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Why are there so few ice age megafaunal kill sites?

As a general rule, I care much more about the consequences of extinction than the causes. Even though I work on past landscapes, my mind is firmly rooted in the present, and I’d my work to have conservation relevance. We know what’s causing extinctions today, for the most part. What comes next — the longterm implications of extinction — are an unknown, and something I think the paleorecord can really speak to.

Despite that, my work does touch on the causes of ice age extinctions, and I often find myself drawn into the long-standing debates on whether climate change or humans* killed off the mammoths, giant ground sloths, and other mega-beasts. It’s a topic of great interest to the general public, and one that has generated staunch advocates on either side (there are relatively few people arguing for some synergistic middle ground).

Me? I tend to fall in the pro-human category, for several reasons. The megafaunal extinctions are characterized by a few global patterns: 1) they were time-transgressive, happening first in Eurasia and Australia, then North America, South America, and finally on islands. 2) They were taxonomically restricted, only hitting land mammals for the most part (few birds, reptiles, etc.). 3) They were size selective, weeding out about half the animals larger than an adult German Shepherd here in North America, but leaving the smaller animals relatively untouched. Plus, we’ve had a couple dozen or more ice ages in the last 2.5 million years. What’s different about this one? People.

Clovis points from the Rummells-Maske Site, 13CD15, Cedar County, Iowa, from the Iowa Office of the State Archaeologist collection. CC-BY-SA-3.0; Released under the GNU Free Documentation License.

Clovis points from the Rummells-Maske Site, Cedar County, Iowa. The large grooves in the bottom were once thought to be blood channels but are now thought to be places where the point can be attached to a wooden shaft. CC-BY-SA-3.0; Released under the GNU Free Documentation License.

Lots of folk disagree, and staunchly. For a long time, it was thought that humans weren’t in North America until about 12,000 years ago, and thus they didn’t overlap long enough with megafauna to have made an impact — especially with small populations (we’ve since pushed the arrival of humans to the Americas to at least 15,000 years ago). It’s understandably difficult to imagine small bands of humans wiping out large populations of megaherbivores and megacarnivores in a relatively short time (though see this classic Alroy paper).

Perhaps the biggest challenge, however, is the lack of unambiguous kill sites, which is a line of evidence that featured prominently in some of the more strident debates. It is, in my opinion, the single best argument against a human cause in the climate-only argument. It’s something of a puzzle, because we know that the first humans in the Americas made specialized stone tools to hunt large beasts, and that they utilized megafaunal resources. So why don’t we have more than a handful of mammoth, mastodon, horse, and camel remains with distinct signs of butchering?

"Hebior Mammoth" found in Wisconsin bearing tool butcher marks. CC BY-SA 3.0 Triebold Paleontology Incorporated.

“Hebior Mammoth” found in Wisconsin bearing tool butcher marks. CC BY-SA 3.0 Triebold Paleontology Incorporated.

Some of this could be an artifact of resistance in the archaeological community to data, similar to the long-standing resistance to accepting that humans were here before 12,900 years ago (a period know as the Clovis culture for the distinctive stone tools found from that time). I haven’t really widespread evidence for this, though. And, if anything, I’m glad that fossil remains are held to really high standards– I wouldn’t want to start calling any bone with scratches or grooves a “kill site” any more than I’d want robust evidence tossed out because of overly restrictive definitions. Folks who are critical of the overkill model often cite the lack of stone tools associated with fossil bones. In other words, it’s not just that we find few bones with cut marks, it’s that we also find few stone tools with them.

