Mating Systems Flashcards
Review of parent-offspring conflict
- Trivers posited that conflict was both sibling-sibling and parent-offspring
- But parent-offspring conflict has proven challenging to demonstrate experimentally
- Places it can be studied: birds and clutches, + pregnancy
- Sometimes, birds lay synchronous broods (developmentally synchronized, hatch at the same time)
- Sometimes, they lay asynchronous broods (hatch in a sequence)
- David Lack hypothesized that:
- Asynchronicity produces a clear brood hierarchy - simple to reduce the brood if food is scarce and focus on just a few healthy survivors
- Synchronicity produces no clear hierarchy - parents might waste resources on producing many, but poor quality offspring
What did David Lack hypothesize about asynchronous and synchronous broods?
- Asynchronicity produces a clear brood hierarchy - simple to reduce the brood if food is scarce and focus on just a few healthy survivors
- Synchronicity produces no clear hierarchy - parents might waste resources on producing many, but poor quality offspring
Rob Magrath’s Blackbird Experiments (synchronous/asynchronous broods)
- Blackbirds feed their chicks worms, which are scarce when it’s dry
- Magrath experimentally made broods of four chicks:
- Synchonous (all at the same time)
- Asynchronous (clear size hierarchy)
- Results upheld Lack’s predictions: under poor conditions, asynchronous broods better, but under good conditions, synchronous broods produced more chicks
Conflict during pregnancy
- In humans, during placental development, embryonic cells invade the arteries of the mother that supply the embryo with nutrition
- These cells break down arterial smooth muscle and nerves
- This prevents the mother from constricting the arteries and so increases the supply of nutrients to the embryo
- In short, the embryo has evolved to extract more resources from its mother than the mother is favored to give
Netsling begging displays: a stable resolution?
- In theory, offspring should increase demand with need
- But, as we’ve seen, offspring should demand more than the parental optimum
- As a counter-adaptation, to avoid being tricked into over-caring, parents should base their assessment of need on an ‘unfakeable’ signal
Kilner:
- Canaries
- Nestlings beg more vigorously when hungry, parents provide more food as begging increased
- Provisioned pairs of chicks, but one had to beg for 10 seconds before being fed, the other for 60 seconds
- Unrewarded begging was costly, impacted chick growth –> begging is costly, which restrains chick selfishness
Table showing diversity of mating systems
Emlen and Oring 1977
- In 1977, Emlen and Oring wrote a highly influential paper about mating systems
- Viewed mating systems as outcomes of the behavior of individuals competing to maximise their reproductive success
Proposed that mating systems are:
- Influenced by resource availability, mate competition, and parental care
Male and female dispersion in space and time:
- When resources are clumped, so are animals, and males can easily control large harems
- How easily can a male exclude competitors?
- A species’ movement patterns affect how easily an individual male can monopolize a mate or multiple
Patterns of desertion/care by either sex (depending on the costs and benefits of parental care):
- If parents can increase reproductive success by seeking additional mates, they can desert
- If one parent can successfully raise young, the other may desert, promoting polygamy
- Availability of resources may determine if one parent can successfully raise young alone
According to Emlen and Oring, mating systems are influenced by what 3 factors?
- Resource availability
- Mate competition
- Parental care
Emlen and Oring: Male and female dispersion in space and time
- When resources are clumped, so are animals, and males can easily control large harems
- How easily can a male exclude competitors?
- A species’ movement patterns affect how easily an individual male can monopolize a mate or multiple
Emlen and Oring: Patterns of desertion/care by either sex
- If parents can increase reproductive success by seeking additional mates, they can desert
- If one parent can successfully raise young, the other may desert, promoting polygamy
- Availability of resources may determine if one parent can successfully raise young alone
Mating systems with no MALE parental care
Theoretically arise from a two-step process:
1) Female reproductive success limited by access to resources, so female distribution depends on resource distribution
2) Males should distribute themselves in relation to how the females are dispersed
- Males compete for females directly (A) or by competing for resource-rich sites (B)
Female distribution in space and time
Experimental evidence about female distribution in grey-sided voles
Experimental evidence: resources -> female distribution -> male distribution
Grey-sided voles:
- Female, then male, distribution influenced by food
- Did males respond to changes in food or in females?
