The Evolution of Variation in Back Patterns in Anolis

Many species of anoles exhibit distinctive dorsal patterns, including spots (e.g. A. sabanus), stripes (A. krugi) or chevrons (A. sagrei) (Figure 1). Dorsal patterns are highly variable in anoles, presenting not only variation across species, but also within species (sexual dimorphism) and within sexes (polymorphism). So why is there such a large variation in dorsal pattern?

Figure 1. Examples of dorsal pattern in Anolis lizards. A, A. sagrei, B, A. krugi, C, A. sabanus (photograph by B. Falk). D, A. pulchellus. Photographs A, B and D by D. L. Mahler.

Previous posts (1,2) explain the extent of the variation in dorsal pattern within females, a phenomenon known as female-pattern polymorphism (FPP), where females are more likely to present variation in dorsal patterns than males. Other studies have tried to explain within-population variation in dorsal pattern in several Anolis species with montane and xeric distributions. These studies suggest that habitat and crypsis could be an important factor explaining variation in dorsal pattern in Anolis.

Anoles are famous for having evolved convergent ecomorphs in different islands in the Caribbean. Each ecomorph is associated with a suit of adaptive traits that has evolved in response to their ecology. Some years ago, I went to the Losos Lab to explore, using several species of Anolis and hundreds of museum species, whether ecomorphs could explain variation in dorsal pattern. Namely, we wanted to know whether differences between ecomorphs could explain the degree of sexual dimorphism in dorsal pattern and female polymorphism, using 36 species of Anolis from the Greater Antilles.

In our paper, published on early view in the Biological Journal of the Linnean Society, we built a matrix with 11 different characters that described dorsal pattern. We used this matrix to construct a principal coordinate space, and in this space we calculated distances between male and female dorsal pattern for each species (amount of dorsal pattern sexual dimorphism) and the variation in dorsal pattern within each sex (amount of polymorphism within sex).

We found that species perching closer to the ground have higher degrees of sexual dimorphism, and males and females from these species usually present different patterns (Figure 2). For example, in A. bahorucoensis, a grass-bush species, females present a dorsal stripe, while males have chevrons. We also found that size dimorphism is correlated to dorsal pattern dimorphism, and species perching closer to the ground have larger differences in size and dorsal pattern between sexes, suggesting that both types of dimorphism are evolving together. We suspect that larger differences in habitat use between males and females in low-perching species may explain why some species are more dimorphic in dorsal pattern that others.

Figure 2. Association between sexual dimorphism in dorsal pattern and ecomorph in 36 species of Anolis. A, Phylogenetic tree with coloured branches representing values of dimorphism in dorsal pattern (Euclidean distance). Circles at tips represent ecomorph and the colour legend is the same as in (B). B, Values of dorsal pattern dimorphism according to ecomorph class.

On the other hand, ecomorph could not explain why some in some species there is higher variation in dorsal pattern in females (FPP). In our study, 44% of the species presented significantly higher female polymorphism than male polymorphism, reflecting how widespread is this phenomenon, but this was not related to ecomorph type. However, species with higher female polymorphism also had males that were more variable, suggesting that they might be under similar selective pressures. More precise information on habitat preferences within sexes, especially in females, will be required in order to fully understand the mystery of female-biased polymorphism.

Reference

Medina, I., Losos, J.B. & Mahler, D.L. 2016. Evolution of dorsal pattern variation in greater Antillean Anolis lizards. Early view, Biological Journal of the Linnean Society.