This is a contest for people portraits, not animals. SIlhouettes offer an opportunity to focus on the outline, the shape of a subject, and in portraiture that means the shape of the whole body for full length portraiture, or if a bust portrait then often a side-on portrait that shows the contours of the face. Silhouettes are traditionally shot such that the subject is very dark, and the background lighting accentuates that, either by being correctly exposed, over-exposed, or through some clever lighting of the subject in the case of the header image for the contest.