Male and Female Sex Strategies

Question 1

Male Sex Strategies

Contest and Confrontation

Answer 1

Male-male competition for mates can happen through two main processes:
1) Aggressive interactions to gain access (and limit the access of other males) to a set of females. Leads to traits which improve success in fights such as large size, strength, weaponry, agility, or threat signals)

Assessment of competitive ability:

Clutton-Brock et al. (1979):
Tim Clutton-Brock and his colleagues studied male-male competition in roaring behavior in red deer off Scottish coast
23% of harem-holding males have injuries
6% were permanently damaged
Roaring advertises competitive ability –reduces risk of injury (?);
- Occurs during rut (mating season)
- Harem holders roar more
- Roar frequency predicted win frequency
- Playbacks: increase roar response

all about the pitch of the roar that is the indicator of the body size (thus how strong) of the stage. in the second stage they present a parallel walk, they size each other up as they walk up and down, comparing the opponents size and potential strength.

There is a logical procedure for this:

• two potential rivals approach each other, sometimes they don’t but most often they then begin a roaring contest which is the most common outcome of the two. and based on this one stag that its outmatched and choose to withdraw at which point the contest ends, but if they are evenly matched they usually then start the second phase of the assessment
• the parallel walk; after which one stage may wish to withdraw, but again if they are evenly matched and only if that’s the case, then a fight ensues.

Size and competitive ability:
Male reproductive success in northern elephant seals.
Reproductive skew: only 8% of males mated, but some males inseminated >100 females (Le Boeuf & Reiter, 1988)
Le Boeuf found not only that alpha (dominant) males mounted females more often, but that the females protested their mounts less often than mounts by non-dominant adults or sub-adults
Dominant male defends exclusive harem of ~40 females.

Looked across different species - sexual size dimorphism in pinnipeds:
Lindenfors et al. (2002)
Predicted that species with larger harem size should have larger size dimorphism
(corrected for phylogenetic correlation, so that each point represents a comparison between two closely related species)
there was a positive correlation between harem size and size dimorphism or the relevant in different size between males and females.

This is evidence that competition between males is responsible for increasing male body size, it is a clear example of a phenotypic or physical characteristic being selected for by contest and confrontation between males.

Clearly, interruption of the copulation attempts of males by other males involves male-male competition, but in the elephant seal example, this competition is likely initiated by a female to increase her probability of mating with the highest-ranking male in the area

Question 2

Male Sex Strategies

Alternative Strategies

Answer 2

We can divide these strategies further into three main forms.

Genetic polymorphisms:
Two or more alternative strategies with equal fitness payoffs
For example, seen in Bluegill sunfish where males arrive at the mating site first and scoop out small bowls in the sand, ready for females to lay eggs in them. They defend these eggs vigorously. They speak to females as they approach and females enter the nest circling around the male to assess him and the ball. if she stays the pair come to rest alongside each other in the middle of the nest. the male stays in an upright posture while they touch bellies and the female expels eggs into the bowl as the male expels sperm at the same time to fertilise them. this may happen several times over, after which males chase a few miles away and guard the eggs for up to 30 days before they hatch. However, he isn’t the only morph around.
- Sneaker males (B) Smaller, non-territorial, tend to look like females. Sneakers do “hit and run” sperm deposits as a visiting female spawns
- Satellite males (C) Deceptive morphs, similar to females. Also deposit sperm alongside spawning pair
These are alternative morphs: fixed for life, but produce fitness payoffs (egsneakers fertilise between 0 and 58% of eggs, depending on frequency in the population).

Another example comes from the Jack salmon. They breed at 1 year (vs 3 for normal males). Provides genetic exchange between cohorts, with bet hedging for uncertain environments. Genes differ between morphs (especially for brain and body development) Aubin-Horth et al. 2005. alternative strategy as;

• Greater proportion may return having spent only the 1yr in the dangerous ocean
• it commits 1yr old fish to mate with the 3yr old fish who are more likely to be genetically diverse and unrelated to them
• in the long term this division into two or more sections of the population ensures survival of at least some in particularly difficult and catastrophic years.

