An online preview version of this paper was published Nov. 4 in the Biological Journal of the Linnean Society.
I began this project in late 2012 as a research assistant to Shane Campbell-Staton, now a Postdoctoral Fellow at the University of Illinois Urbana-Champaign. As part of his dissertation on Anolis carolinensis, Shane saw an opportunity for an interesting side project regarding its morphological variation. The lizard’s geographic range is massive – ranging from Florida to Texas in the east, and north to Tennessee – but surprisingly few studies had examined the way limb and body traits vary between populations, let alone over its broad distribution. Given evidence for Caribbean relatives adapting to variable environmental conditions even over short distances, we were curious whether the same would hold true for the green anole.
Using a set of samples Shane had collected from 14 locations around the southeast (Figure 1), I set out to answer a few questions about geographic variation in the green anole: which traits vary most in this species? How is this variation distributed, and does it correlate with environment? We were also interested in the degree to which this species conformed (or didn’t) to Bergmann’s and Allen’s rule, two eco-geographic principles well studied in reptiles.
Figure 1: Density and distribution of sampling in this study.
The process started, as always, with data collection – in this case, taking X-rays of over a hundred specimens, extracting a set of 26 morphological traits, and pairing them with environmental and genetic data for each site in our study. The resulting dataset was large and multidimensional, and required several iterations of analysis to find a clear and logical approach to test our hypothesis (as an undergraduate, this process of analysis and re-analysis taught me a valuable lesson in troubleshooting, data management, and experimental design).
Looking at our results, we did end up finding a high degree of morphological variation in this species, mostly driven by head width and length. These features marked out several highly distinct populations and generated some striking visual comparisons (Figure 2). Previous studies by Herrel, Lailvaux, Corbin, and McBrayer suggest that this kind of variation may be driven by the role of bite force and head shape in prey capture and combat, and future work on A. carolinensis should follow up on this possibility. We also recovered some morphological clustering among non-proximal populations, which opened the door for examination of possible convergence as a result of environmental similarity over the species’ range.
Figure 2: Head shape variation between an anole from Cedar Creek, OK (left) and one from Punta Gorda, FL (right).
We found that, in general, anoles in more seasonal and colder climates of the north tend to have to have relatively longer limbs and wider and shorter heads than those from less seasonal/warmer locations in the south. With regard to limbs, this pattern may be related to an observed “reversed” Allen’s rule – that appendage length would actually increase in colder climates as a way to more rapidly uptake heat. This explanation is similar to that of the “reversed” Bergmann’s rule previously proposed for some lizards, but for which our data were inconclusive.
In the end, I believe the patterns of variation and environmental correlation that we found in the study will help to establish A. carolinensis as a strong candidate for further studies of morphological variation over a large range, especially with the recent publication of the species’ genome. As an undergraduate, I felt lucky to make a contribution to the literature and to have the opportunity to see through a project from start to finish.
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Finally, please reach out to me with any questions or comments about the study! My code and data are archived on my github page.
Chris
What a cool study! I’ve wondered this myself for a while; I’m glad someone has tested it. It now makes me wonder if we’d find similar patterns in A. sagrei. Using sagrei, you’d also be able to infer the rate of evolution and the strength of the selective pressures because of how recent their invasion is. Maybe a good follow up for you. Good luck!