This sort of sexual deviancy was thoroughly covered in Charles Darwin's lesser tome, "The Origin of Feces".
Rangerwil shall doubtless be along soon to speak on this and other topic he's interested in such as why "dog years" makes age moot in man and animal (aka: "manimal") relationships and his ever poopular treatise on "Male Baboon Butts & Scrotums and how their color/shape denotes readiness for ravaging" to name just two of his fave subjects.
Animals are not nearly as smart as people and yet some people choose to behave like animals.