Symposia and Oral Abstracts

Endogenous rhythms are thought to enhance fitness by facilitating proper timing of biological functions with environmental conditions. The majority of research on circadian rhythms however have been conducted in the lab, where the fitness relevance of endogenous rhythms cannot easily be determined. The pineal hormone melatonin is an important component for the entrainment of circadian rhythms with the organism's diel environment. The current study aimed to begin to uncover the effects of disrupting endogenous rhythms via manipulation of melatonin rhythmicity in a free−living vertebrate. Three weeks prior to egg−laying, wild male great tits (Parus major) received a silastic implant either left empty or filled with melatonin; this manipulation is known to elevate daytime melatonin to levels comparable with normal night peak levels. Additionally, all individuals received a radio−transmitter, and using automated telemetry recording, the precise timing of daily behavioral activity onset was recorded. Individuals were followed throughout the breeding season and measures of reproductive success and extra−pair paternity were assessed. Male great tits receiving melatonin implants delayed daily activity onset compared with control birds, presumably due to the inability to use the melatonin rhythm to entrain endogenous rhythms, and were thus unable to anticipate sunrise. These birds, however became active during the morning light. Control birds became active before sunrise during the dawn chorus. Melatonin−treated birds were more likely to be cuckolded and tended to sire fewer total offspring. These results indicate a strong selective advantage for a functional circadian system capable of anticipating important diel environmental transitions