The collared lizard's imperfect adaptations

By Jeff Mitton

A male collared lizard, Crotaphytus collaris, surveyed its territory from atop a small boulder.

I approached to get a better look, but it moved to hide in a clump of grass. I approached again, and it moved to another boulder, but here it took a stand, doing pushups to signal that it was ready to defend its territory.

I had two immediate impressions. First of all, this male was a dandy, brightly colored and ornately patterned. Second, its slender body, long legs and enormous feet clearly indicated that it was a runner.

Several selective forces influence the pattern and colors of collared lizards. Juveniles and females are cryptically colored, and their colors and textures vary from site to site, depending on the color of the local soil and rocks. This selection is administered by a long list of predators, including red-tailed hawks, Swainson's hawks, northern harriers, Cooper's hawks, ravens, crows, kestrels, bull snakes, pine snakes, black rat snakes, coachwhips and racers.

An experimental study demonstrated the adaptive value of crypsis for collared lizards. Clay models were set out in three different collared lizard localities, and the models were monitored to record predator attacks. Three types of models were deployed at each locality, mimicking juveniles, adult females and adult males. Predators struck at the brightly colored models of males but never struck the cryptic models of juveniles and adult females.

Juvenile males are cryptically colored, but they assume bright colors as they reach sexual maturity. Although bright male coloration increases the risk of predation, bright colors are absolutely necessary to attract mature females to a territory. In the arena of natural selection and evolution, a male that survives to old age but never mates is an abject failure; his genes will be lost. So males risk predation to attract mates.

Adult females, on the other hand, can afford to by cryptic, for there is no shortage of eager males. Juveniles must be cryptic, for they must evade predators so that they can survive to reproductive maturity and mate.

Collared lizards run in three different circumstances. First, they often stalk prey and then run to snag an insect or smaller lizard. Second, they run to the safety of a burrow when a predator threatens. Finally, adult males run to chase intruding males from the adult females in their territories.

Adults run faster than juveniles, and everyone runs faster from a predator than they do in pursuit of prey. But males chasing intruding males from their territories attain the fastest speeds. Collared lizards are even able to run upright, on their back legs, balancing with their long, agile tails.

Natural selection favors the adult males that run the fastest. Among adult males, both territory size and the number of females within a territory increase with the male's top speed. Consequently, the fastest males have the most offspring; in the parlance of evolutionary biology, fast males have the highest fitness.

Some writers, considering the natural world and how it came to be, describe cases of optimal design and perfect adaptation. But natural selection, driven solely by differences in reproductive success, can neither detect nor strive for perfection.

The collared lizard's adaptations are excellent examples of what natural selection can achieve. The cryptic colors and patterns of juveniles and adult females are not perfect, but adequate matches to the local substrate colors and textures so that predators do not detect many of them. The color and patterns of adult males are adequate compromises between the crypsis needed to avoid predators and the flashiness needed to attract females.

Because selection for fast males is unopposed by other forms of selection, males are extraordinarily fast; but the fittest males are not infinitely fast, just faster than most other males.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This piece originally appeared in the Boulder Camera.

June 2012