Kangaroo teeth grow forever – and keep a record of their owner’s age and sex

William Parker, Monash University and Alistair Evans, Monash University

How do you find out the age of a wild animal? For some Australian marsupials, we have discovered you can tell from their teeth.

In a new paper published in Archives of Oral Biology, we show that the front teeth of kangaroos record their age in a number of different ways – and they can even tell us if the roo is male or female.

Long in the tooth

Finding out the age of a wild animal can be important for vets, ecologists and conservationists. Wildlife welfare and assessing the overall health of a population both depend on knowing the age of the animals involved.

With no-one counting birthdays in the bush, scientists often turn to the teeth of wild animals to work out how old they are.

Most of Australia’s marsupials are members of a group called Diprotodontia. This name refers to the animals having large, straight incisor teeth in their lower jaws.

Kangaroos, wallabies, koalas, wombats and possums are all diprotodontian marsupials. In our study, we measured the growth of these incisor teeth in kangaroos and honey possums and found they never stop growing.

We can use this continuous growth to age marsupials by exactly how long they’ve grown in the tooth.

Tree rings and tooth lines

Much like trees have growth rings, teeth have growth lines. These lines form as the different hard tissues that make up a tooth (enamel, dentine and cementum) are added over time.

We looked at the growth lines in kangaroo incisor teeth to see if there’s a record of age there as well. It turns out that yearly growth lines can be found in two different regions of these teeth.

Marching molars

Another weird way we can tell the age of a kangaroo is by measuring the movement of its molars.

Because eating grass can rapidly wear teeth down, kangaroos have a special adaptation where their molar teeth move forward in their jaws over time. Old, worn teeth are pushed forwards and fall out to make way for new, unworn teeth that are much better at chewing. It’s a bit like a conveyor belt of teeth. This process keeps going until the oldest kangaroos have only a couple of teeth left.

Scientists have measured the rate at which molar progression happens and found that it corresponds accurately with age. Elephant teeth move in a very similar way and this technique works to age them as well.

There are several ways to estimate the age of a kangaroo from their teeth. William Parker

Teeth tell more than age

As part of our study, we looked to see if there were differences in the incisor teeth between male and female kangaroos. We found incisors belonging to male kangaroos generally grow faster and can wear down more quickly than the incisors of females.

Information like this is important for understanding animal ecology, as it points to males and females foraging and feeding differently in the wild. Across the animal kingdom, teeth can tell us a remarkable amount about feeding behaviours, different diets and patterns of evolution.

Insights into the lives of ancient kangaroos

There are four species of kangaroo alive today. The largest species is the red kangaroo, and the biggest males grow to around 90 kilograms.

Thousands of years ago, Australia had a wonderful diversity of giant long- and short-faced kangaroos. Some of these likely ran instead of hopped and weighed around 250 kilograms.

Our new methods will help scientists learn more about the lives of these extinct giants. It can be very difficult to determine the age of an extinct animal from a fossil and to work out if that fossil came from a male or female – but we hope that our new methods will bring insight from incisors.

William Parker, PhD Candidate, Monash University and Alistair Evans, Professor, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Jaws of death: how the canine teeth of carnivorous mammals evolved to make them super-killers

Tahlia Pollock, Monash University; Alistair Evans, Monash University, and David Hocking, Monash University

Carnivorous animals come in all shapes and sizes, from the 500-gram quoll to the 500-kilogram polar bear. This disparate group of mammals shares a common feature: canine teeth at the front of their jaws.

Canine teeth are long and pointed, with a sharp tip and, in some cases, bladed edges. These fearsome weapons are what make carnivores such effective killers. In fact, our new research out today reveals how evolution has shaped canines into unique forms to suit each predator’s way of life.

We applied state-of-the-art 3D methods to measure the canine teeth of more than 60 predators including lions, cheetahs, grizzly bears, dingoes and Tasmanian devils. The research represents the first comprehensive analysis of canine tooth shape in predatory mammals.

We discovered canine teeth have evolved in special ways to help each species kill and eat their favourite prey – helping to make mammals some of nature’s most successful predators.

A lion, meerkat, grizzly bear, and African wild dog bearing their canine teeth. Lion Petr Ganaj, meerkat Joshua J. Cotten, grizzly bear mana520, African Wild Dog Matt Burke all via Unsplash

Born to kill

When carnivorous mammals snarl, they reveal four long canine teeth at the front of their jaws - two at the top and two at the bottom. These teeth are the first point of contact between predator and prey, and are used to stab, kill and dismember a catch.

Not all carnivores have the same diet. Grizzly bears eat meat, fruit and plants, while meerkats feed mostly on invertebrates like scorpions and beetles. Big cats, like cheetahs, stick to meat.

Carnivores can also kill in myriad ways. Tigers suffocate their prey with a targeted throat bite, while wolves use multiple slashing bites to tear apart their prey. Small canids such as the red fox snap up and violently shake their prey, while wolverines can kill with a single, crushing skull bite.

