Planet Earth

Can Evolution Explain All Dark Animal Behaviors?

Many actions that would be considered heinous to humans — cannibalism, eating offspring, torture and rape — have been observed in the animal kingdom. Most (but not all) eyebrow-raising behaviors among animals have an evolutionary underpinning.

By Tim BrinkhofMar 9, 2021 12:00 PM
evil looking chimp - shutterstock
(Credit: Sharon Morris/Shutterstock)

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“In sober truth,” wrote the British philosopher John Stuart Mill, “nearly all the things which men are hanged or imprisoned for doing to one another, are nature’s everyday performances.” While it is true that rape, torture and murder are more commonplace in the animal kingdom than they are in human civilization, our fellow creatures almost always seem to have some kind of evolutionary justification for their actions — one that we Homo sapiens lack.

Cats, for instance, are known to toy with small birds and rodents before finally killing them. Although it is easy to conclude that this makes the popular pet a born sadist, some zoologists have proposed that exhausting prey is the safest way of catching them. Similarly, it’s tempting to describe the way African lions and bottlenose dolphins –– large, social mammals –– commit infanticide (the killing of young offspring), as possibly psychopathic. Interestingly, experts suspect that these creatures are in fact doing themselves a favor; by killing offspring, adult males are making their female partners available to mate again.

These behaviors, which initially may seem symptomatic of some sinister psychological defect, turn out to be nothing more than different examples of the kind of selfishness that evolution is full of. Well played, Mother Nature.

But what if harming others is of no benefit to the assailant? In the human world, senseless destruction features on virtually every evening news program. In the animal world, where the laws of nature –– so we’ve been taught –– don’t allow for moral crises, it’s a different story. By all accounts, such undermining behavior shouldn’t be able to occur. Yet it does, and it’s as puzzling to biologists as the existence of somebody like Ted Bundy or Adolf Hitler has been to theodicists –– those who follow a philosophy of religion that ponders why God permits evil.

Cains and Abels

According to Charles Darwin’s theory of evolution, genes that increase an organism’s ability to survive are passed down, while those that don’t are not. Although Darwin remains an important reference point for how humans interpret the natural world, he is not infallible. During the 1960s, biologist W.D. Hamilton proposed that On the Origins of Species failed to account for the persistency of traits that didn’t directly benefit the animal in question.

The first of these two patterns –– altruism –– was amalgamated into Darwin’s theory of evolution when researchers uncovered its evolutionary benefits. One would think that creatures are hardwired to avoid self-sacrifice, but this is not the case. The common vampire bat shares its food with roostmates whose hunt ended in failure. Recently, Antarctic plunder fish have been found to guard the nests of others if they are left unprotected. In both of these cases, altruistic behavior is put on display when the indirect benefit to relatives of the animal in question outweighs the direct cost incurred by that animal.

In Search of Spite

The second animal behavior –– spite –– continues to be difficult to make sense of. For humans, its concept is a familiar yet elusive one, perhaps understood best through the Biblical story of Cain and Abel or the writings of Fyodor Dostoevsky. Although a number of prominent evolutionary biologists –– from Frans de Waal to members of the West Group at the University of Oxford’s Department of Zoology –– have made entire careers out of studying the overlap between animal and human behavior, even they warn against the stubborn tendency to anthropomorphize nonhuman subjects.

As Edward O. Wilson put it in his study, "The Insect Societies," spite refers to any “behavior that gains nothing or may even diminish the fitness of the individual performing the act, but is definitely harmful to the fitness of another.” Wilson’s definition, which is generally accepted by biologists, allows researchers to study its occurrence in an objective, non-anthropomorphized manner. It initially drew academic attention to species of fish and birds that destroyed the eggs (hatched or unhatched) of rival nests, all at no apparent benefit to them.

Emphasis on “apparent,” though, because –– as those lions and dolphins demonstrated earlier –– certain actions and consequences aren’t always what we think they are. In their research, biologists Andy Gardner and Stuart West maintain that many of the animal behaviors which were once thought spiteful are now understood as selfish. Not in the direct sense of the word (approaching another nest often leads to brutal clashes with its guardian), but an indirect one: With fewer generational competitors, the murderer’s own offspring are more likely to thrive.

For a specific action to be considered true spite, a few more conditions have to be met. The cost incurred by the party acting out the behavior must be “smaller than the product of the negative benefit to the recipient and negative relatedness of the recipient to the actor,” Gardner and West wrote in Current Biology. In other words, a creature can be considered spiteful if harming other creatures does them more bad than good. So far, true spite has only been observed rarely in the animal kingdom, and mostly occurs among smaller creatures.

The larvae of polyembryonic parasitoid wasps, which hatch from eggs that are laid on top of caterpillar eggs, occasionally develop into adults that are not just infertile but have a habit of eating other larvae. From an evolutionary perspective, developing into this infertile form is not a smart move for the wasp because it cannot pass on its genes to the next generation. Nor does it help the creature’s relatives survive, as they are then at risk of being eaten.

That doesn’t mean spite is relegated to the world of insects. It also pops up among monkeys, where it tends to manifest in more recognizable forms. In a 2016 study, Harvard University psychology researchers Kristin Leimgruber and Alexandra Rosati separated chimpanzees and capuchins from the rest of the group during feeding time and gave them the option take away everyone’s food. While the chimps only ever denied food to those who violated their group’s social norms, the capuchins often acted simply out of spite. As Leimgruber explains: “Our study provides the first evidence of a non-human primate choosing to punish others simply because they have more. This sort of ‘if I can’t have it, no one can’ response is consistent with psychological spite, a behavior previously believed unique to humans.”

Beyond the Dark Tetrad

Of course, spite isn’t the only type of complex and curiously human behavior for which the principles of evolution have not produced an easily discoverable (or digestible) answer. Just as confounding are the four components of the Dark Tetrad — a model for categorizing malevolent behaviors, assembled by personality psychologist Delroy Paulhus. The framework's traits include narcissism, Machiavellianism, psychopathy and everyday sadism.

Traces of all four have been found inside the animal kingdom. The intertribal warfare among chimpanzees is, first and foremost, a means of controlling resources. At the same time, many appear to actively enjoy partaking in hyperviolent patrols. Elsewhere, primate researchers who have made advances in the assessment of great ape psychology suggest the existence of psychotic personality types. As for Machiavellianism, the willingness to hurt relatives in order to protect oneself has been observed in both rhesus macaques and Nile tilapia.

Although the reasons for certain types of animal behavior are still debated, the nature of these discussions tend to be markedly different from discourse around, say, the motivations of serial killers. And often, researchers have a solid understanding of the motivations and feelings of their own study subjects but not those outside of their purview. Regardless of whether the academic community is talking about humans or animals, however, the underlying conviction guiding the conversation — that every action, no matter how upsetting or implacable, must have a logical explanation — is one and the same.  

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