"I'll scratch your back if you scratch mine" is not a phrase that scientists have always associated with non-human primates in the past. But a new study shows that this kind of give-and-take could explain much of their selfless behaviour.
Primates exchange grooming for other things, such as food, protection and sex.
But doesn't it still boil down to the point that dogs have mixed with humans, and have therefore learned to read human emotions? Chicken and egg, perhaps?
Good point, and I guess one wouldn't necessarily want to domesticate a great ape. It might make a mess of the curtains.