By Paul Rincon BBC News Online science staff

A large group of gravid (pregnant) female rattlesnakes

A US biologist has shown that timber rattlesnakes lead rich social lives and may even form family groups.

Females form birthing rookeries with other snakes when pregnant, care for their young and associate more with their sisters than unrelated snakes.

Details are published in the Royal Society journal Biology Letters.

Mr Clark claims the study is the first to demonstrate an ability to distinguish relatives from unrelated individuals - kin recognition - in a snake species.

"They seem to have a real, strong social aspect to their lives that has been ignored," Rulon Clark, a biologist at Cornell University in Ithaca, US, told BBC News Online.

"Maybe it's the body form, or the lid-less eyes, but they seem like very alien creatures. Because of that, we don't ascribe social-type feelings to them."

They seem to have a real, strong social aspect to their lives that has been ignored Rulon Clark, Cornell University

Overlooked abilities

Mr Clark tested the associations formed between 10 female timber rattlesnakes (Crotalus horridus), reared in a laboratory from birth.

The position of each individual was recorded four times a day, at three hour intervals for three days. The entire study was carried out over a period of two months.

He found that females from the same litter of snakes associated with each other more closely than females from different litters - evidence they could identify kin.

Rattlesnakes exhibit other characteristics in the wild that are consistent with animals that form family groups, such as group defence behaviour and maternal defence of young.

The snakes are venomous, but only attack when aggravated

Reptiles and amphibians are exothermic which means their metabolism and body temperature is dependent on the temperature of their environment.

"They have adapted to live in such a way that they do things more slowly than we do," Mr Clark explains.

"We're too quick for them in some ways, we don't recognise important things that could be going on.

"You really have to either be very patient or set up your experiments in such a way that you see results regardless of your timescale."

Harsh environment

Environmental factors could predispose the rattlesnakes to social behaviour.

"They live in a northern environment where they're required to find some safe place during winter.

Snakes are regarded as the least social of all vertebrates

"Once they find some rocky outcrop with good basking areas, a south-facing slope and crevices deep enough for them to get beneath the frost line in winter, they'll be faithful to it for most of their lives.

"The young are born near these sites and follow the adults there. So there may have been a selection for this social behaviour at the beginning."

Timber rattlesnakes have a wide geographical distribution in the US, from New Hampshire in the north to Texas in the south. They are venomous, but do not generally attack unless aggravated.

Mr Clark said male rattlesnakes may also exhibit social behaviour at certain times of the year. But they become aggressive when placed together in a lab environment.