THE RELATIONSHIP BETWEEN PLASMA TESTOSTERONE CONCENTRATIONS AND THE SEASONAL-VARIATION IN VOLUNTARY FEED-INTAKE IN FALLOW BUCKS (DAMA-DAMA)

In two experiments conducted at Badgery's Creek, NSW (Latitude 34 degr ees S) the roles of testosterone and the thyroid hormones triiodothyro nine (T-3) and thyroxine (T-4) as possible initiators of the change in voluntary feed intake (VFI) associated with the seasonal reproductive behaviour (rut) in male fallow deer were investigated. In Expt 1, the association between changes in these hormones with the onset of the r ut was assessed in deer in which the timing of this event was manipula ted by changing the photoperiod, or by melatonin administration. Group s were maintained under either natural photoperiod (n = 6; control), a constant long daylength (16 h light:8 h dark; n = 9; LD group) or a c onstant long daylength and implanted with melatonin capsules (n = 5; L D + M group) for 7 months from December until the following July. Bloo d samples were obtained weekly and VFI recorded. Feed intake decreased by 94% from the last week of March for 3 weeks in the control group. Changes of a similar magnitude were measured in the LD and LD + M grou ps but these changes were advanced by 1 and 9 weeks respectively in th ese groups. In all groups, circulating testosterone concentrations inc reased markedly at a time corresponding with the decrease in VFI. The concentrations returned to basal levels with the resumption in VFI. A distinct decrease in plasma concentrations of T-3 and T-4 in all three groups was associated with the decrease in VFI, however, the relation ship with T-3 was less apparent. In Expt 2, the role of testosterone i n the regulation of the decline in VFI was investigated. Fallow bucks were treated with testosterone enanthate every 4 or 5 days for 28 days up to 6 weeks prior to the expected onset of the rut. Plasma testoste rone concentrations, which were increased 13-fold, resulted in a decli ne in VFI which was comparable to that observed in the subsequent rut. Plasma free fatty acid concentrations were correlated negatively with the decline in VFI. Thus, the seasonal increase in circulating testos terone concentrations plays an important role in initiating the fall i n VFI associated with the rut. As the rut was still apparent in animal s maintained under an extended photoperiod, it is possible that factor s other than decreasing daylength act as the cue for the timing of the rut.