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

Abstract

SUMMARYThe metabolic cost of the negotiation of obstacles, and the influence that this has on route selection, are important determinants of an animalʼs 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 ) 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.

Effect of the land area elevation on the collective choice in ants

“…The relief of the branches seemed to be a determining factor leading to a clear collective choice in the case of asymmetrical DH condition in both studied species, but no information was available here to discuss the impact of this factor on the modulation of pheromone deposition at the individual level. In addition, although previous work showed the effect the of gravity on forager movement 51 and travel speed 36 , 40 , 57 , we were not able to highlight an effect of slope (ascending or descending) or nutritional status (empty or full crop) on the average moving speed as no difference was revealed whether on back or forth travel on either branch; the average speed remained constant.…”

Effect of the land area elevation on the collective choice in ants

“…The relief of the branches seemed to be a determining factor leading to a clear collective choice in the case of asymmetrical DH condition in both studied species, but no information was available here to discuss the impact of this factor on the modulation of pheromone deposition at the individual level. In addition, although previous work showed the effect the of gravity on forager movement 51 and travel speed 36 , 40 , 57 , we were not able to highlight an effect of slope (ascending or descending) or nutritional status (empty or full crop) on the average moving speed as no difference was revealed whether on back or forth travel on either branch; the average speed remained constant.…”

Geckos decouple fore- and hind limb kinematics in response to changes in incline

“…Duty factor often increases on uphill surfaces, allowing greater time in contact with the ground to provide the extra work required to move against gravity. Collectively, this results in decreased speed on uphill surfaces [ 18 , 19 , 21 , 58 – 61 ], but we do not observe this in the geckos in our study. Russell and Higham [ 21 ] examined species of pad-bearing and padless geckos, and found that a trade-off exists between adhesion on inclines and running speed.…”

A model-based approach to study ant energetics from trajectory data

“…Empirical data were extracted from Refs. ( 6 , 7 , 10 , 60–70 ). Overlapping data between motion and the energy consumption at the individual-level is currently lacking in the literature—leading to an inability to validate predictions directly.…”

Effect of the land area elevation on the collective choice in ants

“…The relief of the branches seemed to be a determining factor leading to a clear collective choice in the case of asymmetrical DH condition in both studied species, but no information was available here to discuss the impact of this factor on the modulation of pheromone deposition at the individual level. In addition, although previous work showed the effect the of gravity on forager movement 51 and travel speed 36 , 40 , 57 , we were not able to highlight an effect of slope (ascending or descending) or nutritional status (empty or full crop) on the average moving speed as no difference was revealed whether on back or forth travel on either branch; the average speed remained constant.…”

Geckos decouple fore- and hind limb kinematics in response to changes in incline

“…Duty factor often increases on uphill surfaces, allowing greater time in contact with the ground to provide the extra work required to move against gravity. Collectively, this results in decreased speed on uphill surfaces [ 18 , 19 , 21 , 58 – 61 ], but we do not observe this in the geckos in our study. Russell and Higham [ 21 ] examined species of pad-bearing and padless geckos, and found that a trade-off exists between adhesion on inclines and running speed.…”

A model-based approach to study ant energetics from trajectory data

“…Empirical data were extracted from Refs. ( 6 , 7 , 10 , 60–70 ). Overlapping data between motion and the energy consumption at the individual-level is currently lacking in the literature—leading to an inability to validate predictions directly.…”

Effect of the land area elevation on the collective choice in ants

“…The relief of the branches seemed to be a determining factor leading to a clear collective choice in the case of asymmetrical DH condition in both studied species, but no information was available here to discuss the impact of this factor on the modulation of pheromone deposition at the individual level. In addition, although previous work showed the effect the of gravity on forager movement 51 and travel speed 36 , 40 , 57 , we were not able to highlight an effect of slope (ascending or descending) or nutritional status (empty or full crop) on the average moving speed as no difference was revealed whether on back or forth travel on either branch; the average speed remained constant.…”

Geckos decouple fore- and hind limb kinematics in response to changes in incline

“…Duty factor often increases on uphill surfaces, allowing greater time in contact with the ground to provide the extra work required to move against gravity. Collectively, this results in decreased speed on uphill surfaces [ 18 , 19 , 21 , 58 – 61 ], but we do not observe this in the geckos in our study. Russell and Higham [ 21 ] examined species of pad-bearing and padless geckos, and found that a trade-off exists between adhesion on inclines and running speed.…”

A model-based approach to study ant energetics from trajectory data

“…Empirical data were extracted from Refs. ( 6 , 7 , 10 , 60–70 ). Overlapping data between motion and the energy consumption at the individual-level is currently lacking in the literature—leading to an inability to validate predictions directly.…”