Scientists Rush to Understand the Murderous Mamas of the Monkey World

For any species hoping to survive in the wild, the lifetime to-do list is agreeably brief: eat, mate, defend your turf and, above all, protect your young. It’s that last one that seems the most primally encoded, and for good reason: it’s hardly possible to pass on your genes if your babies die before they’re old enough to have offspring of their own. And yet not only do animals sometimes fail to protect the young of their species — they often kill them themselves.

Infanticide is disturbingly common in nature. It’s typically committed by males that take over a pride or pack and kill whatever babies are present to make room for the ones they plan to father. It’s not nearly as common for parents to behave murderously toward their own babies, and it’s much rarer still for a mother to be the attacker — especially among primates. Now a study in the journal Primates has revealed that in a species of monkey known as mustached tamarins, the mamas can be a deadly menace indeed — and their infanticidal tendencies can provide some insight into human behavior too.

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Mother tamarins do not have an easy job. Gestation is long — about 150 days — and they usually have twins, and those twins are usually big. It’s up to the mother to carry that double load around until the babies are old enough to navigate the forest canopy themselves. The only thing that makes the work tolerable is that tamarin troops cooperate to rear young, but the conditions have to be right. There must be plenty of males to do the protecting and provisioning, and there can’t be too many other females with babies of their own that also require attention.

An international team of researchers led by primatologist Laurence Culot of the University of Liège, in Belgium, ventured into the Amazonian lowlands of Peru to observe four troops of wild mustached tamarins to determine what distinguishes the babies that survive infancy from the ones that don’t. The data they collected made those differences immediately clear.

When there were at least three assisting males in the troop, the researchers found, the survival rate for infants was an impressive 75%; when there were two or fewer males, the number fell to 42%. When a mother-to-be was the only gestating female in a group, the baby she gave birth to had an 80% chance of surviving at least three months. When there were two or more pregnancies, that forecast plunged to just 20%. “Births must be spaced by three months or more,” the authors wrote, “in order to allow efficient helping behavior.”

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Some of the deaths that resulted from bad spacing or inadequate helpers were accidental or from unknown causes. But in a number of cases the researchers observed, infanticide was to blame. And unlike mothers in the marmoset, meerkat and social-carnivore species who kill the competing females’ offspring, the tamarins killed their own. In some cases, a baby would fall from a tree and receive only indifferent care when it hit the ground — which suggested that the mother either dropped it deliberately or wasn’t much concerned when it fell accidentally.

In one especially gruesome case, the mother killed her baby with a bite to the head, then devoured its brain and part of its upper body — a quick protein hit that she then went to sleep off. At the time of the killing, there were fewer than three helper males in the group, and another female was just a month away from giving birth to her own babies. “Before that,” the authors wrote, “the perpetrator had given birth to twins three times successfully when four or five adult or subadult males were present in the group.”

The explanation for such pitiless behavior is as cold as it is unavoidable: tamarin mothers are simply very good at balancing their genetic ledgers and know when they’re heading for a loss. If they’re raising babies that have a poor chance of surviving anyway, why make a pointless investment of time, resources and calories trying to keep them alive? Better to cut their losses, bag the babies and wait for a better season to breed.

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Humans recoil at such stark genetic number crunching, but while infanticide among our species is socially and criminally proscribed, it does happen — and far too often. And when mothers are the perps, they are often facing some of the same kinds of pressures as the tamarins: uncertain resources (read: money) and an absent or unreliable male. The overwhelming share of mothers in such situations do nothing so heinous as killing their babies, but one provocative 1999 study argued that postpartum depression (PPD) is an adaptive strategy to achieve the same genetic culling by different means.

Anthropologist Edward Hagen of the University of California at Santa Barbara, who conducted the work, looked at all of the things his sample group of PPD mothers had in common and was able to rule out some of the obvious variables, such as unemployment or lack of education. Repeatedly, he found that the two things that correlated the most strongly were the health of the infant and the amount of child-care support the mother was getting. “Mothers with PPD mother less,” Hagen wrote in his paper. Their depression informs them “that they have suffered a reproductive cost and that successfully motivates them to reduce that cost.” (The heat Hagen took for a study that put so unsettling a spin on a psychological condition for which mothers themselves are not to blame discouraged him from pursuing the line of inquiry much further.)

Clearly, depressed or unsupported mothers have options tamarins don’t, and in our species at least, nothing excuses willful neglect, never mind murder. But excusing something is not the same as explaining it. We may be the highest of the primates, but we remain members of that sometimes brutish club, and our lower cousins still do have plenty to teach us.

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