|European (or "common") mole|
There are a number of disadvantages to this lifestyle. You have to constantly dig for one, and your fur and other body parts will get constantly clogged with earth. There is no light, and its difficult to find food or mates. On the other hand, there are also some clear advantages. For one, burrowing animals are safe from most (though not all) predators. Anything small enough to get into their burrows is probably small enough not to want to eat them, and, being underground, they're pretty difficult for anything above ground to find. In addition to that, unless the ground really gets saturated, they don't have to worry much about the rain, and they are protected from extremes of heat and cold.
Although there are a lot of mammals that dig burrows, there are four main groups that have taken the subterranean lifestyle to an extreme. Perhaps the most familiar example are the moles. The mole family consists of forty-one species, although a number of them aren't truly underground creatures, and two of them are semi-aquatic. The group seems to have diverged from its closest relatives, the shrews, not long after the dinosaurs went extinct, and possibly even earlier, making them quite an ancient group. The true underground lifestyle probably evolved at least twice, because the "common mole" of Europe (Talpa europaea) and the "common mole" of North America (Scalopus aquaticus) are not particularly close relatives within the family.
European "American" Star-nosed
& Asian moles moles mole, etc.
^ ^ ^
| | |
| | | Desmans
| --------------- ^
| | |
| | | "True"
---------------------- | shrew-moles
| | ^
| | |
The chart above is somewhat simplified; the "American" moles in fact include one species from China, while the close relatives of the star-nosed mole include some species commonly called "shrew-moles". Brown represents fully subterranean species, blue the semi-aquatic species.
Moles are reasonably widespread, being familiar animals across Europe, much of Asia, and North America. They avoid the colder environments where the ground freezes solid for much of the winter, and so truly subterranean moles are not found in places like Norway or most parts of Canada. They also avoid the deserts, which explains their absence in the Middle East, North Africa, and the central US, despite the presence of two species in the eastern US and three along the west coast. They've never colonised Africa or (less surprisingly) Australia, although there are other mammals with a similar lifestyle on both continents, which I plan on discussing in later posts. Perhaps more surprising is their absence over much of India and in South America; it's not that they dislike tropical environments because there are moles in South East Asia, from Vietnam and Myanmar down to southern Malaysia. For its size, there are also rather a lot of mole species in Japan, along with several species from China, Korea, and the warmer parts of southern Siberia.
True moles are well adapted to their burrowing life. They have powerful front legs with spade-like claws for digging through the earth, and their velvety fur clogs with soil less than that of most other animals would. Unlike bats, which can usually see pretty well when they need to, the eyesight of moles really is quite poor. The European mole (that is, the one we call the "common mole", and which is the only one found through most of Europe north of the Mediterranean) has eyes that superficially appear quite normal, but the cornea is degenerate, and has been compared to that found in humans with the rare visual disorder keratoconus.
Nonetheless, while it prefers to feel its way about with the help of sensitive whiskers and specialised touch receptors on the snout (the ears are also very small, again, to stop them clogging with dirt) the European mole can see, if not very well. The Iberian mole (Talpa occidentalis) of Spain and Portugal, and the eastern mole (that is, the American "common mole", which is found throughout much of the eastern US) are a different case. Their eyelids never open, permanently protecting their eyes from the dry gritty soil of their homelands at the cost of their vision. Still, while the fact that their eyes are permanently shut presumably prevents them from making out any shapes, they aren't completely blind.
The structure of the eyes in Iberian moles has been studied in some detail. The cornea is even more degenerate than in European moles, the iris has no muscle fibres to allow it to widen or narrow in response, and in any event is stuck to the lens, the lens itself has a disorganised structure and is less transparent than in most other animals, and the retina is unusually small. Nonetheless, apart from its size, the retina does look relatively normal, which suggests that it must be at least partially functional. Indeed, Iberian moles do respond to light - by running away from it, which makes sense if you live underground, where there really shouldn't be any.
Nor does the strangeness of moles end with their poor eyesight. In a further bid to stop holes filling with dirt, the female's vagina is sealed shut for most of the year, and only opens when they are sexually receptive. Rather odder, though, is the fact that female moles are, in a certain sense, actually hermaphrodites. This is perhaps rather stretching the term, but it is the case that the ovaries contain a large amount of what certainly looks like testicular tissue - indeed, there's rather more of it than there is ovary. It doesn't produce any sperm, but it does produce male hormones. Of course, all female mammals produce some male hormones - they're made by the adrenal glands - but the female mole produces considerably more than usual.
|Eastern (or American "common") mole|
Quite what the point of this is is a little unclear, although the glands do shut down during the breeding season and when the female is pregnant. Being awash with male hormones for the rest of the year though, does lead female moles to develop some masculine anatomy as they grow; they have an enlarged clitoris, and what appears to be a prostate gland. None of this hampers their ability to be good mothers, though, because moles have a higher reproductive effort (that is, the mass of offspring one mother can produce in her lifetime) than their closest relatives, the shrews. This seems to be due, at least in part, to the production of some very nutritious milk, and may offset the high death rate of young moles when they leave home and make the dangerous journey above ground to find somewhere to establish their own burrow.
They have to make their own burrow, because moles are pretty anti-social animals, and adults normally only meet up to mate. Having said that, they don't need particular large burrow systems, so there can be several moles in a relatively small area. Population densities of 5 to 25 moles per hectare have been reported for the European mole, and 5 to 12 for the eastern mole. Moles, of course, are mostly carnivorous, eating insects and worms, largely, it would appear, by hoping that they accidentally wander into the tunnels.
While the less subterranean members of the mole family are basically nocturnal, its a little more complicated for the true moles. Day and night aren't really an issue for them, and they sleep three times each day for most of the year - although males can be active for rather longer periods when they're looking for a mate.
Because of the protection that hiding below ground offers them, and the fact that they generally don't mind agricultural land, most mole species are not overly threatened by human activity - which isn't to say that farmers don't poison them, just that they don't do it enough to place entire species at risk of extinction. The clearest exception is the Echigo mole (Mogera etigo), which lives in only one small area on the west coast of Japan's main island, Honshu. It prefers soft soils, and changes in agriculture in the region make those less common than they used to be, and the animal is officially considered Endangered.
[Pictures from Wikimedia Commons. Cladogram adapted from Mikko's phylogeny archive and Motokawa 2004]