Life high above the Arctic Circle gets kind of trippy in summer. For months on end, it never becomes totally dark. The sun stops setting altogether for a while. Humans get a little weird—and so do birds. A new study examines the activity patterns of four birds that migrate to northern Alaska and finds that there's no single way they cope with 24-hour light. Some rest every night; some are active all the time. The patterns even vary within species and can change over time—apparently depending on whether the bird is caring for eggs.
Animals have internal clocks, but they have to be synchronized by external cues like sunlight. Researchers have found a total absence of daily rhythms in some animals that live in the Arctic year-round. It seemed like the ability to keep a daily schedule might have disappeared through evolution. But nobody knew what happened to migratory animals that go only to the far north in summer.
Behavioral ecologist Bart Kempenaers works most of the year at the Max Planck Institute for Ornithology in Seewiesen, Germany, where the sun sets every night like it's supposed to. Since 2003, he's been doing fieldwork at 71°N in Barrow, Alaska. The long days and nights had an impact on his team. "We noticed in ourselves that our timing was becoming strange after a while," he says. By the end of the field season, the researchers were eating dinner at midnight or later. And they had no problem staying up all night to catch birds; the scientists just didn't seem to get that tired.
But how do the birds themselves react? Kempenaers and his colleagues picked four species that all spend their winters in tropical or temperate zones and breed in the same environment, the tundra at a research station near Barrow, littered with antennas, electric cables, and debris. "All of the logistics are really good to do this type of work, but the environment itself is a little bit ugly. But the birds don't care and there are lots and lots of them," Kempenaers says.
To track the birds' activity patterns, the researchers caught 142 of the animals by walking around on the tundra and tossing nets over them—the birds aren't afraid of humans—and glued a radio transmitter to each, the size of a small bean. Receivers constantly scanned and recorded the strength of each bird's signal; if it remained the same from minute to minute, the bird wasn't moving.
One of the four species was a songbird, the Lapland longspur (Calcarius lapponicus), included because songbirds' circadian systems have been studied a lot. It stuck to a regular schedule, a few hours of rest every night between midnight and 4 a.m. The other three were spindly-legged shorebirds with different mating systems. The researchers thought that might mean different activity patterns, and they were right. At the beginning of the breeding season, none of the shorebirds had regular schedules. But things changed when the eggs were laid.
The male red phalarope (Phalaropus fulicarius) and female pectoral sandpiper (Calidris melanotos) are the egg-sitting parents in their respective species—and both had a rest period at night. Although "night" might seem like an irrelevant concept in the Arctic summer, it does get colder and dimmer when the sun is low, around midnight. The opposite sex, on the other hand—the female red phalarope and male pectoral sandpiper—were gallivanting about at all hours, probably looking for new mates.
With semipalmated sandpipers (Calidris pusilla), both parents sit on the eggs. And they did something totally unexpected. While they have eggs, the male and female both show regular activity patterns, with a few hours of rest in a row. But the patterns are in non-24-hour cycles, and they're out of sync. So if the male takes his daily rest an hour later every day, the female might be active a couple of hours earlier every day. "Nobody knew that this existed," Kempenaers says. He and his colleagues speculate that the different patterns come about as a kind of social negotiation over who's going to be active when—because it would be tough if one parent is always stuck foraging at night. Puddles and ponds can freeze, and it probably gets hard for the birds to find the invertebrates that they eat.
All of this suggests that circadian clocks can be cued by social roles and that the rhythms can be much more complicated than scientists thought, the team reports online today in the Proceedings of the Royal Society B. "People seem to think about daily activity patterns as something that's more or less fixed in a species," Kempenaers says. "What we show in this paper is it's much more flexible."
The idea that animals can have different circadian rhythms based on their social role is "amazing," says Qing-Jun Meng, a circadian biologist at the University of Manchester in the United Kingdom. He says that scientists in his field are starting to realize that lab experiments don't say much about how animals' internal clocks work in the real world; last year, for example, researchers reported that fruit flies in their natural environment have a third active period each day, in addition to the two that were known from the lab.
"It suggests, overall, that these systems are much more [flexible] than we thought," says John Lesku, a comparative sleep physiologist at La Trobe University, Melbourne. "And that there's much more diversity than we thought." And he agrees with Meng that this shows the value of studying animals in their natural environment. "If you took any of those birds and put them in a box, they're not going to do this."