Once and future cats

Sabercats were magnificent, powerful predators – what does their extinction tell us about the future of life on earth?

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Illustration by Terryl Whitlatch

Illustration by Terryl Whitlatch

Brian Switek is a science writer whose work has been published in Slate, The Wall Street Journal and New Scientist, among others. His latest book is My Beloved Brontosaurus (2013).

There are mammoths along Wilshire Boulevard in Los Angeles. They stand out against the constant crush of urban traffic, a reminder of the city’s recent past. One hapless behemoth stretches her trunk toward the sky, her reflection shimmering on a gently bubbling asphalt pool. Her herd mate looks on impassively, while an infant reaches out with a trunk that’s too short to bridge the terrible distance. A mild reek of asphalt and decay drifts out from the black-tar pond: the scent would have been much riper when mammoths, dire wolves, giant ground sloths and other Pleistocene animals were trapped in this mire between 40,000 and 12,000 years ago.

From this menagerie of megamammals, the Page Museum — built in 1977 beside the La Brea tar pits on Wilshire Boulevard — has selected the singular Smilodon fatalis, the great sabercat, as its mascot.

Just to the right of the museum’s main entrance, two Smilodon grapple with each other on a pedestal. One of the muscular cats buckles under the piercing bite of the other, the loser’s fangs thrown back in a pained snarl. They don’t look terribly different from big cats living today, or even Bob, the museum’s tailless calico who prowls among the ongoing fossil excavations. In general terms, a cat is a cat is a cat. Yet there is something special about Smilodon. These were the last of the sabercats, powerful predators which disappeared frustratingly close to us in time. Why are they — and so many remarkable animals that lived alongside them — gone? That deceptively simple question is what led me to drive 12 hours from my home in Utah to see the place that is not only their grave, but the site that’s bringing them back to life. For Smilodon is both an emblem of the past and a clue to the future extinction or survival of life as we know it today.

Smilodon ranged over much of North America, and one species — the bulkier Smilodon populator — actually crossed the Panama land bridge to terrorise the herbivores of South America until about 10,000 years ago. But the tar pits of La Brea contain the richest accumulation of these cats anywhere. More than 2,100 individuals represented by more than 166,000 bones have been dredged from this place, making the Smilodon second only to Canis dirus, or the dire wolf, in terms of its abundance in the sticky deposits. And that’s without considering all the sabercat remains still sitting in the glop beneath the city’s hard surface.

To understand what happened to these charismatic predators, I scheduled an appointment with John Harris, chief curator at the Page Museum. When I told him I wanted to find out why there are no more sabercats, Harris chuckled. ‘The answer seems to be simple,’ he told me. ‘They ran out of food.’ The bigger mystery, and perhaps a key to why the sabercat disappeared, is why we lost the horses, camels, mastodon and ground sloths who shared their world.

Smilodon is just one of many long, storied lineages that were snuffed out at the fuzzy boundary between the prehistoric and the modern, leaving us with an impoverished world. In the dimmed fossil halls of the Page Museum, the bones of the sabercat’s neighbours glint beneath the exhibit lights, their skeletons reconstructed in the chocolate ‘La Brea brown’ colour. None of the skeletons were found complete. Each one is made up of isolated bones pulled from the various pits the museum crews have worked in since the early 20th century. The fact that there are enough bones of each species to reconstruct everything from adult dire wolves to a baby mastodon and prehistoric birds testifies to the riches of this Pleistocene boneyard.

Some species are familiar. The coyotes that still haunt Los Angeles had Pleistocene predecessors that loped over a wilder landscape, and the huge Ice Age bison was not so different from the one that grazes over western grasslands today. Other species simply seem out of place. Fossil horses and camels would appear to represent an Asian influence in the deposit, but in fact both the equid and camelid lineages originated in North America — these herbivores were among the last of their kind on the continent of their birth, until horses were brought back by Spanish explorers in the 16th century. But the real stars of La Brea are the bizarre, unfamiliar creatures that hark back to a wild world that slipped away not so long ago. While superficially similar to the huge Columbian mammoths found in the same pits, the American mastodon was actually a more distantly related form of shaggy elephant that mashed up plant food with characteristic ‘bubby-toothed’ molars. The giant ground sloths were even stranger, a lineage that emigrated from South America and proliferated in Central and North America.

