Diversity of primate mating systems

Remember the terminology:
(x means multiple)


Prototypes from each of our primate groups:

Prosimian- at least one species of tarsier.
New world monkey- titi monkey, night monkey, callitrichids.
Old world monkey- langur/mentawi island leaf monkey.
Ape- Gibbon.

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.

New world monkey- marikis.
Apes- chimps.

What factors shape the evolution of mating systems?

Sexual selection:
Think about parental investment. Females do more than males especially in primates since they do gestation and lactation. So females become a limiting resource for males. Males can't increase their reproductive success by caring for their kids since they don't know who their kids are, but they can seek more matings.

In polygyny, males put forth more mating effort while females put forth more parental effort. In monogamy, both do more parental effort. In polygyny there's stronger or more intense sexual selection, but in monogamy it's not so intense. Usually in primates you don't see equal parental efforts because females have the ability and tendency to put more into parental effort since they gestate and lactate.

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.


Two preconditions for the evolution of polygyny
1. It must be economically feasible for males to defend females. (Costs vs. Benefits.)
2. Males must be able to capitalize on potential for polygyny. This depends on characteristics that have evolved in their phylogeny, for instance, how much male parental care is required to raise offspring.

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)

Why most primates are polygynous

In most primate species, given the distribution of resources, females do usually congregate. You know, primates are mostly social. Because they're clustered but not in huge groups, they're both possible and beneficial to defend. Also important, females have long intervals means that they're receptive more or less one at a time so they're easier to defend.

To sum up:
"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."


Ok, so if polygyny is so great then why do some end up being monogamous? It's certainly not very common:
Birds- 90% monogamous.
Mammals- under 5% monogamous
Primates- 37/200=~18% monogamous.
(Traditional human societies are about 20% monogamous.)

Characteristics of monogamous primates

  1. Limited mating opportunities
  2. Male investment in offspring is high
  3. Male confidence in paternity is high (we're just talking probability here, not mental awareness)
  4. Little sexual dimorphism
  5. Territoriality & sex-specific aggression

Hypothesis for the evolution of monogamy

Evolves when males can only economically defend one female. This could be because of ecological constraints or because of the demands of parental care. Quite different from birds where parental care is the main factor. Someone always has to be brooding the eggs so they have to take turns so they can eat. This doesn't apply to primates so monogamy is driven more by the spatial distribution of females. Usually it's just too hard for males to defend more than one female.

To sum up:
"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."

#1 seems to be the case in new world monkeys. Crucial point is body size. They're really small but they have offspring that are large relative to the adult size to its really hard for a lone female to raise offspring. Males could abandon and mate more often, but they wouldn't end up passing along more genes.

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.


Generally happens when males don't succeed in monopolizing access to any females. This may be because of the minimal mammalian social system where males are ranging widely through the home ranges of several females. Because they don't have defended territories, they can't monopolize any females. This is seen in the orangutans as well as some prosimians.

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.


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:
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.