In the tropics, we take light and darkness for granted. But when the Sun does not set for four months in summer and does not rise for four months in winter, new coping mechanisms emerge in the wild.
The temperature rose steadily until our car’s dashboard showed 12º C. It was 10 pm and the Sun was out in full glory, shining so bright that we had to squint to see the road ahead. My husband and I were driving into Oulu, a small town on the Baltic coast of Finland, looking for owls in June. At least six species were known to nest in this region in the summer. For the next two hours, we wandered around a patch of forest wondering what it meant to be nocturnal in the land of the midnight Sun. A heavy torch lay in my backpack mocking us.
In nature, timing is everything. Most organisms sleep and wake at the same time every day; mate roughly during the same period every year; and migrate to warmer and colder places in the same season every year. Indeed, the cyclic nature of these processes has led to an entire discipline dedicated to the study of timekeeping in nature, called chronobiology.
One cycle that has particularly fascinated chronobiologists is the circadian rhythm. It refers to any set of periodic activities that occurs roughly once every 24 hours (i.e., one Earth day). We usually think of circadian rhythms as something to do with our sleep cycles, but many biological processes like our daily ablutions and the production of important hormones are circadian in nature.
Just happening once every 24 hours doesn’t make something circadian. To get there, the rhythm needs to be controlled internally, by a (figurative) clock ticking inside the body. In all mammals, this clock is located in a part of the brain called the suprachiasmatic nucleus (SCN). Birds have multiple circadian clocks: in the SCN, in the pineal gland and in their retinae.
There are specific genes that control these circadian clocks. The 2017 medicine Nobel Prize was awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young for isolating some of these genes.
Like an old grandfather clock that must be set every few days, circadian clocks need to be synchronised with the external environment. They do this with the help of some cues. Jurgen Aschoff, a German physician and a pioneers in this field, called these cues zeitgebers, ‘time givers’ in German. Light and dark are the most common zeitgebers that our circadian clocks use, to time activities like sleep so that we are awake when it is light and resting when dark.
In the tropics, we take light and darkness for granted. But in the Arctic, where the Sun does not set for about four months in summer and does not rise for four months in winter, what happens to the circadian rhythms of its inhabitants?
The willow warbler is a small insectivorous bird found throughout northern Europe in the summer. I was excited to see and hear the first willow warbler of my trip but that quickly turned into exasperation. The bird was everywhere – whether at a Finnish nature reserve at 6 am or over the icy Norwegian tundra at 9 pm. Did the willow warblers ever sleep – or did the light drive them to insomnia?
In 1962, R.G.B. Brown, an ornithologist at Cambridge University found that willow warblers in Northern Norway have regular rest periods as well as that their sleep cycles ran for 24 hours, under the the constant light. Brown also observed a distinct ‘rest’ period usually between 9 pm and midnight, after which the birds would become active and resume typical breeding activities like singing (a courtship ritual) and, later, building nests with mates and feeding their young. But it was not clear if this cycle was being controlled internally.
Four decades later, some evidence emerged of an internal mechanism. In most vertebrates (including humans), the onset of sleep is marked by an increase in a hormone called melatonin in the bloodstream. Its release is controlled by the pineal gland, which houses one of three circadian clock in birds. Melatonin levels rise when it’s dark out and fall during the day.
When scientists from the University of Gothenburg, Sweden, measured melatonin levels in willow warblers living in the constant light of northern Sweden, they found that the hormone levels increased in the birds roughly around the same time Brown had observed them resting. In Alaska, studies of Lapland longspurs, a plump finch that breeds throughout the Arctic circle, showed similar results. The birds made the most of the midnight sun – but they did rest for a few hours every night, and their melatonin levels rose and fell accordingly.
But given the absence of darkness, what cues did the birds use to decide when it was night in the Arctic? Barbara Helm, a chronobiologist at the University of Glasgow and one of the members of the melatonin study in Sweden, suggested that changes in temperature and light colour through the day could be important zeitgebers for willow warblers. Several studies have shown that, even with bright sunshine, between 9 pm and midnight was often the coldest and dimmest part of the day in the Arctic.
In animals in lower latitudes, melatonin release is triggered by the onset of darkness. In the Arctic, the hormone levels could have been rising because of the reduced light intensity in this period. Lower temperatures at night inhibit insects, the main food for willow warblers and Lapland longspurs. So it made sense for the birds to call it a night even if it was still light out.
For other birds, activity rhythms depended on their reproductive biology. Researchers from two German institutes – the Max Planck Institute of Ornithology and the University of Konstanz – and the Princeton University, New jersey, studied three species of wetland birds in Alaska during the summers of 2007 and 2008. All species of birds had different mating and parenting systems. The semipalmated sandpiper is monogamous and its adults shares parenting duties. The pectoral sandpiper, a wader that breeds in the polar regions of North America and parts of Siberia, is polygynous: the male mates with several females and the females care for the young exclusively. The red phalarope was perhaps the most interesting species of the three, as it was a rare example of polyandry: the female mated with several males and laid eggs, but it was the male phalarope that cared for the young.
