Comments on the Body Mass Trend 



of Ondatra zibethicus (Rodentia: Muridae) 



During the Latest Pleistocene 



Curtis C. Bentley and James L. Knight 



South Carolina State Museum, 301 Gervais Street 



P.O. Box 100107, Columbia, South Carolina 29202-3107 



ABSTRACT — Martin (1993) suggested, in his investigation of the 

 phyletic evolution in the rodent genus Ondatra, that an increase 

 in body mass through time has occurred in Ondatra zibethicus, 

 with the increase in size being concentrated in the last 600,000 

 years before present (y.b.p.). Ondatra zibethicus apparently obtained 

 its greatest body mass during the latest Pleistocene, followed by 

 a sharp decrease in body mass into Recent times, referred to as 

 a "dwarfing event." We examined fossil muskrats from late Pleistocene 

 sites in South Carolina, Florida, and additional Recent material 

 which do not support the proposed "dwarfing event" of O. zibethicus 

 at the close of the Pleistocene. As more fossil material becomes 

 available, future research could provide a clearer picture of the 

 body mass trend in Ondatra zibethicus. 



METHODS 



Two recently discovered late Pleistocene sites, Crowfield local 

 fauna and the Ardis local fauna (Bentley et al. 1994) yielded fossils 

 of Ondatra zibethicus. These remains, coupled with unpublished 

 material from the Aucilla River, Florida, and Recent specimens from 

 Iowa and Georgia provided new data elucidating body mass trend for 

 this taxon during the latest Pleistocene. It is not within the scope of 

 this paper to do a thorough review of the literature on muskrat body 

 mass nor extensive studies of museum collections. A project of this 

 magnitude would encompass an undertaking much larger than this 

 note. The data published here are intended to elicit further research 

 into the trends of muskrat body mass at the close of the Pleistocene. 



Boyce (1978) noted a small degree of sexual dimorphism in 

 muskrats with males slightly larger than females. However, because 

 this difference is slight, and sex cannot be determined from fragmen- 

 tary fossil material, the effects of sexual dimorphism can not be assessed 

 in this study. 



All measurements were done on the first lower molar (ml). 

 Measurements were taken three times with calipers and rounded to 

 the nearest 0.01 mm. 



Brimlevana 21:37-43. December 1994 37 



