shrew party

May 8, 2007

in Natural history

Shrew_2The phrase, “live fast, die young” could be dedicated to shrews. These small carnivorous mammals have supercharged metabolisms, and spend all their waking moments hunting and eating. Despite being quite numerous in the right habitat, shrews are rarely encountered — alive, anyway. I typically find shrews post-mortem, lying in the path in front of me as if they had just simply given up the ghost mid-stride. Actually, that may be the case for those I’ve found, like the one in the photo. Shrews usually live less than 16 months and with a heart rate measured at 800 beats a minute, I think they just burn themselves right out.

A lifeless shrew, then, is the antithesis of what shrews are all about.  So I was fortunate indeed last week when I not only encountered a living shrew, but a congregation of shrews. Shrews are generally solitary animals, territorial and aggressive toward their own kind. The common shrew in my area, the Masked Shrew (Sorex cinereus), is noted for being especially voracious and intolerant of companionship. I had paused to listen for a bird song when I heard a persistent rustling in the leaf litter off the trail. I glanced over, and saw a shrew darting from a fallen log to a tangle of vines. The next moment, another scooted through the dried leaves. A few feet away, a sleek pointed snout poked out from under a shard of bark, then quickly disappeared. Over the next few minutes, I watched the speedy comings and goings of at least four animals before the forest floor returned to silence. Yes, I knew this was a special occurrence, because this experience was the second in my field career.

Ten years ago, I happened upon the same type of trailside hubbub. That time, I found myself standing amid at least two dozen energetic shrews. They covered an area of about 20 square feet, and ran chasing each other under leaves, over logs, across my boots, in and out of dappled sunlight.  And they were all squeaking at each other in high, whistled voices (most species communicate through vocalization, and Masked Shrews also use echolocation). Despite their quick, zig-zagging actions, their activity did not seem frantic, but vigorous and spirited.

As you may have guessed, these shrew aggregations are presumed to be mating parties, but little is known about the behavior as it is seldom observed (my list of literature and references therein make up most of the published accounts). How did they know to meet in this place?  How did they know when?  How could these furtive, thumb-sized mammals carry on their spring rites, fearless of the tremors of footsteps and the not-so-quiet presence of a huge human?

And how privileged am I to twice have had the opportunity to see this rare, delightful insight into the lives of these secretive animals?

Hieshetter, D. 1972. A concentration of masked shews in Ingham County, Michigan. Jack-Pine Warbler 88:63.

Maier, T. J. and Doyle, K. L. 2006.  Aggregations of masked shrews (Sorex cinereus): density related mating behavior? Mammalia 86-89. (PDF)

Vispo, C. R. 1988. An observation of a wild group of masked shrews, Sorex cinereusCanadian Field- Naturalist 102:731-733.

Woodfenden, G. E. 1959. An unusual concentration of Sorex cinereus. Journal of Mammology 40: 437.


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