On their knees: distal femur asymmetry in ungulates and its relationship to body size and locomotion

ABSTRACT

Asymmetry of the knee (distal femur), where the medial trochlear ridge is larger than the lateral one, has been observed in certain ungulates, but until now there has not been a comprehensive study of this anatomy across a range of extant and extinct mammals. A degree of knee asymmetry is present in most ungulates: this asymmetry increases in extent with increasing body size, and is also more prominent in taxa with a preference for open habitats, independent of body size. However, camelids and proboscideans have symmetrical knees, regardless of size. We propose that the correlation of degree asymmetry with habitat type, as can be observed in extant ruminants, is related to the preferred type of locomotion, with the gallop gait being preferred in open habitats. Neither camelids nor proboscideans employ the gallop as a habitual gait. The allometric increase observed in knee asymmetry, in combination with the correlation with locomotor preference, likely reflects anatomy that resists muscular forces pulling the patella medially when the animal is supporting its weight on a single hind leg during fast locomotion. Inclusion of fossil taxa leads to interesting interpretations about the behavior of extinct ungulates: extinct brachydont equids of presumed closed habitat preference have less asymmetric knees than extant equids, or extinct hypsdont equids of presumed open habitat preference; the knee anatomy of giant deer, Megaloceros, indicates a preference for open habitats, whereas the knee anatomy of the brontothere Megacerops indicates a proboscidean (versus rhino-like) type of fast locomotion.

ACKNOWLEDGMENTS

We thank the following curators for access to specimens in their collections: J. Chupasko (Mammalogy Collections, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts), J. Cundiff (Paleontology Collections, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts), J. Galkin and J. Meng (American Museum of Natural History, New York, New York), R. Secord (University of Nebraska State Museum, Lincoln, Nebraska), C. Norris (Yale Peabody Museum, New Haven, Connecticut), and A. Lister (Natural History Museum, London, U.K.). R. Thompson did the drawings of the knees. Comments from B. Shockey and an anonymous reviewer improved the quality of the manuscript. This project was supported by a Bushnell Foundation Grant (Brown University) to C.M.J.

Handling editor: Blaire Van Valkenburgh