Sexual Conflict and Infanticide: When the Battle of the Sexes Gets Ugly

Imagine you are a theoretical male mammal.  You recently hit puberty and are on the prowl.  By this, of course, I mean you go out looking for females to have sex with.  But woe is you: all the females are either already pregnant or nursing and in not in the mood for love.  You could wait around patiently for the nursing female to finish up and become receptive -- or you could do something absolutely sinister.  You could bite her young in the head and kill them.

"Egad! That seems rather churlish!" you might be thinking (if you are the sort to use words like "churlish").  But for a male mammal, such a dastardly deed could have evolutionary payoffs: when a female mammal loses her offspring she quickly becomes fertile once more.  Of course there is no payoff to the killer if she scampers off to have sex with somebody else.  But in many species it is precisely the killer of her offspring that she mates with.

The question is: why?

A new study in the April issue of Journal of Theoretical Biology attempts to answer this question, at least for primates.  Because we are primates - and yes we have been known to do a little infanticide now and then - the study may be of particular relevance.

James Lyon and his colleagues at the University of South Florida developed a little game that theoretical primates could play.  (biologists like to play little theoretical games: remember John Maynard Smith from my last post)  In this game there are three contestants: females with infants, potential fathers of the infants, and newcomer males.  Strategies for males in the game included either attacking, defending, or remaining neutral toward offspring, whereas strategies for females were to mate either with the insider male or the newcomer male.

While the math is a bit complex, the outcome is fairly straightforward: the best strategy for females is to mate with both males because it reduces the likelihood of infanticide.  A male, on the other hand, only won the game with infanticide if he was a new arrival (in which case he could not be the father of any offspring) or if he was a resident who had recently risen to higher rank (but only if he stands a good chance of mating with the mother of the offspring he killed).

The model appears to be spot on: females of many group-living primates are indeed polyandrous (they have sex with multiple males - see my earlier post on the geometry of promiscuity).  And when do males commit infanticide?  Rarely if there is any chance they might be the father of any offspring they kill and generally only when they are likely to be the father of future replacement offspring.

Lest you think this sort of behavior is the domain of furry brutes without the higher intellect reserved for our own brand of primate, I invite you to search Wikipedia for the word "infanticide" ...

Here, let me do it for you.