Akodon cursor (montensis) - Cursor Grass Mouse

 Photo by Christopher Yahnke, collection of the Field Museum of Natural History

Description:

Akodon montensis is a short heavy-bodied vole-like mouse, with a short tail and limbs.  The pelage is soft and full.  The cursor grass mouse is reddish-brown to olive-brown dorsally, fading to tan on the sides and gradually becoming a reddish-tan to grayish-orange on the venter.  The feet are tan and the tail is sparsely haired and somewhat bi-colored.   The face shows some blackish hairs (Eisenberg and Redford 1999, Nowak 1999).  The ears are rounded and small, and the muzzle is pointed with fine, short whiskers, which do not reach the tips of the ears (Emmons and Feer 1990).     Total length is 90-136 mm; tail length, 32-98 mm; hind foot length, 17-28mm; and ear length, 11-21 mm.  Juvenile males weigh 28g and juvenile females weigh 30g.  The sub-adult male weights range from 28-45g, and female sub-adults weight from 30-40g.  Adult males weigh around 45g and the females can weigh more than 40g (Emmons and Feer 1990, Gentile and D'Andrea 2000).  Akodon montensis has a karyotype of 2n = 24 (Geise and Canarez 1998).

The nasals are long and project behind the maxilla-frontal structure. The zygomatic plate is short.  The mandible is slender and elongated; it has a posteriorly-projecting condylar process and an inflated capsular projection (root of incisor) (Liascovich and Reig 1989).

Distribution:

Akodon montensis is distributed through southeastern and central Brazil, Uruguay, Paraguay, and northeastern Argentina (Eisenberg and Redford 1999).  Akodon montensis is the correct name for populations occurring in Uruguay, southern and eastern Paraguay, southeastern Brazil, and the Misiones and Chaco provinces of Argentina.  Akodon cursor is the correct name for populations occurring in Misiones, Argentina and Sao Paulo, Brazil (Eisenberg and Redford 1999, Liascovich  and Reid 1989).

Ontogeny and Reproduction:

Reproduction occurs during wet and/or dry months, depending on habitat location and elevation, but cursor grass mice will reproduce year-round if conditions are suitable.  Reproduction is related to resource availability.  Reproductive activity does decrease during the colder months.  Most births occur during the dry months, but the mice don’t have a clear pattern of reproductive activity (Bergallo and Magnusson 1999, Gentile and D'Andrea 2000).

Gestation lasts for approximately 23 days.  Females usually have 2 litters per year, with each litter averaging 3-5 young.  Some litters have up to 10 young.  The offspring are weaned after 1-15 days, and sexual maturity is reached around 32-37 days for males and 39-51 days for females.  Short gestation time, early maturity, and short life expectancy are traits that result in rapid population turnover and in rapid responses to environmental variations (Eisenberg and Redford 1999, Gentile and D'Andrea 2000, Feliciano and Fernandez 2002).  Akodon montensis will hybridize with other species within the same genus, especially with Akodon cursor.  The hybrids are infertile (Geise and Canavez 1998).

Ecology and Behavior:

Akodon montensis is the most common rodent species of the forest-grassland ecotome.  It is nocturnal and completely terrestrial.  It favors sites of dense herbaceous cover and increased litter cover.  Akodon montensis travel in tunnels under the leaf litter and nest in burrows (Emmons and Feer 1990).  This species is susceptible to habitat disturbances and is more habitat-selective (Gentile and D'Andrea 2000).  Akodon montensis is restricted to evergreen forest patches along streams and rivers.  Occasionally, they may be found near the edges of cultivated fields and/or open grasslands (Nitikman and Mares 1987, Emmons and Feer 1990, Figueiredo and Fernandez 2004).  Akodon montensis and Akodon cursor are segregated by elevation.  Akodon cursor occurs from sea level to 800 meters, while Akodon montensis is found above 800 meters (Geise and Smith 2001).

Akodon montensis has a small home-range size with individuals occupying no more than 100 square meters.  Most dispersal occurs within 30 meters of the nest site.  No significant differences between male and female movements patterns have been observed.  Researchers are discovering that their geographic position is expanding.  This may result in an increased evolutionary independence between populations over time.  The different patterns of geographic structure might reflect the degree to which each species has reached a geographic equilibrium, with isolation reflecting the balance between dispersal and drift in local populations (Geise and Smith 2001).

Akodon montensis blends well with its environment and utilizes litter and herbaceous cover to conceal itself from predators.  A variety of raptors and other carnivores feed on this species (Eisenberg and Redford 1999).  This species is an omnivore with a diverse diet.  It eats leaves, seeds, fruits, and insects such as coleopterans, lepidopterans, dipterans, and leaf litter arthropods (Emmons and Feer 1990, Eisenberg and Redford 1999, Figueiredo and Fernandez 2004).

Remarks:

The taxonomy of Akodon cursor and Akodon montensis is currently being examined.  Akodon montensis was originally described as a subspecies of Akodon cursor.  Although distributional, chromosomal, morphologic, and molecular evidence support the species status, confusion remains because these two taxonomic entities are difficult to distinguish by external or cranial characteristics.  One way to distinguish the two is that Akodon cursor has a gall bladder, whereas Akodon montensis does not.  A second way to tell them apart is by their karyotype.  Akodon montensis is 2n = 24, while Akodon cursor is 2n = 14/15 (Geise and Smith 2001, Pardinas and D'Eila 2003).  An interesting feature about Akodon montensis is that fertile XY females are found in natural populations (Pardinas and D'Eila 2003).

Akodon montensis is a natural host to the parasite Besrioitia sp.  The parasite is a thick, white, glistening cyst that encloses the host’s cell nuclei. The Besrioitia sp. is found in skeletal muscles and several organs including the intestines, lungs, and liver.  (Grisard and Elsaid 1997).          

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