4) Social Systems - Mating Systems

Mating systems have 3 components:

• the number of mates an individual takes

• whether the male and female form a pair bond

• how long the pair bonding lasts

The evolution of these 3 components (# mates, pair bonding, length of bond) is driven by…

Sexual selection and

Demography

Sexual selection results from the relative investment inreproduction…

Males contribute (in most cases) only spermFemales contribute through egg production (more costly than sperm), gestation or incubation, and parental care of neonates.

Thus, sexual selection is normally female choice of mate(s).

At one time mating systems were thought to representcooperation between the sexes.

More recently mating systems have been recognized toalso represent conflict between the sexes.

The conflict arises because…

• males can maximize reproductive success by mating many times

• females maximize their reproductive success byproducing well-adapted, healthy offspring, (i.e. picking the best mate, not by mating multiple times

Let’s compare the energy expenditure on mating and parental care for (at least) monogamy and polygamy…

Mating effort consists of energy spent on courting, mating,and mate defense or guarding

Parental effort is that spent on feeding and caring for offspring.

There are a number of different mating systems…

Mating systems in vertebrates:

Class Parental Care Mating System

Birds male & female monogamy

Mammals female polygamy

Fish & usually the male polygamy Amphibians

Lizards absent variable

There are different forms of polygamy

•Polygyny – a male mates with more than 1 female.

Not all males reproduce, there is important variancein male reproductive success.

Likely to evolve when there is clumping, either ofresources or females, and when there is not highsynchrony in the receptive times of females.

When females clump, the polygyny is called:female defense polygyny

Example: female gorillas travel in groups to avoid predation by leopards. Males defend these groups from other males.

When resources are clumped, the polygyny is called:resource defense polygyny

Example: an African bird, the orange-rumped honeyguide feeds on beeswax. Males defend bee colonies from other males. Females come to the bee colony as a food source, and copulate with the male defending it.

When a male defends a territory containing enoughresources, he gains in fitness by increasing the number ofmates.

Polygyny arises when a female increases her own repro-ductive success by sharing the male with other femalesdue to the high quality of the territory, rather than havinga monogamous relationship in a resource-poor area.

The threshold resource level (or territory quality level)where this transition in optimum strategy for the femalearises is called “the polygyny threshold”...

There is one more form of polygyny - lekking

Males defend only a small area within a communal displayarea. Females come to the display area and choose among males.

In each lek there is a mating center, where copulationoccurs. Only males displaying adjacent to the copulationcenter get to mate. They are the older, stronger, moreexperienced males. Young males move from the peripheryof the lek toward the center as they age.

Fallow deer

Lekking shows that in some territorial species, someindividuals may not breed at all.

In these cases, territorial behaviour may limit populationsize. Jenkins showed this in research on red grouse. “Floater”males living in poor habitat and not holding territories did not breed.

Jenkins did 2 experiments to demonstrate that territorieslimited population size…

In the first he removed 60 males from their territories.55 of them were re-occupied (by floaters) the same day.

In a 2nd study area he removed 119 males - and 111 werere-occupied within a day.

As a control, another group of 269 males were not removed.During an entire summer only 3 males were lost (probablyto predators).

Therefore, territorial defense limits the number of individuals that breed, and this limits the maximum size of a population.

The birds moving into the vacant territories are floaters, not previously able to breed.

Under natural conditions they fill in when a resident disappears, which is rare.

Other lekking species include bats and birds of paradise

in a bat lek there were 85 males, but only 5 of them accounted for 79% of all matings.

In a grouse lek 2 of 8 males accounted for 70% of allmatings.

Thus it really is key to reproductive success to be adominant male in a lekking species.

What are the costs of polygyny?

For the males…• high mating effort

elephant seals may live 20-30 years, but theydominate and mate for only 2 or 3 years

• for all this effort, subordinate males may “steal copulations (if you know classic blues, think of Howlin’ Wolf’s “Back Door Man” and other, similar songs)

For females…• high parental effort

In species that are not polygynous, the most commonmating system is monogamy. In monogamy, the pair bondcan be short or last throughout adult life.

About 90% of birds are monogamous (at least for the season), and less than 5% of mammals.

Monogamy occurs when resources are not heavily clumpedand/or when female synchrony in receptivity is high. It alsooccurs when males provide a substantial part of parentalcare.

Even in monogamy some “cheating” goes on. Many broodscontain offspring sired by males from neighboringterritories, so-called Extra-Pair Copulations (EPCs).

The next mating strategy is polyandry. In it, one femalemates with multiple males.

It is rare, since usually the female has made a great investment, and the male little investment in reproduction.

The one situation that seems to lead to polyandry is afemale holding a large territory with multiple nest sites.Males each defend one nest, and raise the young. Thisreverses the usual energy expenditure pattern of the sexes.

