Rattlesnakes Show Strong Family Bonds, Study Says

James Owen
for National Geographic News
February 23, 2004

Solitary, aggressive, and dangerous to know—that's how most people see rattlesnakes. Yet scientists are beginning to reveal a seemingly caring, family-loving side to these deadly reptiles.

This reappraisal is highlighted by a new study of timber rattlesnakes (Crotalus horridus) in the eastern United States. Researchers suggest that, as adults, the rattlesnakes can recognize their siblings, even after being separated at birth.

The finding marks the first time kin recognition has been observed in snakes.

Rulon W. Clark found that female timber rattlesnakes from the same litter associate much more closely with each other than other females, even after being raised in isolation for more than two years. Clark is a Ph.D. candidate at Cornell University in Ithaca, New York.

His findings, recently published in the journal Biology Letters, suggest there must be significant benefits for timber rattlesnakes to stick in family groups.

A small number of reptiles, including green iguanas (Iguana iguana) and common lizards (Lacerta vivipara), have been shown to exhibit kin recognition. But until now the behavior hasn't been observed in snakes, which are often regarded as the least social of all vertebrates.

Yet this impression of insociability is looking increasingly misplaced.

Scientists have found that rattlesnakes exhibit other characteristics associated with advanced sociability, including group defense, pheromone alarm signals, and maternal care of young.

Limited hibernation-den areas and basking sites may cause some snakes to live in groups. But scientists say snake groups also occur in the absence of such factors.

Snakes are subtle animals whose social lives have remained a mystery, said Rick Shine, snake expert and evolutionary biologist at the University of Sydney in Australia. "Recent studies have revealed unsuspected complexity in lizard sociality. [Now] Clark's studies on rattlesnakes hint that we will find the same kinds of complexities in snakes as well," he said.

Snakes do have very sophisticated sensory stsems, Shine said. For example, "with a single flick of his tongue, a male garter snake can not only tell the species of another snake, but also its sex, its body size, its body condition, and whether or not it has mated recently," he said. "Clark's work lets us know that—like lizards—these animals can also tell whether or not the other individual is a close relative."

Clark's study involved 24 timber rattlesnakes born in the laboratory from mothers captured in the wild in Pennsylvania. The young snakes were isolated from each other for two and a half years before being introduced in pairs of either same-sex siblings or non-siblings. The responses of each pair were then monitored.

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