Expedition To Swan Island: In Quest Of The Giant Brown Anole

From photographs, the island seems pretty typical of many in the Caribbean; scrubby and somewhat xeric, with some palm trees. One curiosity is that almost the entire length of the island is bisected by an airstrip, presumably put in by the US during the halcyon days of the 60’s and 70’s. Little Swan island has a much different aspect, with karst rock formations and covered with cacti.

So, who cares?

The endemic and extinct hutia of Little Swan. Photo from http://www.redorbit.com/media/uploads/2004/10/42_8cf650ddccd939f96fdf01fb8572efa0.jpg

The interesting thing about the Swans is that, despite being in the middle of nowhere, they’re chockful of reptiles and amphibians and other interesting creatures. Amazingly, Little Swan, all 0.9 square miles of it, used to harbor an endemic hutia, that super-sized guinea pig-esque breed of rodents that proliferated throughout the Caribbean! But, as you may have guessed from the occurrence of this post on this blog, there are more than just general zoological curiosities in the Swans. That’s right–there’s an endemic anole, A. sagrei nelsoni. And it’s more than just any endemic anole–it’s far-and-away the largest A. sagrei known, and has a wide range of interesting attributes.

Anolis sagrei nelsoni from Little Swan Island. Photo by Steve Busack.

From Lister. Evolution 30:677-692.

Just what am I talking about? Well, first, it’s far and away the largest natural population of sagrei known. A mean size of 75 mm svl sticks in my head, but I can’t find the reference for that at the moment (interestingly, Todd Campbell has documented introduced that some introduced populations of sagrei in Florida nearly reach this size). Moreover, in other attributes, it’s also very un-sagrei/un-trunk-ground anole-like. In a pair of classic papers in Evolution in 1976 [1,2], Brad Lister looked at a variety of anole species and populations, comparing those that occurred on islands by themselves to those that occurred in sympatry with other species. His hypothesis was that the loners would experience ecological release and would exhibit broader habitat and resource use and a more generalized morphology. These hypotheses were generally confirmed, and A.sagrei nelsoni was a shining case in point. For example, it perched higher than sagrei from multi-species locales and had much larger toe-pads (see figure to left). Kevin de Queiroz and I re-examined anole ecological release two decades later and came to the same conclusion: sagrei nelsoni’s morphology is intermediate between that of typical trunk-ground and trunk-crown anoles. Finally, check out the dewlap color in this photo above. Not your father’s sagrei!

So, the Swan Island anoles have been studied despite their obscure domain. What remains to be learned?

For one thing, how they got there. The map below, from the first of Lister’s pair of papers, illustrates just how far the Swans are from Cuba, the sagrei homeland. The map suggests they got to Swan by way of the Cayman Islands, but there’s no actual evidence for that. Brown anoles also occur on the Central American coast in Honduras and Mexico, so another possibility is that they took the roundabout route from the mainland. And, of course, they may have taken the express train straight from Cuba. This should be easy to sort out with molecular data, but as far as I’m aware, nelsoni has not been collected in nearly four decades, and thus no genetic samples are at hand. In addition, the environments on Little and Great Swan apparently are remarkably different despite their proximity—are the anoles correspondingly differentiated? And, lastly, just how ecologically and behaviorally generalized are these anoles? Sure, they perch higher than your typical sagrei, but are they just high-riding trunk-ground anoles, or do they show other behavioral tendencies more typical of other ecomorphs? Will they go up into the canopy? Spend a lot of time on leaves? Inquiring minds need to know. And, lastly, of course, why are they so big? It can’t just be because they’re the only anole in town—sagrei occurs on lots of other islands by itself without attaining such gynorminity.

From Lister. 1976. Evolution 30:659-676.

As you may have guessed, we’re headed to the Swans this week. We is me, Alexis Harrison, Randy McCranie, and Leonardo Valdes Orellana. I have wanted to visit the Swans for decades, but whenever I made inquiries, I couldn’t even find out whom to talk to about getting permission or making arrangements (remember, it’s in possession of the Honduran military). For this reason, I jumped at the possibility when Randy—arguably the world’s leading authority on Honduran herpetology—said he could put a trip together (it’s one of the last places he needs to visit before finishing his book on Honduran reptiles and amphibians).

Finally, some information on the herpetofauna of the Swans, courtesy of Randy McCranie:

Five nominal forms with type localities on the Swan Islands

Anolis sagrei nelsoni Barbour, 1914. Placed as a “subspecies” of A. sagrei by Ruibal, 1964.

Aristelliger nelsoni Barbour, 1914. Placed as a “subspecies” of A. praesignis by Hecht, 1951.

Sphaerodactylus exsul Barbour, 1914. Placed as a “subspecies” of S. notatus by Schwartz, 1966.

Cubophis brooksi Barbour, 1914. Described as Alsophis brooksi, placed as a “subspecies” of A. cantherigerus by Lando and Williams, 1969. McCranie, 2011 recommended full species level (as Cubophis brooksi) based on morphology.

Ameiva fuliginosa Cope, 1892. Extinct. One paratype in MCZ collection (MCZ R-20294).

Other reptile species reported from the Swan Islands:

Epictia magnamaculata. Tentatively placed in this species by McCranie, 2011. Molecular data needed to resolve this allocation.

Iguana iguana.

Leiocephalus varius. An apparent recent introduction from Grand Cayman Island.

Hemidactylus frenatus. Recent introduction.

Hemidactylus mabouia. Recent introduction.

Literature Cited

Barbour, T. 1914. A contribution to the zoögeography of the West Indies, with especial reference to amphibians and reptiles. Mem. Mus. Comp. Zoöl. 44(2):209–359.

Cope, E. D. 1892. A synopsis of the species of the teïd genus Cnemidophorus. Trans. Amer. Philos. Soc. 17(1):27–52.

Hecht, M.K. 1951. Fossil lizards of the West Indian genus Aristelliger (Gekkonidae). Amer. Mus. Novitates 1538:1–33.

Lando, R.V., and E.E. Williams. 1969. Notes on the herpetology of the U.S. Naval Base at Guantanamo Bay, Cuba. Stud. Fauna Curaçao Carib. Isl. 31(116):159–201.

McCranie, J.R. 2011. The Snakes of Honduras: Systematics, Distribution, and Conservation. Soc. Study Amphib. Reptiles, Contrib. Herpetol. 26:i–x, 1–714.

McCranie, J.R. In prep. The lizards, turtles, and crocodilians of Honduras.

Ruibal, R. 1964. An annotated checklist and key to the anoline lizards of Cuba. Bull.  Mus. Comp. Zool. 130 (8): 473–520.

Schwartz, A. 1966 (dated 1965). Geographic variation in Sphaerodactylus notatus Baird. Rev. Biol. Trop. 13(2):161–185.