Board Thread:Off Topic/@comment-26397825-20160524033118/@comment-26397825-20160601055013

Human physiology also cannot support a second setof ears growing out the top of the head, and yet Velvet and Blake exist.

Minor changes to physiology is allowed, such as ears on top of the head, feathers growing in place of hair, retractable claws. It's when the physiology would have to undergo a ton of changes, such as the amount needed to make wings a viable trait on a human body, that we draw the line.