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Responses to death vary greatly among the creatures that inhabit the Earth. Every living thing has an expiration date, and the care those left provide in death's aftermath can be both moving and curious. Seeing these death rituals, we have a tendency to anthropomorphize (assign human motivations to non-human species) in an effort to relate. Some of these undertaking behaviors, after all, appear to mimic our own. For instance, when an ant dies, other members of the colony might pick up the deceased and carry it to rest with other dead ants, like a graveyard. But, while it is easy to attribute complex emotions to these practices, the truth is that these are formic behavioral adaptations. Ants that perform the death ritual of carrying another away are enacting behaviors that evolutionarily benefit the colony by ridding a potentially diseased individual from the group.
Edward O. Wilson was studying the strange behaviors of ants back in the 1950s. When one ant died, at first others would move around the corpse as though nothing out of the ordinary was occurring. To Wilson and any other observers, it was clear that the ant was deceased, as the corpse lay upside down with their limbs sticking up. But after a couple of days, the ants would seem to pay more attention, eventually carrying the corpse to what appeared to be a designated graveyard full of other dead ants. Wilson hypothesized that the dead ants must provide some signal to the others to elicit this behavior.
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Social immunity
As it turns out, Wilson was on to something. After many experiments, he found the chemical siren: oleic acid. After placing a drop of oleic acid on a small object, like a piece of paper, the ants would act towards it in the same manner that they would a corpse, carrying and depositing the object outside of the nesting area. Interestingly, when Wilson introduced pieces of paper contaminated with oleic acid to an entirely different ant species, they exhibited the same behavior.
These findings were revolutionary and demonstrated that when the ant body decomposes, it releases oleic acid, which can be detected by others. When another ant happens upon this signal, they investigate the body with their antennae before picking it up and moving it. Such a behavioral adaptation is known as social immunity and serves to protect close-knit groups from the spread of disease.
While social living can have many benefits for survival, there are some tradeoffs, including a higher risk of spreading illness or infection. This is particularly apparent in groups that have low genetic diversity, as they have a higher probability of being susceptible to the same pathogens. By removing the corpse, ants protect themselves and their colony by reducing the interaction with a potentially diseased member of the group. Interestingly, other sorts of undertaking behaviors and death rituals have been observed among other social species, like termites and bees. Perhaps our own cultural funerary practices are in fact adaptive, as well as emotionally salient.
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Not all ant species handle their dead in this way. The process described above is referred to as necrophoresis, which is derived from the Greek words for death (necro) and transport (phoresis). In contrast, while not considered burial, some ant species appear to entomb the dead. Specifically, fungus-growing ants will create chambers into which they place waste and corpses when many fellow colony members have perished. On the other hand, termites, which are also eusocial, cover their dead with pieces of surrounding soil — burying them in the surrounding earth, much like many humans create graves in which to bury the dead. Additionally, termites can also build walls around the corpse to section them off from the others.
Some ants exhibit necrophagy, which is the consumption of the dead. However, this is rare among ants and more commonly seen in termites, which tend to recycle resources and have less nutrient-dense diets. In ants, necrophagy appears to occur when the population is particularly high and resources are low. Since necrophagy carries the risk of disease, some research has suggested that the ant species that exhibit this behavior are better at detecting infections.
Although it is important to avoid anthropomorphizing, we can learn a lot from the stereotyped behaviors of eusocial insects. These observations provide us with insights into social adaptations and disease prevention. Moreover, necrophoresis in ants is a great representation of how chemical signals play into the responses of insects.
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