The Incredible Shrinking Dewlap!

Photo by Bonnie Kircher

Here in north-central Florida, summer is giving way to fabulous fall weather. While this change means an infinitely more comfortable bike commute, it also means that the anoles which were abundant throughout the summer are starting to disappear. Although pedestrians can still find lizards basking in the afternoon sun, Floridians are much less likely to see anoles at every turn. The lizards that are still out and about are also far less likely to be strutting their stuff, keeping their dewlaps tucked away, as they are not needed for mating or competition until the next breeding season. When the dewlap is little used for such an extended period of time during the non-breeding season, could the morphology of this structure be altered?

Indeed, studies have demonstrated that there are marked changes in dewlap size between breeding and non-breeding seasons. Specifically, this already amazing structure seems to change in size, being larger in the summer when it gets the most use, and smaller in the non-breeding season! Simon Lailvaux and colleagues first hypothesized that changes in dewlap size might be correlated with variation in resource availability throughout the year. However, the group found that changes in dewlap size do not correlate with resource availability at all! Recently, following the results of the dietary restriction study, Simon Lailvaux et al. (including yours truly) again asked the question, “Why?” More specifically, are there instead physiological changes that cause dewlap size to expand in the summer and shrink in the non-breeding season?

Lailvaux et al. first asked whether dewlap size was changing because of inherent changes in lizard physiology between seasons or, instead, if changes were due to the extensive use of the dewlap during the breeding season. The authors captured male A. carolinensis lizards before the onset of breeding season and constrained the dewlap in half of the lizards so that the lizards could not extend their throat fan. They found that lizards with unconstrained dewlaps had larger dewlaps in the summer that shrunk again in the fall. The constrained males, on the other hand, had smaller dewlaps in each consecutive season. These data suggest that changes in dewlap size stem from the behavioral use of the dewlap – when a dewlap is extended more often, it gets bigger!

Apparatus for measuring skin elasticity. Photo from Ecology and Evolution. Volume 5, Issue 19, pages 4400-4409, 19 SEP 2015 DOI: 10.1002/ece3.1690

Next, the authors tested the hypothesis that dewlaps change in size due to seasonal changes in skin elasticity that correlate with the increased seasonal behavioral use. One of the authors, materials engineer Jack Leifer, developed a novel technique for measuring skin elasticity that involved pulling a piece of lizard skin on a machine that measures force until the skin sample sheared (see picture).The authors compared the force it took to break pre-breeding, breeding, and post-breeding dewlap skin, using measurements taken from belly skin as a control. They found that dewlap skin is more elastic than belly skin and that both belly skin and dewlap skin are more elastic in the summer. These results support the idea that dewlap skin is inherently stretchier than other skin!

Thus, it seems that changes in dewlap usage, coupled with changes in skin elasticity across the year, make the dewlap a dynamic signal. This work does not demonstrate any mechanism for these changes and leaves the door open for many exciting follow-up studies. Why is dewlap skin more elastic than belly skin overall? How are changes in skin elasticity regulated between breeding and non-breeding season? What are the ecological implications of a dewlap that changes in size over the course of the breeding season?