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 mo
st important? To examine trail route choice in the wood ant Formica rufa, w
e used a series of bridges, each with two alternative routes allowing the c
omparison 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 br
idge combinations, we were able to separate the relative differences in tim
e versus energy saving represented within a series of successive choices. C
ontrary 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 pote
ntial energy costs encountered. (C) 2001 The Association for the Study of A
nimal Behaviour.