Another example of this strategy comes from the Ruff, a bird which mates on leks, which are a special breeding grounds where males defend very tiny territories. Lek species with 3 genetic morphs;
- dominant/territorial males with dark collars (86%)
- satellite males with white collars (15%) - less successful than the dominant males, but survive longer and have longer reproductive careers. because they compete less intensely and they are also smaller so they don’t need much food to survive.
- rare ‘faeder’ males (1%), permanent female mimics gaining sneaky matings. they obtain mating territories together with the females and steal matings when the females crouch to solicit a population from the roughed male. Although they don’t grow the same elaborate breeding plimige as the other males, they have much larger internal testees, therefore can produce more dense sperm.
Both strategies benefit from each other (white males are tolerated as their presence increases male density and hence the attractiveness of the lek; satellites don’t pay costs of territory defence)

Mixed Strategies:
Genetically monomorphic (same appearance), individuals play different “tactics” depending on current payoffs

For example, the male scorpionfly uses 1 of 3 different tactics for securing mating:
(a) some males defend dead insects as a nuptial (mating) gift that attract receptive females which feed upon the carrion;
(b) others secrete saliva materials on leaves and wait for females to come and consume their pretend nuptial gift; (c) others offer their females nothing at all but attempt to force copulation (raping females).
Males use whichever tactic yields the higher reproductive success, if they can.

Conditional Strategies:
Genetically monomorphic, individuals switch in response to condition/status (more long-term)

For example, older male orangutans
Older, flanged males – hold (defend) territories
Unflanged males – rove and force copulations, this strategy is viable and is called conditional because whatever strategy the male adopts depends on their condition.
Unflanged males reproductively successful (Utami et al. 2002)
Unflanged males can switch when social suppression due to flanged males is removed

Second example comes from a gelada (former baboon). they form large herds within which males form harems of up to five or six females. But the conditional element describes how and when they do it.
scenario of gelada with 3 females; how do you go about securing a harem? Essentially, there are two options;
- Follower Strategy - young male may choose to follow one or sometime two undefended females forming small harem of his own to which he may eventually add one or more additional females.
- Takeover Strategy - unseat a successful harem holding male, thereby gaining a full sized harem in an instant. more difficult and dangerous strategy.
Robert Dunbar evaluated the two options as part of his long term study on Gelada in the late 1970s and early 1980s.
follower - 0% probability of wounds
100% success rate
start reproductive career 6.2yrs old (so followers gain almost 1yr and a half head start on the business of producing offspring)
takeover - 50% probability of wounds
70% success rate
start reproductive career 7.7yrs old

overall there is equality in the payoff which maintains these two strategies in the same population at the same time
Dunbar (1984)

Question 3

Male Sex Strategies

Insemination and Implants

Answer 3

more generally known as sperm competition (competiton between sperm over fertilisation). In general, it extends to other behaviours that prevent other males from mating with that female subsequently.

this particularly occurs in those with internal fertilisation and especially even more so when females can store sperm from multiple males (e.g. honeybees and chickens).

Young honeybee queens have a single mating in their lifetime, which may last over three years in the wild. this happens on her first mating flight when she first emerges and on the wings, she can mate with as many as 40 different males. then returns to the colony where she then never leaves except for swarming, never mates again, but stores those sperm for the rest of her lifetime.

In chickens, sperm are viable for a rather shorter period, but still a long time, up to a month. they are stored in the sperm storage tubules at the uterus, and then released at a time they need them to fertilise an egg and the sperm may compete within the uterus and tract. one obvious way for males to engage in the business of sperm competition is to adjust their ejection at size. invest more sperm when;
(1) there is greater risk of competition (e.g. rats)
(2) there is greater potential reproductive benefit to a given mating
This depends on the reproductive value of the female
- Chickens: females with larger combs or a novel female
- Dung fly: longer copulation with females with more eggs

Prevent insemination by rivals:

