On the radio I heard complex birdsong that had been recorded at midnight in summer in Finland. The Last Word has discussed what life is like for humans in polar regions, but how does wildlife cope with 24 hours of daylight?
• Everything that lives permanently inside the Arctic circle has adapted to the unusual conditions. If organisms don’t maximise the benefits of the long summer, they will not survive the long winter.
With 24 hours of daylight in summer, photosynthesis never stops, so plants produce abundant leaves, flowers, pollen, nectar and seeds, and plankton blooms occur in the sea. Plant growth is exceptional: Arctic dandelions (Taraxacum arcticum) look more like rhubarb. Insects, birds, mammals and fish exploit this opportunity, with the added benefit that with constant daylight, everything can see what everything else is doing.
Songbirds are programmed to produce a dawn chorus in which, at various times, they try to outperform rivals, attract mates, establish territories and reassure their mates sitting on eggs. The nearest thing to dawn in the Arctic summer comes around midnight, when the sun is closest to the horizon. After this, the light intensity starts to increase again.
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In winter, small mammals hibernate, but they must build warm nests and store seeds to keep them going. Arctic foxes (Vulpes lagopus) don’t hibernate, but they have developed incredible hearing and can locate and catch small mice moving under the snow.
Another interesting adaptation is that young children experiencing the midnight sun will play outdoors at 2 am without getting tired or grumpy. It is also strange that even though the air temperature may only be 5°C, people will happily wear shorts.
Andrew Carruthers, Beaconsfield, Quebec, Canada
• This is an area of active research. Animals have internal clocks, synchronised by external cues such as sunlight. When laboratory rats are isolated from these cues and exposed to continuous light, they develop a 26-hour cycle of rest and activity known as the Aschoff effect, after the German scientist who first observed it in the 1960s.
When G. Edgar Folk studied other rodents in the wild during the Arctic summer – the nocturnal North American porcupine (Erethizon dorsatum) and the diurnal Arctic ground squirrel (Spermophilus parryii) – he found that their circadian rhythms were exactly 24 hours. This suggests that the animals are conscious of where the sun is in the sky, which allows them to tell the time.
Behavioural ecologist Bart Kempenaers and his colleagues investigated the sleeping patterns of four species that migrate to the Arctic in summer to breed. The Lapland longspur (Calcarius lapponicus), a songbird, rests for a few hours each “night”, sometime between midnight and 4 am. Even though the sun doesn’t dip below the horizon, this is when it is at its lowest in the sky, making this the coldest, dimmest part of the day.
The three other species studied were shorebirds and none had a regular rhythm until their eggs were laid. Then the egg-sitting parent in two of the species – the male red phalarope (Phalaropus fulicarius) and female pectoral sandpiper (Calidris melanotos) – rested at “night” while their mates stayed up, possibly looking for other mating opportunities. With semipalmated sandpipers (Calidris pusilla), the parents take turns to incubate their eggs. They appear to negotiate a duty rota, possibly to ensure they both get a chance to feed. If one parent were always foraging at night, he or she might go hungry if the habitats in which they catch invertebrates were frozen over.
A better understanding of the circadian rhythm and what can subvert it may help solve human problems like jet lag, shift-work fatigue and seasonal affective disorder (SAD).
Mike Follows, Sutton Coldfield, West Midlands, UK
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