Imagine sperm cooperating by the thousands in the great race to an egg–even though only one teammate gets the prize. Researchers now report such teamwork for the first time in a placental mammal.
The sperm of the European wood mouse, Apodemus sylvaticus, hook together in long, thick trains that can double an individual’s speed, say Harry Moore of the University of Sheffield in England and his colleagues in the July 11 Nature.
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In pondering altruism, theorists some 25 years ago proposed sperm cooperation for species in which females mate with multiple males. In that brutally competitive world, collaboration between sperm from the same source may be worth some sacrifice from individual sperm. The new study suggests that’s the case in the wood mice.
Clustering sperm have turned up in some insects and squid, and most New World marsupial sperm pair up to improve their mobility. Those partnerships pale beside the story that’s unfolding for the European wood mouse.
This common mouse ranks as a “supreme sexual performer among rodents,” comments Roger V. Short, a reproduction biologist at the University of Melbourne in Australia. Both males and females mate repeatedly, he says. A male wood mouse devotes about 4.5 percent of its total weight to testes, one of the highest percentages among rodents.
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Moore’s group released wood mouse sperm in a fluid used for in vitro fertilization and discovered long trains of speeding swimmers. After wood mice mated in the laboratory, the researchers recovered such trains from the females’ reproductive tract. A hook grows out of each sperm head and snags either another hook or a sperm tail to form an elongated tangle of thousands of swimmers.
About half an hour after a train forms, many of its sperm lose their front membrane. In this premature version of the normal step in fertilization called the acrosome reaction, sperm give up their chance of fertilizing the egg. The ruptures break up the train and free any intact sperm for action.
What Moore would like to know now, he says, is whether trains always include sperm from just one male.
“No one’s ever seen anything like these enormous sperm trains in a mammal,” says Short. “For the first time, this makes it clear that within an ejaculate, we’re all boys together.”