The function and evolution of anole dewlaps have been the focus of studies for decades. As flashy, showy displays of color, it’s no wonder that dewlaps even captured the attention of Darwin. However, most studies to date have focused solely on male dewlaps leaving female dewlaps much neglected (but see Johnson & Wade, 2010 and Vanhooydonck et al., 2009). While the possession of dewlaps is less common in females than males, and female dewlaps are often rather diminutive compared to the male’s, the mere presence of female dewlaps in a good number of species, combined with striking variation in color, pattern and size across the genus, begs the question of what is driving the evolution and maintenance of such female ornamentation?
In a new paper, Harrison and Poe (2012) tackle this largely neglected topic. Focusing on dewlap size variation in females, they use a comparative analysis on 228 anole species to test four hypotheses:
- Female dewlaps have evolved merely as a genetically correlated by-product of selection on male dewlaps. If this is true, Harrison and Poe predict that female and male dewlap size should be positively correlated.
Alternatively, the size of female dewlaps may be the result of selection acting directly and independently on females through:
- Social selection (male mate choice or female-female territorial defence). Using sexual size dimorphism for a proxy for male-male competition, the authors predict that female dewlap size should be negatively correlated with sexual size dimorphism.
- Selection for species recognition. As such, dewlap size should be reduced in single-species communities.
- Sensory drive, whereby selection favors those signals that are effectively transmitted through the local habitat. As a result, there should be a correlation between dewlap size and microhabitat use.
Harrison and Poe find support for two of their hypotheses. First, their results find support for the social selection hypothesis: in species with little sexual size dimorphism, females have large dewlaps, while in species where dewlaps are absent or small, there is a wide range of sexual size dimorphism.
Secondly, their results from a subset of their dataset, comprising only Greater Antillean anoles, support the sensory drive hypotheses. Female dewlap size differed significantly among habitat specialists, with crown-giant and twig anoles having larger dewlaps than the other ecomorphs. The authors suggest that perhaps these ecomorphs evolved larger dewlaps for their signal to stand out against the dense habitat of their respective microhabitats or, as crown-giant and twig anoles commonly occur in low densities, large dewlaps would be effective signals for longer range communication.
This paper is a valuable addition to what little is known about the evolution of the female dewlap, and provides a good launching pad to hopefully much more research to come. Another exciting avenue of research regarding female dewlaps has already come up on AA: the curious sexual dimorphism in dewlap color and pattern in some species is particularly intriguing. Who’s game?