Science: What sends a lemming over the edge?: Malcolm Smith reports on a likely explanation for the mass suicides of these northern rodents

Devotees of the computer game 'Lemmings' know how difficult it is to ensure that these stubby-tailed rodents arrive safely at their destination. In the wild - in Scandinavia and north-west Russia - it is impossible.

These little rodents, no more than five inches in length, undergo one of the best-known cyclical fluctuations in their numbers that nature ever invented. Wildlife documentary after wildlife documentary has shown close-up footage of thousands of the little, vole-like mammals careering headlong into rivers and the sea - frequently to certain death. The question is, why?

Lemmings are cold-climate mammals. They live in moist, stony, tundra vegetation consisting of sedges, willow scrub and dwarf birch. For half the year - often longer - their environment is frozen and snow-covered.

Northern Europe and north-west Asia have three species, of which the best-known is the Norway lemming - the film star. Usually nocturnal, it has a stumpy tail, short legs, and a round body covered in thick, yellow-brown fur patterned with dark brown streaks and patches.

Every three to five years, lemming numbers peak, then suddenly decline again. At these population peaks, the excess number of rodents often move down from their mountain tundra homes to valleys and spread out, sometimes en masse. Eventually, some reach the sea and attempt to continue this frenzied dispersal. With uncanny timing, the wildlife film-makers are usually on the spot.

All sorts of theories to explain these enormous population fluctuations have been put forward. They include predation, nutrition, disease, parasites, competition and genetic changes. None has been proven. Another theory - that lemming population swings are related to chemical changes in the plants they eat - had not been tested - until recently.

Tarald Seldal and Goran Hogstedt of the University of Bergen, and Knut-Jan Andersen of Haukeland Hospital in Bergen, have done just that. Their findings provide very strong circumstantial evidence that chemicals produced by the plants grazed by Norway lemmings control the ups and downs these rodents have to cope with.

Lemmings graze plants like stiff sedge and cotton grass. Both are abundant in the Scandinavian tundra, and both produce defensive chemicals when damaged. The heavier the grazing - in other words the damage - the more of these chemicals they produce. Tundra plants are not unique in this; many plants produce such defensive substances.

Some of these chemicals are proteins which inhibit the activity of proteases (enzymes which break down protein in food) in the intestines of mammals. The most important feature of these inhibitors is that they put a stop to the action of trypsin, a protease secreted by the pancreas.

Grazing mammals ingesting these inhibitors can't digest proteins in their food. To make matters worse, reduced protein digestion stimulates more production - by the pancreas - of proteins which, in turn, are inhibited and lost in the faeces. These enzymes contain large amounts of essential dietary amino acids which are rare in the plants eaten. So lemmings and other grazing mammals, though they may eat voraciously, slowly starve. An enlarged pancreas is an associated symptom.

The Norwegian researchers found that levels of the inhibitor in these plants rose considerably in known years of peak lemming numbers and remained high during the following year of their decline, falling back to a very low level in the next year when lemming numbers again returned to a population low.

Lemmings taken from a declining population had pancreases nearly three times the normal size. They also had retarded growth, 25 per cent of them dying within 10 days of capture despite consuming more than 10 times their own body weight in food each day.

It is probable that the high trypsin inhibitor levels in peak and early decline lemming years can also explain the rodents' delayed sexual maturation, compressed breeding season and high dispersal rates at such times.

To cause the cyclical changes in lemming numbers, there has to be a link between high rodent numbers and the quantity of the inhibitor produced. There is, and it is related to grazing densities.

Cotton grasses and sedges grazed very occasionally produced inhibitors for only a short period of time, peaking around 30 hours after each bit of damage. But, if lemming numbers are high, and the plants are frequently grazed, more inhibitor is produced and it stays at high concentrations for as long as the grazing continues.

So low densities of lemmings can eat, digest their proteins and reproduce to their hearts' content. But once their populations start to climb, as inevitably they do, their food plants take defensive action. Mass starvation, and a frenzied search for pastures new, is the result.

If it is any consolation, the roller-coaster of lemming control at least guarantees that there will always be plants for them to live on. But, you might well ask, isn't it a wonder that evolution hasn't seen to it that the plant and the plant-eater reach a more stable accommodation? Of course not, otherwise what would the wildlife film-makers do with themselves?

