Sabre-toothed cats had weak bites

The sabre-toothed cat is one of the most famous prehistoric animals and there is no question that it was a formidable predator, capable of bringing down large prey like giant bison, horses, and possibly even mammoths. The two massive canines – the largest teeth of any mammal – are a powerful visual. But while they were clearly powerful weapons, scientists have debated their use for over 150 years.

Now, a new study shows that Smilodon, the most iconic of the sabre-tooths, had a surprisingly weak bite. They were a precision weapon that were used to deliver a single, final wound to an already subdued victim – the equivalent of an assasin’s stiletto rather than a swordsman’s blade.

Earlier suggestions pictured Smilodon using its teeth to hang onto the back of large prey, to slash their abdomens open, or to impale them at the end of a flying pouce. One of the most popular theories said that the cat would have used its teeth to sever arteries and airways with a decisive bite to the throat – a quicker technique than the suffocating neck bites used by modern lions.

Working out how strongly Smilodon could bite would go a long way towards deciding on one of these theories and to do that, palaeontologists have studied the animal’s fossilised skull. Even then, opinions have gone either way depending on which bit of the skull they looked at. The muscle attachment points suggest it has small jaw muscles, but the bite could have been powered from the neck. The lower jaw is smaller, but strongly built, lending weight to the idea of a powerful bite.

To get some clearer answes, Colin McHenry and colleagues from the University of Newcastle, Australia decided to put Smilodon‘s skull through a digital crash-test. They used a technique called ‘finite element analysis‘ or FEA, which is typically used in mechanical engineering and crash-testing for cars.

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Bone-crushing super-wolf went extinct during last Ice Age

Being confronted with a pack of wolves is bad enough, but if you happened to be in Alaska some 12,000 years ago, things would be much, much worse. Back then, the icy forests were patrolled by a sort of super-wolf. Larger and stronger than the modern gray wolf, this beast had bigger teeth and more powerful jaws, built to kill very large prey.

The gray wolf - smaller than the Beringian variety, and with weaker jawsThis uber-wolf was discovered by Jennifer Leonard and colleagues from the University of California, Los Angeles. The group were studying the remains of ancient gray wolves, frozen in permafrost in eastern Beringia, a region that includes Alaska and northwest Canada.

These freezer-like conditions preserved the bodies very well, and the team found themselves in a unique position. They could not only analyse the bones of an extinct species, but they could extract DNA from said bones, and study its genes too.

For their first surprise, they found that these ancient wolves were genetically distinct from modern ones. They analysed mitochondrial DNA from 20 ancient wolves and none of them was a match for over 400 modern individuals. Today’s wolves are clearly not descendants of these prehistoric ones, which must have died out completely. The two groups shared a common ancestor, but lie on two separate and diverging branches on the evolutionary tree.

Hunter/scavenger

The genes were not the only differences that Leonard found. When she analysed the skulls of the Beringian wolves, she found that their heads were shorter and broader. Their jaws were deeper than usual and were filled with very large carnassials, the large meat-shearing teeth that characterise dogs, cats and other carnivores (the group, not meat-eaters in general).

The overall picture is that of a skull specially adapted to bite with tremendous force. These ancient wolves were hypercarnivores, specialised for eating only meat and killing prey much larger than themselves. Leonard even suggests that the mighty mammoths may have been on their menu.

The eastern Beringian wolf was a formidable hunter that could also turn to scavenging - just like modern hyenas.Once prey was dismembered, the wolves would have left no bones to waste. With its large jaws, it could crush the bones of recent kills, or scavenge in times between hunts. Today, spotted hyenas lead a similar lifestyle.

The wolves’ teeth also suggest that bone-crushing was par for the course. The teeth of almost all the specimens showed significant wear and tear, and fractures were very common.

Their powerful jaws allowed the Beringian wolves to quickly gobble down carcasses, bones and all, before having to fend off the competition. And back then, the competition included many other fearsome and powerful hunters, including the American lion and the short-faced bear, the largest bear to have ever lived.

Evolution of a super-wolf

Leonard suggests that the ancestor of today’s gray wolf reached the New World by crossing the Bering land bridge from Asia to Alaska. There, it found a role as a middle-sized hunter, sandwiched between a smaller species, the coyote, and a larger one, the dire wolf.

When the large dire wolves died out, the gray wolf split into two groups. One filled the evolutionary gap left behind by the large predators by evolved stronger skulls and teeth. The other carried on in the ‘slender and fast’ mold.

The extinct super-wolf would have been able to hunt prey even larger than this bison.But in evolution, the price of specialisation is vulnerability to extinction. When its large prey animals vanished in the Ice Age, so too did the large bone-crushing gray wolf. Its smaller and more generalised cousin, with its more varied diet, lived to hunt another day.

