Kairuku goes for a Swim

Penguins are amazing swimmers.  There plump bodies and specialized feathers provide perfect streamlining – penguins create even less drag than computer models predict they should when cutting through the water.  They also reach impressive depths. We see a lot of artwork of fossil penguins standing on land, but like modern penguins they were birds of the oceans.  Below is a great reconstruction of Kairuku waitaki out at sea.

Three Kairuku waitaki penguins head out to sea. Artwork by Chris Gaskin, owner and copyright owner: Geology Museum, University of Otago. Used with permission.

How well did giant penguins swim?  This is an interesting question.  At such different sizes and shapes, it is not safe to assume extinct species like Kairuku swam in an identical fashion to living penguins.  In fact, some researchers have suggested that giant penguins were not as capable of diving as living penguins.  This idea was based primarily on the flipper bones of some fossil species like Anthropornis, which were considered more angled than those of living penguins.  However, the idea that giant penguins had to stay closer to the surface is wrong for three reasons.  First, not all giant penguins have angled flippers.  When the bones are articulated at their joints, some giant penguins show a strongly angle between the humerus and the more distal wing bones (those closer to the tip of the wing).  However, in the species  Icadyptes salasi  the flipper skeleton is nearly straight when articulated.  Not all giant penguins had the same type of flipper!  Regardless, this is somewhat irrelevant to diving capacity. Many modern birds hold their wings in a partially folded position and still make it down to impressive depths. The Thick-billed Murre (a relative of puffins and razorbills), which weighs only about two pounds, can reach depths of up to 180 meters, angled wing and all.  Perhaps the most compelling evidence that giant penguins were accomplished divers is the general relationship between size and diving depth in tetrapods.  In marine birds and mammals, there is a very strong positive correlation between body mass and maximum dive depth. Provided penguins don’t “break the rules”, it’s pretty safe to assume giant penguins were capable of deeper dives than their living relatives.  Emperor Penguins delve deep into the dark seas, and have been recorded  foraying 500 meters below the surface.  Amazingly, this record is a minimum estimate because the gauge recording the penguins depth broke under the pressure!  I suspect Kairuku could manage similar depths, though of course most of the time this would not be necessary because much of its prey was probably higher in the water column.