reproduction
How human disturbances → female mate choice disturbances → genetic disturbances
increasing hybridization (lake victoria cichlids)
Human impact: lake pollution
Mate choice impact: turbidity = female cichlids in Lake Victoria cannot distinguish between colour patterns of males of their species vs other species, so they mate with males from other species
Genetic impact: higher rates of hybridization in Lake Victoria cichlid species
How human disturbances → female mate choice disturbances → genetic disturbances
Sexual imprinting
Human impact: conservation refuges or zoos
Mate choice impact: managing animal populations can eliminate suitable models on which to sexually imprint, which is important for establishing mate preference
Genetic impact: individuals may imprint on adults from the wrong species = maladaptive behaviours
How human disturbances → female mate choice disturbances → genetic disturbances
danger and decreased mating
Human impact: disturbances from censusing, habitat manipulation, etc.
Mate choice impact: animals may interpret this as cues of danger, causing females to spend more time being vigilant for danger and less time choosing mates, so low quality males end up with mating opportunities they would not normally get
Genetic impact: under extreme circumstances, this increased vigilance could cause females to skip breeding altogether
How human disturbances → female mate choice disturbances → genetic disturbances
Mating and parental investment in managed populations:
Human impact: conservation and management practices
Mate choice impact: in zebra finches, when the number of males is low, females may breed with males they wouldn’t normally choose, and these choices ripple into the next generation. If high-quality males are absent from a population, females may invest fewer resources in their offspring
Genetic impact: conservation biologists may force females to mate with low quality males, so females devote fewer resources to their offspring, making offspring less healthy. Children may also be less healthy and suffer adverse effects such as weakened immune systems.
Direct benefits model
selection favours females that have a genetic predisposition to prefer mates that provide them with tangible resources (above and beyond sperm) that increase their fecundity, so this phenotype should increase in frequency over time
I.e. females that choose males that provide an important resource (food, safe shelter, parental care, etc.) do better than less choosy counterparts
Good genes models + applies to
selection favours that females choose the males with genes best suited to their particular environment (e.g. superior foraging skills, ability to fend off predators). In doing so, they receive indirect benefits, as their offspring receive the good genes that led their mother to choose a particular male as a mate in the first place
Applies to: mating systems where the primary benefits received by females is the good genes their offspring receive due to their choice
Which male traits should females accept as honest indicators of a male’s genetic quality?
- Honest indicator traits should be costly to produce (e.g. peacock tail, colour)
- Costlier trait = more difficult to fake = likely true indicator of good genes
Parasite Resistance and Good Genes
Parasite resistance is a costly trait:
- Females that choose males with strong resistance to parasites receive indirect benefits by passing parasite resistance genes to offspring
- Sexual selection should favour using other male traits that correlate with parasite resistance (e.g. body colouration) since the parasites might not be visible
Hamilton-Zuk hypothesis (parasitism)
females often choose the most colourful (and least parasitized) males
Sticklebacks and MHC cues experiment
- females use proximate chemical cues to assess the MHC allele quality/quantity of potential mates
- MHC peptide ligands are assessed via odour
Exposed a female to two different water columns: water from a male’s tank, or the same water, but supplemented with MHC peptide ligands
- If a pair had suboptimal number of MHC peptide ligands: addition of synthetic ligands made the odour on that side more attractive, suggesting that # of peptide ligands was the cue being used to select males with good genes
- BUT If a pair had optimal number of MHC peptide ligands: addition of ligands made the odour associated with that column less attractive
Sticklebacks and MHC peptides of former mates
foraging females were repelled by the MHC peptide ligand odour of their former mates, as this reduces the chance that they accidentally raid the nest of their mate and cannibalize their own eggs (females raid nests for eggs to eat after giving birth which are guarded by those males)
Runaway sexual selection models
One locus houses alleles that code for female preference, and the other houses alleles associated with the male trait that females prefer. Over evolutionary time, specific alleles from the two genes become associated with each other—when one allele is present in male offspring, the other allele is likely to be present in female offspring from that clutch
Sensory bias model
when a male trait first emerges it is preferred by females because it elicits a neurobiological response that is already in place in females, and that such a response initially is not associated with mating preferences
