Adelie penguins regularly steal stones from fellow nest-builders to fortify their own, even though they get pecked and chased in the process. More surprising yet is the recent discovery that some females resort to peddling their bodies in exchange for the precious pebbles.
That's right. Penguin prostitutes...
There's some denial going on among the academic community, though:
"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."
And to conclude:
Other animals have been seen trading food for sexual favours but only within a partnership.