Arctic summers mean migrating animals, a bounty of breeding opportunities, and 24 hours of sunlight. Many plants and animals experience 24-hour cycles telling them when it's time to rest and when it's time to get up—called the circadian rhythm—that are often tied to light cues. So what happens when the sun never sets?
For four species of migrating birds that breed in the Arctic, new research shows that "anything goes," said Bart Kempenaers, a behavioral ecologist with the Max Planck Institute for Ornithology near Munich.
Lapland longspurs (Calcarius lapponicus) exhibit a 24-hour cycle, while semipalmated sandpipers (Calidris pusilla) and pectoral sandpipers(Calidris melanotos) are active around the clock. Red phalaropes (Phalaropus fulicarius) shift from a roughly 21-hour cycle to a 29-hour cycle.
The type of cycle each displays depends on the species, an individual's sex, and their social circumstances.
Arctic residents like the reindeer and a bird called the ptarmigan don't really have a 24-hour cycle, said Kempenaers, co-author of a recent study published this week in the journal Proceedings of the Royal Society B.
Scientists think this enables the permanent residents to take advantage of the midnight sun to feed around the clock. (Read about Scandinavians who walk with reindeer in National Geographic magazine.)
But Kempenaers and colleagues were curious to see what happened with Arctic migrants.
So the scientists studied the activity patterns of four bird species that migrated to the same area near Barrow, Alaska (map) to reproduce during the Arctic's five-week breeding season.
They attached 0.03-ounce (one-gram) radio transmitters to 142 individuals of Lapland longspurs, semipalmated sandpipers, pectoral sandpipers, and red phalaropes.
The radio tags allowed researchers to continuously monitor individual activity levels, which the team then verified with behavioral observations in the field.