Because these arguments come up frequently, I’ve spent a lot of time pondering, and I have some ideas. This is where I love the power of blogs — I get to speculate without having to worry about the fact that I’m not an archaeologist (though I do read the literature). I also have zero data, partly because I’m not an archaeologist and partly because I’m trying to explain the absence of evidence, which is tough. With all that in mind — that I’m a paleoecologist with no data– here’s what I think:

After a lot of thought, it no longer surprises me that there are few kill sites, in terms of bones or bones and tools together, for two reasons: taphonomy** and efficiency:

Let’s talk about taphonomy first, because it’s huge: In order for an animal to become part of the fossil record, it’s got to be buried fairly quickly after death, and ideally in a place without a lot of oxygen (to inhibit decomposition) or acidic conditions (which will dissolve the bones). If you’re hunting an animal for food (criterion one for a kill site: kill animal) and then butchering it (criterion two: leave marks on bones), you’re likely in the open. The bones you don’t take with you and use as tools, or process for marrow, are going to be scavenged by coyotes and other wild animals. Those first humans in the Americas may or may not have had domesticated wolves (there’s evidence of dogs in Europe going back to 30,000 years ago, and east Asia going back 16,000 years ago), which would have had an impact on carcasses, too.

Hadza men return from the hunt in-- they're are one of the few contemporary African societies that live primarily by foraging. These guys are professionals. Photo by Andreas Leaderer, CC BY 2.0.

Hadza men return from the hunt in– they’re are one of the few contemporary African societies that live primarily by foraging. These guys are professionals. Photo by Andreas Leaderer, CC BY 2.0.

Plus, bones on the surface don’t last very long. They get eaten, dragged off, or eventually decompose or dissolve if they’re not buried. It’s remarkable that things end up as fossils at all, when you think about it. Here in Maine, the soils are way too acidic, so the only fossil bones we get are in shell middens on the coast, in offshore areas, or in lakes or sandy riverbanks. If a hunter shot a moose and wasn’t able to track it down, it probably wouldn’t become a fossil. Our acidic, peaty soils make quick work of the remains. That’s not true everywhere, obviously, but there’s a reason that a large number of ice age fossils are associated with streambeds, caves, lake sediment, marshes, or other environments where they’d be buried quickly or preserved well.

As for human behavior, this includes both the efficiency of using as much of an animal as possible (which we see in many hunter-gatherer cultures) as well as the fact that tools are valuable items. When I build a shed in my backyard, I don’t leave my hammer behind when the shed is finished. Similarly, I find it hard to imagine an ice age hunter-gatherer spending a lot of time making a stone tool only to leave it behind, intact, with the butchered carcass. Stone tools make up a huge amount of the archaeological record from this time, because stone preserves well. We don’t have clothing from animal skins, or baskets, or other cultural artifacts made from soft tissues– but that doesn’t mean we don’t think humans made those items and used them. We may have limited evidence of direct interactions with megafauna here in North America, but that could just as easily be because people were resourceful, intelligent, capable hunter-gatherers — smart, efficient people who knew what they were doing when they went on a hunt. The absence of unambiguous kill sites may not be that surprising, when you think about it that way.

*We’re not going to talk about comets or hyper-diseases or other fringe theories.
**The study of the process of fossilization

Categories: Commentary

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Jacquelyn Gill

14 replies

    1. Thanks!

      I’ve heard two theories: First, that since humans evolved in Africa with megafauna, African megafauna was not “naive” to the impacts of these smart, capable hunters, and could adapt to human hunting pressure early. Then, as humans moved into new places, megafauna couldn’t adapt quickly enough to survive a novel predator.

      Another, more recent idea, is that there were in fact megafaunal extinctions in Africa, but they were much earlier (e.g., around a couple million years ago, when hominins first started using tools).

      Liked by 1 person

  1. Thank you for this – really a very interesting piece.

    It made me wonder if secondary effects of human presence could play a role too, not just direct hunting? You know, the presence of bands of humans could change behaviour patterns of prey animals – just look at how a few wolves change when and where deer are to be found, thus changing the rate of sapling getting to grow up. Likewise, gathering some plants and not others might easily have an influence on the botanic ecology of places. And once fire, and the burning of whole stretches of land come into play – and why should only farmers do that if it drives off dangerous predators for example? – one could easily picture whole landscapes changed. And the megafauna on top of the food chain would suffer first.

