All aboard the sperm train

Most animals exhibit cooperative behaviour, yet the wood mouse takes this to extremes. A new study has revealed that its sperm organise themselves into long 'trains' in order to travel faster and so beat the competition

Sperm whales, sperm banks, and now "sperm trains". The image this conjures up is bizarre, but it is true; there are sperm trains, and they are odd. A British scientist has discovered that thousands of individual sperm cells can hook up to form long trains of swimming cells that can move faster through the female reproductive tract. It is a unique act of cooperation between "brothers" in the vital race to be the first to fertilise the egg. The sperm trains were found in male wood mice, but they may teach us something about wider issues of cooperation and competition in nature.

Sperm whales, sperm banks, and now "sperm trains". The image this conjures up is bizarre, but it is true; there are sperm trains, and they are odd. A British scientist has discovered that thousands of individual sperm cells can hook up to form long trains of swimming cells that can move faster through the female reproductive tract. It is a unique act of cooperation between "brothers" in the vital race to be the first to fertilise the egg. The sperm trains were found in male wood mice, but they may teach us something about wider issues of cooperation and competition in nature.

Human society is also based on an uneasy mix of conflict and cooperation. Among men, competition is often over women, and cooperation is frequently about finding partners, too.

From an evolutionary perspective, animals cooperate because they hope that any help they give will be reciprocated, or because of blood ties. Genes bind us into cooperation, and studies of animal behaviour in the past 20 years have revealed that kinship holds many social groups together. That English woodland bird, the long-tailed tit, starts off breeding in standard pairs. But if their nests fail, they give up trying to breed themselves and help another pair – providing that they are relatives.

Selfless, cooperative behaviour extends further than individual relatives, though. It involves the cells of our bodies. As the Oxford zoologist Richard Dawkins has argued, we are clusters of cooperating cells – without this cooperation we would fall apart.

Why should sperm cells be any different? Sperm are effectively motorised vehicles for carrying the DNA of males. They travel in large groups that we call ejaculates. Sperm from the same ejaculate are closely related, and their common goal is to penetrate and fertilise an egg. But sperm can compete if the female they are inside has mated with another male. Sperm from unrelated males fight to fertilise a female's eggs.

Because the females of most animals are promiscuous, sperm competition is ubiquitous in the animal kingdom. But for a long time, since Darwin, females were assumed to be sexually monogamous, a view that coincided with Victorian prudery. Only in the 1970s, as the "selfish gene" revolution took place, did the female-monogamy myth begin to crumble. Reproduction was no longer seen as a cooperative venture between male and female; it was a case of each sex selfishly trying to exploit the other.

It all started with a yellow fly: Scatophaga, the dungfly. Geoff Parker, now at Liverpool University, watched amazed as female dungflies mated with one male after another. Putting two and two together, Parker predicted that the sperm of the females' partners would compete to fertilise her eggs. He realised that once a male had mated with a female, the dungfly would do best, in terms of fertilisation, by preventing his partner from copulating with anyone else. On the other hand, a male that encounters an already-mated female would do best by diluting, displacing or disabling the sperm of any competitor inside the female.

Evolution, then, simultaneously favours "sperm defence" and "sperm offence". Parker's insight was that these opposing forces would result in the rapid evolution of features associated with fertilisation – adaptations to sperm competition.

This idea is applicable to more than dungflies. Adaptations to sperm competition are everywhere. Mate-guarding, a form of sperm defence, involves hanging on to a female after you've finished mating, to make sure that no other male gets to mate with her before her eggs are fertilised. Think of piggy-back ladybirds, tandem-flying butterflies and baboon consortships, in which males follow their fertile partner's every move; they're all mate-guarding.

With sperm offence, a common strategy is to pump in more sperm. Males of many animals seem to be able to ejaculate more sperm when required to. More male competitors around? Pump in more sperm. Partner been away for a while? Pump in more sperm. The same is true among different animals. In the species in which females are more promiscuous, males transfer more sperm during mating, because they have tremendous testicles.

Female chimpanzees are promiscuous, and if human testicles were the same relative size as the chimps', they would be as big as grapefruits. The male chimp's titanic testicles allow him to copulate frequently and transfer a huge number of sperm each time. Their cousin, the gorilla, lives in a world almost bereft of sperm competition because male aggression keeps sexual competitors at bay. He has tiny testicles. If men's testicles were the same size, they would be the size of broad beans.

What about ourselves? Do we experience sperm competition? Undoubtedly we do, although judging from our plum-sized testicles, less often than chimps but more often than gorillas. In the Nineties, Robin Baker and Mark Bellis, then at Manchester University, began to study sperm competition in people. They came up with some ideas about how human sperm might fight. Their seminal idea was based on the fact that a human ejaculate is a motley collection of cells – big sperm, small sperm, fat sperm, slim sperm and so on. The level of variation had been dismissed by other researchers as mere production errors.

Baker and Bellis turned the production-error idea on its head and proposed that the different sperm "morphs" each had a role in fertilisation. Some sperm, they said, were natural-born killers, designed by evolution to destroy the sperm of rival males, but in so doing destroying themselves for their kin in an act of cooperation. But the idea failed the test of scientific scrutiny – research revealed that human ejaculates do not contain altruistic killers.

But it now appears that altruistic sperm wasn't such a daft idea. A new study by Harry Moore at the University of Sheffield, and colleagues, published in last week's Nature, shows that the wood mouse has very strange sperm. Moore is a reproductive biologist, specialising in the function of sperm. That the wood mouse was worth studying occurred to him when his pet cat dropped a male on the carpet. The mouse's testicles were huge – a sign of promiscuity. When Harry examined its sperm under the microscope, he saw something unique. Some of the sperm had organised themselves into long trains; strings of sperm hundreds or thousands strong, with their tails thrashing wildly and the whole thing zipping along at a startling rate. Using a bit of sophisticated computer technology known as a "sperm tracker", Moore found the sperm trains to be travelling much faster than the solitary sperm.

At high magnification, mouse sperm look different from our own tadpole-esque structures, and bear a long hook on their tip, with which the sperm clasps the tail of a sperm-mate and forms trains. Once the sperm are within the female reproductive tract, and have traversed the cervical junction, they let go, for an egg can be fertilised only by a single sperm. In letting go, some of the sperm die – firing off their acrosomal enzymes in their heads and blowing their own chances of fertilising an egg. All so that one of their relatives might make it to the egg.

Females are rarely monogamous across the animal kingdom, and in many species they mate with several different partners. For males, the consequence of this is competition between the sperm of rival males. This conflict pushes evolution into overdrive, creating some of the most extraordinary adaptations in the animal kingdom. The wood mouse's sperm train seems to be another bizarre outcome of the competition between males to fertilise eggs.

Tim Birkhead is professor of behavioural ecology at Sheffield University