Kathryn Knight

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A rumbly tummy is always a good sign that you need to get some food, but how do you know that you're hungry when you forage on behalf of a collective stomach? That is the problem faced by social insects. According to Elva Robinson from the University of York, UK, Temnothorax albipennis foragers are always the leanest occupants of the nest. However, ‘It's not as simple as the lean ants know when they're hungry and that triggers foraging’, explains Robinson, recalling that instead of consuming their honeydew load, slender foragers selflessly give it up to fatter nest mates. Robinson and her colleagues, Ofer Feinerman and Nigel Franks, wondered whether the skinnier ants forage because their mass has fallen below a critical threshold or whether the ants are motivated by their previous foraging experience and their slenderness is merely a side-effect of their frenetic lifestyle (p. 2653).

‘But those things are generally almost impossible to untangle’, says Robinson, adding, ‘So that is where technology came in’. Realising that they could use minute radio frequency ID tags to selectively open the nest's exit, the team designed an experiment where they could produce tubby and slender foragers to find out which factor – thinness or experience – sent foragers searching for food.

Teaming up with Feinerman to build a computer-controlled door that was activated by the ants' ID tags, Robinson initially allowed all of the nest's occupants to roam freely around an experimental arena, which she equipped with a well-stocked feeder for 1 h a day. However, as soon as the first successful foragers returned, Robinson used a reader to scan their IDs and enter them onto a blacklist that banned them from embarking on further outings. Deprived of food, the nest then motivated the next cohort of foragers to depart, but they too were blacklisted upon their return.

Over a week, Robinson photographed the foragers to determine their fat levels and built up the blacklist so that at the end of that period the active forgers were fatter than the foragers had been on the first day. Then she wiped the blacklist clean to see which ants would be the first to leave: the leanest foragers with out-of-date experience that had suffered a week of setbacks, or the tubbiest foragers with the most recent experience.

The results were very clear. Robinson recalls that as soon as they could open the door again, the leanest ants were back out foraging. And, as she rebuilt the blacklist over the course of a second week, the foragers' girth increased as the slimmest forgers were systematically prevented from exiting the nest again.

So, an ant's leanness is the most important factor in deciding whether it goes out to forage. However, when Robinson took a closer look at the ants' activity patterns, she realised that experience also contributed to a lesser extent to the decision. ‘For any given leanness, if the ant had got any foraging experience at all in the previous week it was more likely to come out than ones which didn't have any experience at all,’ she recalls.

Considering the rationale of the ant's strategy Robinson says, ‘There are good reasons why you would expect it to be important for lean ants to forage’. Explaining that lean ants are more mobile and less valuable than their corpulent nest mates she adds that the mechanism is self-regulating. Nests on the verge of starvation will have many foragers to satisfy their hunger, while foragers in well-fed nests will lose interest in foraging as they gain mass when forced to digest their honeydew cargo destined for the already full collective stomach.