Introduction
Altruistic suicide is the self-destruction of an individual to protect or enhance the survival of others, commonly seen in microbes and social insects, and even at the cellular level in multicellular organisms. It is the most extreme form of altruism, in which the altruistic actor gains nothing for themselves; it serves for the benefit of others only.Self-sacrifice seems to be a glaring contradiction to Darwin's theory of evolution, where every individual seeks its own advantage. Usually, such altruistic patterns are explained by close kinship and the promotion of one's own genes. But there are also many examples where the individuals are not directly related to each other.
A few examples where altruistic suicides can be observed
The slime mold Dictyostelium discoideum lives as single-celled amoebae when food is abundant. When food runs out, thousands of amoebae aggregate into a multicellular “slug” and then a fruiting body. About 20% of the cells in the slug differentiate into a rigid, dead stalk of the fruiting body. These cells die in the process and never reproduce, but their sacrifice lifts the remaining spore cells high enough to be dispersed more effectively.This parasitic trematode Dicrocoelium dendriticum (Lancet Liver Fluke) has a complex life cycle with several hosts: from snails to ants to grazing mammals. Within the infected ant’s brain, one 'mind-control' metacercaria manipulates the ant to climb to the tip of grass blades and clamp down, making it more likely to be eaten by a grazing mammal (the definitive host). The other metacercariae stay dormant in the ant’s body, ready to infect the mammal once eaten. The brain-dwelling parasite is sacrificial: it usually dies when the ant is eaten or even before, but this behavior benefits the others inside the ant that will complete the life cycle.
The bacterium Salmonella typhimurium is a gut pathogen. When it invades a host’s intestinal lining, some cells trigger inflammation, which disrupts the normal microbiota and favors Salmonella growth. Therefore a subpopulation of Salmonella invades epithelial cells and expresses a Type III secretion system (T3SS-1). This provokes a massive immune response that kills these invading bacteria but clears competitors and releases nutrients, allowing the non-invading Salmonella to flourish in the inflamed gut. In other words, a few bacteria sacrifice themselves to trigger host inflammation, benefiting others who remain in the lumen.
Social insects: Some worker ants or honeybees perform suicidal defense. For instance, Japanese honeybees can overheat and kill an invading hornet by swarming it, even though the high temperature is lethal to many of the bees. In some ant species, workers rupture their own bodies to release sticky or toxic substances that immobilize predators (“autothysis”).
Multicellular organisms (cellular altruism): Even within a single organism, some cells undergo apoptosis (programmed cell death) for the benefit of the whole organism — removing damaged or dangerous cells that might otherwise harm the body. This is considered a form of altruism at the cellular level.
References and further reading
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