The Short-Tailed Shrew, an evolutionary superstar

He [Raven] looked about and thought there was nothing on the land as lively as the fish in the water, so he made the shrew-mice, for he said, “They will skip about and enliven the ground and prevent it from looking dead and barren, even if they are not good for food.”                                                                                     – from Clara Bayliss’s 1909 collection A Treasury of Eskimo Tales.

Northern Short-Tailed Shrew (Blarina brevicauda) sniffing the air for prey

Creation myths aside, our early mammalian ancestors were little omnivorous insectivores very much like modern shrews.  Mammals evolved some 200 million or so years ago and lived alongside the dinosaurs, but did not grow to large size until millions of years after the dinosaurs went extinct some 65 million years ago.

Today we are a diverse group of flying, swimming, and running creatures ranging in size from the blue whale 120 feet long with a heart the size of a small car down to shrews no more than a few inches long, weighing less than an ounce.  We don’t know much about our early ancestors, small terrestrial creatures rarely leave fossil remains, but from what we can tell it seems that the shrew body design and hunting strategy is extremely successful and has remained a persistent mammalian body plan.

Shrew-like creatures live all over the world and are renowned for their ferocity, appetite, boldness, and their unusual (at least in mammals) venomous bite.  Here in Vermont the shrew you are most likely to encounter is the Northern Short-Tailed Shrew (Blarina brevicauda).  As near as I can make out, Blarina roughly translates to “nose-necked” and brevicauda actually does mean “short-tailed”, making this one of the few animals that has a matching common and scientific name.  Shrews are in the Soricidae family, which just means, “the shrew family.”

Short-Tailed Shrew running over freshly cut grass

Shrews move rapidly, driven by their rapid metabolism and resulting need to eat constantly.  They move with a peculiar blend of short, jerky twitches and weasel-like fluidity.  The shrew in the photos was hunting for insects and earthworms in freshly cut grass.  I followed it for perhaps 200 feet, and during that time it only stopped to sniff out prey, to eat it, and to hide from the dog that wanted to know what I was watching so avidly.

Like many predators, shrews are curious, engaging in high risk, high reward activities.  Their eyesight is poor, but their sense of smell is excellent.  How good their hearing is seems to be uncertain, some people thinking it is good, others poor.  I suspect that it is pretty good and that they are sensitive to vibrations via their whiskers and feet.

Searching for food

Unlike many animals the shrew had no fear of me what-so-ever, not even flinching when I stroked its back as it ate one of the 4 earthworms it caught while I was watching.  Every small animal nearby, on the other hand, was terrified of the shrew.  Insects froze into immobility, antennae twitching and heads slowly tracking as the shrew passed by.  Well should these creatures be wary of the shrew.  If a shrew goes more than a few hours without eating it will starve to death.

Short-Tailed shrews are tiny, massing between 2 and 5 US quarters (about .5 to 1 ounce), but they are perfectly capable of killing and eating prey several times their size.  There is a 3 minute National Geographic TV video of a shrew exploring a garden, then killing and eating a garter snake much larger than itself.  Most of the time shrews will content themselves with insects, worms, and seeds.

The dense fur of a shrew

Shrew fur is thick and dense, like the fur of an otter, but lacking the oily guard hairs.  The fur is so dense that it is waterproof, allowing some species of shrew to hunt underwater.  Shrews need this dense fur to keep warm through the winter.  Their small size means they lose heat quickly, necessitating both a rapid metabolism and good insulation.  In winter they remain beneath the snow as much as possible, eating cached food, keeping activity to a minimum, and burning brown adipose tissue (what we commonly call “brown fat”) to keep warm without resorting to shivering.

They have as many predators as they have prey, but their venom and unpleasant musk helps to keep some mammalian predators at bay.

Shrews are tremendously strong for their size.  I could see the back and neck muscles bulging as this shrew pulled earthworms from the soil.  It pulled a short section of the worm from the ground and ate it, pulled another short bit, ate that, and continued, as though it was eating Twizzlers at the movies.

