Passage IV

NATURAL SCIENCE: This passage is adapted from the article "The strangest Bird" by R. Ewan Fordyce and Daniel T. Ksepka ©2012 by Scientific American).

That the earliest penguins have turned up in New Zealand is probably no coincidence. Until humans arrived, less than 1, 000 years ago, the islands there formed a temperate seabird paradise on the margins of the South Pacific and Southern oceans. The region was free of terrestrial predatory mammals and afforded space for breeding colonies, with abundant food in the surrounding seas.

Geologic evidence suggests that the area would have been equally conducive to the seabird way of life at the end of the Cretaceous. New Zealand today is the largest exposed area of a submerged mini continent known as Zealandia that broke off from the ancient supercontinent of Gondwana perhaps 85 million years ago. Thus liberated, Zealandia drifted northeast into the Pacific, carrying plants and animals, including dinosaurs, to its resting spot about halfway between the South Pole and tropics. As Zealandia drifted, it cooled and sank. Shallow seas flooded the land, and a broad continental shelf formed around its perimeter. Despite its isolation from other landmasses, Zealandia did not emerge from the end-Cretaceous extinction unscathed. Many of its marine and terrestrial organisms perished in that die-off. Yet what was bad for those creatures was good for penguins. With marine reptiles such as mosasaurs and plesiosaurs out of the picture, early penguins could swim the waters around Zealandia free of competition or predation.

Having gotten their sea legs in Zealandia, penguins soon expanded their domain dramatically, dispersing across thousands of miles and into new climate zones. Fossils of Perudyptes devriesi from Peru show that penguins arrived close to the equator about 42 million years ago, settling in one of the hottest places on earth during one of the hottest times in the planets history. By 37 million years ago the birds had spread to almost every major landmass in the Southern Hemisphere.

Yet why, after restricting themselves to Zealandia for millions of years, did penguins suddenly start spreading across the Southern Hemisphere. around 50 million years ago? Recently Daniel T Ksepka discovered an important clue to this mystery: a long-overlooked feature on the surface of fossil flipper bones. The humerus bears a series of grooves that are easy to miss among the markings associated with tendons and muscles.

Those grooves form at the spot where a cluster of arteries and veins presses against the humerus. These blood vessels make up a countercurrent heat exchanger called the humeral arterial plexus, which allows penguins to limit heat loss through the flippers and to maintain their core body temperature In coId water. In live penguins not blood leaving the heart gets cooled by the plexus before reaching the flipper tip, and cold blood returning from the flipper gets warmed before approaching the heart.

The identity of the grooves on the fossil flipper bones shed some surprising light on the origin of penguin thermoregulation. One of the most amazing aspects of modern penguin biology is the birds’ ability to tolerate extreme cold. One would logically assume that the plexus evolved as an adaptation to frigid environments. But fossils suggest otherwise. Penguins such as the Delphinornis from Antarctica show that this feature evolved at least 49 million years ago. The early Waimanu penguins from Zealandia show no hint of the trait at 58 million years ago, however. The plexus therefore must have evolved in the intervening time, when the earth was far warmer than it is today. Back then Antarctica lacked permanent ice sheets and instead offered a temperate forested environment; Zealandia was even toastier.

What use did early penguins have for a heat-conserving plexus in this greenhouse world? Although sea-surface temperatures were high, early penguins probably foraged in cool upwelling regions, which are rich in nutrients and thus support a bounty of prey, including fish and squid. But because heat is lost more quickly in water than air, a warm-blooded animal such as the penguin risks going into hypothermia even in warm seas if the water is below core body temperature. Reducing heat loss through the flipper would have helped them conserve body heat on long foraging swims in chilly waters.

The humeral plexus may have also allowed penguins to survive the long open-water journeys by which they initially dispersed from Zealandia to other continents. Only much later would modern penguins co-opt this mechanism to invade the sea ice shelves that formed when the planet cooled.