Day's Grass Mouse

Day's Grass Mouse is a small South American sigmodontine rodent belonging to the species-rich genus Akodon, commonly known as grass mice or field mice. It inhabits the Andean Yungas forests and adjacent montane grasslands of Bolivia, occupying a relatively restricted range along the humid eastern slopes of the Andes. Like other members of its genus, it is a terrestrial, semi-fossorial mouse adapted to dense ground-level vegetation. It is considered Least Concern by the IUCN, suggesting a relatively stable population with no immediate major threats.

Bolivia, primarily in the Yungas region along the eastern Andean slopes, possibly extending into adjacent areas of Peru and northwestern Brazil.

Endemic or near-endemic to Bolivia, with records concentrated in the departments of Cochabamba and Santa Cruz along the eastern Andean foothills. Its range may marginally extend into adjacent regions of Peru or Brazil, but this remains poorly documented.

Altitude and depth

Typically recorded between approximately 500 and 2500 m above sea level, corresponding to the Yungas altitudinal belt on the eastern Andean slopes.

Seasonal movements

No documented seasonal migrations. Like most small Akodon rodents in the region, it is likely active year-round, with possible fluctuations in population density linked to seasonal rainfall and food availability.

Primarily montane humid forests (Yungas) and ecotonal areas between cloud forest and high-Andean grassland (puna). It favors dense herbaceous undergrowth, leaf litter, and scrubby vegetation where ground cover provides shelter. It may also occur in secondary growth areas along forest edges.

Adaptations to its environment

Its compact, robust body and short limbs are suited for movement through dense ground vegetation and leaf litter. Dense pelage provides insulation against the cool, moist Yungas climate. Generalist dietary habits allow it to exploit variable food resources across seasons.

Least Concern (LC) on the IUCN Red List. The species is presumed to have a sufficiently large and stable population within its Bolivian Yungas range, with no evidence of rapid decline meeting threatened thresholds. Its habitat, though subject to ongoing pressure, still supports viable populations.

Estimated population

No precise global population estimate is available. The population trend is assumed to be stable based on IUCN LC classification, though detailed surveys are lacking.

Habitat loss and fragmentation due to agricultural expansion, coca cultivation, logging, and road construction in the Bolivian Yungas are the primary threats. Climate change may alter cloud forest distributions and reduce suitable habitat over time.

Diseases and parasites

Like other Akodon, it is likely a host for hantaviruses (a notable concern for the genus in South America), as well as fleas, ticks, mites, and intestinal helminths. Specific parasite surveys for this species are not well documented.

Portions of its range fall within protected areas such as Carrasco National Park and Amboró National Park in Bolivia, which provide some degree of habitat protection. Bolivian national legislation offers general wildlife protections applicable to this species.

Recommended actions

Comprehensive field surveys to better define population size and range limits; monitoring of habitat integrity within Bolivian Yungas protected areas; strengthening enforcement against illegal deforestation in buffer zones; and taxonomic/genetic studies to clarify species boundaries.

Dorsal fur is brownish to grayish-brown, often with a slightly darker mid-dorsal stripe or grizzled appearance. The ventral surface is paler, typically grayish-white or buffy. The tail is relatively short compared to body length, lightly bicolored, and sparsely haired. Ears are small and rounded.

Size

Total body length approximately 90–130 mm; tail length roughly 60–95 mm; hindfoot length approximately 18–24 mm. No significant published size data specific to this species; estimates are based on morphologically similar congeners.

Weight

Estimated 15–35 g, typical for Akodon species of similar size. No documented sexual weight dimorphism data specific to this species.

Distinctive features

Distinguished from closely related Bolivian Akodon species (e.g., Akodon boliviensis, Akodon cursor) by subtle cranial morphology, dental proportions, and molecular characters. Precise field identification requires careful measurement and often genetic analysis, as external appearance is broadly similar across the genus.

Differences between males and females

Sexual dimorphism is minimal in external appearance, as is typical for Akodon. Males may average slightly larger than females, but the difference is not diagnostically reliable in the field.

Likely solitary or loosely social outside the breeding season, consistent with most Akodon species. Population densities can fluctuate markedly with resource availability. Territorial behavior around burrows or core foraging areas is probable.

