Nearly all organisms display circadian rhythms, and these rhythms are thought to enhance fitness by facilitating proper timing of behavioral and physiological events with environmental conditions. The majority of studies 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 environment. The current study aimed to begin to uncover the effects of disrupting endogenous circadian rhythms via manipulation of the melatonin rhythmicity in a free-living vertebrate. Prior to the onset of the egg-laying, wild great tits (Parus major) received a silastic implant either left empty or filled with melatonin; this manipulation is known to abolish diel melatonin rhythms by elevating daytime levels to levels comparable with the night peak of melatonin in control birds. Additionally, all individuals received a small radio-transmitter (<0.5g). Using an automated telemetry-recording unit, we recorded the precise timing of onset of daily behavioral activity, as well as the total amount of activity per day from all individuals. Focal individuals were followed throughout the breeding season and measures of reproductive success and extra-pair paternity were assessed.
Results/Conclusions
Male great tits receiving nighttime-like melatonin levels displayed disruption of their circadian rhythms; these birds were unable to anticipate sunrise and only became active during the morning light, while control birds became active before sunrise during the dawn chorus. Male great tits receiving melatonin sired fewer total offspring (with-in pair and extra-pair offspring combined) that successfully fledged. These results indicate a strong selective advantage for a functional circadian system able to anticipate important diel environmental transitions.