    These are just the home-spun theories of someone who is by all means not an expert but very much a layperson. So I’d rather phrase this as a question – are such indirect influences being considered in the megafauna exctinction debate? Or is it over the top to think humans so invasive, so capable of changing ecosystems even when they are “just” hunter-gatherers?

    I remember reading about the way earliest agriculture changed the landscape of central Europe. It is amazing how big that change was in the course of just a few hundred years. Those neolithic farmers didn’t change the tree populations by consciously aiming to cut down certain species but by unconsciously changing the evolutionary pressures working on woods by doing what they did (semi-settled agriculture, moving their villages around every few years if I got that book correctly).

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    1. Great comment — and you bring up a lot of reasons why it’s really tricky to tease apart cause and effect in the paleoecological or archaeological record. There are many indirect effects, like you mention — changing behavior, the role of fire, etc. They are talked about, but it’s very tricky to get a good handle on them, though some folks are trying with a method called “agent-based modeling,” which tries to account for the role of these kinds of interactions.

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  2. Good to see this topic and your thoughts. Here’s a few of mine (note: Paul S. Martin was my mentor and co-author), so yes, I am biased.

    1. If you kill a big animal, you process it (and sometimes eat it where it lays); thus, with the exception of cliffs where bison were run off, a kill site is a one-time event.

    2. If you have dogs, you will likely not use your tools to get even close to a bone. Eat the best meat yourself and throw the rest to the dogs.

    3. In warm times of year, you are in a race to eat the beast before it rots. Much of it you must leave behind — certainly anything near a bone.

    4. Big predators tend to be local. If I and my family made a kill, and especially during warm times, I would remove just what we had time to eat before it rotted and carry it off a good distance so that the local predators could gravitate to the kill site and I would be safe. Hence, teeth marks on bones, not stone tools.

    5. I recall reading somewhere that the best/safest way to “kill” a megafaunal creature was simply to penetrate the gut area, puncturing an intestine. In a few days, septicemia will set in, so you just follow the wounded animal at a distance, waiting for it to weaken or die on its own. In post-glaciated regions, where bogs are prevalent, the fever of septicemia likely would induce the mastodon to seek relief in a bog.

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  3. Being a layperson who just follows topic out of interest, am I allowed to ask silly and uniformed questions? 🙂

    (This is purely rethorical, I mean to ask them one way or the other. Here they are: )

    – would Neanderthals and Denisovans and possibly other H. erectus-offspring in Asia or wherever not count as “extinct megafauna” too? They did become exctinct suspiciously soon after we arrived, in Europe, anyway? (Don’t know about Denisovans actually, I don’t even know if anybody really knows when they perished.) And might not the reasons for the wooly mammoth’s demise and the Neanderthals’s demise be similar, even? (just a question)

    – could H. sapiens not have caused the demise of all those megafauna species in other ways than just by hunting them? When our ancestors arrived on a scene, they changed the whole setup very much, I would imagine: changing migration routes maybe. Outcompeting the top predators. Pushing populations into different, poorer habitats with more difficult conditions. Reducing reproduction rates by hunting infant specimens preferentially. Even altering the landscape by using fire (or in other ways? Can “hunter gatherers” change the landscape, by changing the plant structure, maybe)?Could the massive die-offs possibly be a secondary effect?

    – I have read about Gravettian dwellings made of many, many tusks. The text said that it isn’t even clear if the mammoths were hunted or the tusks just picked up from carcasses – but this might be an indicator of massive hunting. Or is that wrong?

    I realize this is just armchair theorizing. But maybe someone who knows more about these matters than I do has an opinion? I’d love to hear it. (And sorry for being so wordily verbose in asking …)

    Oh, not to be forgotten: a great blog post, I very much enjoyed reading it. Thank you!

    S.

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    1. There are a few definitions for megafauna. By the definition of “adult body size >50 kg” (which is about an adult German Shepherd), then yes, hominins are definitely megafauna! For ice age animals, though, we often use a 1000 kg cutoff (so, not in that case). We don’t know much about other hominins, but Neanderthals are thought to have died off potentially due to competition, but also due to interbreeding with humans — if you’re from northern Europe, you likely have ~2.5% Neanderthal DNA in your genome!

