The influence of testosterone on the development and fixation of the form of displays in two age classes of young black-headed gulls

Abstract

Young black-headed gulls, Larus ridibundus, first perform incomplete forms of display before they show the adult-like stereotyped complete form. Two hypotheses concerning the mechanism involved in the development of display were tested. (1) Experience with the performance of incomplete display is a necessary step in the development of the complete motor pattern. (2) The complete form can be triggered early in ontogeny if the proper motivational state for agonistic behaviour is present. The motivational state was manipulated by changing the blood levels of testosterone, known to be positively correlated with aggressive behaviour in young gulls. First, testosterone plasma levels were determined in the blood of young gulls at several stages in the development of display. Second, young gulls of two age classes (6-day-old chicks and 10-week-old gulls) were implanted with pellets of testosterone proprionate; from several birds the pellet was removed after the complete display emerged. The findings indicate that (1) testosterone influences the gradual emergence of complete display in young birds; (2) the neural coordination of the complete motor patterns is present early in ontogeny; (3) the effect of testosterone on anatomy and behaviour is age-dependent; and (4) later in ontogeny the displays become independent of this hormone. © 1992 The Association for the Study of Animal Behaviour.