• may insert Copulatory plugs into her vaginal tract, it physically prevents any further copulation for several hours before it eventually falls out.
• In one species of intestinal worm, the males sometimes copulate with rival males and seal up their genital regions as well
• In the wasp Cotesia rubecula, males prevent subsequent inseminations by mimicking females and soliciting copulations from rival males

The strange case of Xylocorismaculipennis:
Some species have “traumatic insemination”, also called hypodermic insemination. a mating practice where the male pierces the females abdomen and injects his sperm through the wound into her abdominal cavity.
Flower bugs, X.maculipennis, sperm swims around the female’s body until they find and fertilize her eggs.
But males also inject sperm into rival males. Sperm enter victim’s testes, where they wait to be passed on in subsequent matings.

Rhesus Macaques; a female and male; have two forms of evidence for sperm competition. in the female they is a copulatory plug, and in the male his testes are very large. producing a lot of sperm, these large testes and therefore the size of the testes may indicate the level of sperm competition.
Sandy Harcourt tested this across different primate species. as body size increase, so does testes size which varies among the social system. it is shallower with species which lives with males and females, impaired living species or in species with unique male groups. the slope is steeper with those who live in multimode groups like Chimps and the Gelada. in this test the more males there are in the group, the greater the male on male competition and the larger the testes.

There is evidence for sperm competition in humans.
Baker & Bellis:
Sperm count increases with time spent away from partner
100% of time together: 389 million sperm per ejaculate
5% of time together: 712 million sperm per ejaculate

Also proposed the Kamikaze Sperm Hypothesis:
some sperm appear to be healthy and straight while others appear to be really immobile and end up sticking to each other to form a non-moving max. these exist not to fertilise at all, but to wait around in the uterus and tract and to intercept and stick to other means in the event of a subsequent copulation.
In this experiment, men abstain from any sexual activity and then collected their ejaculate while they showed pictures denoting either a low degree of competition or a high degree of male on male competition. They found that when the risk of perceived sperm competition was high, men produced ejaculate that contained higher percentages of motile sperm. Doesn’t support hypothesis.

Mate guarding:
- Reduces sperm competition risk
- Protection of female from harassment
the female is sent off to defend the territory by placing scent secretion from glands near the eye onto these branches that you can see. males do this too, but only after the females scent and only the exactly the same branches. They provide an olfactory signal that the female has mated and therefore unavailable.

Infanticide:
Infanticide (killing) of the 1st male’s offspring and re-insemination by the 2nd
Male eliminates previous males progeny and replaces it with his own
rats & mice 21 species, carnivores 66 species, ungulates 11 species, seals 21 species, primates 58 species
——————————————————————————
Above and beyond the physical genitalia that are necessary for the act of mating, males often possess other traits that play an important role in attracting mates. These traits are referred to as secondary, or
epigametic, sexual characteristics, and include ornamental plumage, bright colors, and courtship displays. The underlying genetics of such epigametic characteristics are the subject of much experimental work (Wilkinson et al., 2015). For example, male fruit
flies “sing” to females during courtship by vibrating their wings.
Their courtship song, which is an epigametic trait, not only influences female mate choice but also may be important in the process of speciation in fruit flies (Spieth and Ringo, 1983; Tomaru and Oguma,
1994). One particular form of song in fruit flies, called “pulse song,” is very conspicuous during courtship, and the interval between pulses (the interpulse interval, or IPI) appears to affect female fruit flies’ choice of mates (Arthur et al., 2013;Ewing and Bennet-Clark, 1968;
Ritchie et al., 1999; Schilcher, 1976a,b).
Early work on the genetics of courtship song in Drosophila suggested pulse song might be controlled by a large number of different genes, each of which contribute a small amount to the expression of the song. Subsequent work, however, suggests that the genetics of song appear to involve three loci that account for much of variance in courtship song (Gleason et al., 2002).

