Calomys Musculinus

Calomys musculinus

Drylands Vesper Mouse

Description:

Members of this genus are small mostly granivorous rodents (Salazar-Bravo et al. 2001). The characteristics of C. musculinus include a mouse-like body form, with prominent ears, narrow hands and feet, and a moderately haired tail. The total body length ranges from 60-125 mm; tail length 30-90 mm. Generally, the tail is shorter and the head and body are rarely longer. In specific localities this species does not normally exceed 100mm, which can be used as a diagnostic character (Braun and Diaz 1999). The pelage is thin, and coloration varies from buff, tawny, grayish buff, grayish to dark brown. Ventral coloration is typically gray to white (Nowak 1999).

Distribution:

Range of C. musculinus includes northern and central Argentina (Nowak 1999). Formally it was thought that this species was only found in eastern Paraguay; however, research has found C. musculinus inhabiting the central Paraguayan Chaco region (Myers 1982; Yahnke 1999). There may also be reason to suspect that the range includes south central Bolivia to central Brazil (Salazar-Bravo et al. 2001).

Ontogeny and Reproduction:

Calomys musculinus like other members of the family Muridae possesses a Type III ovarian cycle. This is typified by a combination of spontaneous ovulation, short luteal phases in females that did not copulate and the development of fully functional progestational phase if copulation occurs. This reproductive strategy may have evolved due to predation pressure increasing the probability that females will become pregnant by returning to estrus quickly if breeding does not take place (Cutrera et al. 1998).

The typical breeding season includes most of the summer continuing into the autumn. The average time of reproductive stages was determined by laboratory studies of captive colonies. The average estrous cycle is 5.7 days. The gestation is 24.5 days, and their litter size is 5.4- 7.60. Females reach sexual maturity at 72.5 days, and males reach sexual maturity at 82 days (Nowak 1999; Mills 1992). These values characterize the opportunistic life history of the species, enabling it to colonize unstable crop habitats (Mills 1992).

Several studies have provided evidence in support of the hypothesis that C. musculinus is a polygamous species with a solitary social structure (Laconi and Castro-Vazquez 1999). In contrast, the closely related C. laucha is a more monogamous species. C. musculinus females have been observed to interact with more than one male in their home range, which overlaps the range of several neighboring males (Laconi et al. 2000). Copulation with more than one male may take place. The presence of a male, even the siring male, is deleterious for the young, both in reduced survival and protracted growth. Laboratory results show that maternal nesting behavior includes aggressively excluding the male both pre and postpartum (Laconi et al. 2000). Again compared with C. laucha, C. musculinus had a lower frequency of co-occupancy (Laconi and Castro-Vazquez 1999). Aggressive behavior such as combat may be responsible for the reduced growth rate and survival due to the heat loss of the highly altricial pups during maternal absence.

Seasonal changes influence reproduction in C. musculinus. Male reproductive systems are responsive to unfavorable environmental conditions of fall and winter, yet some reproduction does take place during this time (Mills et al. 1992). Animals, males in particular, that are born in the autumn do not usually become active until the following spring (Mills et al. 1992). Mass is more significant than age when determining sexual maturity (Mills et al. 1992).

Growth takes place in three distinct phases: the maximum instantaneous growth rate located between the birth and 30 days old, a lower growth rate between 60 and 90 days, and then a progressive decline in growth rate from 90 to 180 days. These growth ratestend to differ between the sexes (Provensal and Polop 1993).

Ecology and Behavior:

This nocturnal, secretive, rodent selects open vegetation habitat, with a wider niche than other coexisting rodent species. Expansion of human agricultural activities may be contributing to an increase in the abundance of C. musculinus by indirectly creating more edge or a boarder habitat (Busch & Kravetz 1992). C. musculinus preferentially selects boarders of fields over crop fields indicated by differences in abundances (Busch et al. 2000). Boarder habitats are more attractive for rodent species because of increased stability (Ellis et al. 1998). Activities that cause mortality such as sowing and harvesting do not affect these areas (Busch & Kravetz 1992). Crop type influences the abundance of this species, with higher densities present in corn than in soy beans, with population dynamics synchronized with corn (Busch et al. 2000). Vegetation reducing predatory risk may be more of an influence than dietary limitations of this more omnivorous, generalist species. The availability of green plant cover may be favored by reproductive individuals in early autumn when the boarders provide a more protected microhabitat for large, above ground nests that would otherwise be exposed in crop fields (Yunes et al. 1991). Large above ground nest may serve a thermoregulatory purpose in addition to their breeding purposes (Yunes et al. 1991). Nest building for thermoregulatory purposes may correspond to breeding, since the insulation conserves energy, which can then be delegated to reproduction (Yunes et al. 1991).

Seasonal distribution within the habitat may be influenced by the presence of other species, such as Akodon azarae and Calomys laucha, through competition. During winter and autumn, the presence of C. lucha and A. azarae has anegative relationship to C. musculinus (Busch et al. 2000)Distribution within the more arid portions of its range may be dictated by proximity to water (Cutrera et al. 1998).

Seasonalitymay also dictate the proportions of major dietary items such as insects, leaves and seeds the animals consume (Ellis et al. 1998). Leaves make up a small portion of the diet throughout the year. During the winter, seeds make up a larger proportion of the diet compared with arthropods. In the spring and summer, this is revered with the arthropods making the largest contribution to diet. The diet is the most diverse in the spring and summer and autumn; lowest in winter and spring. Corn and soybeans are the most important plants in the diet of C. musculinus, and they areconsumed in high proportions during the fall and winter. Other plants growing in boarders made more of a contribution in the spring and summer. Specific preferences are not present with most plant species consumed in proportion to availability. Dietary studies have yielded information about parasites with the nematodes as the most common parasite of the stomach (Ellis et al. 1998).

Remarks:

This species serves as a vector for Argentine Haemorrhagic fever (Junin Virus). The mode of transmission is thought to be predominantly through airborne virus-containing aerosols of rodent excreta. First recognized in 1955, the disease was initially localized in rural populations in the northwestern Buenos Aries province of Argentina. Annually 100-4,000 cases are reported, with the exception of 1993 when 24,000 cases were reported. Outbreaks of the disease are seasonal corresponding to the harvest of corn in March through June. Disease outbreaks have been strongly correlated with rodent density. Significant increases in the density of C. musculinus immediately preceded an outbreak of the disease (Mills et al. 1992). The common symptoms of the disease include gradual onset of fever, sore throat, myalgias, low back pain, and abdominal pain. The fatality rate is 15-30% (CIDRP 2004).

In addition to the Junin Virus, C. musculinus may also become infected with Trypanosoma cruzi, the protozoan parasite responsible for Chagas-Mazza disease (Cutrera et al. 1998).

Nomenclature:

The drylands vesper mouse belongs to the murid genus Calomys Waterhouse 1873 Rodentia: Sigmodontinae (Salazar-Bravo et al. 2001). Synonyms for Calomys musculinus, Thomas 1913 include, Calomys cordovensis, Calomys cortensis, Calomys murillus. Members of this genus, including C. musculinus, have been referred to as “lauchas” or vesper mice (Salazar-Bravo et al. 2001). A frequently used alternative common name for this species is the corn mouse.

Phylogenicallythere are two distinct clades within Calomys.C. musculinus belongs to the clade containing three total species, including C. lepidus and C. sorellus (Salaza-Bravo et al. 2001).

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Reference written by Sarah Orlofske, Biol 378 (Mammalogy), University of Wisconsin – Stevens Point: Edited by Kim Moore. Page last updated 12-20-04.