Have sex and die: that鈥檚 the lot of a shrew-like marsupial that can鈥檛 afford to waste time picking out the ideal mate but seems to produce great offspring all the same. It looks like the trick might be down to some high-performance sperm.
Antechinus stuartii is a small Australian marsupial with a bizarre sex life. Males gather in a nest, wait for females to turn up and then attempt to mate with as many as possible in what for both sexes is a once-in-a-lifetime sexual orgy. Two weeks later, the males鈥� immune systems fail and they die, while the females go on to produce what is usually their one and only litter before dying a few months later.
When Diana Fisher of the Australian National University in Canberra and colleagues took wild animals and allowed females to mate either once with each of three males, or three times with just one male, they found females with multiple mates had three times as many offspring that survived till weaning.
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Promiscuous these animals may be, but that doesn鈥檛 mean anything goes genetically speaking. Paternity tests and follow-up experiments revealed that some males sired far more surviving offspring than others.
That would make sense, say the researchers, if better-quality males also produce sperm that is able to outcompete the sperm of other males. If so, females do not need to pick out the best males: instead, they can mate with multiple partners and let the males鈥� sperm fight it out (Nature, vol 44, p 89). 鈥淭here have been clues that this might happen in other animals, but this is the first time this has been shown in a comprehensive way,鈥� says Fisher.
Could the same processes be at work in other mammals? Females are thought to select male partners to reduce the chances of contracting disease and because they tend to invest more in fewer offspring compared with males. Antechinus鈥榮 unusual reproductive habits make it an oddity, with more evolutionary pressure than usual for 鈥済ood sperm鈥� to win out. However, the benefits of multiple mating shown in this study are striking and suggest that females of other species might also benefit from such behaviour, says evolutionary biologist Tom Tregenza of the University of Exeter in the UK.
