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Andalgalomys pearsoni -  Pearson’s Chaco Mouse

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

Physical Description:

Andalgalomys pearsoni is a medium-sized mouse that has a relatively long tail.  The tail is slightly hairy and bi-colored with a pale brown dorsal surface.  The rest of the mouse is reddish-brown with strands of black hair scattered throughout the dorsal region (Nowak and Paradiso 1999).  The face is essentially the same color as the dorsum but lacks the black hair and is outlined in an orange color (Eisenberg and Redford 1992).  The bottoms of the feet are white with no hair or cushions.  The ears are slightly hairy and medium-sized (Nowak and Paradiso 1999).  They appear dark brown on the outside with pale edges (Anderson and Yates 2000).  A. pearsoni resembles other members of the same genus but differs in dorsal coloration.  A. olrogi has a yellowish-brown dorsum and A. roigi  has a grayish-brown dorsum (Braun and Mares 1996).  The total body length of this species is 236 mm with a tail length of 124 mm, a hind foot length of 25 mm, and an ear length of 20 mm (Myers 1977).        

The skull tends to be somewhat slender in the tribe Phyllotini (Anderson and Yates 2000).  The supraorbital borders (which are clearly ridged) in genus Andalgalomys are generally far apart and come together towards the back of the skull. The temporal ridges are well-defined and tend to hang over the orbits.  The zygomatic plate of Andalgalomys is slightly concave when compared to other species in the tribe Phyllotini.  The auditory bullae are relatively large in this genus.  The tooth morphology is brachydont.  The penis contains preputial glands, which are derived characteristics exclusive to the tribe Phyllotini (Tree of Life 2004).


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


A. pearsoni is a species endemic to the chaco of South America, found exclusively in the western region of Paraguay (Yahnke 1999).  Other species in the Andalgalomys genus are located farther south.

Ontogeny and Reproduction:

Andalgalomys pearsoni tend to breed often and have a short gestation period.  The young are tiny and numerous, with an average litter size of 5.  Parental care is limited.  Winter breeding is thought to occur, as females trapped in July exhibited no signs of breeding.  The males that were captured at this time did, however, have large scrotal testes (Myers 1977).

Ecology and Behavior:

Andalgalomys pearsoni inhabit "islands" of dry grasslands within the Chaco (Myers 1977).  There is an overall complexity to the distribution of animals in the Chaco. The Pearson's mouse is affiliated with areas of bare soil.  The ground coverage consists of low herbaceous plants and very few woody plants. They do not appear to choose their habitat based on a high amount of litter.  They are exclusive to the regions of the western Chaco that are dry and sandy, possibly due to the division of resources (food) among the various rodent populations (Yahnke 1999).  A. pearsoni feeds mainly on plant material, as do all members of the tribe Phyllotini.   Arthropods are an insignificant portion of their diet (MacDonald 2001).


A. pearsoni is a known carrier of hantavirus pulmonary syndrome (HPS).  A large outbreak of the disease occurred in Paraguay in 1995 (Yahnke 1999).  This disease is deadly to humans.  It is transmitted by various means, including through the droppings, urine or saliva of the infected rodent (National Center for Infectious Diseases 2004). 

Andalgalomys was formerly included in the genus Graomys, as of its discovery in 1977.  In 1978 the genus was changed to Andalgalomys (Braun and Mares 1996).  There is still strong support for changing the mouse back to the genus Graomys (Braun and Mares 2000).  DNA evidence, however; shows that these two genuses are not the same, so the two should not be placed in synonymy.  One of the DNA disparities is the differing number of diploids (Anderson and Yates 2000).

Literature Cited:

Anderson S., and T.L. Yates.  2000.  A new genus and species of Phyllotine rodent from Bolivia.  Journal of Mammalogy 81(1):18-36.

Braun, J. K., and M. A. Mares.  1996.   A new species of Phyllotine rodent, genus Andalgalomys (Muridae: Sigmodontinae) from Argentina.   Journal of      Mammalogy  77(4): 928-941.

________________________.   2000.  Graomys, the genus that ate South America:  a reply to Steppan and Sullivan.  Journal of Mammalogy 81(1):271-276.

Eisenberg, J. F., and K. H. Redford.  1992.  Mammals of the Neotropics-The Southern Cone Volume 2.  University of Chicago Press, Chicago, Illinois.

MacDonald, D.  2001.  Encyclopedia of Mammals.  Andromeda Oxford Limited, New York, New York.

Myers, P.  1977.   A new Phyllotine rodent (genus Graomys) from Paraguay.  Occasional Papers of the Museum of Zoology, University of Michigan.  676:1-7.

National Center for Infectious Diseases.  2004.  (Accessed 3 December 2004).

Nowak, R. M. and J. L. Paradiso.  1999.   Walker’s Mammals of the World, 4th ed. Johns Hopkins University Press, Baltimore, Maryland.

“Tree of Life Web Project:  Phyllotini.” 2004.  (Accessed 3 December 2004).

Yahnke, C. J.  1999. Community ecology and habitat associations of small mammals in the endemic region of Hantavirus Pulmonary Syndrome in the central Paraguayan Chaco.  Ph.D. Dissertation,  Northern Illinois University.

Reference written by Marisa Abrahamson, Biol 378 (Mammalogy), University of WisconsinStevens Point:  Edited by Chris Yahnke. Page last updated 7-25-05.

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