IV.

Fauna

The same or closely related species of animal tend to be found in tundra environments around the world. Musk oxen, caribou, and reindeer are the main large grazers feeding on grasses, sedges, willows, and lichens. Hares and lemmings are common consumers of grasses and sedges, while predators include polar bears, wolves, lynxes, foxes, eagles, hawks, falcons, and owls. Many migratory birds breed on the tundra during the brief summer, feeding on the flush of seeds and berries and/or the rapidly emerging populations of mosquitoes, black flies, blow flies, springtails, weevils, beetles, and spiders. Alpine tundra supports mountain goats, bighorn sheep, ibex, chamois, foxes, wildcats, pikas, marmots, ground squirrels, rabbits, voles, many summer-visiting birds, and the resident ptarmigan, a high-altitude version of the red or willow grouse that feeds mainly on willow buds and the succulent parts of other dwarf shrubs. Flies are scarce on alpine tundra, but butterflies, beetles, and grasshoppers are often abundant.

Animals have evolved a number of ecological strategies and physiological adaptations to cope with the harsh tundra environment. The most obvious strategy is to migrate to the tundra when the climate warms in the summer and when food supplies are at their maximum, and leave when temperatures begin to fall. Barren-ground caribou, for example, migrate in large herds from the fringes of the taiga to feed on the flush of plants hurriedly flowering and setting seed during the short summer growing season. Dwarf willows, blueberries (bilberries), cranberries, grasses, sedges, and carpets of lichen all develop rapidly at this time providing fodder for grazers and berries and seeds for other mammals and birds. Predatory wolves track the herds on their northward migration, while foxes clean up carrion and the sickly.

The migration of close to 100 species of breeding bird to the tundra coincides with the spring flush of vegetation and the emergence of insects from the soil and from beneath the regenerating carpet of plants. The herbivorous willow grouse makes only a short journey to the tundra from the evergreen forests to the south, but Arctic terns travel 40,000 km (25,000 mi) twice a year from the Arctic to the Antarctic and back again. Visiting birds include grouse, terns, ducks, geese, and waders that, along with rapidly multiplying populations of small rodents, draw in predatory eagles, peregrines, merlins, and owls.

The emergence of insects on which many summer visiting birds and their chicks rely is triggered by the springtime melting of ice at the soil surface that is unable to drain away because of the impermeable barrier of underlying permafrost. This creates boggy conditions ideal for invertebrate larvae, which hatch from eggs deposited in the previous year. Each small patch of tundra may be home to millions of herbivorous springtails, plant-sucking weevils, carnivorous beetles and spiders, and detritus-eating blow flies, dung beetles, and burying beetles. The most notorious insects of the tundra, however, are the mosquitoes and black flies, which rise in summer from the surface of pools and marshy areas like clouds of smoke. Female mosquitoes possess the piercing mouthparts used to suck blood from their victims. They can prove such an irritation to caribou that some individuals are driven to distraction.

A few animals are hardy enough to stay on the tundra all year round. The largest resident grazers are musk oxen, which survive the bitter Arctic winter by insulating themselves with thick layers of fur and fat. Young musk oxen are born with huge deposits of heat-producing (thermogenic) fat in their bodies and regularly hide beneath their mothers’ shaggy skirts of fur for extra protection from the penetrating winds. Adult musk oxen also huddle together in groups for warmth and, when threatened by predators, will often form a protective circle with the young calves at the centre.

Arctic foxes are also resident on the tundra, feeding mostly on the lemming population, which finds refuge in the winter beneath the insulating mantle of snow. Relative to their size, Arctic foxes have the thickest pelts in the animal kingdom and are stockier than their temperate counterparts with snub noses and tiny ears to reduce the surface area of skin in contact with the cold air. Like many other cold-climate animals, Arctic foxes have evolved an ingenious method of preventing heat escaping from their paws into the freezing snow. A fine network of blood vessels at the top of the leg called the rete mirabile (“wonderful net”) carries cool blood back to the body from the feet and warm blood in the opposite direction. The blood vessels come into such close contact in the rete that heat flows from the leg-bound blood, cooling it before it reaches the feet, while at the same time warming the ascending blood before it re-enters the torso. In this way, the paws of an Arctic fox are kept constantly at around 0^o C (32° F) or slightly higher, while blood in the animal’s body remains at 38^o C (100° F). Because there is little difference in temperature between paws and ground (that is, a negligible thermal gradient) little heat escapes from the animal’s legs into the environment. Caribou have similar heat-exchangers and a special type of fat in their lower legs which, unlike the fat in their bodies, remains pliable at low temperatures. Inuit have long appreciated the difference and used the marrow-fat at the top of Caribou legs as a solid food and the fat extracted from the feet as a fluid lubricant.