The oldest fossil martens date from the Pliocene; the epoch immediately before the great Pleistocene Ice Ages. By this time, they had already begun to develop some of their distinctive features. Martens are much larger than true weasels, and within, or slightly above, the size range of the largest mustelines, the European polecats. Where mustelines have evolved narrow bodies for chasing prey down into their burrows, the martens have never needed to do so, and while they have the short legs typical of most members of the family, their bodies are noticeably more compact. They are generally brown in colour, and most species have a highly visible 'bib' of paler - usually yellowish - fur on their chest.
The most distinctive feature of the martens is that they spend much of their lives in the trees, quite a different habitat from other members of the family. By heading up into the trees, they have been able to find a source of food quite away from their ground-dwelling kin, and over much of their range (although by no means all), they are the only small carnivores that hunt among the branches.
Their adaptations for the arboreal life include hairy paws with long, semi-retractile claws, giving them a good grip on branches. Indeed, the whole shape of their feet is different from that of true weasels. The latter are "digitigrade", walking on tip toe, as cats and dogs do. Being digitigrade gives an animal speed, a useful feature in a carnivore that must sometimes chase its prey, but it means that only a small part of the foot is in contact with the ground. That's not an issue for an animal on a steady surface, but grip is especially important to martens. As a result, while they don't walk fully on the soles of their feet as humans or bears do, and often rise into a digitigrade stance while running along the ground, their feet are arranged so that they can at least partially use their furry soles to better clasp onto branches when they need to.
Martens also have long, bushy, tails, and this, too, is an adaptation to their chosen lifestyle. The tail acts as a counterbalance while moving swiftly along or between branches, aided by the fact that their bodies are less elongated than those of weasels. Taken together, these features enable martens to perform remarkable feats of athleticism, leaping from branch to branch with great alacrity.
Perhaps the best known marten is the pine marten (Martes martes). Despite their name, pine martens are as at home in broadleaf deciduous forest as they are in pine forest, and they are found almost everywhere in Europe that there is sufficient woodland, avoiding only the drier Mediterranean climates of Greece, southern Portugal, and most of Spain. They are, however, found on many of the Mediterranean islands, including Sicily, Sardinia, Corisca, and the Balearic Islands. It's possible that they were introduced to these islands by humans, and have prospered there largely because there's nothing native there that's any better a dry woodland carnivore than they are, even if it isn't an ideal environment for them. On the other hand, it has been argued that the pine martens of Minorca are distinct enough to be considered their own subspecies, which would imply that they have been there a very long time indeed.
Outside of Europe, pine martens are found across Turkey, and as far east as the more fertile parts of eastern Iraq and northern Iran. Less surprisingly, perhaps, they also inhabit the forested slopes of the Caucasus Mountains and the great woodlands of northwestern Russia.
Although pine martens need forest to survive, they don't like the woodland to be too dense. Instead, they prefer a more open habitat, with sizeable gaps between at least some of the trees, allowing enough sunlight to filter down to the forest floor to encourage a dense undergrowth to form. That's because pine martens don't just hunt in the trees, but also on the ground. While their favourite food always seems to be rodents, for example, in parts of their range they eat more voles than squirrels.
Indeed, in general, pine martens seem to be quite adaptable animals, and what they eat depends a lot on where they are. They are also willing to change their diet throughout the year, taking advantage of what's available. They are less purely carnivorous than weasels, and, in parts of their range will switch to eating fruit in the autumn. During winter, they are more likely to eat carrion, or even earthworms, and they may prefer insects in the spring. In some places, they will take a lot of birds, while in others, there are sufficient rodents for birds simply not to be worth the effort, although they will still take the occasional one - or their eggs - if a good opportunity presents itself.
Pine martens are nocturnal, spending most of the day sleeping in cozy dens made inside tree hollows or other locations safely off the ground. Like most members of the weasel family, they are fairly antisocial outside of the breeding season, jealously marking out a patch of forest about five kilometres across for their own use, and driving out any adult members of their own sex that they find there. The territory is marked out with dung and with scent from their anal glands, and will include a number of hidden food caches, in addition to their favoured sleeping den.
