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Why lizards may inherit the Earth

Monitor lizards extract oxygen both when they inhale and exhale, perhaps explaining why they are so successful.

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Cheryl Ertelt

The savannah monitor lizard has a respiratory system that makes air circulate one way.

A lizard captures oxygen from air both when inhaling and exhaling — a feat normally associated with birds. Many scientists believe birds developed the adaptation to cope with the enormous requirements of energy needed to take flight, and the discovery of ‘unidirectional breathing’ in the savannah monitor lizard1 raises questions about when and why the trait first evolved.

“To go and find a similar air-flow pattern in animals as distantly related [to birds] as monitor lizards is mind blowing,” says Mathew Wedel, an evolutionary biologist at Western University of Health Sciences in Pomona, California, who was not involved in the discovery.

Mammals and many other vertebrates breathe tidally, which means that air travels into the lung to gas-exchanging cavities called alveoli and then back out via the same path. Not birds, which store some of the air they inhale in specialized sacs. When they exhale oxygen is extracted from this air.

In 2010, a team led by Colleen Farmer at the University of Utah in Salt Lake City reported that alligators and other crocodile-like animals, too, practise unidirectional breathing2. The discovery hinted that dinosaurs might have also breathed the same way. But Wedel says that “it wasn’t super surprising that crocs might be a bit bird like”, because their lungs resemble those of birds.

Now a team led by Farmer and her colleague Emma Schachner, an evolutionary biologist at the same institution, report in Nature that savannah monitor lizards (Varanus exanthematicus) use the same breathing mechanism1.

Monitor lizards are a group of 70 or so species that includes Komodo dragons, the largest lizards on Earth. On the surface, their lungs look like they would use breathing, Schachner says. “When you pull the lungs out it just looks like a bag with chambers. It doesn’t look anything like the bird lung.” But computed tomography (CT) scans revealed a large chamber, with a series of up to 11 brachial tubes branching off in parallel and linked to one another via perforations — a set-up that could enable one-way flow (see picture).

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Our reporter speaks to Colleen Farmer about her surprising discovery on monitor lizards.

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To test this possibility, the researchers dissected the lizards' lungs and filled them with water containing suspended spheres, to better track how the water flowed. The water flowed tidally through the large chamber, but unidirectionally in the smaller brachial tubes. Schachner’s team confirmed that air followed these patterns during breathing as well by implanting sensors in the lungs of five lizards and measuring air flow as the animals breathed.

The discovery of unidirectional breathing in monitor lizards could either mean that the trait evolved in the common ancestor of birds, crocodiles and lizards — an animal that lived roughly 270 million years ago and resembled an iguana — or that the feature evolved independently in each evolutionary branch, Schachner says. To determine which scenario is correct, Schachner’s team plans to study the breathing patterns of still more reptiles, such as iguanas, geckoes and bearded dragons.

Her team’s study also raises questions about why unidirectional breathing developed in the first place. Farmer has hypothesized that it helps animals obtain oxygen while they’re holding their breath because unidirectional breathing allows more oxygen to be extracted from air — something that many lizards do when startled. Crocodiles can hold their breath for upwards of 20 minutes, and ancient marine reptiles may have found the trait useful for long dives, Schachner says. The trait could have also been an adaptation to lower oxygen levels on Earth, she says. During the early Triassic era 250 million years ago, oxygen made up 12% of air, compared with 21% today.

“It might explain something about why monitor lizards are so successful,” says Wedel. These lizards get more oxygen from air than any other reptile and they live in environments ranging from parched desert to tropical forest. “Who knows when the next asteroid hits, maybe monitor lizards will inherit the Earth,” Wedel says.

Schachner believes that no one found unidirectional breathing in lizards because the trait is so hard to measure, especially in wild animals. Wedel hopes the discovery by Schachner's team will inspire others to eschew conventional wisdom. “Now everybody who wakes up tomorrow and reads this paper will look at familiar organisms with a bit of curiosity and mistrust.”

REF. 1

In monitor lizards, air flows out of the lungs through special sacs rather than backtracking on the path it took on its way in, as shown by measurements in live lizards and in the lungs of dissected animals.

Journal name:
Nature
DOI:
doi:10.1038/nature.2013.14355

References

  1. Schachner, E. R. et al. Nature http://dx.doi.org/10.1038/nature12871 (2013)

  2. Farmer, C. G. and Sanders, K. Science 327, 338340 (2010)

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