Sensory bias model - berry example
Red berries are the most nutritious food source available to a bird species, so natural selection favours the neurobiological circuitry in females that gives them an innate preference for red. Now, if red feathers should suddenly arise in males of this normally species, birds with red feathers may be chosen as mates because the female’s nervous system is already set to respond preferentially to red objects
Frogs and Sensory Biases
Physalaemus pustulosus and Physalaemus coloradorum:
- Males call to attract females with a high-frequency “whine”
- Females detect this through their basilar papilla
- Pustulosus males add a low-frequency “chuck” to the end of their call, and pustulosus females prefer this sound
- Females detect this through the amphibian papilla
The preference for chucks in female pustulosus was the result of an already in-place sensory bias in favour of such low-frequency sounds
Common ancestor of both frogs did not use a chuck call
- preferred chuck as soon as it appeared
Sexual conditioning results
After exposure to a conditioned sexual stimulus, males are quicker to copulate, become better competitors with other males, display higher levels of courtship, and produce more sperm and progeny
Sexual imprinting
When young individuals “imprint” on the behaviour and morphology of adults (almost always parents) and use these characteristics to guide their subsequent selection of mates (often restricted to a small developmental window)
2 ways of experimentally examining sexual imprinting
- Cross fostering: test if offspring copy mating preferences of adoptive parents
- Novel trait approach: offspring are raised by parents that have some novel trait introduced by an experimenter
Novel trait approach on the mannikin bird - red feather
Would offspring raised by adults with red feathers display a sexual preference for individuals with red feathers when they matured?
YES
Males with a red feather mother preferred females with red head feathers
Females with a red feather father preferred males with red head feathers
Suggests that young mannikins imprinted on the red head feather of their parents, and expressed a preference for such birds when they matured
Neurobiological underpinnings of imprinting - dendritic spines and zebra finches
- More dendritic spines = better at learning about potential mates
- Decrease in dendritic spine density soon after imprinting
Zebra finch: found decreased spine density after exposure to a female on which it completed the process of imprinting, as it was less reliant on learning about whom to choose as a mate
japanese quails
Adult male Japanese quail learn to stay in areas where they have the opportunity to mate with a female
Experiment:
a. Brown adult males who already sexually imprinted on brown females (because they were raised with other brown quail) was presented with a brown and blond female
b. In the lab, males were only given the opportunity to mate with blond females, so males learned that the presence of a blond female meant a mating opportunity, but the presence of a brown female did not
adult-stage learning about who was likely to be a receptive mate overrode the effects of early sexual imprinting
Mate-Choice Copying in Black Grouse
occupy leks
Placed a dummy female on a male territory and had females observe
Females were more interested in mating with a male that had copulated with other females on his territory, even if they were model females
Mate-Choice Copying in Sage Grouse
The unanimity of female mate choice would increase as more females mated on a given day, because more opportunities to observe and copy mate choice would exist
= mate-copying
Role of oxytocin in mouse mate-choice copying
OT knockout mice: appear to learn normally, except in the context of social learning and mate choice
- did not copy female mate choice, but showed no other differences in their ability to learn
Oxytocin plays a role in mate-choice copying in mice
Oxytocin (OT)
a neurohormone that plays a role in social behaviours including mate choice, maternal bonds, and individual recognition
Song Learning and Mate Choice in Cowbirds
Most songbirds learn the songs they sing from tutors
When mature, birds mated with others from the same rearing regime (IN or SD parents) in which they were raised
Males copied the songs of the adult males with which they were raised
Females preferred songs that were like those of the males with which they were raised (either copying song preferences of adults females in their population or imprinting )
Interference (male-male competition)
form of male-male competition where one male interferes with a second who is attempting to mate with a female (common in amphibians and insects)
E.g. European earwig: heavier males more successfully interrupt copulation
Male-Male Competition by Interference: Elephant Seal
In some cases, females solicit males to remove a rival during copulation
Elephant seal: males form large harems where a dominant male that is larger than females defends the females
- if not dom male tries to copulate, female protests and dom. male usually comes and fights off rival
What is the relationship between sexual size dimorphism and male-male competition in pinnipeds (seals, walruses, sea lions)?
