So here's the legal question: how did the copyright get assigned to Caters? I can't see how there's been a legal transfer. The monkeys were unlikely to have sold or licensed the work. I'm assuming that it's likely that the photographer, Slater, probably submitted the photos to the agency, and from a common sense view of things, that would make perfect sense. But from a letter-of-the-law view of things, Slater almost certainly does not hold the copyrights on those images, and has no legal right to then sell, license or assign them to Caters.
I don't think anyone is worrying about the monkey filing an infringement suit, but it is an interesting question. Link -Thanks, Marilyn!
Update: Caters News Agency contacted Techdirt, and asked them to take down the two pictures. Someone in the comments offered to pay for the monkey pictures with a picture of a spider. Link -Thanks, wernerna!
(Image credit: an unnamed Indonesian Macaque)