- Kept females in cages and moved their positions each day to simulate movement on a home range
- When females were spaced out, males became dispersed - when females were clumped, males aggregated
- When males were kept in individual cages, distribution of females was not affected
Experimental evidence about female distribution in blue-headed wrasse
- Females spawn at favorite sites: individual females return to these sites every day to lay eggs
- Males compete to defend territories at these sites
- Experimentally, Warner removed either all of the breeding males or all of the breeding females from a site and replace them with males and females from a different site
- Males replaced, spawning sites unchanged
- Females replaced, spawning sites changed
- Females choose spawning sites -> males compete to defend sites that females prefer
Evidence in mammalian mating systems: Solitary females, range size defensible by males
- Economics of monopolizing a female is influenced by three main factors: female group size (sociality), female range size, and seasonality of breeding
- Solitary females, range size defensible by male
- Over 60% of mammal species: solitary female, male defends territory
- Small female ranges -> polygyny, large female ranges -> monogamy (but still usually abandons young)
- Female ranges are small enough for a male to defend, but a male could NOT defend a large enough area to have >1 female -> obligate monogamy - male might provide parental care, as in canids and marmosets
Evidence in mammalian mating systems: Solitary females, range size NOT defensible by males
- When females wander more widely, males too have to roam over wide ranges, and may associate temporarily with females when they are in oestrus
Examples of females being social
Leks
- In leks, males defend only a tiny patch of territory that has almost no resources
- Instead, males put more energy into complex advertisements
- Mating success in leks is highly skewed
- Has arisen in 7 species of mammales, 35 species of birds, some frogs and insects: not a common mating system
- Thought to be favored when males are unable to defend females themselves or the resources they require
- Widely dispersed resources
- High population density -> high rates of interference between males makes resource defense uneconomical
When is devoting energy to advertisements as opposed to resource and territory defense thought to be favored?
When males are unable to defend females themselves or the resources they require
Why do males aggregate on leks? Hypothesis 1: males aggregate on ‘hotspots’
Hypothesis 1: Males aggregate on ‘hotspots’ (places where the female encounter rate is particularly high)
- Example: lekking sandfly
- Males aggregate on vertebrate hosts, which are hotspots (females must visit them to obtain a bloodmeal)
- Several hundred males occur in a lek, each within a territory (2cm radius)
Why do males aggregate on leks? Hypothesis 2: males aggregate to reduce predation
- Example: in the neotropical frog, calling males suffer high predation from bats, who hone in on male calls
- Calling males are safer in larger choruses because of predator dilution
Why do males aggregate on leks? Hypothesis 2: males aggregate to increase female attention
- Males may gain from ‘stimulus pooling’
- To be a viable hypothesis, however, increase in pay-offs per individual needs to increase with lek size
- Shelly (2001) varied lek size in two species of Tephritid flies, where males aggregate on leaves and emit pheromones or acoustic signals to attract females
- Placed varying numbers of males in small pots covered with mesh and released hundreds of females nearby
- Larger leks (18-36 males) did attract more females per male than smaller leks
- BUT, in nature, males tend to exist in smaller leks!
Why do males aggregate on leks? Hypothesis 4: males aggregate around ‘hotshot’ males
- If some males have particularly effective displays (‘hotshots’), then it could benefit poor signalers to cluster around them and parasitize their attractiveness
- If this hypothesis is true, we can make two predictions:
1) If you remove the hotshot male from a lek, males will rearrange themselves to aggregate around the next most hotshot male
2) If the hotshot male changes his physical location, other males should follow him and set up new locations too
Why do males aggregate on leks? Hypothesis 5: male aggressions facilitate female mate choice
- Females may come to leks to assess male quality
- Leks may facilitate comparison among potential mates – viewing all males at once rather than sequentially
Obligate monogamy: why is monogamy the predominant mating system in birds?
- David Lack: monogamy is the predominant mating system in birds because each male and each female will, on average, leave most descendants if they share in raising a brood
- This hypothesis definitely explains monogamy in some species
- E.g. in seabirds and birds of prey, males and females share in incubation or males bring food to females on the nest – both sexes are essential to chick feeding
- Death or removal of one partner leads to complete breeding failure
Obligate monogamy: do pairs function better as they’re together longer, or do individuals just become better parents with age?
- Oystercatchers
- Socially monogamous, pairs breed year after year
- Newly-formed pairs have low reproductive success
- But, success increases with duration of the pair bond (up to 5-7 years)
- Pair bond duration influences reproductive success independently of male or female age
- If you remove either male or female, reproductive success gets lower with the newer partner, but then increases again over the subsequent four years
So why do birds divorce?
