"as it is jeopardising his livelihood" -- So if a monkey hadn't taken a selfie, he'd be starving and homeless.
The monkey took "hundreds of pictures", increasing the likelihood of producing a few quality shots. Its quantitative approach has seemingly yielded the better results compared to whatever Mr. Slater has applied.
Mr. Slater can not be attributed the director role either, as the macaque has "hijacked a camera", disregarding his setup and direction.
But how could the macaque benefit from holding the copyright on the picture?
"For every 10,000 images I take, one makes money that keeps me going. And that was one of those images." This is what it should be about. We should want him, as any other professional doing his job, to make a living. Instead of spending money on beating him on court, let's spend the money to buy rights to the picture to put it up under a creative commons license.
Why are we spending so much energy on finding out who's "right", instead of just doing "the right thing", in cases where there is a win-win situation like this.
Legal precedent
Here's another. If I leave my laptop running Visual Studio out and a monkey bangs on the keyboard to make a program (by accident, just as with this photo), I don't own the copyright to that program.
http://www.copyrightcompendium.com/#202.02(b)
The Compendium reflects the rulings of the Copyright Office and a court might see things differently. Or the Copyright Office might agree that this photograph was not authored by the human. If they don’t agree, then at the very least you can’t register the copyright and get statutory damages. (Unless you sue the Copyright Office to force them to register.)