Cool Critters: Shrews use shrewd tricks to survive, icluding shrinking their heads

The shrew – North America’s tiniest mammal – will boggle your mind. Ten species live in Washington state. And as you read this, thousands of them are darting around the ground like over-caffeinated furballs while searching for their next meal.

These tiny mammals with pointed snouts and poppy seed-sized eyes don’t hibernate to survive wintry weather. Instead, they shrink their skulls. And that’s just one of their superpowers.

No larger than a human thumb, shrews are everywhere but seldom seen. Now that winter weather has arrived, they’re zipping throughout woodlands, among sagebrush, along roadsides and probably not far from your back door.

“They are difficult to see and find because they’re so small and fast and they blend in with leaf litter and woody debris on the ground,” said Conservation Northwest wildlife scientist Jordan Ryckman, who has published research on shrews in Washington state. In winter, they spend a lot of time scurrying around trees and shrubs and in tunnels below the surface of snow cover, she explained.

Although shrews resemble field mice endowed with Pinocchio noses, they aren’t rodents. They’re insectivores, a group of small mammals including hedgehogs that eat mainly insects, spiders and worms. Being that they have poor eyesight, they often use echolocation to detect prey, somewhat like bats. Their long, sensitive whiskers help them probe as well.

Similar to hummingbirds, shrews are high strung and possess a whip-fast heartbeat. At 1,200 beats per minute, “shrews have the fastest heartbeat and one of the shortest life spans (11 to 13 months) of any mammal,” according to the National Wildlife Federation. Nearly constantly in motion, “they only sleep for a few minutes at a time.”

To fuel its amped-up metabolism, the shrew must eat every hour or so, which can be a struggle in winter when prey is scarce. They confront that problem head on, so to speak, by shrinking their heads – including the brain – up to 20%, according to research published in the journal Current Biology in 2017. Scientists believe the shrinkage reduces the shrew’s high energy demands, thereby lowering its need for near-constant food. Then, when temperatures warm up in spring, their heads start returning to normal size – as does their ravenous need for food.

They’re still highly active, but less so than during summer months, Ryckman said.

“Shrews have adapted incredible mechanisms for survival,” she said. They’re even equipped with scent glands on their sides that emit a “repulsive odor” to repel many predators, she added. A cat, raccoon or fox might attack a shrew but then spit it out in disgust.

There are some 385 shrew species worldwide, with 39 in North America. The 10 species in Washington state can be found in a variety of habitats, ranging from forests and grasslands to sagebrush country and gardens. (As consumers of insects, snails, slugs and even ticks, shrews are considered a gardener’s friend.) There’s even a semi-aquatic species capable of walking on water.

Yep, that’s right.

Stiff hairs on the feet of the northern water shrew enable it to scamper across the surface of water. What’s more, they can hunt small aquatic prey underwater using a technique called “bubble sniffing,” where they blow tiny bubbles of air through their nostrils to detect scent particles in the water, Ryckman said.

As a pathway for treating human diseases such as Alzheimer’s and multiple sclerosis, scientists are trying to figure out the chemicals responsible for the shrew’s incredible shrinking brain that grows back at the onset of spring.

The shrew – one of the smallest, fastest, hungriest mammals on the planet – practically lives under our noses. Small minded? Not this fellow.