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
