The sleep conundrum

How many hours of night sleep do you get? Are you ever surprised at how many more/less hours other people sleep? Well, if you are, you might find comfort knowing that the variation in number of sleep hours across species is huge and, so far, very much an evolutionary mystery.

Common thought is that sleep provides us with a much necessary "recharging" and common that it has an evolutionary advantage. However, it takes time away from foraging/preying and mating, and makes individuals more vulnerable to predators. Furthermore, the huge spread of sleeping hours across species makes it harder to pin point whether it does have a selective advantage or not.

A new paper published in Science [1] shows that at least in one bird species, pectoral sandpipers (Calidris melanotos), being able to sleep less confers an advantage: males compete for fertile females over a period of 3 weeks, during which they sleep very little. Lesku et al. showed that males who slept less were the ones who produced the most offsprings.

The researchers' main hypothesis was that

"variability in sleep duration observed across the animal kingdom reflects varying ecological demands for wakefulness, rather than different restorative requirements. According to this hypothesis, animals can evolve the ability to dispense with sleep when ecological demands favor wakefulness."

Pectoral sandpipers mate with multiple females and are not involved in the raising of their offsprings. Therefore, their mating success is determined solely on how many fertile females they have access to. Furthermore, in the high Arctic, the sun never sets during mating period. Males are awake longer than females and engage in courtship behaviors, competitions with other males, and defending their territory (don't know why, high school just came to mind. . . ahem). Their total wake time was a strong predictor of how many offsprings they ended up having. A caveat the researchers put forward is that this conclusion seems to be at odds with the hypothesis that there is a genetic basis to the duration of sleeping times, as shorter durations would be selected and hence the variation in wake time would progressively lessen. Instead, still a great variation was observed across males.

The bit that I found most intriguing is that at lower latitudes at some point darkness falls, and since most of courtship is based on visual displays, this limits the daily amount of time males can engage in such activities. However, the study was conducted in Alaska, where the sun never sets during mating season. I wonder if the same study on pectoral sandpipers that live at lower latitudes would have yielded the same results. I also can't help but wonder: this behavior was obviously selected by a favorable environment, in this case the fact that the sun never sets during summer days. I think it was this past summer that my dad said, "Would birds have ever learned to fly if it weren't for the wind?"

[1] John A. Lesku, Niels C. Rattenborg, Mihai Valcu, Alexei L. Vyssotski, Sylvia Kuhn, Franz Kuemmeth, Wolfgang Heidrich, & Bart Kempenaers (2012). Adaptive Sleep Loss in Polygynous Pectoral Sandpipers Science DOI: 10.1126/science.1220939