TESTOSTERONE AND THE CIRCADIAN SYSTEM OF THE MALE CANARY (SERINUS-CANARIA)

Effects of exogenous testosterone on the circadian system of the male domestic canary (Serinus canaria) were studied in light:dark (L:D) cyc les with 16 h of bright light (ca. 35 lx) and 8 h of dim light (ca. 3. 0 lx) during the nonreproductive (regressive) phase of the annual gona dal cycle. In addition, effects of testosterone implants were tested u nder L:D cycles (35:3.0 lx) with different L:D ratios and periods (T> 24 h). The question was asked whether male sex hormones af fect entrai nment properties of the circadian pacemaker system and whether they di ggerentially influence circadian parameters of various behavioral func tions (e.g, perch hopping, feeding, vocalization). During the regressi ve phase, at low levels of endogenous plasma testosterone, application of exogenous testosterone increased locomotor activity and vocalizati on (songs and calls) to the levels measured previously during the repr oductive (progressive) phase. Differential effects of testosterone on amount and ''activity time'' (alpha) of perch hopping and feeding were not accompanied by differences in phase relations and entrainment pat terns of the two rhythms with the L:D cycle close to the upper limit o f the range of entrainment(T> 24 h). It is concluded that exogenous te stosterone influences effect or processes (locomotor activity, vocaliz ation) but does not directly affect entrainment properties of the circ adian pacemaking system in the male canary.