Akodon toba - Chaco Grass Mouse

Description:

Akodon toba is a vole like medium sized mouse, with short limbs, and a short tail.  “The pelage is soft and full with a reddish brown to olive brown color dorsally, fading to more of a tan on the sides and gradually becoming a reddish tan to gray washed with orange on the venter”(Higdon 2003).  The tail is sparsely haired.  Akodon toba has tan feet and the face shows blackish hairs.  There is some sexual dimorphism with adult males weighing  around 45grams and females about 40 grams (Higdon 2003).

Distribution:

Akodon toba is found in the chaco of western Paraguay, eastern Bolivia, northwestern Argentina, and the western most Mato Grosso state in central Brazil (Christoff et al. 2000).  Much of the area where Akodon toba is found is subtropical and semixeric.  Much of their range (the chaco of Paraguay) exhibits pronounced seasonal fluctuations is precipitation with the dry season between April and September and the wet season between November and February.  Temperatures range from an average high of 35 degrees Celsius in the warmest month to a average low of 23 degrees Celsius during the coldest month (Yahnke et al. 2001).

Ontogeny and Reproduction:

Akodon toba reproduces year round, but reproduction peaks during the rainy season.  Food availability seems to be the main factor determining reproduction (Bergallo et al. 1999).  Litter size is typically 3, and average gestation time for this genus is 23 days.  Litter size variation has been shown in other species in the genus and is likely to occur in Akodon toba.  “Variable litter size may be expected in species that live in fluctuating environments” (Suarez et al. 2004).  Also, for this genus young are normally weaned at about 14 days.  Sexual maturity is reached sooner is males than females.  In males sexual maturity is reached at about 32-37 days and at about 28 grams, females reach sexual maturity at around 42 days and about 30 grams.  Like most rodents this species is r-selected with short life span, early maturity, and short gestation time.  These characters allow for quick responses to changes in environmental factors (Higdon 2003).

In the Akodon genus there is a particularly high ratio of XY females up to 30% in some species.  This results from the Y chromosome failing to initiate the male pathway.  The breeding genetics naturally select against XY females so that the frequency of these individuals should naturally decrease.  However, it has been suggested that mutation, meiotic drive, and natural selection are mechanisms for the maintenance of high proportions of XY females in naturally occurring popualations (Hoekstra and Hoekstra 2001).

Ecology and Behavior:

Akodon toba has a relatively small home range of no more than 100 m. square (Geise et al. 2001).  This species seems to prefer pasture habitat, in one study they trapped Akodon toba in four different habitats, but 81% of the captures were in pastures (Yahnke et al. 2001).

Remarks:

Some of the information on this species was taken from the data on the species Akodon cursor which is a synonym for Akodon toba.  Some information on reproduction was taken from information on the genus Akodon.

Literature Cited:

Bergallo, H.G., and W.E. Magnusson. 1999. Effects of Climate and Food Availability on Four Species in Southeastern Brazil. Journal of Mammology 80:472-486.

Geise, Lena, M.F. Smith, and J.L.Patton. 2001. Diverstification in the Genus Akodon (Rodentia: Sigmodontinae) In Southeastern South America: Mitochondrial DNA Sequence Analysis. Journal of Mammology 82:92-101.

Higdon, L. 2003. “Akodon cursor (cursor grass mouse)” (Online), Avialable http://animaldiversity.ummz.umich.edu/site/accounts/information/Akodon_cursor.html>.  Accessed December 5, 2005.

Hoekstra, H.E., and J.M. Hoekstra. 2001. An Unusual sex-determination system in South American field mice (Genus Akodon): The role of mutation, selection, and Meiotic drive in maintaining XY females. Evolution 55(1):190-197.

Suarez, O.V., M. Busch, and F.O. Kravetz. 2004. Reprodutive strategies in Akodon azarae (Rodentia, Muridae). Canadian Journal of Zoology 82:1572-1577.

Yanhke, C.J., P.L. Meserve, T.G. Ksiazek, and J.N. Mills. 2001. Patterns of infection With Laguna Negra Virus in the Central Paraguayan Chaco. American Journal of Tropical Medicine and Hygiene 65:768-776.

Reference written by Shane Zertler, Biol 378 (Mammalogy), University of Wisconsin – Stevens Point:  Edited by Chris Yahnke. Page last updated 7-31-06.