The Descent of Penguin

Penguins are world-class free-divers capable of diving to depths exceeding 50m. African penguins dive regularly to 30m and have a depth record of 130m. Adélie Penguins swim almost all the time between the surface and 50m although their deepest recorded dive is 240m. Larger penguins such as the Emperor have been recorded diving to 535m and can hold their breath for fifteen minutes! This species is not short on endurance either — a group of researchers observed one Gentoo penguin making 450 dives in 15 hours! Using time-depth locator transponders other investigators have tracked Emperor’s swimming up to 65km per day.

Like humans, the main problem the penguin faces is being unable to breathe underwater. Having a relatively small body size compared to seals penguins are more restricted in the amount of oxygen they can store while diving. The underwater pressure compresses the air held in the lungs and air-sacs, and consequently these airways only provide about a third of the oxygen requirements needed for each dive.

One of the compensatory mechanisms employed by penguins to overcome oxygen requirements is found in the unique lung structure. Human lungs conduct air in when we inhale and out when we exhale. This means that mammal lungs have blind ends; air enters, stops, and then comes right back out. Penguin lungs, however, were designed so that air never stops. Before examining how penguins dive so deep and stay down so long it is useful to review the structure of their respiratory system.

Early in growth and development of humans and penguins the respiratory system begins as a diverticulum (part of the digestive tract). Later in development the digestive tract and respiratory tract separate. The respiratory diverticulum then forms two lung buds. At this stage, human and penguin lung development are fairly similar. But not for long! Shortly after this stage penguins develop additional organs called air sacs.

Penguins have to breathe air just like mammals and therefore they have to come above water while swimming. Unlike the human free-diver who breathes in a static position on the surface, the penguin takes in lungfuls of air at speed. While swimming, the penguin will alternate underwater locomotion with intermittent jumps out of the water — porpoising – about every 30m to 50m. The jump phase lasts about half a second and the penguin uses this time to breathe. Other species also employ porpoising but it usually means they are in danger!

Once they have taken a breath air enters the nares (we call them nostrils) and travels to the lungs via the trachea. Penguin tracheas are much like the human trachea and are composed of mucous tissue, muscle, and cartilage as depicted in Diagram 1 below.

Diagram 1. Cross-section of a penguin trachea.
Photo credit: Encyclopedia Britannica

The light blue area at the top of the image is the lumen or tunnel portion of the trachea. Immediately below the lumen is the respiratory epithelium that is only one cell thick and beneath this is a thin layer of muscle. The two layers (rings) that are visible are cartilage.

Diagram 2. Penguin Respiratory System
Encyclopedia Britannica

The penguin’s pulmonary system is represented above. Common to the human system are the trachea, two bronchi and two lungs but there are several other features that make up the penguins’ pulmonary system. The easiest way to explain the advantages of the extra features of this efficient breathing equipment is to continue to follow the passage of air as it makes its way through the system.

As it passes through the trachea air runs through a distended portion known as the syrinx an extension of the trachea that enables the penguin to vocalize. The syrinx is also the site of tracheal bifurcation, each channel connecting the syrinx to a lung. However, air first entering the respiratory system immediately flows into the posterior air sacs. This sequence of events occurs during the first inhalation.

When the penguin exhales, air situated in the posterior air sacs flows into the lungs via structures called the ventrobronchi and dorsobronchi (depicted in the third diagram below). Air flows through these respiratory passageways which subdivide and interconnect with smaller bronchi called parabronchi, the site where oxygen is exchanged. Up to this point the process if fairly similar to human respiration but unlike human respiratory physiology the air doesn’t stop here; it keeps flowing!

When the penguin inhales again the sequence of events described above begins again but this is still not the end of the story since air still in the lungs travels further along the system to the anterior air sacs. On reaching this stage the muscles of the abdomen contract, the penguin exhales again and air is forced through the system. Air that remains in the anterior air sacs is funneled into the interclavicular air sacs – essentially a part of the trachea — and air is expelled via the trachea. This final stage represents the final part of the respiration ‘loop’.

Diagram 3. Penguin respiratory system

Diagrammatically it is possible to see the structures that facilitate the circular respiration pattern. This design essentially ensures a continuous flow of air is passed through the lungs, allowing a constant supply of fresh oxygen. The reason the oxygen is fresh is because the oxygen-blood exchange membrane always has oxygen flowing over it, unlike the human situation that is reliant upon an in-out movement.

The structure of the penguins respiratory system also explains why this species do not suffer nitrogen narcosis. While descending the air sacs do not resist pressure but become compressed. The air is in these sacs and NOT in the lungs so there is not too much air in the lungs and no bubbles (the ones that lead to the ‘bends’) can be formed under increasing water pressure. Just to be on the safe side however, penguins do not employ vertical free ascents when free-diving, choosing instead to veer at an oblique angle, thus significantly slowing their ascent. This allows time for any nitrogen that might be under lower pressure to return to the air inside their body cavities.

One more surprise design feature of this avian free-diving species is the interclavicular sac, an air pocket that is continuous with bone, allowing penguins to fill bone cavities thereby creating an extra air reservoir — a possible challenge for genetic engineers seeking to design a human free-diving hybrid maybe?

The complete physiology of penguins is still unknown. For example, researchers do not know whether penguins are able to consciously control or limit blood flow or if this occurs automatically. Whatever the science, these birds have evolved some impressive solutions since abandoning the skies over 50 million years ago.