Boza Ivanovic style with zoo animals - how?

I came across Boza Ivanovic's zoo animal portraits last week as I was preparing to go to my local zoo to get some ideas/inspiration. I have been wracking my brain for the last week trying to figure out how he can get the lighting in his images. I can find no info on his technique.

I'm assuming he can't set up studio lights in the cages, and off camera lighting at a public zoo is pretty difficult to accomplish, so how is he getting the look he gets? When I think about lighting (artificial), I know I can increase my shutter speed to make the background disappear, or create only a lit silhouette by killing ambient and using a back light. Etc...

He, he may be using some flash, as there may be flash reflections in the eyes, but I can't tell if it's flash or possibly the sun I see reflecting. Is it possible to do that type of lighting with ambient only?

The light is most likely sunlight coming through openings of some sort.

I expect the animals are in enclosures that have dark corners that allow the animals to get out of the light.