Q: I found a little black mammal dead on a trail in Muir Woods. My friend says it’s a shrew mole. It looks more like a shrew to me. What’s the difference?
Well, I’ll try not to make a mountain out of a molehill with your question. I shrewdly planted a mole in a university mammalogy department who should be able to help me answer. But a warning—those little feisty carnivores are really hard to tame. I know, I know, groan.
Stephen, I also find dead shrew moles, especially in the winter.
The 461 species of shrews (not shrew moles) are found nearly everywhere on earth and are the fourth-largest mammal family after the murids (rats and mice), cricetidae (hamsters and voles), and vespertilionidae (microbats). Shrews are extremely common, with some estimates putting global population numbers as high as 100 billion individuals. Shrews tend to be tiny: mouse-size or smaller. They are very, very active—some can make 12 moves per second and have up to 1,500 heartbeats a minute! That’s even more than a hummingbird. Shrews need to eat constantly and may eat twice their weight or more every day. Not surprisingly, they have very short life spans—live fast and hard, die young, like James Dean. A few of them dig below the ground, but most stay right on the surface, tracking down and voraciously eating spiders, centipedes, insects, and even vegetation with their large canines or incisors and fine chopping teeth. Some shrews, including one of our local species, the vagrant shrew, even have the ability to echolocate to investigate their environment.
In the Bay Area we have five shrew species: the aforementioned vagrant, or wandering, shrew; Trowbridge’s shrew; the ornate shrew; the fog shrew and Bendire’s shrew. They molt twice a year: gray in the winter and dark brown in summer.
A close relative of shrews are moles. We have but one species of New World mole here: the northern broad-footed mole. (However, there are subspecies, including one on Angel Island.) These animals mostly stay underground. They are supremely adapted for their fossorial world with cylindrical bodies, reduced eyes and ears, small hips, powerful digging front feet (they even have an extra thumb to help), and a pelage (coat) of super-soft, velvety fur. This unique pelage enables the animal to easily go backward within its tight burrow without the hairs catching on the sides of the tunnel. Moleskin, by the way, is a cotton product, not actual mole fur.
Like shrews, moles are intense predators with high caloric needs. Moles create two types of tunnels. Superficial tunnels, with raised ridges, are the ones we often see, but they’re infrequently used. The primary and extensive mole freeways are a foot or more below the surface and are used over and over again. Mole tunnels are basically insect traps. When a mole comes across a worm or a beetle grub while tunneling, or when prey simply falls into the tunnel, the mole’s sensitive whiskers detect the critter and the mole moves quickly to dispatch it. Moles also have toxic saliva, and they’re able to paralyze prey and keep them alive and fresh. There’s a persistent rumor of one mole larder with more than 1,000 earthworms!
Finally, there are shrew moles, which do look like a cross between a shrew and a mole. However, a shrew mole is a mole, the smallest of the American moles. Shrew is an adjective in this case; these moles are dark gray to shiny black, with barely discernible eyes, a bit of a snout, and smaller front feet than those of their close relative the broad-footed mole. While those two moles share many characteristics, the shrew mole spends more time aboveground and is a bit more flexible in its dietary choices: like the shrew, it will even, when desperate, eat vegetation. Unlike the solitary broad-footed mole, shrew moles live in loosely connected groups of 10 or 15. And apparently, they are often found dead in the middle of trails. Right, Stephen?