Morphological variation and population structure of Finnish muskrats, Ondatra zibethica (L.)

Abstract

muskrat carcasses (я=637) collected in spring from trappers in four areas in Finland. In the cohort born during the previous summer males were larger than females (body and carcass weight, skull length); in animals that were one year older (old), sexual dimorphism was less prominent in weight but significant in skull length. Tooth wear was greater in males (young), resulting in lower molar index values. Muskrats showed greater body dimensions during the first half of the trapping season (April) than during the second half (May), probably because of intraspecific competition (social dominance). The animals differed in size and molar wear between the localities and also between several habitats within a local population. Large size correlated with low molar wear, both characteristics depending on the availability and quality of food in the habitat. In order to describe habitat suitability, growth index values were calculated by dividing the carass weight by the amount of the tooth wear. The relative densities of muskrat populations were highest and the animals were largest and had the highest growth indices in habitats with a good supply of high quality food. In southern Finland Scirpus lacustris is a valuable food for muskrats. Besides macrophyte species composition, the animal condition depend on mussel supply and on water level fluctuations (especially combined with a shallow waterside profile). Muskrats of largest size and high growth index were trapped in rivers with high diversity and abundance of preferential food plant species, relativelity good mussel supply and moderate water level fluctuations. Most (91 %) individuals in the spring populations were young; 9 % were one year older. Only one-individual seemed to have overwintered three times. Males predominate over females in catch numbers in both cohorts. The sex ratio remained constant during the spring trapping period.