Abstract
The ratio of the bodyweights of male and female rats, together with the weight of the females, seems to give valuable information as to the biological value of an experimental food as a whole and to the lifespan expectancy. Extremes of the bodyweight ratios are 0.9 and 2.0, the lower ratios applying to younger age groups and to old animals, the higher ratios to the ages in between. There is evidence that diets which are most favourable with regard to longevity give rise to maximum ratios between 1.5 and 1.6, which are maintained during later life. The females give always less response to dietary measures and are less disturbed by very bad quality rations than the males. Their lifespan is often longer than that of the males.
The bodyweight ratio has within reasonable limits the same numerical value as the ratio of food efficiencies of male and female animals; the weight ratio is always very simple to obtain, whereas measurement of food-efficiency ratio requires much more work.