ECOVIEWS: Why do shrews travel through snow?

February 2, 2019

When my grandson Parker was invited to go skiing in Vermont in early January, he wanted to see a moose. Though his encounter with a mammal was a bit lower to the ground than he had hoped, it formed an intriguing part of his adventures. On two occasions he saw a shrew scrambling across the top of the snow and disappearing down a hole. He asked me, “What’s going on? Don’t they hibernate up there?” Simple answer is that shrews do not hibernate anywhere. Several species are known to be active beneath snow cover during the winter, occasionally venturing to the surface.

These tiny, secretive little mammals, which are not rodents, belong to a group of animals informally called insectivores. Among their closest relatives are moles and hedgehogs. Native to all continents except Australia and Antarctica, the known number of shrew species exceeds 375. The eastern United States has about a dozen species. The ability of shrews to survive in cold climates is evidenced by Vermont having eight kinds of shrews and Alaska 10, compared to Alabama with six and Florida with only three.

Shrews are distinctive in many ways. One kind, the short-tailed shrew of the eastern United States, is the only venomous mammal in the world other than Australia’s duckbill platypus. Short-tailed shrews can be further singled out as the only mammals to deliver venom by biting and in which both sexes produce venom. The shrew’s saliva, produced by glands in the mouth, can paralyze prey such as an earthworm or small snake. Because the poisonous substance is forced into prey by biting, it can technically be considered venom. In the platypus, venom is delivered by a sharp spur on each hind foot connected by a duct to a gland found only in males. The technique is not used to capture prey.

The 2-inch-long pygmy shrew is found from southern Canada to Alaska and as far south as northern Alabama and Georgia. It is one of the smallest mammals in the world, with some adults weighing less than a nickel. Shrews have ravenous appetites and must eat constantly or die. Shrews kept in captivity have been observed to eat their weight or more each day. Their primary foods are insects, including grubs and other larvae, spiders, slugs and earthworms They will not hesitate to eat small mice, amphibians or reptiles, especially those found in a dormant state during winter. Sometimes shrews eat fungi and vegetation. They have the highest vertebrate metabolisms, rivaled only by some hummingbirds.

Shrews are among the most abundant mammals in the world. Because of their small size and mainly subsurface activity, they go unnoticed by most people, even those who spend a lot of time outdoors. Ecologists use small mammal traps to capture rodents, but many shrews are reluctant to enter them. The widespread abundance of the southeastern shrew was not discovered until the 1960s when herpetologists at the University of Georgia’s Savannah River Ecology Laboratory began using pitfall traps to capture reptiles and amphibians. Pitfall traps are cans or buckets buried in the soil with the open end parallel to the ground. I still remember the amazement of mammalogist colleagues when I brought in several of these little brown southeastern shrews that had fallen into our pitfall traps. I had never seen one before. They had not seen many either.

We have much still to learn about the ecological mysteries surrounding shrews. Each species has traits and abilities not possessed by others. How any can survive a Vermont winter beneath the snow, find an adequate supply of food to fuel themselves, and stay alive until spring returns is perplexing. Considering that many kinds of shrews are able to do so, scientists clearly are a long way from knowing how each one manages. What an exciting time to be alive knowing so many ecological mysteries are out there yet to be discovered and explained.

Update hourly