Centennial Museum gecko logo

Desert Diary
Mammals/Two Arms


Living things, naturally, have to obey the laws of physics. Some of these govern the relationship between power and speed, where there's usually a trade off between strength and rapidity. Think about the gears of an automobile. In low gear, we have lots of power, but creep along; in high gear we speed but, as a steep hill quickly advises us, not a great deal of power.

The same principles govern animal bodies. The upper arm bones of moles are short and stocky, with various projections giving favorable angles for powerful muscles. The forelimbs of moles are digging tools, adapted to moving heavy, consolidated soil during the construction of their burrow systems. But no one that I know of has ever accused a mole of being fast. On the other hand, ever had a cat swipe at you so fast that you saw only a blur? That cat's upper arm bone is long and slender, while its claws more than make up for its relative lack of power. Most animals are armed for their special roles in life.
pen and ink


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.

cat and mole humeri

Humeri of Domestic Cat and Coast Mole presented as if of approximately the same length.