Locomotion on a slope in leaf-cutter ants: metabolic energy use, behavioural adaptations and the implications for route selection on hilly terrain.

The metabolic cost of the negotiation of obstacles, and the influence that this has on route selection, are important determinants of an animal's locomotor behaviour. We determined the gross metabolic cost of locomotion on slopes of different gradients, ranging from -90 to +90 deg, in leaf-cutter ants (Acromyrmex octospinosus) in a closed-circuit respirometry system. Ants were able to select their preferred speed for each gradient. The gross metabolic energy expenditure per unit distance travelled on the slope (C(path)) was calculated from the rate of CO(2) production and the speed of locomotion. These data were used to predict the optimal slopes for minimising the vertical cost of locomotion and vertical journey time. The gross rate of CO(2) production was approximately constant (1.7 ml g(-1) h(-1)) and was not significantly affected by slope. Ants moderated their speed with slope (P<0.05), travelling the fastest during level locomotion (2.0±0.1 cm s(-1), N=20) and increasingly slowly with increased gradient (both on an incline and a decline). C(path) varied significantly with slope, being lowest during level locomotion (646.0±51.2 J kg(-1) m(-1)) and increasing with increasing gradient. These results suggest that ants adapt their locomotor behaviour to keep metabolic rate constant despite changing mechanical demands. It is predicted that when undertaking a journey involving vertical displacement that ants will select routes with a gradient of between 51 and 57 deg during ascent and with a gradient of between -45 and -51 deg during descent, in order to minimise both vertical journey time and vertical cost of locomotion.