Why do they stand around in the sun when there's some perfectly good shade around?

It's hot here. Hot enough to make an icecube in the freezer nervous. Air cons are already cranked up to maximum.

On the way home just now, I passed this big paddock with lots of cows in it. There were heaps of really big shady trees in there, but all of the cows were standing around in the sun.

Then when I get home, one of my dogs is laying out on the back deck in the sun! (He's not even wearing sunscreen or a hat!)

Why do they do this?