Growth variability in the rat.

Growth of rats during the first 40 days of postnatal life was investigated in relation to the parameters of head-body length, weight and cranial length on the basis of standard growing litters of 6 young and experimental litters of 12-14 young--the latter exhibiting a range of differences attributable to nutritional suppression. A characteristic triphasic growth spectrum manifested in all the parameters of both sexes and litter type within the preweaning period (day 1-20) of development. In the postweaning period (day 21-40) three phases of growth were also detected during analysis. Growth was shown to be at its greatest potential during the preweaning period but did not exhibit any sexual dimorphism in either type of litter. Differences in growth potential existed between the standard litter and the experimental litter but these did not detract from the overall triphasic pattern apart from slight alterations in the individual phasic length. Analysis of the postweaning period of development showed that growth was considerably slower compared to the preweaning period with sexual dimorphism appearing in the parameters. In addition, relative growth (allometric) analysis of the three parameters indicated that the growth rates differed throughout the whole of the developmental period (day 1-40) irrespective of the conditions imposed on the litter types indicating that each parameter possessed an individual growth response.