Palaeeudyptes-one of the ‘giant’ penguins that lived during the Oligocene, about 28 million years ago-may have evolved a means of retaining heat when they were still living in warm climates, reveals a new study.

A key adaptation that helped modern penguins to invade the cold waters of Antarctica within the last 16 million years is the so-called humeral arterial plexus, a network of blood vessels that limits heat loss through the wings.

The plexus routes blood coming into the body from the wings past the blood travelling from the body to the wings. As such, the cooler blood from the wings, which get cold in the water, is heated up by warmer blood from the body, thus conserving heat.

To find out more about how this anatomical structure evolved, scientists investigated seven live penguin species and 19 fossil ones.

Surprisingly, they found the plexus arose at least 49 million years ago, when the planet was going through a warm “greenhouse Earth” phase due to vast amounts of global warming gases that got pumped into the atmosphere, perhaps by volcanism.

“I began this work thinking we would relate heat retention in penguins to the global cooling that took place at the Eocene-Oligocene boundary [about 34 million years ago], whereas in fact, penguins were cold-water-tolerant millions of years earlier,” Live Science quoted Daniel Thomas, a palaeontologist at the University of Cape Town, as saying.

The earliest known penguins to feature the plexus lived on the lost continent of Gondwana, on what is now Seymour Island in Antarctica.

The researchers suspect the plexus first evolved to help penguins save energy during long foraging trips in the cold water, as the structure evolved in concert with dramatic skeletal changes that promoted buoyancy and reduced drag, thus improving deep-diving and long-distance swimming.

As global climate cooled, the plexus then found a new use, proving key to the penguins’ invasion of Antarctic ice sheets.

The findings were published in the journal Biology Letters.

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