One of the basic rules of biodiversity is that species diversity increases toward the tropics and decreases toward the poles.

An extension of this is that species in the tropics tend to have very small ranges compared with species in the boreal or tundra zones of the Arctic. In the tropical forest region of Brazil, the branches of a river are a sufficient barrier, a strong enough isolating force, to isolate individual species of tamarins, marmosets and other small primates in small pockets of habitat. A tropical plant species may have a range restricted to one particular valley on one range of hills.

The tropical regions of the world, in South America, Africa and Southeast Asia, though rich in species, are so far apart geographically that they share few species in common. Years of experience in the tropical forests of Southeast Asia are no preparation at all for a walk in a tropical forest in South America: The species one finds in these areas are all totally different.

Visit the far northern regions though, the great boreal or taiga forests of Eurasia or North America, or the narrow strip of tundra around the same northern continents, and one finds considerable overlap in species -- even on different continents. These northern landmasses have relatively recently all been connected in one way or another; the Bering land bridge between Alaska and Siberia, for example, existed until about 10,000 years ago.

These regions are so far north and the conditions so tough that relatively few species survive here. Those that are successful find similar environmental conditions stretching from Siberia to Scandinavia, and across Alaska and Canada, so if they can survive on one continent they can almost certainly survive on the other. Some of the most familiar northern mammal species, such as the polar bear, the musk ox and the Arctic fox occur right across this Holarctic region.

The cross-continental distribution is not confined to mammals. On my first journey north from my British homeland to Iceland, I encountered a low, heathlike plant called crowberry, which is common across the treeless Icelandic moorland. The crowberry's small black berries are an important autumn food for birds and mammals. Later, I found the same plant growing in similar habitat in northern Norway and northern Finland. Then last summer, while botanizing on the tundra of northeast Siberia and on various islands off northwest Alaska, I came across the very same species.

There were other species in the same plant community that I recognized. I had never been to the region before, yet I found a number of familiar species, many thousands of kilometers from where I had first encountered them.

Recently in Jasper National Park, Canada, I encountered another continent-spanning species in the shape of a very confiding female moose and her calf. The first snows of the winter had fallen in the Maligne Valley and the cow and calf were clearly more concerned about finding tasty bushes to browse on than they were about me and my camera.

The moose (Alces alces, or in Japanese, hera-jika) is the largest member of the deer family. These enormous creatures have a most distinctive shape, tall at the shoulder, but sloping down to the hips; long-legged, short-necked, and with a massive rather horselike head. Another strange feature is the fleshy dewlap, known as the "bell," which hangs from the throat. They stand 1.5 to 2 meters tall at the shoulder, measure up to 2.7 meters long and weigh up to 820 kg, which makes them the second largest native North American mammal after the bison.

For all their size, they are usually shy. It is not often that one needs to back off from a moose to obtain a good shot.

If you are familiar with the confectionery confusion between North America and England (biscuit = scone; cookie = biscuit, and so on) then you may not be surprised by the confusion in common names of mammals. What is a moose in North America is an elk in Europe, while an elk in North America is an entirely different animal, also known as the wapiti, which is closely related to the animal the Europeans call the red deer.

Whatever the name, though, the North American moose and Eurasian elk are one and the same species, and this is another species with an extraordinarily large range spanning the northern parts of both great northern continents.

Moose prefer being close to water and they will often wade into lakes and streams to feed on submerged aquatic plants (and, in summer, to escape from the unwelcome attention of biting flies). They have also been known to dive underwater to reach aquatic plants.

Their main diet consists of grasses, herbs, tree bark and, like the mother and calf I watched in the Rockies, they will browse on buds and the growing tips of shrubs and bushes.

Although the females and calves lack antlers, the males make up for them by carrying enormous palmate racks averaging 160 cm across, with as many as 30 projecting tines, and weighing an incredible 20 kg. Despite their enormous size, the bulls shed their antlers each year. Once the autumn rut is over they drop the old rack, in November and December, and grow a new set the following year. At first the new antlers are covered with a smooth skin of velvet which protects the supply of blood in its network of vessels, though this later dries and is scraped off, leaving the new antlers to be used to establish their dominance among other males, and during the mating season to attract females.

When they were intensively hunted for their trophy antlered heads and meat, the range of the Eurasian elk and North America moose contracted considerably. Now, with hunting legally restricted, their numbers are increasing in many areas and they are re-extending their range. Some are finding their way into suburban and even urban areas, especially during the winter.

If the modern moose seems large, consider the extinct Megaceros of Europe, many fossilized remains of which have been unearthed from the bogs of Ireland, giving rise to the name "Irish elk," though it was distributed throughout Europe. It was even larger than the moose, measuring 3 meters in length, weighing over 800 kg and with antlers spanning well over 2 meters. Though they survived to be contemporary with early humans, for some reason still unknown Megaceros died out, leaving their smaller cousins, the moose, to be the largest of the world's deer.

Moose are usually solitary, with the exception of cows traveling with their calves. When they are not solitary, what should we call them? After all, the plural of mouse is mice, so when I saw that cow and calf together did I really see a pair of "meese?"

If you have questions or comments, or know what the collective term for a group of moose is, you can reach me via e-mail at [email protected] or via snail mail care of The Japan Times.