Sampling locations of the populations of study across the Caribbean. (1) Soroa (Cuba), population 1; (2) Soroa (Cuba) population 2; (3) Grand Cayman; (4) Santa Clara (Cuba); (5) South Bimini; (6) Chub Cay; (7) Andros; (8) Crooked Island; (9) Acklins; (10) San Salvador; (11) Staniel Cay; (12) Pidgeon Cay; (13) Grand Bahama; (14) South Abaco; (15) Cayman Brac; (16) Little Cayman; (17) Jamaica.
The dewlap is arguably one of most fascinating features of anoles. For me, it is the baffling diversity in dewlap size, coloration, and use —both among and within species— that makes it so interesting. However, understanding the origin and evolution of dewlap diversity in Anolis has proven a daunting task (Nicholson et al. 2007; Vanhooydonck et al. 2009). In an attempt to make (a little more) sense of the drivers and constraints of anole dewlap variation, a team of Belgian researchers from the University of Antwerp, led by evolutionary ecologist Tess Driessens, decided to look at dewlap diversity in Anolis sagrei. They surveyed 17 island populations of A. sagrei across the Caribbean and quantified dewlap design (color, size) and dewlap display behavior of both males and females.
Last year, Driessens and colleagues published their findings on how variation in abiotic factors (such as precipitation, temperature and other climatic variables) could explain much of the observed inter-island variation in dewlap design and use in A. sagrei (‘signal efficacy’ hypothesis). In a paper that came out last week, the team reports on the role of the biotic environment in driving dewlap diversity in the brown anole. Inspired by the wonderful study of Vanhooydonck et al. (2009), the researchers tested whether among-population dewlap variation could be (at least partially) assigned to variation in predation pressure (estimated by island size, tail break frequency, presence/absence of the predatory curly-tailed lizards, clay model attack rate), sexual selection (using sexual size dimorphism), and/or species recognition (number of syntopic Anolis species). Overall, they found only limited support for the idea that the extensive interpopulational variability in dewlap design and use in A. sagrei is mediated by variation in their biotic environment. Although they did find that males from larger islands show higher dewlap display intensities than males from smaller islands, and that males are more likely to have a ‘spotted’ dewlap pattern when co-occurring with a high number of syntopic Anolis species, the direct connection with predation pressure and species recognition remains ambiguous and demands further investigation.
In another recent paper, focusing only on the size of the male dewlap and their maximum bite capacity, the Belgian researchers asked a different question: does dewlap size signal fighting capacity (estimated by bite force) in A. sagrei, and is this true for all 17 sampled populations? And, does the level of signal honesty (that is, the steepness of the dewlap size-bite force relationship within a population) vary among populations, and is it linked with the strength of intrasexual selection? Their results showed that absolute dewlap size is an excellent predictor of bite force in all A. sagrei populations. However, relative dewlap size was only an honest signal of bite performance in 4 out of the 17 populations. Surprisingly, the level of signal honesty did not correlate with the strength of intrasexual selection.
Male brown anole biting on a purpose-built force plate. Photo by Tess Driessens
While the work of Tess Driessens and her team sheds new light on the drivers and constraints of dewlap diversity in A. sagrei, there is still plenty of study material left for future dewlap fanatics.
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