Pine martens mate in the summer, but, like many weasels, the embryos do not begin to develop until much later, allowing them to be born eight months after conception, in the spring, when food is abundant. Litters of between three and five pups are common, although they can be larger. The young don't open their eyes for about six weeks, but begin weaning only shortly after that, eventually leaving their mother in the autumn, to find a home of their own.
The American marten is, however, smaller than European pine martens; not counting the tail, the latter can be nearly two feet long (55 cm), whereas American martens never exceed a foot and a half (45 cm), and are often smaller. Their fur is somewhat richer in colour, with the yellowish throat 'bib' of pine martens sometimes turning to a brighter, orange, hue in American martens. Even though their litters don't seem to be any larger, another difference is that the females have eight teats, rather than just four.
Unlike the European species, while they can tolerate other forms of woodland, they greatly prefer coniferous forests, something that has restricted their advance southwards. They are found across much of Canada and Alaska, reaching as far east as Newfoundland, and venture into the north of Maine, Michigan and Minnesota. In the west of the USA, however, the pine forests of the Rocky Mountains have allowed them to reach much further south, and they are found as far afield as northern California and Colorado. Some researchers have even considered the martens of the US Rockies to constitute a separate species, although this has yet to be widely accepted.
In most other respects, the biology and habits of American martens are very similar to those European pine martens. They breed and give birth at around the same time, age at about the same rate, and have the same sort of social system. They do seem to rely less on fruit and the like to supplement their meat diet, which consists largely of squirrels, chipmunks, voles and, where they can get them, hares, but they, too, can be fairly said to be omnivores. They are also more likely to den on the ground that pine martens, often finding rocky crevices or other shelter, rather than tree hollows.
That trend is, however, taken much further by the beech marten (Martes foina), to the extent that it's just as commonly known by the alternative name of stone marten. Beech martens are also forest dwelling animals, and are found across much of Europe, but they seem to actively avoid pine forests, and so are absent from much of Scandinavia (except Denmark) as well as the British Isles. Their ability to tolerate drier, rockier, terrain puts them in better stead in the Mediterranean, where they are found across Spain, Portugal, and Greece, as well as in those areas frequented by pine martens. Although there are evidently many broad-leaf forests in which both species can prosper, pine martens probably only survive on the islands of the western Mediterranean because beech martens are absent there. The two species are found together in the Caucasus and many parts of the Middle East, but beech martens have also spread much further, and are common as far afield as Mongolia and Tibet.
Pine and beech martens are not easy to tell apart. In theory, beech martens are supposed to have darker fur, with a white, rather than a yellowish, bib. In practice, both species show considerable variation, and there can be a fair degree of overlap in appearance. The best way to tell them apart is by examining the third molars in their upper jaws, which have a different shape - but that's far from practical under most circumstances.
Although their forest habitats are endangered by logging and encroaching farmland - beech martens may quite like houses, but they do avoid open fields - like many other members of the weasel family, all three species of pine and beech marten are hunted for their fur. Here, beech martens have something of an advantage, since their fur is coarser and less valuable than that of either of the pine martens, which may explain their preference for slightly warmer climates. While pine martens don't turn white over winter, as stoats do, their fur does become exceptionally thick and luxuriant before the snow arrives, something that has made them favoured targets for hunters since at least the middle stone age.
Over the last few decades, trapping of martens has become less common, and, despite low populations early in the twentieth century, none of the species are currently in any danger of extinction. Locally, problems may be more severe, especially on islands where populations were already restricted. Pine martens once ranged freely across the British Isles, but they are now all but vanished from England and Wales, and only found patchily in Ireland. In contrast, modern forestry practices in Scotland seem to have favoured them, and they are making something of a comeback there, as they are in some other parts of their range on the continent.
In one instance, martens have used their value to humans to exploit a new habitat. In the 1940s, some beech martens escaped from a fur farm near Milwaukee, and their descendants now live wild in the forests of Wisconsin, thousands of miles from their European and Asian homelands.
[Pictures from Wikimedia Commons. Cladogram adapted from Koepfli et al. 2008.]