Altruism is a behavior by an animal that benefits the species more than it benefits the individual performing the behavior.

In a species of ground squirrel, an individual calls out when it sees a predator, thereby signaling the other squirrels to take cover.

The predator will spot and try to kill the squirrel giving the alarm. If this cooperative gesture is risky to the individual, why does it sound the alarm?

By being altruistic and warning others of predators, the squirrel protects them, thereby helping ensure their reproductive success.

Other members of the species will survive, and their genes will be passed on to future generations.

A caste is a specialized form of an animal that carries out a particular function in a group or community.

Species with castes that cannot reproduce are extreme examples of altruism. Many species of insects and one species of mammal, the naked mole rat, live in large communities. Only the queen and a few males reproduce in these communities. Most female bees are sterile.

From a genetics perspective, individual animals are vehicles that perpetuate genes. But how can sterile animals which do not reproduce, perpetuate their genes?

Relatives have many genes in common. For example, your brother or sister is likely to have many of the same genes that you have. When your relatives reproduce, they are helping pass on the genes you share.

In social insects and naked mole rats, workers are closely related to the reproducing queens. By helping the queen reproduce, workers help pass on copies of their genes. This idea is called kin selection.

When a male honey bee mates with a female, he explodes. His genitals become lodged into the female. By this adaptation, the male is able to fertilize the female and prevent other males from copulating with her. The male, however, dies.

To the whole species, the survival of the individual male honey bee doesn’t matter, as long as reproduction takes place and the bee's genes are successfully passed on to the next generation.

We’ve seen that some behaviors seem to benefit the individual, while others benefit the whole species. Ultimately, behaviors follow the same rules as other properties of organisms – a constant process of evolution selects for traits that increase the likelihood that the organism’s genes will be replicated and will survive to the next generation.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education