Similar things happened in other groups of meat-eaters. The American lion and sabre-toothed cats went extinct, but the more adaptable puma and bobcat lived on. The massive short-faced bear disappeared, while the smaller and more opportunistic brown and black bears survived.

Leonard’s findings suggests that the casualties of the last Ice Age extinction were more numerous than previously thought. What other predators still remain to be found in the permafrost?

Reference: Leonard, Vila, Fox-Dobbs, Koch. Wayne & van Valkenburgh. 2007. Megafaunal extinctions and the disappearance of a specialized wolf ecomorph. Curr Biol doi:10.1016/j.cub.2007.05.072

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Of dogs and devils – the rise of contagious cancer

While human cancers cannot be transmitted from person to person, scientists have recently identified two types of contagious cancers in animals. In Tasmanian devils and domestic dogs, cancer cells have evolved into independent parasites, jumping from animal to animal like an infectious virus.

Cancer cells are, for all intents and purposes, immortal. Having broken free of the rules and strictures that govern other cells, they are free to grow and divide as they please. In a short space of time, a lone cancer cell can form a mass of identical clones – a tumour. Theoretically, cancers could exist indefinitely, but as always, there is a catch. Those that spread quickly and aggressively do so at the expense of their host, who usually ends up dying, taking the tumour with it.

But there is one way a cancer could escape this fate and carry on its selfish reproduction – by finding another host. It could become contagious.

In humans, cancers are definitely not contagious. You can’t catch cancer from someone who has it. At most, you can inherit a higher risk of developing cancer, because of faulty genes passed on from your parents. But recently, scientists have found some startling exceptions to this rule.

Even devils get cancerTasmanian devils are plagued by a contagious facial cancer

Earlier this year, Australian researchers Anne-Marie Pearse and Kate Swift found that a facial cancer plaguing the local Tasmanian devils (right) was caused by contagious cancers.

The condition, known as devil facial tumour disease, is spread when an infected devil bites another. The devils’ boisterous temperaments and their propensity for squabbling over carcasses mean that such bites are common.

Once infected, the animals develop grotesque tumours that stop them from feeding properly, and they usually die of starvation within six months. As a result, the cancer is decimating the already small population.

Going to the dogs

At University College London, Robin Weiss and Claudio Murgia have found another example of an infectious cancer – a disease called canine transmissible venereal tumour (CTVT), or Sticker’s sarcoma.

CTVT is transmitted through sex or close contact between infected dogs. It was first described 130 years ago by a German scientist called Novinski and its origins have been debated ever since. Some scientists suggested that it was caused by a virus, much like human papillomavirus (HPV) causes cervical cancer in humans today.

But in a study published this month, Weiss and Murgia have put these theories to rest. They and their colleagues analysed tumour samples from 40 dogs across five continents. All these samples shared identical and distinctive genetic markers that uninfected tissues from the same dogs did not.

The explanation was clear – these cancers had not developed in the usual way from the cells of the host animals. The cancer cells themselves were spreading from dog to dog.

Becoming contagious

CTVT evolved in an old Asian dog line, like the huskyThese rogue cells have become parasites in their own right, evolving from a single ancestor into a dynasty that has colonised the globe aboard canine vessels. How this process began is still a mystery, but Weiss’s analysis provides some hints as to where and when.

The original cancer cell must have developed in either a wolf or an old Asian dog lineage, such as a Husky (left). It evolved anywhere between 200 and 2500 years ago and may well have been around for even longer.

In fact, the CTVT cancer cell is very likely to be the oldest lineage of mammalian cells still in existence. The cells that Weiss is studying today are most probably direct clone descendants of the same cells that Novinski identified 130 years ago – genetically identical great-granddaughters of the original tumour.

To kill or not to kill

When that original cell gained independence, it became truly immortal, long outliving its original body and lasting for centuries. So far, we don’t know of any human cancer cells that have pulled off a similar trick. But Weiss feels that if they did, the best place to look for them would be in people with weaker immune systems including transplant patients and those with HIV.

His group have found evidence that evading the host’s immune defences is a key part of CTVT’s strategy for finding another dog to infect. The cells accomplish this by switching off some key immune system players – a group of genes collectively called dog leukocyte antigens (DLAs). They also secrete a protein called TGF-β1 that strongly blocks any immune responses.

But slipping past immune sentries would do the cells no good if the host died before infecting another dog. Infection requires sex, which may not happen for some time. So CTVT is a merciful parasite.

At the start of infection, it grows rapidly, but within 3-9 months, it regresses of its own accord. By never killing its carrier, the cells ensure that they can spread to as many new hosts as possible.

