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.

Female Siblings

Clark found female siblings were the most intimate. On average, they rested just 6 centimeters (2.3 inches) apart. Forty-four percent of the time, Clark observed the snakes entwined. By contrast, non-sibling females kept an average distance of 14 centimeters (5.5 inches) and spent just 18 percent of their resting time entwined.

Clark's finding is supported by previous studies conducted in the wild, which found timber rattlesnakes that shared the same hibernation den were more closely related to one another than to snakes in neighboring dens.

This latest study found that male pairs, both kin and non-kin, never entwined and hardly ever touched in the lab. Clark says this may be because the male timber rattlesnakes were well fed and on a constant light cycle, causing them to be reproductively active year round.

When male timber rattlesnakes "are reproductively active, they are more agonistic toward other males," he said. "In the wild, males can sometimes be found aggregating together outside the mating season. So they may also exhibit kin recognition."

Clark says timber rattlesnakes and other pit vipers may be the most social of all snakes. Timber rattlers are often observed in groups during and after gestation and also when shedding their skins. Females also exhibit close parental care and lay scent trails to help their young locate winter hibernation dens.

This suggests that the benefits to rattlesnakes to keep together outweigh the costs. Those costs include a greater susceptibility to parasites, disease, and predation as well as increased competition for mates and food.

"This may be especially true when groups are at a fixed location like a den or rookery," where females give birth, Clark said. "Predators can repeatedly return to prey on group members. However, this cost may be offset by the enhanced ability of a group to defend against such predators."

Rattlesnake enemies include badgers, coyotes, eagles, foxes, opossums, raccoons, skunks, wild pigs, and turkeys. Clark added: "I even had an adult female I was radio-tracking last year killed by a red-tailed hawk."

Defense Strategy

Faced with so many predators, group defense may well be the best strategy for snakes. But because of the potential for high losses, scientists believe only closely related snakes are likely to benefit. The defense strategy is rooted in "inclusive fitness theory," whereby an individual considers not only its own survival, but also that of its kin.

Individuals are willing to sacrifice themselves, given the likelihood that a high proportion of genes related to them would live on through family members, the theory says.

Whether other snakes tend toward family groups is uncertain. Other snake species, such as the Butler's garter snake (Thamnophis butleri), often gather in numbers. But to date, no evidence has been found that kinship plays role.

John Pickrell contributed to the reporting for this article.

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