Many animals use brightly colored patches of skin to communicate with other individuals of the same or opposite sex during breeding. This phenomenon has been especially well studied in lizards. In many lizard species, different individuals within the same population have different colors. This type of variation is called a polymorphism, and the different colors are called color morphs. In several species of color polymorphic lizards, different color morphs also show differences in behavior. Recently, researchers have started to find that, in color polymorphic species, there is also often variation among different populations in how many morphs are present, which morphs are present, and how common particular morphs are. These types of differences could make it less likely that individuals from different populations interbreed, which can contribute to the formation of new species.

        We studied the mesquite lizard (Sceloporus grammicus), which lives primarily in mountain habitats in Mexico. Other researchers thought that the mesquite lizard might represent a case of new species formation in progress, because there are unusual genetic differences among populations. Studies of how different populations were related to one another genetically had yielded some confusing results, however. There was anecdotal evidence that both male and female mesquite lizards had throat color polymorphisms, but no one had yet done a formal study of them. We thought studying differences among populations in throat color might help explain some of the confusing results regarding which populations interbreed with which other populations. Studying differences among individuals within populations could also help us understand how color conveys information about behavior in animals.

       We sampled 22 populations and found that there are two types of throat color polymorphism in the mesquite lizard. In some populations, males can have orange, yellow, or blue on their throats; whereas females can have orange, yellow, or white. These colors also occur in several related lizard species. In other populations of the mesquite lizard, however, both males and females can have orange, yellow, or white on their throats. We never found blue and white males together in any population. We used DNA to make a phylogeny, or "family tree," of the populations we sampled, which showed that the change from blue  males to white males has occurred at least twice during the evolution of the mesquite lizard.

      We focused on two particular populations, one with blue males and one with white males, to study whether males of different throat colors behaved differently, as had been found for many other species with color polymorphisms. We found that males of different color morphs differed in aggression in both populations. Even though orange and yellow color morphs occurred in both populations, the colors didn't mean the same thing about a male's aggression in the two different populations. We also found that blue and white males were behavioral opposites: blue males were more aggressive than other males in their population, whereas white males were less aggressive than other males in their population.

      We used the same two populations as we used for the male aggression study to ask how the presence of blue males in one population and white males in another population affects a female's ability to determine whether a male is from the same population as she is or not. We found that females more strongly rejected the courtship attempts of "foreign" males of unfamiliar morphs (blue or white) than those of "foreign" males of familiar morphs (orange or yellow).

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