Air flows through lizard lungs in one direction, a finding that may prompt a rethink about how some species evolved following Earth's biggest mass extinction, a study says.
Humans and most other animals in contrast have a so-called "tidal", or two-way, breathing system, in which air is drawn into the lungs until it reaches a dead end - bunches of cells called alveoli.
There, oxygen is drawn into the bloodstream and exchanged for carbon dioxide, which is then breathed out with the used air.
Birds, though, are a notable exception to this, with the air entering the trachea, or windpipe, flowing through the lung in one direction and exiting the way it came in.
This unusual pattern of breathing has raised much debate among biologists.
One theory is that a one-way flow is a highly efficient way of providing oxygen for strenuous activity: it helps birds cope with the exertion of flying and dealing with thinner concentrations of oxygen at altitude.
But, surprisingly, lizards - a sleepy group of land-loving animals - can also be counted as members of the club, according to the new probe released on Wednesday.
University of Utah biologist CG Farmer led a team that carried out 3-D scans of the lungs of African monitor lizards (Varanus exanthematicus) and then implanted flow metres in five of the animals to see how the air moved.
The investigators found that air enters the lizards' trachea and then splits into two airways - one for each lung.
It then winds its way through a series of chambers in each lung, passing through perforated walls, before doubling back.
A similar one-way flow also appears to exist among American alligators, helping them quite literally to hold their breath, whether they are under water or not, Farmer said in a phone interview.
If so, some of the thinking about unidirectional breathing - that it helps to sustain high metabolism - has to be scrapped.
"I think it's probably not an adaptation allowing an animal to be active," Farmer said.
"In fact, (it's likely to be) an adaptation to allow them to sit quietly for long periods of time without moving, without breathing," she said.
