I think Slater is approaching this all wrong. He should do what book publishers have been doing for years for public domain works. They add notes and commentary and translations to works in the public domain and claim copyright over their EDITION of the work. The original work is still in the public domain but new EDITION enjoys copyright protection.
So in this case, the raw unedited photo is in the public domain because as Wikimedia states animals cannot claim copyright but Slater's edition with his cropping and post processing is copyrighted by Slater. Anyone can use his unedited photo for free but not his edited one. All he has to do is make sure his raw unedited photo never makes it to the Internet.