Joseph A. Chapman

Harmonic Mean Measure of Animal Activity Areas

A new method of calculating centers and areas of animal activity is presented based on the harmonic mean of an areal distribution. The center of activity is located in the area of greatest activity; in fact, more than one “center” may exist. The activity area isopleth is related directly to the frequency of occurrence of an individual within its home range. The calculation of home range allows for the heterogeneity of any habitat and is illustrated with data collected near Corvallis, Oregon, on the brush rabbit (Sylvilagus bachmani.)

Effects of Trapping and Weather on Body Weights of Feral Nutria in Maryland

(1) Rates of change in body weight of nutria from Maryland are calculated based on changes in weight and are expressed as the changes in weight per unit weight per unit time. (2) A model of nutria rate of change in body weight is fitted to the data and used to predict the weight gain patterns of male and female nutria. (3) The rate data are compared among the factors of sex, trapping method (leg trapped v. live trapped), season (where winter is defined as December through February and summer as the rest of the year), and year (March 1975-February 1976 v. March 1976-February 1977) in an analysis of covariance with inverse body weight as the covariate. A regression was made of mean fractional rates, adjusted for the covariate inverse body weight, against mean seasonal temperature and mean seasonal snowfall. (4) Rates of change in body weight were significantly greater in 1975-76 than in 1976-77 reflecting the severe winter of the latter year. (5) Leg trapped nutria grew consistently slower than live trapped nutria, although the difference was not significant at P < 0.05. (6) There was a significant interaction between sex and season. In the summer, females grew faster than males, although this may be due, in part, to pregnancy. The pattern was reversed in the winter. (7) There were significant correlations between mean adjusted rates of growth and both mean seasonal temperature (r = 0.83) and mean seasonal snowfall (r = 0.80). These factors could reduce food availability and make greater demands on metabolic energy.

A Model for Predicting Age-Specific Body Weights of Nutria Without Age Determination

A model proposed by Dixon et al. (1979) for predicting body weights of nutria is tested against a separate set of data on body weights of nutria aged by standard methods. Separate models are tested for males and females with regressions showing no significant differences between observed and predicted body weights. A slight decrease was observed in female body weights at ages greater than 2 years but the sample size was small for these ages.