eli5: What triggers the swap between winter and summer coats in animals?


What makes an animal swap between summer and winter coats?

Is it timed by a biological clock?
Is it temperature?
Or is it just diet maybe?

Take an arctic fox for example. It sheds its winter coat going into summer, if you move that fox to a warmer climate will the foxes fur stay perpetually in “summer mode”.

In: Biology

Or maybe its a combination of the above and other factors

Or maybe the mechanism varies species by species rather than try to follow some rule of hand

For many species, it’s triggered by the days becoming shorter/longer. Temperature typically is not a factor.

The entire nature shifts between seasons. That includes the plants, trees and flowers. All fauna and flora.

It’s a biological clock. They sense temperature changes, sunlight, duration of day and night, angle of sun in the sky and shadows. Its a complex, synchronized system….

(Humans are among the very few species that can reproduce all year long. For most species reproduction is seasonal).

It’s controlled by changes in the length of the day. As winter approaches, the days get shorter, and this triggers molting into winter fur (and vice versa when the days get longer in the spring). Mind you, it’s not the light intensity or the total amount of sun energy received that’s registered, but the relative lengths of the light and dark phases. This isn’t a biological clock as such (but the mechanism that controls it is possibly connected to the *circadian* biological clock, which tells the body what time of day it is).

Because days are longer in the summer (and shorter in the winter) the farther you go from the equator, populations living at a certain latitude are genetically adapted so that they change to/from winter fur at a specific daylength that corresponds to the time of year when this is appropriate. So what happens if you move an animal like this to a warmer location depends on how you move it. If you move it somewhere that’s significantly warmer but located to the east or west of its home, it’ll keep changing its fur at the same dates as it used to. This is also true if the fox stays in place but the climate gets warmer, so that winter now comes later and spring comes earlier than before: the fox will still follow its genetically coded sun-based calendar, so it’ll spend a steadily increasing part of the year wearing the wrong fur, with shitty camouflage as a result. This is a problem that’s affecting snowshoe hares in North America right now.

If you move the animal north or south, things get a bit more complicated, because different day-lengths now occur at different times of year. Let’s say you’ve got an arctic fox living in central Sweden, where days get down to 10 hours in mid-October; maybe that’s a good time to start growing a winter fur. Move that fox down to France, you don’t get to 10-hour days until the first week of November, so the fur change gets delayed. Move the fox to Nepal, days *never* get as short as 10 days even in mid-winter, so the fox would probably stay in its summer fur constantly, despite the abundant snow.

When an organism responds to the length of the day, it’s called photoperiodism. It happens in lots of things. Insects use it to control when to go into hibernation; plants use it to control when to flower or prepare next year’s buds. I’m a researcher working with photoperiodism in insects, so let me know if you have other questions about it.