Question 4

Male Sex Strategies

Viability and Vivacity

Answer 4

Mate choice: what should males select?

a) Complementarity
Mating your own species!
- Cross-species breeding rarely leads to viable offspring
- Processes of sexual imprinting & phenotypic matching ensure that individuals mate within-species (e.g. critical period for learning who is an appropriate mate).

However, males still participate in Indiscriminate sexual behaviour (e.g. Male toad mates finger, Beetle attempts to copulate with beer bottle)

b) High fecundity or fertility (the potential to produce offspring in the future)
Mormon cricket males produce a spermatophore (nuptial gift + sperm all in one!). by Gwynne (1981)

Spermatophore are costly to produce (up to 27% of mass) – large paternal investment for an insect

So there is strong selection pressure for choosing larger females who will produce more eggs.

Overall, there is an emphasis on choosing a mate that is young, has high fertility, and attractive.

Question 5

Female Sex Strategies

Female Mate Choice

Answer 5

Bateman’s Principle underlined Darwin’s suggestion showing that compared to males, females are more limited by their rate of reproduction. so that since they can’t increase fitness by producing more offspring than biology allows, the best way they can increase their fitness is by making the most of each breeding opportunity. they can do this by being choosy in the male they select. But do they choose? demonstrating that choice actually occurs isn’t completely straightforward.

An experiment on African widowbird:
Small birds with tails reaching ~0.5m
Do females choose males based on tail length? (Andersson, Nature, 1982)
At start of breeding season, males:
- Moult to achieve full breeding plumage
- Establish territories (of variable quality, e.g. more food)
- Build starter nests to attract females
- They offer no paternal care

Andersson wanted to see first whether tail length correlates with mating success; see whether the males with longer tails produce more nests and whether they had isolated them first, because we understand that the best are always picked first.
He found tail length predicts mating success, but 2 possible reasons:
(i) Female choice
(ii) Or just male-male competition – longer tail males just win better territories
Overall the classic experiment demonstrates that females can and do actively choose between males.

But …Why?

Important questions remain:
- Do females improve their fitness by choice (either in number or quality of offspring)?
- How? What are the benefits of choice that lead to increased fitness?
- How do they choose (discriminate) the best male (are there general rules/mechanisms)?
——————————————————————————–
Do females improve their fitness by choice (either in number or quality of offspring)?

Fruit flies (Partridge, 1980)
Two conditions:
(1) had a female that was caged with several males, so the female had the choice in who to mate with any other condition
(2) the female had one male, so they had no choice of mate
she then measured the competitive success of resulting offspring from the first instar larvae that were produced until adulthood. She found that the offsprings of females who had the choice were on average, better competitors. so the answer is yes choice did increase fitness.

Zebra finches (Burley, 1980)
Females given choice had chicks that had higher survival rates

No paternal care (Partridge’s conclusion):
“Flies may be able to detect heritable fitness in members of the opposite sex…”

Paternal care (Burley's conclusion):
Chosen males fed chicks more than males in the no-choice condition

choice do increase fitness, but there may be different kinds of benefit
These need not be mutually exclusive!
——————————————————————————
How? What are the benefits of choice that lead to increased fitness?:

There are four different types of evolutionary models of female mate choice:

• Direct benefits
• Runaway selection
• Good genes
• Sensory Exploitation

Direct benefits:
The direct benefits model of mate choice hypothesizes that selection favors females that have a genetic predisposition to prefer mates that provide them with tangible resources—above and beyond sperm —that increase their fecundity 
Benefits to mother. So pick males
......who help rear offspring
...who provide resources
...who are healthy
...who protect mother/offspring
Therefore not the kind of belief Patridge looked at, because those were indirect benefits.

nuptial gifts (e.g. food such as a blowfly) - Many insects (flies, crickets). Males present prey, sometimes encased in silk or froth. Randy Thornhill and his colleagues found that males court with the gifts, and females choose based partly on size of gift. Up to about prey of about 20 mm2, copulation length and sperm transfer were positively correlated with nuptial prey size. But note for long-term mating: Has to be reliable, also indicating future provision. A female that actively chooses males that bring large nuptial gifts produces more eggs and, in all likelihood, has a longer life span,both because of the nutrition she receives directly from the nuptial gift and the decreased amount of time she must allocate to hunting. Selection pressure on males to bring large nuptial gifts has also resulted in males stealing large prey from one another

Indirect benefits:
Increased offspring fitness. So pick males who will produce attractive offspring and have good genes. This is the kind of benefit we seen in the flies.