Familiar or alien, all these creatures — and many others — disappeared from North America between 50,000 and 8,000 years ago. On a human scale, this seems a long time, but in the fossil record it marks a calamitous, rapid decline. Paleontologists and archaeologists have invoked many possible causes for the catastrophe, from disease and a wayward comet to climate change and hungry, hungry humans. Today, only the latter two mechanisms are taken seriously, but exactly how the dispersal of humans conspired with the onset of a warmer, wetter global climate to drive so many species to extinction is still fiercely debated. Figuring out how the disaster played out is critical to understanding the future of life on Earth. This was not a just a last stand of Ice Age animals against the beginning of the Anthropocene, or age of humans. The extinction of the megamammals is part of a drawn-out process that continues to tatter and imperil what’s left of the planet’s wilderness.

Most scientists have believed that, as the mammoths, horses and bison of the Ice Age disappeared — whether killed by humans in need of meat, by habitat loss, or by a combination of factors — the abundant populations of Smilodon simply ran out of food on the hoof. Jaguars, grey wolves and other carnivores survived, but North America’s last sabercat apparently could not cope with the changes that arrived at the close of the Pleistocene.

So their extinction has been understood in terms of top-down ecological stress, a victim of ‘trophic cascade’, just as the top predators of the ocean today are dying off because populations of prey fishes are collapsing beneath them. The plight of today’s big cats also seems to echo the downfall of Smilodon: we know that leopards, tigers, jaguars and other big cats require large swathes of habitat that are connected through ecological corridors, providing them with plenty of ground to stalk, and enough prey to survive. Decrease the habitat and food supply, and the cats suffer.

But what if we could trace the clues back the other way? What if the extinction of Smilodon could help us understand what wiped out so many of the species it relied upon for food? New research on this question could help us untangle the frighteningly mysterious nature of extinction — in the past and future — itself.

My fascination with Smilodon and other megafauna springs from a slightly selfish lament. As wonderful as today’s striped and spotted great cats are, I can’t help but feel cheated that none of the powerful saber-toothed cats are alive today. The fact that they disappeared a scant 10,000 years ago is all the more frustrating. Humans undoubtedly saw them in their dying days — and possibly pulled the ecological rug out from under their mighty paws. Not only that but the long-fanged hypercarnivores were already prowling the landscape when the earliest humans took their first tentative steps away from the safety of the trees. The sabercats watched our evolution from the shadows and tall grass, only to disappear just short of the modern age.

Ever since the early days of palaeontology, we’ve seen sabercats as terrible icons of the prehistoric wild. In A History of British Fossil Mammals and Birds (1846), the British anatomist Richard Owen cited the sabercat Machairodus and other prehistoric carnivores as evidence that humans could not have lived while such fierce animals prowled. Prehistoric people, he wrote, would have been ripped to shreds and deposited as dung in dank prehistoric caves:
When we are informed that, in some districts of India, entire villages have been depopulated by the destructive incursions of a single species of large Feline animal, the Tiger, it is hardly conceivable that Man, in an early and rude condition of society, could have resisted the attacks of the more formidable Tiger, Bear, and Machairodus of the cave epoch. And this consideration may lead us… to conclude that Man did not exist in the land which was ravaged simultaneously by three such formidable Carnivora.

Owen was wrong, of course. By 1859, the same year that Charles Darwin’s On the Origin of Species was published, archaeologists and paleontologists confirmed that humans really did live at such an age. Decades of additional finds pinned the emergence of the first humans to around 6 million years ago in Africa, at which point sabercats had already been honing their skills on other prey for millions of years.

In general form, Smilodon had standard cat traits of shearing teeth, retractable claws, a short muzzle, and arms better suited to catching than running. These features had become established by about 20 million years ago in Pseudaelurus — a fossil carnivore thought to represent the beginning of true cats, and the last common ancestor of today’s living cat lineages and the extinct sabercats. However, cats were not the only creatures to evolve saber fangs. Another group of superficially catlike predators, the nimravids, evolved earlier, and overlapped with cats. There were even marsupial predators — such as the ‘pouch saber’ Thylacosmilus in South America — that independently evolved elongated canines. Yet the fangs that have most forcefully sunk themselves into my imagination belong to the true sabercats.

