Black garden ants (Lasius niger) on a cotton bridge. Photo credit: Audrey Dussutour.

Black garden ants (Lasius niger) on a cotton bridge. Photo credit: Audrey Dussutour.

Every family has a picky eater, but imagine feeding a family of 20,000, where some siblings eat nothing but pasta and others expect steak. This is the situation faced by forager ants on a daily basis. ‘The queen and larvae want protein for growth, but the workers want carbohydrates for energy,’ explains Audrey Dussutour from the University of Toulouse, France. This is fine when food is plentiful, but Dussutour and her colleague Stephen Simpson, from the University of Sydney, Australia, wondered how well ant colonies cope when a staple vanishes from the menu and they are threatened with starvation.

‘I love collecting ants, it's like digging for treasure’, chuckles Dussutour, who remembers returning from the nearby village of Marquefave with 24 colonies of black garden ants (Lasius niger), ready to test their resilience when their diets became unbalanced. Dussutour established 64 well-fed mini colonies – with 200 residents each – and then assigned each colony to one of four diets: a high carbohydrate/low protein diet that was perfectly balanced for the ant's nutrition; a poorly balanced diet of low carbohydrate and high protein that would produce malnutrition; a dilute well-balanced diet – the ant equivalent of thin soup; and a starvation diet. Then she waited to see how well they fared.

However, Dussutour was in for a shock. ‘Usually we use foragers to do these studies because they are supposed to be older so they are supposed to die earlier’, she says. However, the foragers that were fed the optimal diet lived for almost as long as the inner nest workers, with one surviving an incredible 409 days while the last worker held out for 436 days. And some of the foragers on the well-balanced thin soup diet even survived for 300 days. However, the foragers on the high protein malnutrition diet struggled, only surviving an average of 21 days, and the foragers that were placed on a starvation diet were decimated. ‘They lasted a week and a half, but the inner nest workers survived for over 100 days’, says Dussutour. So, when their diet took a turn for the worse, the foragers were always the first to die, even though they were the colony's sole supplier of nutrition.

But why did the foragers suffer first? Dussutour was puzzled: ‘We wondered if it was because they had different tasks or if it was because they were different ages’. Foragers are usually the oldest ants in the nest and workers are the youngest, so Dussutour used a cunning trick to create youthful foragers and elderly nest workers to test whether age or role was the cause of the forager's demise. Testing the ants’ longevity on a starvation diet, Dussutour was amazed to discover that the foragers always died first. ‘Even the foragers that were young died earlier than the inner nest workers that were old’, she says.

Having ruled out age as the cause of the foragers’ deaths, Dussutour wondered whether they were more vulnerable to starvation because they carry less fat than worker ants. She measured the fat content of foragers’ and workers’ bodies as they switched between roles, and no sooner had a worker switched to become a forager than its body fat content dropped to 15%. Meanwhile, the fat content of foragers that assumed a worker's role rocketed to 30%. ‘The colonies that had consumed more carbohydrates were more fat and we were able to show that [for ants] the fatter you are the better it is for your survival,’ chuckles Dussutour.

So it seems that lean foragers die first when food is scarce, although Dussutour explains that foragers have to be skinny: ‘If they are not starving, they won't be motivated to go out to forage’, she laughs.

S. J.
Resistance to nutritional stress in ants: when being fat is advantageous
J. Exp. Biol.