Logical Flaws in Studies of Pleistocene Carnivore Tooth Wear

Van Valkenburgh co-authored a study comparing rates of broken teeth suffered by large modern carnivores with those experienced by fossil carnivores excavated from the Rancho La Brean tar pits.

http://www.cof.orst.edu/leopold/class-reading/Van%20Valkenburgh%20and%20Hertel%201993.pdf

She found that large carnivores from the La Brean tar pits suffered 3 times as many broken teeth as modern day carnivores.  The fossils she examined from the pits included saber-tooth (Smilodon fatalis), American lion (Panthera atrox), cougars (Puma concolor), bobcats (Lynx rufus), dire wolves (Canis dirus), and coyotes (Canis latrans).  The modern species that she compared them to were lions, jaguars, leopards, cheetahs, bobcats, timber wolves, hyenas, and African hunting dogs.  The specimens from the tar pits dated to between ~36,000 BP-~12,000 BP.  She concluded that competition for prey was more fierce then than it is among modern large carnivores.  She believed increased competition for food forced large carnivores to utilize more of the carcass, and this led to more broken teeth from gnawing on bone.  She dismissed the alternate explanation that the higher rates of broken teeth among large Pleistocene carnivores were the result of attempting to take down larger prey than modern carnivores usually attack.  Dr. Valkenburgh suggested the larger size of the Pleistocene carnivores compensated for the larger size of the prey.

I never paid much attention to this study because using rates of durophagy as a proxy for carnivore competition seemed like a dubious assumption.  (Durophagy is just a fancy word for bone-eating.)  I think different species either eat more bone than other species because they have different nutritional needs, or they eat more for some other unknown reason that has nothing to do with competition.

Last year, a new study was published that completely contradicts Dr. Valkenburgh’s study.

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052453

Several scientists looked at teeth from Rancho La Brean predators and extant carnivores using Dental Microwear Textural Analysis (DMTA).  DMTA requires a lot of fancy equipment and procedures including a white light confocal profilometer, a scale sensitive fractal analysis, and a scanning electron microscope.  The scientists are able to see the teeth on a computer screen in 3D.  This eliminates observer measurement error.  Carnivores that avoid bone, such as cheetahs and lions, have different types of microwear on their teeth than those that eat a lot of bone, such as African hunting dogs and hyenas.

This is the equipment used to look at tooth wear in animals.

These scientists found little difference in rates of durophagy between the extinct and extant carnivores, meaning the competition between predators in the late Pleistocene in this region was no more or less fierce than that of today’s Africa.  Of course, I don’t buy the whole proxy assumption in the first place.  Most of the broken teeth were canines rather than molars.  Canines are more likely to break when taking down prey; molars are more likely to break when chewing bones, therefore they conclude the larger size of prey was the factor that explains the higher incidence of broken teeth among large Pleistocene carnivores.

One of the conclusions of this more recent study has a logical flaw.  The scientists authoring this 2nd study used DMTA to look at the teeth of Smilodon and Panthera atrox over time.  Different tar pits hold fossils of different spans of time ranging from ~36,000 BP-~12,000 BP, so they looked at 5 specimens of each species from pits of chronologically different ages.  They found no difference in tooth wear between the older specimens and the young specimens that dated closer to the time of extinction.  Many scientists think large Pleistocene carnivores became extinct because the animals they preyed upon became extinct.   They expected evidence of increased durophagy among the most recent saber-tooths and lions as they were forced to utilize more of the carcass.  But they found no evidence of this.  I think this doesn’t disprove the likelihood that carnivores did die out because their prey disappeared.  The final sentence in the abstract seems to imply that it does though.  It states : “The difference in DMTA attributes from older to younger deposits offers little evidence that declining prey resources were a primary cause of extinction for these large cats.”  Brian Switek, who writes an online blog for National Geographic, discussed this study and even goes so far as to ask what caused the extinctions of large Pleistocene carnivores, as if this study somehow disproves declining populations of suitable prey was the cause.  In my opinion this is flawed thinking.  Besides the dubious assumption that certain species of carnivores utilize more bone if prey is scarce, it seems unreasonable to expect to find the last nutritionally-stressed members of a population to be represented in the fossil record.  The odds of an animal becoming fossilized is so rare that it’s extremely unlikely that a member of the last remnant of a species headed for extinction would become fossilized.  The sample size–just 5–is also way too insignificant to detect whether or not this was occurring.  Moreover, big cats that eat mostly meat and organs and avoid bone are more likely to die of starvation before they chew on many bones.

