We all know that certain species of animals which can have a social life are altruistic : human, elephant... All are vertebrate and mammals. But what about invertebrate ?
We know that certain species of social insects can have altruistic behaviors. The example of the kamikaze bees is well-known (Hamilton, 1964). The behavior of the bees can be explained altruistically : they are dedicated to their community. And despite the fact that this behavior can contribute to the extinction of the species, the sterile kamikaze bees have a genetic material which is selected by evolution. Since the bees have only one set of chromosomes (the same mother, no father), they have a common genetic heritage with their sisters. This is precisely why it is interesting for them to sacrifice themselves rather than trying to preserve themselves. By sacrificing themselves, they lose their genetic material, but they save the one of their sisters. Of course, this type of selection may be applied to human being, more specifically, in the context of parental love.
Jurgen Heinze and Bartosz Walter decided to study a difficult case -difficult because occasional, non-systematic, and difficult to predict- : why, in certain cases, an individual from a certain society isolate himself from the others, in order to die ? The question is quite general because it can be applied to elephants, human beings and other species. But they did a thorough examination of the question with the help of a colony of ants (Temnothorax unifasciatus).
They introduced in the vivarium a lethal parasitic fungus called Metarhizium anisopliae. And the result of the observation is univocal : a large part of the workers who died from the fungal infection left the colony before death (hours or days before), and died isolated, far from the colony.
But the researchers had to make sure that the fungus was not the cause of this behavior. Because it is a parasite that manipulates the host to improve its transmission. It happens that this fungus "transforms" the ants into zombies, impelling them to climb up a stem where they die. From this elevated position, in the wind, the fungus is well distributed to others. But, by exposing the surviving ants to CO2, and significantly reducing the lifespan of the ants, they observed that ants left the colony before death took hold (which leads to the interesting observation that agonizing ants are not carried away from the colony by others, they leave by themselves). Thus, this altruistic behavior is not caused by the manipulation of the fungus, but by death.
But why can this behavior be considered "altruistic" ? I think that the researchers used a very intuitive definition of altruism : a behavior is said to be altruistic if the consequences of the behavior are the well-being of the others rather than its own. In this case, the consequences of the dying ants are the well-being of the others ants, rather than death in a social environment. The dying ants leave the colony and face death alone rather than risking to infect the others. They may struggle against the impelling need to climb up a stem, as the parasite required, to face death in a remoted and isolated area. Behaving this way, they are choosing the well-being of the others rather its own.
Hamilton, W.D. 1964. "The Genetical Evolution of Social Behaviour". I & II. Journal of Theoretical Biology, 7 pp.1-52.
Chapuisat, M. 2010. "Social Evolution: Sick Ants Face Death Alone". Current Biology, Volume 20, Issue 3, pp. 104-105.
See also : Current Biology and a summary at the BBC website.