The Adelie penguin, a species that inhabits Ross Island in Antarctica, ensures the survival of its species in a curious manner. Though scientists claim that this species mates for life, an exception is apparently made where nest building is concerned.
The female Adelie penguin, desperate to obtain the stones she uses to build her nest, visits the nests of bachelor Adelies, goes through the entire courtship routine, mates with him, and waddles off with the prized pebbles she worked for.
The harsh, frigid Antarctic climate makes it necessary for the female Adelie to use unconventional means to obtain the rocks she needs to build a nest for her offspring. Trees and grasses cannot survive in this climate, and the stones that the female penguin does find, are likely to be frozen solid in the ice or in the mud.
So rare are the pebbles the penguin needs that she sometimes even risks her neck by robbing another females' nest of the stones.
This explains why the date and mate method is the one female Adelie penguins prefer. She follows the courtship ritual, which includes repeatedly dipping her head and, if the unattached male expresses interest in her, she lies flat on her back, as an invitation to mate. Once the two have mated, the female collects the stones she came for as a sort of payment, and returns home to her mate for life!
Sometimes, especially cunning female Adelies engage in the courtship ritual, minus the mating part, grab the rocks, and go home. According to Fiona M. Hunter of the University of Cambridge, the males of this species do not bear a grudge, and do not become aggressive when they get nothing for something. This researcher even observed one bold and masterful female make off with 62 stones from one single male in one hour, without engaging in anything but the head-bobbing ritual.