Do whales practise infanticide?

Studies have demonstrated infanticide in three species of dolphins: the bottlenose dolphin (Tursiops truncatus), the Indo-Pacific humpback dolphin (Sousa chinensis) and the Guiana dolphin (Sotalia guianensis). In the first species, newborns and juveniles have been found dead in the eastern and western North Atlantic. All had scars suggesting that they had been killed by members of their own species. Similar scenarios have been documented for the other two species in their respective ranges, i.e. the western North Pacific for the humpback dolphin and the South Atlantic for the Guiana dolphin. All these cases confirm that the practice of infanticide is well established in certain species of dolphins.

The only case of infanticide in killer whales ever reported occurred in 2016 in British Columbia waters of the North Pacific. During their field work, experts in killer whale behaviour quickly realized that they were observing an interaction between two distinct family groups. The first was a family of a few individuals including a newborn just a few days old. The second family group was composed of two individuals: a 32-year-old male and his mother, both of which were unrelated to the other group. After hearing a commotion, the scientists saw the 32-year-old male under their boat with the lifeless newborn in his mouth. A chase between the newborn’s mother from the first group and the male orca from the second ensued, but to no avail. In what the researchers dubbed “teamwork,” the 32-year-old’s mother appeared to be helping him by fending off the calf’s mother and preventing her from pursuing her offspring’s aggressor.

Élise Huchard, a researcher at the Centre for Functional and Evolutionary Ecology (CNRS, Montpellier), combed through 30 years of studies relating to a list of 260 land mammals. Of these, 119 are believed to practise infanticide. Lions and baboons are high on the list, as they are well known for their propensity to kill the offspring of other parents. The researcher claims that this nefarious behaviour is motivated by the desire to increase one’s fitness, which is proportional to the individual’s ability to reproduce. Indeed, by killing the young of another group or family, the dominant male can control the origin of the juveniles under his watch. Ms. Huchard explains that “this way he maximizes his chances of fathering offspring of his own.”

In closing, several hypotheses have been put forward in an attempt to explain infanticide. The first is that of cannibalism, also called predation, which would tend to occur in populations facing a food shortage. Cannibalism, however, seems to be a secondary benefit of certain infanticides, which are better explained by the hypothesis of sexual selection (killing a young animal to make its mother become fertilizable more quickly). Science also mentions the elimination of potential competition including competition for resources, or the elimination of young animals that are in poor health or that could represent a burden. In order to adapt to this destructive behaviour, females have developed a clever technique: mating with multiple partners so that the potential male does not know whether or not the individual he is about to kill is one of his own!