I contacted the main author of this study, Dr. Larisa Desantis, and pointed out the logical flaws of this conclusion but she never responded.  I tried to comment on Brian Switek’s blog but he wouldn’t even allow my comment to be published.

I believe competition with humans drove saber-tooths and American lions to extinction.  Humans directly hunted them and overhunted their prey, and it was this combination that made it impossible for these species to survive.  To expect to find evidence of this by looking at a handful of fossil teeth under a microscope is ridiculous.

Advertisements

Tags: , , , , , , ,

6 Responses to “Logical Flaws in Studies of Pleistocene Carnivore Tooth Wear”

  1. Mike Says:

    Mark, I posted link to your post at Google+ community ‘Life in the Cenozoic Era’.

  2. Mark L Says:

    Mark, do you know how much of a generalist the American lion and the smilodon were? I can’t see a pre-Columbian group eliminate all the different prey items of a generalist carnivore, or the carnivore itself…see coyote and FL pythons as examples, even with all our tech and weapons. On the counter, see how easy it was to starve specialists, even human ones like plains indians with buffalo removal. I guess jag would also be a good generalist also (till guns took them out in NA) .

  3. markgelbart Says:

    Lions and saber-tooths were not generalist carnivores…they required large species of megafauna for sustenance. Native-Americans did completely wipe out mammoths, mastodons, ground sloths, horses, Pleistocene species of bison, camels, llamas, and peccaries. (This is the only logical cause of Pleistocene extinction in my opinion.) Isotopic studies suggest saber-tooths and lions excavated from Rancho La Brea mostly relied on horses, bison, and camel–all 3 were wiped out in this region. Later, bison recolonized much of North America.

    Rabbits and a few deer don’t provide enough meat to sustain a viable population of 500 pound cats.

    Coyotes and Burmese pythons are not good analogies for large Pleistocene carnivores. Coyotes under pressure from hunting produce larger litters. That is how they’ve survived the presence of humans. Humans have only been battling pythons in south Florida for a decade. Pythons live in remote inaccessible swamps where humans have difficulty penetrating.

    Plains Indians were not specialists. Homo sapiens are an omnivorous species.

  4. Mark L Says:

    Your comment on the remoteness of terrain for Burmese pythons kind of makes my point that pre-Columbian humans were not able to kill off a widely distributed species (even a large cat) but were only able to eliminate some of it’s prey. Llamas and camels are another example as they ranged in places humans did not easily reach in decent numbers until recently. There is no way for the limited number of humans to eliminate that many ‘hard to reach’ species in that many locations over that time scale unless they were incredibly effecient hunters (and they weren’t). In between the hunts the animals reproduce as usual. Or, there were more people hunting than we thought. Or…something else got them all at once (disease?) and severly took down their numbers (like bats with WNS).
    Admittedly it would take time for a large carnivore (or onmivore) to acclimate to human hunters, but obviously some did (jags, panthers, red wolves, black bears). Whether the others did or didn’t would be a good question to ask. Like we discussed before, hunting a mastodon is not as easy as just jamming a stick in it’s butt and running. If the mastadon knows you are coming, it has some options. That takes a lot of planning and people to address.

  5. markgelbart Says:

    There’s no evidence that llamas or camels ranged into remote places where humans couldn’t reach them until recently. Llamas did survive in the Andes Mountains, but Indians there had a taboo against overhunting them, possibly explaining why they survived there.

    Jaguars and tapirs survived the end Pleistocene extinctions, but some of them lived in remote areas such as jungles and deserts where the population of humans wasn’t very high.

    I don’t recall discussing hunting mastodons with you, but I believe hunting a mastodon was as easy as sticking a spear up its butt and running. There is evidence for exactly that from a fossil site in Venezuela.

    I think the evidence is overwhelming that humans overhunted the megafauna to extinction. Megafaunal populations collapsed exactly the same time that humans colonized North America. Scientists were able to determine this by analyzing the abundance of dung fungus spores in lakes located in New York and Indiana. This collapse does not correspond with any climate change or asteroid impact. Humans began overhunting these beasts about 15,000 years ago, causing an immediate collapse of the populations within 1500 years. Most of the megafauna didn’t become completely extinct until between 11,000 BP-7500 BP. It did take Native Americans about 7500 years to wipe these species out.

    Evidence from studies of dung fungus spore abundance supports a protracted overkill scenario rather than a blitzkrieg model of extinction.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


%d bloggers like this: