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."
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.
SOURCES AND RELATED WEB SITES