Effects of season and external testosterone on the freerunning circadian activity rhythm of european starlings (Sturnus vulgaris)

Summary

• 1. Results obtained from activity recordings of birds living under natural photoperiodic conditions have suggested the hypothesis that the daily activity-time (α) and the period (τ) of the circadian locomotor activity rhythm are affected not only by external variables but also by internal changes in the hormonal balance related to the annual cycle of reproduction.

• 2. To test this hypothesis, in experiment 1, groups of male European starlings were transferred at 4 times of the year from outdoor aviaries to constant condition chambers where their circadian locomotor activity rhythms were recorded for a period of about 3 months in both continuous dim light of about 0.7 lux (LL) and continuous darkness (DD). Testicular size was established in all birds prior to the transfer to experimental conditions and subsequently at about monthly intervals.

• 3. The results show a close relationship betweenα and testes size: in birds which were transferred to LL or DD during the reproductive season (when testes were large),α during the first 30 days of the experiment was significantly longer than in birds which were transferred to these conditions at other times of the year (when the testes were regressing or small; Fig. 2,3,4). Moreover, birds in which testes size increased in the course of the experiment showed a corresponding lengthening inα (Fig. 2,5). — On the other hand, seasonal effects onτ are suggested only by the results obtained in continuous light: birds transferred to LL during the reproductive season showed slightly shorterτ values than birds transferred to these conditions at other times of the year (Fig. 3).

• 4. The results of experiments 2 and 3, in which the effects of castration and external testosterone on the circadian locomotor activity rhythm of male starlings were studied, confirmed the conjecture that at least a is under hormonal control. Whereasα increased steadily in intact birds whose testes grew in continuous dim light (about 0.2 lux),α remained essentially constant in castrated birds kept under the same conditions. This inhibiting effect of castration onα could be restored by injections of testosterone (Figs. 6, 7). — No clear effects of either castration or testosterone treatment onτ could be detected.

• 5. On the basis of these findings some of the peculiarities of the circadian locomotor activity rhythms of birds living under natural lighting conditions can partly be explained. The results confirm the hypothesis that the increased activity time during the reproductive season in spring is due to changes in the circadian system induced by hormonal variations. It is proposed that similar endocrine effects may be responsible for the drastic alterations in the daily pattern of locomotor activity which occur in nocturnally migrating, but otherwise day-active, birds at the beginning of the spring migratory season.