There’s been little research into the associations between canine tooth shape, function and evolution. Our research sought to determine what canine shapes are best for each predator diet.

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Read more: New research reveals animals are changing their body shapes to cope with climate change

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Lion using its long sharp dagger-like canines to deliver a targeted neck bite and taking down an Oryx in the Kalahari Desert. Lion canines Mike van den Bos and hunting Thomas Evans both via Unsplash

A bite worse than its bark

We scanned and compared the canine teeth of more than 60 carnivores, including tigers, coyotes, polar bears, wolverines, raccoons and even quolls. We then looked at the association between canine shape and function.

We found tooth shape varies depending on the types of food a carnivore regularly bites into – just like we choose different kitchen knives depending on what we want to cut up.

Big cats such as lions, tigers and cheetahs have some of the sharpest canine teeth in the animal kingdom. These long, dagger-like weapons are used to stab – biting down deeply into the throats of prey to bring them down.

Take a 3D look at the canine teeth of a cheetah in the interactive below.

Cheetah skull with long sharp canines by Evans EvoMorph Lab on Sketchfab

Other species, such as the coyote and red fox, have slender, curved canines. These teeth act as hooks to help hold small prey and prevent it slipping from the mouth when shaking.

Animals that eat a lot of “soft” prey, or those that deliver throat bites, often have sharp, slender canines. The sharp tips make a crack in the prey and as the animal bites down, the long, sharp edges of the tooth help penetrate deeply into the catch.

Species with a tougher or more varied diet have stout, robust teeth that don’t break when crunching bone or other hard foods. These species include scavengers such as the Tasmanian devil, and generalists such as the honey badger.

The bluntest upper canine tips we examined belong to the crab-eating mongoose. As the name suggests, the species feeds on crabs and other hard prey such as reptiles, snails and insects.

We also found canine teeth with blunt tips and edges were found in animals that kill prey with crushing bites to the skull, such as the American martin or wolverine. Blunt tips are better able than sharp tips to withstand the stresses produced by such heavy force.

Canine teeth can be long and sharp, slender and curved, or blunt and robust. These differences relate to how these teeth are used during hunting and feeding. Image by Tahlia Pollock

Something to chew on

The research helps establish new links between tooth shape and ecology that may shed light on the diet and behaviour of extinct species.

For example, the thylacine (or Tasmanian tiger) had curved canines, which suggests it may have snapped up and shaken smaller prey. This supports recent research on thylacine skull shape which found that, contrary to previous theories, the thylacine likely hunted small rather than large prey.

By studying canine teeth up close, we’ve discovered just how well evolution shaped even the smallest animal features to suit the niches they fill in nature.

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Read more: Who would win in a fight between a wedge-tailed eagle and a bald eagle? It's a close call for two nationally revered birds

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Tahlia Pollock, PhD candidate, Monash University; Alistair Evans, Associate Professor, Monash University, and David Hocking, Adjunct Research Associate, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Tasmanian Tiger was not the 'big bad wolf'

The Tasmanian tiger was hunted to extinction as a 'large predator' – but it was only half as heavy as we thought

Smithsonian Institution/colourised by D.S. Rovinsky

Douglass Rovinsky, Monash University; Alistair Evans, Monash University, and Justin W. Adams, Monash University

Until it was hunted to extinction, the thylacine – also known as the Tasmanian tiger or Tasmanian wolf – was the world’s largest marsupial predator. However, our new research shows it was in fact only about half as large as previously thought. So perhaps it wasn’t such a big bad wolf after all.

Although the thylacine is widely known as an example of human-caused extinction, there is a lot we still don’t know about this fascinating animal. This even includes one of the most basic details: how much did the thylacine weigh?

An animal’s body mass is one of the most fundamental aspects of its biology. It affects nearly every facet of its biology, from biochemical and metabolic processes, reproduction, growth, and development, through to where the animal can live and how it moves.

For meat-eating predators, body mass also determines what the animal eats – or more specifically, how much it has to eat at each meal.

Catching and eating other animals is hard work, so a predator has to weigh the costs carefully against the benefits. Small predators have low hunting costs – moving around, hunting, and killing small prey doesn’t cost much energy, so they can afford to nibble on small animals here and there. But for bigger predators, the stakes are higher.

Almost all large predators – those weighing at least 21  kilograms – focus their efforts on prey at least half their own body size, getting more bang for the buck. In contrast, small predators below 14.5 kg almost always catch prey much smaller than half their own size. Those in between typically take prey less than half their size, but sometimes switch to a larger meal if some easy prey is there for the taking – or if the predator is getting desperate.

The mismeasure of the thylacine

The March 14, 1868 edition of the Launceston Examiner featured tales of a ‘hyena’ that managed to kill 25 sheep. trove.nla.gov.au

Few accurately recorded weights exist for thylacines – only four, in fact. This lack of information has made estimating their average size difficult. The most commonly used average body mass is 29.5kg, based on 19th-century newspaper accounts.

