It may sound like a meme, but that’s its name. That or “Ecuadorean cotton rat”, if you’re feeling less adventurous. But for once I can tell you the origin of its bizarre name, and that alone is unexpected.

Before you wrinkle your nose, cotton rats (genus name Sigmodon) look more like voles than your average creepy sewer rat, and rather than feeding on rubbish, they prefer much healthier foods like seeds and grasses.

That doesn’t prevent any farmer fist-shaking, of course, since they’re also partial to crops and carry diseases. Over time we’ve turned the latter to our advantage, so the pandemic likely sucked for them as well.

“Very cute Hispid Cotton Rat (Sigmodon hispidus) eating stalk of grass near the St. Marks NWR lighthouse”. Image by James Young, via Flickr.

Just to give a(n unpleasant) taster, hispid cotton rats (Sigmodon hispidus) were used for polio research back in 1937; to develop a typhus vaccine during World War II; and to this day are one of the most “popular” rodents for infectious disease research. And popular is one of the last things you’d want to be during a pandemic. For several reasons.

Fortunately, our unexpected Ecuadorean friend has managed to swerve all that. It isn’t well known and so far we’ve only given 11 of them a scientific poking. We think it follows the usual cotton rat playbook and is active both day and night, living solo unless it’s a mum, and forming “runs” in grasses in between feeding, nesting and hiding places.

When he discovered it in 1924, Anthony thought the frosty fur along its back and sides was one of its main characteristics. However, other species were found to have this too, and this was one of the reasons it was demoted to a subspecies of hispid cotton rat in the 1950s. (“Uh-oh,” – the unexpected, and at-the-time lab-free cotton rat.)

It was only in the 1990s, when Voss came along and pointed out that parts of its skull, teeth and genetics made it unique, that it regained its title (“Phew!” – the unexpected, and continuously lab-free cotton rat). That wasn’t the unexpected part, though.

Cotton rats are “New World” rats – found in the southern US and northern regions of South America – and until Anthony’s discovery, they were the poster child for tropical and subtropical areas. So it was more than a little surprising to find one scampering about at 4,500m/14,763 feet on the highest mountain in Ecuador.

Chimborazo, Ecuador.
Image by Alejandro Miranda from Pixabay

In case your physical geography is as terrible as mine, that would give our “unexpected” cotton rat a literally frosty coat some mornings, since Mount Chimborazo regularly sees snow. So how did it get there? A gradual change in real estate, of course.

Reig thinks its ancestor was an off-shoot of the Andean swamp rat (Neotomys), and while it was migrating from northern Peru, the land shifted upwards. Not in some catastrophic event of course, but somewhere between 5.3 million and 11,700 years ago in the Plio-Pleistocene period.

It’s since carved itself a nice little ecological niche, although it has deforestation, farming expansion and the superior numbers of the soft grass mouse (Akodon mollis) to contend with. And since it’s considered Vulnerable by the IUCN, its continued survival isn’t unexpected.


Unexpected Cotton Rat Fact File

Imagery © 2022 NASA, TerraMetrics, Map data © 2022 Google, INEGI United Kingdom

What? A vole-like rat that lives at high altitudes in central Ecuador.

Scientific name? Sigmodon inopinatus.

Sigmodon refers to the s-shaped cusps of its molars, found in other cotton rats, and inopinatus means “surprise” or “unexpected”. So a straight-forward name for a change!

Where? The often chilly slopes of Chimborazo, central Ecuador, at 3,500-4,500m /11,482-14,763 ft elevation.

How big? From head to tail, about 25 cm/9.8 inches long.

First recorded? In 1924 by H.E. Anthony.

Diet? Most likely grasses and seeds, like other types of cotton rat.

Behaviour? Not sure. Probably diurnal and solitary, again like other cotton rats. One female was found with four embryos, but that’s expected to be the lower end of the litter size.

Endangered? Considered “Vulnerable” by the IUCN thanks to the usual suspects of deforestation, farming and population fragmentation.

Just to prove I’m not fibbing:

American Museum of Natural History. No date. “Plio-Pleistocene“.

Anthony, H.E. 1924. “Preliminary Report on Ecuadorean Mammals. No 4“, American Museum Novitates 114(1-6).

Barnett, Adrian A. 1999. “Small mammals of the Cajas Plateau, southern Ecuador: Ecology and natural history“, Bulletin of the Florida Museum of Natural History, 42(4):161-217.

Curlee, Joseph F. et al. 2012. “The Laboratory Rabbit, Guinea Pig, Hamster, and Other Rodents“.

Editors of Encyclopaedia Britannica. No date. “Cotton rat“.

Eisenberg, John F., and Redford, Ken H. 1999. “Mammals of the Neotropics, Volume 3: Ecuador, Bolivia, Brazil“. University of Chicago Press.

Gardner, Alfred L. et al. 2015. “Mammals of South America: Volume 2: Rodents“. University of Chicago Press.

Harris, Arthur H. 2014. “Sigmodon sp. – Cotton rats“. UTEP Biodiversity Collections, Department of Biological Sciences, and Centennial Museum, University of Texas at El Paso.

Hulin, Mark S. et al. 2006. “The Laboratory Rat“.

Naylor, L. & Roach, N. 2018. “Sigmodon inopinatus“. The IUCN Red List of Threatened Species 2018: e.T20214A22355983.

Swier, V. J. et al. 2009. “Patterns of Chromosomal Evolution in Sigmodon, Evidence from Whole Chromosome Paints“, Cynogenetic and Genome Research, 125(1): 54–66.

Wilson, Don E. and Reeder, DeeAnn M. 2005. “Mammal Species of the World: A Taxonomic and Geographic Reference: Volume 1.” The Johns Hopkins University Press.

Zeballos, H. & Vivar, E. 2016. “Akodon mollis“. The IUCN Red List of Threatened Species 2016: e.T748A22383156