What type of bones do penguins have that helps them stay underwater?

Penguins have solid bones, which help them remain submerged while swimming underwater.

Most flying birds have lightweight, air-filled bones that reduce body mass for flight. Penguins are flightless birds, so their denser skeletons are useful in a different environment: they reduce buoyancy and make underwater pursuit of prey more efficient.

Solid bones are only one part of penguin swimming ability. Their streamlined bodies, powerful flippers and strong muscles also help them maneuver through water. Their wings have evolved into flippers rather than structures capable of powered flight.

A common mistake is to say that penguins have no air spaces at all. The key comparison is that penguin bones are relatively solid and less pneumatic than those of many flying birds; this increases density without being the sole explanation for diving.