How Big Were Prehistoric Penguins?

Lay the upper wing bone of an early penguin along your arm and it covers the whole forearm. A single foot bone from the same kind of bird outmeasures a human hand. Bones like these belonged to penguins that stood a head taller than the biggest emperor alive today, and the heaviest of them outweighed an emperor several times over.

For a long stretch of their history, penguins were giants. The small, quick birds we know are what remained after the giants lost their ocean.

The short answer

The largest penguin yet described, Kumimanu fordycei, weighed an estimated 159 kilograms: more than two adult men combined and nearly four times the weight of a modern emperor penguin. An Antarctic species, Palaeeudyptes klekowskii, may have measured two metres from beak to tail. Giants like these were common for millions of years after the dinosaurs died out, then faded once toothed whales and seals moved into the same waters and competed for the same fish.

The heaviest penguin ever found

For years, scientists treated the emperor penguin's size as roughly the upper limit for the group. A modern emperor reaches roughly the height of a six-year-old and weighs around forty kilograms. Fossil digs in New Zealand and Antarctica then opened a much deeper record, and the ceiling moved.

Kumimanu fordycei came out of a New Zealand beach and was formally named in 2023. Its describers, writing in the Journal of Paleontology, estimated its living weight from the dimensions of its humerus, the upper wing bone, by comparing it against living penguins. Their two estimates landed at about 148 and 160 kilograms, depending on which bone measurement they used. Either figure makes it the heaviest penguin on record by a wide margin.

Length is a separate contest. Palaeeudyptes klekowskii, whose fossils come from Seymour Island near the tip of the Antarctic Peninsula, may have measured two metres from beak tip to tail. Researchers took to calling it the colossus penguin, only half as a joke.

Giant penguins were the standard model

The giants were a pattern repeated across lineages and coastlines. For roughly the first ten million years of penguin history, large species appeared wherever shallow, fish-rich seas gave them room, all across the Southern Hemisphere:

• New Zealand held Crossvallia and Kumimanu biceae, a close relative of the record holder.
• Antarctica held Anthropornis and the colossus penguin.
• Peru held Icadyptes, a large penguin with a long, spear-like beak.

The pattern matters more than any single species. Size was the normal outcome for early penguins, which means something about their world rewarded it.

Why were early penguins so big?

The answer starts with an extinction. Sixty-six million years ago, the asteroid impact that ended the dinosaurs on land also removed the great marine reptiles from the sea. Mosasaurs and plesiosaurs were gone, and the top of the ocean food web stood largely empty for a long stretch. Marine mammals had not yet evolved into ocean hunters. Early penguins, whose wings still resembled those of their flying ancestors, were already specialist divers, and they swam into a sea with room at the top.

In that open ocean, a big body paid off in three ways.

More oxygen for deeper fish

A larger body carries more blood and muscle, and so more stored oxygen. That buys longer dives and the muscle to drive down to fish that smaller divers cannot reach.

Slower heat loss in cold water

Heat escapes through the skin, and a bigger animal has less skin for every kilogram of body inside it. A giant penguin cooled more slowly than a small one in the same water, which made long hunts in cold seas cheaper.

Room to be expensive

Large bodies are costly to run. In a sea with no big competitors, that cost was easy to cover, because the fish were there for whoever could reach them. That condition is the one that eventually changed.

Why did giant penguins disappear?

That window shut around thirty million years ago. Toothed whales and the first seals spread through southern waters. They were quick, warm-blooded hunters, and they wanted the same fish. The leading explanation for the giants' decline is simple arithmetic. A huge body needs a huge food budget, and once a mammal could outswim and outeat the giants, size stopped being an advantage and became a liability.

Over the next few million years, the giants thinned out of the fossil record and then disappeared from it. What came through were leaner, faster, cheaper-to-run penguins, the forerunners of the eighteen species alive today.

Why penguin fossils survive so well

We know about these giants partly because of the same trait that ended penguin flight. Penguin bones are dense and heavy, built as ballast for diving. Dense bone sinks, resists being broken up and scattered, and holds together long enough to fossilize. The thin, air-filled bones of flying birds tend to break down before rock can preserve them.

So the deep history of penguins sits in stone because the animal was built to weigh. Every emperor walking across the ice today carries a modest version of a frame that once produced the largest seabirds known, and new pieces of that record keep turning up on cold southern beaches.

Key terms

Humerus

The upper bone of the wing or arm. In fossil penguins, its length and width are used to estimate how heavy the living bird was.

Mosasaurs and plesiosaurs

Large marine reptiles that dominated the oceans until the extinction that also ended the dinosaurs.

Toothed whales

The branch of whales that hunt with teeth, including today's dolphins and sperm whales. Their spread put them in direct competition with penguins for fish.

Related field notes

For how the modern penguin body works, from its flipper stroke to its cold-weather defences, browse more Penguins Decoded field notes.

This article is adapted from "The Penguins Taller Than a Man," the second chapter of Penguins Decoded, which traces the rise and fall of the giant penguins bone by bone.