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The gallant ant (Image: Rob Snow)

It’s the season of selflessness, so meet some unlikely altruists – the ants that work tirelessly to rescue their sisters

YOU don’t have to be a cynic to realise that the bonhomie and gift-giving at this time of year is not entirely motivated by a desire to bring pleasure to others. Go on, admit it, sometimes it has more to do with assuaging guilt, improving our standing in the eyes of the recipient, or simply hoping for something better in return. And there’s nothing wrong with that. Even if we are not entirely altruistic, at least humans have regular orgies of niceness. By contrast, just a handful of examples of genuine altruism have ever been recorded in other animals.

But before we become too complacent, let’s bring in Elise Nowbahari. She believes that if evidence of genuine altruism is rare in the wild, that is simply because biologists have not searched for it systematically. She is part of a new vanguard of researchers exploring selfless behaviour in animals in the lab. Nowbahari’s experiments at the University of Paris-North in France are certainly systematic – if a little unkind – and .

Altruism is a behaviour of an individual that benefits another at its own expense. Biologists have been arguing over its prevalence among animals for decades, but Nowbahari was blissfully unaware of the debate when she stumbled into the minefield. She studies the behaviour of ants and, having spent 30 years collecting the insects from sandy soils in southern France, has often observed them digging around trapped individuals and pulling at their limbs to free them. She decided to investigate what motivates ants to undertake these dangerous missions, where they risk getting trapped themselves or, worse, eaten by predatory antlion larvae, which dig pits and lurk, semi-concealed, at the bottom with their jaws wide open. Such apparently selfless rescue behaviour is seen by many as one of the purest forms of altruism.

The first challenge was to create an effective trap in the lab. What was needed was an ingenious way of restraining the “victim” ant – which might be as small as 3 millimetres long – without damaging it or preventing it from releasing the pheromone that signals its distress to others. After much trial and error involving glue and pincers, Nowbahari, Karen Hollis of Mount Holyoke College in South Hadley, Massachusetts, and their team hit upon the solution. Take a piece of cotton thread, create a loop by sewing it through a small square of filter paper, then pop your ant in and tighten the noose around its waist before tying off the ends. Finally, place your handiwork near the entrance of an ant nest, cover the filter paper with a sprinkling of sand, and leave the victim to “scream” (chemically) like a classic Hollywood damsel-in-distress tied to the railroad tracks.

“The ant is left to ‘scream’ (chemically) like a damsel-in-distress tied to the railway tracks”

It was fiddly, but the team soon had ant-restraint down to a fine art and could sit back and watch the rescuers in action. The first species under their spotlight was the sand-dwelling Mediterranean ant, Cataglyphis cursor. The team was expecting to see limb-pulling and sand-digging – two behaviours that have long been observed in the wild – but to their delight, the ants also adopted other strategies. If digging and pulling didn’t do the trick, they would transport sand away from the victim, returning many times until they had exposed the snare, and then bite or tug at the thread. This suggests they understood the nature of the problem at hand, say the researchers, and were not simply enacting a fixed response. The selflessness shown by some rescuers was quite remarkable, too. “I have seen a helper ant try for 6 hours to help a trapped one,” says Nowbahari. “She might stop from time to time, but she kept coming back.”

Workers to the rescue

Repeating the entrapment using four more species native to the Mediterranean region, the team discovered that not all ants are so good to each other (). The most devoted rescuers belonged to species that live in loose sand where they forage individually, are prey for antlions and are liable to be trampled by hoofed animals. By contrast, two species that live in hard, compact soils where the risk of collapse and predation is lower do not rescue at all. “They don’t attack the victim,” says Nowbahari, “but they do throw her, still attached to the filter paper, outside the area occupied by their nest-mates.”

The researchers conclude that a species’ propensity to rescue reflects the ecological conditions in which it evolved. To corrupt a phrase: necessity is the mother of intervention. In fact, this axiom also appears to underpin differences among individual ants in the most selfless species. Forager ants – older workers that roam far from the nest, putting themselves in positions where they may need rescuing – were most disposed to help one another. Nurses, which are younger and more sedentary, sometimes rescued other ants. And, in the experiments at least, inactive juveniles, which do not leave the nest and cannot rescue, were almost never rescued themselves.

Ants also discriminate on the grounds of relatedness. In all the species Nowbahari and Hollis have studied, there is only one fertile queen per nest, meaning that all the sterile female workers are her daughters. Workers generally only go to the rescue of their sisters, ignoring or even behaving aggressively to the victim if it belongs to another nest or species. Or, more subtly, they might pull at a victim from a different nest but in a less careful way, taking hold of its neck or delicate antennae as well as its limbs. “We have never seen an ant pull on a sister’s antennae,” says Nowbahari.

Being nice to relatives is not pure altruism because they share your genes so, by helping them, you promote your own genetic heritage. This is known as kin selection. Nevertheless, Nowbahari describes the behaviour of her ants as altruistic because they will put themselves in danger to rescue another. What’s more, she thinks that we have underestimated such altruistic rescue behaviour. “It probably isn’t in all animals, but we believe it’s much more widespread than was thought.”

So, do ants empathise with one another’s pain? show that they too will help cage-mates that are trapped, and Jean Decety at the University of Chicago, who has studied rats, calls their behaviour empathic. But rats have a limbic system – the part of the mammalian brain associated with emotional responses – and ants do not. Besides, Nowbahari’s ants still went into a frenzy of helping even when the victim was anaesthetised, indicating that they are not aware of each other’s mental states.

Rather than empathy, ants seem to be driven by an evolved urge to help members of their group – what biologists call pro-social behaviour. Their selfless acts suggest that complex, cognitively motivated behaviour might come about through simpler mechanisms than we had thought. Or, put another way, exactly what makes ants so nice to each other is a bit of a mystery. No doubt the same can be said for humans.

This article appeared in print under the headline “The gallant ant”