There is a growing consensus that polyandry (mating with multiple males) is beneficial, despite potential costs like disease or injury. One mechanism may be that genetically better males have greater fertilisation success when their sperm compete with those of inferior males. Surprisingly, there is no unequivocal evidence for this because of the difficulties of experimental design. Enter the brown antechinus, Antechinus stuartii. The unusual life history of this mouse-sized marsupial, centred on a mating season of a fortnight, makes it possible to design experiments to resolve the problems. The results show conclusively that polyandry can increase net offspring fitness in the wild, and males that sire more offspring through sperm competition also have much greater offspring survival.

Fisher, D.O., M.C. Double, S.P. Blomberg, M.D. Jennions and A. Cockburn. 2006. Post-mating sexual selection increases lifetime fitness of polyandrous females in the wild. Nature 444: 89-92 (2nd November).

Some links to stories about this paper:
New Scientist. Mate like crazy and let the sperm fight it out.

Scientific American. Marsupial Mating Proves Promiscuity Pays.
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Livescience. Swinger Marsupials Produce Healthier Offspring.

UK Metro. Sleep around for healthier babies (for fans of Little Britain).

iafrica.com. Free love breeds better babies.

ABC Science. Marsupials sleep around for babies' sake.

Fox News / AAP. Promiscuity breeds better babies: study.
http://www.news.com.au/story/0,23599,20687133-36398,00.html

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