Onychomys arenicola
Mearns's grasshopper mouse

Onychomys arenicola, Mearns grasshopper mouse, has a limited distribution in the southwestern portion of North America. It can be found in south-central New Mexico, extreme western Texas, and northeastern Mexico. (Clary, et al., 15 July 1999; Davis and Schmidly, 1997; Hinesly, 1979)

Mearns' grasshopper mouse habitat in Texas includes low, arid, sandy or gravelly desert areas where the principle vegetation consists of creosote brush, mesquite, yucca, lechuguilla, and condalia. In New Mexico it is associated with desert grasslands dominated by black grama and mesa dropseed with occasional soaptree yucca and Mormon tea, annual and perennial forbes and grasses.

This mouse is found at elevations ranging from 1,340 m to 1,580 m. (Davis and Schmidly, 1997; Hinesly, 1979; Kay, 1998)

Onychomys arenicola is a small mouse with a fat tail. It has a cinnamon-pink or grayish dorsal side and a white ventral side. At the base of each ear on the front side, there is usually a white or grayish tuft of fur. The nose, cheeks and sides are white. The tail of this mouse is sparsely haired and bicolored, with dark above and white below. The tail makes up more than about 1/3 of total length of the mouse, and is 2.5 times as long as the hind foot length. This species is similar to Onychomys leucogaster, but smaller, with a relatively longer tail and smaller teeth. It is also similar in appearance to Onychomys torridus, but is smaller in every measurement except the nasal length of the skull.

Juveniles look like adults, but their upper parts are bluish-gray.

The total length of adults averages 146 mm, the tail averages 52 mm, and the hind foot averages 21 mm. The weight of adult males ranges between 24 and 30 g, with an average of 26.5 g. Females are lighter, typically weighing between 22 and 28 g, and averaging 25 g. (Davis and Schmidly, 1997; Hinesly, 1979)

The mating system of these mice has not been reported. They are presumed to be polygynous, although in congeneric species, males and females are known to associate in pairs throughout the year, and may be monogamous. (Nowak, 1999; Nowak, 1999)

Breeding begins in late January or early February and continues into September. Gestation period is 26 to 35 days. Litter size varies between two and seven. Half-grown young have been captured in April, June, July, and August, suggesting these mice produce two to three litters per year. (Davis and Schmidly, 1997)

Although data are scarce for O. arenicola, in general, mice in this genus weigh about 2.6 g at birth. Their eyes open at about 2 weeks of age. Young mice are typically weaned around 3 weeks of age. (Nowak, 1999)

Females in this genus become sexually mature at seven to eight weeks of age. A young female can give birth to her first litter at four months of age. (Davis and Schmidly, 1997b)

The young are born without much ability to move around. Their eyes are closed. Females nurse their offspring and care for them in a nest. It is not known whether the male participates in parental care. (Nowak, 1999)

Grasshopper mice kept in pairs in captivity reached an average age of barely four years, five years at the most. The lifespan of thse mice in the wild has not been reported. (Niethammer, 2001)

The behavior of O. arenicola is similar to that of O. leucogaster. These mice are primarily solitary, and predatory with proportionally large home ranges (about 2 ha). These mice never remain in one place for very long, constantly moving around their home ranges in search of food.

These mice live in burrows which they have either entered after the burrow has been abandoned by its original owner, or which they sometimes invade and overtake by force.

Onychomys arenicola shows very strong aggression toward other members of the same species and cannibalism has been observed much more frequently than in other rodents. It has been suggested that these mice may live in male/female pairs year round, although this would seem to shorten the lifespan, as one of the pair inevitably kills the other.

Aggression within the species is so strong that in 90 tests, in which two animals of the same sex were placed in 5x5 m cages, 70% of fights ended with the death of the loser within twenty-four hours.

Mating behavior has been described as quite complicated, surely a result of avoiding fatal aggressive encounters.

Onychomys arenicola hunt their prey like most sophisticated predators, stalking prey in an almost feline manner. Prior to attacking, or when faced with an adversary, these mice stand on their hind limbs, point their nose upward, and let out a shrill, piercing cry which lasts between 0.7 and 1.2 seconds. The function of this vocalization is not known. Humans can hear this cry up to 100 m away. It has been compared with a wolf's howl. After vocalizing, the mouse will rush toward and seize its prey, killing the hapless victim with a bite to the head.

Because of their short legs and chunky bodies, these grasshopper mice are not very quick footed. However, they are expert at dodging, twisting, and turning. In close quarters they can easily capture and overpower other mice of the same size or larger, a skill that probably helps them in their encounters with conspecifics. (Clary, et al., 1999; Davis and Schmidly, 1997; Niethammer, 2001; Nowak, 1999)

The home range of these animals is approximately 2 hectares.

Members of the genus Onychomys are known to make different vocalizations in different social contexts, including a "chit" sound during aggression, and a soft "chirp" during mating. This indicates that acoustic communication is important in this species. Fighing behavior most certainly could be considered a form of tactile communication. Tactile communication is probably also important between a mother and her young and between mates. It is likely that there are also visual and chemical signals passing between conspecifics, as is common in most mammals. (Nowak, 1999)

The feeding habits of Mearns' grasshopper mouse are similar to those of Onychomys leucogaster.

Their diet consists mostly of other animals (about 90%) but also a variety of plants (10%). They feed mostly on insects (including grasshoppers, scorpions, beetles, and crickets), but will attack and eat other small mammals, including other grasshopper mice.

The stomach of this species is interesting, because the peptic and muriatic glands are disposed in a special pocket, perhaps to protect them from damage by the chitin in their food. They are reported to cache food. (Davis and Schmidly, 1997; Davis and Schmidly, 1997b)

Onychomys arenicola avoids predators by spending daytime hours inside burrows. Their most active time coincides with little to no moonlight; that is before the moon rises, after it sets, during new moon phases, or during overcast conditions. (Davis and Schmidly, 1997; Niethammer, 2001)

Given its habitat and nocturnal lifestyle, the most likely predators of this species are owls, snakes, and carnivorous mammals like coyotes and bobcats.

As a predator, O. arenicola probably helps to regulate certain insect populations. As a prey species, abundance of O. arenicola may help to regulate populations of its predators. However, because it does not occur at very high densities, it is not likely to be a very important component of the diet of any predator.

Due tot heir seed caching behavior, it is possible that these animals also help to disperse seeds of certain plants.

Due to their strong territoriality and predatory nature, O. arenicola can be beneficial to people. O. arenicola can be beneficial by negatively impacting populations of small rodents and insects, species that may have negative affects on human practices such as agriculture.

Members of the genus Onochomys are reported to make docile and fascinating pets, indicating that these animals present some value to the pet trade. (Davis and Schmidly, 1997; Nowak, 1999)

When the supply of food is low, grasshopper mice have been known to eat and or cache plant matter. This may result in negative affects on human agriculture. (Niethammer, 2001; Nowak, 1999)

No special conservation issues are currently recognized, but due to the geographic isolation and ecology of this species, habitat loss, fragmentation, and degradation could significantly influence population dynamics.

In previous articles O. arenicola was combined with O. torridus; however, recent genetic work has recognized Mearns grasshopper mouse as a separate species. In southwestern New Mexico, O. arenicola was found to be specifically distinct from O. torridus when a contact zone between the two species was found and no hybridization or back crosses, based on karyotypes, occurred. (Davis and Schmidly, 1997; Hnida, et al., 1998)