Female Mate Choice

Our guest lecturer for female mate choice was Rebecca Dowan. She's a doctoral student here at U of Michigan doing her thesis on female mate choice, so she's been studying up on it and reading all about it and everything.

Remember Darwin's two components- competition within one sex, like male-male competition, and choice of one sex for the other sex, as in female mate choice. Females invest more in offspring so they have more to lose by making a bad mate choice so you expect them to be more choosy. Females become limiting resource for males. We're going to be talking about female mate choice in primate species who live in social groups with more than one male in it, but we must remember that it also undoubtedly occurs in other types of social groups. In gorillas, for example, a female choosing which male's group to join also makes a mate choice.

Note that this is not necessarily a conscious decision; it's just behavior. Imagine some species of fish. Females who are about to spawn swim in shallow water, and will mate with anyone who is there. Males compete to establish territory in the shallow water. So, the bigger guys get the women cause they can edge out the other, smaller fish. Females aren't necessarily consciously choosing the big guys, it's just the way they behave.

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."

Mate Choice Mechanisms

Active solicitation
Seen in the brown capuchin- females in estrous will follow males around, making distinct vocalizations and facial expressions.

Refusals to mate
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.

Subtle signals
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.

What traits might a female select for in a mate?

Dominance rank
Traditional sexual selection theory predicts that females will mate with dominant males- they've shown themselves to be superior, so they must have superior genes. For example, in brown capuchins, the females solicit exclusively the dominant male, only switching to subordinates after peak fertility. This has little to do with the males' genetic qualities and more to do with their willingness and ability to protect children they have fathered. Proof: Recently deposed adult males are suddenly no longer attractive to females. Their genes haven't changed, only their rank.

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.

In langurs, patas monkeys, redtails, and blue monkeys (all polygynous species), females have been observed to mate selectively with newly transferred males. Why? Maybe to avoid harmful effects of inbreeding. In groups where known males are likely to be related to the females, it's better to mate with foreign guys.

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.

"Good genes"
This is the most intuitively solid theory, but there's little evidence for it. In theory, females could prefer males whose genes would give their offspring good traits like health or strength. Or, they could be choosing males with some other type of (ornamental) characteristic which makes them attractive. Male squirrel monkeys, for example, seasonally increase their body size by like 25% using water and fat. Females will actively choose the really big males so maybe this seasonal weight gain is a case of "good" ornamental genes.

Variety of mates
Having a variety of mates is good for two reasons. First, it may prevent infanticide because since she did it with so many guys they don't know for sure that the kid isn't theirs. Second, it could induce multiple males to take care of the kid.

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.

Return benefits
From Smuts' work with savannah baboons; certain male-female baboon pairs spend a lot of time grooming and spending time together at times when she's not sexually active, like when she's pregnant or lactating. these pairs are called friends. Females get defense from the males- males protect both her and her infants even when there's no way he could be the father. Males also hold, carry, and groom kids of their friends, as well as allow them access to good feeding areas. The males benefit by using their friends' kids as buffers against attacks from other males. Also, when the female does eventually come back into estrous, she'll be more likely to mate with her friend than with some other guy. So, she may chose who to mate with based on benefits she got or expects to get from this behavior.

The difference between choice and preference

This is an important side note. Sexual selection is a hot topic in animal behavior these days, but it's only recently that we've moved from studying male-male competition to female mate choice. This is due in part to empirical problems with figuring out how to separate the effects of female mate choice and male behavior.

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.

So how might male behavior constrain female choice?

Male-male competition; In baboons, it's especially intense. Dominant males have a lot of influence over what lower males do. Lower males might not be allowed to get near females so an observer can't tell whether the females prefer anyone but the highest-ranking males.

Male coercion of females; It is very common for males attack estrous females. In one group being studied, it occurs an average of 5 times a week. One in fifty attacks results in serious injury. Males will often use aggression to initiate and maintain consortships, as well. So, females might be constrained by this too.
But none of this has been tested. This is what our lecture set out to figure out;

To what extent do male tactics constrain female preferences?

The study group was a captive population of baboons in University of Washington's primate field station. The baboons live in a four-acre enclosure in a naturalistic setting.

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.