Why are animals killed? It may have been believed that only humans kill others on purpose, but it has long been known that this is not the case. Even when Jane Goodall shook with her tale of chimpanzee “wars”, it was understood that there were animals capable of cruelty killing another member of the same species, even a close friend. But also when an animal kills another without that deliberate purpose, for example, in a confrontation for being the alpha, which causes incurable injuries, what is the evolutionary meaning of that death, if at all?
“It is a phenomenon to which not enough attention has been paid,” laments José María Gómez, one of the three researchers who have systematically looked at this dark profile of mammals for the first time. And to his surprise, it is a much more widespread reality than he imagined. Of the 1,000 species of mammals analyzed, just over a third (352) had cases of adulticide, that is, deaths caused to another adult specimen of the same species. At least 280 with records of deaths caused in the wild. In addition, they consider that these numbers will surely be underestimated due to lack of scientific registration of deaths in other animals. “We were surprised by the number of species, more than those that register infanticides,” says Gómez, a scientist at the CSIC.
Infanticide, killing young, is a much more studied circumstance because it is considered an evolutionary and reproductive strategy of many animals. Males of many species kill female offspring in order to dispose of them freely, but they also kill themselves due to lack of resources, for example. It is understood that it has a function. “The main reason why the adulticide there is no conceptual framework, which is the one we propose here ”, explains Gómez, who even highlights the novelty of the use of that term.
From the analysis of this thousand species, at least four reasons (not exclusive) are discovered why mammals kill each other: attacks between males to mate, to defend valuable resources, to protect their young and to take advantage of their congeners. .
There are groups of mammals with much greater tendencies to kill each other, which may not match well with the image we have of killer animals: deer and deer, shrews, kangaroos and primates. In contrast, bats, whales and dolphins or rabbits and hares are hardly killed.
Differences between the sexes
Another thing that surprised the researchers (who published their study in Proceedings of the Royal Society B) is that females also kill a lot. But for very different reasons: while males commit adulticides to increase their opportunities to reproduce, females do so mainly to defend their young from the threat of infanticidal congeners. They, to have offspring; them, to keep them alive.
“We observed robust correlations,” says Gómez, “females do not kill in the same context, but are associated with the defense of their progeny. But there is no scientific literature on this phenomenon, only reports ”, he laments.
Deaths are perpetrated mainly by males (there are 320 species with male adulticides compared to 133 with females that kill), a fact that contrasts strikingly with infanticide, where the prevalence is similar in both sexes (119 compared to 89 species). In most cases, the victim of the males was another male (in at least 232 species); males kill females in 42 species, females kill males in 30 and females kill each other in 35 others.
“Traditionally it has been considered that mortality among adults is anecdotal, with no intention of killing, but a simple by-product of competition between males, an uncontrolled escalation in competition for a resource: the female”, summarizes Gómez. And he gives as an example a fight between deer, which “got out of hand and one of them accidentally dies before the loser surrenders.”
But his work opens the door to analyze this phenomenon from a broader point of view: “Regardless of the specific causes, adulticide in mammals can have important evolutionary consequences,” says the study. For example, to better explain the development of weapons or other strategies, such as the one that leads sea elephants to recognize themselves by voice to avoid confrontations in which they are close to death.
Or to understand the proportion of deaths perpetrated by females, which are not anecdotal, but repeated. They are capable of killing to defend their young, among other reasons, because the investment of energy dedicated to a young is enormous. The male wants to reproduce and all his efforts are focused on that. But what about the female who has devoted so many months, resources, energy, to gestation and lactation?
A recent study calculated, for a female chimpanzee who was killed a week after birth, that infanticide resulted in a loss of 473 days of her fertile life, in some cases up to 20% of that period in which you can have offspring. For a long time it was thought that females who used violence in their environment, like many types of rodents, did so because they were territorial or defended food, until later studies illustrated this reality: they were aggressive towards their congeners defending the burrow so that they will not kill their young.
In addition, the researchers detail that there are 47 species in which deliberate deaths of members of the same species have been recorded, such as when chimpanzees go in command to kill members of a rival clan. By groups, these murders occur in 24 species of rodents, in 13 of primates and only 10 in carnivores. This suggests that cannibalism does not mediate adulticidal evolution in mammals. For example, compared to other animal groups (spiders, scorpions, some fish), it is very rare to kill another animal of the same species with the intention of eating it. Therefore, adulticides do not stand out among carnivores and there are many herbivores killing each other.