Worm Salad

Remarkable Reptile #10
December 2, 2008, 5:01 am
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I have always been a fan of island  biogeography and one of my big passions in biology are the weird things that animals do on islands. From giant flightless birds to enormous tortoises to pygmy elephants, the variety of life on islands never ceases to amaze me. And while the science has its practical uses, beyond initial wonderment and amazement, from predicting alpine responses to climate change to managing wildlife preserves, my fascination with islands still stems primarily from their wondrous diversity of life.

(Incidentally, if this description of a fascinating science has piqued your interest at all, may I recommend David Quammen’s The Song of the Dodo to you?  It’s a really remarkable, well-written look at the scope of the science, and its practical implications, and while I may hold reservations about some of his assertions, it’s still a very good book.)

And so, we start my look at remarkable reptiles with one species that really exemplifies the magnificent and diverse beauty of islands — the common boa, Boa constrictor.

The subspecies that you’re probably familiar with is the one sold in pet stores across the world; the Colombian red-tailed boa, a locality specimen of B. c. imperator. With their charming disposition and fairly impressive size of up to about nine feet, they are an instantly recognizable snake. But, just in case you’ve never seen one, here’s a picture of one of these mainland beauties:

Mainland Colombian Boa, showing typical coloration

Mainland Colombian Boa, showing typical coloration

So, if you’re interested in what these beauties do when they get onto islands, read more after the jump.

The most obvious difference between island boas and mainland boas is their size. Mainland boas can break ten feet in exceptional cases; most can top out at a hefty eight feet. On the other hand, their island counterparts rarely exceed five feet in total length, and weigh just one fifth of what a mainland boa does. Some island populations have different shapes of the head, with larger eyes and longer snouts — the males even have longer tails.

These changes have happened relatively recently, and seem to help the boas adapt to a more arboreal lifestyle, where birds are a significant prey source. Furthermore, these boas can have remarkably small population sizes — ecosystems with just eight animals have been recorded, but unfortunately, those numbers might be skewed by over-harvesting for the pet trade.

Since these boas produce very small litters (fewer than twenty young, compared to their mainland counterparts, who may drop sixty babies at a time), the recovery of these populations may be in severe jeopardy. Since these small litter sizes are probably related to their secondary adaptation to an arboreal lifestyle on these island havens, it is ironic that that which makes them remarkable may also doom them as a species.

But why are these animals so desirable to the pet trade in the first place? Because these boas, in addition to having evolved a unique morphology, diet and lifestyle when they came to these islands, also evolved some radically beautiful color patterns to better adapt to their local insular microclimate and substrate. Ultimately, captive, private breeding of these animals might be the only way to maintain these remarkable animals for any period of time, as their habitats are being ransacked at an alarming rate, and even in those which are protected, the boas reproduce slowly, perhaps too slowly to ever recover. (Ironically, more recent boa invasions of islands seem to have an opposite effect, in which the invading, rapidly reproducing giant snake unleashes an ecological nightmare in its new home.)

I don’t have nearly enough freely licensed photos to show you the enormous variety and diversity of these animals, but Rio Bravo Reptiles has an amazing page showcasing the remarkable diversity of insular boas. And, well, I’ll end this post with a photo of my very own insular rarity: a Hog Island Boa by the name of Gawain. You can see exactly how diferent he is from a mainland boa — pale, speckled, and very tiny (although, to be fair, he is not yet an adult).

A boa from the island of Caya de los Cochinos

A boa from the islands of Cayos Cochinos

Okay, so maybe it was a little unfair for me to showcase my pets. Don’t worry … I have something much more remarkable for you tomorrow than just a little bit of fancy island evolution.

Relevant Papers:

Natural History and Conservation of Island Boas (Boa constrictor) in Belize
Scott M. Boback, Copeia, December 2005

A Morphometric Comparison of Island and Mainland Boas (Boa Constrictor) in Belize
Scott M. Boback, Copeia, May 2006

Life-History Adaptations to Arboreality in Snakes
Lígia Pizzatto et al., Ecology, February 2007

Boa constrictor, an introduced predator threatening the endemic fauna on Cozumel Island, Mexico
M.A. Martínez-Morales and A.D. Cuarón, Biodiversity and Conservation, July 1999


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