Sexual size dimorphism (males larger than females) should be greatest in species where harem size is large, as this is where male-male competition is usually most intense
Sexual selection for large body size should act more strongly on males, so as harem size increases, female body size should remain constant
Relative size of males also increased as harem size increased, but the relative size of females stayed fairly constant
Cuckoldry
when a male tries to outcompete others by sneaking into a parental male’s territory and cuckolding the parental male
Male-Male Competition by Cuckoldry: Bluegill sunfish
Three different reproductive strategies:
1. Parental morph
- territorial
- large
- wait for eggs to hatch, defend and oxygenate them
- Sneaker morph
- smaller and less aggressive
- don’t hold territories
- hide near a parental male and swim quickly into a territory while the parental male and female are spawning, shed their sperm, and swim away
= CUCKOLDING - Satellite morph:
- look like females, so they position themselves between a spawning pair, releasing his own sperm
Sneakers invest in producing many short-lived, lower-quality sperm (large testes)
Parentals invest in producing fewer but higher-quality sperm
Male-Male Competition by Cuckoldry: Springtail bugs
- In the presence of other males, male springtails produce fewer but more attractive spermatophores (trade off between quantity vs attractiveness)
- Males modify the number and attractiveness of their spermatophores as a result of indirect competition from others
= subtle male-male competition
Spermatophore + direct or indirect benefits from choosing
a capsule of sperm that sits at the tip of the stalk. Later, a female chooses which spermatophores to pick up in their ovipore
Females pick up only one spermatophore and receive indirect benefits from discriminating between spermatophores
Aggression
- occurs when
- what is not aggression
occurs when animals either send threatening signals (e.g. animal flashes its canine teeth) and/or engage in some sort of physical combat (agonistic behaviour)
Ethologists generally do not consider predator-prey interactions as aggression
Result of aggression
Outcome of aggressive contests can determine where individuals rank within a dominance hierarchy, and that subsequently affects access to tangible resources such as food and mates
Aggression and Habitat Choice in the Banded Kokopu Fish
Pool with dominance hierarchy
top of the hierarchy have preferential access to food in the pool
- Subordinate fish had lower growth rates than more dominant fish + were more likely to leave their group and move to another pool in their stream
- Growth rate of subordinate fish was higher after such a move, likely due to fewer competitors
(Even though they received the same amount of food)
What do animals fight over?
- Obtaining food
- Securing space (territories, home ranges)
- Providing safety for their family members
Animal body weapons - found on (m/f) + indicate _____
- Most often found on males
- Usually honest indicators of: male size and fighting ability
Dominance hierarchies: how are they established + benefit of being at the top
rank orderings of individuals in a group based on the results of pairwise aggressive interactions
Individuals at the top: more food, more mating opportunities, safer territories
Are cooperation and aggression opposites?
NO
Individuals in groups often cooperate with each other in order to compete, often aggressively, with individuals in other groups
Ex. grooming behaviour to solidify within-group bonds
Fight or flight response
a surge in adrenaline and norepinephrine produces a quick increase in blood sugar, this sugar and oxygen are delivered to strategic areas such as the brain, skeletal muscles, and heart
- digestive and reproductive systems temporarily shut down
How does an individual decide whether to fight against conspecifics (members of the same species)?
or just fight in general
Ultimate perspective:
- when benefits of victory outweigh the costs of fighting, natural selection favours aggressive behaviours
Proximate perspective: endocrinological underpinnings:
1. Dominants: increased circulating androgens (e.g. testosterone) = more likely to fight and win
2. Subordinates: increased glucocorticoids (e.g. cortisol) = more likely to flee or lose
When do dominants and high-ranking individuals have increased glucocorticoids
Doms: when energy is spent fending off competitors
High-rankers: when challenged by many subordinates in their group
Colour as a dominance hierarchy signal
after losing a fight often signal subordination to reduce future costs, and the signal may inhibit its aggressive behaviours
Atlantic salmon:
- Dominant males: develop dark vertical eye bands
- Subordinate males: develop darker body colour
Swordtail:
- Dominant males: develop red lateral stripe
- Subordinate males: develop black lateral stripe
Salmon fight behaviour
- During fights, males go through a sequence of behaviors, including circling, charging, and biting.
