It's probably better to be safe than sorry.
But having said that, be safe, not boring.
In animal photography I frequently see photos classified as 'animal portraits'. However in the general photography mindset, I don't know how widely that is accepted. There will always be things that blur the boundary however. If it's a photo of a dog in a primarily urban landscape, then how do we classify that? Is it really 'nature'? Or is it an 'animal portrait'? Or perhaps it doesn't fit any of the categories at all because there isn't a category such as 'urban' or 'street scene' etc.
It's always debatable as to where things fit. Personally, I would accept an 'animal portrait' as a 'portrait', however I do not know if the organizers think the same. It'd be interesting to have some input on this.