Anole Annals

Much has been written about how differences in relative limb length allow anoles to successfully occupy distinct portions of the arboreal habitat. Most research has focused on large-scale patterns of diversity, which are presumably the result of natural selection. But limb length could also be more finely tuned using an alternative process, whereby an individual’s morphology responds to environmental cues experienced throughout development. This process is known as phenotypic plasticity. We already know that anole limbs exhibit plasticity in the lab (e.g., Losos et al. 2000, 2001 – A. sagrei; Kolbe and Losos 2005 – A. carolinensis), with lizards raised on narrow perches having shorter limbs than those raised on broad perches.  But what about lizards in their natural environments; could plasticity in limb length allow lizards to become more specialized to their particular microhabitat in the wild?  This was the question that Trinity University (San Antonio) undergraduate Alisa Dill asked in summer 2010, working with fellow undergraduate Andrew Battles, regular AA contributor Thom Sanger, and me.

We studied green anole lizards in three plots in Palmetto State Park, in southeastern Texas (Figure 1).

Figure 1 from Dill et al. In press. Study plots in Palmetto State Park. The star on the small
map of Texas (bottom left) indicates the location of the park.

The three plots differed dramatically in perch availability – one was in a dense dwarf palmetto stand in a closed forest, another was trees and bushes surrounding an open field, and the third was in a light forest with many small vines and twigs. We found that female lizards in the third plot (with the narrowest perches) had significantly shorter hindlimbs than females in the other two plots, although males did not differ in limb length among the plots.  Juveniles also did not differ in limb length across the three plots, consistent with the idea that the female’s developmental experience on varying perches may influence their adult limb length.

Why this difference between the sexes?  (This difference is also consistent with results from the laboratory studies mentioned above.)  We also observed where males and females were found in their habitat and how they locomoted through it to determine if differences in behavior between the sexes could affect the way the developing lizards interact with their environment. We found that females performed fewer locomotor behaviors, spending more time on particular perches; thus, perhaps the perches had a stronger influence on female limb length than on males, who used many more perches as they move through their environment.

There is much more work to be done to further test the hypothesis of plasticity in a natural environment. After all, the previous lab experiments were performed by growing lizards on either a wooden 2×4 or a small dowel, a much simpler environment than a forest with many perch options. It would be ideal to perform a “common-garden” experiment where juvenile lizards are transplanted between environments with different perch diameters. We also don’t yet understand the developmental mechanisms that cause limb plasticity in anoles, but more information about those mechanisms will help determine why the sexes respond differently to differences in their microhabitat.

You can read more about our study in our recent publication in the Journal of Zoology, available online now in Early View.

References:

Dill, A.K., T.J. Sanger, A.C. Battles and M.A. Johnson. In press. Sexually dimorphisms in habitat-specific morphology and behavior in the green anole lizard. Journal of Zoology.

Kolbe, J.J. & Losos, J.B. (2005). Hind-limb length plasticity in Anolis carolinensis. J. Herpetol. 39, 674–678.

Losos, J.B., Creer, D.A., Glossip, D., Goellner, R., Hampton, A., Roberts, G., Haskell, N., Taylor, T. & Ettling, J. (2000). Evolutionary implications of phenotypic plasticity in the hindlimb of the lizard Anolis sagrei. Evolution 54,301–305.

Losos, J.B., Schoener, T.W., Warheit, K.I. & Creer, D. (2001). Experimental studies of adaptive differentiation in Bahamian Anolis lizards. Genetica 112-113, 399–415.

Anolis heterodermus. Photo by J. Losos.

We all know that habitat fragmentation and destruction have devastating consequences on biodiversity. Yet, one of the reasons that Caribbean anoles have been so intensively studied is that some species do extremely well in human-disturbed habitats and, because they have become so ubiquitous, they are extremely good subjects for ecological and behavioral studies.

In fact, it gives pause to realize that Caribbean islands were mostly cloaked in forest before the arrival of man, and thus many of the common anole species which are abundant in open, disturbed habitats–brown anoles, for example–were probably much less abundant in pre-historic times. In other words, it seems likely that some species are actually doing better today than in the past, but there are very few relevant data.

Anolis (Phenacosaurus) heterodermus probably occurs at higher altitudes than any other anole and has a very large altitudinal range. It’s natural history is almost unknown, and until recently, nothing had been published on its ecology and behavior since Miyata’s J. Herp. paper 30 years ago. However, that has now changed. Rafael A. Moreno-Arias has just completed his master’s degree at Universidad Nacional de Colombia on populations of this species near Bogotá, and the first paper from this work was recently published in Biotropica.

The habitat near one of the study sites in Tabio, Colombia. Photo by J. Losos.

