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

Voices
voicesGood for Lana Del Rey for helping kill that myth, writes Grace Dent
Sport
The Pipes and Drums of The Scottish Regiments perform during the Opening Ceremony for the Glasgow 2014 Commonwealth Games at Celtic Park on July 23, 2014 in Glasgow, Scotland.
Commonwealth GamesThe actor encouraged the one billion viewers of the event to donate to the children's charity
Sport
Karen Dunbar performs
Entertainers showcase local wit, talent and irrepressible spirit
Sport
Members of the Scotland deleagtion walk past during the opening ceremony of the 2014 Commonwealth Games at Celtic Park in Glasgow on July 23, 2014.
PROMOTED VIDEO
Arts and Entertainment
The Tour de France peloton rides over a bridge on the Grinton Moor, Yorkshire, earlier this month
film
Life and Style
fashion Designs are part of feminist art project by a British student
News
Very tasty: Vladimir Putin dining alone, perhaps sensibly
news
Life and Style
Listen here: Apple EarPods offer an alternative
techAre custom, 3D printed earbuds the solution?
Arts and Entertainment
Top guns: Cole advised the makers of Second World War film Fury, starring Brad Pitt
filmLt-Col Steven Cole is the man Hollywood calls when it wants to borrow a tank or check a uniform
News
The University of California study monitored the reaction of 36 dogs
sciencePets' range of emotions revealed
News
Snoop Dogg pictured at The Hollywood Reporter Nominees' Night in February, 2013
people... says Snoop Dogg
News
ebookA unique anthology of reporting and analysis of a crucial period of history
Arts and Entertainment
A scene from Shakespeare in Love at the Noel Coward Theatre
theatreReview: Shakespeare in Love has moments of sheer stage poetry mixed with effervescent fun
News
Joining forces: young British men feature in an Isis video in which they urge Islamists in the West to join them in Iraq and Syria
newsWill the young Britons fighting in Syria be allowed to return home and resume their lives?
Arts and Entertainment
The nomination of 'The Wake' by Paul Kingsnorth has caused a stir
books
News
i100
Life and Style
food + drinkZebra meat is exotic and lean - but does it taste good?
Independent
Travel Shop
the manor
Up to 70% off luxury travel
on city breaks Find out more
santorini
Up to 70% off luxury travel
on chic beach resorts Find out more
sardina foodie
Up to 70% off luxury travel
on country retreats Find out more
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

BI Manager - £50,000

£49000 - £55000 per annum + competitive: Progressive Recruitment: My client is...

BI Project Manager - £48,000 - £54,000 - Midlands

£48000 - £54000 per annum + Benefits package: Progressive Recruitment: My clie...

VB.Net Developer

£35000 - £45000 per annum + competitive: Progressive Recruitment: If you're pa...

SAP Business Consultant (SD, MM and FICO), £55,000, Wakefield

£45000 - £55000 per annum + competitive: Progressive Recruitment: SAP Business...

Day In a Page

Screwing your way to the top? Good for Lana Del Rey for helping kill that myth

Screwing your way to the top?

Good for Lana Del Rey for helping kill that myth, says Grace Dent
Will the young Britons fighting in Syria be allowed to return home and resume their lives?

Will Britons fighting in Syria be able to resume their lives?

Tony Blair's Terrorism Act 2006 has made it an offence to take part in military action abroad with a "political, ideological, religious or racial motive"
Beyoncé poses as Rosie the Riveter, the wartime poster girl who became a feminist pin-up

Beyoncé poses as Rosie the Riveter

The wartime poster girl became the ultimate American symbol of female empowerment
The quest to find the perfect pair of earphones: Are custom, 3D printed earbuds the solution?

The quest to find the perfect pair of earphones

Earphones don't fit properly, offer mediocre audio quality and can even be painful. So the quest to design the perfect pair is music to Seth Stevenson's ears
US Army's shooting star: Lt-Col Steven Cole is the man Hollywood calls when it wants to borrow a tank or check a military uniform

Meet the US Army's shooting star

Lt-Col Steven Cole is the man Hollywood calls when it wants to borrow a tank or check a military uniform
Climate change threatens to make the antarctic fur seal extinct

Take a good look while you can

How climate change could wipe out this seal
Should emergency hospital weddings be made easier for the terminally ill?

Farewell, my lovely

Should emergency hospital weddings be made easier?
Man Booker Prize 2014 longlist: Crowdfunded novel nominated for first time

Crowdfunded novel nominated for Booker Prize

Paul Kingsnorth's 'The Wake' is in contention for the prestigious award
Vladimir Putin employs a full-time food taster to ensure his meals aren't poisoned

Vladimir Putin employs a full-time food taster

John Walsh salutes those brave souls who have, throughout history, put their knives on the line
Tour de France effect brings Hollywood blockbusters to Yorkshire

Tour de France effect brings Hollywood blockbusters to Yorkshire

A $25m thriller starring Sam Worthington to be made in God's Own Country
Will The Minerva Project - the first 'elite' American university to be launched in a century - change the face of higher learning?

Will The Minerva Project change the face of higher learning?

The university has no lecture halls, no debating societies, no sports teams and no fraternities. Instead, the 33 students who have made the cut at Minerva, will travel the world and change the face of higher learning
The 10 best pedicure products

Feet treat: 10 best pedicure products

Bags packed and all prepped for holidays, but feet in a state? Get them flip-flop-ready with our pick of the items for a DIY treatment
Commonwealth Games 2014: Great Scots! Planes and pipers welcome in Glasgow's Games

Commonwealth Games 2014

Great Scots! Planes and pipers welcome in Glasgow's Games
Jack Pitt-Brooke: Manchester City and Patrick Vieira make the right stand on racism

Jack Pitt-Brooke

Manchester City and Patrick Vieira make the right stand on racism
How Terry Newton tragedy made iron men seek help to tackle their psychological demons

How Newton tragedy made iron men seek help to tackle their psychological demons

Over a hundred rugby league players have contacted clinic to deal with mental challenges of game