MALE wood mice have come up with an ingenious strategy to maximise their chances of fatherhood 鈥� altruistic sperm. Thousands of their sperm line up head to tail, making a long express train that can wiggle its way to an egg much faster than a lone swimmer.
鈥淲hen I saw it I couldn鈥檛 believe my eyes. And I鈥檝e been working on sperm for 30 years,鈥� says Harry Moore, a biologist from the University of Sheffield. He thinks the sperm have evolved the trick in a bid to outcompete sperm from other wannabe dads.
Common wood mice (Apodemus sylvaticus) are highly promiscuous. To help pump up sperm production to meet the demand, they have large testes for their body size 鈥� a feature often found in animals that face serious competition in fathering offspring. 鈥淭heir testes count for over 4 per cent of their body weight. They鈥檙e really quite big,鈥� says Moore. He reasoned that they might also have developed other traits to help their sperm successfully fertilise eggs.
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Under the microscope, Moore and his team found that the wood mice sperm are hook-shaped. This lets them join up head to tail a few minutes after ejaculation, forming chains 50 to 2000 sperm long. The longer chains are so big they are visible to the naked eye.
The 鈥渟perm train鈥� travels up to 50 per cent faster than lone sperm, and is better at manoeuvring through the thick fluids found in the cervix and uterus (Nature, vol 418, p 176).
Other animals have also adapted their sperm. Marsupials have sperm that pair up side by side to increase their swimming ability. Guinea pig sperm clump together in packs, although these tend to swim around in circles and don鈥檛 seem to confer an advantage. And there is some evidence that human sperm tend to follow each other, like cyclists sheltering in the slipstream of the leader of the pack. But no one has ever seen such a blatant show of altruism, with genetically similar sperm helping each other out in a bid to see off sperm from rival males.
About an hour after forming the train, about half of the wood mouse sperm undergo a premature acrosome reaction, a process that normally helps sperm burrow into an egg. That allows the chain to break apart, leaving some sperm incapacitated but giving theothers a good shot at fertilisation.
Moore鈥檚 team now plans to investigate whether there鈥檚 an optimum length of sperm train, or if there鈥檚 a selective pressure still pushing for longer and longer chains.