The most extensive areas of natural forest are the tropical rainforests of the Amazon, Congo and Indonesia and the boreal forests, or Taiga that extend across the northern regions of Asia, Europe and North America. The tropical rainforests are often described as jungles, but this word has gone out of favour among academics. Because they are very warm and very wet for most of the year, they support a very high density and very high complexity of plant and animal life in three dimensions, the vertical dimension extending from underground burrows to the top of the tree canopy.

Plant and animal life in the boreal forests is restricted by poor soil and long, freezing winters. The number of species that can thrive in this habitat is low and can be considered within three categories: herbivores, like deer and caribou that can exist on leaves and lichen; their predators, such as wolves; and animals ranging from ground squirrels to the grizzly bear who have adapted to the stress of winter by retreating into shelters and going into hibernation.

The Boreal Forest

As a habitat for animals, the Taiga, or boreal forests are best considered together with the open tundra. The main flora of the forests are the coniferous trees, pine, spruce and larch, the leaves of which have little nutritive value, even for ruminants. A few deciduous trees, willows and birches provide digestible leaves in season. Ground cover below conifers is sparse: a few shrubs and grasses, mosses and lichens. In the summer, when the snow melts, large areas of the forest are under water and this aquatic environment supports the growth of highly nutritious vegetation. At the northern limit of the boreal forest, the trees become progressively sparse and peter out altogether at the tree line, north of which is the tundra, the zone of permafrost that, during the short summer when the snow recedes, reveals the muskeg, a water‐saturated peat bog held together by sphagnum moss and supporting the growth of some dwarf shrubs, sedges, grasses and lichens. The sodden summer environment of the forest and tundra also supports a prodigious quantity of blood sucking insects such as mosquitos and ticks. The slightly less severe southern regions of the boreal forest contain a higher proportion of deciduous trees, shrubs, aquatic plants and fruiting plants that favour bears, beavers and some of the more static Cervidae like the moose.

Cervidae

Most of the large herbivores of the boreal forest and tundra are the Cervidae. These are given different names in Eurasia and North America, and this can cause confusion. The Eurasian, semi‐domesticated reindeer is the same species as the wild North American caribou. A moose in North America is an elk in Europe. What the Canadians call an elk, or wapiti, is a large subspecies of what Scots would call a red deer.

The largest population of the Cervidae are the caribou, or reindeer. Taxonomists identify many subspecies. For our purposes, they may simply be classified into three groups: the migratory caribou, the settled forest caribou and the semi‐domesticated reindeer. The migratory caribou of North America spend the frozen winter months in the forest, where the cold and wind are less severe, browsing dead and dying leaves from the few deciduous trees and shrubs, and foraging (‘rustling’) under the snow for digestible lichens and mosses. Like most other ruminants, they exhibit nutritional wisdom. Lichens are higher in energy and mosses higher in protein. Pregnant caribou that need more protein to sustain the developing embryo, select a higher ratio of mosses to lichens. As the thaw sets in, they move out onto the tundra and gather in enormous groups, ranging in numbers from 50 000 to 500 000. During summer, they migrate across the new growth of vegetation on the tundra, travelling 20–50 km/day, perhaps 5000 km over the entire season at speeds up to 60–80km/hour. Calves born during the migration are up and running within minutes of birth. But why run so far and so fast? The conventional explanation is that the density of vegetation in the tundra is sparse and therefore they need to forage over a wide area. However, they could save a lot of energy by simply ambling forwards over the muskeg, grazing as they went. The strong inelastic motivation to run must be driven by a powerful, presumably aversive stimulus. It is difficult to escape the conclusion that the main reason they run, and run is in a vain attempt to escape the plague of external parasites, including flies and blood‐sucking insects such as mosquitoes. The behaviour of fly ridden caribou is similar to that seen in cattle when the warble fly (oestrus ovis), otherwise known as the gad fly, was endemic in the UK. This behaviour has always fascinated me as an expression of mental state. The warble fly has a distinctive sound, but it does not cause any physical discomfort to the animal at the time. Its sole purpose is to lay eggs discretely on the hair of the legs. Pain comes much later, when the egg becomes a large larva that migrates through the muscles of the animal to emerge months later through the skin of the back. The panic reaction (‘gadding’) of ruminants like cattle and caribou to these insects appears to be a hard‐wired response to a sound stimulus associated with a painful sensation that will occur well into the future.

One subspecies, the boreal forest caribou, is said to be sedentary, a strange description of an animal that spends most of its life on its feet. These animals live in the forest throughout the year where the microclimate is better, but food is scarcer. The forest caribou is recognised as an endangered species, partly as a result of loss of habitat. However, population numbers were always low compared to the caribou that spend the summer months on the tundra. The migrating caribou is undoubtedly the more successful subspecies but at a much greater cost to the individual. Predators include the wolf packs who have to run long and hard to get a kill, and golden eagles that can snatch a young calf. However, the main natural killers of the caribou are parasites, both internal and external. Heavily parasitized animals become weaker and less able to escape the wolf pack. While the wolves may have struck the final blows, the parasites were the agents of their destruction. In strictly Darwinian terms, the migrating caribou must be classified as a highly successful species, but it is a strategy that carries a high cost to the individuals. One cannot attribute their success to a highly developed mind.

The reindeer of northern Eurasia are the same species as the North American caribou. They have been domesticated for at least 3000 years, initially as a source of meat and hides. Habitat and diet are essentially the same in the Eurasian and American Arctic, the difference being that in Eurasia, migration between summer and winter grazing sites is managed by the reindeer herders. For the most part, the animals are herded en masse and given little or no personal attention, yet individual reindeer can be trained to be ridden, carry loads, pull sleighs and entertain children as Santa’s helpers. Selection over millennia for genetic traits favouring domestication will have led to a divergence in personality between the amiable reindeer and the wild caribou but this does not address the question as to how this domestication began. One possibility is that the human population was higher in the Eurasian arctic so that humans rather than wolves became the apex predator. In these circumstances, it would have favoured both species to evolve a form of coexistence based on herding rather than hunting.

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