Mammals Killing Babies Leads to Bigger Testicles and More Promiscuity

Mammals have evolved all kinds of gruesome compulsions, but few are more disturbing than the rampant infanticide observed across species. It is extremely common for male mammals to ensure paternity of their offspring by killing a female's existing babies by another male, only to be supplanted by the next baby-killer down the line.

Believe it or not, there's a silver lining to this violent behavior. According to a study published today in Science, females in many mammal species respond to male infanticide by becoming much more promiscuous, which in turn creates uncertainty over the offspring's paternity.

This means that males are less likely to kill the resulting brood because they don't know whether or not they sired it. As a result, they must evolve their own response to this "paternity dilution," as the study's authors call it, to ensure they still have good odds of locking down fatherhood. Instead of axing babies, they develop larger testes to hold more sperm. Basically if they can't outcompete their rivals post-birth, the best option is to outcompete them right at conception.

"The increase [in testicle size] is present in the various primate groups, the felids, the seals, the squirrels, and the mice," lead author Dieter Lukas, a behavioral ecologist at the University of Cambridge, told me.

"Plus, as we are showing in the study, whenever infanticide by males is being lost, which also occurs in various lineages, testis are always larger than expected for a given body size," he added.

It's an sexually driven evolutionary battle with incredibly high stakes, and it has interesting manifestations for each individual species—especially when it comes to primates. For example, the Tonkean macaque is the only member of the macaque family that seems to have done away infanticide entirely.

"This might have to do with ecological factors such as predation," Lukas said. "Tonkean macaques are the only macaque species in which there seems to be an equal ratio of adult females and males in the social groups, whereas in the other macaque species females outnumber males."

"Maybe in the other species, in which the adult sex ratio is biased towards females, dominant males can still maintain sufficient control of several females such that infanticide has more benefits than costs," he said.

Another strategy that female primates have employed is a sort of sliding scale of sexual access. "In many primate species, females show extensive sexual swellings, obvious colourful skin structures that change size over the menstrual cycle," Lukas said. "These swellings appear to signal the likelihood of conception, being largest in size right around ovulation."

"It is a graded signal, with dominant males guarding females during their periods of maximum swelling, but other males having an opportunity to mate at other times," he continued. "Since the signal is graded, and not a perfect advertisement of estrus, females might be able to have a high chance of having offspring with the strongest, potentially preferred males, while creating sufficient confusion among other males to avoid harassment and infanticide."

Clever girls. Indeed, this wily outmaneuvering of infanticidal males is not just an assertion of maternal protection, but in a side result, shows just how much influence females have over the future of a species. "In the chacma baboon, up to 50 percent of the infants might be killed by males in these populations, a massive impact, more important than disease or predation," Lukas said. "It does show the strength of selection on females to develop counter-strategies."

At the risk of anthropomorphizing, the brutal bloodlust of these would-be fathers conjures up dynastic struggles in our own species's history. Maybe I've just been reading up too much on the Plantagenets recently, but it is unsettling how often the children of supplanted monarchs are unceremoniously purged to make way for new bloodlines. Perhaps the diversity of mammalian relationships to infanticide has cultural ricochets as well.

But that is a tangent for another study. Moving forward, Lukas and his co-author Elise Huchard, a behavioral ecologist at the Centre National de la Recherche Scientifique, will be tackling an even stranger form of mammalian infanticide, this time perpetrated by females.

"While we were collecting the data, we noticed that there are at least as many species in which females kill offspring of other mothers than there are species in which males kill offspring," said Lukas. "The two are not linked, and we are now interested to understand the factors that increase competition between females to the extent that they start killing each other's offspring."

If the dynamics behind that mystery are anywhere near as combative as those observed in this study, we're in for another round of delightfully uncomfortable revelations about our mammalian heritage. Bring it on.