Andean Vesper Mouse

The Andean Vesper Mouse is a small rodent endemic to the high-altitude puna and paramo grasslands of the central Andes. It is one of the most cold-adapted members of the genus Calomys, thriving at elevations where few other small mammals persist. This species plays an important role in high-altitude ecosystems as both a seed disperser and a prey item for Andean predators. It is notable for its resilience to extreme diurnal temperature fluctuations typical of high-altitude environments.

High Andes of Peru, Bolivia, Chile, and Argentina

Distributed along the high Andes from southern Peru through Bolivia and northern Chile into northwestern Argentina, generally restricted to the altiplano and surrounding highland plateaus.

Altitude and depth

Typically found between 3,000 and 5,000 meters above sea level, among the highest elevational ranges of any Calomys species.

Seasonal movements

Breeding activity and population density fluctuate seasonally, generally increasing during the wetter austral summer months when vegetation and food availability are higher.

Found primarily in puna grasslands, high Andean steppe, and rocky outcrops with sparse bunch grass (ichu) vegetation. It also occupies agricultural edges and disturbed areas near human settlements at high elevation.

Adaptations to its environment

Possesses dense fur for insulation against cold high-altitude temperatures and physiological adaptations for efficient oxygen use at low atmospheric pressure. Its burrowing behavior provides thermal refuge from extreme daily temperature swings characteristic of the puna.

Listed as Least Concern by the IUCN due to its wide distribution across the high Andes and presumed large, stable population without major range-wide threats.

Estimated population

No precise global population estimate exists, but the species is considered common and widespread within suitable habitat, with a presumed stable population trend.

Localized threats include habitat degradation from overgrazing by livestock, agricultural expansion, and mining activities in some parts of its range; climate change may also alter puna grassland composition.

Diseases and parasites

Like other Calomys species, it may host ectoparasites such as fleas and mites, and some Calomys species are known reservoirs for arenaviruses, though specific pathogen data for this species is limited.

No species-specific conservation programs exist; the species benefits indirectly from protected areas within the Andean altiplano region such as national reserves in Bolivia and Peru.

Recommended actions

Recommended actions include monitoring population trends in response to climate change, protecting puna grassland habitat from overgrazing, and further taxonomic study to clarify subspecies status.

The fur is soft and dense, generally grayish-brown to yellowish-brown on the dorsal surface with a paler, whitish to grayish underside. The tail is relatively short compared to body length, and the ears are small and rounded, an adaptation to reduce heat loss.

Size

Head-body length typically ranges from 60 to 90 mm, with a tail length of approximately 50 to 75 mm.

Weight

Adults typically weigh between 10 and 20 grams, with little difference between sexes.

Distinctive features

Distinguished from other Calomys species by its shorter tail relative to body length, denser pelage, and specific cranial features adapted to its high-altitude habitat.

Generally solitary or found in loose family groups, with individuals maintaining separate burrow systems though home ranges may overlap.

When it is active

Primarily nocturnal and crepuscular, though some activity has been observed during daylight hours, likely influenced by extreme cold nighttime temperatures at high altitude.

Diet

Omnivorous, feeding mainly on seeds, grasses, and other plant material, supplemented occasionally with insects and other small invertebrates when available.

Communication

Communication likely involves ultrasonic vocalizations, scent marking via urine and glandular secretions, and tactile interactions typical of small rodents, though specific studies on this species are limited.

Interactions with other species

Serves as prey for Andean predators such as foxes, owls, and raptors; may compete with other small sympatric rodents like Phyllotis species for food and burrow sites.

Role in its ecosystem

Functions as an important prey species in the high Andean food web and contributes to seed dispersal and soil turnover through its burrowing activity in puna grasslands.

Breeding appears to peak during the wetter season, with females producing litters of typically 3 to 6 young after a gestation period of approximately 20 to 25 days, consistent with related Calomys species.

Raising the young

Females provide primary care for offspring, nursing and sheltering young in burrows until weaning, which occurs at a few weeks of age similar to other small cricetid rodents.

Maturity and lifespan

Sexual maturity is likely reached within 2 to 3 months, typical for small cricetid rodents; wild lifespan is probably less than 1 to 2 years due to high predation pressure.

Classification

Class Mammalia > Order Rodentia > Family Cricetidae > Genus Calomys > Species Calomys lepidus

Subspecies

Several subspecies have been described historically (e.g., C. lepidus lepidus, C. lepidus vulcani) but their validity is debated and many are now considered synonyms of the nominate form.

Scientific synonyms

Hesperomys lepidus (original combination); Calomys lepidus vulcani has sometimes been treated as a separate subspecies

Recent studies

Recent phylogenetic studies using molecular data have helped clarify the taxonomic relationships within the genus Calomys, including the placement and validity of subspecies of C. lepidus.

Genome and DNA

Specific chromosome number data for Calomys lepidus is limited, though related Calomys species generally have diploid numbers ranging from 2n=36 to 2n=64 depending on species.

Compared with similar species

Compared to its lowland relative Calomys musculinus, the Andean Vesper Mouse has denser fur and a shorter tail, reflecting adaptations to cold high-altitude conditions rather than temperate lowland habitats.

This mouse lives at some of the highest elevations recorded for any rodent in the Americas, surviving in environments with intense UV radiation and near-freezing nighttime temperatures. It is remarkably adapted to low-oxygen conditions found at altitudes above 4,000 meters.

Where and how to see it

Best observed at dusk or during early morning hours in open puna grasslands of the Bolivian or Peruvian altiplano; patience and use of small live traps or careful nighttime spotting with a flashlight increase chances of sighting.

Risks to humans

No notable risk to humans, though as with other rodents, caution should be taken regarding potential zoonotic disease transmission through contact with droppings or urine.