Circannual and circadian rhythms in the concentration of corticosterone in the plasma of the edible frog (Rana esculenta L.).

For a period of 21 months between May 1974 and September 1976, circadian variations in the plasma concentration of corticosterone were studied by competitive protein-binding techniques in mature male and female edible frogs living in their natural environment. Blood samples were taken from 8 to 12 frogs six times daily and conventional and cosinor methods were used for statistical analysis. Circadian rhythms were not detected during February and March (time of hibernation). Circannual rhythms were detected in three parameters of the circadian rhythm. The mean concentration of corticosterone over a 24 h period (24 h mean) reached a peak on 1 May (between 15 April and 15 May; 95% limits of confidence); the annual mean value of the 24 h means was 1.97 +/- 0.25 (S.E.M.) microgram/100 ml, with an amplitude of 0.66 microgram/100 ml (0.53--0.79 microgram/100 ml; 95% limits of confidence). Circadian variations in the concentration of corticosterone were largest in May (peak of reproductive activity). The times at which the peak concentration of corticosterone occurred showed circannual variations: peak values were detected around 24.00 h in May, 19.00 h in July and 08.00 h in November. Both circadian and circannual variations have therefore been demonstrated in an endocrine function of an amphibian in its natural habitat.