graphic

Watch an ant scurrying around searching for food, and you're looking at the tip of an ant iceberg; each forager is just one individual in a well-oiled foraging network that fuels their entire community. Species that survive on nectar make repeated foraging trips to flowers and other nectar sources, but many return with their crops only partially filled. Pablo Schilman and Flavio Roces explain that this apparently inefficient nectar-delivery has long puzzled scientists, and while the nectar's concentration seems to influence the ants' decision about how much to fill up, it wasn't clear whether the semi-loaded foragers were minimising their energetic foraging costs, or limiting the amount of time spent supping to maximise the return on their nutritious load. Schilman and Roces decided to measure the metabolic rates of freely feeding Camponotus rufipes as they fuelled up on sugar solutions over a range of concentrations delivered at various flow rates(p. 4091).

Surprisingly there was no correlation between the insect's metabolic rate and their final load; no matter how heavy their nectar burden, the insect's metabolic rate was relatively unaffected. Instead the team found an `increase in individual metabolic rate with increased sugar flow rate' explains Schilman, which he suspects is caused by the insect's increased activity levels as they become more motivated upon finding an abundant nectar supply.

The team also compared the amount of energy carried back by a forager with the energy expended during the trip to find out if the ants cut their foraging trip short to conserve energy, but even the weakest sugar solution was enough to sustain the intrepid insects over distances far exceeding the average foraging trip. The ants weren't limiting their loads to conserve energy, it seems that `decreasing their foraging time is more important than increasing individual energetic efficiency', says Schilman. He suspects that the insects cut time filling up to give themselves more time to spread the good news when they return to the nest.

References

Schilman, P. E. and Roces, F. (
2006
). Foraging energetics of a nectar-feeding ant: metabolic expenditure as a function of food-source profitability.
J. Exp. Biol.
209
,
4091
-4101.