Two particular words that mean male donkey and female dog, respectively, have, for centuries, been used as nasty descriptions of their specific genders in humans. So have 'snake', 'dog', 'worm', 'leech', 'rat', 'vulture', 'crow', 'ox', and 'slug', for both sexes. How has this happened? The practice doesn't fit because it combines incomparable species. Obviously the idea is to imply that we have no more control, knowledge, or grace than other creatures. However, animals can't help the ways that they are, so using their behavior to degrade our kind bears no weight. It would make much more sense to berate the actions of humans with literal summaries and negative connotations. What automatically occurs in nature has nothing to do with decisions. Also, what has there been about male donkeys and female dogs to turn their titles in to insults in the first place?