Altruism

Altruistic actions are generally seen as 'noble'. Yet some 'lowly' organisms are apt to match the most heroic human acts of devotion and self-sacrifice. To use a widely-quoted example, consider Dicrocoelium dendriticum, also known as brainworm. These parasites spend some of their stages in the innards of cows, exit in the feces and, in the form of cercaria, are eaten by ants a few stages later. Once ingested, a gang of cercaria will break through the ant's stomach wall. One of them makes it to the brain of the ant, and causes it to climb on the tips of grass blades, thus exposing itself to be taken up by the grazing cattle. The other cercaria form cysts in the ant's body, ready to pursue their life-cycle within the cow that swallows them. But the one who made it to the brain — the 'brainworm' — dies without leaving offspring. It has effectively sacrificed itself for the survival of its gang. In humans, comparable feats would be the stuff of epic poetry. Small wonder that evolutionary biologists feel challenged by such behaviour and see it as a high priority aim 'to take the altruism out of altruism'. To begin this task, they define the term 'altruism' in purely Darwinian terms, devoid of any moralistic undertones. An action performed by individual A and affecting individual B is said to be altruistic if it increases the fitness — the reproductive success — of the recipient B, and decreases the fitness of A. In this context, one may as well give names to the other possible scenarios: if the action increases the fitness of both A and B, one speaks of cooperation; of spite, if it decreases both fitnesses; and of selfishness if A's fitness is enhanced and B's fitness diminished. Both altruistic and spiteful traits lower the reproductive success of their bearers and seem at first inconsistent with the action of natural selection. Yet they abound. The suicidal behaviour of the brainworm is a spectacular example of reproductive altruism. The other cercaria could not even reach their egg-laying stage otherwise. Further forms of reproductive altruism occur in the brood care of social insects. The worker castes consist of sterile individuals representing a dead end for the germ line. Nevertheless, it is their task to take care of the brood and even to commit suicide for the community's defence. In a wide variety of …