Sigmodon arizonae plenus

Colorado River Cotton Rat

Family: Cricetidae · Class: Mammalia · Order: Rodentia

Conservation status: G5T2T3 S1S2

The Colorado River cotton rat (Sigmodon arizonae plenus) is a subspecies of Arizona cotton rat endemic to the Colorado River valley. Adults typically measure 95-120 mm (3.7-4.7 inches) in head-body length with a tail ranging from 85-110 mm. The fur is grayish-brown to brownish-gray on the dorsal surface with lighter gray undersides. Like other cotton rats, this subspecies has small eyes, short rounded ears, and a relatively short tail compared to body length. The pelage is coarse and somewhat harsh to the touch, with individual hairs having dark bases and lighter tips creating a grizzled appearance. Historically, the Colorado River cotton rat occurred along the Colorado River from southern Nevada through western Arizona and into extreme southeastern California. The subspecies was documented in riparian areas of the lower Colorado River valley, including portions of Imperial and Riverside counties in California. Current distribution appears significantly reduced from historical range, with the species experiencing substantial population decline due to habitat loss and degradation. This subspecies inhabits dense riparian vegetation along the Colorado River and its tributaries. Preferred habitats include areas with thick understory vegetation such as arrowweed (Pluchea sericea), mesquite (Prosopis species), and salt cedar thickets. The species typically occurs in areas with permanent water nearby and requires dense ground cover for nesting and protection from predators. Elevation range is generally below 500 meters (1,640 feet) in the desert riparian zone. Colorado River cotton rats are primarily nocturnal and crepuscular, remaining active during cooler parts of the day. They construct runway systems through dense vegetation and create spherical nests woven from grasses and other plant materials. Diet consists primarily of green vegetation, seeds, and occasionally insects. Reproduction occurs year-round in favorable conditions, with females producing multiple litters annually. Gestation period is approximately 27 days, with litter sizes ranging from 2-6 young. The species has a relatively short lifespan of 6-12 months in the wild. According to USFWS ECOS database, the Colorado River cotton rat is currently listed as a Species of Concern, reflecting significant conservation concerns (USFWS ECOS 2025). The subspecies has a global rank of G5T2T3, indicating the full species is secure globally but this subspecies is imperiled to vulnerable. In California, it carries a state rank of S1S2, meaning it is critically imperiled to imperiled within the state. Primary threats include habitat destruction from agricultural conversion, urban development, water diversions that alter natural flood cycles, and invasion of non-native vegetation that degrades riparian habitat quality. The construction of dams and water control structures along the Colorado River has fundamentally altered the natural hydrology that maintained suitable riparian habitats. Recovery efforts focus on habitat restoration and protection of remaining riparian areas through the Lower Colorado River Multi-Species Conservation Program.

Data from The California Species Project — 14,000+ California species with verified data from CNPS, CDFW, USFWS, and more.