Most species only have to worry about eating for one, but when you're a forager ant you're eating for the community. You have to listen not only to your own nutritional needs, but the needs of your coworkers too. Or do you?Audrey Dussutour and Steve Simpson from the University of Sydney, Australia,wondered whether the colony as a whole was able to regulate its nutritional intake. Dussutour explains that ants' dietary requirements change as they grow; adults are able to survive several weeks without food, while hungry larvae need a constant supply. Would the colony's ability to regulate nutritional intake vary when larvae came along? Dussutour began monitoring the carbohydrate intake of green headed ant colonies to see if the collective regulated its nutritional intake(p. 2224).
Isolating individual adult-only colonies and supplying them with either concentrated (18%), medium strength (9%) or dilute (4.5%) sucrose solutions,Dussutour filmed the ants' comings and goings over a 6 week period to see how many foragers were recruited by their colonies to gather food. `Ants have a sweet tooth,' says Dussutour, so the insects mobbed the concentrated solution during the first few hours of the week, but recruited fewer and fewer foragers as the colony's occupants `filled up' during the week. The colony was regulating its carbohydrate intake, but not particularly well. However, it was a different matter for the insects provided with a weak sucrose solution. According to Dussutour, they didn't regulate their intake at all during the first few days, preferring to starve rather than consume the unpalatable weak sucrose.
But when larvae started coming along, things changed; `the colony got better at regulating their intake,' says Dussutour. Even though the ants didn't like the weak nectar supply, they started sending out more and more foragers over the 6 week experiment, doubling the volume of sucrose solution supplied to the nest. The colony was regulating its carbohydrate intake, but why? Was the colony responding to the larvae's hunger?
Dussutour increased the colony's size by adding either larvae or adults and monitored the volume of sucrose gulped down by the foragers. The colonies only sent out enough foragers to cover the extra mouths when the adult numbers increased. But it was a different matter when Dussutour increased the number of larvae. The colony not only sent out more foragers, but also massively increased the volume of dilute sucrose carried back to the nest. The colony was regulating its nutrient intake to satisfy its demanding young.
Finally, Dussutour tested whether individual ants could independently regulate their individual sucrose uptake by isolating groups of 25 adults from each colony, supplying them with sucrose for 1 h a day over 5 days, and measuring the amount each ant consumed by weighing it. The insects successfully regulated their sucrose intake. Ants supplied with concentrated sucrose gorged on the first day, but reduced their intake on subsequent occasions, while the ants fed on unpleasantly dilute sucrose barely touched it on the first day, but gradually increased their intake as they became hungrier.
Having found that ant colonies respond to their young when filling the`collective stomach', Dussutour is keen to find out how the larvae communicate with the colony to satisfy their hunger.