Meeting Abstract
Nearly all organisms display daily changes in physiology and behavior. Yet, substantial variation among individual daily rhythms exist; some individuals naturally begin their daily activity early (e.g. morning larks) while some begin their daily activity later (e.g. night owls). The mechanisms giving rise to individual variation and the relationships between behavioral rhythms and other important biological processes, such as seasonal and daily timing of breeding behaviors, are still not fully understood. Reproductive steroid hormones are excellent candidates for mediating timing of reproductive-related behaviors as these hormones are known to influence daily locomotor rhythms in captivity, and they display daily plasma rhythms. Little is known, however, about whether these daily hormone peaks (which generally occur at night) are repeatable, and whether variation in daily peak levels are related with variation in the basic organization of the hypothalamic-pituitary-gonadal axis. Here, we describe research from our group demonstrating peak baseline levels of testosterone during the nighttime, high repeatability of these levels, and strong relationships between nighttime levels and peak-induced levels following a ‘challenge’ with gonadotropin-releasing hormone. Further, we will describe correlative studies in female songbirds that seek to link daily variations in behavioral and hormone rhythms with observed variation in seasonal clutch initiation.
