Physical laws work on organisms just as on inanimate matter. Thus, the functions and structures of animal parts necessarily go together. Cursorial mammals are those that are specialized for running, and their skeletons reveal their adaptations for this function. Two of the obvious physical requirements for rapid running are the weight distribution of a limb and the length of the stride.
The horses of the Chihuahuan Desert region, fossil or living, are good
examples. The main muscle masses are drawn up close to the body, since any weight
distant from the body requires more energy to move (try swinging a 10-lb weight back
and forth at the end of your extended arm and you'll see). The feet are very
elongated, and the toes have been reduced to one per foot; furthermore, horses run on
the tips of their single toes—the whole result being a very long limb, allowing a
single stride to carry it far. How long? Well, that knee-like structure up high on the
hind limb? That's not a backwards pointing knee—it's the heel!
Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
Anatomical Adaptation for Running