This
animal
-
controls
the rhythm of animal life on the tundra, because it is an important
food for ermines, arctic foxes, Snowy Owls, Gyrfalcons, and
jaegers
-
remains
active throughout the arctic winter without freezing to death
-
can
reproduce within weeks of its birth
-
has
roughly four year cycles of drastically fluctuating populations
Description
Lemmings are mouselike rodents that live in treeless areas of northern
Canada. They have short ears, largely hidden in the fur, short legs,
and short tails. Adult brown lemmings are about 150 mm in total
length, including about 20 mm of tail. Their body weight varies
from about 55 g in some years to about 115 g in others. Their fur
is a full brown and grey summer and winter. Collared lemmings are
the same overall size as brown lemmings but with a shorter tail
(about 15 mm). Their colour changes with the seasons (hence their
other common name, varying lemming). In summer, a collared
lemming has a black nose, grey cheeks, tawny ear spots, a chestnut
collar, and a more or less prominent black dorsal stripe. With the
autumn moult, however, the summer coat is replaced by a solid white
winter one and the front feet develop two greatly enlarged claws,
presumably to help dig through the hard-packed tundra snow.
Habitat
and habits
The smallest of the mammals of the High Arctic, lemmings are key
species in arctic ecosystems. For unknown reasons, lemming populations
fluctuate drastically, peaking about every four years and then crashing
almost to extinction. Because the small bodies of lemmings are important
food for ermines, arctic foxes, Snowy Owls, Gyrfalcons, and jaegers,
this mysterious cycle controls the rhythm of animal life on the
tundra.
Most
of the range occupied by lemmings is underlain by permafrost, or
soil that is always frozen, often within a few centimetres of the
surface. This means that the lemmings are unable to dig deep burrows
for shelter even in summer. Where the soil contains much water,
however, seasonal freezing and thawing creates ridges and depressions
that lemmings use for burrows and as travel routes, respectively.
Brown and collared lemmings in the same general area tend to choose
different habitats in summer. Collared lemmings use higher and drier
sites, and brown lemmings the lower and wetter ones. This segregation
coincides with the distributions of preferred forage; for example,
depending on what is available, collared lemmings might seek out
willows and cranberries, and brown lemmings prefer sedges, arctic
cotton, and certain mosses. In winter, the habitat segregation tends
to break down as collared lemmings move to lower ground where the
snow is deeper and provides more shelter.
The
long arctic winter is a critical time for lemmings because, unlike
many species of temperate rodents, they do not hibernate. It is
amazing that these small, warm-blooded animals remain active throughout
the arctic winter without freezing to death. Their short appendages
(ears, legs, tails) are an adaptation to reduce heat loss, and their
winter fur is thicker than that of summer. As winter approaches,
lemmings make large, globular nests of finely shredded grasses and
sedges on the surface of the ground, which provide additional insulation
when they are not out hunting for food. Snow provides critical insulation.
Lemmings forage in the space that forms between soil and snow, known
as subnivean space, almost never appearing on the surface. In the
High Arctic, temperatures at the soil-snow interface are not exactly
warm (-25°C), but they are better than those above the snow,
and the difference is a key to lemming survival.
Unique
characteristics
Lemming
populations have long been known to fluctuate drastically. Peak
numbers tend to recur about every four years. Furthermore, numbers
are high over a huge area: for example, 1960 was a "lemming
year" for almost all of the Canadian Arctic. All sorts of reasons
for the cycles have been suggested, from changes in the number of
sunspots to snow conditions. Weather is a likely, but still unproven,
trigger. Winter creates problems for lemmings, the amount and timing
and distribution of snow mitigate those problems, and peak numbers
occur only following winter breeding. Unfortunately, no one has
yet studied the role of snow cover in sufficient detail to prove
that it causes the cycle. We do know that on Devon Island in Nunavut,
collared lemmings bred during winter 197273, when the temperature
under the snow fell below -20°C, and their population peaked
the following spring.
