One of the treats of a snowy winter is wandering around looking for animal tracks. When I counted up the species for which we’ve found tracks, I saw that one taxonomic family was disproportionately represented—the Mustelidae. Five species of mustelids are likely to leave tracks in snow in our region: ermine, mink, marten, river otter, and wolverine. I’ll first present some basics about mustelids in general, and then some specifics about each of these five species.
Mustelids are a widespread family, occurring on every continent except Australia and Antarctica. There are over fifty-five species, ranging in size from the diminutive least weasel, weighing as little as one or two ounces, to the sea otter, reported to reach over a hundred pounds. They tend to have relatively long, thin body shapes, although some, such as badgers, are stockier; legs are generally short. The claws do not retract (unlike most cats), but again there is an exception: the claws of the fisher are partially retractable. Males are generally larger than females of the same species; their home ranges are larger and tend to overlap those of several females.
They are typically carnivores, preying on a variety of small or middle-sized animals, and sometimes scavengers, although some, such as marten, also eat fruit (and serve as seed dispersers). Much of their ecology is related to availability of food: population abundance, litter size and survival, frequency of reproduction, rate of maturation of juveniles, adult survival (starvation is reported to be a common cause of death in wild populations).
Along with males of many other placental mammals, male mustelids possess a baculum or penis bone. The size of this bone in different species has been suggested to relate to the length of the copulatory act: long bacula are correlated with long copulations. Extended copulations are not generally possible when penile erections depend entirely on hydraulics, i.e. blood pressure. This begs the question of when and where long copulations are adaptive or, conversely, when and where short ones are adaptive. As you might imagine, the subject has attracted some discussion but with no definitive answer.
Most mustelids also share a reproductive habit that (to humans) seems odd: After copulation, the fertilized egg divides just a few times and then rests; it does not implant immediately in the uterine wall, so no placenta is formed and the embryo does not develop further for some time. The delay of implantation lasts several months, during which the few-celled embryo just floats around in the uterus. Eventually, however, it does implant, a placenta develops, and active gestation (just a few weeks long) begins. Thus, the time of mating and the time of active pregnancy are well separated, and birthing is therefore postponed to a season well after the mating season. Delayed implantation is typical of numerous other mammals, including bears and seals.
The adaptive value of a seasonal separation of mating and birthing is often discussed. Most explanations address the importance of rearing young at times of year when food and other conditions are optimal.
This leaves unanswered the reason(s) why mating occurs so long before the season for rearing offspring, and I have not discovered good explanations. In some cases, other aspects of the life history may have created limitations on the convenience of getting male and female together; for instance, bears hibernate for the winter and sexual encounters shortly before birthing are not likely, summertime is surely more convenient; or males take advantage of freedom from child care to go roaming and foraging while females tend the young. I’d like to find a serious analysis of the conditions that favor the seasonal separation of mating and birthing.
Here are a few interesting factoids about our resident mustelids:
River otter—They are very aquatic, eating mostly fish, other aquatic animals in open water or tide pools, and sometimes capturing floating birds from underwater. They can dive to about twenty meters, staying under up to four minutes or so. Being heavy-bodied, they must tread water or scull with the tail to stay afloat. Otters are reported to forage cooperatively in some locations (e.g., Prince William Sound). Otters often travel overland between bodies of water, sometimes sliding over the snow. Their home ranges are said to be smaller on the coast than in the interior, presumably because food sources are more abundant. Otters usually mature at age two years.
Wolverine—They often favor remote areas but use a variety of habitats. In winter, this large mustelid mostly forages by scavenging carcasses left by other predators; its powerful jaws can crack the bones of moose. It is sometimes said that wolverines are too big to survive very long on small prey, too small to kill large game animals regularly, and too slow to chase fast prey. So scavenging becomes a good way to feed. In summer, carrion is less available and wolverines eat more small mammals and birds. They commonly den under deep snow in alpine areas, but commonly travel widely (many miles) to find food. If they get lucky, they will cache surplus food in a handy location. They are slower to mature than our other mustelid residents, usually maturing when three to five years old.
Marten—Denizens of old-growth and mature forests, they are highly arboreal. They commonly feed on small mammals, as well as birds, eggs, and carrion, and are said to need the equivalent of at least three voles per day. However, they can also kill hares and marmots. They mature at age one or two years, depending on food supply. There are two species in Southeast; detailed genetic studies have shown that Kuiu and Admiralty islands are home to a distinct and strictly coastal species (which also occurs on Haida Gwaii and Vancouver).
Ermine—They eat almost anything that moves and need to eat almost continually; they are good swimmers and climbers. They make cozy nests, often usurping the nest of a prey mammal (after eating it), even lining the nest with fur of the prey. Well-insulated resting places are necessary for this small, slender predator that needs this help to keep warm in winter. Sometimes they cache their prey near the nest. Juvenile females can become sexually mature while still in the natal nest, at an age of only one or two months. So, when their mother mates after giving birth (which is the custom with these animals), sometimes the same male will fertilize her daughters as well! They have a short life span in the wild, often living less than two years. Ermine are represented by three distinct genetic lineages in our area, and one of them, with a very limited distribution, is considered to be of conservation concern.
Mink—Comfortable on land and in water, they eat fish, crayfish, various other small aquatic critters, and birds—they are said to be especially fond of bird eggs. They make short dives but usually forage in the shallow water or on land. Mink (and wolverines, ermine, and marten) are adept at climbing. They, like squirrels, are able to descend from a tree rapidly and skillfully, because they can rotate their hind ankles so the claws engage with tree bark. They breed as yearlings, and seldom live longer than three years in the wild.
The populations and historical geographic ranges of marten, wolverine, and river otters in North America have been seriously restricted by human activity: habitat loss including deforestation, over-trapping, pollution (especially otters), reduction of their prey populations. In some cases, reintroductions have restored local populations.
Footnote: (There are three other mustelids in Alaska but we don’t generally see evidence of them here. Least weasels live up north and do not occur here. Sea otters live in the sea, yes, and seldom come ashore. Fishers have only rarely been recorded in Southeast and, in any case, are very elusive.)