Male sandpipers do better by choosing sex over sleep

Jerry Oldenettel
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The need to sleep has long plagued scientists. Why do we—and in fact every other animal with a nervous system—spend such large portions of our day sleeping? After all, there are so many other aspects of life that need our attention. Studies have suggested that sleep may function to consolidate memories, help us solve difficult problems, and boost our immune system. However, there’s still no conclusive answer to why sleep is so vital.

This week, one hypothesis is gaining ground. It suggests that sleep is a “state of adaptive inactivity”that conserves energy when activity is either not required or is not particularly advantageous.

In the most recent issue of Science, a group of scientists from the Max Planck Institute for Ornithology tested this theory by looking at a system where near-constant activity, and therefore a lack of sleep, might benefit fitness. In pectoral sandpipers, male fitness is determined by access to females, and at the high Arctic latitudes in which these birds live, extremely long days enable males to engage in near-constant mating displays during periods of high female fertility. If giving up sleep to spend more time wooing potential mates increases the reproductive success of males, sleep in this species might depend more on the value of wakefulness, rather than the benefits of resting.

The researchers recorded electrical activity from the brains and the muscular systems of several male sandpipers via electroencephalogram and electromyogram dataloggers, in order to determine the amount of time the males spent sleeping each day. The sleeping patterns of males varied greatly; some males slept as little as 2.4 hours per day, while others slept more than three times as much. Males that spent the least time sleeping during periods of high female fertility were rewarded; those that slept less interacted with more females, sired young with more females, and sired greater numbers of young overall than males that slept more.

Because male pectoral sandpipers generally return to their breeding grounds yearly, it was possible for the scientists to assess survivorship by using return rates as a proxy; males that didn't return the next year were likely to have died. If sleep deprivation during the breeding season is detrimental to long-term health, males that give up sleep might be less likely to survive until the next year. However, the researchers found that males that were successful in breeding the previous year—those that slept less—were ten percent more likely to return the next year than other males. This suggests that males were not rendered less healthy over the long term by giving up sleep; in fact, they actually returned to breed at greater rates than birds that took more time out of their breeding season to sleep.

The take-home message is surprisingly simple: self-imposed sleep deprivation is positively correlated with both reproductive success and survival in male arctic sandpipers. Furthermore, this study lends some credence to the theory that sleep may not be particularly beneficial when animals have something better to do.

However, the period during which females are fertile is a relatively short three-week window. So males may be able to “catch up” on their sleep after the breeding season when females are not fertile. Additionally, the study did not test any short-term health risks of this strategy, or whether sleep deprivation in these males led to any type of reduced cognitive performance. It is possible that there are ramifications for sleep loss. But, in the grand scheme of biology, reproductive success is what matters, as it determines what traits are passed on to the next generation. And for male pectoral sandpipers, sacrificing sleep for sex appears to be a good strategy.