A
remarkable feature of the lemming cycle is the extreme scarcity
of individuals at the "low point" of the cycle. Although
several species of small rodents that live in temperate climates
also reach peaks of abundance about every four years and some of
them reach much higher densities at the peak than lemmings do, none
can equal the extreme scarcity of lemmings at the low point. Such
extreme scarcity raises the possibility of extinction. But passing
through a population "bottleneck" probably strongly favours
the individuals best adapted to survival in harsh arctic conditions.
The cycle of every four years or so may be a device to keep selection
abreast of the changes continually going on in the Arctic.
An
early theory was that regular cycles of scarcity and abundance resulted
from the interaction between a predator and its prey. When the prey
became numerous, the predators brought down their numbers, which
then resulted in death by starvation for the predator. However,
the shoe now seems to be on the other foot. We know that nesting
success of Snowy Owls and survival of arctic fox pups are both related
to lemming abundance. Both owls and foxes produce very few, if any,
surviving young except in "lemming years." The generation
of foxes born in a "lemming year" sustains the fox population,
even though its numbers gradually decline, until the next lemming
peak.
Another
early theory was the obvious one of epidemic disease periodically
sweeping through the lemming population. The larger the population
and the more contact between individuals in overcrowded conditions,
the easier the spread of an infection. Unfortunately, no one has
found a disease that is rampant in all declining lemming populations.
During some declines disease is virtually absent.
Another
obvious candidate is the interaction between lemmings and their
food supply. As lemming numbers increase so does damage to the vegetation.
Ultimately, the food supply is no longer able to sustain the population.
Following a massive die-off of lemmings, the vegetation is able
to recover, which sets the stage for a new cycle. The quantity and
quality of available food are known to vary with the stage of the
lemming cycle, but proof of cause and effect is still lacking.
In
recent years, researchers have focused on changes in the animals
themselves. The first measurable evidence came from noting changes
in average weights of individuals in different phases of the cycle.
In a number of species of small mammals, the largest individuals
are found in the spring of the peak year. Researchers are now looking
for more subtle changes. For example, increasing density produces
more social interaction between individuals, which induces stress,
which results in altered hormone levels, which may interfere with
reproduction or alter behaviour. Stress itself may lead to increased
mortality. Lemmings tend to be aggressive toward one another. If
the behavioural alteration were in the direction of an increase
in aggression, fewer lemmings would be born and more would be killed
by their own kind.
In
Scandinavia, lemmings become restless in years when their populations
are high. In the mountainous terrain of Norway, for example, when
lemmings begin to move they tend to go downhill and get funneled
into valleys. The result is that large numbers eventually reach
the sea or a large lake. They may proceed onto sea or lake ice or
jump into the water, which has given rise to the popular conception
that they are committing mass suicide to relieve a problem of overpopulation.
There is, however, no authentic account from the North American
Arctic to back up such a belief. Most of our Canadian lemmings live
on rather flat terrain and too far from the ocean to make such migrations
possible. The Inuit have no legends about migrating lemmings and
it is difficult to believe that they would have overlooked such
an event, especially if it occurred repeatedly.
It
is certainly true that in the spring of a high population year individual
lemmings will often be seen on lake and sea ice, but they do not
move in an orientated manner, all going north or all going south
like migrating birds, and large groups are never seen. Once on the
ice, individuals run rapidly and tend to move in straight lines.
Lemmings have been seen on sea ice as far as 55 km from the nearest
land. We do not understand why lemmings would move onto sea and
lake ice in the spring of peak years, but spring is a time of social
upheaval caused by the environmental changes associated with snow
melt, and the physiological changes associated with onset of the
breeding season.
Range
There
are three lemming species in the Canadian Arctic. Two species occur
on the mainland tundra west of Hudson Bay and in the southern part
of the arctic archipelago: the brown lemming Lemmus sibiricus, whose
range extends southwards in mountainous areas (see map), and the
collared (or varying) lemming Dicrostonyx torquatus, which has colonized
the Queen Elizabeth Islands right to the northern tip of Ellesmere
Island. The collared (or varying) lemmings that inhabit the Ungava
Peninsula are usually considered to be a separate species Dicrostonyx
hudsonius.
