These prostitutes have sex with unattached males and take a pebble from the male's nest after having sex. In an actual study the researchers speculate that the female has bent over to grab a stone and the male has misinterpreted the gesture—she hasn't changed her mind or performed a trick.
But researchers are still studying the phenomenon, and a consensus has not yet been reached; it is assumed that either the female is baiting, or that the male deliberately chooses to misinterpret the gesture, as coercive sexual copulation is common among these penguins. The BBC further reported Hunter as saying that the female penguins probably didn't engage in prostitution only for stones.
Hunter believed "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". This behavior was also suggested as a mate choice process by which the females might find a possible future mate.
This would provide a female penguin with another male penguin should their current mate die. The male penguins, the study speculates, were engaged in sex with the prostitute females only for sexual satisfaction. According to Hunter's observation, the number of prostitute penguins was very low, and she approximated this as "only a few percent". The data show that when extrapair copulation occurs at the male's nesting site, the female takes one or more stones; but when the extrapair copulation occurs at the female's nesting site, the male never takes a stone.
Clearly a male who has copulated with a female benefits his progeny when she takes a stone. Sometimes copulation doesn't occur, but the female still takes a stone.
But both males and females steal stones: The benefit of gaining stones without a fight is clear, but the female is not always willing to copulate to avoid a fight. The researchers speculate about the possible genetic fitness advantages and disadvantages of the practice, and aren't altogether sure that the female copulates mainly in order to obtain a stone.
A study conducted by the Max Planck Institute for Evolutionary Anthropology in Leipzig , Germany , and published online in the Public Library of Science , attempted to support the meat-for-sex behaviour hypothesis, according to which, in early human societies the best male hunters had the maximum number of sexual partners. Unable to study early humans, researchers studied chimpanzees.
According to Cristina Gomes of the Institute, the study "strongly suggests that wild chimpanzees exchange meat for sex, and do so on a long-term basis". The data reveal that chimps enter into communities of hunting and sharing meat with each other over long periods of time and females within the meat-sharing community tend to copulate with males of their own meat-sharing community.
Direct exchange of meat for sex has not been observed. The discs could be exchanged by the monkeys for various treats. During one chaotic incident, a researcher observed what appeared to be a monkey exchanging a disc for sex. The monkey that was paid for sex immediately traded the silver disc for a grape. The researcher took steps to prevent any possibility of coins being traded for sex after his suspicions were aroused.