Anole Annals

Geographic variation in dewlap coloration in A. distichus on Hispaniola (from Ng et al.)

Animals regularly need to communicate with one another (both within and between species) and have developed a variety of signals, some quite elaborate, for doing so.  In some cases, we see extensive variation in these signals across the range of a species, raising the questions of how and why this occurs.  As Julienne Ng, Emily Landeen, Ryane Logsdon, and Rich Glor explain in a new Evolution paper, there are essentially three possible explanations.  Signals may diverge due to random drift, the pressures of sexual selection, or adaptation to local signaling conditions.  The latter possibility, in which signals evolve to match local habitat or environmental conditions, is a particularly interesting scenario.

In their study, Ng et al. examined geographic variation in the dewlaps of Anolis distichus, which vary from yellow to orange/red across Hispaniola.  They recorded reflectance spectra from the dewlaps of 36 different populations, extracted annual precipitation, surface temperature, and percent tree cover variables from GIS data layers, and tested for associations between dewlap and environmental variation.  Because dewlap variation could potentially be influenced by the relatedness of two populations in space or through shared ancestry, Ng et al. also corrected their data sets to remove the effects of spatial autocorrelation and phylogenetic relationships, important extra steps that will hopefully become commonplace in future studies.

It turns out that in drier habitats, A. distichus display smaller, brighter, yellow dewlaps, whereas in wetter habitats, they display larger, less bright, orange dewlaps.  Dewlaps also tended to be more orange in cooler environments with more tree cover.  Interestingly, this pattern is actually opposite that observed by Leal and Fleishman (2004) in A. cristatellus on Puerto Rico, which have brighter dewlaps in drier areas.  Thus, like any good study, this one raises a series of interesting new questions in the course of answering several others.  As Ng et al. point out, it will be interesting to see what future studies tell us about the mechanistic underpinnings of environmentally-associated dewlap divergence.

Finally, I think that the first line in Ng et al.’s paper is an especially good one: “Signals involved in sexual selection and species recognition – the peacock’s tail, the rhinoceros beetle’s horn, and the swordtail’s sword, to name just a few – are some of evolution’s most spectacular outcomes.”  Hopefully, with the impressive recent work done on its ecologically and evolutionarily important variation, researchers in other systems will take note that the anole’s dewlap clearly deserves to be added to this list too.

Ng, J., Landeen, E. L., Logsdon, R. M. and Glor, R. E. 2012. Correlation between Anolis lizard dewlap phenotype and environmental variation indicates adaptive divergence of a signal important to sexual selection and specie recognition. Evolution. doi: 10.1111/j.1558-5646.2012.01795.x

Leal, M., and Fleishman, L.J. 2002. Evidence for habitat partitioning based on adaptation to environmental light in a pair of sympatric lizard species. Proc. R. Soc. Lond. Ser. B 269:351–359.

Proportion of population genetic divergence accounted for by isolation-by-environment and isolation-by-distance in 17 Anolis species (from Wang et al.)

Identifying the factors contributing to population genetic divergence is important for understanding how many evolutionary processes play out in geographical space. Plus, it’s just plain interesting. In a new paper in Ecology Letters, Ian Wang, with Anole Annals stalwarts Rich Glor and Jonathan Losos, tested the roles of environment and distance in determining spatial patterns of population genetic divergence of 17 anole species on the Greater Antilles. To give the game away (spoiler alert!), the short answer is that both play a role, with some interesting variations among islands and species. However, it’s not just Wang et al.’s results that are interesting (more on those later), but also how they went about getting them.

Wang et al. tested two (not mutually exclusive) hypotheses for population genetic divergence. The first was isolation-by-distance (IBD), where distance and dispersal barriers prevent gene flow among populations. The second was isolation-by-environment (IBE), where there is either selection against dispersers, or a preference to remain in the environment where individuals are locally adapted. To test these hypotheses for each species, the authors first quantified environmental dissimilarity among populations using the Worldclim dataset, MODIS vegetation data, and elevation. Next they measured geographic distances among populations, but with a twist. To incorporate the idea that certain environments will be easier to disperse through than others, Wang et al. constructed environmental niche models. They then used the resulting (reverse) suitability values as a proxy for the ‘resistance’ of an area to movement and calculated the weighted distance between populations using two methods: least-cost pathway and all-possible-paths (circuit distance).

Armed with these measures of environmental dissimilarity and geographic distance, Wang et al. used structural equation modeling to determine the contribution of IBE and IBD to genetic divergence (they redid the analysis a few other ways, to ensure their results were robust. Short answer: they were). They found that both IBE and IBD had a role, but that distance was of greater importance, with collinearity being much less of an issue than I, at least, initially guessed. Their results were relatively consistent across species and islands, though a few species, mostly Hispaniolan, were exceptions (you’ll have to read the paper to find out which ones). Regardless of whether you’re more interested in the general pattern across species (and islands), or in the exceptions, Wang et al.’s study will undoubtedly generate more research questions and spur future work.

Lastly, one of the paper’s aspects I liked best was how the authors used environmental niche models. Species distribution/environmental niche/ecological niche/spawns-of-hell models get a lot of flak from a lot of sources. Much of this is even deserved – however, this is often more the fault of the modeller than the model. As Wang et al. have shown, such models can still provide useful and interesting insights into ecological and evolutionary process. In fact, anole biologists are leaders in new and informative ways to exploit such models. Wang et al.’s paper certainly continues this (emerging) tradition.

