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Platythyrea punctata workers and brood. Workers darken their cuticle colour with age. Photo credit: Abel Bernadou.

Life in Platythyrea punctata ant nests is all about the pecking order: from the dominant egg layers down to their lowlier sisters, which toil on behalf of their nestmates, and the humblest foragers. ‘Young workers establish dominance hierarchies and rank orders’, says Abel Bernadou from University of Regensburg, Germany, adding that the stay-at-home workers seemed to carry more fat than the leaner foragers, in addition to living longer. ‘However, it wasn't clear how the body fat levels of P. punctata workers play a role in regulating the transition from inside to outside nest activities’, says Bernadou. The team also knew that it was possible that other factors, such as the ants’ fertility, could be responsible for their abrupt lifestyle change. So, Bernadou, Elisabeth Hoffacker, Julia Pable and Jürgen Heinze decided to find out how much fat corpulent nest workers and lithe foragers carry.

After collecting ants at different stages of life, from the youngest nest workers to elderly foragers, and measuring the amount of fat they were carrying, it was clear that the youngest housekeepers were the fattest. They had body fat levels around 20%, while the oldest housekeepers’ fat content fell to ∼10% and that of the elderly foragers crashed to only 5%. But had the slim foragers’ fat loss triggered their change of role, or were other factors at play?

Wondering whether the ants’ ovaries might contribute to their lifestyle change, Hoffacker diligently followed the foraging antics of 90 workers, ranging in age from newly emerged adults to elderly foragers, for several days. Analysing the activity levels of the fattest ants, the condition of their ovaries (whether they were well or poorly developed) had no effect. The fattest ants spent little time foraging, regardless of their ovary development, so the ovaries do not trigger the ant's transformation from nest worker to forager. However, when Bernadou plotted the amount of time the ants spent out-and-about against their thorax fat content, there was a clear switch point. Ants with a thorax fat content above 4% barely ever left the nest, while the slimmer ants below the 4% threshold rarely spent any time at home. Corpulence, not fertility, seemed to be the key factor in the move to an outdoor lifestyle, but is body fat the trigger that converts an indoor worker to an intrepid outdoor forager?

To answer this question, Pable collected young adults that had just emerged from their pupae and slimmed down half of them, by only feeding them once in 10 days, while feeding up the remaining ants on a daily diet of Drosophila. Then, she paired up a slim and a tubby youngster in a tiny nest and kept an eye on their activities to identify which of the two spent most time outdoors. During the first 3 days, the skinny youngsters spent twice as long outside as their well-fed sisters. Even though the under-fed ants were much younger than normal foragers, they were as keen to venture out as the elderly thin ants. In other words, body fat is the key to the ants’ transition from worker to forager, and Bernadou says, ‘Leanness is not a consequence of foraging activity but has a causal effect on the transition from duties inside the nest to outside tasks’.

The team adds that the fat switch makes sense on several levels. Only sending out the slimmest members of the nest protects the collective fat reserves, as the foragers may not return, in addition to ensuring that the youngest workers are well upholstered to invest energy in producing the eggs that ensure the nest's future.