When it is active

Primarily nocturnal to crepuscular, with most foraging activity occurring after dusk. Daytime rest periods are spent sheltering under logs, dense vegetation, or in shallow burrows.

Diet

Omnivorous; diet likely includes seeds, plant material (stems, roots, fungi), and invertebrates (insects, larvae, worms). Dietary composition probably shifts with seasonal availability of resources, as documented in closely related species.

How it feeds and hunts

Forages by actively searching through leaf litter and ground vegetation for invertebrates and seeds. May use olfactory cues to locate buried seeds or soil-dwelling invertebrates. No pursuit hunting; captures prey by pouncing or digging.

Communication

Communication likely involves ultrasonic vocalizations for mother-pup interactions, scent marking via urine and glandular secretions to demarcate territories, and tactile and visual signals during conspecific encounters.

Interactions with other species

Predated upon by owls (e.g., Tyto alba, Strix species), snakes, and small felids (e.g., Oncilla). Competes with sympatric rodents such as other Akodon species and Oligoryzomys for food and microhabitat. May serve as host for ectoparasites and endoparasites shared with other small mammals in the Yungas community.

Role in its ecosystem

Plays a role as a seed disperser and consumer of invertebrates in the Yungas forest ecosystem. As prey for raptors and carnivores, it functions as an important link in the local food web. Its burrowing activity may contribute modestly to soil aeration.

Breeding likely peaks during the warmer, wetter months (austral spring and summer, approximately October–March), consistent with other Andean Akodon. Gestation is approximately 20–23 days in the genus. Litter size is typically 2–6 pups.

Raising the young

The female is the primary caregiver, nursing and protecting pups in a nest. Young are altricial at birth (eyes closed, hairless) and are weaned at approximately 14–21 days. Males show little to no direct parental involvement, as is typical in the genus.

Maturity and lifespan

Sexual maturity is typically reached at 2–3 months of age in Akodon species. Lifespan in the wild is estimated at 1–2 years; captive individuals may live up to 3–4 years under optimal conditions.

Classification

Mammalia > Rodentia > Cricetidae (subfamily Sigmodontinae) > Akodon > Akodon dayi

Scientific synonyms

Originally described by Osgood in 1916; no widely recognized synonyms, though earlier specimens may have been attributed to related Akodon taxa before formal species delimitation.

Recent studies

Molecular phylogenetic studies of the Akodon boliviensis species group have helped clarify the taxonomic status and boundaries of A. dayi relative to closely related taxa in the eastern Andean foothills. No landmark discoveries specific to this species have been widely published as of 2023.

Genome and DNA

The Akodon genus is renowned for remarkable karyotypic variability, including cases of XY females in some species. The chromosome number for A. dayi has not been extensively published; diploid numbers across the genus range from 2n=10 to 2n=44. Genetic studies placing A. dayi within Akodon phylogenies have been limited.

Compared with similar species

Akodon dayi is most similar to Akodon boliviensis and Akodon sylvanus, which share overlapping or adjacent ranges in Bolivian montane habitats. It is distinguished primarily by cranial measurements and molecular markers. The broader Akodon genus (~40 species) is one of the most speciose mammalian genera in South America.

The Akodon genus contains some of the only mammal species in the world where a significant proportion of genetic females carry an XY sex chromosome pair rather than XX — a phenomenon linked to a non-functional Y chromosome variant. Day's Grass Mouse shares its Yungas home with an extraordinary diversity of endemic species, making the Bolivian cloud forest one of the world's most biodiverse regions per unit area. Despite being relatively understudied, A. dayi has contributed to broader understanding of Andean small mammal biogeography.

Where and how to see it

Best observed during nocturnal surveys in the Yungas forests of central Bolivia (e.g., around Carrasco National Park or the Chapare region) using live-trapping with Sherman traps baited with oats, peanut butter, or seeds. The rainy season (November–March) tends to coincide with higher activity and population peaks. A permit from Bolivian wildlife authorities is required for trapping surveys.

Risks to humans

No notable direct risk to humans. As with other wild rodents in South America, there is a theoretical risk of hantavirus exposure through contact with urine, feces, or saliva; standard precautions (gloves, masks) are advised when handling wild individuals.