Similar
lemming species are found in other circumpolar countries, such as
Norway, Greenland, and Russia. In taxonomy, or the science of classifying
organisms, brown and varying lemmings are classified as microtines,
along with the muskrats, bog lemmings, and voles of southern Canada.
Breeding
Lemmings of both sexes are able to reproduce within weeks of their
birth. The proportion that reproduces in the summer of their birth
varies widely from year to year, and seems to be related to population
density. After a year, a female is capable of producing three litters
of young even in the short arctic summer, but most fail to do so.
The length of the summer breeding season is related to population
density. When numbers are low, breeding continues into September;
when numbers are high, breeding may end in July.
Lemmings
sometimes breed in the winter, but there is always a pause in spring
and fall separating summer and winter breeding. How such a small
mammal, already under a severe thermal stress, can muster enough
energy to breed in an arctic winter, and what factors determine
when winter breeding will occur, are still mysteries.
Conservation
Although lemmings have lived for up to three years in captivity,
probably no lemming survives more than one winter in the wild. Wild
predators likely kill most of them (except perhaps during major
die-offs, when other factors, such as starvation, may come into
play). In the snow-free season, arctic foxes, ermines, Snowy Owls,
jaegers, and Gyrfalcons all take their toll. Wolves may take the
occasional individual, and even caribou and fish have been known
to prey on lemmings. Although life is more secure under the winter
snow, several investigators who were on the tundra when the snow
was melting have discovered remains of lemming nests that showed
signs of ermine predation. The ermine is completely at home under
the snow. Researchers who spent four winters in the early 1970s
on Devon Island, in todays territory of Nunavut, found that
between 5 and 16 percent of lemming nests had been attacked by ermines.
Even higher rates of predation by ermines have been recorded on
Banks Island in the Northwest Territories and in Alaska. Snowy Owls,
the only raptors, or birds of prey, present in winter, are poorly
equipped for digging through snow, so an owl could only strike a
lemming on the snow surface. The small arctic fox can dig through
the wind-packed tundra snow, but the energy cost is high for such
a small meal.
In
general, lemmings are not threatened by human activity except locally
around villages, mines, oil wells, and other industrial sites. Adverse
weather conditions probably kill a fair number. In fall, early onset
of low temperatures in the absence of snow is potentially lethal.
And in spring, during snow melt, when the insulating quality of
snow declines, lemmings can find themselves at the mercy of the
elements if the weather turns nasty. Winter nests may be full of
water, and summer burrows plugged with ice due to refreezing of
the melting snow. Although lemmings are known to suffer from a number
of infectious diseases and to harbour a variety of parasites, relatively
few lemmings die of diseases or parasites.
Inuit
do not eat lemmings, nor do they make any use of their small skins.
However, those Inuit who support themselves in whole or in part
by trapping benefit indirectly from the "run" of arctic
foxes that follows each lemming peak.
One
of the Inuit names for the collared lemming is kilangmiutak, which
means "one-who-comes-from-the-sky." The legend of lemmings
falling from the sky is common to Inuit all across the North American
Arctic and Scandinavia. It probably arose because of the sudden
appearance of lemmings when the snow melts following a winter of
intensive reproduction. Lemmings, particularly the collared lemming
with its presumed origin from the sky, were sometimes used by shamans
as a source of supernatural powers.
Lemmings
are a vital part of the rather simple web of life on the tundra,
and they help to teach us how intricate even that simple ecosystem
is. Their burrowing changes the arctic soil. Their feeding habits
influence the composition of the plant community on the tundra.
And trappers appreciate them for another reason. Because arctic
fox numbers rise and fall according to the abundance of lemmings,
the income of people who depend on fox trapping for a livelihood
is linked to lemmings. The influence of lemmings extends far beyond
the Arctic. A lemming population decline may produce a surplus of
Snowy Owls that will, for a time, flood into southern Canada to
the delight of bird watchers. Meanwhile, the tundra ecosystem is
gathering its resources in preparation for the outburst of energy
that will come with the next lemming year.
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