The main cause of infant mortality in some mammal species isn't disease or predators, researcher report, but rather infanticide practiced by adult males of the species.
There have always been reports of infanticide -- the killing of infant or young animals -- by adults of numerous species, including primates, but most scientists considered it an aberrant, unimportant pathology.
Not so, says primatologist Sarah Blaffer Hrdy, who as long ago as 1974 suggested such infanticide is the product of mammalian evolution.
It might even be a common feature of mammalian life, she has said in the intervening decades, spurring generations of researchers to document the behavior in hundreds of species.
Now a new study of infanticide across all mammal species has given support to her assertion.
Evolutionary biologists Dieter Lukas and Elise Huchard of the University of Cambridge say their study suggest only certain conditions favor the evolution of infanticide -- exactly the conditions Hrdy had originally proposed.
Infanticide is common in mammals that live in groups where reproduction is monopolized by a small number of males that often cannot keep their dominant position for long due to many challengers.
By killing off babies fathered by competing males, a dominant male -- or a new male that has successfully driven him off -- could improve the chances of having more of his own offspring, Hrdy has long suggested.
That's supported by Lukas and Huchard, who say out of 260 species studied, in nearly half of them males were observed killing young animals not related to them.
Males kill babies fathered by others to make the dead infant's mother available for mating, Huchard says.
In species found to practice infanticide, animals live in groups where females greatly outnumber males, and where females can give birth year-round, rather than just once a year.
Infanticide was not observed in mammal species with seasonal reproduction because it offers no benefit to males that still would have to wait for a subsequent breeding season for females to become fertile again, the researchers noted.
"There's no sense for a male to kill the offspring in the previous year, because he has to wait anyway," Lukas says.
Hrdy says she is pleased to see her views so strongly supported by the new study by Lukas and Huchard.
"My main comment is, 'Well done,'" she says, adding she considered the study particularly noteworthy for the scope of the species studied, ranging from opossums to lions.
