Snakes' Family Values

Thicker than water. Timber rattlesnakes recognize each other even if they've been reared apart.

Snakes are often thought of as loners. But new findings reveal that sisters of at least one rattlesnake species recognize each other and prefer each other's company over that of strangers. Experts say the discovery is further evidence that snake behavior is more complex than previously suspected.

Despite snakes' reputation as antisocial, there have been a few reports over the decades hinting at social tendencies in these often reclusive and hard-to-observe hunters; for instance, herpetologist Harry Greene of Cornell University and others found in 2002 that maternal care of newborns is widespread in pit vipers.

Cornell University wildlife biologist Rulon Clark had followed radio-tagged adult timber rattlesnakes (Crotalus horridus) in the field for years. He found them far more gregarious than expected; for instance, females often basked together. In a follow-up study in the lab, he raised 24 of these snakes--10 females and 14 males--from three litters. He then housed pairs of snakes together for periods of 3 days in four combinations: sisters, unrelated females, brothers, and unrelated males.

In a paper published online by the Proceedings of the Royal Society Biology Letters on 13 February, Clark reports that when resting coiled, on average, females kept within about 6 centimeters of a sister, but stayed twice as far from unrelated females (~14 cm). (Males, on the other hand, kept at least 20 cm distance from each other, whether they were related or not.) Sisters also touched each other significantly more (~78% of observations) than did unrelated females (~43%).

The findings show that the rattlesnakes can recognize their relatives, despite being raised in isolation from each other for more than 2 years. It suggests the species, and potentially others, "may lead much richer social lives than previously thought," Clark says. Family groupings may protect snakes from predation by safety in numbers as happens elsewhere in the animal kingdom, says Clark, who next plans to see how the rattlers hang out in the wild and how they recognize each other.

Snakes in Australia exhibit "social" behaviors as well, notes evolutionary biologist Rick Shine of the University of Sydney. "It's likely these will prove to be widespread," he says.

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