What animals are darker on the bottom side than the top?

[Tamerlane]

My favorite - the Black-Bellied Salamander. One bit me :).

[Snickers]

@dracoi

I’m not so sure I’d dismiss a function right off the bat. Belly fur can be substantially different from the upper coat in terms of length, thickness and texture as well as in coloration. At least some of those differences have to result from selection pressures.

I can imagine that light-colored fur would have slightly different thermal properties than dark-colored fur. Perhaps a light belly is useful in environments where the ground gets hot the during day and one would like to reflect infra-red. (The back fur, being driven more by camouflage needs might have to stay darker regardless of temperature).

That’s just a WAG, but I sure wouldn’t chalk it up to fish ancestores without more analysis.

Our guide in the Serengeti was of the opinion that this is the case for the Thompson gazelle - their white bellies reflect the heat radiating from the plains surface and helps keep them cooler.

[hammos1]

Silverback gorilla

[Colibri]

@Snickers

Our guide in the Serengeti was of the opinion that this is the case for the Thompson gazelle - their white bellies reflect the heat radiating from the plains surface and helps keep them cooler.

That doesn’t make a lot of sense physiologically. More radiation is coming in on the animal in the form of sunlight from above than is re-radiating from the ground. And any heat radiating from the ground will be in the form of infrared, that is, outside the visible spectrum. The white color of the underparts is due to reflectance in the visible part of the spectrum, not the infrared. You would have to show that the underparts were reflective in the infrared as well as the visible for that to work.

[Wiggie]

I have several of these dudes in my fishtank.

They skim the surface of the water eating the bugs floating on top. They spend the majority of their adult lives upside down. Hence, the reverse shading.