This suggests the thylacine would probably have taken relatively large prey such as wallabies, kangaroos and perhaps sheep. However, studies of thylacine skulls suggest they didn’t have strong enough skulls to capture and kill large prey, and that they would have hunted smaller animals instead.

This presented a problem: if the thylacine was as big as we thought, it shouldn’t be able to live solely on small prey. But what if we’ve had the weight wrong the whole time?

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Read more: Why did the Tasmanian tiger go extinct?

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Weighing an extinct animal

Ben Myers of Thinglab scans a Museums Victoria thylacine. CREDIT, Author provided

Our new research, published today in Proceedings of the Royal Society B, addresses this weighty issue. Our team travelled throughout the world to museums in Australia, the United States, the United Kingdom and Europe, and 3D-scanned 93 thylacines, including whole mounted skeletons, taxidermy mounts, and the only whole-body ethanol-preserved thylacine in the world, in Sweden.

Based on these scans, we created new equations to estimate a thylacine’s mass, based on how thick their limbs were – because their legs would have had to support their entire weight.

We also compared the results of these equations with a new method of digitally weighing 3D specimens. Based on a 3D scan of a mounted skeleton, we digitally “filled in the spaces” to estimate how much soft tissue would have been present, and then used our new formula to calculate how much this would weigh. Taxidermy mounts were easier as there was no need to infer the amount of soft tissue. The most artistic member of our team digitally sculpted lifelike thylacines around the scanned skeletons, and we weighed them, too.

Building and weighing a thylacine. Scanned skeletons (lop left) were surrounded by digital ‘convex hulls’ (top right), which then had their volume and mass calculated. The skeletons were then used in digitally sculpting lifelife models (bottom left), each with their own unique stripes (bottom right). Rovinsky et al.

Our calculations unanimously told a very different story from the 19th-century periodicals, and from the commonly used estimate. The average thylacine weighed only about 16.7 kg – not 29.5 kg.

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Read more: Friday essay: on the trail of the London thylacines

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Tall tales on the tiger trail

This means the previous estimate, based on taking 19th-century periodicals at face value, was nearly 80% too large. Looking back at those old newspaper reports, many of them in retrospect have the hallmarks of “tall tales”, told to make a captured thylacine seem bigger, more impressive and more dangerous.

It was based on this suspected danger that the thylacine was hunted and trapped to extinction, with private bounties already placed on them by 1840, and government-sponsored extermination by the 1880s.

Thylacines were much smaller in stature than humans or grey wolves. Rovinsky et al., Author provided

The thylacine was much smaller than previously thought, and this aligns with the smaller prey size suggested by the earlier studies. Predators below 21 kg – in which we should now include the thylacine – all tend to hunt prey smaller than half their size. The “Tasmanian wolf” probably wasn’t such a danger to Tasmanian farmers’ sheep after all.

By rewriting this fundamental aspect of their biology, we are closer to understanding the role of the thylacine in the ecosystem – and to seeing exactly what was lost when we deliberately hunted it to extinction.

Douglass Rovinsky, PhD Candidate, Monash University; Alistair Evans, Associate Professor, Monash University, and Justin W. Adams, Senior Lecturer, Department of Anatomy and Developmental Biology, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

A New Spiky Mesozoic Mammal

Last week a new species of mammal from around 125 million years ago was described in the journal Nature called Spinolestes xenarthrosus. It had a pile of unusual and perhaps advanced features that gives more evidence that mammals in the Mesozoic were diverse in their morphology and ecological niches. The spectacular preservation of the skin and hair meant that the soft ears and the spine-like hair structures were preserved!

Al Evans commented on the new finding in the online news site The Verge.

National Geographic News

Lab members recently contributed to discussions about science news covered in National Geographic.

David Hocking revealed some of the secrets of leopard seal feeding when discussing new critter-cam footage of leopard seals.

http://news.nationalgeographic.com/2015/08/150807-leopard-seals-antarctica-behavior-ocean-animals-science/

Al Evans talked about some of the amazing features of our native marsupials, including the tiny babies of red kangaroos - and how kangaroos can put embryos in suspended animation during a drought!

http://news.nationalgeographic.com/2015/08/150828-baby-mammal-size-differences-panda-kangaroo-science/

Late nights at the Synchrotron

Earlier this month a few of the Lab members helped out at the Australian Synchrotron working with Museum Victoria scanning fossils, including dinosaurs and Ediacaran animals. Waverley Leader caught Anton Maksimenko, Tom Rich, Lap Chieu and Pat Vickers-Rich in the act (of not doing very much):

We hope this will give us cool things like internal structures, and the ability to 3D print full-size dinosaur skeletons never-before seen, because they are still encased in rock!

Welcome to EvoMorph!

We're very pleased to announce the unveiling of the new web site for the Evans Evolutionary Morphology Lab - evomorph.org. We will share interesting news from our work and other topics in morphology, anatomy, evolution, evo-devo and palaeontology.

Come back often to see what we've been up to!

Al Evans, Lab Head