The Tasmanian devils are not so lucky. Their cancers are spread through biting, a more frequent event than sex, and as such, they can afford to be more aggressive. But the devils’ population is small and their genetic diversity is low. This combination may spell the end for both devils and cancer cells, unless some vaccine can be found.

 

Reference: Pearse & Swift. 2006. Nature 439: 549.
Murgia, Pritchard, Kim, Fassati & Weiss. 2006. Cell 126: 477-487.

 

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The fox and the island – an Aleutian fable

 

Island-dwelling animals across the world have been devastated by predators introduced by man. In the Aleutian islands, this age-old problem has gone one step further. There, the introduction of Arctic foxes has changed the very nature of the land itself.

 

Nizki island has changed. If you had visited Nizki in the 19th century, you would have been greeted by the chorus of massive colonies of seabirds, and set foot on verdant grassland. But travel to the island now and you would find a land transformed. The tall grasses and most of the seabirds have gone. The landscape is now tundra, dominated by low-lying shrubs and suffering from poor soil quality.

And if you looked carefully, you could probably spot the perpetrator behind the altered terrain – the Arctic fox. According to literature and folklore, the fox had exceptional powers of cunning and trickery. But science now reveals that they have another trick up their sleeve – the power to change entire landscapes.

The foxes arrive

The Arctic fox takes another Aleutian birdNizki Island is part of the Aleutian archipelago, a band of sub-Arctic islands that spans the gulf between Russia and Alaska.

In the late 19th century, the island chain was visited by fur traders. Seeking to forestall losses from declining sea otter numbers, the traders introduced Arctic foxes to the islands to act as a readily available future source of fur.

A century later, Donald Croll and James Estes from the University of California, Santa Cruz, were carrying out conservation work in the Aleutians. They noticed that islands infested by foxes had changed from grassland to low-lying tundra and wanted to work out how the furry predators had affected the Aleutian archipelago.

Thankfully, the fur traders had failed to introduce foxes to some of the islands, and many remain fox-free to this day. They had unwittingly set up a massive natural experiment, which Croll and Estes took advantage of. Backed by a team of researchers, they surveyed 18 islands, comparing those that were ridden with foxes and those that lacked them.

Goodbye seabirds, farewell gauno

They found that when foxes first invaded the islands, they began doing what natural selection had designed them to do – killing. Their prey were the local seabirds, and only species that nested on unreachable cliff faces escaped them.

Burrow-nesters like puffins and surface-nesters like gulls were easily taken and their populations were decimated. Today, seabirds are a hundred times more common on fox-free islands than on their fox-infested neighbours.

The landscape changes from grassland to tundra without guano

Bird droppings, or ‘guano’ were the main source of fertiliser for the Aleutian vegetation. By feeding in the productive ocean waters and defecating inland, the birds transferred nutrients from the rich sea to the poor land.

As the seabirds died, guano levels fell by over 60 times, and the soil was quickly rendered infertile. With foxes around, the levels of phosphorus – a key nutrient in most ecosystems – on an island plummeted by three times.

This newly depleted land could not longer support lush grassland, and shrubs became the dominant plant as the grasses died out.

As a final test of their theories, the scientists artificially added fertiliser to parts of fox-infested islands over three years, to mimic the effect of guano. On the fertilised terrain, grass rapidly re-established itself as the dominant plant group, increasing in numbers by 24 times.

Killer immigrants

The problem of introduced predators is, sadly, not uncommon. All around the world, people have transferred predators to places they don’t belong with devastating consequences. The problem is especially serious on islands, where the local wildlife is naïve about the threat of predators, or has lost defensive adaptations such as flight.

In Australia and New Zealand, foxes, cats and stoats have hunted their way through local bird populations, driving many to the brink of extinction. And as the Aleutian problem demonstrates, the effects of introduced killers can ripple out to affect more than just their prey. In this example, entire ecosystems can be changed over a very large area.

Deporting the problem

In the Aleutians at least, the problem seems solvable. The US Fish and Wildlife Service has been removing foxes from Aleutian islands for over 35 years. As a result, the seabirds are staging a comeback and the lush vegetation is returning. Even so, it may take several more decades for the islands to return to their former glory.

Until then, the Aleutians serve as a stark reminder of the disastrous effects of placing top predators where they don’t belong. Conserving these animals in their original homes is just as important – other studies have shown that removing top predators can wreak equally dramatic changes in an ecosystem.

Many of the world’s key predators – sharks, big cats, polar bears and many more – are facing extinction across a wide range of habitats. The need to conserve these decisive and often charismatic animals has never felt stronger.

Reference: Croll, Maron, Estes, Danner & Byrd. 2006. Science 307: 1959-1961.

 

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Images: (Photos from Anthony DeGange and Donald Croll)