Muskrat Body Mass Trend 39 



Fossil specimens from the Ardis local fauna, deposited in the 

 collections of the South Carolina State Museum (SCSM), are designated 

 by the base number S. C. 93.105. and cited in this paper by the two 

 digits following the base number. Specimens from the Crowfield 

 local fauna and Recent unaccessioned specimens at the South 

 Carolina State Museum are denoted by SCSM. Fossil material from 

 the Florida Museum of Natural History is indicated by the accession 

 numbers of 132680 - 131318. 



We used a weighted, Wilcoxon rank analysis test to search for 

 statistical significance between our data and Martin's. The data were 

 analyzed over three time intervals: 1) 20.000 y.b.p. to Recent; 2) 

 20,000 y.b.p. to 10,000 y.b.p.; 3) 20,000 y.b.p. to 15,000 y.b.p. The 

 Wilcoxon rank analysis test was used to compare sample means, as 

 Martin's raw data were not available to us. 



INTRODUCTION 



Martin (1993) derived a regression formula based on the length 

 of the ml to estimate the body mass of arvicolines: M = 0.71 (I) 359 , 

 where M is body mass in kg and L is the length of ml in mm. He used 

 the formula to help determine the trend in body mass for the polytypic 

 genus Ondatra during the last 3.75 million years. Martin noted that 

 most of the change in body mass occurred during the last 600.000 

 years. Muskrats reached their greatest size (1.75 kg) during the latest 

 Wisconsin between 20.000 and 10,000 y.b.p. This larger form was 

 Ondatra zibethicus floridanus (Lawrence 1942). synonymized with 

 O. zibethicus by Martin (1993). Martin conjectured that approximately 

 10,000 years ago it appears that body mass dramatically decreased to 

 the levels he recorded for Recent samples. Martin referred to this 

 decline in body mass as a "dwarfing event.*' He was unclear as to the 

 cause but mentioned human culling and/or natural selection as possible 

 explanations. Applying Martin's regression formula to fossils from 

 several recently collected late Pleistocene sites in South Carolina and 

 Florida, as well as recent specimens from Iowa and Georgia (Table 

 1). gives more resolution to this short, but apparently dynamic time 

 interval. 



RESULTS AND DISCUSSION 



The Ardis local fauna (19.000 y.b.p.) (Bentley et al. 1994) 



yielded 18 Ondatra mi's for which measurements could be taken 



(Table 1). producing a mean body mass of 0.95 kg. An unpublished 



fauna from South Carolina, the Crowfield local fauna (80.000 y.b.p.), 



