I was reading up on portrait photography and came up with the following:
1. There is formal and informal portraiture, as well as environmental portraiture. Formal portraiture refers to posed-for photos usually taken in a studio. Informal portraits are more casual in setting and mood. Environmental portraiture refers to photos that capture both the surroundings and the subject.
2. Portrait photography almost always have humans as subjects whether individually or in groups. I did not find any direct reference to animals as subjects of portrait photography.
When animals are photographed in their natural environment (i.e., a bird perched on a tree branch), it is easy to categorize it as nature photography. But when one takes a photo of a clownfish inside a saltwater aquarium, is it nature photography?
In the same manner, when one takes a photo of a cat at rest inside a house, is it nature photography or is it portraiture?
it is easy to classify the photo above as an informal portrait. But what about the two photos below?
*Note: The first photo was scanned from a print; the second and third photos were taken with a Canon EOS 350D.