More Words, Deeper Hole

I'm going to have to look up details of capybara brains now.

But...why are you so eager to discount the arboreal environment as a driver of brain size in its own right? Think about it; although a terrestrial animal has to be aware of what is going on in three dimensions to some degree, it is largely moving in two, with activities like jumping or climbing being only occasional -- and it is negotiating what is for the most part a single, continuous surface.

Arboreal animals, on the other hand, are fully mobile in three dimensions, and rather than negotiating a single large surface they are negotiating a series of narrowly restricted horizontal and vertical surfaces, many of which interact in highly complex ways and are mobile in their own right. And a misstep 40 feet above the ground is frequently going to have more dire consequences than a misstep at ground level --Gravity Is Not Always Your Friend. I would have thought that arboreal environments are fundamentally more challenging to any animal which is attempting to move at speed (so leaving aside sloths, then) in terms of perception, physical coordination and necessary speed of reaction. Why would that not be a spur for a more developed set of sensory and motor cortices?