Small Arctic Mammals Entrain to Something during the Long Summer Day

There are several journals dedicated to biological rhythms or sleep. Of those I regularly check only two or three of the best, so I often miss interesting papers that occur in lower-tier journals. Here is one from December 2006 that caught my eye the other day:

Mammalian activity - rest rhythms in Arctic continuous daylight:

Activity - rest (circadian) rhythms were studied in two species of Arctic mammals living in Arctic continuous daylight with all human-induced regular environmental cues (zeitgebers) removed. The two Arctic species (porcupine and ground squirrel) lived outdoors in large enclosures while the Arctic summer sun circled overhead for 82 days. Would local animals maintained under natural continuous daylight demonstrate the Aschoff effect described in previously published laboratory experiments using continuous light, in which rats' circadian activity patterns changed systematically to a longer period, expressing a 26-hour day of activity and rest? The outdoor experiments reported here, however, showed that under natural continuous daylight, both species (porcupine and ground squirrel) had specific times of activity and rest on a nearly 24-hour scale, and their activity peaks did not come later each day. The daily rhythms of the two species were recorded using implanted physiological radio capsules, and from direct observation.

You may recall that I wrote about a similar study in a much larger Arctic mammal - the reindeer, which loses the overt behavioral rhythmicity during the long summer. Apparently, these two small mammals, the porcupine and ground squirrel are different.

In the press release, they explain:

It seemed that although the scientists were very careful not to provide time cues of any sort, the animals had managed to latch onto something that gave them regularity.

"I have written for years that experimental animals seem to be hungry for cues, or time signals, to keep on a regular cycle," Folk said. "So we tried to figure out what cue the wild animals were using, and we could find only one thing that kept a 24 hour periodicity. At Barrow, the sun travels in a circle overhead for 82 days, but at midnight the circle is tipped to the north.

"We postulate that the animals are conscious of where the sun is in the sky and that the nearness of the sun to the horizon could be a clue to animals, and even plants, to keep on a 24-hour schedule."

This is an interesting hypothesis: not just using the clock to orient by Sun, but also using the Sun posiiton to entrain the clock. I hope this gets tested and that this was not just a case of investigators missing an alternative environmental cue. Changes in the Earth's magnetic field show daily oscillations and are potentially one of such alternative cues that animals could use. Just like Dr.Folk states in the article, I'd also like to see this study replicated in Arctic birds, as they are known to be sensitive to the magnetic field which they can use for migratory orientation.