3 min read

Chipmunk Unmentionables

Why we don't talk about chipmunk identification
Yellow-pine chipmunk
Our local yellow-pine chipmunk is one of the more brightly colored species. Photo by David Lukas

I feel fortunate that I now live in an area where there is only one species of chipmunk. It makes my life as a naturalist much, much easier.

As I work hard to bring you this newsletter on the 4th of July, please click here to make a donation or consider upgrading to a paid subscription to help support the newsletter. For the cost of a single coffee once a month you can make a huge difference. Thank you!

I used to live, and travel widely, in the Sierra Nevada, where there are eight chipmunk species that are virtually impossible to separate. Even experts struggle to tell these chipmunks apart, and if it's this hard for experts, imagine what it's like for the chipmunks themselves.

On the west side of the mountain crest, I could find this different species of chipmunk, but I'd be hard pressed to separate it from my local chipmunk. Photo by David Lukas

The risk of mating with the wrong species goes up dramatically when multiple species look the same and live in the same geographic area. This risk is even higher for small animals, like chipmunks, that are pumped full of hormones, hyperactive, and interacting with brief flashes of intense energy during the height of their breeding season.

Chipmunks are fast moving and can be aggressive and territorial during the breeding season. Photo by David Lukas

Chipmunks probably have a mix of clues they pay attention to—including odors, behaviors, and vocalizations—to determine if they're attempting to mate with another chipmunk of the right species. But chipmunks also have a fail-safe backup that keeps them from making a mistake in the heat of the moment: the male of each species has a uniquely shaped penis bone that will only fit, like a lock and a key, with a female of the same species.

chipmunk bacula
Examples of species-specific variations in chipmunk bacula. Illustration from "The Baculum in the Chipmunks of Western North America" by John White

These penis bones, also known as bacula, are found in many mammals. In some species they allow for extended periods of copulation, and in other cases (such as chipmunks) they help distinguish similar species.

hidden chipmunk
Chipmunks don't necessarily want to be seen, much less identified by a human. Photo by David Lukas

Unfortunately for chipmunks, if a scientist comes along and wants to know which species you are, they might need to cut out your penis bone to make sure they've positively identified you!

As we dive into the heart of summer I apologize in advance. Where I live in the North Cascades, we have an extremely short season for hiking and backpacking, and to keep my soul alive I need to get outside as much as possible. There may be weeks when I won't have time to write a newsletter, or I may end up producing an abbreviated version of the newsletter. This breaks my heart, but I also need to keep my batteries charged and I'm trying to find the right balance for everyone. Thank you for your patience and continued support.