Effects of Experimental Manipulation of Testosterone Levels on Parental Investment and Breeding Success in Male House Sparrows

Abstract

-Breeding male House Sparrows (Passer domesticus) were implanted with testosterone (T), the antiandrogen flutamide (F), or an empty capsule as a control (C). Parental feeding rates by C-treated males were high until nestlings reached 10 days of age, then declined significantly. This is the typical temporal pattern of parental behavior for free-living males. In contrast, F-treated males fed young at a high rate throughout the nestling stage, while T-treated males fed young much less frequently and were more involved in male-male competition during this period of time. There was a significant decrease in the breeding success of T-treated males resulting from increased starvation of their nestlings. Despite lowered levels of testosterone, F-treated males were able to maintain control of their nest boxes and exhibited normal sexual behavior. During the subsequent brood, breeding success of T-treated males again was reduced by nestling starvation. Our results demonstrate that high levels of testosterone inhibit the expression of parental care in male House Sparrows. Moreover, they suggest that the typical pattern of testosterone levels in males (high when mate guarding and low when feeding young) represents an optimal compromise between allocation of effort to male-male competition vs. parental care. Received 10 July 1986, accepted 1 March 1987. CORRELATIONAL and experimental studies on House Sparrows (Passer domesticus) have suggested that the activities of breeding males represent a trade-off between investment in malemale competition vs. parental care. In males, circulating levels of testosterone reflect this trade-off, being elevated when male-male competition is high and low when parental investment is high. We have suggested that this pattern maximizes an individual's overall reproductive output, even if it results occasionally in a reduction of fecundity (Hegner and Wingfield 1986a, 1987). In this study, we tested experimentally whether the variable pattern of male investment represents an optimization by artificially altering that pattern. This was achieved by altering the endocrine and behavioral state of males with subcutaneous hormone implants. We tested specifically whether elevated levels of testosterone during the last portion of the nestling stage, the interbrood interval, and the egglaying stage are critical for (1) defense of the I Present address: Department of Zoology, NJ-15, University of Washington, Seattle, Washington 98195 USA. nesting site, (2) normal sexual behavior, and (3) successful mate guarding. We also tested (4) whether lowered levels of testosterone during incubation and the early portion of the nestling stage are necessary for the expression of parental care.