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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).
Mexico's governmental Centro Nacional de Referencia Fisanitaria has a web page entitled "Colección Biológica de Roedores Plaga del Centro Nacional de Referencia Fitosanitaria" listing "pest rodents" documented in Mexico. A pest rodent is one that might appear in an agricultural or ranch setting. They list four identified Sigmodon species for Mexico, of which only one exhibits the large ears, dark gray underside, occurs at our elevation, and is found in our area, and that's the West Mexican Cotton Rat, SIGMODON MASCOTENSIS.
West Mexican Cotton Rats are endemic just to western Mexico, from southern Nayarit and Zacatecas states as far east as western Hidalgo and western Puebla, south to northwestern Oaxaca, at elevations from 0-2250m (7400ft).
The 2017 study by Sara T Martínez-Chapital and others entitled "Sigmodon mascotensis (Rodentia: Cricetidae)," reports that our rat occurs in many habitats, from mangrove swamps and tropical rainforest to pine and oak forests and grasslands, disturbed areas and croplands. Recognizing the similarities of our rat with the very widely distributed Hispid Cotton Rat, they mention studies indicating that the Hispid Cotton Rat is restricted to northeastern Mexico, while most cotton rats in southwestern Mexico are West Mexican Cotton Rats.
Our rat doesn't seem to be much of a pest, though often it's seen in and around agricultural fields and pastures.
The above-mentioned study by Sara T Martínez-Chapital and others describes our rat in great detail, from its diet and breeding cycle to the length of its postpalatal teeth. At this writing that admirable work is freely available on the Internet.