The study, published Thursday in the journal Functional Ecology, came from two years of studying a group of 19 male great tits living in a man-made nest box in southern Germany. About half of the group was implanted with tubes containing melatonin, the hormone that controls sleep and wake cycles and is often used as a sleep aid; the others were left alone. Radio transmitters on the birds allowed the team of researchers to track them.
It became apparent that the birds receiving melatonin were confused about when the day started. They woke up ever so slightly later than their more wakeful counterparts — an average of 12 minutes before dawn, as opposed to the typical 22 minutes. It was only a short lag, but it made all the difference when it came to breeding.
That’s because songbirds are “socially monogamous,” meaning that they pair up in long-term relationships for the purposes of gathering food and raising offspring, but are free to look outside the nest, so to speak, when it comes time for mating.
For these birds, that time is early in the morning. While the melatonin-impaired males slept, the other male birds in the study were able to mate with the partners of the late-sleepers. The earlier birds fathered slightly more offspring (the researchers conducted paternity tests on all the nestlings), and were much less likely to find their own nests filled with chicks that weren’t their own. On average, the late-sleepers wound up caring for about 2 “extra-pair” nestlings, while the average for the early-risers was less than half that.
“If you are not around when she becomes active, she may seek opportunities from neighboring males,” lead author Timothy Greives, a biologist at North Dakota State University, said in a news release. “Our data suggest that making sure you are active before the females may impact the number of offspring you sire with your mate.”
To be fair, the late-sleeping birds can’t really be blamed for their predicament — they were drugged (in accordance with NIH guidelines for experimenting on animals), after all. The study says less about individual lazy birds than it does about the significance of their circadian rhythm, the physiological cycle that controls feelings of sleepiness and wakefulness in animals, plants, fungi and even bacteria.
The fact that the early birds had more reproductive success indicates that these birds’ circadian rhythm has been sexually selected over years of evolution. Birds that wake up that crucial 22-minutes before dawn will pass on their early rising genes; those that don’t will see their laziness lost to the forces of natural selection. It’s a powerful incentive to wake up on time — and stay away from scientists wielding melatonin implants.
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