SYNOPSIS: More on the various critters in Krugman's backyard

The trees are, of course, still bare, and the ground is still mostly snow-covered, so it's easy to see all the squirrels rushing about - and a lot more pleasant than doing the nth and, I hope, final revision of Chapter 11.

But why are all the squirrels black?

Until now, I thought of gray as the default color for squirrels. I did notice lots of black squirrels in Canada, but saw mostly gray squirrels in Cambridge; I was surprised to see black as the main color here. It seems especially odd given the other species in clear view - not deer today, but hawks, several of them, circling overhead. If I were a black squirrel running on the white snow, I'd be feeling pretty exposed.

Of course, a little work on Google reveals the answer. I learn from the  Animal Diversity Web that black squirrels conserve heat better in the cold:

"Some interesting clines occur in both skull size and coat color. There is a decreasing cline southward in skull size, though toothrows and mandible sizes remain the same (possibly due to stabilizing selection on those characters involved in mastication). Also, more black-coated squirrels occur in the north. Studies have shown that black animals have 18% lower heat loss in temperatures below -10 degrees Celcius, along with a 20% lower basal metabolic rate, and a nonshivering thermogenesis capacity 11% higher than grey morphs".

A little more searching reveals that black squirrels tend to be more common in cool, woodland areas - like Princeton, the deer capital of central New Jersey - while gray squirrels predominate in urban areas. And apparently my move to the exurbs trumps the fact that I've moved south.

And hey, hawks have to make a living, too, right? But not while I'm watching, please.

Originally published on the Official Paul Krugman Site, 3.8.03