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Excerpts from Jim Conrad's
Naturalist Newsletter
entry dated August 23, 2022, issued from near Tequisquiapan, elevation about 1,900m (6200 ft), N20.565°, W99.890°, Querétaro state, MÉXICO
WEST MEXICAN COTTON RAT
For most of the last year during my daily walks into the grossly overgrazed, thin-soiled scrublands consisting of patches of clumpgrass amid mostly knee-high, spiny shrubs and the occasional mesquite and prickly-pear tree cactus, I've been glimpsing rats. Nowadays, with the beginning of the rainy season (which has arrived two months late), I'm seeing many more. On a recent walk a friend came along with a much better camera than mine and at a fair distance was able to take the above picture.
It's tricky to identify rodents from such pictures because often it's necessary to see the teeth and measure parts. However, the salt-and-pepper gray fur on this fairly large rat and the long tail reminded me of the Hispid Cotton Rat, Sigmodon hispidus of the US, so I started looking for Sigmodon rats found in semiarid upland central Mexico.
On the Internet plenty of images of the Hispid Cotton Rat are available because it occurs throughout the Americas, from Brazil to the central US, and some of those pictures look like our rat. However, our rat appears to have larger ears than most rats identified as Hispid Cotton Rats. Also, most Hispid Cotton Rat pictures show whitish underbellies, while our rat's underparts are dark gray, plus the AnimalDiversity.Org website places its upper elevation limit as 1130m (3700ft), while we're at about 1,900m (6200 ft).
*UPDATE: In 2022 the best information found about Mexican rats was a Mexican government webpage entitled "Colección Biológica de Roedores Plaga del Centro Nacional de Referencia Fitosanitaria. That page listed Mexico's "pest rodents," of which only one appeared to resemble our rat, and that was the West Mexican Cotton Rat, Sigmodon mascotensis. In 2025, that webpage has disappeared. However, with our picture uploaded to the iNaturalist website, which has become the website for naturalists worldwide to document and identify their observations, two iNaturalist users, "jpietra" and "mauro_rojo" recognized the closely related White-eared Cotton Rat, SIGMODON LEUCOTIS. Moreover, iNaturalist's constantly improving AI Identification App and its excellent distribution maps indicate that this ID most likely is correct.
The White-eared Cotton Rats are endemic just to Mexico, from the uplands of southwestern Chihuahua and southern Nuevo León south to central Oaxaca. It is not considered to be a threatened species.