Data from: Costs of sleeping in: circadian rhythms influence cuckoldry risk in a songbird

1. Circadian (i.e. daily) regulation of behaviors is thought to provide
fitness benefits to organisms by enabling them to anticipate diel changes
in the environment, such as sunrise. 2. A common behavior among socially
monogamous songbirds that usually takes place in the early mornings is
extra-pair mating, i.e. copulating with partners outside of the social
pair bond. 3. Thus, variation in when individuals begin their daily
activity may influence their reproductive success; early risers may be
better able to gain copulations guard their partners thus minimizing their
risk of being cuckolded compared to late risers. Sexual selection may thus
play an important role in shaping circadian behaviors, but this assumption
has yet to be tested in free-living animals. 4. Here we experimentally
weakened endogenous circadian rhythmicity and thus anticipation of dawn in
male great tits (Parus major) in the wild through the subcutaneous
administration of implants filled with melatonin shortly before egg-laying
began in this population; melatonin is a hormone released during the dark
phase of the day, and is one important cue animals use to entrain their
circadian clock. 5. Experimental individuals delayed onset of daily
activity compared with controls, and were more likely to be cuckolded
compared with control males. Manipulation did not alter other behavioral
traits observed; no difference between treatments was observed in activity
levels during the day or in the end time of daily activity. 6. These
results strongly support the assumption that selection, particularly
sexual selection, shapes circadian phenotypes of wild vertebrates which
enable anticipation of important and predictive diel changes in an
individual's biotic and abiotic environment.