A fossil penguin named Kumimanu biceae stood about 1.65 meters tall and weighed about 100 kilograms; size estimates for fossil penguins can change, because many are known only from a few arm and leg bones. The Kumimanu fossil is about 55 to 60 million years old, close in age to the oldest known penguins; the name Kumimanu, given to a giant fossil penguin, means "monster bird" in Maori. Early penguins lived in warm seas, so penguins did not begin in the cold; the king penguin is the second largest penguin. The gentoo penguin is the third largest penguin; the little penguin is the smallest kind of penguin. An adult king penguin stands 85 to 95 centimeters tall.

The emperor penguin is the biggest penguin; a grown emperor penguin stands about 1.1 meters (45 inches) tall. A big emperor penguin can weigh about 40 kilograms (88 pounds); penguins as big as or bigger than emperors lived between about 50 and 20 million years ago. Fossil bones suggest penguins began in the Zealandia region, the land around New Zealand; the giant penguins vanished from the fossil record about 20 million years ago; one idea blames the rise of seals and toothed whales. Leopard seals and killer whales hunt penguins in the water; emperor penguins can swim up to 20 kilometers an hour, and Adelie penguins swim 4 to 8 kilometers an hour. An emperor penguin can dive more than 500 meters deep; the longest emperor penguin dive on record lasted 22 minutes. Most dives last 3 to 6 minutes; penguins swim by pushing themselves along with their flippers.

Emperor penguins build up fat before winter and live on it while they fast during breeding; emperor penguins breed on sea ice and also hunt along its edge, so they depend on it. Most emperor colonies sit on fast ice, sea ice that is frozen to the land or to grounded icebergs. Emperor penguins rely on stable fast ice for about eight months of the year, from breeding until chicks leave; penguins huddle close together to stay warm. Huddles can hold from under 200 emperors to 5,000 or more, packed as tightly as 10 birds to a square meter on very cold days. As of April 2026, satellite images show about a 10 percent loss of emperor penguins across Antarctica between 2009 and 2018. As of November 2026, climate change is the main cause of the fall in emperor penguin numbers.

As of November 2026, emperor penguins are officially listed as at risk of extinction: threatened under the United States Endangered Species Act and Endangered on the IUCN Red List. As of November 2026, the main reason is shrinking sea ice, which is expected to keep shrinking for decades. As of November 2026, about two-thirds of the 18 penguin species are listed as threatened by the IUCN. As of November 2026, the Galapagos penguin is endangered. As of November 2026, only a few thousand Galapagos penguins are left; counts differ, from 1,200 to about 2,000-3,000. As of November 2026, African penguins are critically endangered, and their numbers have been falling since the early 1900s. Penguins are indicators of the health of the ocean and of climate change; at least one emperor penguin colony has all but disappeared as the sea ice changed.

Words to know

• Zealandia: New Zealand region
• fossil: old remains
• estimates: careful guesses
• icebergs: floating ice
• indicators: signs