Testosterone does not affect basal metabolic rate or blood parasite load in captive male White‐plumed Honeyeaters Lichenostomus penicillatus

Abstract

We examined physiological and morphological responses of reproductively active male White‐plumed Honeyeaters Lichenostomus penicillatus to exogenous testosterone (T). Birds used in the experiment were held in captivity for two weeks and then were given subcutaneous implants which were either empty (controls) or T‐filled silastic capsules (T‐treated). Basal metabolic rates (BMR) measured just prior to implantation and again 4‐weeks later were not affected by treatment and were indistinguishable from BMR measured in free‐living White‐plumed Honeyeaters, averaging 3.2 ml O2/g/h. Plasma T levels measured at the time of the final BMR evaluation were 1.2, 1.8, and 5.5 ng/ml in free‐living, control, and T‐treated birds, respectively. Body mass was unaffected by T treatment. All captive birds used in the experiment displayed Haemoproteus sp. infection, but blood parasite loads did not differ temporally between T‐treated and control birds during their 6‐week captivity. Corticosterone secretion during a 60‐min capture‐handling protocol did not differ between control and T‐treated birds, but corticosterone levels at each sampling period in captive birds were significantly lower than those of free‐living males. Three morphological variables of White‐plumed Honeyeaters were affected by T treatment: T‐treated birds had significantly higher haematocrits and larger testes than control birds and also showed no change in bill colour. Control birds, by contrast, showed substantial lightening of the proximal half of their bills. We conclude that BMR, resistance to Haemoproteus infection, and stress responsiveness are not directly affected by T levels in male White‐plumed Honeyeaters.