A shove from Big Mama is enough to shrivel your ovaries if you’re a female marmoset. But at least you get to help raise her kids.
When it comes to bearing young, the marmoset strategy is less is more. In colonies of these tiny New World monkeys, only one female gives birth, while other group members, both male and female, help tend the twins she typically bears. This unusual arrangement depends on an even more unusual physiological adaptation. The dominant female actually makes her subordinates infertile: their ovaries shrivel and stop releasing eggs. After many years of research, biologist David Abbott has figured out how such dominance is enforced--it’s a combination of pheromones, visual cues, and simple bullying.
Abbott discovered ovarian suppression among marmosets in the mid-1970s. Field studies had suggested that marmoset colonies somehow limited their birthrate. In a laboratory in his native Scotland, Abbott conducted a study in which he could observe the way marmosets behaved in various groupings. I noticed that when I formed groups, only the dominant female would ovulate, he recalls. At first he thought the phenomenon might just be an artifact of the animals’ confinement. Then bang! The penny dropped--I realized this is something the animal actually did.
Since then ovarian suppression has been observed in mammals as varied as mongooses, jackals, and the naked mole rat. According to Abbott, who is now a researcher at the Wisconsin Regional Primate Research Center, the phenomenon seems to crop up where animals have to cooperate to survive in a niche. For instance, the marmosets that Abbott has studied, Callithrix jacchus, live in the forests of northeastern Brazil, where they must band together to defend themselves from hungry birds and snakes. Because marmoset babies are nonetheless prone to getting picked off, the animals must have twins to ensure the survival of their genes--which makes cooperation even more necessary. Rearing marmoset twins in treetops is hard work; a marmoset mom needs other colony members to stand guard, forage, and help carry the young brutes. Unless they cooperate and have a division of labor and reproduction, says Abbott, they’re not going to make it.
But how does the dominant marmoset enforce the division of labor she wants? Most primates are competitive breeders: they hinder reproduction among subordinates not by ovarian suppression but simply by fighting. Among female baboons, for example, battles are common, and low-ranking females bear the brunt of most attacks. Although these long-suffering subordinates ovulate normally and mate just as often as high-ranking females, they have longer intervals between births. The bloody battles not only impair their overall health but also make it harder to conceive and bear offspring, possibly because of periodic or chronic increases in stress-related hormones such as cortisol and prolactin that somehow disrupt the animals’ normal reproductive function. But when Abbott looked for such things among marmosets, he found few signs of aggression and no increases in stress- related hormones. This made sense: since marmosets must rely on the help of others for successful breeding, they can’t afford to disturb the peace.
A gentler way of communicating dominance is pheromones--scent signals--and for some time Abbott and other researchers assumed that pheromones are what a dominant female uses to suppress ovulation in her subordinates. An experiment Abbott did showed that pheromones are indeed involved. When he plucked individual subordinate females from a colony and housed them in isolation, he found that they ovulated within 10 days, which is normal for a marmoset. But when he exposed them to the pheromones of the dominant female, the onset of ovulation took roughly 30 days.
Recently, however, Abbott has found that pheromones are far from the whole story: the mere sight of the dominant female is also enough to delay ovulation in a subordinate. When he isolated a subordinate female from a colony in a transparent plastic cage from which she could see but not smell the dominant female, it took her 30 days to ovulate--the same delay that had been induced by pheromones. Moreover, in both experiments the subordinate did eventually ovulate, indicating that although both scent and visual cues can alter the ovarian cycle, neither is powerful enough on its own to suppress ovulation completely. Such partial effects, says Abbott, make sense in the wild: if scent cues, for instance, could suppress ovulation entirely, a dominant marmoset that had either migrated or died could continue to suppress the reproduction of others through her lingering scent.
In fact, even scent cues and visual cues in combination are not enough to keep subordinate marmosets permanently in reproductive bondage. The key to lasting ovarian suppression, says Abbott, is a third factor: the physical presence of the dominant female. Once she has established her dominance through aggressive posturing and occasionally through actual fighting, she continues to throw her weight around in more subtle yet still effective ways--a gentle shove here, an intimidating scowl there. According to Abbott, it is the combined effect of these redundant cues--pheromones, visual cues, and physical bullying--that allows the queen to maintain her reproductive hegemony economically and peacefully. Unlike a female baboon, say, she does not have to do constant battle with her subordinates.
The details of the subordinate marmosets’ physiological response remain obscure. It appears, though, that their hypothalamus (a region of the brain associated with basic drives) develops a heightened sensitivity to estrogen, the female sex hormone. The estrogen induces the hypothalamus to slow its secretion of a crucial hormone, gonadotropin-releasing hormone, that ordinarily starts the cascade of biochemical events that ends in ovulation.
Human females don’t engage in ovarian suppression, but they do produce the same hormones as marmosets, and their ovulation may also be influenced by the stresses of their environment. What we know about the marmoset--as well as evidence from lactational studies in humans--suggests that the neuroendocrine mechanisms might in fact be exactly the same, says Abbott. Some women who have trouble conceiving, he thinks, may be suffering a physiological response to social or psychological stress. It’s possible there may be an evolutionary advantage in a physiological mechanism that makes it harder for a woman to conceive when she is under stress that could make her less able to care for her offspring.
The great thing about the marmosets’ way of reducing their birthrate is that it is peaceful. Unlike mammals that duke it out for reproductive dominance, marmosets have neatly organized who gets to reproduce when. As for the suppressed subordinates, they gain the protection of the group and a chance at one day taking the dominant female’s place. Instead of being beaten to a pulp, says Abbott, they get to survive and perhaps breed another day.
