Which animals living as prostitutes?

Adelie penguins build nests in gravel to lay their eggs. When nearby pairs are distracted, these penguins do not hesitate to steal their stones. And some females are engaged in prostitution to earn some stones, even though they already have a long term partner. There are single males who also make their nests in the hope to have a family, females visit them to have sex and afterwards leave taking couple of gravel stones.


SOURCE: http://www.nwf.org/News-and-Magazines/National-Wildlife/Birds/Archives/2005/Mating-in-a-Material.aspx
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