The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology (2015)

Animal Altruism

Charles Darwin (1809–1882), William D. Hamilton (1936–2000)


The notion of altruism—the selfless concern for others—is a traditional virtue in many cultures, and “the Golden Rule” is a core belief in many religions. Humans are said to be altruistic if the act is performed with the conscious intention of helping another. But acts that would be considered altruistic are performed by animals not perceived to be capable of conscious thought. When analyzing altruism, animal biologists look at the consequences of the act and not its conscious intention.

Moreover, the behavior performed may benefit other organisms at a great cost to the altruist. From an evolutionary perspective, some of these acts seem at odds with Darwin’s theory of natural selection, which anticipates that animals will act to enhance their own survival and successful reproduction, thus providing themselves with a competitive advantage. Evolutionary success for an organism depends upon leaving behind as many of its genes as possible. Consider then that worker bees evolve without the ability to reproduce and only exist to benefit the hive and guarantee the propagation of the sole queen bee, defending her to the death from attack.

In 1964, W. D. Hamilton, one of the great evolutionary theorists of the twentieth century, proposed the inclusive fitness or kin selection hypothesis to explain this altruistic behavior. Hamilton postulated that the abnormally close genetic relationship of the sterile female bee altruists in the same colony encourages them to ensure one another’s survival—and, by extension, the survival of the non-sterile queen—as the probability of spreading their genes is more dependent on the survival of the queen than on their own individual survival. Similarly, vervet monkeys, squirrels, and American robins sound a vocal alarm when detecting the presence of a potential predator, at the cost of revealing their own location and exposing themselves to attack. Vampire bats share their blood with less fortunate colony members who have not fed. Altruistic behavior is also seen among nonrelatives of the same species, who exchange favors, which is called reciprocal altruism: “you scratch my back and I’ll scratch yours,” both literally and figuratively.

But from an evolutionary perspective, it is difficult to explain why dogs adopt orphan cats and squirrels, or why dolphins have saved humans from attacking sharks. Perhaps a good deed is its own reward.

SEE ALSO: Darwin’s Theory of Natural Selection (1859).

Kin selection theory predicts that animals will behave more altruistically toward their relatives than toward unrelated members of their own species. Moreover, research studies have shown that the closer the relationship, the greater degree of altruism.