A bug's life – how ants maintain constant energy expenditure over hilly terrain

Holt N., Askew G. (2012). Locomotion on a slope in leaf-cutter ants: metabolic energy use, behavioral adaptations and the implications for route selection on hilly terrain. The Journal of Experimental Biology, 215: 2545-2550 Details

Customer study highlights

Ants – more often associated with insecticide measures than popular science – may have more in common with humans than previously thought.

When walking, humans have a tendency to seek out the shortest route between two points (i.e. a straight line), presumably because this will incur the least time and energy expenditure. The presence of obstacles and changing gradients creates deviations from the shortest route, generally resulting in route optimization. Interestingly there is evidence of certain animals adopting this strategy too. According to a recent study this now includes ants!

Holt and Askew (2012) describe a tendency for leaf-cutter ants (Acromyrmex octospinosus) to adjust their walking speed with changing slope gradient, which they interpreted as evidence for minimizing gross metabolic energy expenditure.

To show this, ants from a single colony were tested one at a time inside a variable-incline, sealed 'running tube', designed to measure CO2 production, temperature, pressure, and mass air flow. Data were acquired using a PowerLab and LabChart software.

Ants' CO2 production was unchanged by slope gradient (±90°) with an average of 1.7 ml g-1 h-1 for all slopes. From this, the gross metabolic rate was calculated to be approximately 47 J. h-1 (compared to a resting metabolic rate of 293,000 J. h-1 in an average human). In contrast, walking speed was significantly affected by slope: ants travelled at a mean 2.0 cm s-1 during level walking, which was reduced to approximately 0.7 cm s-1 for 90° inclines and declines (i.e., forming an inverted U shape speed-slope relationship).

This behavioural adaptation may confer survival advantages, for instance by allowing foraging for longer periods with increased yields for the colony. The advantage for humans is less clear – perhaps this facilitates our busy lifestyles? Or perhaps we have optimized our ability to be lazy.

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