The influence of social cues on the reproductive endocrinology of male brown-headed cowbirds: field and laboratory studies.
Captive male brown-headed cowbirds exposed to long days exhibit gonadal growth and have elevated plasma testosterone (T) levels. This photoperiodic response is enhanced if males are housed with female cowbirds: Photostimulated males with females increase plasma testosterone levels sooner than do individually housed photostimulated males. Peak plasma T levels are similar in both groups, although peak levels are maintained longer in males housed with females. The gonadal cycle is similarly affected; males in the presence of females have earlier gonadal recrudescence and maintain mature gonads longer than do photostimulated males without females. Plasma corticosterone levels increase in the unpaired males, suggesting that removal of social cues is stressful for these birds. Free-living paired males have significantly higher plasma testosterone levels than do unpaired/unknown males early in the season, when social relationships are being established; the levels are similar thereafter. There is no difference between the two groups in testicular maturation rates; nor do they differ in plasma corticosterone levels at any time of the season. These results suggest that social stimuli are important in modulating the secretion of testosterone in males early in the season when pairing occurs, and possibly late in the season as well, probably to prevent termination of breeding prior to that of females.
D000818:Animals; D001717:Birds / Q000502:physiology*; D003345:Corticosterone / Q000097:blood*; D003463:Cues; D008027:Light; D008297:Male; D010507:Periodicity; D012621:Seasons; D012931:Social Environment; D012934:Social Isolation; D013737:Testis / Q000254:growth & development*; D013739:Testosterone / Q000097:blood*
D013739:Testosterone; D003345:Corticosterone