Meeting Abstract
Experimental manipulation of testosterone has established it as a potent pleiotropic regulator coordinating morphology, physiology and behavior. However, the relationship of field-sampled, unmanipulated testosterone with traits of interest is often equivocal. Circulating testosterone varies over the course of the day. Reports indicate that testosterone peaks during the night in birds, yet most field studies sample testosterone during the morning. Sampling at times when levels are low may be one reason relationships are not always observed. Testosterone is regulated by the hypothalamic-pituitary-gonadal axis, with gonadotropin-releasing hormone (GnRH) initiating the endocrine cascade. Researchers have begun to examine GnRH-induced testosterone levels with traits of interest, yet the relevance of these induced levels are not fully clear. Using photostimulated male great tits (Parus major) we test the hypotheses that testosterone levels peak during the night and that GnRH-induced testosterone is related to nightly testosterone peaks. Blood was sampled during the first, middle or last third of night. One week later, baseline and GnRH-induced levels were sampled during mid-morning. Morning baseline testosterone levels were low compared with night-sampled levels that peaked during the first third of the night. Further, GnRH-induced testosterone was strongly correlated with levels observed during the first third of the night. These data suggest that morning testosterone samples likely do not reflect an individual’s endogenous peak, and GnRH-induced levels approximate an individual’s nightly peak and may be an alternative for birds that cannot easily be sampled at night in the field.
