Altruistic behaviors are “unselfish” behaviors—those that help another individual at the expense of the individual carrying out the behavior. Despite the negative consequences for the altruistic animal, these behaviors are thought to have evolved for several reasons.

Reasons for Altruistic Behaviors

First, most altruistic behavior occurs between individuals that are related. Altruism is particularly pronounced in animals that live in colonies with one reproductively capable female—the queen—such as bees and naked mole rats.

In these eusocial animals, all the members of a colony are very closely related. The non-reproductive members—for instance, worker bees—care for the queen and her offspring by engaging in altruistic behaviors, such as bringing back food and defending the colony. In the course of these activities, they may even sacrifice their own lives for the good of the queen and the colony.

However, because they are closely related to the queen, their self-sacrificing behavior increases the chances that the genes they share with her will be passed down through her offspring—ultimately preserving altruistic behavior in the population.

Altruism also occurs between related animals that are not eusocial, such as squirrels who warn other members of their group—some of whom are related to them—with alarm calls when a predator is near. This puts the squirrel giving the call at risk, but helps the group, and its genes, survive.

Altruism can also occur between unrelated individuals within a social group, such as when primates groom each other or share food. Although these behaviors may be costly to the altruistic animal in the short-term, they can be beneficial if the favor is repaid later. This is called reciprocal altruism and primarily occurs in animals that live in stable social groups in which individuals have many opportunities to “repay” individuals who have helped them in the past.