In the best studied case, the bird’s clutch size is fixed(called a determinate clutch). The only way this femalecan gain fitness is by defending a large enough territoryto attract multiple mates to multiple nests.

Generally, in polyandrous species the sex roles arereversed…

• females are larger than males

• females defend territories

• females are brightly coloured to attract males

• males are dull coloured and do not defend territories

Polyandry occurs, for example, in arctic spotted sandpipers.

With the limited growing season (and thus food availability), females maximize egg production by ‘tricking’ males into incubating eggs.

The female abandons the male and the nest, moves on, and ends up mating with several males. In one month she produces five clutches of eggs (20 total eggs). The total egg biomass is 4-5x her body weight!

The last possibility is a variable mating system within asingle species.

Bachelor, monogamous, and polygynous males are found together in the same population.

In some migratory bird species:In the spring…• males set up territories• males escort females around the territory• females choose the territory in which to nest

Male behaviour (and mating system) depends on male status:

Status Territory defense Plant cover

bachelor weak sparse

monogamous stronger medium

polygynous very strong dense

Territories with dense vegetation have the most insects that birds use to feed their young.

Now think about female choice…

Is she better off choosing a territory with an unmated maleor one in which the male already has one or more females?

Already mated males have good, productive territories, butshe has to share it with other females.

Unmated males have lower quality territories. However,she doesn’t have to share, and may get help raising theyoung (Yeah, sure!)

Verner & Willson (1966) studied marsh wrens in western North America. This species migrates there from more tropical areas each spring. Males arrive first and set up territories.

Dominant males defend the best territories, and typically attract 2 or 3 females. Some males are able to defend territories of only intermediate quality. They are monogamous.

Some males on low quality territories are unmated (remain bachelors).

What determines territory quality?

Answer: the abundance of insects that are the marsh wrens’ food.

The abundance of insects is determined by the abundance of plants along the edge of the marsh. The abundance of plants determines the abundance of insects that feed on them.

How do the females choose?

Females choose after visiting the territories of several males.

Once the choice is made, it’s final. She stays within the chosen territory.

Males will accept any females that choose to settle within their territory. Usually, no more than three females will settle within a single male’s territory.

Why only three? What are the considerations that determine the female’s choice?

A female could choose to mate with an unmated, bachelor male. The result would be a monogamous pair.

Or –

She can choose to mate with a male who already has one or more females living within the territory.

If she mates with the bachelor -Benefits:• she has the territory to herself with the male• the male will help raise the young, but…Cost:• the territory defended by the male is of lower quality

If she mates with a male who has one or more mates –

Benefit:• the territory defended by this male is of higher quality. There is more food in the territory for her babies.

Cost:• She shares the territory with other females• She gets no help in parental care from the male

So, her decision is based on the difference in the quality of the territories among which she can choose.

If territories are of essentially equal quality, she should always choose to mate with a bachelor. There is no advantage to sharing the male.

Here’s a model developed by Orians (1969) for howa female should make the choice to maximize her fitness…

A female’s fitness (reproductive success) increases with anincrease in male territory quality.

Given 2 territories of equal quality, a monogamous female will have higher fitness than a polygamousfemale.

If a female has to share a territory, her fitness will belower than if she occupied the territory alone.

But clearly there can be a balance…

A polygamous female on a very high quality territory couldhave a higher fitness than a monogamous female on a poorerone.

What we should find in nature is that males with betterterritories should mate with more females and have moreoffspring.

And…

Polygamous females should have equal or greater fitnessthan monogamous females.

Here are plots of relevant data...

Now consider sexual selection…

In natural populations, as you’ve seen, individuals of onesex often compete for mates. Typically it is males competingfor females.

Males that win in this competition have a larger numberof mating opportunities, and are favored in natural selection.

Darwin called the process sexual selection.

The process results in:territorial defenseelaborate form, colour, and behaviourstructural adaptations like antlers, bright feathers, & mating dances

In the widowbird, females selectmales with longer tails. This happensin birds of paradise, as well. Theresult is incredibly elongated andelaborated tails. This is calledrunaway sexual selection.

Scientists finding runaway sexual selection have developedthe handicap principle.

The male that can thrive while carrying one of these tailsor another structure resulting from runaway sexual selectionmust be superior physiologically and genetically. If it weren’t, it couldn’t survive carrying around this handicap.

References:

Paton, D.C. and Ford, H.A. 1983. The influence of plant characteristics and honeyeater size on levels of pollination in Australian plants. In Handbook of Experimental Pollination Biology. C.E. Jones and R.J. Little eds. Scientific and Academic Editions, N.Y. N.Y.

Verner, J. and Willson, M.F. 1966. The influence of habitats on mating systems of North American passerine birds. Ecology, 47:143-147.