The humble shrew hiding in your garden

by Russel Barsh

Kwiaht

Weighing less than a nickel, a shrew can eat 7 pounds of slugs and insects in a year!

Islanders most often encounter shrews as trophies brought home by cats, who delight in playing with these tiny, frenetic mammals but disdain to eat them. Shrews play an important but unseen role in island ecosystems, however, as consumers of many garden “pests.” Indeed, they alone fill a niche that, in mainland western Washington, is shared by a number of small rodents, and a greater diversity of reptiles and amphibians than we have in San Juan County.

Shrews are not rodents, but rather insectivores, the ancient mammal family that also includes the hedgehogs and moles. Modern shrews evolved some 41-48 million years ago and now comprise over 340 species worldwide. Three species account for most of the shrews living around the Salish Sea: the masked shrew (Sorex cinereus), the dusky or montane shrew (Sorex monticolus) and the vagrant or wandering shrew (Sorex vagrans) that seems to be dominant in the islands. Salish Sea shrew species are superficially much the same in color and appearance, having separated from each other less than a million years ago.

Our shrews live very short, intense lives for mammals. Their life expectancy is about 6 months, although some individuals survive over one or even two winters. Shrews are born blind like kittens, but they wean in about three weeks and mature quickly. At about three months, young shrews disperse and claim territory for themselves. In the Salish Sea, shrew territories average less than a tenth of an acre; as many as 25 individuals can share one acre in springtime when prey is most abundant. Breeding consists of brief, chance encounters between April and August. Nests are small cups woven of leaves, grass, moss, hair and sometimes feathers.

With a sky-high metabolism rate, shrews need to eat at least every two hours, day and night. They only take brief rests, and they are not picky eaters. In our area, earthworms, spiders, caterpillars, maggots, slugs, woodlice, beetles, ants and other insects are major menu items. If a shrew succeeds in subduing a large meal such as an earthworm, what remains uneaten will be cached for later enjoyment. Shrews will also eat some foliage and seeds, though they cannot survive on plants alone.

Shrews have limited vision, but they are endowed with exquisite hearing and long, flexible noses and swirls of sensitive whiskers. Foraging shrews often stand on their hind legs and sniff the air, then grub in the soil like pigs using their long whiskery noses. Shrews have large scent glands, moreover, and appear to use scent to distinguish kin from intruders. Amazingly, shrews also echolocate like bats. The American zoologist E.R. Buchler discovered a half-century ago that when vagrant shrews’ ears were plugged, they twittered more loudly but were unable to find their way back to a familiar bait station.

Vagrant shrews are well-suited for the islands because they are unusually adaptable ecologically. They have been found in swamps, bogs, riparian corridors, wildflower meadows and many different kinds of coniferous forests, as well as gardens. They prefer younger, broken-canopy woodlands to dark mature forests, and are more likely to colonize pastures and lawns than other Salish Sea shrew species, although like other shrews they seek snags, logs and other woody debris for shelter.

Predators of our island shrews are mainly owls (which are also the main predator of island bats) and domestic cats. Unlike some larger shrew species that eat nestling mice, such as the short-tailed shrew (Sorex brevicauda) found in northeastern North America, our Salish Sea shrews are not venomous and do not have the means to fend off a predator other than speed and boldness. Shrews are also parasitized by diverse mites and fleas, and here in the islands by a tiny, specialized native tick, Ixodes soricis, that poses no danger to other mammals.

The life of an island shrew is short, and filled with danger! You can make your garden more shrew-friendly by leaving a fallen log and some brushy patches undisturbed. (This is also an invitation for native bees to move into your garden.) Shrews will also nest in rockpiles. They also benefit from hedgerows for evading detection by owls; and from compost piles that are rich stores of worms.