- Losers, who often have increased levels of cortisol, swim close to the surface to avoid future aggressive interactions with individuals that have defeated them
Serotonin’s Effect on Aggression and social status - for different species
Fish:
More serotonin = less aggression + subordinate status
Mammals:
More serotonin = less aggression + high social status
Crustaceans:
More serotonin = more aggression + high social status
- When losers are administered serotonin, they become more aggressive
- If serotonin inhibitor is injected at the same time as serotonin, the effect disappears, suggesting an important role for serotonin in lobster aggression
Breeding programs accidentally leading to aggression
Captive-bred males showed much higher levels of aggression than males from the natural population, which was greatest in high-density populations
- Surprising because structured environments typically reduce aggression
due to accidental selection for aggressivity
Captive breeding
animals are bred for many generations in a controlled environment to minimize inbreeding + increase population size to a large enough number that endangered animals can be introduced back into their natural environments
Game theory models of aggression are used when…
- Models used when fitness of an individual depends on both its own behaviour and the behaviour of others
Resource value
Either straightforward (an item of food) or difficult to calculate (access to reproduction)
Net value affects decision to fight and duration of fighting
Individuals may value a resource differently
- A starving individual values food more than a fed one
- One who has learned (by investing time and energy) about their territory may value it more than an intruder
The hawk-dove game - 2 behavioural strategies
Hawk = a player escalates and continue to escalate until either it’s injured or its opponent cedes the resource
Dove (aka mouse strategy) = a player displays as if it will escalate, but retreats and cedes the resource if its opponent escalates
Bourgeois Butterflies example (hawk-dove model)
- Speckled wood butterflies do not have territories fixed in space, instead they are 3D well-lit areas
- Very little aggression bc it’s not worth the cost of a prolonged fight over the benefit of the short-lived sun patch
Sometimes the males that come realise it’s occupied and just leave
Only fight when they both think they are the patch owner (didn’t realise each other’s presence and entered around the same time)
The war of attrition model
Model where agonistic encounters are settled by displaying aggressively but not actually fighting
Assumes:
- Individuals can choose to display aggressively for any duration of time
- Display behavior is costly—the longer the display, the more energy expended
The model predicts that all contest lengths from this ESS function (2/V)e^-2x/v
- any choice of display time from this function leads to equal fitness gains to individuals
The war of attrition model dungfly example
- Females arrive at fresh dung patches to lay eggs, and males aggregate at such patches for access to females, where male-male competition occurs
Results: stayed different amounts of time (exponentially distributed) following model prediction
- Time to move from old to new patch was 4 mins
- Model then predicted equal fitness for all males, regardless of stay time
- That is what was seen
The sequential assessment model
Analyzes fights in which individuals continually assess one another in a series of “bouts”
- Assess fighting abilities in each bout = like statistical sampling
- More sampling = lower error rate = more confidence in estimates of fighting ability
Individuals should begin with the least dangerous type of aggressive behaviour and sample (probe) each other with respect to that behaviour over some time
more evenly matched sample longer
Nannacara Anomala fish sequential assessment model example
- Males for hierarchies and aggressive interactions range from changing colour to tail beating, biting and mouth wrestling, to circling while biting (most dangerous)
- Behaviour matched predictions
- Fights lasted longer when evenly matched and behaviours appeared in the same order (shorter fight = less behaviours seen)
Winner and Loser Effects in Copperhead Snakes
- No winner effects
Losers: were more likely to lose again + cede access to females. Two-time losers lost all contests with smaller opponents, suggesting that loser effect outweighed positive size advantage
Plasma corticosterone was significantly greater in losers than in winners or controls
= act subordinate
Winner and Loser Effects in Rivulus Marmoratus fish
penultimate thing
- WW fish were significantly more likely to win a fight than LW fish
- Wins and losses two moves back in time (penultimate) had less effect than wins and losses one move back in time, but still had an effect
Bystander effects + important for ___
when the observer of an aggressive interaction changes its assessment of the fighting abilities of those it has observed. Through observation, bystanders learn about possible future opponents (eavesdropper effects)
Important for hierarchy formation
Bystander Effects in the Green Swordtail Fish
Do swordtails who observe aggressive interactions between others change their own behavior when interacting with those they have observed?
