Infanticide May be Behind the Evolution of Sex Behavior and Sex Roles in Mammals

Infanticide may be behind the evolution of sex behavior and sex roles in mammals.  Scientists have found that these brutal acts may be strategic, and that males want to ensure that the maternal investment of females is directed toward their own future offspring for the longest time possible. Yet mothers have a counterpoint; female mouse lemurs have evolved a highly-effective strategy to ward of males.

"In species in which infanticide occurs, testis size increases over generations, suggesting that females are more and more promiscuous to confuse paternity," said Dieter Lukas, the lead author of the new study, in a news release. "Once sperm competition has become so intense that no male can be certain of his own paternity, infanticide disappears-since males face the risk of killing their own offspring, and might not get the benefit of siring the next offspring."

When it comes to the mouse lemur, the females have as many mates as possible in a short amount of time. This confuses the paternity of the infants, which works in the females' favor. This particular strategy can evolutionarily be seen in the mouse lemur; the testes of male mouse lemurs may swell five to ten times during the breeding season.

When females use this strategy, it favors males that have sperm that outcompete those of others. This leads to males producing every larger quantities of sperm, leading in turn to increases in testis size.

In this latest study, the researchers examined 260 mammalian species to show that male infanticide occurs in species where sexual conflict is most intense, and reproduction is monopolized by a minority of males; in other words, infanticide occurs in animals where the females don't employ the same strategy as mouse lemurs.

"While it had previously been suggested that infanticide might be an evolutionary driver in mammalian societies-leading to females allying themselves with other females or forming bonds with a specific male in order to defend their offspring-we've now shown that this isn't the case: male infanticide is a consequence of variation in sociality, most commonly occurring in species where both sexes live together in stable groups, said Lukas.

The findings are published in the journal Science.