Penguins are turning to prostitution. But instead of doing it for money, Antarctic dolly-birds are turning tricks to get rocks off their menfolk.

Stones are essential for penguins to build their nests. A shortage has led to the unorthodox tactics.

"Stones are the valuable currency in penguin terms," said Dr Fiona Hunter, a researcher in the Zoology Department at Cambridge University, who has spent five years observing the birds' mating patterns.

Prostitution is described as the world's oldest profession. But Dr Hunter is convinced it is the first time it has been seen in animals.

Penguin partners

All of the female penguins Dr Hunter observed trading sex for stones had partners.

Penguins stick to the same mate, she said, but none of the males twigged what was happening.

"There was no suspicion on the part of the males. Females quite often go off on their own to collect stones, so as far as the males are concerned there is no reason to suspect."

She added: "It tends to be females targeting single males, otherwise the partner female would beat the intruder up."

Dr Hunter and Dr Lloyd Davis of the University of Otago watched the penguins at work on Ross Island, about 800 miles from the South Pole as part of a Antarctica New Zealand programme.

Tricking their prey

On some occasions the prostitute penguins trick the males. They carry out the elaborate courtship ritual, which usually leads to mating.

Having bagged their stone, they would then run off.

"The courtship display is a head-bowing display," Dr Hunter said. "It usually starts with the male, who bows his head and looks out the corner of his eye."

She said she does not think the female penguins are doing it just for the stones.

"The female only takes one or two stones," she said. "It takes hundreds to build the nest to get their eggs off the ground.

"I think what they are doing is having copulation for another reason and just taking the stones as well. We don't know exactly why, but they are using the males."

She said the female penguins could also be testing potential future mates, in case their existing partner died before the next mating period.

The single male penguins appeared to have only their own pleasure as a motive.

The action takes place during a three-week mating period starting in late October.

The most stones Dr Hunter saw a single female taking was 62, although she said she suspects her final total was higher.

The number of prostitute penguins is quite low, she said.

"It's probably only a few percent," she said. "I was watching opportunistically, so I can't give an exact figure of how common it really is."

Other animals have been seen trading food for sexual favours but only within a partnership.