Remember the terminology:
(x means multiple)
New world monkey- howlers, cebus monkey.
Old world monkey- most of leaf monkeys, langurs, family presbytis, macaques.
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.
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.
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.
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.