“It’s not that these events are weird one-off things … this is actually a pretty significant source of mortality,” says Eli Strauss, a behavioral ecologist at the University of Nebraska–Lincoln. Strauss and his colleagues scoured three decades of data on spotted hyena populations in Kenya to study deaths of cubs less than a year old (SN: 4/23/02). Of 99 observed deaths, 21 could be attributed to infanticide, always by female killers. Starvation and lions also took many young cubs’ lives.
The infanticide observations made the team wonder why hyenas kill within their own group. It “seems sort of counterintuitive if animals benefit from living socially,” Strauss says. Though hyenas spend much of their time alone, group living allows them to defend their turf against rival hyena clans and to gang up against threatening lions, he says.
Hyena mothers give birth in an isolated den. But typically within a few weeks, they move their cubs to a communal den. Such dens shelter little ones from large predators that can’t enter the sanctuary’s small access holes, says Ally Brown, an environmental biology student at Michigan State University in East Lansing. But the communal den presents other risks — all the cases of infanticide occurred in its vicinity, documented by researchers who either found the dead cubs or observed the clans from cars that serve as mobile blinds (SN: 4/23/02).
Female hyenas kill cubs in the same way that they attack small prey. A hyena “would just go up to a cub and grab it by the skull and crush it,” says Brown, who presented the work in a poster at the Ecological Society of America’s 2020 meeting held virtually the week of August 3. And close kin weren’t necessarily immune — one female targeted her sister’s two cubs, coaxing them out of the den before killing both.
A hint at what spurs such slayings lies in the hyenas’ ranks. In hyena societies, males may come and go while females stick around as permanent members (SN: 3/28/16). Aggressive interactions and alliances help determine which hyenas are on top, and all individuals know where they stand, Strauss says.
Female cubs that reach adulthood can grow a maternal line, which helps boost that family group’s rank. In nearly all cases studied, killers ranked higher than the victim’s mother. That suggests that some females may use infanticide to keep their rivals’ bloodlines down.
In 11 out of 21 infanticides, dead cubs were eaten. Since timings of cub killings didn’t correlate with prey availability and since hungry males weren’t killing cubs, the researchers concluded that consuming the dead cubs wasn’t the main motivation for the attacks. Strauss, Brown and Michigan State behavioral ecologist Kay Holekamp also shared their results May 2 in a preprint posted at bioRxiv.org.
“Infanticide is a very difficult to observe phenomenon,” says Elise Huchard, a behavioral ecologist at the Centre National de la Recherche Scientifique and the University of Montpellier in France who was not involved with the work. The number of cases with known cause of death and quality of the observations contribute to the strength of the study, Huchard says.
More is known about why males kill young, Huchard says. In other species, such as lions, for instance, males may kill young animals so that that the mothers will become sexually receptive sooner. Similar behavior has been observed in dolphins (SN: 7/21/15). This study now also shows that in some species, females compete to pass on their genes, but through offspring survival instead of by vying for opportunities to mate. With high competition for reproduction, “males and females will do anything to promote their own offspring, including killing the offspring of others,” Huchard says.
Despite their treatment of others’ cubs, female hyenas can be “very attentive and diligent mothers,” Strauss says. Hyena moms nurse their cubs for around 14 months and help them get enough food even after they’re weaned. And mothers seem to recognize their loss when a cub is killed, Strauss says, sometimes making distressed sounds or grooming the dead cub.
