THE DIFFERENTIATION OF ANIMAL BODY WEIGHTS

1. In an even-aged population individuals differ in weight. Differences may be due to variability in embryo size, variability in growth rates or to unequal (contest) competition between individuals. Observation of the weight distribution within a population, and how it changes with time and density, can distinguish between these causes. 2. Plant populations generally show positively skewed weight distributions, with the skew increasing with both time and density. This suggests that, in plants, contest competition is the cause of the observed variability in weights. 3. This study estimates the skewness of the weight distribution in animal population age classes using data on the minimum, maximum and mean weights of populations. For birds and mammals only a small proportion of studies (12 and 8%) were estimated to possess a positively skewed weight distribution. The proportion was higher in fish populations (28%). 4. There was no general tendency for the skew to increase with time or density. 5. Some reasons why animal populations may tend to possess more normally distributed weight distributions, in contrast to plants, are discussed. Simple models which predict skewed weight distributions in animal populations fail because they omit important features of real populations such as emigration and size-dependent mortality.