Distinguishing discrete polymorphism from continuous variation in throat colour of tree lizards, Urosaurus ornatus

Within population colour variation in animals is ubiquitous and can range from discrete polymorphism to continuous variation. Distinguishing discrete polymorphism from continuous variation can be challenging, and this hinders our ability to test hypotheses about colour variation. We tested whether throat colour variation in the ornate tree lizard [Urosaurus ornatus (U. ornatus); Baird & Girard, 1852) is discrete or continuous using photographs of 617 individuals from seven study sites in the Chiricahua Mountains of Arizona, USA. Using principal component analyses on ten colour variables derived from photographs, we found throat colour to be highly repeatable for both males and females. Cluster analyses suggested that there are different groups of individuals based on their throat colours in males and in females, but distinguishing between the groups was difficult due to significant overlap in colour. Therefore, it appears that there is a significant amount of continuous variation in both male and female tree lizard throat colours. We suggest quantifying trait variation before assuming a colour trait is discrete. By using numerical descriptors of colour, more information is retained than by using discrete groups. Quantifying individual variation in colour is important for linking colour with other traits such as reproductive strategy, immune function, and size and for testing hypotheses about the evolution and maintenance of colour polymorphism.