Published online 4 March 2008 | Nature | doi:10.1038/news.2008.641

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When snakes fall from trees

Cold snaps make for unbalanced reptiles.

Snakes make good climbers on narrow branches.Punchstock

Researchers investigating the agility of arboreal snakes at cold temperatures have started to detail how and when reptiles tumble out of their trees.

Snakes, like all cold-blooded animals, cannot control their body temperature. Instead, they tend to go along with the conditions of their environment, slowing down their movement as the temperature drops. In particularly cold conditions, some reptiles become completely immobile, which can cause problems. A cold snap in Florida this past winter, for example, caused iguanas to fall out of their arboreal homes.

To investigate, biologist Gary Gerald and his colleagues at Miami University in Ohio recorded the velocities, body postures, and balancing abilities of corn snakes (Elaphe guttata) on horizontal beams at 10 °C, 20 °C, or 30 °C. The snakes were exposed to the temperatures for 2 hours before being encouraged to cross the ‘branches’, which had diameters of 3, 6, or 10 centimetres, in a climate-controlled laboratory. Spotters stood below to catch the snakes if they fell, and a safety cushion was placed beneath the artificial tree in case the spotters missed or dropped a falling snake.

Snakes on a limb

The team reports in the Journal of Experimental Zoology1 that at high temperatures snakes moved faster and were more stretched out along branches of all diameters. At lower temperatures, snakes moved much slower and adopted a looped posture — their bodies meandered along the branch, improving stability.

The snakes were about ten times more likely to fall at the coldest temperature than at the warmest temperature, even though they were still mobile. The falling and the looped posture hint that temperature might have an effect on balance as well as movement, the team says.

The snakes in colder climes were not particularly cooperative to the experiment. Perhaps knowing what was in store for them, they would often not even try to cross the branches. In these cases of insubordination the researchers had to tickle the back of the snakes' tails to prod them along.

Thin and loopy

Counterintuitively, snakes in colder temperatures tended to fall from thin branches less often than from thick ones, perhaps because they could make more loops along the narrower surface.

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Their agility on thin branches give snakes something of an advantage over other tree-dwelling animals, says Gerald. "People often think of snakes as being at a disadvantage by not having legs, but in trees there is really no question that they have a leg up," he says.

The study may help to explain some aspects of reptile ecology. “That so many of the snakes fell when cold may help to explain why there are so few arboreal snake species outside the tropics," says biologist and flying snake expert John Socha at the University of Chicago in Illinois. 

  • References

    1. Gerald, G. W., Mackey, M. J. & Claussen, D. L. J. Exp. Zool. 309A, 147-156 (2008).
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