Corpse Warfare in Ant Kingdom

by Ellie Kravets

Formica cinera, the main focus ant species of this study. Image credit © Artengalerie

It’s a well-established fact that ants are insane. But it seems that scientists are only starting to realize the degree to which these omnipresent insects are absolutely, mind-numbingly crazy. (And, side note, there is such a thing as a crazy ant: Nylanderia fulva. To read about the problems surrounding that species, go here.)

When was the last time you heard of a group of organisms forming an intricate and complex social structure which allows them to wage chemical warfare and take slaves from rival groups (and without any form of ruler or power hierarchy)? You could make a case that humans have created a similar social structure, but last I heard we hadn’t been classified as viscoelastic materials. (And if you click on none of the links in this article, please click on that one. It’s fascinating stuff.)

But while there’s been a ton of institutional attention on the social habits of ants, there hasn’t been much studied about their so-called “social-defensive” strategies. Think about it: for all of the perks that living in a large group will bring you (mainly, that you get more food and better shelter because you can efficiently divvy up the workload) there are also an enormous number of risks.  If the worst should happen and your colony was attacked by a predator, a much larger number of your species is put at risk. Infectious disease rates tend to skyrocket in highly social environments, due to crowding in close-quarters. So if your species is going to adapt to living in social environments, then you’d better also evolve some strategies to protect yourself from its risks.

Enter Dr. István Maák. He and his team of researchers from the University of Szeged in Hungary just published a paper that discusses these “social-defensive” strategies of ants. Specifically, Maák and his team looked deeply into the ways that ants dispose of and treat their dead.

Corpses are a huge infection hazard, as they are a breeding ground for deadly bacteria. Ants aren’t going to just leave the corpses of their nest mates rotting in the colony tunnels. Some ant species, then, build special burial chambers on the outskirts of the main nest area. Others make distinct and structured piles of corpses somewhere outside of the nest area. Still others may create a line of corpses marking territorial boundaries or even pile the corpses at the entrances of rival nesting sites as a precursor to raiding the rival nest. What Maák and his team found was evidence that indicates that different ant species treat their dead differently to communicate with other ants in the area.

A Plectroctena ant carrying a corpse of its own species. Image credit © Wikipedia Commons

The study breaks down like this: When ants found corpses of rival species (for example, one that was especially aggressive or was known to take slaves) within their home colony’s territory, they immediately set out to move the corpses into the colony. Remember, some ant species use their nest mate’s corpses as a sort of intimidation tactic before setting out on a raid, so this frantic “Get these things underground!” response was probably to “unmark” their colony as a raid target. Once the corpses were out of sight, one of two things probably happened: 1) the rival corpses were dismembered and spread around the colony in order to quickly disseminate information about the rival colonies to the worker ants (similar to how police dogs can be given a shirt to smell in order to get information on a whole person) or 2) the rival ants were eaten. (Hey, they would be an extra source of food. Waste not want not.)

When ants found corpses from their own colony within their territory, they also quickly moved the corpses underground. The thought goes here that—even if rival ants hadn’t specifically marked the home colony out for a raid—having mounds of corpses right around the nest area doesn’t exactly exude subtlety. Nest mate corpses were moved underground in order to better hide the colony entrance. (And probably weren’t eaten, as direct cannibalism within a species is just asking for diseases.)

The most lackluster response happened when the ants found corpses of a non-rival ant species. The ants moved very few of the non-rival corpses back into the nest; instead, they simply pushed the corpses farther toward the border of the home territory, in the manner of a street sweeper pushing trash out of the main road.

And, finally, when ants found corpses of their own species that were “foreign” (i.e., from an area far away from the home colony) the ants didn’t really know what to do. Some of the foreign corpses were brought underground, some were completely ignored, and some were pushed to the edge of the home territory.

The take away is that the ants studied by Maák and his team were able to tailor their responses to each individual species. Instead of wasting their time bringing all available ant corpses back into the nest for study/eating/information dissemination, the ants only spent that kind of time and energy on the species which posed a threat to the colony. This behavior reveals another small, but crucial, element of ants’ psyche. They are able to determine worthiness/unworthiness of an action based on predetermined traits. In the animal world, that’s a big step. In the insect world, it’s a monumentous one.

For more on ants and their dead, check out:
How Ants Bury Their Dead
Dead or Alive, for Ants It’s the Scent That Counts

For more on ants and their behavior:
Ant colonies: Behavioral Variability Wins
The rebellion of the ant slaves

To learn how and why ants rule the world:
Why Ants Rule the World

To prepare for the oncoming zombie plague:
Zombie Snipers at the Doorstep

Maák, István et al.  (March 2014). Cues of meaningless objects? Differential responses of the ant Formica cinera to corpses of competitors and enslavers. Animal Behavior, 91, 53-59.