The court appeared to be doubting the validity of the claim that a monkey had proceeded with to sue for copyright protection.
A federal district judge in San Francisco decided previous year that Naruto lacked standing to sue.
Hearing oral arguments in the case yesterday (12 July), Judges Carlos Bea, Randy Norman Smith and Eduardo Robreno questioned counsel for both parties in the litigation, which centres on a selfie taken by Naruto and then claimed by the owner of the camera, David Slater. It wants to administer all proceeds for the benefit of the monkey.
Attorneys for a wildlife photographer whose camera was used by a monkey to snap selfies asked a federal appeals court to end a lawsuit seeking to give the animal rights to the photos.
PETA lost that case in 2015 and is now appealing that decision at the US Court of Appeals.
Angela Dunning said the monkey named Naruto couldn't hold a copyright because he cannot benefit financially from his work.
Slater published the photos in 2014 in a book he created with software from San Francisco-based Blurb.
The item has been corrected to show the photographer's company owns the copyright, not the publishing company. The animal rights organizations says the monkey drew the connection between pressing the shutter release and the change in his reflection, and made different facial expressions while pressing the shutter release.
The US copyright office ruled in Slater's favor, saying that animals can not own copyright.
Crested macaques are part of the monkey family and have been listed as a critically endangered species by the International Union for Conservation of Nature.
The 9th Circuit proceedings will be broadcast online.