Early bird catches more than just the worm

Most organisms -- including humans -- experience daily rhythms driven by a body clock. Results of this study provide insight into the evolution of the body clock. Researchers found that birds that start their day earlier father more offspring. The early risers used that time to mate with other birds not in their social pair. You could call it the "you snooze, you lose" effect.

In "Costs of sleeping in: circadian rhythms influence cuckoldry risk in a songbird," researchers from the United States, Germany, United Kingdom, Italy and the Netherlands tracked the songbirds Parus major (commonly called great tits) and their nests in Germany over a two-year period.

Timothy Greives, assistant professor of Biological Sciences at North Dakota State University, Fargo, North Dakota, USA, served as lead author of the study. Researchers included: Michaela Hau, Max Planck Institute for Ornithology, University of Konstanz; Sjouke Kingma, University of East Anglia, University of Groningen; Bart Kranstauber, Max Planck Institute for Ornithology; Kim Mortega and Martin Wikelski, Max Planck Institute for Ornithology, University of Konstanz; Kees van Oers and Christa Mateman, Netherlands Institute for Ecology; Glen Ferguson, National Bioenergy Center; and Giulia Beltrami, Università di Ferrara.

In the study, one group of birds received small implants of tiny, flexible tubes containing melatonin to change their body clocks, while another control group did not. Melatonin is a hormone that is released from the pineal gland in the brain, during the dark of night. The release of this hormone is inhibited by daylight. The pattern of nightly hormone release results in tuning of the bird's body clock.

Radio transmitters on the birds allowed tracking and blood samples confirmed paternity.

"We found that birds that received the implants filled with melatonin woke up slightly later than birds that received an empty implant. Further, we found that males that received an implant filled with melatonin were more likely to be found raising young in their nest that had been sired by another male," said Dr. Timothy Greives.

"This study suggests that being active during this pre-dawn period may be important for not only gaining reproductive success through extra-pair mating attempts, but may also be important for copulating with your mate. If you are not around when she becomes active, she may seek opportunities from neighboring males," said Greives about results of the study. "Our data suggest that making sure you are active before the females may impact the number of offspring you sire with your mate."

The birds in the study were males living in an established nest box population near Radolfzell, Germany. Their daily activity patterns were recorded in the field for up to 19 days. Data was recorded from 10 control implanted and 9 melatonin implanted birds. In the 2010 and 2011 field seasons, all nest boxes were checked regularly for active nests and followed through incubation and nestling rearing.

With mixed up melatonin levels, one group of birds in the study was slightly confused about when the day started, experiencing unnatural continuous night-like melatonin levels both day and night. The melatonin-implanted birds did not sire as many birds, both within their social pair and outside of it. Though treated with melatonin, it did not affect the males' ability to fertilize eggs or to help raise their offspring.

Nestlings born to males treated with melatonin showed no size difference compared to the size of the young birds raised by males in the control group. Nestlings of melatonin-treated males, however, were more likely to be sired by an extra-pair male, compared to those of control individuals.

In Brief

Results of this study show that sleeping in had its costs. Male birds that were asleep in the nest while their mate awoke, later found themselves taking care of nestlings fathered by an early riser.

Further experiments will address how female birds assess male wake-up times, and determine if there are relationships between natural variation of these rhythms and reproductive success, without manipulating melatonin. Researchers also want to study whether the rhythms expressed by males are related with reproductive hormones.

The research was funded by the National Science Foundation International Research Fellowship Program 0852986, by a North Dakota Experimental Program to Stimulate Competitive Research award to Timothy Greives, and by The Max-Planck-Gesellschaft to Michaela Hau.