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Calomys laucha - Small Vesper Mouse


Calomys laucha is similar in appearance to a house mouse, with a tail length approximately 40% of total body length.  The dorsal pelt is a mixture of light brown, orange/gold, and black.  The ventral pelage is grey/white.  An identifying characteristic of Calomys laucha is a small area of white fur behind both ears.  The mean length of total body, tail, hind foot, and ear are 127.0 mm, 54.9 mm, 16.5 mm, and 13.1mm respectively (Redford & Eisenberg 1992).  Calomys laucha adults weigh approximately 12.8g (Yahnke et al. 2001).


Calomys laucha is found in the Chaco regions of South America, including those of Paraguay, central Argentina, southern Bolivia, southeastern Brazil, and Uruguay (Bilenca and Kravetz 1999).  The Chaco can be divided into two sub-regions, the Chaco Austral of Argentina and the Chaco Boreal of Paraguay and Bolivia. The topography of the Chaco is primarily flat.  It includes a wide variety of vegetative habitats.  Calomys laucha is thought to primarily inhabit the western Chaco where conditions are drier and the soils sandier (Myers 1982). 

Ontogeny and Reproduction:

The small vesper mouse has a fairy short oesterous cycle, which makes them a popular choice for research in reproductive physiology.  Females are easily bred, have large litters, and ovulate multiple times throughout the year.  The males also have a noteworthy reproductive characteristic.  The sperm of the small vesper mouse has a flat, rather than the more common hooked head.  The tail of the spermatozoa is attached to the center of this flat head, and the nucleus of the sperm is asymmetrical (Lasserre et al. 2000).  Calomys laucha produces an average of 5.15 young per litter.  The most productive periods for Calomys laucha are the colder months of the year (Yahnke et al. 2001).  Nests are cup-shaped and made of soft materials including grasses, leaves, stems and various man-made items.  The small vesper mouse has a monogamous mating system (Chiappero et al. 2002). 

Ecology and Behavior:

The habitat selection of Calomys laucha changes seasonally.  During the summer the small vesper mouse selects habitats with high levels of vegetation.  In the winter months the small vesper mouse takes refuge in border habitats with less vegetative cover and more open spaces (Bilenca & Kravetz 1999).  Calomys laucha are found at the highest densities in crop fields, possibly because of the high level of disturbance experienced there.  Many studies have shown that Calomys laucha are broad-based omnivores, with diets including seeds, leaves, grains and arthropods.  A food item that Calomys laucha seems to prefer is the E. indica seed (Castellarini et al. 2003).  The diet of the small vesper mouse is hypothesized to be a function of their relationship with coexisting species with similar diets as well as the disturbance level of their habitat.   Many rodent communities of South America are constructed in a hierarchical manner.  There have been significant findings supporting the concept of interspecific competition as a determinant of niche habitat partitioning between Calomys laucha and its conspecifics.  The Akodon azarae, for example, seem to out-compete Calomys laucha in the winter months for control of the ideal habitat space.  Calomys laucha is forced to wait out the winter months in less ideal habitats (Bilenca & Kravetz 1999, Castellarini et al. 2003).

As with other members of the order Rodentia, Calomys laucha plays a crucial role in ecosystem dynamics.  The members of order Rodentia are the “main course” to many predators higher up on the food chain.  The predators of the small vesper mouse include birds of prey and other carnivores.


Hantavirus is a zoonotic disease, which means that it spread from animal reservoir to humans.  One such reservoir is Calomys laucha.  Small vesper mice males that have reached reproductive age are physically aggressive with one another.  This aggression leads to biting and other physical injuries, which is how some researchers believe the Hantavirus is spread within the Calomys laucha population (Yahnke et al. 2001).  The virus is found in rodent urine, saliva, and feces.  The inhalation of micro-particles (dust) is the most common means of infection, although fomite surfaces (dirt) can also harbor the virus.  The virus can be destroyed with disinfectants, such as bleach or alcohol.

Literature Cited:

Bilenca, D. N. and F. O. Kravetz.   1999.  Seasonal changes in microhabitat use and niche overlap between Akodon Azarae and Calomys laucha in agroecosystems of central Argentina.  Study of Neotropical Fauna and Environment 34:129-136.

Castellarini, F., C. Dellafiore and J. Polop.  2003.  Feeding habits of small mammals in agroecosystems of central Argentina.  Mammalian Biology 68:91-101.

Chiappero, M. B., A. Blanco, G. E. Calderon, M. S. Sabattini and C. N. Gardenal. 2002.  Genetic structure of populations of Calomys laucha from central Argentina. Biochemical Systematics and Ecology 30:1023-1036.

Lasserre, A., E. Cebral and A.D. Vitullo.  2000.  Successful capacitation and homologous fertilization in vitro in Calomys musculinus and Calomys laucha.  Journal of Reproduction and Fertility120:41-47.

Myers, P.  1982.  Origins and affinities of the mammal fauna of Paraguay.

Mammalian Biology in South America 6:85-93.

Redford, K. H. and J. F. Eisenberg.  1992.  Mammals of the Neotropics, Volume 2.  University of Chicago Press, Chicago.  pp 279-280.                

Yahnke, C. J., P. L. Meserve, T. G. Ksiazek and J. N. Mills.  2001.  Patterns of infection with Laguna Negra virus in wild populations of Calomys laucha in the central Paraguayan Chaco.  American Journal of Tropical Medicine and Hygiene 65(6):768-776.

Reference written by Jahna Cook, Biol 378 (Mammalogy), University of Wisconsin – Stevens Point:  Edited by Chris Yahnke. Page last updated 07-25-05.


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