Trading in sleep for sex can seem like a pretty good deal at first. Who needs sleep anyway? That’s what caffeine is for. But after a week of curtailed slumber, your priorities are likely to shift. Exhaustion is a powerful force capable of trampling even reproductive urges, at least in our species. But for some male pectoral sandpipers, sleep during the mating season is largely expendable. And with good reason; those birds that sleep the least father the most offspring.
Male pectoral sandpipers (Calidris melanotos) have their work cut out for them. The breeding period is fleeting, the ladies choosy, and the competition fierce. Rather than coupling up to incubate eggs, childcare in this species is left entirely to the females*, leaving males free to mate again as many times as they can manage before the available females run out. But each mating requires time-intensive wooing. Females need to be persuaded with courtship displays and the suitor’s ability to fight off rivals. Fortunately the birds breed in the Arctic tundra. Summers here offer an orgy of non-stop sunshine, so no time need be lost to lack of light. Sleep is thus the only thing standing in the way of a male devoting himself fulltime to the pursuit of mates.
In an article recently published online in Science, researchers examined the sleep habits and mating successes of groups of pectoral sandpipers during their sun-filled breeding periods. By measuring both muscle and brain activity, they found that males didn’t waste any time sitting around taking in the scenery or sulking over rejections. As long as there were still fertile females to be had, males were either actively engaging in mating behaviors or taking naps. But the duration of these siestas varied between individuals. While all males were more active than females, some males were particularly devoted to their reproductive agenda. The most zealous breeder was active over 95 percent of the time for a 19-day stretch.
Males that spent less time snoozing and more time courting managed to produce the most offspring. And the sleep-deprivation didn’t appear to take a toll on the birds. While we might expect the shortest-sleeping males to promptly doze off at the wheel on their return migration and fly into a tree, they were actually more likely to turn up at the same mating site the following year than were the less reproductively successful longer-sleepers. However, the authors note that the overall rate of return to the breeding ground was extremely low, and that the short-sleepers might just be more attached to the breeding ground where they previously got lucky.†
How impressed you are by these birds may depend on your ideas about sleep. Humans spend a good chunk of time sleeping (one third of our lives, by some estimates) and plenty of evidence suggests that something important goes on during these listless hours of snoring. When deprived of sleep we’re uncoordinated, slow-witted, and irritable. We do sloppy work with copious typos. It seems our performance would deteriorate altogether if we didn’t stop at some point and catch up on lost sleep.
But an alternative hypothesis exists that sleep is merely a period of “adaptive inactivity”. That rather than being essential to functioning, it’s more like the dormant state that bacteria in mayonnaise enter when the jar gets stuffed into the fridge – with reproduction and metabolism being put on hold until it reaches the greener pastures of a picnic-bound egg-salad. (Or like hibernation in mammals, if you prefer a non-bacterial analogy.)
If the need for sleep is, as this hypothesis suggests, contingent on the circumstances, it would be reasonable for male pectoral sandpipers to dispense with the activity during the breeding season. And yet none of the males shunned sleep completely. Additionally, the shortest-sleeping males appeared to be partially compensating for their sleep loss by sleeping more deeply during their power naps (as measured by time spent in deep “slow wave” sleep).
It’s possible that sleep does provide some vital benefit to these birds, but that certain members of the species have evolved the ability to sleep minimally when the situation calls for it. The real curiosity then – given that short-sleeping sandpipers are more likely to pass on their genes – is why the longer-sleeping trait hasn’t died out already. Perhaps we’re still missing a crucial detail that would show that the longer-sleeping approach also confers benefits. Or maybe sleep durations for mating pectoral sandpipers are like 100-meter sprinting times, with each new generation striving to shave a few seconds/minutes off the previous record.
* Not all sandpipers espouse these gender roles. In the spotted sandpiper species (Actitis macularius), males are tasked with the incubation of eggs while females hunt for additional mates.
† The returning short-sleepers also fared better than average males in the second breeding season.