The first of the long-lived and varied sabercat branch of the felid family tree evolved about 13 million years ago. But not all sabercats were alike. Paleontologists have identified three general forms — the scimitar-toothed cats, the dirk-toothed cats, and the cookie-cutter cat. The differences between the three go beyond tooth shape. While scimitar-toothed cats had relatively short, serrated fangs and long, slender legs suggestive of hunters that chased down prey, the dirk-toothed cats — such as Smilodon — typically had longer canines and heavily built forelimbs that aided the carnivores in bringing down prey by force. The cookie-cutter cat, represented by the appropriately named ‘alien knife’ Xenosmilus, combined the short canines of one group with the burly proportions of the other in serrated incisor teeth that could have ripped huge chunks of flesh from prey.

Our environmental dominance today is very recent: our ancestors were prey for millions of years, and in parts of the world humans still are

Of all these cats, however, no group has been given as much attention as the dirk-toothed cat, and Smilodon in particular. And even though there were three species of Smilodon that together spanned a period from 2.5 millions years ago to 10,000 years ago, it’s the latest North American species — Smilodon fatalis — that is found at La Brea and has come to symbolise sabercat biology. This cat was about the size of a Siberian tiger, albeit with a few tweaks to its anatomy. Smilodon might have lacked the tiger’s long, balancing tail, but it had longer canines and even more heavily muscled forelimbs.

How all those traits evolved has been fuel for rampant paleontological speculation, particularly since this particular sabercat could have stalked our prehistoric predecessors. Our fascination today is a continuation of the awe and terror our ancestors must have felt when these cats still lived. The sharp chill I get when I’m camping in the dark forest and hear a twig snap in the woods must be much the same as my hominin forebears felt as they tried to sleep in trees or caves, aware that they were not well-suited to the night.

There’s actually a technical term for such unease. John Laundré, ecologist at the State University of New York at Oswego, sees these pangs of intense, vigilant awareness as a response to the ‘landscape of fear’ created by predators. From grasshoppers to elk, prey species change their behaviour when dangerous carnivores are present. Prey are wary of ambush, avoiding foods that grow in habitats that leave them vulnerable: instead, they shift their diet to safer fare. The mere presence of a predator can change the landscape, and humans are not exempt. Our environmental dominance today is very recent: our ancestors were prey for millions of years, and in parts of the world humans still are. We learnt to mitigate our fear only when we became predators too, and we learnt by living in close quarters with some of the most formidable carnivores of all time. To chart how fear turned to instruction and even admiration, we must go back to the origin of the human family.

Smilidon-Fatalis-Body

At the outset, our ancestors were small apes that split their time between the trees and the ground. The ability to walk upright was a critical, early facet of human evolution, later refined as more time was spent striding over the terrestrial realm. While walking and foraging on the ground, the early hominins were in the haunts of large carnivores. In a 2007 survey of large African carnivores who lived between 6 million years ago and the present, the paleontologists Adrian Treves of the University of Wisconsin at Madison and Paul Palmqvist of the University of Málaga in Spain counted at least four genera in the 6-million-year range — including long-legged hyenas, a large bear, the ‘false sabertooth’ Dinofelis, and the sabercat Machairodus. By 3.5 million years ago, Africa’s carnivore guild had expanded to include the sabercats Homotherium and Megantereon, the giant hyena Pachycrocuta, and the forerunners of wild dogs, lions, leopards, cheetahs, and spotted hyenas. The large carnivores of Africa today are just a remnant of the great profusion of carnivores the continent hosted right up until about a half a million years ago.

Of these ancient predators, sabercats stand out as especially frightening. Did they actually kill hominins? Direct evidence has been difficult to find. Anna Behrensmeyer, a paleontologist at the Smithsonian National Museum of Natural History in Washington DC, has speculated that an entire group of Australopithecus afarensis — the hominin species to which ‘Lucy’, the half-complete skeleton of a female found in 1974, belonged — was slaughtered by a sabercat at a spot in prehistoric Ethiopia around 3.2 million years ago. But such cold cases are hard to crack, and the lack of direct evidence could mean that sabercats didn’t pounce on our ancestors very often. Or maybe the cats were just especially fastidious about finishing all of their meals.

