Like American soldiers on the shores of Normandy during World War II, salmonella bacteria sacrifice themselves for the greater good — a phenomenon that may illuminate the evolution of altruism.
When salmonella enter the digestive tract, they fare poorly: other bacteria have already established their positions. But by sending an advance group digging into intestinal tissues, they set off an inflammatory reaction in their host, sweeping away the other bacteria. The advance group also dies, but the intestine is wide-open for colonization by their brethren. The population flourishes because of the selflessness of a few.
From a Darwinian viewpoint, the existence of altruism in nature is at first sight puzzling, as Darwin himself realized. Natural selection leads us to expect animals to behave in ways that increase their own chances of survival and reproduction, not those of others.
Here’s another example of sacrificiing for the greater good. Vervet monkeys give alarm calls to warn fellow monkeys of the presence of predators, even though by doing so they attract attention to themselves and increase their chance of being attacked. Biologists argue that the group that contains a high proportion of alarm-calling monkeys will have a survival advantage over a group containing a lower proportion, thereby encouraging this trait to continue and evolve among individuals. The Vervet monkey crier is Nature’s Hero. And Nature’s heroes are our real altruists.
Helping a relative is another example of animal altruism.
The above photo shows nest mate workers engaging in the social sharing of liquid food. This behavior does more than merely transfer food. Ants also use it to pass chemical signals among each other, and research has shown that sharing food helps the colony maintain a cohesive identifying odor.
Why are worker ants sterile? In most species, the balance between male and female is 50-50. But there are exceptions. In some ant species, for example, the ratio is around three daughters for every son. That is because the sterile female workers invest more into female larvae than males. The workers ants are more closely related to their sisters than to their brothers. A female ant may be able to spread more genes by helping to raise her queen mother’s eggs than trying to lay eggs of her own.
And finally interspecies animal adoptions are another example of altruism.