The colors, mating strategies of side-blotched lizards

By Jeff Mitton

I saw something unexpected while hiking in Little Wild Horse Slot Canyon — a shaft of light illuminated the severed right forewing of a monarch butterfly.

First of all, I was unaware that monarchs migrated over the San Rafael Swell. But once that sunk in, I wondered which predator caught and dismembered the monarch.

Ravens had been soaring overhead all morning and I heard canyon wrens singing their iconic songs — either could have taken the monarch. But when I examined the canyon wall directly above the wing, I saw another, perhaps more likely predator. A side-blotched lizard was watching me closely.

Side-blotched lizards, Uta stansburiana, are 2.5 inches from the tip of the snout to the base of the tail, but total lengths are more than 5 inches when their tails are added. They eat mostly insects and are common on the Colorado Plateau, in the Great Basin in the southwestern deserts, California and northern Mexico.

This individual was a male, for it had diffuse blue spots on its back. In contrast, females do not develop spots but retain a series of black and white lateral lines and chevrons developed in their youth. I took the time to notice the lizard's coloration because it plays a major role in their behavior and natural selection.

Males come in three distinct, genetically determined color morphs. The throat and sides can be colored orange, blue or yellow. The colors are not inconsequential adornments — they advertise both the position of the animal in the dominance hierarchy and the male's reproductive strategy.

Barry Sinervo and Curt Lively documented the nexus of interactions among the three color morphs and likened their mating strategies to a game of paper, rock and scissors. In case you have forgotten, two or more players thrust out a hand held flat to represent a sheet of paper, two fingers representing scissors or a fist to represent a rock. Rock breaks scissors, scissors cut paper, paper covers rock. Each is beaten by one and beats one.

Orange males have the highest levels of testosterone, they defend the largest territories, have the largest harems and dominate both blue and yellow males. Orange males mate those in their harems but also invade the adjacent territories defended by blue males to mate with their neighbor's females.

Blue males defend smaller territories with fewer resident females, and although they cannot vanquish orange invaders, they are able to drive yellow males away.

Yellow males are at the bottom of the dominance hierarchy and do not defend territories and therefore do not have resident females to mate. However, they are sneakers, and when orange males are off mating with the harems belonging to blue males, yellow males sneak into the momentarily undefended orange territories to mate the resident females.

The interactions among the three male color morphs cause the morph frequencies to vary over time. When orange males become common, they have few blue neighbors to take advantage of, so orange mating success begins to drop. At this time, the sneaking yellow males have many mating opportunities and high reproductive success, so the frequency of the yellow morph increases.

When yellow males become common, they put relentless pressure on the orange but are not able to mate females on blue territories, for the blue males are vigilant and dominant to yellow. At this time, blue males have a distinct advantage, and their frequency increases.

When blue males are most common, dominant and aggressive orange males raid their harems, so the frequency of blue males declines while the frequency of orange males increases.

This game of paper, rock and scissors in side-blotched lizards causes the male morph frequencies to cycle incessantly through time.

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

February 2014