There are numerous advantages in being big—along with many disadvantages. One of the good things about being a giant is that it's not easy to be bullied, and if it comes to a push and shove type of competition, size usually wins out. Likewise, most predators shy away from prey animals that reach a size where risk outweighs likely benefits. Thus, when we look at the fossil record, we often see trends where medium-sized animals become bigger and bigger through time. Of course if you're small to begin with, increasing your size doesn't have a immediate advantage in most cases, and thus cannot be selected for.
The disadvantages of large size cannot be disregarded. You need more
food, of course, but also larger size generally means a slower reproductive rate.
That's fine as long as you're big enough to fend off predators. But with the
coming of man, too often the attrition rate has come to exceed reproduction.
Accordingly, the California Condor, once common through our desert, has slowly faded
away—retreating to its last stronghold in the Far West.
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.