Homo erectus could have waited for the cat to leave a dripping, meaty carcass behind, and quickly swept in to scavenge whatever they could carry away before other carnivores showed up

Of course, food provision might have run the other way. For our entire history, we have been omnivores. Our primate lineage is characterised by a dental toolkit capable of chewing flesh as well as leaves, and a pivot from a more plant-based diet to one that included greater amounts of meat has often been considered as an essential moment in human evolution. Big brains are energetically expensive to grow and maintain, and so protein in the form of flesh would have provided us with the dietary resources to underpin the evolution of a larger brain. The question is how humans acquired meaty meals, and when they started doing so. The very earliest humans might have caught small prey, as modern chimpanzees do, and good evidence for butchery of large carcasses goes back to about 2.5 million years ago. But in between those milestones some palaeoanthropologists have suggested that the sloppy eating habits of sabercats eased our entrance into the predatory guild.

For a long time, vertebrate paleontologists thought that the elongated fangs of sabercats hindered their ability to feed. Thus sabercats were thought to be soft-food specialists only — avoiding the hard bones that could damage their fragile teeth. In the arithmetic of predation, picking off only the accessible soft tissues would have left plenty of food behind for hominins on the lookout for protein. Prehistoric humans such as Homo erectus could have waited for the cat to leave a dripping, meaty carcass behind, and quickly swept in to scavenge whatever they could carry away before other carnivores showed up.

Alfonso Arribas, an archeologist at the Mining Museum in Madrid, and Paul Palmqvist argued in 1999 that the relatively common, wide-ranging sabercats might have deposited a trail of carcasses that prehistoric humans followed. Assuming that sabercats such as Homotherium and Megantereon left a great deal of flesh on their kills, and that the cooler, drier conditions of ancient Eurasia allowed carcasses to last longer on the landscape, Arribas and Palmqvist argued that there could have been a great deal of relatively fresh meat for hungry humans to exploit. Using a toolkit of worked stones, these humans could have cut flesh from carcasses and broken bones to get at marrow. Rather than striking out on their own out of wanderlust or some other internal motivation, they might have simply walked out of Africa in the footsteps of messy carnivores.

However, with new archaeological finds and better dating of human-occupied sites, this particular out-of-Africa hypothesis has fallen out of synch. Cats did not leave a bloody trail of meat scraps for us to follow. What’s more, we have new evidence that sabercats were not the wasteful eaters anthropologists once thought.

About 20 miles north of San Antonio in Texas, Friesenhahn Cave is a 20,000-year-old Homotherium den. This cat is the most abundant carnivore found in the cave, represented by at least 33 individuals. The number of sabercat bones here is exceeded only by the number of damaged bones from young mammoths — herbivores that were roughly two years old and about the size of a bison, represented by parts of at least 34 individuals. Homotherium kitten bones found in the cave indicate that the cats were probably living there, and the adults probably dragged the mammoth parts back to their shelter. Curtis Marean, professor at Arizona State University, and his fellow palaeoanthropologist Celeste Ehrhardt argued in 1995 that the disarticulated mammoth bones which litter the cave, and the particular patterns of damage to them, show that sabercats were efficient eaters after all.

Many of the mammoth remains were limb bones that show pits, gouges and scrapes consistent with the damage that carnivores such as big cats and hyenas leave on herbivore bones today. Likewise, the upper incisor teeth of the Homotherium in the cave were often broken and worn. Homotherium, like Smilodon, had a battery of short incisors that jutted further forward of the canine teeth than in modern cats. Based on the bone damage and the wear on the teeth, it seemed that the sabercats used their incisors to scrape tissue right off the bone.

