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The Emerald Tree Boa and Paleogeography

Enjoy this blog from March 16, 2012.

In honor of St. Patrick’s Day, we couldn’t think of a better animal to highlight than the emerald tree boa.

Wait, you say, the patron saint of Ireland was no friend of snakes, legendarily driving them out of the Emerald Isle.

We’ll get to that.

An emerald tree boa curls its sinuous body around a branch at Shedd Aquarium.

First, let’s look at our 3-foot emerald tree boa in Amazon Rising. While he’d be perfectly camouflaged on Ireland’s rolling greensward, he’s adapted to the equally verdant canopy of the Amazon rainforest. Neatly coiled in concentric loops on a branch, with head in the middle, and absolutely motionless, he disappears into the green like just another vine. The white zigzag pattern along his back imitates the mottled sunlight that penetrates the forest’s dense leaves.

Emerald tree boas, Corallus batesii, are nocturnal hunters. At night, an emerald tree boa’s pencil-thin vertical pupils expand to admit as much light as possible so the snake can detect the movement of birds, bats and rodents among the branches or on the ground.

More important than vision, however, is the snake’s ability to register the presence of its warm-blooded prey through deep heat-sensing pits around its mouth. These labial pits give the reptile a thermal image of its surroundings.

With its long prehensile tail securely wrapped around a branch, the slender but powerful snake can lunge up, down, or out, adjusting its speed and direction to intercept its prey in mid-air. Then, gripping the prey with its recurved teeth, the boa throws a few coils of its body around it to quickly suffocate it by constriction. (Boas are not venomous.) Draped on a tree branch, the snake then slowly engulfs its meal with its loosely connected, expandable jaws. Digestion can take a few days, and one meal will tide over a snake for several weeks or even months.

Shedd’s boa eats a feeder mouse (purchased frozen, to be thawed and warmed before serving on tongs) every two to three weeks.

Emerald tree boas seldom come down to the ground unless it is to move to another tree, deftly climbing the trunk to aerial safety again. Or at least relative safety; they are hunted by birds of prey.

Two emerald tree boas coil their long vividly green bodies into tight coils over and around a tree branch in Amazon Rising.
A close up of an emerald tree boa's wedge-shaped head, seated in profile in a nest of its own coils wrapped around a branch. The snake's small eyes are set near the top of its head, its long jaw stretching almost the entire length of its skull.

Snakelessness

Now, as to why Ireland has never, ever had any native snakes: Blame it on paleogeography, the last ice age and, ultimately (with only a few exceptions), snakes’ inability to cross salt water.

Snakes evolved about 100 million years ago, when the Earth’s dry land was bundled up in one supercontinent. Through tectonic movement, this huge land mass slowly tore apart to become Antarctica, Africa, Australia, India and South America. Those areas are where you will find fossil snakes and, with the exception of Antarctica, a fascinating array of living snakes.

The other large land areas we know today—North America, Europe and the British Isles—were part of the ocean floor then. But as the world’s climate became drier, starting about 65 million years ago, the northern continents were exposed and further arose through geological processes. In addition, changing sea levels opened and closed land bridges. All this started a land rush by snakes (and other animals) into every imaginable biome: grasslands, deserts, wetlands, forests and mountains.

Only Antarctica, Iceland, Greenland, New Zealand—and Ireland—were, and to this day remain, snakeless, because they were isolated. And, looking specifically at Ireland, iced up by the series of glaciers that gripped much of the Northern Hemisphere on and off between 3 million and 15,000 years ago. Snakes being ectothermic—literally getting their heat from outside sources, that is, their environment—they were frozen out even when glacier-era land bridges did link Ireland with Britain and the European mainland. As the last glaciers melted and sea levels began to rise again, Ireland was severed from Britain before Britain was cut off from the continent. Snakes established themselves in Britain, a few species getting as far north as Scotland, but were kept from Ireland by 12 miles of cold ocean.

As the late Dr. Archie Carr, herpetologist extraordinaire, wrote, “(T)he snakelessness of Ireland is pretty much the same sort of thing as the lack of elephants there.”

So by St. Patrick’s time, in the fifth century, Ireland’s lack of snakes had long been a done deal. Most likely, snakes were a metaphor for paganism, which the missionary was also credited with banishing. Ireland has not barred the door against the reptiles, however, so you will find snakes, doubtlessly including the popular emerald tree boa, in Irish zoos and the homes of reptile enthusiasts.

—Karen Furnweger, web editor