22 Sep 2017

How kangaroos avoid dehydration with their nose

Red kangaroos are the Conor McGregor of kangaroos, and it’s not because of their hair colour. They are tough, really tough. Unlike grey kangaroos, which typically seek shade in woodlands and mostly depend on human-built water holes, red kangaroos don’t shy away from living in the driest, hottest deserts.

To cool down their bodies and avoid overheating (and death), kangaroos may pant, sweat and even lick themselves. But all these strategies to lower body temperature come with a price: they use up body water. And when you’re living in a place where water is a rare commodity, licking yourself profusely might not always be the best idea… So how do red kangaroos manage to avoid overheating and save body water at the same time?

To answer this question, Dale Nelson, Gavin Prideaux, and Natalie Warburton from Flinders and Murdoch Universities decided to take a close look at the red kangaroo’s nose. Yes, the nose.

Red Kangaroo (Macropus rufus).

Mammals have complex noses with narrow, curled spongy bones that work as an air conditioning system. These so-called turbinate bones are lined with thin blood vessels that make a temperature gradient along the nasal cavity—from cooler near the exterior to warmer internally. As we inhale, the incoming air is quickly warmed as it travels down the nasal passages, and when we exhale, warm air coming from the lungs is cooled, which saves body heat. Turbinate bones have another function though, and this is where red kangaroos come back into the story.

In the late 1970s, a few research teams noticed that desert mammals have extravagantly long turbinate bones. Camels, for example, have very long, convoluted turbinate bones that swirl round and round like a corkscrew. This scrolled shape increases the nasal surface area to about 1000 cm2, which is over six times the nasal surface area of humans. But what’s the advantage of having such extreme noses in the desert?

Scientists back then suspected it must have something to do with saving body water, and they were right. It turns out that in these animals the water vapour in exhaled air condenses as it contacts the cooler nasal surface, turning into liquid water. For example, giraffes may save up to 3 liters of water a day by condensation in the nose. But as impressive as this may sound, how this water is reabsorbed into the body has remained a mystery for over three decades.

Now, Nelson, Warburton, and Prideaux add the first piece to this puzzle in a new study published in the Journal of Zoology.

During work on fossil kangaroos at Flinders University, the team started wondering how the many shapes of noses in different species of kangaroos and wallabies might be related to their environment and behaviour. They were especially intrigued by the bulging noses of red kangaroos.

“Red kangaroos are the most adapted to the very hot, arid conditions of the Australian outback”, says Warburton. “Previous studies had described some aspects of nasal morphology in kangaroos, but we still didn’t really understand how these related to the biology of the animals in the wild.”

They set off to examine the internal bones and tissues of these animals expecting to find very long, coiled turbinate bones, like in other desert mammals, but they discovered something that “has never been found before”, Warburton says.

Digital images from CT-scans, the same technology used in hospitals to image internal body structures, revealed a pocket of bone within the floor of the nasal cavity.  This small hole in the bone was unusual, so the researchers used histological techniques to look carefully at the tissues lining it. To their surprise, they found that the bone pocket was filled with lymphatic vessels.

Lymph vessels are responsible for returning fluid from tissues into the circulating blood, so this pocket could be used to reabsorb the water condensed in the nose into the body.

“The condensation of water vapour from air as animals breathe out is known to […] conserve water in arid environments, but this is the first time that a possible mechanism for the reabsorption of that condensed water has been found in the nose of any mammal”, says Warburton.

Kidneys are the main site of water reabsorption in the body, and this is why in hot days we need to visit the WC less often. Desert mammals including kangaroos have special kidneys that produce very concentrated urine, which helps to save body water.

Nelson and colleagues may have discovered a new mechanism of water reabsorption in the nose that helps explain how desert mammals cope with the harsh conditions of their environment, but “further physiological testing is necessary to see if this is what is really going on”, Warburton claims.

In the future the team also plans to look at fossils of kangaroos to try and understand how extinct species adapted to changes in their environment.

“Through understanding how animals interacted with the environment in the past, we are able to better predict how they might adapt to environmental changes in the future”, Warburton concludes.

Nelson, D. P., N. M. Warburton, and G. J. Prideaux. "The anterior nasal region in the Red Kangaroo (Macropus rufus) suggests adaptation for thermoregulation and water conservation." Journal of Zoology (2017).

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