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)

PROMOTED VIDEO
Arts and Entertainment
tv
News
Katie Hopkins appearing on 'This Morning' after she purposefully put on 4 stone.
peopleKatie Hopkins breaks down in tears over weight gain challenge
Life and Style
fashionModel of the moment shoots for first time with catwalk veteran
Life and Style
fashionAngelina Jolie's wedding dressed revealed
News
i100
Sport
Alexis Sanchez, Radamel Falcao, Diego Costa and Mario Balotelli
footballRadamel Falcao and Diego Costa head record £835m influx
News
i100
News
ebooksAn unforgettable anthology of contemporary reportage
Arts and Entertainment
Madame Vastra and Jenny Flint kiss in Doctor Who episode 'Deep Breath'
tv
Arts and Entertainment
Olivia Colman topped the list of the 30 most influential females in broadcasting
tv
Life and Style
techIf those brochure kitchens look a little too perfect to be true, well, that’s probably because they are
Arts and Entertainment
Danish director Lars von Trier
tvEnglish-language series with 'huge' international cast set for 2016
Life and Style
tech
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

Secondary supply teachers required in Wisbech

£21000 - £35000 per annum: Randstad Education Cambridge: Secondary teachers ne...

PPA Cover Teachers Required in Doncaster

Negotiable: Randstad Education Sheffield: Primary PPA Teachers required for wo...

Maths teachers needed for supply work in Ipswich

£21000 - £35000 per annum: Randstad Education Cambridge: Maths teachers requir...

Executive Assistant/Events Coordinator - Old Street, London

£35000 - £38000 per annum + benefits: Ashdown Group: Executive Assistant/Event...

Day In a Page

'I’ll tell you what I would not serve - lamb and potatoes': US ambassador hits out at stodgy British food served at diplomatic dinners

'I’ll tell you what I would not serve - lamb and potatoes'

US ambassador hits out at stodgy British food
Radio Times female powerlist: A 'revolution' in TV gender roles

A 'revolution' in TV gender roles

Inside the Radio Times female powerlist
Endgame: James Frey's literary treasure hunt

James Frey's literary treasure hunt

Riddling trilogy could net you $3m
Fitbit: Because the tingle feels so good

Fitbit: Because the tingle feels so good

What David Sedaris learnt about the world from his fitness tracker
Saudis risk new Muslim division with proposal to move Mohamed’s tomb

Saudis risk new Muslim division with proposal to move Mohamed’s tomb

Second-holiest site in Islam attracts millions of pilgrims each year
Alexander Fury: The designer names to look for at fashion week this season

The big names to look for this fashion week

This week, designers begin to show their spring 2015 collections in New York
Will Self: 'I like Orwell's writing as much as the next talented mediocrity'

'I like Orwell's writing as much as the next talented mediocrity'

Will Self takes aim at Orwell's rules for writing plain English
Meet Afghanistan's middle-class paint-ballers

Meet Afghanistan's middle-class paint-ballers

Toy guns proving a popular diversion in a country flooded with the real thing
Al Pacino wows Venice

Al Pacino wows Venice

Ham among the brilliance as actor premieres two films at festival
Neil Lawson Baker interview: ‘I’ve gained so much from art. It’s only right to give something back’.

Neil Lawson Baker interview

‘I’ve gained so much from art. It’s only right to give something back’.
The other Mugabe who is lining up for the Zimbabwean presidency

The other Mugabe who is lining up for the Zimbabwean presidency

Wife of President Robert Mugabe appears to have her sights set on succeeding her husband
The model of a gadget launch: Cultivate an atmosphere of mystery and excitement to sell stuff people didn't realise they needed

The model for a gadget launch

Cultivate an atmosphere of mystery and excitement to sell stuff people didn't realise they needed
Alice Roberts: She's done pretty well, for a boffin without a beard

She's done pretty well, for a boffin without a beard

Alice Roberts talks about her new book on evolution - and why her early TV work drew flak from (mostly male) colleagues
Get well soon, Joan Rivers - an inspiration, whether she likes it or not

Get well soon, Joan Rivers

She is awful. But she's also wonderful, not in spite of but because of the fact she's forever saying appalling things, argues Ellen E Jones
Doctor Who Into the Dalek review: A classic sci-fi adventure with all the spectacle of a blockbuster

A fresh take on an old foe

Doctor Who Into the Dalek more than compensated for last week's nonsensical offering