Frequency distribution of body mass in groups of highly standardized laboratory rats are characterized by negative...

8,600 laboratory rats (outbred and inbred, SPF) of 3 independent breeding and experimental units, divided into groups of in 80-100 animals of equal sex and age, were housed in groups of 5 animals of the same sex and age per cage under highly standardized (SPF) conditions. The body masses were determined monthly between 8 and 56 weeks of age. The frequency distributions of body mass per group (coefficient of variation 7-12%) very frequently showed small negative kurtosis (-0.12 to -0.76) but no general tendency to skewness. Kurtosis is small and seems to have been acquired before birth and had no genetic reasons because it was also present in groups of inbred strains and in animals not selected artificially after birth. It may be caused by shortages of both tails of the frequency distribution of the body mass.