- There’s an initial cost to breeding with a new partner - all else being equal, oystercatchers should always stay with the partner with whom they have the most experience
- But there’s an annual divorce rate of 8%. Why?
1) Oystercatchers that desert get a better territory - deserter (initiator) benefits
- victim pays cost of parenting again
2) Oystercatcher is forced to change mates because they are usurped by a competitor - victim forced into a new territory
- initiator (usurper) presumably benefits
- other partner: loses old mate, gains new
In general, the trend is that the initiators of divorce gain, while victims lose
The situation is different in songbirds
- Songbirds are socially monogamous
- But if you remove the male during the nestling period, females are still able to raise some of the young on their own
- Thus, males increase reproductive success but are not essential
- Predominance of monogamy in these species likely does not arise (as suggested by Lack) because each sex has the greatest benefit as a result
- Instead, it arises because each sex has limited opportunities for polygyny
- These limited opportunities might arise because:
1) Strong competition among males may make it difficult for a male to access a second female
2) Females are likely to suffer in polygyny by losing out on male help, and so may be aggressive toward other females
Reducing polygyny in burying beetles
- Females lay eggs on a carcass: both parents et and regurgitate carrion to feed their young
- On small carcasses, males cooperate to feed offspring of one female
- On large carcasses, males attempt to attract a second female with a ”headstand” display
- A second female increases the male’s reproductive success, but decreases the female’s, so
females physically attempt to disrupt the male’s headstand posture
Resource defense polygyny
- Males monopolize females indirectly by controlling scarce resources that females need, like food or nest sites
- Especially common when food and nest sites are patchily distributed, and males can defend the best patches to gain the most mates
- Resource defense polygyny can arise i various ways, with different costs and benefits to each sex:
- Very occasionally: there are no costs to females if a male mates polygynously
- This usually happens if males contribute very little parental care. Thus, little is lost if they mate multiply
Polygyny with costs to females
- In most cases, females do suffer costs from polygyny because they have to share either the resource a male controls (food, nest sites) or share the male’s contribution to parental care
- Females may be forced to accept a polygynous system if males control all the suitable breeding habitat
- Face the choice between ‘accept polygyny’ or ‘forego breeding’
- In marsh wrens, females settle with mated males only after all the bachelor males have paired, later settling females have no choice but to accept the costs of polygyny
The polygyny threshold model
Evidence for the polygyny threshold
- Red-winged blackbirds
- Breed in marshes - females build nests in vegetation
- Males defend territories and attract females with singing and red epaulets
- Experiments by Pribil et al. studied a population in Ontario and tested four key predictions of the polygyny threshold model
Evidence for the polygyny threshold: is there a cost to polygyny?
- Removed females from 40 pairs of male territories -> one territory in each pair had two females and the other had only one
- Polygynous females suffered high predation, and their young were fed less frequently
- Polygynous females had significantly lower reproductive success than monogamous females
- Conclusion: yes, there is a cost
Evidence for the polygyny threshold: is female settlement influenced by male mating status?
- Removed females from some territories to give arriving females a choice between two males w/ territories or similar quality, but one w/ a female and one without
- Arriving females had a choice: polygyny or monogamy
- In all 16 cases, the first female to settle chose the monogamous option
Evidence for the polygyny threshold: can increased territory quality induce females to choose polygyny?
- 16 pairs of male territories matched for quality - removed females so one territory in each pair had one female, the other zero
- For territory w/ one female, built nest platforms over the water
- For territory w/ zero emales: built nest platforms over ground
- In 14/16 pairs, new females settled
- In 12 of those cases, she chose polygyny in good territory
Evidence for the polygyny threshold: Is polygyny on a good territory adaptive?
- Calculatred the benefit of nesting over water = 1.02 more young compared to nesting over land
- Calculated cost of polygyny = 0.62 fewer young raised compared to monogamy
- Benefits of nesting on a good territory -> costs of polygyny
- Conclusion: polygyny on a good territory is adaptive
But how realistic is the polygyny threshold model?
- Assumptions made here are similar to the Ideal Free Distribution model
- Resource patches available = male territories of different qualities
- Females assumed to be “free” to settle wherever they choose
- Under “ideal” conditions, they should settle where their reproductive success will be
greatest - But Ideal Free conditions don’t always hold in nature! (Despots!)
- it may pay 1st female to prevent a 2nd from settling
- And, it may pay males to change their mating system to benefit themselves