The fossils from Friesenhahn Cave showed that sabercats were adept at taking apart and processing carcasses. The same would have been true for Smilodon: in 2012, when Larisa DeSantis, a paleontologist at Vanderbilt University in Florida, and her colleagues looked at the pattern of microscopic damage on the teeth of both Smilodon and American lion specimens from La Brea, they discovered that the sabercat probably chewed bone more often than the lion did. Microscopic scratches and pits on the teeth of Smilodon indicate that the cat dispatched relatively small prey with its long fangs, but had to work around bones to get everything it could from the carcass.

Smilodon and kin have been variously portrayed as slashers, stabbers, slicers, ambush predators, and vampires

Tooth damage brings us a step closer to visualising how Smilodon fed. But how did it employ those characteristic canines in killing? Ever since paleontologists uncovered the first sabercat skulls in the 19th century, there has been ceaseless debate and speculation about sabercat killing techniques. Smilodon and kin have been variously portrayed as slashers, stabbers, slicers, ambush predators, and vampires. A consensus about how the cats actually hunted is only just now coming into focus, and makes the circumstances of their disappearance even stranger.

As I walked the La Brea grounds with Harris, I was reminded of my childhood encounters with Smilodon. The carnivore was brought to life in the fossil books I’d bring home from school and in the low-budget documentaries I watched again and again. One programme in particular stuck in my mind. The short show brought Ice Age animals back to life by way of stilted stop motion animation. Smilodon was one of the stars, shown terrorising an armoured glyptodont — effectively an enormous armadillo — and the giant ground sloth Megatherium. The cat’s fangs made it seem like it could take on anything, but, the narrator explained, its most prominent characteristic was also its undoing: Smilodon was a runaway evolutionary train, and was so successful at tearing into the hides of ground sloths and armoured glyptodonts that the cat’s canines kept growing until it could no longer close its jaws.

At the time, I had no idea that this was utter bunk and had been discarded by scientists decades earlier. The idea of ever-growing Smilodon teeth was an example of weird, non-Darwinian evolutionary mechanisms that many paleontologists favoured during the late 19th and early 20th centuries. In place of the undirected and unpredictable evolutionary pattern Darwin proposed and that we now accept, many researchers at the turn of the century preferred mysterious internal evolutionary drives that propelled species along predetermined anatomical routes. The tragic fate proposed for Smilodon fit into this general pattern, with long sabers acting as a sign of ‘senescence’ of a species near death — at least until paleontologists such as William Diller Matthew pointed out in the early 20th century that the scenario made no sense. Not only did sabercat species exist for millions of years with no sign of excess in their evolution, but natural selection would have prevented such a self-defeating trait from ever evolving. ‘[I]t is impossible to believe that any innate tendency to evolution — if such exists — could so far overcome the repressive influence of selection as to produce finally a race that would be self-extinguishing,’ Matthew wrote.

Even so, paleontologists continued to struggle to explain the striking curvature of Smilodon fangs. The notion that big teeth were required to dispatch large and heavily armoured prey was the most obvious answer, although some naturalists generated odd hypotheses. The paleontologist Jacob Wortman, for one, speculated at the turn of the 20th century that, in addition to killing prey, the teeth of the sabercat ‘may also have been used to assist the animal in climbing, and in this way attained their great size’. Other experts supposed that Smilodon speared fish, sucked blood from deep holes gouged in large prey, or was actually so inept at hunting that it solely scavenged on mammoth and sloth carcasses.

For those who argued that the cat was an active hunter, it was almost impossible to understand how they did it. Some experts supposed that Smilodon attacked closed-mouthed, slamming its chin against prey to repeatedly puncture the skin. Others presumed that Smilodon sunk its canines in open-mouthed, and then yanked its head backward to slice through flesh. Nobody knew the real answer, and the jerking animatronic diorama in the Page Museum exemplifies the dilemma. Clinging to the back of a ground sloth, a Smilodon tilts its head this way and that over the shaggy victim’s neck, as if trying to figure out what to do with its fangs. The two megamammals are locked in a perpetual state of confusion, waiting for paleontologists to choreograph what must have happened.

But what distinguishes science from belief or comfy ‘just so’ stories is the critical matter of testing. That is where the new palaeontology comes into play. To understand how sabercats fed, what they fed on, how our arrival on the evolutionary scene affected their food supply, and what led to their demise, we have to understand them, and the world they lived in, at a much deeper level. There is no better place to do so than at La Brea.

