Analysis of morphometric measures in relation to the age classes. Rats were allocated to 3 age classes as described in the text. Body weight (A) where analysis was initially by 3-way GLM, but sex and its interactions were not significant and therefore was removed from the model. In a 2-way GLM with host species and age as factors, for main effect of host species F1,479 = 565.8, P < 0.001; for main effect of age F2,479 = 908.9, P < 0.001 and for the interaction F2,479 = 90.0, P < 0.001 and the model adjusted R2 = 0.83; head and body length (B) where analysis is by 3-way GLM, with host species, sex and age as factors, and this gave for the main effects of host species F1,477 = 370.4, P < 0.001, age F2,477 = 337.5, P < 0.001, host sex F1,477 = 19.2, P < 0.001 and for the interaction between host species and sex F1,479 = 7.8, P = 0.005. No other interactions were significant with the model adjusted R2 = 0.68.