A considerable body of evidence suggests that sleep is essential to sustain the brain, with insufficient slumber linked to deficits in attention, motivation, sensory–motor processing and memory. Alternatively, some scientists propose that sleep may simply serve to conserve energy at times when activity is not constructive. In this manner, animals might evolve the ability to forego sleep when ecological demands favour wakefulness. The pectoral sandpiper (Calidris melanotos) is a polygynous Arctic seabird with strong sexual dimorphism. Males of this species expend extensive time and energy defending their territories and displaying for their less enthusiastic female counterparts. With no investment in their young post-copulation and, at such northerly latitudes during this season, no darkness to limit the chances to attract females, mating effort is all or nothing for these males. As John Lesku from the Max Planck Institute for Ornithology and his European colleagues realized, time spent dozing might limit opportunities for male sandpipers to pursue fertile females. As male reproductive fitness in these birds is determined by access to fruitful females, they hypothesized that sexual selection may favour the ability of males to forego sleep without experiencing the ill effects of sleep deprivation.
Lesku and his team deployed customized radiotelemetry-based systems to record activity patterns and log male–female interactions in a population of pectoral sandpipers on the Arctic tundra at times when females were fertile and post-fertile. They found that males were livelier than females in both periods, and that male activity decreased once fertile females were no longer available. One of the males was even active over 95% of the time for a 19 day stretch! Next, the researchers used a novel biologger to simultaneously measure brain and muscle activity – the latter indicative of neck movement – to determine whether sluggish males were actually sleeping more instead of just sitting peacefully. These results showed that the sandpipers quickly transitioned from being actively awake to sleeping, with no intermediate period of quiet restfulness: so inactivity and activity are accurate proxies for sleep and wakefulness in this bird.
Focusing on the birds' activity patterns, the team found that the time spent sleeping per day was highly variable, ranging from 2.4 to 7.7 h and, in line with their hypothesis, males that slept the least sired the most offspring. Although the males that slept less did also sleep more deeply, they remained sleep deprived overall. This demonstrates that these males maintained the high level of performance necessary to ‘score’ a mate despite their sleep deprivation, challenging the view that sleep loss incurs inevitable costs. Continuing the project over several years, the researchers discovered that the birds that fathered more offspring were more prone to return to the study site. If sleep loss corresponds to reduced survivorship, the opposite should be true. In addition, males that returned were more likely to sire offspring in the subsequent year. This implies that reproductively successful males either have increased long-term survival or at least greater site fidelity. Lesku's team revealed that sacrificing sleep ensures paternity in pectoral sandpipers, suggesting that the capacity to forego sleep is adaptive for males of this species. For these birds, to snooze is most certainly to lose!