Penguins come in many sizes, but one shape fits all. Moving through water requires much more energy than moving through air, as water is denser. For swimming marine animals – whether fish, seal, whale or penguin – a spindle shape has proved the best at minimising the drag.
Penguins’ feet are tucked away at the rear, where they function as rudders. Their feathers lock together to form an insulating cover that directs water over the body with little disturbance.
Breeding for speed
Experiments in wind and water tunnels show that coefficients of drag for penguins are lower than those for any car. Natural selection may permit penguins to indulge in a yellow crest or two, but not to mess with their basic design. Those that don’t conform are not as efficient, and as a consequence breed less successfully and survive less well.
Northern hemisphere: a penguin-free zone
Penguins are found only in the southern hemisphere. If being a spindle-shaped diver is the best way to exploit the riches of the deep, why are there no penguins in the northern hemisphere?
The answer is twofold. Firstly, flightlessness is only likely to evolve in places free of land-based predators – especially mammalian ones – such as the isolated islands of New Zealand and the subantarctic. In contrast, being vulnerable to the bears, wolves and other predators of the northern hemisphere makes flight too valuable to give up. Secondly, tropical seas are generally too unproductive and the prey too dispersed, creating a barrier to penguins moving further north.
Penguins need food that is concentrated in swarms, such as krill, fish or squid, as they cannot cover large distances rapidly, compared to many flying seabirds. Despite this, some enterprising penguins have got as far north as the equator where the cold, nutrient-rich Humboldt Current reaches the Galapagos Islands, creating upwellings where prey is abundant compared to most of the tropics.