Wang, IJ, Glor, RE & Losos, JB. 2012. Quantifying the roles of ecology and geography in spatial genetic divergence. Ecology Letters. doi: 10.1111/ele.12025

Anolis cristatellus from Puerto Rico. Photo taken by Liam Revell

Anolis lizards are a model system for studies of evolutionary ecology because they are remarkably adaptable creatures. We know from long-term studies conducted by Jonathan Losos, Dave Spiller, Tom Schoener, and others that anoles can rapidly adapt their behavior and morphology over ecological timescales. For example, the presence of a ground-dwelling predator (Leiocephalus carinatus) forged a strong selective gradient in favor of A. sagrei with longer hindlimbs within a single generation. Interestingly, in a follow-up study the long-term effect of this predator is that A. sagrei evolves shorter hindlimbs, as they will tend to perch higher off the ground, where the perch diameter is narrower than near the ground. These studies of rapid morphological evolution puts anoles in the a very exclusive club with the likes of stickleback fishes, Peromyscus beach mice, guppies from Trinidad, Galapagos finches, and few others, as vertebrate systems in which evolutionary change on ecological timescales has been confidently demonstrated.

A notable exception to Anolis ‘evolvability,’ however, is thermal physiology. The thermal physiology of reptiles is generally evolutionarily conserved – taxa separated by millions of years and found in very different thermal environments will often share similar physiological patterns. But recent research has suggested that some physiological metrics may not be as static as previously thought, and that Anolis invasions provide an excellent opportunity to see how labile physiology actually is.

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Three stereotyped head-bobbing patterns of A. carolinensis. The black area represents the amplitude of the head as it moves up and down. Although amplitude can vary within and among individuals, the cadence remains constant for each of the display types. The hatched area refers to times when the dewlap is displayed. Length and number of dewlap displays and associated headbobs at the end of the display can be quite variable. This figure is a modification from Jenssen et al. (2000) that appeared in Lizards in an Evolutionary Tree.

Anole displays have proven to be as frustrating as they are fascinating. These displays, which are species-specific and typically involve headbobs and/or pushups that may or may not be accompanied by dewlap extensions, are as varied as one might expect within a group as diverse as Anolis. However, despite concerted efforts by an energetic group of researchers aimed at understanding the form, context and meaning of these varied adult male displays, a complete understanding of this complex signal eludes us. The stereotyped displays exhibited by many anole species are of particular interest, and of these, arguably the best-studied are the A, B, and C displays of that lab rat of the anole world, Anolis carolinensis.

In a recent paper, Jenssen and collaborators report a characteristically detailed and rigorous field study on the use of such stereotyped displays in both breeding and postbreeding free-ranging A. carolinensis males. Through painstaking analysis of videotapes of thousands of undirected male displays in nature with no obvious receiver, they show, among other results, that breeding males in “monitor” mode (i.e. lizards that signal while stationary) used mostly C displays, but increased use of A and B displays while moving or “travelling.” Monitoring lizards also exhibited an overall lower display rate than travelling males, and used more and longer volleys of displays. Additionally, about a third of all displays had an extra “shudderbob” tacked on. These patterns held through the postbreeding season. Interestingly, Jenssen et al. note that these undirected displays aimed at no-one in particular are most similar to aggressive signals used by males engaged in contests. The implication is that these undirected displays are in fact directed at an unidentified (or undetected) rival male audience, rather than being for the benefit of any single lady lizards in the area.

The notion of males displaying aggressively just in case any rivals might be present makes sense for a species such as A. carolinensis that defends areas harbouring females, rather than trying to attract them. One wonders if this result would hold for species that place less emphasis on territory defense and that have been rumoured in the past to exhibit signs of female preferences (Anolis valencienni, anyone?).

Annual conferences are a major way for scientists to get their research out to a broad audience and to find out what is new and emerging in different fields. For those of us who study Anolis lizards, there are two annual conferences that are a major draw for our community – the Society for Integrative and Comparative Biology (SICB) meeting in January and the SSE/SSB Evolution meeting in June. There are other conferences, as well, that meet less often, such as the World Congress of Herpetology and the Anolis Symposium, which are also important gatherings for our growing community.

Last year, we were pleased to report that Anolis research was prominently featured throughout the SICB conference in Charleston, South Carolina. In addition to more than a dozen talks and posters, there was also an open forum on the Anolis genome and evo-devo research, in light of the publication of the A. carolinensis genome. The online schedule for SICB 2013 has just been published and a preliminary search using the keyword Anolis returns a list of 18 talks and posters. There is a great diversity of topics explored this year, including phylogenetic frameworks for evolutionary convergence, aggressive behavior, locomotion, thermal ecology, and parasitism, among others.

One of the cool things we did last year was blog live from SICB in Charleston (1, 2, 3). Because the conference was hosted in South Carolina, Marc Tollis shared some pictures of actual anoles at the conference center. We plan to blog live from the conference in San Francisco and provide you with information about all the interesting research being done on anoles. Stay tuned for more!