In his office, surrounded by books and stacks of paleontological papers, Harris explained that the richness of La Brea’s bones was apparent in the 19th century when oil exploration had already started turning up many curious bones. Even though naturalists and interested passersby had poked around the tar pits around the turn of the 20th century, systematic excavations didn’t start until 1913. Many of the bones in the museum’s collection were extracted from about 100 different pits over the course of two years. The paleontologists and their assistants collected the better part of a million bones, even though they mostly had their eye on ‘more impressive things’ such as skulls and large mammal bones.

A century after those early excavations, the scientific philosophy at La Brea has changed. Smilodon and the ground sloths might be the stars of the site, but a great deal of the work that goes on at La Brea now involves tiny creatures — insects, invertebrates, small reptiles and amphibians, and other creatures whose remains are preserved in and around the megafauna bones. These less charismatic fossils form the setting for understanding how Smilodon lived and, perhaps, how it died off.

By discriminating what lies in the asphalt at a much finer level, Harris estimates that museum researchers — including scads of volunteers — have doubled the list of species found at the site. ‘La Brea really does collect a huge diversity of the biota from the Late Pleistocene,’ Harris told me. ‘We have well over 600 species of animals and plants from the last phase of the last Ice Age, and there’s greater diversity preserved here than any other deposit of similar age.’ And the sample size of many of those species is staggering. ‘If people want to study living wolves,’ Harris said, ‘they’re lucky if a museum has 20. Here, we have more than 4,000 dire wolves.’

As the camel picks, chews, swallows and starts to digest the shrubby plants, isotopes of carbon and nitrogen from those plants become assimilated into its tissues and bones

Unlike the charismatic megamammals, many of the 600 species pulled from La Brea are still around. ‘Most animals smaller than a coyote are still living today,’ Harris explained, ‘but they’re not in this neck of the woods.’ For example, a particular species of snail found in the asphalt is today only found in cooler, wetter habitats at elevations about 5,000ft in California and Arizona. This incredibly detailed understanding of the plants and animals of the time is made possible by La Brea’s natural asphalt. ‘Asphalt acts as a wonderful preservative,’ Harris says. In most cases, bones trapped in asphalt are not even mineralised, and are in fact the original bone of the actual animals. They still count as fossils, but they are not like stone dinosaur bones. When a museum volunteer uncovers a Smilodon skull, they are touching the actual, stained bone of the big cat.

Early 20th century paleontologists boiled the bones in kerosene to clean off the tar and generally paid little attention to the microfossils that littered the sludge that came off the skeletal parts. Today, researchers at La Brea take great care to get every fossil they can. Not only are the bones so carefully collected and documented that the entire bonebed can be reconstruct from notes, but the matrix surrounding those bones is also labelled so that its association with a particular grid square is recorded. That matrix is then treated with biodiesel to reduce many pounds of sediment and asphalt into siftable slurry that yields small fossils. These are some of the most important clues in the collection.

Sabercats and mammoths attract the most visitor attention but the small fossils fill out the ancient environment with a much greater degree of resolution. And even when bones go into the lab, volunteers spend a considerable amount of time picking out those little fossils that have been preserved inside skull cavities or in the matrix clinging to bones. As my friend and Page Museum volunteer Herb Schiff told me while showing off a dire wolf skull: ‘They chain you to your chair until you identify all the little bones.’ A pain in the ass for volunteers, perhaps, but a bonanza for researchers.

Despite the wonderful preservation of these bones, it is incredibly rare to discover even a partial skeleton of one individual animal at La Brea. As animals became trapped in the asphalt, their bodies fell apart and settled as isolated bones in the black slurry. ‘We don’t find skeletons,’ Harris says. ‘Out of one million bones, you can count the number of associated skeletons on two hands.’ Rare exceptions include ‘Zed’, an enormous Columbian mammoth found during excavations of the neighbouring art museum, and ‘Fluffy’, an American Lion found nearby.

