In the rainforests of Borneo, there lives a reddish brown ant by the name of Colobopsis explodens that really knows how to go out with a bang. When locked in battle with ants from another colony, C. explodens workers bring the fight to a swift end by ripping themselves open and spewing noxious fluid on the enemy. The workers die while pulling off this power move, but their sacrifice protects the rest of the colony from marauding predators like the weaver ant.
This is not typical animal behavior. Most creatures behave in ways that give themselves the best shot of surviving and passing their genes on to their own offspring. The workers of C. explodens achieve neither of these things by self-destructing—yet they aren’t the only insect to do it. They belong to a group called exploding ants that are found across Southeast Asia. And self-sacrifice shows up among a number of insects that live together in colonies including other ants, termites, bees, and certain aphids.
“One small worker does probably not cost very much to lose, but at the same time the benefits of deterring an intruder such as a big vertebrate predator that might potentially destroy the whole colony, or even other insects that might engage in a raid of the colony, are potentially huge,” says Olav Rueppell, a biologist at University of North Carolina at Greensboro.
Self-sacrifice can get pretty elaborate, and in some cases happens even when there isn’t a battle raging. Here are some of the most extreme moves that insects pull to defend their nestmates, from eviscerating themselves to leaving the safety of the nest to die alone.
Disembowelment
You might have been on the receiving end of insect self-annihilation if you’ve ever been stung by a honeybee, which tears apart its own body to leave a stinger embedded in your flesh. But often, insects that disembowel themselves are aiming at other bugs.
Such is the case for C. explodens, which also goes by the nickname “yellow goo” in reference to the color of its signature chemical weapon. It’s thought that these ants “explode” by contracting their muscles until their abdomen and internal glands rupture and ooze sticky fluid, says Alice Laciny, an entomologist at the Natural History Museum Vienna.
For the predatory ants they face off against, getting slimed is downright deadly. If an enemy bites into the poisonous goo, it loses control of its limbs and dies in seconds. Otherwise, the goo will gum up its joints and immobilize the hapless insect. And even after it dies, C. explodens‘s mandibles do not release their grip on the enemy’s body. “Then it’s sticky and it has a dead ant hanging from its antenna or leg,” Laciny says. “It probably won’t survive very long in that state.”
Self-destruction doesn’t really give the colony a numerical advantage during a skirmish. “It’s not so much about one-on-one confrontation, so an eye for an eye,” Laciny says. She suspects that behavior has more to do with keeping contaminants at bay. Exploding ants seem to be particularly dependent on the bacteria and fungi in their rainforest habitat and die if they are taken away from this microbiome. Any insect that threatens to get into their nests or too close to their foraging territory will carry foreign spores and bacteria.
The bright yellow gunk, which can sometimes be seen shimmering between the plates of the ants’ exoskeletons, could serve as a warning signal to enemies. “It seems to be well-known among other insects in the rainforest that they should just stay away,” Laciny says. Few insects are willing to venture onto trees where C. explodens colonies reside during the day, when the volatile ants are awake.
Although researchers have been spotting the ants and their yellow goo for decades, it was only recently that Laciny and her colleagues examined the species closely enough to give it a scientific name, which they reported in the journal ZooKeys on April 19. In fact, C. explodens is the first new exploding ant described since 1935.
That’s partly because different species of exploding ants can be nearly impossible to tell apart, while members of the same colony can look completely different, Laciny says. The tiny exploding members of C. explodens colonies were long thought to be a separate species from the other, larger workers. This ability to create different castes is one reason that self-sacrifice works so well for insects that live together in colonies. “Insects are quite plastic in their body plans,” Rueppell says. That means a colony can invest in a few members that self-destruct in very specialized ways.
And it’s not just ants that do this. Workers belonging to a termite species called Neocapritermes taracua found in French Guiana grow “explosive backpacks” as they age and become less valuable members of their society. When these termites rupture their bodies, blue crystals from the pouch on their back come into contact with secretions from the salivary glands. By mixing two chemicals to create an especially toxic brew, the termites’ final act is really “a step up in terms of sophistication,” Rueppell says.
Refusing help
Scientists recently discovered an entirely different kind of battlefield altruism in the matabele ant of sub-Saharan Africa. These ants are known for hunting termites, which put up a fierce fight. It’s not uncommon for workers to lose limbs during a raid. These injuries are not always fatal, though, says Erik Frank, an evolutionary biologist at the University of Lausanne in Switzerland. Wounded ants are carried home by their nestmates and have their injuries licked clean so they can heal. Before long, they can run as fast as a healthy ant and go raiding again, even short a few legs.
However, Frank realized there was more to the story one day after he accidentally drove over a column of raiding ants in Côte d’Ivoire’s Comoé National Park. When he stopped the car and got out to see how the ants were doing, Frank noticed something odd. “On ‘ground zero’ the nestmates were investigating all the injured ants but only helping the ones that still had a chance to survive,” Frank said in an email. He later presented ants with five legs amputated to their nestmates, and saw that these severely injured workers were rarely picked up.
