What is your view on using animals as the main character in young adult/adult literature?
Well, I think it should be okay, as long as you handle it correctly. My mother has a friend who wrote a novel with animals as main characters that was most definitely meant for an adult audience. Her overall story was clearly shown to be for older readers through her plot and plot description (the bit on the back or inside cover of a novel). I would be sure to specify in the description at least one or two of the themes you intend to explore that are not suitable for a younger audience, just to be on the safe side. How long are you thinking you'll be making this novel? If it ends up being rather long that could also be an indicator that the book is not for children. While there are certainly adult novels that are not particularly long (take Animal Farm or Lord of the Flies, for example), many of them are significantly longer than normal children's books.
If you're more concerned about the story coming off as juvenile to adult readers in the sense that it will drive them away, I think the key would just be to show the story has a serious plot line. To me, I view a book as juvenile when the problems are more shallow or the solutions are less developed. (For example, a shallow problem might be that a person can't decide between two people for who they want to go to a dance with. A shallow solution would be if they were mostly saved by someone else, or that things seem to just get better without much indication of how they got better. If a person was struggling with their parents and without actually talking about the problems or explicitly discovering how to deal with their feelings in a healthy way they magically are all happy and dandy again, that indicates the simplicity I associate with a children's book.)
In short, I don't see a problem with animals as main characters in adult literature. c: