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.
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.
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.
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From "The Egyptian" of Mika Waltari:
From his sons he went to Princess Baketamon’s room and said to her, “My royal consort, you have shone in my thought like the moon during these past years, and my longing has been very great. Now my work is done, and you shall soon sit by my side as your sacred blood entitles you to do. I have shed much blood for your sake, Baketamon, and for your sake cities have burned. Have I not earned my reward?”
Baketamon sweetly smiled at him, and stroking his shoulder shyly, she said, “Truly you have earned your reward, my consort Horemheb, great warrior of Egypt! I have built in my garden a pavilion the like of which has never been seen, to receive you as you deserve. Every stone in its walls I have collected myself in my great longing for you. Let us go to this pavilion, that you may have your reward in my arms and that I may give you joy.”
Horemheb exulted at her words, and Baketamon led him into the garden. The members of the court hid and held their breath at what would follow. Slaves and stableboys fled also. Thus Baketamon led Horemheb to the pavilion. When in his impatience he would have seized her, she defended herself gently and said, “Bridle your manhood for a while, Horemheb, that I may tell you with what great toil I have built this pavilion. I hope you remember what I said when last you took me by force. Look carefully at these stones. Each one of them—and they are not few—is a memorial of my pleasure in another man’s embrace. I have built this pavilion with my own pleasure, and in your honor, Horemheb. This great white stone was brought to me by a gutter of fish who was enchanted with me; this green one was given me by an emptier of latrines in the charcoal market; and these eight brown stones set together were brought by a vegetable seller who was quite insatiable and who warmly praised my accomplishments. Have patience, Horemheb, and I will tell you the history of every stone. We have plenty of time. Many years lie before us, but I believe the story of these stones will last me until my old age, if continued each time you seek my embrace.”
At first Horemheb would not believe her words but took them for some grotesque joke, and Baketamon’s modest demeanor deceived him. When he looked into her oval eyes, he saw there a hatred more terrible than death, and he believed what she told him. Mad with rage he seized his Hittite knife to slay the woman who had so hideously dishonored him.
She bared her breast to him and said mockingly, “Strike, Horemheb! Strike the crowns from your head! For I am a priestess of Sekhmet—I am of the sacred blood—and if you kill me you will have no right to the throne of the Pharaohs!”
Her words brought Horemheb to his senses. She held him bound, and her revenge was complete. He dared not tear down her pavilion, which confronted him whenever he looked out from his rooms. After reflection he saw no other course than to appear ignorant of Baketamon’s behavior. To tear the building down would have been to betray to everyone his knowledge that Baketamon had let all Thebes spit upon his couch, and he preferred laughter behind his back to open shame. From then on he laid no hand on Baketamon but lived alone. To Baketamon’s credit be it said that she embarked on no more building works.