But it wasn’t not for lack of trying. The healthy ants attempted to help, but their fallen comrades refused to cooperate. Instead of tucking their legs in and lying motionless when touched, the injured ants flailed around violently until their rescuers gave up. In other words, these ants perform a kind of self-triage that allows helpers to focus on other ants with less debilitating injuries.
It’s pretty unlikely that the ants are actually trying to sacrifice themselves, though. “This is not a conscious decision by the ants,” says Frank, who reported the behavior in February in the journal Proceedings of the Royal Society B. Normally when an ant gets hurt, it will stand up again, then call for help by releasing pheromones and allow its nestmates to scoop it up. When a seriously injured ant flails around, it is probably trying to stand up and failing, over and over again. “If you are able to stand up you are likely not too heavily injured so that you are still useful for the colony,” Frank says.
Preemptive action
Sometimes, an insect will give up its own life to help its fellows even when there is no imminent danger. This happens every night in colonies of the Brazilian ant Forelius pusillus. When sunset falls, the ants seal up the entrance to their nest, leaving one to eight workers outside to finish the job.
“The ants trapped outside were not accidental victims, but rather were part of a deliberate strategy of entrance closing,” scientists wrote in The American Naturalist after watching the doomed ants covering the nest with sand until it was completely concealed. As the night wore on, the ants were blown away by gusts of wind, attacked by other kinds of ants, or simply wandered away. When morning arrived, none of these ants were ever found near the entrance of the nest.
Self-sacrifice might have become a routine part of life in F. pusillus colonies because these ants are particularly vulnerable to attack, Rueppell says. F. pusillus makes its nest on bare soil with little plant cover. What’s more, it lives in tropical areas, which tend to be crawling with all kinds of ants. An ant’s worst enemy is usually other ants, which means F. pusillus might have to take extreme steps to keep the colony hidden.
This isn’t the only time insects have been spied sacrificing themselves to ward off future threats. Scientists in Japan have found that an aphid by the name of Nipponaphis monzeni springs into action when its home is threatened. The bugs live together in swellings found on the outside of trees called galls; when the researchers drilled holes in these galls, the aphids within immediately began secreting bodily fluids to repair the damage. The aphids that were in charge of plastering over the hole shriveled up and in at least some cases died.
This might actually be a good thing for the remaining aphids in the gall. “Several nymphs were buried in the plaster, like ‘aphid sacrifices,'” the scientists reported in Proceedings of the Royal Society B. The entombed carcasses likely made the repair job sturdier, Rueppell says.
Most aphids—including the kind feasted upon by ladybugs—wouldn’t be able to use this trick. They are solitary insects that do not share a home base, Rueppell says. Self-sacrifice works best for insects that live together in nests or galls walled off from the outside world. In less isolated societies, it would be all too easy for freeloaders unrelated to the martyred insect to move in and reap the benefit of their sacrifice. “Any altruistic system is prone to exploitation,” Rueppell says.
Exile
Parasites and diseases can have a field day with big, enclosed nests of insects. But scientists suspect that ants and bees may have a way of halting germs from spreading too far. When these insects become sick, they leave the colony and go into exile to await their deaths.
In one experiment, Rueppell and his colleagues dosed honeybees with carbon dioxide and hydroxyurea, a drug used to treat sickle cell anemia and some cancers. “We wanted to really make them feel very sick,” he says. The bees that survived this treatment abandoned the hive, even though their fellows did not try to kick them out. Other scientists have seen rock ants stop socializing with their nestmates and head into seclusion when sickened with carbon dioxide, infected with fungal spores, or sickened by unknown ailments.
Scientists don’t really know what prompts these insects to start behaving in ways that will inevitably cause their own demise. Whether and how the brain might be overriding an insect’s self-preservation instincts when it goes into quarantine or rips itself to pieces is a mystery. “Is there something special about this, or is it just a continuation of normal defensive behaviors?” Rueppell wonders.
Still, it’s probably a safe bet that self-sacrificing insects are not consciously laying down their lives for the greater good. “I don’t think we can call an ant that sacrifices itself a ‘good’ ant,” Rueppell says.
In some ways, insect colonies resemble a single living organism instead of thousands of separate individuals. “Immune cells in our body are to some degree self-sacrificial as well,” Rueppell says. If we lose a few cells, the rest of the body doesn’t care. The workers that die to ensure the colony’s survival are also easily replaced—but their sacrifice isn’t entirely selfless.
The members of an insect colony are very closely related, and workers often don’t reproduce. If an insect can keep the queen and the rest of the colony from being wiped out by enemies or illness by laying down its own life, there’s a better chance that some of its genes will be passed on by relatives.
“It’s not the individual that counts—otherwise we wouldn’t see these self-sacrificial behaviors,” Rueppell says.
