August 2018 Natural Mystery Answered

We had a terrific response to last month's Natural Mystery. It looks like skulls are a popular subject, and one that many of you know quite well. Nearly everyone who submitted an answer correctly identified this as a vesper (evening) bat skull. Most people narrowed it down to a bat based on the teeth. Among mammals with tiny skulls, rodents have distinctive incisors and lack canines, while shrews teeth are more uniform in size and stained. Within the vesper bats, guesses included little brown bat (aka little brown myotis), silver-haired bat, and big brown bat. Some of the differences between these species are subtle, but five of you teased it out correctly.People used a variety of resources to narrow down the options further, including Animal Skulls by Mark Elbroch; Mammals of Mississippi by Alisha A. Workman; the Smithsonian's Mammals of North America website; and skull keys for bats of Minnesota.

The size of the skull was perhaps the biggest clue. Karen Kaehler and Emily Culhane both used dentition to narrow their choices, but found that a key difference between little brown bats (Lasiurus myotis) and big brown bats (Eptesicus fuscus) isn't visible in this photo. Little brown bats have a single upper incisor on each side, while big brown bats have two. While there are a few other differences between these two species that are visible in the photo, such as the profile of the forehead and the height of the coronoid process of the lower jaw (sorry for the jargon), those differences are fairly subtle. The easiest way to distinguish the two species is by size. As Kirsten Welge notes, little brown bat skulls range from 13.7-15.6mm long, while big brown bat skulls can measure from 15.1-23 mm.

Skull shape also proved to be a key. As Allison Holzer explained:

"I went Mark Elbroch's book Animal Skulls--at the back there is a table of measurements for North American species, so I looked for a bat species where the measurements we were given overlapped with the measurements in the book (which are based on a large-ish sample size). Hoary bats and Big Brown bats both overlap, but after looking at the illustrations, I saw that the shape of a hoary bat's skull is much more squat looking. Therefore I conclude Big Brown bat."

And, indeed, this is the skull of a big brown bat, Eptesicus fuscus. Congratulations to Karen, Emily, Kirsten, Allison, and everyone else who keyed this out correctly.