Next time we will have a guest speaker on female mate choice.
Remember it was male-male competition or female mate choice that
determined sexual selection, and we did male-male competition last time
so you might think it would be logical to do female mate choice this
time, but we're skipping ahead to mating systems since the guest can't
make it 'til Wednesday.
About the test: It is written, not scantron or multiple choice. Mostly short answers, with a few essays. The emphasis will be on lectures. We're responsible for the readings, but they will not be emphasized.
Polygyny:
Prosimian- lemur.
New world monkey- howlers, cebus monkey.
Old world monkey- most of leaf monkeys, langurs, family presbytis,
macaques.
Ape- gorilla.
Polyandry: (rare in primates)
New world monkey only- some of the callitrichids are "facultatively"
polyandrous. This means they're not always so, but they can and in some
cases tend to be.
Promiscuity:
New world monkey- marikis.
Apes- chimps.
So, females go where the food is and males go where the females are. This is a good generalization for cases when male involvement is minor.
Ecological factors:
Environmental Potential for Polygyny (EPP)- how
resources are distributed.
Females choose their limiting resource which is food and males choose theirs which is females. So males will always be under selection to monopolize multiple females. For instance whether they'll be able to get access to females depends on how females are dispersed in space and in time. If they're evenly distributed in space, it'll be hard to hold on to more than one female so there's little potential for polygamy. But if resources, and therefore females, are clumped then one male can control access to many females. In this case there's a lot of potential for males to remain polygynous.
Also females' distribution in time affects the EPP- if they are synchronized in breeding /estrous, then it's more difficult for a single male to monopolize females since they're all receptive at once and it's too easy for other guys to sneak in while he's working on one. If they're spread out in time then it's easier for the same male to control the few females who are ready at each time. Note that these two kinds of clumping have the opposite effect of each other. Synchrony reduces defendability of mates while spatial clumping increases defendability.
So, like, there might be high polygyny potential, but males might not be
free to take advantage of it because babies will die without their
contribution. Ecological factors determine both EPP and ability to
capitalize on the EPP. Phylogenic factors also determine how well the
male can capitalize on the EPP. Economic feasibility and capitalization
ability both determine the degree of monopolization of mates which
determines mating system.
(There's a flow chart diagram for this which makes it easier to
understand than mere words)
"In most primate species, females congregate spatially in small, stable groups.
In addition, long interbirth intervals create a situation in which there are only a few reproductively active females per sexually active male.
These factors set the stage for intense male-male competition for the limited number of fertilizable females, and polygyny typically results."
Characteristics of monogamous primates
"Monogamy evolves when either sex has the ability to monopolize multiple members of the opposite sex either because of ecological factors do not permit them to or because of the constraints imposed by parental care.1. Monogamy evolves when male parental care is indispensable to female reproduction.
2. Monogamy evolves when aggression by mated females leads to their spatial separation and prevents males from acquiring additional mates."
In gibbons however, females are very evenly distributed in the environment and this seems to be because of mutual aggression between females. So a male might like to have a harem but the females won't have any of it. This has been difficult to prove experimentally, but they have done playback experiments. When you play female sounds then the female of a pair will charge over to the speaker to attack but the male will just sit there and be a dork. So the females reduce males' options until they have no choice but to be monogamous.
It also happens in large groups with multiple males and females. There may be two factors happening- it may not be economically feasible to restrict access to a large group, too big a job for one male. It has also been suggested that in like chimps and maybe marikis(brachyteles) it's that males need to reduce aggressive interactions between themselves because they've got to cooperate to defend joint territory from outsiders. They suppress their natural competition otherwise they'd all lose all the females. At least when they share they get some females. So this is all why males might be into promiscuity. What do the females get out of it?
Females must have a reason for mating with more than one male, too. Females' reasons haven't been studied as much. One reason might be to ensure fertilization. Also it might confuse paternity so it's not sure which male fathered her offspring so they're less likely to kill the kid. Or, maybe it's easier for a female to just mate with a male than to be subjected to aggression if she refuses. More on this later, but maybe it's just less risky for females to lie back and have done with it.
Polyandry
Rare in mammals in general, but occurs facultatively in tamarins.
Data from six populations of saddleback tamarins studied over 4
years.
Percentage of groups displaying each type of composition:
(X=multiple)
1f-1m 22%
1f-Xm 61%
Xf-Xm 14%
single-sex 3%
So maybe they're not completely polyandrous but it's the most common result. This may occur because they have unusual reproductive biology. For one thing they usually have twins. Offspring are quite large at birth; their combined weight can be up to 25% of the mother's weight. (OUCH!) By the time they wean, each offspring is about half the size of the mother. So she's providing enough milk to provide enough for two who together weigh as much as her! So females don't usually even carry offspring after second week. Males do this, or previous offspring. So males do much more carrying and seem to be completely necessary especially to escape from predators. Male parental care is limiting to reproductive success, and it's even higher with more than one male helping, so sometimes it takes two males just to raise one female's offspring.
