40 Curtis C. Bentley and James L. Knight 



under study by Fred Grady of the National Museum of Natural 

 History, produced a sample of six muskrat mi's giving a mean of 0.92 

 kg. In Florida, an unpublished, late Pleistocene site (12,000-10,000 

 y.b.p.), from the Aucilla River (Priscilla site, Little River section), 

 under study by S. David Webb (Florida Museum of Natural History, 

 personal communication), yielded 15 mi's with a mean of 1.04 kg. 

 The age of this site is based on numerous radiocarbon dates (Dunbar 

 et al. 1989; S. D. Webb, personal communication). 



The senior author obtained 22 muskrat carcasses from a fur 

 buyer in Roselle, Carroll County, Iowa, in December 1992. The sample 

 represents muskrats from populations within a 100-mile radius of Roselle 

 and were probably collected from many different sites. Using Martin's 

 regression formula, these samples produced a mean of 0.94 kg. Measure- 

 ments of two modern mi's from Georgia (no other data) had estimated 

 mean mass of 1.27 kg. 



Comparing these data (Fig. 1) with those of Martin (unpublished 

 data) would suggest that the increase in body mass for the time span 

 covered by our samples was much more subtle than Martin's data 

 would indicate. After Martin's initial body mass increase at 600,000 

 y.b.p., the upward change in size until Recent is almost negligible. In 

 addition, the statistical analysis of the three time intervals yielded no 

 significant difference between our data and Martin's. Thus, our data 

 do not support Martin's ''dwarfing event" at the close of the Pleistocene. 



We view the data on which the dwarfing hypothesis was based 

 as having several problems, the most significant of which is sample 

 size. The four samples between 10,000 and 20,000 y.b.p. which constitute 

 the much larger forms of O. zibethicus (compared to Recent specimens) 

 are made up of a total of seven mi's, from four sites (Table 2), an 

 extremely small sample size. Furthermore, all of Martin's 10,000- 

 20,000 y.b.p. specimens were recovered from cave faunas (Table 2), 

 a habitat in which modern muskrat populations do not naturally occur. 

 This would suggest that various selection pressures, probably predatory, 

 must be taking place in order for these remains to occur in cave 

 deposits. Thus, the samples from these caves may not be a true representation 

 of nearby local populations. 



Assuming that Martin's regression for body mass holds true, it 

 seems highly probable that a "dwarfing event" has not occurred between 

 the latest Pleistocene and contemporary times based on this new material. 

 However, this has little consequence for the overall trend in muskrat 

 body mass over the past 3.75 million years. When this time period is 

 re-scaled to a common interval length, Martin (1993) states that the 

 dwarfing event ". . . appears to be only moderately pronounced." 



