Figure 1. Day/night variations in pineal and plasma melatonin in shrews from different ages.

Shrews were grouped as young (<7 mo), adult (7–12 mo) and aged (>12 mo) as indicated on the abscissa. Values are expressed as the mean±Standard Error of the Mean (SEM). The number of individuals is indicated below the corresponding column. There was a significant effect of age on both pineal gland and plasma melatonin content (SRH test: P = 0.001 and P = 0.003 respectively). Pineal gland and plasma concentrations of young and adult animals were significantly different between night and day, but this marked diel rhythm disappeared in old animals as a result of a loss in the nocturnal production (different letters indicate significant differences, Mann-Whitney U test, P<0.05).