More likely to avoid the winner of an observed contest, regardless of how badly they defeated their prior opponent
Subtle bystander effects when eavesdroppers interact with losers: eavesdroppers were less likely to initiate aggressive behavior and win against (1) losers that had persisted in their fights or (2) losers that had escalated their aggressive actions
Bystander Effects in the Cichlid Fish (Oreochromis mossambicus)
androgens
When males observe a fight between a pair of other males their androgen levels rise (significant increase in testosterone levels in this study)
- Increased testosterone may better prepare the eavesdropper for future aggressive interactions by indirectly affecting attention, learning, and memory in ways that may be beneficial
Audience effects
individuals involved in aggressive interactions change their behaviour if they are watched
Audience Effects in the Chimpanzees
“victim screams” during pairwise aggressive interactions function to entice support from observers (e.g. to intercede and break up the fight)
- Audience present = victim screams were longer + more intense
- Audience effect was seen only when at least one of the audience members held a rank in the hierarchy that was equal to or above the rank of the aggressor
= Successful screaming strategy
Social networks
groups within which information flows between individuals
Social network analysis (SNA)
incorporates visualization techniques, descriptive measures, modelling, and simulations to examine the dynamics of social networks
How Social Network Theory Can Inform Aggression in Pig-Tailed Macaques
young male policing
Policing: when a small number of males in a group break up fights
Removing policing males: increased aggressive behaviours + decreased affiliative behaviours
SNA findings:
Those remaining had fewer play partners and fewer grooming partners
Speed of information flow within the group decreased, reducing prosocial behaviours
Led to the formation of smaller subgroups
A. burtoni cichlid fish observation of aggression and mate choice
- impact on males fighting
- Males fought more intensely when watched by another male
- Watching or being watched during aggressive interactions had a similar effect on c-fos and egr-1 gene expression patterns in the brains of male fish
- When observers were larger than the fighters, changes in c-fos and egr-1 gene expression in fighting males were pronounced in brain areas associated with stress
= also fought less - fought more when watched by smaller or same size
Same-sex sexual behaviour (SSB)
same-sex courtship, pair bonding, and copulation
Prevalence: numerous cases in species of mammals, birds, reptiles, amphibians, insects, molluscs and nematodes
Distinctions in SSB terms
Sexual orientation: term used for humans only
Sexual preference: can be measured in many animals; may be labile (subject to change/easily altered)
Same-sex sexual behaviour: homosexual behaviour
Problem with the question “Why would animals engage in sexual behaviors that do not directly result in reproduction?”
