Why Do Mammals Kill Each Other?

Humans run the animal kingdom’s only criminal courts. We alone bicker over the difference between murder and manslaughter, and plumb the ethical depths of intent. Other animals still kill their own kind, but whether they do so deliberately, with any semblance of malevolence or premeditation, is up for debate—mostly because we don’t have any real way to tell. “Humans answer when you ask those questions,” Elizabeth Hobson, a behavioral ecologist and evolutionary biologist at the University of Cincinnati, told me. “They might lie, but at least they can answer.”

José María Gómez, a biologist at the Arid Zones Experimental Station, in Spain, has spent years trying to work around this problem and suss out the motivations behind other mammals’ most violent acts. By carefully archiving and analyzing animals’ aggressive behaviors, he and his colleagues hope to better understand the evolutionary roots of lethal violence—what drives one animal to take the life of another. Their latest work is a grisly census of mammals that commit what Gómez calls “adulticide,” the act of one fully grown creature killing another of its own kind.

About 6,500 species of mammals are known to science; by Gómez’s count, at least 352 of them have been observed snuffing out one another. “This behavior is probably rare and uncommon,” he told me in an email. The paper includes data from thousands of other research studies, but isn’t meant to be a comprehensive catalog of mammalian slaughter. Even well-studied mammals can hide extreme behavior, so some species were probably left out. Gómez’s study is also more a survey of what’s possible for an animal, rather than what’s typical: Some of the killings the researchers assessed happened in captivity or were recorded as one-offs, so they’re not necessarily the species norm. But Gómez hopes that the findings will inspire more research into animal aggression in its ghastliest forms. These behaviors are among the acts that researchers least understand; bridging that gap requires a few unflinching looks.

The study cast its net intentionally wide, Gómez said, so he and his colleagues could try to tug out some patterns. Unsurprisingly, carnivores such as lions, tigers, and bears (oh my)—some of the usual suspects—are pretty prone to killing one another. So are primates; shrews; marsupials such as kangaroos and wallabies; and even-toed ungulates, a group that includes hippos, giraffes, and deer. But scientists almost never see bats, whales, dolphins, rabbits, or hares taking one another’s lives.

Gómez and his colleagues also found that the primary perpetrators of adulticide were males, which were fingered as killers in some 90 percent of the species that made the team’s lethal list. And when they did the deed, their victims were also male. These killings commonly involved sex, or at least the promise thereof, with death occurring at the paws, claws, or jaws of a competing suitor violently vying for access to a mate. Hard-headed pronghorn and tusk-wielding walrus might fatally gore one another during fights; giraffes might brutally batter one another with their über-long necks. “Some things just happen—there’s a bad fight and one gets stabbed worse than the other,” Hobson, who wasn’t involved in the study, told me.

Only about a third of the species on the researchers’ list included femme fatales. Here, too, sex seemed to be a factor, but on a different timeline: Females were especially wont to kill other adults—males and females alike—after they’d mated, as a way to keep their offspring from harm. That checks out with the mammalian way of life. To pass her genes onto the next generation, a mom has to survive the grueling rigmarole of gestation, labor, lactation, and caring for a fairly fragile infant. “That’s extremely energetically costly,” Tyus Williams, a carnivore ecologist at UC Berkeley who wasn’t involved in the study, told me. A threat to a kid could mean restarting that entire reproductive shebang—a cost worth shedding some blood to avoid. “Instead of going through that again, they want to defend the one they have,” Williams said. Female ring-tailed lemurs, for instance, have been seen lethally retaliating against pup slayers. Lady mountain lions will also go to the mat with other adults over their cubs.