Runaway Selection:
Sir Ronald Fisher proposed an idea known as runaway sexual selection. Models of runaway sexual selection center on the relationship between alleles at two loci. In such 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 in a clutch, the other allele is likely to be present in female offspring from that clutch.

By choosing certain males, females are able to produce attractive offspring:
Fisher (1934) pointed out that females who mate with attractive males will have attractive sons, provided that the attractiveness is inherited
He suggested that initially females are attracted to features which have survival value and that these characteristics become exaggerated
This is known as Runaway Selection (or “sexy sons” hypothesis; though can also apply to female traits!)

Choosing good genes:
Good genes models propose that selection favors that
females choose the males with genes best suited to their particular environment—for example, genes associated with superior foraging skills or the ability to fend off predators. . In doing so, the females
receive indirect benefits, in the sense that their offspring receive some of the good genes that led their mother to choose a particular male as a mate in the first place.

Improved growth and survival of offspring of peacocks with more elaborate trains (found by Marian Petrie).
Peahens mated with males having different tail eyespot density. One such proxy cue is body coloration which has been studied extensively in birds and fish. Healthy males tend to be very colorful, while infected males have much duller colors.
Eggs were then artificially incubated (no female behavioural favour to different offspring)
Chicks were raised, released and outcome observed
(Petrie 1994, Nature)

Indirect benefits in pronghorns - Offspring from males that had large harems (green line) had higher survival rates than offspring from other males (orange line),
suggesting that females were selecting males based on some measure of a male’s genetic quality. (From J. Byers and Waits, 2006, p. 16344).

Color, parasites, and good genes - One reason stickleback females may prefer the most colorful (red) males is that color intensity is positively correlated with
resistance to parasites.

Sensory Desensitisation:
Sensory bias models posit that 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.
—the sensory bias hypothesis centers on how a female preference initially arose in a population, not how it was maintained in a population by natural selection over evolutionary time.

female mate choice is more prevalent, likely because females stand to lose much more than males by making a bad choice of mates. 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. In addition, in species with internal gestation, females typically devote a great deal of energy to offspring before they are born, and so should be under strong selection pressure to choose good mates that will produce healthy offspring.

How do they choose (discriminate) the best male (are there general rules/mechanisms)?

Indicator traits - Phenotypic (physical) characteristics that reveal individual quality. Need not be visual, they can be olfactory and auditory.

How can females take short cuts in their choice?

Uganda kob: females choose males at centre or in positions with the lowest predator risk (Deutsch 1992)
Males compete for central territories

Mate choice copying: copying another female on her mate choices, useful strategy for young ones that may miss out on a quality they did not see before.

Question 6

Female Sex Strategies

Multiple Mating and Maternal Effects

Answer 6

Jennions and Petrie (2000) suggests different ways that multiple mating increases reproductive success:
1) Trading-up: mating with subsequent males to improve on father quality (e.g. extra-pair copulations to secure “good genes”)
2) Post-copulatory control of paternity (e.g. some birds) if pre-copulatory choice is limited. Appear to be able to choose which sperm fertilises the eggs. this can be useful if their choice if their previous choice was limited so they can mate with multiple males and choose later.
3) Genetic bet-hedging – if cannot discriminate between male quality. especially valuable if females can’t discriminate between male quality or doesn’t have the opportunity to do so.

Inciting male competition:
- Loud, low frequency unmistakeable calls that attract attention
- Where vocalisations occur in both sexes, females initiate much earlier and for longer
- All females call, but low-ranking males keep silent
- Calls may lead to interrupted copulation by higher-ranking males
therefore females use these calls to manipulate male behaviour to make sure they get the higher ranking males.

