Comparing inclined locomotion in a ground-living and a climbing ant species: sagittal plane kinematics.

Formicine ants are able to detect slopes in the substrates they crawl on. It was assumed that hair fields between the main segments of the body and between the proximal leg segments contribute to graviception which triggers a change of posture in response to substrate slopes. The sagittal kinematics of two ant species were investigated and compared on different slopes. Cataglyphis fortis, a North African desert ant, is well known for its extraordinary sense of orientation in texturally almost uniform habitats, while Formica pratensis, a common central-European species, primarily uses landmarks and pheromone traces for orientation. A comparison of these two species reveals differences in postural adaptations during inclined locomotion. Only minor slope-dependent angular adjustments were observed. The largest is a 25 degrees head rotation for Cataglyphis, even if the slope is changed by 150 degrees, suggesting dramatic changes in the field of vision. The trunk's pitch adjustment towards the increasing slope is low in both species. On all slopes Cataglyphis achieves higher running speeds than Formica and displays greater slope-dependent variation in body height. This indicates different strategies for coping with changing slopes. These specific aspects have to be reflected in the ants' respective mode of slope perception.