Seasonal and Age-Related Changes in Plasma Testosterone Levels in Mountain White-Crowned Sparrows

Abstract

Plasma levels of testosterone (T) were measured by radioimmunoassay in Mountain White-crowned Sparrow (Zonotrichia leucophrys oriantha) males of known age and natural history during four consecutive reproductive seasons in the Sierra Nevada of California. Because nesting occurred comparatively early in all of these seasons and mean schedules varied by only 11 days, data were lumped to show the seasonal pattern of T. This pattern was unimodal with a maximum coinciding with the period of competition for territories and mates and with mate guarding. This was followed by a decrease toward a minimum that coincided with the period of parental care and, eventually, postnuptial molt. When these data were analyzed by stage of the reproductive season (rather than calendar date) and by age, two unexpected results emerged. First, T levels decreased significantly between the stages of nest building and laying. Female mates should have been fertilizable and soliciting copulations during both of these stages and intermale conflicts associated with mate guarding should have been frequent. In both situations, high T levels would be predicted by current paradigms. Second, T levels were considerably lower in yearling males than in older males prior to the laying stage. Reasons for this age-related difference are unknown but it was observed that yearling males tended in all comparison intervals to have lower body masses, shorter wings, and shorter testis and cloacal protuberance lengths than older males--they were smaller birds. Yearling males were also less successful in obtaining mates than older males but, once paired, reared just as many offspring.