In strongly seasonal environments, such as in mid and high latitudes, most bird species reproduce during short periods in spring and summer only. During this time, these species also aggressively defend territories. Reproductive activity is stimulated by increasing daylength in spring, which also initiates secretion of reproductive hormones like testosterone to regulate reproductive and territorial behavior. Our understanding of the environmental cues and the hormonal mechanisms controlling reproduction and territoriality in birds comes almost entirely from studies on temperate zone species. However, the majority of bird species live at tropical latitudes, yet for these we have only vague ideas of how they adjust these processes on a seasonal basis. In contrast to most temperate birds, many tropical birds have adopted long breeding seasons and maintenance of permanent territories. Here the Spotted Antbird (Hylophylax naevioides) from the rainforest in Panama will be used as a new model species for tropical endocrinology, to conduct an in-detail investigation of the mechanisms that regulate reproduction and territoriality in tropical birds. Previous work has revealed an unusually high sensitivity to small daylength changes in these birds, which may allow this species to use photoperiod as a seasonal signal in their tropical environment. Experimental manipulations on captive Spotted Antbirds will allow to elucidate the mechanism for such photoperiodic sensitivity and also show the importance of food cues for timing of breeding. Like many other tropical birds Spotted Antbirds can have very low levels of testosterone throughout the year, despite aggressively defending territories. Field and laboratory experiments in which testosterone levels and actions are manipulated will clarify the mechanism that controls aggressive territorial behavior year-round. These experiments will test the validity of endocrine models developed for temperate zone birds. This project will greatly advance our understanding of whether tropical species rely on similar seasonal and endocrine systems as temperate zone birds or whether they have evolved different physiological solutions to cope with their environment. Because tropical rainforests are disappearing at an alarming rate, detailed knowledge of reproductive mechanisms will provide critical baseline information for potentially threatened species.
