HERMANSON, J.W.; BERTRAM, J.E.A.; Cornell Univ.; Univ. of Calgary: Transport and transitions on arid grasslands: a long-legged niche.

It is often said that long-limbed animals such as horses developed their cursorial habits and morphology as a result of predatory pressures. The “arms race” in which predators grow faster and prey species become more elusive is often hinted at in evolutionary documentary. We discuss the form and function of horses, in the context of recent studies on gait and energetics, arguing that equine morphology developed to allow effective long distance travel at moderate speeds allowing the exploitation of widely distributed resources in the prevailing environment. Similar specializations have occurred in several divergent clades to exploit the same general niche of open grasslands, similar to the Miocene savannahs of North America. The distinguishing major features of horses, long limbs with the use of highly coordinated gaits, hypsodont dentition, and hindgut fermentative digestive system, adapted them for a unique although currently restricted niche. Parallels are noted in camels, which had an early tendency toward an elongate distal limb skeleton with specialized muscle form and the use of the pace as an efficient gait for long-distance transport. Camels also exhibited specialized dentition, and a form of ruminating foregut digestion that is similar to, but does not have the lengthy gut transit time of, the rumen of modern artiodactyls. Farther removed phylogenetically from these examples, but exhibiting functionally comparable locomotory and digestive adaptations, are large kangaroos of the Australian continent and ostriches of Africa. We discuss unifying principles of these four groups and, in a subsequent presentation (Bertram and Hermanson) summarize specific features of the musculoskeletal system of the modern horse that provide the means to exploit the niche. Supported by NSF IBN 9819985.