Thomasomys ucucha is a small Andean rodent belonging to the family Cricetidae and the genus Thomasomys, a group of high‑elevation mice endemic to the northern Andes. First described by Oldfield Thomas in 1917, the species is known from cloud forests and páramo ecosystems at elevations of 2,800–3,500 m in the central Ecuadorian Andes, with occasional records near the Peruvian border. Its dense, soft fur is gray‑brown dorsally and lighter ventrally, providing camouflage among moss‑covered understory and rocky outcrops. The species exhibits a relatively short tail, often tufted, and large, dark eyes adapted to the dim, misty habitats. Although its range is restricted, the IUCN currently lists T. ucucha as Least Concern due to its occurrence in several protected areas and an apparently stable population. Ongoing research aims to clarify its exact distribution, genetic diversity, and ecological role as a seed disperser and insect predator within these fragile high‑altitude ecosystems.

Thomasomys ucucha is primarily nocturnal and solitary, with individuals maintaining exclusive home ranges that may overlap minimally with conspecifics. Activity peaks shortly after sunset, when the mouse forages on the forest floor and among low vegetation. Territorial scent marking is achieved through glandular secretions on the cheeks and forepaws, and occasional vocalizations—soft chirps and squeaks—facilitate brief encounters between neighboring individuals. The species displays limited social interaction beyond brief mating encounters; however, mothers exhibit strong maternal care, defending nests constructed of moss and leaf litter. Seasonal movements are minimal, but individuals may shift micro‑habitat use in response to moisture gradients and food availability, especially during the rainy season when insect prey becomes abundant.

Thomasomys ucucha is an omnivorous forager with a strong insectivorous component, reflecting the high protein demands of a high‑altitude lifestyle. Its diet consists of arthropods (beetles, moth larvae, springtails), seeds of native grasses and shrubs, and occasional fruit fragments from cloud‑forest understory plants. Seasonal shifts are evident: during the dry season, the mouse relies more heavily on seeds and dry plant material, whereas the wet season sees a surge in insect consumption, sometimes comprising up to 60 % of stomach contents. Foraging occurs on the ground and among low vegetation, employing rapid, precise movements to capture prey. The species also caches small seed caches near its nest, a behavior that may aid in seed dispersal and provide a food reserve during periods of scarcity.

Sexual maturity in Thomasomys ucucha is reached at approximately 4–5 months of age for both sexes. Breeding is opportunistic but tends to peak during the early wet season (October–December), when food resources are most plentiful. After a gestation period of roughly 28–30 days, females give birth to litters of 2–4 altricial pups. The young are born hairless and blind, relying entirely on maternal milk for the first three weeks. Mothers construct insulated nests in concealed locations, such as under dense moss or within rock crevices, and provide exclusive care, nursing the pups until they are weaned at about 21 days. Juveniles achieve independence shortly after weaning, but they may remain within the maternal home range for an additional month before dispersing to establish their own territories.

The IUCN classifies Thomasomys ucucha as Least Concern, yet the species faces localized threats that could impact its long‑term viability. Primary pressures include habitat loss from agricultural expansion, especially potato and quinoa cultivation, and infrastructure development such as road construction that fragments cloud‑forest and páramo habitats. Climate change poses an additional risk by altering temperature and precipitation patterns, potentially shifting suitable elevational bands upward and reducing available habitat. Although the species occurs within several protected areas (e.g., Podocarpus National Park), enforcement of conservation measures is uneven. Population estimates are lacking, but field surveys suggest a stable trend within protected zones, while unprotected populations may be declining at an estimated rate of 5–10 % per decade. Ongoing monitoring and habitat restoration are recommended to mitigate these threats.

• Thomasomys ucucha is named after the Quechua word “ucucha,” meaning “small mouse.”
• It inhabits one of the world’s highest‑elevation cloud forests, often above 3,000 m.
• Its fur provides excellent insulation, allowing it to maintain body temperature in near‑freezing night temperatures.
• The species plays a dual ecological role as both seed disperser and insect predator, influencing plant regeneration and arthropod populations.
• Despite its limited range, T. ucucha has been recorded in three distinct protected areas, highlighting the importance of conservation networks in the Andes.