In that paper, Moreno looked at six habitat patches, differing in size and degree of fragmentation. By conducting a mark-recapture study,  he found that populations seemed to be increasing in all populations. Moreover, survival and growth rates were calculated to be highest in the most disturbed habitats, perhaps reflecting this species’ adaptation to edge habitats. Although too much habitat destruction is obviously detrimental–without any bushes, the species will not be able to survive–it seems that the species, perhaps like its Caribbean cousins, does just fine in fragmented landscapes. However, Moreno and Urbina-Cardona take a different, more nuanced, view on their findings, as their abstract below indicates.

Abstract:

Habitat fragmentation and loss affect population stability and demographic processes, increasing the extinction risk of species. We studied Anolis heterodermus populations inhabiting large and small Andean scrubland patches in three fragmented landscapes in the Sabana de Bogotá (Colombia) to determine the effect of habitat fragmentation and loss on population dynamics. We used the capture-mark-recapture method and multistate models to estimate vital rates for each population. We estimated growth population rate and the most important processes that affect k by elasticity analysis of vital rates. We tested the effects of habitat fragmentation and loss on vital rates of lizard populations. All six isolated populations showed a positive or an equilibrium growth rate (k = 1), and the most important demographic process affecting k was the growth to first reproduction. Populations from landscapes with less scrubland natural cover showed higher stasis of young adults. Populations in highly fragmented landscapes showed highest juvenile survival and growth population rates. Independent of the landscape’s habitat configuration and connectivity, populations from larger scrubland patches showed low adult survivorship, but high transition rates. Populations varied from a slow strategy with low growth and delayed maturation in smaller patches to a fast strategy with high growth and early maturation in large patches. This variation was congruent with the fast-slow continuum hypothesis and has serious implications for Andean lizard conservation and management strategies. We suggest that more stable lizard populations will be maintained if different management strategies are adopted according to patch area and habitat structure.

Last year, Nicholson et al. proposed splitting Anolis into eight genera in a paper in Zootaxa.  This idea was extensively debated in AA’s pages (e.g., 1,2,3 and links therein). Now, two papers have been published criticizing the methods and conclusions of Nicholson et al. and suggesting that the generic name Anolis be retained for the entire clade.

In a paper just published two days ago in Zootaxa, Steve Poe argues strongly against Nicholson et al.’s proposal on multiple grounds, primarily on the lack of demonstrated monophyly of most of the proposed genera. Poe concludes at the end of the introduction of the paper: “Nicholson et al. (2012) selectively adopted results of their own flawed, unstable, and conflicting analyses, selectively incorporated pertinent published data and results, and changed names for over 100 species that have never been included in a phylogenetic analysis. The proposed taxonomy is unnecessary and unwarranted according to standard taxonomic practice. It should not be adopted by the scientific or nonacademic communities.” The paper is only five pages long and is readily downloaded.

Meanwhile, within the past month, Castañeda and de Queiroz published a paper in the Bulletin of the Museum of Comparative Zoology on phylogenetic relationships within the Dactyloa clade of anoles (pdf, supplementary material). The paper is a follow-up to their 2011 paper on Dactyloa, adding morphological data to the molecular dataset analyzed previously. We’ll have more on this paper soon, but the pertinent part for today is the “Note added in Proof” appended to the beginning of the paper. The authors explain “Shortly after our paper was accepted, Nicholson and colleagues published a phylogenetic analysis of anoles and a proposal to divide Anolis into eight genera… Here, we comment briefly on their study as it pertains to the phylogeny and taxonomy of the Dactyloa clade,” and then go on to criticize Nicholson et al.’s recognition of genera (in this case, Dactyloa) and species groups that are not monophyletic in their own analyses. Moreover, like Poe, Castañeda and de Queiroz present strong critiques of the Nicholson et al. methodology and analyses, concluding “Because our results are based on larger samples of Dactyloa species (for both molecular and morphological data), as well as larger samples of molecular data (with respect to both numbers of bases and numbers of gene fragments, and including both mitochondrial and nuclear genes), and because many of their taxonomic conclusions that differ from ours are either contradicted by their own results or unsubstantiated, we do not consider any of the differences between our phylogenetic results and taxonomic conclusions compared with those in the study by Nicholson et al. (2012) to warrant changes to our proposed taxonomy. In contrast to Nicholson et al. (2012), we refrain from assigning some species to series and treat some taxonomic assignments as tentative because of contradictory results or poorly supported inferences, and we present justifications for all taxonomic decisions pertaining to species not included in our analyses.”

The Castañeda and de Queiroz critique is only two pages long. Read ’em both and decide for yourself.