assumes, wrongly, that heterosexual sexual behaviours typically result in reproduction
ex. extended female sexuality
Extended female sexuality
heterosexual sex with no direct reproductive benefits via conception
common
Examples: scorpion flies (food for sex), old world primates
Extended female sexuality in chimps + benefits
Sexual swelling for 14 days
Fertile only for 3-4 days
- females solicit sex frequently during sexual swelling and mate up to 5 times per h with most males in the group
benefits
- Material benefits (food, protection)
- Social bonding
3 explanations for SSB
- Errors (e.g. fruit flies)
- Used for initiating & maintaining social relationships (e.g. bonobos)
- Parental care by two females when sex ratio is female biased (e.g. Laysan albatross)
Male-male courtship in fruit flies
explained by errors
- Male fruit flies court intensively very young males
- Male fruit flies spend similar times courting immature males & females
- Male courtship of mature males is also prevalent
Same-sex sexual behaviour in bonobos
+ what hormone
Females engage in frequent same-sex sexual behaviour
Males do it but less often
Initiating + maintaining social relationships
- reconciliation + reducing tension
- also occurred independently of agonistic encounters
- Associated with increased levels of oxytocin (hormone mediating social bonding)
- Correlated with female bonding
may provide with material benefits such as food and protection
Bonobos (SSB) vs chimps (no SSB)
food sharing vs. monopolization
Bonobos:
- Two individuals were more likely to co-feed
- More socio-sexual behaviour
- More play behaviour
Chimps:
- A single individual was more likely to monopolize the food in chimps (97%) than in bonobos (59%)
Conclusion:
Suggests that food monopolization is associated with a lack of social bonding (or lack thereof)
Rhesus Macaques
SSB + effect on reproductive success
- Males that mounted each other were also more likely to support each other during conflicts with other individuals
- Same sex sex did NOT reduce male reproductive success
Laysan albatross SSB at kaena point
- Female pairs had courtship displays + mutual grooming behavior
- 59% female (heavily female-biased)
- 31% of pairs were female-female
need 2 parents to raise a child
- Shared a nest and kept only one egg
Fitness correlates of SSB: reproductive success is lower in female-female pairs only because hatching rate was lower due to much lower mating with males. Otherwise, fledging rate was identical
Sexual orientation
Sexual identification
Sexual behaviour
positively correlated in humans
Sexual orientation
One’s relatively enduring sexual attraction to the other sex (heterosexuality), the same sex (homosexuality), both sexes (bisexuality) or neither sex (asexuality)
Prevalence of SSB in humans
- Exclusive same-sex: ~1%
- Same-sex sexual encounter in the past 5 years: 2-11%
- Same-sex sexual attraction but no contact: 8-10%
Sexual identification of non-heterosexual behaviour is increasing
Reason: greater societal acceptance increases identification, behaviour, or both
16% of generation Z (born 1997-) identify as non-heterosexual
- 11.5% as bi
Is same-sex sexual behaviour heritable?
Heritability estimates of 0.2-0.5
- used swedish twin study
Genome Wide Association Study (GWAS) of SSB
loci discovered
Five loci significantly associated with same-sex sexual behavior:
- 2 combined
- 1 female-specific
- 2 male-specific
Manhattan plot
anything above line indicates statistically significant correlation between a gene an same-sex sexual behaviour
Congenital adrenal hyperplasia (CAH) and SSB
Rare mutation (1/15,000) typically in the gene coding for the enzyme 21-hydroxylase
Leads to fetal exposure to high levels of testosterone
Outcome for females:
- Females more likely to show SSB
30% of CAH women aren’t exclusively heterosexual, compared to 5% in the general population
Why would humans engage in same-sex sexual behavior?
- There is no evolutionary dilemma as long as same-sex sex does not reduce fitness
Has it reduced fitness over evolutionary time? We don’t know
Does it reduce direct fitness now? Yes
How are genes with low fitness maintained?
ex. SSB genes
Likely answer: such genes, at certain combinations, have positive effects
Fraternal birth order effect on SSB
- The tendency for older brothers to increase the odds of homosexuality in later born males
- Each additional older brother increases the likelihood of being gay by 33%
Only the # of older BIOLOGICAL brothers significantly predicted homosexual behaviour
- Not relevant either for gay men with no older brothers or for lesbians
Maternal immune response hypothesis for gay sons with older brothers
Maternal antibodies to male-specific proteins alter sexual orientation
- Mothers of gay sons with older brothers will have higher concentrations of antibodies for the neuroligin 4Y protein than mothers of heterosexual sons
- Neuroligins have been linked to social behavior in humans, mice & flies
Mothers of gay sons with older brothers had much higher antibodies for neuroligin 4Y
- Because fetal material enters maternal blood, women may produce antibodies to the foreign neurologin 4Y protein
- Such antibodies enter back into the fetus blood and bind to neuroligin 4Y to alter sexual orientation
But no direct link between neuroligin 4Y and sexual orientation
Conclusions: same-sex sexual behaviour in humans is normal/expected due to…
- Given the enormous genetic variation + environmental variation that determine human cognition and behaviour, anything occurring at a small frequency isn’t puzzling
- 2 fundamental rules of life: complexity + variation
