Running up that hill – how hard is it?

A key area of research for the Centre for Geometric Biology relates to understanding how the size and shape of organisms (the geometry) affects the acquisition and use of energy.

Studies have found that the energy cost of terrestrial locomotion increases with an animal’s size but, when considered per unit mass, the cost is actually lower in larger species.  However most of these data have been collected from animals running on the flat whereas wild animals traversing a landscape will encounter a range of differently sloped terrain.

More recent research has analysed the relationship between the Net Cost Of Transport (NCOT; the amount of oxygen used per metre travelled), the incline and animal body mass across species. These analyses all considered the cost of transport on a ‘pound for pound’ basis which do not reflect the absolute energy expenditure for an animal or, therefore, the ecological consequences to the animal of those metabolic costs.

Lewis Halsey from The University of Roehampton and the Centre for Geometric Biology’s Craig White were interested to discover if examining the effect of body size on the energetics of going up hills, changes when data is included that looks at the NCOT for the whole animal, as well as on a relative basis and, as in previous studies, on a ‘pound for pound’ or mass specific basis.

They reviewed data from published literature where animals ran on a treadmill at more than one incline and compiled data on the oxygen consumption per metre travelled, (i.e. NCOT data), as well as mass values for each species studied.  In total Lewis and Craig were able to collate and analyse data for 24 species ranging in mass from the 30 g mouse to the 492 kg horse.  Data was limited to birds and mammals running up slopes of less than 90 degrees to ensure that there was no association between body mass and incline in the data set (very small species may be able to traverse inclines greater than 90 degrees).

Interestingly, Lewis and Craig found that apparently contradictory interpretations are possible with a single data set because in both absolute (whole animal) and relative terms, lighter birds and mammals experienced a smaller increase in transport costs when walking uphill. In contrast on a mass specific basis or ‘pound for pound’ basis the increase in transport costs were similar for species of different sizes.  When taken altogether these different measures suggest that smaller animals are intrinsically less efficient movers, and so any extra cost incurred from moving uphill, (due to working against gravity), is relatively small.

Relationship between Net Cost Of Transport (NCOT) and gradient, on a per species basis. From left to right the graphs present absolute NCOT (N=26), relative NCOT (N=26) and mass-specific NCOT (N=23) plotted against gradient. Lines link data for the same species. Lightest species are in blue and heavier species in orange.

Because absolute and relative costs of transport are greater for heavier animals moving up slopes, then we might expect that bigger animals are more inhibited in the routes they take across a landscape.  The documented routes of African elephant herds are apparently constrained by the topography of their home ranges: they rarely walk on ground where the slope is more than about 4 degrees.  The routes of smaller animals, however, are less likely to be influenced by the terrain they are traversing as the relative and absolute cost of running up hills is low.

This research demonstrates the value and importance of considering costs of animal locomotion in absolute (whole animal), relative and mass-specific terms.

This work has been published in The Journal of Experimental Biology.