These differences in mating systems seemed to show some link to their activity rhythms. The monogamous semipalmated sandpiper, where both sexes incubate the eggs, had a free-running rhythm: regular sleep-activity cycles that did not follow a 24-hour day. The researchers found that some pairs had 21-hour days while others had 27-hour days. Bart Kempenaers, one of the authors and a behavioural ecologist from the Max Planck Institute for Ornithology, believes that the birds were synchronising their activity cycles with their partners – a social zeitgeber, with activity rhythms synchronising according to social cues. The study pointed out that in the presence of normal light-dark cycles, social cues may be weak zeitgebers – but in the Arctic summer with constant light, these zeitgebers could help animals maintain a rhythmic sleep cycle.
Having a fixed activity rhythm may not always be a desirable trait. In the two polygamous species, the sexes that didn’t raise the young were arrhythmic throughout the summer. The male pectoral sandpiper and the female red phalarope would stay awake as much as they could and mate with as many partners as they could. Indeed, in another study, Kempenaers and his colleagues found that for the male pectoral sandpiper, this behaviour of sleeping less bestowed a reproductive advantage over its rivals. The male that slept the least also reproduced the most. Not having a regular sleep cycle did not seem to affect their physiological condition.
When all potential mates have been impregnated (or, in the case of the red phalarope, when all males have begun incubating their clutch of eggs), Kempenaers speculated that the promiscuous sexes continued to stay arrhythmic. “They just seem to sleep more,” he said. Meanwhile, the partner that took on parental duties became rhythmic, following a daily pattern of activity and rest similar to the willow warbler. “This is most likely a side-effect of the fact that, during night, temperatures are lower, often below zero).” In such a situation, “continuous incubation is more important for embryo development and foraging is less profitable.”
While migratory birds adapt to the Arctic in a variety of ways, Kempenaers believes they quickly synchronise with a light-dark cycle when they move to lower latitudes. So what happens to the year round residents, especially during the polar winters?
Having needed several layers to protect myself in the summer in the Arctic, I’ve no doubt I wouldn’t have fared well in total darkness and temperatures below freezing. Polar residents need to cope with snowfall, blizzards and, of course, persistent spells of darkness. They do so by not moving much in the cold, maintaining their core body temperatures and reserving fat. Some mammals, like arctic ground squirrels and brown bears, do one better: they sleep it off.
Biologists from the University of Alaska, Anchorage, measured the internal body temperature cycles of ground squirrels that hibernate in winter. They found that when the squirrels went to sleep, there were no signs of any rhythms. But shortly before they woke up at the start of spring, body temperatures rose and fell in a 24-hour cycle. Their sleep-activity patterns also switched to a 24-hour rhythm. But how the switch happens and what zeitgebers the squirrels use remains a mystery.
Some Arctic residents could be making do without an active circadian clock. Researchers from the University of Tromsø, Norway, have been studying reindeer in the country for several years. In 2007, one set of researchers examined the activity rhythms of reindeer from Svalbard, an archipelago off the Norwegian coast, at two sites: one located at 79º N and another in mainland Norway at 70º N). In both places, the researchers found that the reindeer remained active throughout the year but in short bouts, showing activity rhythms that were distinctly ultradian, i.e. shorter than 24 hours.
Reindeer are ruminants – the plant matter they were eating was tough and fibrous, and needed to be thoroughly chewed and fermented in their gut for digestion. These plants were also nutritiously poor, so reindeer had to graze for long periods and effectively rendering eating an energy-consuming activity. In winter, the reindeer rested more. In summer, they were more active and rested less. The researchers pointed out that, as herbivores, reindeer needed to feed more frequently during the summer, when food was available, and rest more in the winter, when looking for plants would only take up important energy.
In another study in 2010, a different set of researchers from the same university showed that the clock genes, which would have to be active for an animal to have a circadian rhythm, were greatly weakened in reindeer. Somehow, the herbivores had suppressed their circadian system to function in the Arctic environment. All this information flies in the face of studies that showed that suppressing the circadian clock would lead to physiological stresses or impairment of daily functions. The Arctic reindeer had revealed that timekeeping is a complex interplay of environment and physiology.
Amid all this complexity, it will come as no surprise that our quest for owls in the bright Arctic night ended mostly in failure, with one notable exception – a short-eared owl, which breeds in northern Europe and, during winters, in places like Western India. Around the time when even the willow warblers had fallen silent, a magnificent short-eared owl had swooped past us and perched on a pole, taking in the sunshine and the quiet night. I wonder if, like us, it was also relieved that at least the willow warblers were asleep.
Bhanu Sridharan is a freelance journalist interested in writing about climate change, biodiversity and ecology.