A different angle: comparative analyses of whole-animal transport costs when running uphill.

Comparative work on the cost of terrestrial locomotion in animals has focused on the underpinning physiology and biomechanics. Often, much of an animal's energy budget is spent on moving around; thus, there is also value in interpreting such data from an ecological perspective. When animals move through their environment, they encounter topographical variation, and this is a key factor that can dramatically affect their energy expenditure. We collated published data on the costs for birds and mammals to locomote terrestrially on inclines, and investigated the scaling relationships using a phylogenetically informed approach. We show that smaller animals have a greater mass-specific cost of transport on inclines across the body mass range analysed. We also demonstrate that the increase in cost for smaller animals to run up a slope as opposed to along a flat surface is comparatively low. Heavier animals show larger absolute and relative increases in energy cost to travel uphill. Consideration of all aspects of the cost of incline locomotion - absolute, relative and mass specific - provides a fuller understanding of the interactions between transport costs, body mass, incline gradient and phylogeny, and enables us to consider their ecological implications, which we couch within the context of the 'energy landscape'.