Male Infanticide In The Animal Kingdom: Why Some Mammal Species Kill Their Offspring

For some in the animal kingdom, it's survival of the fittest -- and his offspring. Some male mammals kill the babies of their rivals as a way to ensure that their own offspring endure. Scientists once thought the practiece spurred evolution in mammalian societies, but new research from the University of Cambridge in the U.K. suggests male infanticide – when one male kills off the infants of another dominant male – is strategic and occurs in stable groups in which males and females live together.

The study, published Thursday in the journal Science, found that when a male deposes a rival male in a social group, he will murder the rival’s offspring so that females in the group can invest in the wellbeing of the new male’s offspring. Dominant males in competitive mammal species have short windows of opportunity to sire kids.

Additionally, researchers observed that some mammal species have adopted other strategies to avoid infanticide. Specifically, the females in some species will have multiple male partners in a short amount of time to confuse the paternity of their infants. Scientists call this approach “paternity dilution.” Males in those species tend to have larger testicles, researchers noted, an evolutionary trait that increases the likelihood that their sperm is passed on to females. “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,” Dieter Lukas, from University of Cambridge’s Department of Zoology, said in a statement.

Previous research has shown that infanticide occurs most frequently in species where one or a few males dominate a large group of females. Infanticide has occurred in many types of mammal species, including baboons, dolphins and lions. Scientists have developed several theories as to why that is, including that some males might see newborns as competitors for resources, according to Smithsonian.

Researchers from Cambridge observed 260 mammalian species in which reproduction came down to a few dominant males. Males killing the young of others was seen in 119 of the species.

Researchers believe infanticide evolved later in the mammalian family lineage and was not practiced 160 million years ago, when the first mammals appeared on Earth. “When we started, we weren’t sure if infanticide was present in some ancestral mammal and is just more pronounced in some species and is lost in other species,” Lukas told the New York Times. “Or maybe it just evolved in those species where the conditions were right.”