Maternal effects:

• Maternal effects describe outcomes in offspring phenotype arising from physiological or behavioural influence of the mother (i.e. non-genetic effects)
• Females might allocate more resources to offspring of attractive males… though only limited evidence
• But birds can sometimes choose offspring sex! The mechanism is unknown, but likely enabled by females being the heterogametic sex (ZW, males are ZZ)

evidence of this comes from Pike & Petrie (2005) and her peacocks:
Maternal effects - sex ratio bias
Eyespots removed or added across two halves of a season (attractive or unattractive ‘versions’ of the same male)
Eggs collected after mating
Females produce more males to attractive ‘versions’

Question 7

Female Sex Strategies

Dominance and Reproductive Suppression

Answer 7

Social dominance and fertility:
- Enhance own reproductive success relative to other female group members through;
- Aggression & harassment; targeted against reproductively active, pregnant or lactating females (selfish behaviour)
- Contribute to early death of others’ offspring (eg. hunting dog, lions)

Reproductive suppression:

• In some New World monkeys (marmosets & tamarins) and many social carnivores (wolves, wild dogs, meerkats) only a few mature members of group reproduce
• Other breeders are evicted; or intrasexual conflict acts as a contraceptive in social subordinates
• Infertility maintained by behavioural and physiological mechanisms (stress hormones).

Given little (or zero) survival or reproductive future outside the group, individuals are forced to remain with a group and co-operate (some what forced) to rear infants

Benefits of suppression in meerkats:

• In 12 groups of meerkats: experimentally treated the subordinate females in 6 groups with hormonal contraceptive injection, and the other 6 groups with saline
• In Year 2, repeated this but reversed the group conditions
• Compared behaviour and outcomes in Year 1 and Year 2

Results:

a. Dominant females attacked subordinates less. benefits are that the females gained more weight during pregnancy.
b. Subordinates were less likely to be evicted from the group. Benefits of this is that pups were heavier when they emerged from the breeding den for the first time
c. This increased the adult female: pup ratio
d. And increased the provisioning (feeding rate)for pups of dominant females

These benefits continue after emergence:
pups continued to grow faster
All these results clearly demonstrate the benefits of reproductive suppression, but also how the usual costs of suppressing subordinates by the dominant reduces these benefits

Question 8

Female Sex Strategies

Cooperation in Offspring Care

Answer 8

Allomothering which can include:
- Suckling other’s young – cooperatively.
- Other provisioning (feeding food items), carrying, or protection – known as “helpers at the nest”
- Typically sisters, other female kin, grandmothers
- Found in birds, carnivores, langurs, guenons, callitrichids, elephants, humans

this has positive effects on offspring survival:
Phllis Lee’s studies show that calf survival can be as high as 1 in 5 or 20% in the absence of allomothers, whereas this halves when there are more than 5 allomothers.
Turke (1989) study shows that it can have similar effects in humans in Micronesia in the Pacific. The total number of surviving children of mothers who have come to the end of their reproduction, varies depending on the sex of the first two children. It was higher if the mothers first two children were daughters. the next successful outcome is if the first child was a daughter and the next was a son (early help as soon as possible). Having a son first was correlated with lower reproductive success He argues that this is due to daughters being helpers at the nest in a way that boys are not

Helpers at the nest - Ifaluk:
Girls are more economically useful to the family because:
• They work more than boys
• They stay in their natal households longer because of a matrilocal residence rule
• Food that girls produce is for household consumption, while food produced by boys (fish) is distributed to other households

Helpers at the nest - Grandmothers:

• Vervet monkeys: Infants are more likely to survive when there is a grandmother present in their group (protection of mother & infant from harassment)
• Elephants: Infant survival increases(transmission of knowledge across generations, protection from predators, some allo-suckling)
• Humans: Grandmothers contribute 10% extra food to households in Hadza (increasing infant growth rates, lowering costs to mothers and contributing to infant survival. Grandmothers contribute to family income, especially maternal grandmothers)