Harris suggests we take a walk outside the museum to see some of the pits that are still aggregating bits of plants, as well as insects and small animals that happen to blunder into them. Along the path, dead leaves blanket the edge of a shallow tar seep. ‘On days like this, when it gets hot, the asphalt gets very sticky,’ Harris told me. That’s when seemingly harmless puddles become deadly. Despite the dramatic mammoth vignette in front of the museum, Harris explained that the actual prehistoric asphalt seeps were not giant pools. The amount of asphalt needed to trap an animal was surprisingly shallow — only an inch or two. ‘Animals got stuck like flies on flypaper,’ Harris said. Much like the small puddle we passed by, the seep would be mostly covered by leaves and dirt. Then a horse or camel would blunder into it. ‘That animal is stuck at the top, and dies in a week or 10 days from hunger. Unless it’s unlucky and a sabercat or wolves come along.’ Those predators must have come along quite often — the reason why their remains are so abundant here.

These bones, and new techniques used to study them, have allowed researchers to start bringing Smilodon back to life in a slow-motion manner. One of the first steps was figuring out what the cat actually ate. As a child, I always imagined the sabercat stalking giant ground sloths: documentary narrators often pointed out that those vexing fangs had evolved to take on the biggest of the big Pleistocene mammals. But, as Harris explained to me, new research tells us otherwise.

Picture a Pleistocene camel Camelops, browsing among the chaparral of Los Angeles around 12,000 years ago. As the camel picks, chews, swallows and starts to digest the shrubby plants, isotopes of carbon and nitrogen from those plants become assimilated into its tissues and bones. The plants leave an elemental signature inside the animal tied to particular plants. But the transmission doesn’t end there. Hiding in the tall grass is a Smilodon, which kills the camel. As the cat feeds, the chemical traces get into its own tissues, adding to the predator’s own internal record. Since different herbivores preferred different plants and therefore contained different chemical signatures, the same isotopes in carnivores can act as a proxy of the animals they ate.

In 2004, Harris and Joan Brenner-Coltrain, an anthropologist at the University of Utah, studied the isotopes extracted from the bones of Smilodon, Camelops, and other celebrities of La Brea, encompassing the range of big predators as well as the various types of small to enormous herbivores. Rather than matching the lumbering giants of the Pleistocene, the geochemical signature in the Smilodon bones most closely matched the ruminant herbivores — the camels, bison, and small pronghorn. This isn’t to say that Smilodon didn’t occasionally leap onto the backs of giant sloths or make an attempt at young mammoths but, in general, the cats targeted ruminant herbivores, two of which have living relatives in North America today.

While the future course of evolution is unknowable, there is a possibility that we are only in a short lull between sabercats

Whereas Smilodon was once seen as a rapacious nightmare capable of bisecting sloths and disembowelling mastodons, paleontologists now consider it to have been a careful, strategic hunter that not only discriminated among possible prey, but specifically targeted the most vulnerable parts of potential victims. Biomechanical estimates have found that Smilodon had a relatively weak bite, so probably didn’t use the single-bite killing style of modern cats. Instead, the sabercat relied on powerful neck musculature to help drive its teeth into a victim and then pull to shear away flesh from the neck or belly, severing blood vessels and causing dramatic trauma. The goal was to cut critical lifelines and slice through muscle so that the shocked prey could be quickly brought down. Natural selection had honed the cat’s anatomy to be a specialised killer. Perhaps evolution will do so again.

While the future course of evolution is unknowable, there is a possibility that we are only in a short lull between sabercats. Long killing fangs have evolved so many times in the past 20 million years that there’s every reason to believe that a newly derived sabercat might evolve again. In fact, Per Christiansen, a zoologist at the University of Aalborg in Denmark, argued in 2012 that the clouded leopard — a mid-sized cat that prowls the tropical forests of Indonesia — has relatively elongated teeth and shows a great deal of similarity to true sabercats. Given a few million years, might the saber-toothed descendants of today’s clouded leopards slash at the throats of mid-sized herbivores of the future? Such a scenario isn’t impossible, although it’s contingent upon which animals survive into a human-dominated future that will undoubtedly be warmer than today.