(Photograph omitted)

Start your day with The Independent, sign up for daily news emails
New Articles
tvDownton Abbey Christmas special
Arts and Entertainment
Wolf (Nathan McMullen), Ian (Dan Starky), The Doctor (Peter Capaldi), Clara (Jenna Coleman), Santa Claus (Nick Frost) in the Doctor Who Christmas Special (BBC/Photographer: David Venni)
tvOur review of the Doctor Who Christmas Special
News
peopleIt seems you can't silence Katie Hopkins, even on Christmas Day...
News
news
PROMOTED VIDEO
News
i100
Arts and Entertainment
Left to right: Stanley Tucci, Sophie Grabol and Christopher Eccleston in ‘Fortitude’
tvSo Sky Atlantic arrived in Iceland to film their new and supposedly snow-bound series 'Fortitude'...
Arts and Entertainment
Jenna Coleman as Clara Oswald in the Doctor Who Christmas special
tvForget the rumours that Clara Oswald would be quitting the Tardis
Arts and Entertainment
Japanese artist Megumi Igarashi showing a small mascot shaped like a vagina
art
News
The Queen delivers her Christmas message
newsTwitter reacts to Her Majesty's Christmas Message
Arts and Entertainment
tv
Life and Style
fashion
ebooks
ebooksA year of political gossip, levity and intrigue from the sharpest pen in Westminster
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

Recruitment Genius: Account Manager

£20000 - £35000 per annum: Recruitment Genius: This full service social media ...

Recruitment Genius: Data Analyst - Online Marketing

£24000 - £35000 per annum: Recruitment Genius: We are 'Changemakers in retail'...

Austen Lloyd: Senior Residential Conveyancer

Very Competitive: Austen Lloyd: Senior Conveyancer - South West We are see...

Austen Lloyd: Residential / Commercial Property Solicitor

Excellent Salary: Austen Lloyd: DORSET MARKET TOWN - SENIOR PROPERTY SOLICITOR...

Day In a Page

A Christmas without hope: Fears grow in Gaza that the conflict with Israel will soon reignite

Christmas without hope

Gaza fears grow that conflict with Israel will soon reignite
After 150 years, you can finally visit the grisliest museum in the country

The 'Black Museum'

After 150 years, you can finally visit Britain's grisliest museum
No ho-ho-hos with Nick Frost's badass Santa

No ho-ho-hos with Nick Frost's badass Santa

Doctor Who Christmas Special TV review
Chilly Christmas: Swimmers take festive dip for charity

Chilly Christmas

Swimmers dive into freezing British waters for charity
Veterans' hostel 'overwhelmed by kindness' for festive dinner

Homeless Veterans appeal

In 2010, Sgt Gary Jamieson stepped on an IED in Afghanistan and lost his legs and an arm. He reveals what, and who, helped him to make a remarkable recovery
Isis in Iraq: Yazidi girls killing themselves to escape rape and imprisonment by militants

'Jilan killed herself in the bathroom. She cut her wrists and hanged herself'

Yazidi girls killing themselves to escape rape and imprisonment
Ed Balls interview: 'If I think about the deficit when I'm playing the piano, it all goes wrong'

Ed Balls interview

'If I think about the deficit when I'm playing the piano, it all goes wrong'
He's behind you, dude!

US stars in UK panto

From David Hasselhoff to Jerry Hall
Grace Dent's Christmas Quiz: What are you – a festive curmudgeon or top of the tree?

Grace Dent's Christmas Quiz

What are you – a festive curmudgeon or top of the tree?
Nasa planning to build cloud cities in airships above Venus

Nasa planning to build cloud cities in airships above Venus

Planet’s surface is inhospitable to humans but 30 miles above it is almost perfect
Surrounded by high-rise flats is a little house filled with Lebanon’s history - clocks, rifles, frogmen’s uniforms and colonial helmets

Clocks, rifles, swords, frogmen’s uniforms

Surrounded by high-rise flats is a little house filled with Lebanon’s history
Return to Gaza: Four months on, the wounds left by Israel's bombardment have not yet healed

Four months after the bombardment, Gaza’s wounds are yet to heal

Kim Sengupta is reunited with a man whose plight mirrors the suffering of the Palestinian people
Gastric surgery: Is it really the answer to the UK's obesity epidemic?

Is gastric surgery really the answer to the UK's obesity epidemic?

Critics argue that it’s crazy to operate on healthy people just to stop them eating
Homeless Veterans appeal: Christmas charity auction Part 2 - now LIVE

Homeless Veterans appeal: Christmas charity auction

Bid on original art, or trips of a lifetime to Africa or the 'Corrie' set, and help Homeless Veterans
Pantomime rings the changes to welcome autistic theatre-goers

Autism-friendly theatre

Pantomime leads the pack in quest to welcome all