      Indirect effects were definitely possible, but hunting would have been the most direct pressure.

      I’m here at the Mammoth Site in Hot Springs, SD, and they have a replica of one of those houses that contains around 130 skulls. These could certainly be examples of mass kills (bones on the surface don’t tend to last long, so it’s hard to imagine being able to scavenge that many skulls).

      Thanks for your comments!

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      1. J – Yes, the Mammoth Site in SD is awesome! Its history of discovery and locally-funded preservation is inspiring, too.

        While you are at the site, notice the honey locust trees (Gleditsia) in the parking area. I told the staff that, though that domesticated variety was bred to be thornless and podless, nonetheless they still represent a plant that was deeply influenced by coevolution with the megafauna for millions of years — both to ensure its seeds were dispersed via dung and its inner bark left unstripped by mammoths via thorns.

        The late Paul S. Martin and I converged in 1999 on The Mammoth Site to help celebrate the 25th anniversary of its discovery. Paul did a science slide show and I led a “Mammoth Memorial Service.” When he died, I constructed a “tribute” page that included all my materials/experience with him, plus links to any of his classic papers I could find online (or post myself), plus any other eulogies/tributes I found online, and continuing with additional items people bring to my attention.

        Because I am not an academic, I can be zany and playful with how I do the page. I’ll post the link here, but two things to notice above all:

        (1) Near the top you can click on a youtube video that is the only place, I think, where you can hear Paul’s voice (in a phone interview with me). Title of vid: “Paul S. Martin, Pleistocene ecologist, 1928 – 2010”.

        (2) I videoed the memorial service for Paul in Tucson; it was scheduled one year after his death so colleagues had plenty of time to adjust their schedules to get there. The final component of the vid you can link to has me leading the closing “hymn” — which we sang had sung at the Mammoth Memorial Service in 1999: “Bring Back the Elephants!”

        http://thegreatstory.org/paul-martin.html

        P.S. I think creating tribute pages for these pre-web old scientists is vital. In January, William Catton died, author of the classic 1980 “Overshoot”. My husband gathered testimonials to put on a tribute page (which I created), including a testimonial from Paul Ehrlich. And Catton’s coauthor in the 1970s emailed me jpgs of their 3 classic papers together, which otherwise were unavailable online. Plus I found and linked to the several youtube videos that featured him. And, we communicated with the family and they sent us a pdf of a lovely bio of him that I posted too. So there is a lot that one can do well beyond wikipedia for making it easy for later generations to easily sample the legacies of these ancient ones. You can see the Catton tribute page here: http://thegreatstory.org/william-catton.html

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  4. It’s certainly a pickle, isn’t it!

    The issue with at least some potential kill and butcher sites isn’t just the lack of stone tools, it’s the lack of any sort of cultural materials that might be expected to be left. Stone butchering knives would need to be resharpened, leaving behind a variety of debitage. Use-scars on stone tools indicate that some impact flaking could be expected, like when the knife or point hits bone. To use your example, the guys who built my porch took their tools home, but I still find brads and other smaller metal bits in my yard from the construction (not to mention sawdust, which wouldn’t last long in the record).

    On the other hand, the absence of some of these may reflect investigator bias as well. Resharpening and impact flakes can be very small, easily passing through a standard 1/4-inch screen (and many times soil may not have been screened at all). Then again, I helped excavate a juvenile mammoth with evidence of human-induced cut marks, for which we collected all associated matrix and wet-screened through as small as 1/16th-inch with only one questionable debitage.

    Another issue is collectors. An exposed mammoth or other megafaunal kill site might well have been looted of Clovis and other early Paleo diagnostics (again, this is where the presence of debitage would be helpful). In Texas, almost every county has had at least one Clovis point reported, but there are very few Clovis sites, and even fewer where Clovis and megafaunal remains were both present.

    If I were to speculate, I would suggest that perhaps the climate shift impacted the food chain for the larger creatures, and that humans helped speed the inevitable by picking some off when they could.

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