Pulling an earthworm from the ground

We humans are proud of our accomplishments, but perhaps we should be more humble before the little shrew.  This tiny creature, so easily killed by a careless foot is upholding 200 million years of successful mammalian tradition, wearing a body design that gave rise to all other mammals from humans to whales, bats to elephants, beavers to monkeys.

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6 comments on “The Short-Tailed Shrew, an evolutionary superstar

  1. Isaac says:

    Fascinating writeup for a fascinating creature. I wish they would garner as much attention as the macrofauna of the world. I am especially amazed at their metabolism; a quick search shows one species of shrew to have a resting heart rate of over 800! They just operate at a completely different level than us.

    • EarthKnight says:

      They really are amazing. I forgot to mention that a short-tailed shrew can have a home range of 2.5 hectares, or about 6 acres. That’s a big area for such a tiny creature. Population densities can range from 5-80 per acre, but usually don’t rise higher than about 20 per acre. I wouldn’t be surprised if their impact on insect populations was nearly as great as that of bats.

      • Isaac says:

        Shrews as critical agents of insect control. I like that.

        Going back to the metabolism, do you have any idea as to why it would be so high from an evolutionary perspective? I can understand having a constant body temperature would be advantageous, but it seems to have gone a little overboard.

      • EarthKnight says:

        That’s an interesting question. I think there are a couple of reasons.

        They’re not far off the predicted heart-rate to body-mass predictions. In, “The origin of allometric scaling laws in biology from genomes to ecosystems,” the authors try to figure some of this out. One thing they came up with is the energy costs of pumping blood around. The relevant line is, “Since small mammals dissipate relatively more energy in their networks, they require elevated metabolic rates to generate the increased energy expenditure to circulate the blood.”

        Their predictions are that the minimum body mass that an animal can have and still have blood pulsing is right near a shrew body size. Below that mass the heart rate would rise above 1000BPM, but there would be no pulse, thus no or little blood circulation.

        Short-tailed shrews also may have an increased heart rate due to cold acclimation and their high protein diet.

        Interestingly, they can overheat and die if they are in a 80 degree (F) or higher environment. Probably part of the reason they stay below ground so much of the time.

  2. Bonnie says:

    Yesterday I was speaking on the phone with my neighbor, when I heard my husband say, “there`s a mouse”. It was walking from my family room into my kitchen…. I immediately knew it was a shrew…he ran under my oven,,,and the next thing I knew, my neighbor was at my door with a mouse trap! I said no….we will catch him in a can. Long story short…he was not shy…came out a few time to run away….I felt his soft beautiful coat as I tried to guide him into the can….but he was speedy when he needed to be. My husband was the one who finally placed the can on its side….and when my little friend came out…he actually walked right into it. I have seen shrews in my yard..on their mission, definitely going somewhere intended, but this was quite the visit. My neighbor and husband thought I was nuts,,,I was looking at this little creature for about 2 minutes, probably less, in admiration, his coat was beautiful glistening dark charcoal, I was amazed how adorable he was. Looking back….I never knew they have a venomous bite, as stated in your article, and feeling/petting his coat as I tried to coax him into the can as he ran….well I think I will keep that fact to myself, and not tell my husband. Looking forward to my next encounter only in the garden.

    • EarthKnight says:

      Wonderful, I’m glad you were able to safely catch and move the little fellow. They really are beautiful animals. Few people take the time to look as closely as you did.

      The biting issue is interesting, I’ve never been bitten, perhaps because I am always gentle with the creatures I encounter. Like most small creatures if they can avoid it at all they will not bite, their venom is primarily reserved for dispatching prey and only secondarily as a defensive mechanism.

      Reports of the effects of their venom on humans varies quite a bit, ranging from not much more than a bee sting to several days of pain.

      Good incentive to be slow and treat the little fellows kindly.

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