Anolis carolinensis from south Florida. Photo courtsey of Neil Losin

Anolis carolinensis, the slender green anole from the southern United States is no stranger to most readers of this blog. This species is a model system for reproductive biology (here), is an emerging model for studies of development and Evo-devo (here and here), and is the first squamate lizard with a sequenced and annotated genome (here and here). Anolis carolinensis is, however, only one member of a diverse clade of lizards, though the natural history of many of these species is poorly understood relative to their popular cousin. Furthermore, the vast majority of carolinensis anoles* are known from few museum specimens meaning that robust descriptions of their morphology remain difficult to obtain.

I have recently become interested in the carolinensis series (for reasons discussed below) and have set out to better understand their biology. This post is meant to introduce readers to the diversity of carolinensis anoles and to put out a call for additional information that may be out there. While I have searched the wires for literature on these species, the community may know of hidden gems that have otherwise eluded me.

1) Species diversity and biogeography

The 13 species of the carolinensis subgroup and their localities are listed in the table below. Though many of these species were described in the late 19th and early 20th centuries the list continues to expand as Orlando Garrido and colleagues explore Cuban biodiversity, uncovering local variants to the more widespread species. Garrido has described four new species of Cuban green(ish) anole in the last 25 years and recent molecular analysis suggests that localized variation may be more common than previously appreciated. For example, Rich Glor and co-authors revealed several independently evolving lineages of A. porcatus that are correlated with Cuba’s geological history of partial island submergence (paper here).

The anoles of the carolinensis series. Specimen counts are based on a survey of the big five anole collections. Click to enlarge.

While several species are geographically widespread (A. porcatus, A. allisoni, and A. carolinensis***), the majority of this diversity evolved on relatively small Caribbean islands or in distinct regions of Cuba. For the species inhabiting nearby islands, it appears that Cuba served as a center of diversity with repeated overwater dispersal leading to multiple speciation events (for detailed discussion of A. carolinensis origins see here). At least four dispersal events are necessary to explain the carolinensis subgroup’s diversity, but the origin of A. maynardi, A. fairchildi, A. longiceps, and A. brunneus has not been fully resolved. Molecular dating suggests that the last common ancestor of the carolinensis subgroup was approximately six million years ago.

I should also briefly note that A. longiceps has recently been listed as vulnerable on the IUCN Red List because of its limited geographic distribution. Navassa Island is approximately five square kilometers and is undergoing notable habitat destruction due to an uncontrolled goat population, Haitian squatters, and a history of mining.

2) Morphology

In many respects, the carolinensis anoles are quintessential trunk-crown anoles. They tend to be slender, greenish anoles that are typically found several meters above the ground on medium sized perches. There are, however, several interesting aspects of their biology that deserve further attention. Peculiarly, A. brunneus cannot achieve the rich emerald green color of its relatives for example. What has piqued my interest is the skull morphology of the carolinensis series. As I previously discussed, the carolinensis anoles have highly elongate skulls relative to other species. In fact, some species’ skull shapes are greater than two standard deviations away from the average skull shape of the entire genus – they aren’t just elongate, they have obnoxiously long faces with forcep-like jaws!

Male and female skulls of A. maynardi from Cayman Brac.

We still don’t know what ecological and evolutionary forces have generated this striking diversity in head shape, but several interesting patterns are emerging from the data. I have been struck by the correlation between skull elongation and sexual dimorphism – only males reach the extreme levels of facial elongation while females retain more moderate skull proportions. Furthermore, the species found on small islands tend to reach greater levels of facial elongation and sexual dimorphism than any of the species on Cuba. The exception to this pattern is A. carolinensis which evolved a shorter face and lower levels of dimorphism than A. porcatus following colonization to North America. The history of anole biology is marked by few studies focused on explaining craniofacial diversity and, therefore, I cannot yet explain this pattern using available data. I hope that additional ecological and behavioral studies can bring light to the origins of skull shape diversity in this group.

3) The way forward

Male and female A. maynardi. Cayman Brac 2009

Within the anole world I have often informally thought that the number of preserved museum specimens was proportional to our understanding of a species’ natural history. If that statement is true, then it would also be true that our understanding of the carolinensis anoles is strongly biased towards one species, A. carolinensis. But because of its reversal in head shape, this species may be one of the least informative regarding my quest to understand variation in skull shape and sexual dimorphism.  After serious effort, I find few studies that have surveyed fundamental ecological, behavioral, or morphological questions for most of the carolinensis species. A recent study of A. maynardi and observations by Pat Shipman are starting to shed light on the biology of at least one species, but more multidisciplinary and integrative work is still needed.

I would greatly appreciate ideas, hypotheses, or general thoughts regarding either the carolinensis series or skull elongation among anoles. At the end of this week, I embark on a trip to study A. brunneus on Crooked Island, Bahamas.  Stay tuned for updates from the field with new ideas and observations.

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* Here I use the term “carolinensis anoles” and “carolinensis series” synonymously. While not a formal taxonomic rank, supraspecfic rankes have a long history among Anolis biologists, which is discussed in more detail in Lizards in an Evolutionary Tree. The carolinensis series is made up of two clades, also termed “subgroups,” the carolinensis and isolepis subgroups. Due to the lack of specimens, most of my work and interest has been shaped by the carolinensis subgroup though I hope that this will eventually change.

** Despite the promising number of A. brunneus specimens, only nine specimens can be located in the MCZ collections since the early 1980’s.

*** The distribution of A. isolepis in Schwartz and Henderson (1991) is listed as “possibly in all mountainous regions of Cuba…,” but several question marks accompany points on the locality map. An updated distribution map is clearly needed for this species before making a definitive statement about its abundance.

 

Thomas Sanger
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