Why do humans sit in the shade? Generally to cool off, right? This is part of a process known as 'homeostasis', where humans basically attempt to maintain a constant internal temperature of approximately 98.6F. Now, the thing is that your body actually produces a lot of internal heat, so if you just stood in a room at 98.6F, you'd pretty quickly overheat. So you've actually gotta stay in an environment that's slightly cooler than 98.6F. This 'trying to maintain a comfortable temperature' deal is common to all organisms. Even plants will do it: many temperate plants will curl up their leaves if it gets too cold, or even self-amputate parts if it gets too hot.*
*Don't try this at home. It's not a good way to cool off.
There is an obvious answer to that.
What other reasons are there for them to sit (or stand) in the shade?
To hide? Maybe that too.
It is just a coincidence that they happen to be in the shade right when it is a hot day?
Possible too.
Yes, they stand in the shade for the same reason that we do.
