Images containing animals are often difficult to process automatically because poor and variable lighting, shadows, and overlapping and touching animals lead to indistinct or incomplete boundaries. A snake is an algorithm based on the mechanics of an elastic loop. It can find continuous boundaries when edge data suffer from the above problems. This paper briefly introduces the algorithm and then develops some novel extensions, specially useful for the problem of outlining pigs in a pen. The algorithm is tested on a number of images and shown to give good boundary location in cases where simpler methods fail.