Announcement: All overheads used in lecture are available on reserve
now.
Exam update: It's all short answer, like a few sentences.
Our guest lecturer today is Rebecca Dowan. She's a doctoral student here at U of Michigan doing her thesis on female mate choice. So she's been studying it and reading all about it and everything.
Mate choice may be defined as follows:
"Any pattern of behavior, shown by members of one sex, that leads to their being more likely to mate with certain members of the opposite sex than others."
Seen in the brown capuchin- females in estrous will follow males around, making distinct vocalizations and facial expressions.
In vervet monkeys, females will refuse to mate and will show aggression against courting males. In a study, over half of the courting males were refused, with 10-20% involving bites and chases by the females. Since vervet males aren't much larger, females can refuse pretty easily. Also, females will cooperate to defend other females from males. In rhesus macaques, females have also been found to successfully refuse.
In savannah baboons, there is too much sexual dimorphism for females to refuse the males and exhibit aggression toward them. Males will herd estrous females in consortships, which are short-term pairbonds that can last 1-4 days, while the female is in peak estrous. Consortships are characterized by the male's maintaining close proximity to the female. The female can decide where she wants to go, but the male follows. They are also characterized by a high frequency of mating, usually initiated by males. Also, there is a high frequency of males grooming females. (Usually, females groom males.) Although males in consort herd partners from other males, sometime a female will persistently approach other males, often of a lower rank than her current partner. Our lecturer has been studying whether or not females' approaches affect the likelihood of the consortship being challenged.
In rhesus macaques, females will maintain proximity to the lower ranking males. This has also been observed in Japanese macaques and some baboon populations. Also, in macaques in captivity, blood testes were used to establish paternity and rank was shown to not correlate with number of children fathered. Conclusion: There's a lot of variation.
Another reason could be to prevent infanticide, which is typically
committed by males who have just entered breeding system. Pregnant
females may solicit copulation with the new guy to make him think that
maybe her kid is his. Among the baboons at Gombe, females solicit more
from males trying to enter than from established males. In fact,
sometimes females will go a really long way to solicit new males.
Our lecturer has also seen this in the captive population that she's been
watching. Established males will herd females away from new males, but
the females will be persistent in following and bugging the new guys.
In Japanese macaques, female mate choice is responsible for circulation between groups. As time goes on, males rise in group rank but they also become less attractive to the females. So they leave for a new group after a few years because even though they will be of a lower rank, they'll get more sex.
This may be what is happening in Barbary macaques. Estrous females mate with numerous males every day, and the males do not interfere with each others' copulations. Males are also involved with a lot of infant care. There's not much evidence that males do more protection of infants that they're more likely to have fathered.
You will remember that the different sexes have often conflicting interests. An observed behavior might not represent male and female 's interests evenly. Often female mate choice will be constrained by male's tactics.
Preference is desires or propensities than an individual possesses. Preference does not always result in choice. Compare this to the definition above of choice.
So female mate choice is an action that can be observed and measured- it's what the female actually does. However, preference can't be so easily measured. So far investigators have only been able to measure observable behavior. We don't know how much the choice reflects females' actual preferences, since observed mating is usually a compromise between female and male interests. So female preferences can only be studied experimentally.
She basically collected daily observations, documenting who the females were mating with, who they formed consortships with. The focus was on consort pairs. Outwardly, consort pairs suggest that they're dictated by males more than by females. So she compared this data with that from experiments to test whether females have mating preferences at all, and if so, what they are. Does the female baboon like the guy she's in consortship with?
To do the experiments, she set up a room with three males isolated in cage areas that permit some movement. Dividers prevented the males from touching or seeing each other, but they know of each others' presence through smell and vocalizations, of course. A female is put into the room for a two hour period. Then the observers note who she approaches, who she presents to, who she grooms and hangs out with. In this way, she has greatly reduced the effects of male-male competition and male coercion. Of course, the males can and do still threaten the females, but they can't physically hurt the females.
No statistical results yet because the data collection has just finished, but the females seem to be doing different things than in the group. They do indeed seem to have a preference, often for someone other than their normal consort. Also some females demonstrate a sampler type of preference, checking out all their options pretty equally. Maybe they' like to mate with multiple males. This seems to imply that the females are constrained by male behaviors.
Why do this? Until we know to what extent male tactics constrain female preference, we don't correctly understand female mate choice.
Today we handed in our papers and reviewed some more for the midterm.