Penguins are ancient birds, probably evolving about 65 million years ago, at the time that dinosaurs became extinct. Since penguins are well adapted to the cold, the South American and African penguins feel the heat on land and have evolved various ways to cope with the sun. African penguins have a black stripe curving across the top of the chest. They are insulated by air trapped between their feathers. This makes the birds extremely vulnerable when they are moulting, which they do annually, and for this period of about three weeks (at Boulders about November) they are land-bound, getting thinner and more bedraggled until the moulting process is completed. Before moulting they eat hugely and put on about 30% more fat. Moulting takes about three weeks, during which their weight almost halves.