Both sexes appear to kill out of selfishness, the great evolutionary equalizer. Mammalian males are out to mate with as many females as they can; females, whose reproductive opportunities are more limited, are out to guard the investments they’ve already made. The motivational motifs of adulticide roughly echo those of infanticide, which Gómez thinks is more common and certainly better studied. Males will murder their competitors’ kids to bring the mothers back into heat, a common practice among lions; females will kill babies to hoard scarce resources for themselves, a macabre act that’s been well studied in primates such as marmosets. “They’re just defending their own,” Emily Weigel, an animal-behavior expert at Georgia Tech who wasn’t involved in the study, told me. “Everyone just wants to maximize the number of kids that they have.”

Weigel and others noted that the broad trends described by the study are a great starting point for more investigation, but obscure important subtleties. The paper didn’t set out to define how, or even if, adulticide benefits killers. And plenty of other explanations for the behavior are likely to exist, including several that have a less explicit connection to sex and progeny. There’s more to the narrative than “males are competitors and females are carers,” Brett Frye, a primatologist at the Wake Forest School of Medicine who wasn’t involved in the study, told me. Females, too, need to contend for space and food, outside of their reproductive roles. That might be especially true among meat eaters and ultra-territorial species, says Julie Turner, a behavioral ecologist at the University of British Columbia who wasn’t involved in the study. Wolves and hyenas of both sexes, for instance, will slay one another in real-estate disputes.

Weigel thinks that some of that nuance might have been lost because of science’s historical bias against logging female behaviors, especially ones that trend toward the aggressive—that is, acts typically attributed to males. Gómez’s study, which hinged on data that came before it, is, in a way, as much a dive into mammalian adulticide as it is an exploration of the ways in which scientists have recorded it.

Our understandably human-centric perspective is almost certainly coloring our read of animal adulticide in other ways as well. We can’t help but use other mammals as a mirror for our own behaviors. But while death is a documentable event, the reasons behind it start in the realm of hypothesis, and don’t always leave. Behavioral ecologists end up having to “black-box questions of motivation,” Joseph Feldblum, a primatologist and chimpanzee expert at the University of Michigan who wasn’t involved in the study, told me.

Gómez’s definition of adulticide is cautious with the idea of intentionality, though it roughly parallels what homicide is to us. Straight-up murder, if such a thing exists among nonhuman animals, definitely hit the team’s mark, but so did killings that could be likened to manslaughter—violent acts that incidentally led to a swift and untimely death. That means a stampeding creature that shoved another off the edge of a cliff would make the cut; so would dual deaths that occurred when two antlered animals locked themselves together and couldn’t wrest back apart. (A sick mouse that transmitted a lethal virus to one of its brethren, however, would not.)

Still, Gómez and his colleagues did flag several dozen species as carrying out “deliberate” adulticide—24 rodents (including a couple of mole rats and several squirrels), 13 primates (including gorillas and ring-tailed lemurs), and 10 carnivores (including gray seals, cheetahs, wolves, and polar bears). A marmot killing and cannibalizing its kin can be carnage with a cause. So can a raid led by chimpanzees, which will form belligerent coalitions to usurp the territories of other groups. Feldblum said that after years of watching chimps wallop one another—biting, beating, and stomping until blood is drawn and bones are broken—he’s pretty sure these actions are more than just “run-of-the-mill aggression.” Attempts at lethal violence can be costly for everyone involved, which might be why so many of the cases featured in Gómez’s study were apparently accidents. Many mammals will actually go out of their way to avoid a deadly act. But when sex or survival is on the line, it’s not difficult to see how death could become well worth the risk. “Killing a rival takes them out of contention for good,” Feldblum said.

The knowledge that other mammals commit something akin to murder or manslaughter can be unsettling. It can be a reminder of our own homicidal history, and maybe that’s part of what’s kept us from diving more deeply into these patterns before. “People have this very Disney World idea in their heads when it comes to nature,” Williams, the UC Berkeley ecologist, told me. “But nature is grisly. It’s brutal.” Sometimes one animal’s survival comes directly at another’s expense. Natural selection, after all, doesn’t work if everyone makes it out alive.

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