For now, the exact reason why Smilodon disappeared remains unknown. Loss of food is a likely cause, but that answer only moves the question a step back to why Smilodon’s prey died out. The sabercat was a casualty in a wider extinction at the end of the Pleistocene that marked the end of the Ice Age and the beginning of a world over which our species has disproportionate influence. Some researchers like to call this the Anthropocene, but whether or not such a designation truly fits depends on how long our species lasts. What might the fossil record look like 100 million years from now? The Pleistocene extinction could come to shade into the modern biodiversity crisis with little or no break in between. The close of the Ice Age might have been the beginning of a new age, or it could have been one dramatic blip in an ongoing mass extinction, tracking the rise of human dominance. Some of the garbage that ends up preserved in La Brea’s asphalt might help future archaeologists untangle this mystery.

Yet we can do more than wait for an indifferent universe to place our existence — however long or short — in context. By looking to the past, we can get some sense of the future. Figuring out why Smilodon became extinct is not a frivolous endeavour with implications only for the fossil-obsessed. The sabercat’s story is part of a grander tale of evolution and extinction that is unnervingly close to our own time. It took place in an era when humans were spreading across the planet and rapid climate change was quickly altering habitats that had been in place for tens of thousands of years. Smilodon and other charismatic Pleistocene species contain records of change under pressures that have only intensified. Of course, there are some differences: our prehistoric relatives killed mammoths for food but elephants today have more to fear from the ivory trade and black-market dealers. And while climate change at the end of the Pleistocene was regulated by the ebb and flow of Ice Ages, now it is altered by us in ways we are only just beginning to comprehend. Nevertheless, these dual extinction drivers reach back into the prehistoric past, a reminder of the dynamic nature of life on Earth and how a single species can alter the fate of so many others.

At the end of our tour through the site, Harris and I lingered at the back of the Fishbowl Lab — a glass-lined semicircle where visitors can see volunteers working on the La Brea bones. Surrounded by casts, sculptures and reference bones, Harris recalled how he started working here. ‘Other paleontologists told me not to bother with it, because it’s all been done.’ But Harris and the many volunteers and researchers who have converged on the bonebed have proved those critics wrong. ‘We’re still finding stuff for the first time,’ Harris told me with enthusiasm. Those discoveries are creating an ever more detailed picture of evolution and ecological change from a time just the other side of the divide between prehistory and history.

After stepping out of the lab, I went out to the main museum floor to wander among the skeletons. They’re from a time thousands of years before I was born, but I feel at home among them. More so than among the smoggy crush of Los Angeles streets. They remind me of when my imagination was sharper and I spent long childhood afternoons trying to will the creatures back into existence with my unfettered private speculations. The megamammals are so close to us in time, and so strangely familiar, but I can only get to them in the caricatured outlines that my mind wraps around their bones. The creatures are vestiges of a past I’ll never see, but they are not just that. They and their Ice Age world might be left to us only as bare bones, yet I can’t help but wonder what they will tell us about what’s to come.

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Comments

  • Erin Weeks

    Don't know if the author or staff wrote hed and dek, but sounds like a rip-off of Sharon Levy's 2011 book about Pleistocene megafauna (including sabercats), "Once and Future Giants."

  • Kate

    That was hugely enjoyable, thanks very much to the person who wrote this. One of my earliest memories is a picture of a sabre toothed tiger in an embossed book about prehistory. I couldn't read the words, but I was enthralled.

  • Walter

    Excellent article Brian, but I think you got the top-down & trophic cascade thing upside-down. In food-web terminology, extinction of the apex predators such as Smilodon because of collapse of their prey populations is not a top-down process, is a bottom-up process. Trophic cascades are top-down, so an example would be when producer / basal consumer populations get into trouble because of decline of top predators, the contrary of what you suggested. You can find a few examples here in the paper with DOI 10.1126/science.1205106
    -wsa

  • Cahokia

    For all the analogies that can be made between the Pleistocene mass extinction and the putative die-off today, there's a glaring difference.

    Today most extinctions are of small and obscure species. Then the mass extinction overwhelmingly impacted very large species. While our contemporary megafauna may have very limited distributions, very few have become extinct in historical times.

    Give credit where it's due - modern man is able and willing to stop at the brink, and pull charismatic species away from extinction. Are elephants or tigers going to become extinct in our lifetimes? Not likely - because we care enough to keep them around.

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