An extensive list of physiological parameters from previous experiments was
re-analysed in order to evaluate the effects of enrichment, cage type and group size
on the within-group variation and hence on the number of animals needed in studies
using Wistar rats. The independent factors studied in these experiments included the
provision of aspen gnawing blocks for enrichment, solid bottom cages (SBCs) and grid
floor cages (GFCs) and animal number per cage (varied from 1-4). SOLO power analysis
was used to calculate the smallest number of animals (n) needed to
detect an arbitrarily chosen 20% effect size, when significance was set at
P = 0.05 and statistical power at 0.90. N ratios
(n
larger/n
smaller) were calculated for the effect of enrichment, cage type and group
size to compare the 'treatment group' with the 'control group'. The
n values of adrenal gland, interscapular brown adipose tissue
(BAT) and epididymal adipose tissue (EAT) weights seemed to vary most, whereas final
body weight (FBW) and growth seemed to be the least variable ones. According to
one-sample t-test, the N ratios of most
physiological parameters differed significantly from zero (except the ones of FBW)
indicating that n values in 'treatment' and 'control' groups were
unequal. The results indicate that some of the physiological parameters are
susceptible to variability attributable to environmental modifications in general
whereas some are not. Furthermore, they suggest that the variation of different
parameters may vary from one experiment to another and between different environments
thus hindering the estimations of appropriate number of animals.
