Morphotypic Variation in Throat Coloration: A Continuous or a Discrete Trait? A Survival Analysis on a Polymorphic Lizard
Demographic models are useful for analyzing the effect of selective pressures on populations. Polymorphic populations display dramatic variation in phenotype, and different morphotypes representing alternative strategies are characterized by specific sets of behavioral, physiological, and morphological traits. Coloration is a classic polymorphic trait, and variation in this trait has been linked to other traits, such as aggressiveness, size, and immune responses. Many studies of polymorphic populations have placed individuals into discrete categories, assuming that all individuals of each morphotype have the same performance; however, traits related to color can vary between individuals possessing the same coloration or classified as the same morphotype. Here, we determined the association of survival to a continuous or a discrete classification based on the percentage of colored area in the gular spots in four populations of the Mesquite Lizard Sceloporus grammicus and tested for patterns among populations. In two of our study sites (both located in the volcano “La Malinche”), there was no association of coloration on survival with either a discrete or continuous classification. At the other two study sites, there was a continuous association of color on survival, which suggests that previous studies of variation in polymorphic species might have often been conducted at an inappropriate “resolution” and that predictions in our analyses could be improved.
Map of the distribution of Sceloporus grammicus, modified from CONABIO (Mejía et al., 2014) and from Hall and Selander (1973), and the study sites. The gray polygon represents the distribution of the species in central Mexico. The red circle indicates sites 1 and 2 at “La Malinche” Volcano. The blue circle indicates site 3 at “Cumbres del Ajusco” National Park, and the green circle indicates site 4 at “Nopala” Hidalgo.
Photography and measurement methods. All lizards were placed in a plastic card holder so that their throat could be viewed as clearly as possible. (A) The known size reference used to determine the size of structures in the image; (B) yellow line shows how we measured each color area; and (C) the total gular area that was measured using ImageJ software.
Morphotypes found in our study sites. Panels A to D show pure morphotypes: (A) yellow, (B) orange, (C) gray, and (D) blue. Panels E to H show mixed morphotypes: (E) yellow-gray, (F) orange-gray, (G) yellow-blue, and (H) orange-blue. Each photo has a yellow line, which corresponds to the scale (1 cm).
Survival functions. Models for site 3 data: (A) Model where survival covaries with yellow in cubic form (YE3); (B) model where survival covaries with yellow in quadratic form (YE2). (C) Model for site 4 data, where survival covaries with the color blue in cubic form (BL3).
Contributor Notes
Associate Editor: W. L. Smith.