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.

Reference: 
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).

Why life got so big

About 570 million years ago, large, frond-like creatures suddenly invaded the ocean floors. For over a billion years, the Earth’s oceans were filled with bacteria and microscopic algae, but during the Ediacaran period, from 635 to 541 million years ago, larger multicellular organisms began crowding the seas.

Fossil imprints from the Ediacaran derive from soft-bodied organisms resembling modern-day sea anemones (Cyclomedusa), annelid worms (Dickinsonia) and sea pens (rangeomorphs such as Charnia). Among these bizarre creatures, the rangeomorphs are the most abundant in the fossil record—and also some of the largest.

Artist impression of rengeomorphs (credit: Jennifer Hoyal Cuthill)

Rangeomorphs were unlike any creature on Earth today. Some were as small as a coin, while others could grow up to 2 meters high. They looked like ferns, with branches spreading out from a central stem, but they likely fed by filtering nutrients from the water, similar to corals. Because rangeomorphs were so different from any known life form, paleontologists still don’t agree whether they were primitive animals related to soft corals, some sort of weird fungus or even a new (now extinct) kingdom of life, the Vendobiota.

These ocean dwellers eventually disappeared after the Cambrian explosion, some 541 million years ago, when fast-moving predators emerged (and probably ate them).

Changes in ocean chemistry

Based on the chemical signature of ancient seawater left on rocks, geochemists think there was a sharp rise in ocean oxygen levels soon after the end of the Gaskiers glaciation, about 580 million years ago. These changes in the ocean chemistry could explain the appearance of larger and more complex marine organisms—more food, bigger bodies. However, even though this may seem quite obvious, it’s actually quite difficult to demonstrate.

Jennifer Hoyal Cuthill and Simon Conway Morris, from the University of Cambridge (UK) and Tokyo Institute of Technology (Japan), used an original approach to tackle this problem.

“We wanted to see whether the increase in body size could point to a rise in oxygen, since the type of growth can tells us whether the animals have nutrients available or not”, says Hoyal Cuthill.

They suspected that Ediacaran organisms were large because they had a ‘nutrient-dependent’ type of growth, rather than an evolutionarily new genetic makeup.

‘Seeing’ extinct creatures grow

Many organisms can’t grow beyond a certain size, regardless of how much they eat. Humans for example, will (unfortunately) just get fatter, not taller, because they are genetically programmed to reach a specific maximum height. But for some organisms nutrient availability can affect body size. This type of nutrient-dependent growth is quite common in invertebrates and plants. Some plants will grow almost indefinitely, as long as there are nutrients (and light) available in the environment.

But how do you measure growth in organisms that lived nearly 600 million years ago?

This is where rangeomorph fossils come in handy.

Hoyal Cuthill and Conway Morris had previously worked with several rangeomorph specimens to study the unusual body plan of these animals. During this research it dawned on them that the rangeomorphs’ complex fractal branching shape, with larger older branches at the bottom and smaller younger branches on top, was the key for testing the nutrient-dependent growth hypothesis.

“It’s like looking back at your childhood photographs and comparing your height through your old photos up to the present day”, says Hoyal Cuthill. “We were inferring the history of growth of a rangeomorph by looking at parts of the structure of different ages”.

The researchers could basically “see” in a single fossil specimen how the animals were growing during their lifetime, by comparing the relative size and shape of younger and older branches.

A unique rangeomorph fossil

Fossil of Charnia (Jennifer Hoyal Cuthill)

The new study focuses on an exquisitely preserved specimen of Avalofractus abaculus, one of the last fossils removed from the Trepassey Formation, in Newfoundland (Canada), before strict restrictions were imposed to protect the site (currently called Mistaken Point Ecological Reserve). Hoyal Cuthill obtained a high-resolution cast from the Royal Ontario Museum and scanned it by CT- microtomography, a technique which uses x-rays to make detailed digital 3D reconstructions.

Two other specimens (Charnia masoni and an undescribed specimen from the South Australian Museum) were also analysed based on digital photographs.

Mathematical and computer models comparing the surface area and the volume of younger and older branches showed that growth gradually slowed down as rangeomorphs got bigger, which is exactly what happens in modern organisms with nutrient-dependent growth.

 “… You’re getting less nutrients as you get larger, so you cannot sustain the same rate of growth, and it slows down”, Hoyal Cuthill explains.

But there was more. Nutrient availability can also affect body shape, which is technically called ecophenotypic plasticity. Hoyal Cuthill and Conway Morris also found that rangeomorphs could rapidly change shape to access higher levels of oxygen in the seawater above them, by growing into a long, tapered shape.

Nutrient-dependent growth provides a mechanism to explain why changes in ocean chemistry caused the appearance of these large organisms in the Ediacaran, some 30 million years before the Cambrian explosion.

Hoyal Cuthill next wants to investigate whether rangeomorphs really are animals, and to which modern groups are they related to.

“Rangeomorphs are quite mysterious and were only relatively recently discovered and identified as Precambrian organisms”, she says. “This is an exciting time and many researchers are looking at the biota of the Ediacaran and finding new fascinating things”.


Reference: Hoyal Cuthill, Jennifer F., and Simon Conway Morris. "Nutrient-dependent growth underpinned the Ediacaran transition to large body size." Nature Ecology and Evolution (2017). DOI: 10.1038/s41559-017-0222-7

This article was published originally as a guest post in the PLOS Paleo Community blog with the title "Why Precambrian life got so big" on the 18-07-2017. You can read it here.