Foraging efficiency in the wood ant, Formica rufa:: is time of the essence in trail following?

Central place foraging in ants is expected to maximize colony efficiency by minimizing the lengths of established trails. Short trails reduce both the time and energy costs of delivering food back to the nest, but which is most important? To examine trail route choice in the wood ant Formica rufa, we used a series of bridges, each with two alternative routes allowing the comparison of combinations of horizontal, vertical or time-expensive detours of differing lengths or delays. The ants preferred the shortest, quickest routes in all cases, although when vertical and horizontal deviations were equal in length they preferred the former, probably because this represents a reduced risk in terms of navigation errors. When comparing a range of bridge combinations, we were able to separate the relative differences in time versus energy saving represented within a series of successive choices. Contrary to earlier evidence, savings in energy explained significantly more of the variation in ant trail preferences than savings in time. We suggest that our results can be explained by wood ants adaptively adjusting their speed when walking in the vertical plane in response to the additional potential energy costs encountered. (C) 2001 The Association for the Study of Animal Behaviour.