November 2025 Natural Mystery Answered

Everyone who wrote in about last month's Natural Mystery correctly identified these mandibles to species. Congratulations to Ronnie Hartman, Greg Schayes, Erin Anderson, and Haleigh Ziebol (one of our new facilitators) for carefully examining the characteristics of this tiny animal and reaching the precise conclusion!

These mandibles belong to the northern short-tailed shrew, Blarina brevicauda.

Trackers used Mark Elbroch's book Animal Skulls and Richard Wolniewicz‘s Field Guide to Skulls and Bones as reference guides. 

Haleigh noticed a feature that ruled out herbivores:

"A teensy mandible without a diastema points us to a teensy carnivore."

While Ronnie considered several small mammals:

"I noticed immediately that the mandible was long, thin and flat, that the teeth were very dark on top, and that the coronoid process was at a 90 degree angle from the mandible, which immediately eliminated mice, rats, squirrels, and voles."

Greg immediately recognized them as shrew mandibles:

"Upon first glance I noticed both the small size and the sharp pigmented teeth and narrowed down my candidate list to shrews. In Minnesota, according to the DNR’s website we have 7 shrew species present in the state–Pygmy shrew, Short-tailed shrew, Masked shrew, Arctic shrew, Smokey shrew, Least shrew and Northern water shrew. "

Trackers also noted the size of the mandibles. Erin stated:

"These mandibles are right around 15/16 mm, which is within the appropriate range for [the northern short-tailed shrew.]"

Note that Erin referenced the Wolniewicz guide, which measures the mandible in its entirety. Meanwhile, Elbroch's guide only measures the bone and does not include the incisors. As Greg pointed out:

"Noting that Elbroch’s reference does not include every shrew species present in MN, it became immediately clear that the mandible length was much larger than some of the smallest shrew species at over 12mm!"

Further examination of the morphology offered additional clues to the species. Greg went on to state:

"1) the coronoid process is squared and rises vertically from the posterior of the toothrow. 2) The angular process is much more stout than in other shrew species. 3) There is a step posterior of the angular process."

Meanwhile, Haleigh noted that "shrews of the Sorex genus have a mandible with... generally a longer and thinner body." Erin pointed out that "the incisor points straight out" and "the angle and condyle even with one another."

These are all great characteristics to look for when identifying shrews! While narrowing down to a single species isn't always feasible, in this case the trackers above did a great job sorting through the evidence and precisely identifying the mandibles.

Congratulations again to Ronnie, Greg, Erin, and Haleigh! And big thanks for sharing their processes with all of us.