Air Sac: Evolution, What Animals It Is Found In, Functions

Air sacs are permanent air cavities within organisms. The presence of these sacs in the bones is called pneumaticity, and their formation process during bone development is called pneumatization.

The distribution of these sacs in the body of organisms is relatively varied. They can be present in bones (skeletal pneumaticity) such as the skull, vertebrae, ribs, sternum and others. They are also present as part of the respiratory system of many animals.

Skull of a Tyrannosaurus rex. These animals had massive, fused cranial bones, yet they had numerous small air spaces, which made them lighter. Taken and edited from AE Anderson [Public domain], via Wikimedia Commons.

Scientists believe that the development of air sacs allows animals that present them to have or improve balance, efficiently cool their bodies and increase gas exchange.

The diversity of organisms that these sacs possess ranges from birds and mammals, to insects. Even today it is known that dinosaurs had pneumatic bones, that is, bones with air sacs.

Article index

  • one


    • 1.1

      In humans

  • two

    In which animals are air sacs found?

  • 3


    • 3.1

      On dinosaurs

    • 3.2

      In birds

    • 3.3

      In mammals

    • 3.4

      In insects

  • 4



Of the current animals, birds have the largest number of structures with air sacs in their bodies. From an evolutionary point of view, birds are the current descendants of dinosaurs.

Birds are known to evolve from small carnivorous dinosaurs. These dinosaurs lived in the late Jurassic or later (about 152 to 163 million years ago), according to fossil records discovered in China and South America.

However, the ancestor of all modern birds is believed to have lived in the late Cretaceous or later (about 72 to 100 million years ago). This ancestor survived in the subsequent 65 million years that elapsed after the mass extinction of the dinosaurs.

Researcher Peter Ward, in 2006, suggested that the first air sacs in dinosaurs appeared in organisms that lived in the Triassic period (about 252 million years ago).

This structure was an evolutionary character that allowed the organisms that possessed it to adapt to the low levels of oxygen that existed during that period.

This characteristic is currently maintained in the descendants of dinosaurs, birds. The presence of these sacs in other zoological groups may be due to a mechanism of parallel or paraphyletic evolution.

In humans

On the other hand, human beings have paranasal sinuses. These are a system of air cavities located in the head; in the frontal, ethmoid, sphenoid bones, and in the upper jaw.

There is much controversy about the evolutionary aspects of the paranasal sinuses in Neanderthals and modern humans. It is not known for sure what is the origin and evolutionary function.

One of the proposed hypotheses about the presence of these air sacs is adaptation to extreme cold. Much discussed topic and with many detractors.

The presence of air sacs in hominids and other vertebrates has also been a controversial issue in evolution. These sacs have appeared and disappeared in different groups, even in unrelated lineages.

Various hypotheses have been proposed about the presence of pneumatization or air spaces in the skull of vertebrates. Among these hypotheses stand out: adaptations to cold, dispersion of the chewing force, cranial lightening and storage of substances.

In which animals are air sacs found?

The first organisms known to have air sacs were dinosaurs. They were present in the group of Ptrosaurs (Flying Dinosaurs) and Saurischians.

The latter were represented by the Pteropods (such as the Tyrannosaurus rex ) and the Sauropods (such as the great long necks).

Of the current animals, birds are the ones that have the largest number of air sacs inside the body. They are present in all or most species of birds.

These structures are also found in other vertebrates, such as mammals, both in the bones and in part of the respiratory system.

Other animals that have air sacs or so-called structures are insects. These arthropods have air sacs as part of their respiratory system. They are present as dilations or expansions of the tracheae.


On dinosaurs

Different hypotheses have been put forward on the functionality of skeletal and non-skeletal air sacs in dinosaurs.

The presence of these spaces could indicate that in these organisms it helped them to have a high metabolic activity, since the sacs increased respiratory capacity.

Some authors also propose that the air sacs served to improve balance and to reduce rotational inertia. This last hypothesis, however, is highly disputed, since it does not apply in groups such as the Sauropods.

Another hypothetical function of air cavities is to regulate body temperature. The presence of feathers covering the body, or the activity of flight, could warm the body.

The bags would then function as a means of dissipating excess heat. The presence of pneumatic bones in dinosaurs allowed them to have lighter bones and facilitated the evolution of giant living forms.

In birds

In birds, non-bony air sacs can be seen as small chambers that swell and deflate. This causes a flow of gas into a separate chamber, analogous to what occurs in the lungs.

They also present bony air cavities. The presence of these spaces allowed the birds to have lighter bones. This, in turn, allowed most of these organisms to fly.

In addition, it helped them to have highly efficient respiratory systems, to distribute skeletal mass and it is even believed that it allows some birds to fly at high altitudes.

Non-skeletal air sacs in Birds and Dinosaurs. Taken and edited from CNX OpenStax [CC BY 4.0 (], via Wikimedia Commons.

In mammals

In mammals there are air sacs in the so-called paranasal sinuses. These are diverticula belonging to the nasal cavity. They contain air and are part of the bones of the skull.

There are several hypotheses about the functions of these pneumatized structures. These hypotheses include lightening of the skull, heat exchange and dissipation, and use for mucus secretion.

The study of these functions has been carried out using animals such as horses, ruminants, carnivores, small pets (dogs and cats), even in humans.

In insects

The aerial sacs of insects are chambers with a great capacity for expansion. They are associated with the tracheae and their function is to store air and facilitate flight.


  1. Air sac. Anatomy. Recovered from
  2. Skeletal pneumaticity. Recovered from
  3. The origin of birds. Recovered from
  4. P. Ward (2006). Out of thin air: Dinosaurs, birds, and earth’s ancient atmosphere. National Academies Press.
  5. V. Dressino & SG Lamas (2014). Controversies over the evolutionary role of paranasal sinus Pneumatization in humans and Neanderthals as an Adaptation to extreme cold. Ludus Vitalis.
  6. Matt Wedel: Hunting the inflatable dinosaur. Recovered from
  7. H. Brooks. (2018). Pneumatized bones help scientists study the evolution of breathing in birds and dinsoaurs. Recovered from
  8. R. Zárate, M. Navarrete, A. Sato, D. Díaz, W. Huanca (2015). Anatomical Description of the Paranasal Sinuses of the Alpaca (Vicugna pacos). Journal of Veterinary Research of Peru.

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