In a Cannibalistic Society, It’s Not About Survival—It’s All About Recycling
By Thomas Chouvenc
In this spooky time of year, there are many examples we could draw from insects to give the heebie-jeebies to non-entomophiles. We could talk about mosquitoes, the most dangerous animals on the planet, or ticks, which vector a variety of diseases, but these are not so much spooky as they are dangerous, seeing their dramatic impact on human health worldwide. Let’s keep this post on the light-hearted side, please.
So, spooky bugs. Let’s start with zombie ants. It’s a classic dive into zombie culture while exploring the amazing biology of a host-manipulating fungal parasite. Next in line would be parasitoid wasps, as their gruesome life cycle rips through the host’s organs from the inside while keeping it alive the entire time. Another favorite: the Nicrophorus burying beetles lay their eggs on a decomposing carcass and display extensive parental behavior to their growing larvae as they chew through the putrid flesh of the roadkill. Gruesome, yet full of love. It’s the pinnacle of cute. Halloween could definitely use insects as core material for “horrific” displays in our front yards (with all the classic anatomic fails of course, but this is another horror story). By the way, I was very disappointed when nothing came up from a “parasitoid wasp Halloween display” Google search (someone should do something about this, please).
Another spooky example, often forgotten, is cannibalistic insect societies. I bring your attention to termites. Termites have evolved away from their Cryptocercus-like wood roach ancestor and reached the highest level of social organization while exploiting woody material. The evolution of their biology was therefore constrained by a significant dietary restriction: wood is carbon-rich but notably nitrogen-poor. Termites have therefore perfected a recycling strategy toward nitrogen conservation over evolutionary time: cannibalism.
It also has been suggested that cannibalism could have an essential role in helping a group of termites survive a period of starvation. Workers would cannibalize their nutritionally-dependent nestmates to alleviate their trophic burden. This strategy would therefore reduce the metabolic footprint of a starving group of termites to increase their chance of survival, à la Soylent Green.
However, all previous studies on this topic subjected small groups of termites to starvation. In a recent study, I revisited this concept, but instead subjected full termite colonies to starvation. To make a long story short: termites are terrible in their survival strategy during starvation events because, in fact, they don’t really have a strategy.
Unlike honeybees that store months-worth of honey, termites have a carpe-diem approach to food safety, as they have no internal reserves. Instead, subterranean termites such as Coptotermes gestroi will relentlessly forage for new food sites to prevent food shortage in the first place. But if starvation of the termite colony actually occurs, give it about 30 days.
As the metabolism of the colony progressively ran out of fuel, nutritionally-stressed individuals started accumulating in the colony. Unfortunately for termite larvae and workers, who are hemimetabolous insects stuck in a permanent juvenile molting cycle, the time to molt eventually comes, and the younger the instar, the faster the molting cycle. Have you ever tried molting while completely starving? I would not advise it. These termites failed in their attempt to molt, leading to their death and subsequent cannibalism from nestmates.
Therefore, the brood and young workers were the first ones to be eaten, not as survival rations for the group, but because they were the first ones to die during their molting process. Then, some soldiers started shutting down and were cannibalized, as the remaining workers could not feed them. Toward the end, a handful of workers remained, with the king and queen being the last ones to die, inevitably. So, if termites actually have a survival strategy during starvation, it is this: “Keep the king and queen alive as long as you can.”
Contrary to a previous perception, termites don’t reduce the trophic burden of the colony by cannibalizing the dependent castes. Mortality was not cannibalism-driven; instead, cannibalism was mortality-driven. Termites just do what they always do: if a dead or moribund individual shows up in the group, it is cannibalized to recycle the nitrogen. The excessive mortality resulting from starvation triggered a massive cannibalism wave, which ended up with an accumulation of old workers with staggering levels of uric acid building up in their fat body, resulting from the sudden excess of available nitrogen through cannibalism. Who thought cannibalism could go wrong?
So now you know, if you have a termite infestation in your house, you can now actually say, “My house is being attacked by a horde of cannibals.” It would be almost accurate.
A previous version of this article first appeared on the Insectes Sociaux blog.
Thomas Chouvenc is an Assistant Professor in urban entomology at the University of Florida, UF/IFAS.