Why being fat is an advantage for ants

Published in the Journal of Experimental Biology, the study shows how well ants cope when threatened with starvation or imbalanced diets, shedding light on the complexities of achieving an ideal macronutrient balance for longevity in social insects and other species.

A team of researchers led by Dr Audrey Dussutour from the University of Toulouse and Professor Stephen Simpson, Academic Director of the University of Sydney's Charles Perkins Centre investigated how the lifespan of foragers and inner-nest workers was affected by dietary imbalance.

They examined a collection of 24 colonies of black garden ants (Lasius niger) – which can live in colonies of up to 20,000 workers – to test the ants' resilience to nutritional changes, modifying the ratio of protein to carbohydrate in their diets.

After establishing 64 well-fed mini colonies with 200 residents each, the colonies were assigned to one of four diets to see how well they fared: 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.

Surprisingly, forager ants that were fed the optimal diet lived for almost as long as the inner nest workers, despite typically being older, leaner and dying earlier than worker ants.

However foragers on the high protein malnutrition diet struggled, surviving only an average of 21 days. When faced with negative dietary changes, the foragers were always the first to die, despite being the colony’s sole supplier of nutrition.

"The results show that foragers are especially vulnerable to a nutritionally imbalanced, high-protein diet or to starvation, dying earlier than their nest mates and rendering the colony destitute," said co-author Professor Simpson.

"The nutritional needs of colonies of tens of thousands of ants are served by specialist foragers that have no conception of the differing and particular needs of all the members of the colony, yet somehow they manage to collect exactly what is required. 

"However, if ever the foragers are compromised then the whole colony suffers and may collapse," he said.

To determine whether resistance to starvation was related to body fat content, the researchers constructed daughter colonies and measured the fat reserves of different ant castes as they converted from forager (typically lower in fat) to worker (higher in fat) roles.

When workers switched to become foragers their body fat content dropped to 15 percent. Meanwhile, the fat content of forager ants that assumed a worker's role increased to 30 percent.

"The colonies that had consumed more carbohydrates were fatter and were able to show that [for ants] the fatter you are the better it is for your survival," said lead author Dr Dussutour.

"We confirmed once again that restriction to high-protein, low-carbohydrate diets decreased lifespan in ants, as in other species of insects and mammals," the authors state.

The paper, 'Resistance to nutritional stress in ants: when being fat is advantageous' is available online.