Prostitution among animals
A few studies have been used to promote the idea that prostitution exists among different species of animals such as Adélie penguins, chimpanzees, and crab-eating macaque. Penguins use stones for building their nests. Based on a 1998 study, media reports stated that a shortage of stones led female Adélie penguins to trade sex for stones. Some pair-bonded female penguins copulate with males who are not their mates and then take pebbles for their own nests.
Chimpanzees who seem to be trading food for sex are also said by some to be engaging in prostitution.
Prostitution in animals was first reported in 1998 by Fiona Hunter, a researcher at the University of Cambridge, and Lloyd Davis of the University of Otago, who had spent five years observing the mating behavior of penguins. The study was conducted as part of an Antarctica New Zealand programme on the Ross Island, approximately 800 miles (1,300 km) from the South Pole.
According to the report about the study published by BBC News Online, some female penguins sneak around on their partners. These prostitutes have sex with unattached males and take a pebble from the male's nest after having sex. Or they sometimes perform the courtship ritual as a trick and grab a stone without the sex (in the 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). 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."
While the sensationalized versions of the study emphasize prostitution, the research data itself is less sensational. The data shows 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: sometimes they get away with it and sometimes they are attacked. 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. Researchers observed chimpanzees in the Taï National Park and concluded that a form of prostitution exists among the chimpanzees in which females offer sex to males in exchange for meat. 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 reveals 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. 
According to one researcher who had a paper published in the journal Animal Behaviour, as reported by The Cambridge Student, long-tailed macaque males use grooming to pay for sex. The study showed a correlation between the length of time a male spent grooming a female and her willingness to have sex with him. 
In a 2005 study at Yale–New Haven Hospital, capuchin monkeys were taught to use silver discs as money. One researcher "saw something out of the corner of his eye" that looked like a coin being exchanged for sex. The researcher took steps to prevent any possibility of the coins being used for sex after his suspicions were aroused, so while it is possible that it happened once, no events of this nature were ever repeated. 
- "Penguins are turning to prostitution". BBC. 1998-02-26.
- Prostitution in animals. The Cambridge Student
- Connor, Steve (2009-04-08). "Sex for meat – how chimps seduce their mates". The Independent (London).
- McKee, Maggie (2005-01-02) Mating in a Material World, National Wildlife Federation
- Hunter, E.M.; Davis, S.L. (1998). "Female Adélie Penguins Acquire Nest Material from Extrapair Males after Engaging in Extrapair Copulations". The Auk 115 (2): 526–528. doi:10.2307/4089218.
- Cristina M. Gomes and Christophe Boesch (2009). "Wild Chimpanzees Exchange Meat for Sex on a Long-Term Basis". PLOS One 4 (4): e5116. doi:10.1371/journal.pone.0005116. PMC 2663035. PMID 19352509.
- Gumert, Michael D. (2007). "Payment for sex in a macaque mating market". Animal Behaviour 74 (6): 1655. doi:10.1016/j.anbehav.2007.03.009.
- Monkey Business, by Stephen J. Dubner and Steven Levitt; at the New York Times; published June 5, 2005; retrieved April 10, 2014