Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Not all ants are as busy Alex Wild/alexanderwild.com

Species: Temnothorax rugatulus ants

Habitat: High-elevation forests of the western US and Canada

A group of ants labelled as lazy in 2015 may play a more important role in their societies than we thought, possibly helping to feed their hard-working peers.


The ants belong to a species (Temnothorax rugatulus) that builds nests under rocks in the forests of western North America. Most individual ants are busy with daily duties, but Daniel Charbonneau and Anna Dornhaus at the University of Arizona noticed in 2015 that some were consistently doing, well… nothing.

Now, Charbonneau and Dornhaus, and their colleagues, have studied the lazy ants to find out more. Their analysis of the ants’ anatomy and behaviour suggests they aren’t the freeloaders we might assume them to be.

First, these ants don’t simply do nothing all day, despite their lazy label. They simply behave differently from their peers, says Charbonneau. “These ants walk more slowly, are isolated in colony interaction networks and have the smallest behavioural repertoires,” he says.

They look different too: Charbonneau says they tend to be plumper and more likely to contain egg cells inside their bodies than their more energetic peers.

Living larders?

These observations mean we can rule some things out, he says: lazy ants are not simply old, infirm worker ants, for instance. Instead, lazy ants seem to be immature workers. Their plumper bodies might be evidence that they are storing food in their crops to share with their nest mates later, says Charbonneau. What’s more, the eggs they carry might serve as food for other ants – particularly since other ant species are known to sometimes lay such unfertilised “trophic” eggs.

“Inactive workers [may be] storing food for the colony,” says Charbonneau.

An alternative function of these idle workers is that they may serve as a reserve, says Erik Frank at the University of Würzburg in Germany. “There should be situations in which a larger workforce comes in handy – like defending the nest,” he says. “It also leaves a buffer to replace dead foragers.”

Charbonneau is unconvinced by this idea, though. He says it has been explored in some earlier studies of colonial insects, but these found no clear evidence that colonies have a “reserve worker” community.

Frank adds that there is probably no single explanation for the existence of lazy ants. “As the authors state, there very likely is not just one reason for these inactive ants,” he says. “They probably have various benefits for the colony and reasons for their inactivity.”

Now Charbonneau aims to explore the unusual ants even more thoroughly. He also plans to look beyond this species to see if lazy ants are a more common feature of ant colonies in general. “The next thing is really starting to look at the function and explanation for inactivity across species to see if there’s some main mechanism which facilitates inactivity,” he says.

Journal reference: Integrative and Comparative Biology, DOI: 10.1093/icb/icx029

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