A Pelican Pouch and the Evolution of Lung Breathing

Pelicans and crocodiles bear anatomical resemblance at second glance. The Pelican’s distinguishing throat pouch stores unlucky fish, but it also plays a role in heat regulation. Birds with sun-exposed nests display a behavior called gular fluttering, where they vibrate their neck pouch, mouth open, to dissipate excess heat. A number of crocodiles and lizards display a homologous behavior to help them cool off. The gular pouch in vertebrates is a vestigial trait that tells the evolutionary story of how fishes emerged out of water and started breathing air.

Researchers at Brown University found that a number of reptiles use a related behavior called ‘gular pumping’ when physically exerted or pinned down by prey. Air-breathing fish and amphibians use a ‘buccal pump’ to draw air into a temporary cavity, before sealing their mouth and nostrils to pump air into the lungs. The gular pump is a similar motor action, used in situations when an individual can’t increase their rate of breathing. One study simulated locomotor action by running lizards on a treadmill. They took snapshots of the animal’s breathing and found that the lizard was drawing in multiple breaths into the gular cavity, in between larger exhalations from the lungs.

Understanding the motor action of the reptile and avian throat helps us pinpoint the evolutionary timeline of how lung-breathing developed. Reptiles, birds and mammals developed new traits to overcome a ‘speed-dependent axial constraint’ that limits the respiration rate during locomotion. One of the first traits that emerged was an accessory set of muscles that controlled lung breathing around the rib-cage, instead of through the mouth. In mammals this emerged as the diaphragm, a membrane that gives us more flexible respiratory capability when we’re tired out. Turtles and tortoises use an internal oblique muscle to achieve the same effect, decoupling respiratory and locomotor action.

A mechanism called ‘respiratory-locomotor phase coupling’ allows mammals to coordinate breathing with their stride, birds with their wing flaps. Mammals typically breathe once per stride or once per step, while birds breathe about three to five times per wing flap. Breathe deeply next time you see a pelican catch a fish. Hope you spot one soon.