The chimpanzee sex exchange

Duffy, Wrangham and Silk describe in a short paper the correlation of male mating success and support for the alpha male in a group of chimpanzees. The finding isn't surprising -- it's been long assumed that subordinate male chimpanzees participate in coalitions to attain mating rewards -- but it's a nice quantification. Helping the alpha male maintain his status had a much larger impact on mating success than rank.

References:

Duffy KG, Wrangham RW, Silk JB. 2007. Male chimpanzees exchange political support for mating opportunities. Curr Biol 17:R586-R587. doi:10.1016/j.cub.2007.06.001