New Zealand is the penguin capital of the world. Of the 19 species of living penguins, nine breed in New Zealand or its territories, including the Ross Sea sector of Antarctica, and another six are visitors. New Zealand is home to the greatest diversity of penguins, and has more fossil species than any other region. The first and the oldest fossil penguins were found in New Zealand.
Penguins belong to an exclusive family of birds called Spheniscidae. While many groups of waterbirds include one or two flightless species, penguins are the only group in which all members are flightless. This universal loss of flight suggests that, whatever evolutionary event drove the ancestors of penguins to give up flying, the transition to swimming and diving has been enormously successful.
Flying versus diving
While both flying birds and diving birds are streamlined to minimise the effects of drag in the air or water, the other demands on their bodies are contradictory. Flying birds need to be light, with sufficient wingspan to provide lift to counteract the effects of gravity. In contrast, a diver needs to be dense to offset the buoyancy of water and, for propulsion, the wings need to be short and powerful to move through the denser medium. During the Late Paleocene–Early Eocene period, 50–60 million years ago, the southern seas were temperate to subtropical and offered abundant food to a bird that could dive deep enough.
Bigger size for deeper dives
Successful diving is a function of body size: larger animals have larger oxygen stores and are able to dive deeper and for longer. To get to the deepest fish and squid, the ancestor of penguins needed to become larger. The point at which seabirds with some diving ability became flightless divers was probably when they reached a weight of about 1 kilogram. Beyond that, the contradictory requirements of flying and diving are mutually exclusive: the wingspan that will keep a large bird aloft is too big and fragile for efficient diving.
Diverse sizes of divers
Once free of the constraints of flying, penguins evolved rapidly into many different species (a process termed radiation by evolutionary biologists), with some becoming much larger. The largest New Zealand fossil penguin ever found, Pachydyptes ponderosus, is estimated to have stood 1.4–1.6 metres tall. In contrast, the smallest known penguins, fossil and living, measure just 40 centimetres in length. This diversity suggests that trading flight for diving made it possible to exploit a range of marine environments, from deep Antarctic waters to subtropical surface and inshore waters.
Of all modern penguins, the large emperor penguin performs the most astounding diving feats. Their regular dives are 100–200 metres deep and last around six minutes, but some have achieved dives of 22 minutes and depths of 565 metres. The dives of little penguins, on the other hand, are usually 10 or 20 metres and last 25 seconds, although depths of 60 metres have been recorded.