Nope. No hyaenish things at that place and time.

It is a bit of a misconception to think of a niche as being available but unfilled. Of course a bone-crunching species could have carved space out in that ecosystem, but there was no lineage that went in that direction. A niche doesn't exist in the absence of the species that fills it.

That is, there were not large carcasses strewn about going to waste for lack of a hyena-like thing, those carcasses were being consumed by other things; probably defleshed more thoroughly by things that we would consider more hypercarnivorous (e.g., sabertooths and dire wolves - probably the short-faced bear too) and then the bones could have been chewed on by coyotes and eventually recycled by smaller mammals (e.g., rodents) and other organisms.

Had there been a hyena-like lineage (e.g., a proper hyena or a borophagine), that lineage might have been able to push species around the fringes of their dietary niches (e.g., dire wolves may have gotten to spend less time at a carcass and may have become even more hypercarnivorous) if they could have gained a foot hold.

I suspect all large mammalian carnivores will preferentially scavenge when the opportunity presents itself. Hyaenas are known to kill up to 95% of what they eat. True, hyaenas can consume more of a carcass than a "hypercarnivorous" species (e.g. big cats), but that doesn't make then lesser hunters. Given that, I'm not certain we should expect any particular carnivore to have been an obligate scavenger. Pleistocene carnivores would have certainly utilized carcasses differently though I'm not certain that is good basis for inferring than some scavenged more than others.

I agree. There certainly are no "obligate" scavengers. Nor are there really obligate hypercarnivores - just about anything will eat a fresh carcass if they come across it - even cows!

When we think about this "scavenger" type morphology, what we are really discussing is the morphological abilities. These durophages are ABLE to consume carcass elements much more thoroughly, but that certainly is not their whole diet or even the majority of their diets. As Matt pointed out, one of the three species of durophgous hyenas has been found to be very efficient in hunting. I believe that the same seminal study (Kruuk 1972) found that they lions that lived with those hyenas actually were both less successful hunters and stole carcasses more often from hyenas than the hyenas stole them from the lions.

Let's not forget our friends the teratorns and condors. These certainly would have been able to pick up the scavenging-niche slack, and fit the criteria of a large, terrestrial and specialized scavenger.

Yes, I was thinking of American avian scavengers, which, if I am not wrong, are the largest avian scavengers in the world and represent a proper example of obligatory scavenger. But they are not bone crackers.

I agree that large extinct condors and teratorns were important scavengers. Among mammals, good candidates for bone cracking include the dire wolf and the short faced bear, based on their jaw and tooth morphology. See, for example, "Late Pleistocene giant short-faced bears, mammoths, and large carcass scavenging in the Saltville Valley of Virginia, USA" for some discussion and references.

Anyonge & Baker in their paper "Craniofacial morphology and feeding behavior in

Canis dirus, the extinct Pleistocene dire wolf" say that dire wolf possessed a normal canid craniofacial morphology, without bone-cracking specialization

Figueirido & al. (2010) in their paper "DEMYTHOLOGIZING ARCTODUS SIMUS, THE ‘SHORT-FACED’ LONG-LEGGED AND PREDACEOUS BEAR THAT NEVER WAS" suggest that short-faced bear was a normal omnivore.

Hi Roman,

I chime in with Adam that bone-crushing is semantically defined by the presence of bone crushers! crocuta-like hyenas and borophagine canids were supreme bone crushers, but I suspect that a short faced bear could have easily get the job done with no obvious modification in their molars (or premolars, for what is worth)...and dire wolves were pretty well equipped to hack it as well

Hi, Pasquale,

May be the question sounds better like this: "why the ecotype of bone-crushing scavenger was not present in North American Quaternary Fauna, unlike Eurasian faunas?"

Dire wolves look pretty hypercarnivorous (flesh eaters) by most measures. By my measures (radius of curvature of the premolars and intercuspid notches) they seem more hypercarnivorous than many modern canids including modern wolves.

Short faced bears could have eaten anything they wanted! They had grinding surfaces on their molars, but also tooth morphologies that would be good at flesh slicing or bone crunching. However, when you get that big and strong, there really aren't too many things that you'd have trouble eating.

Matt will be able to tell us if microwear indicates specific diets for them.

As for birds, as an obligate mammalogist, I never give them much thought! However, I would still bet that the terrestrial carnivorans were more significant scavengers (even the hypercarnivores) than were the avians. I haven't seen comparative studies of modern avians vs. carnivorans, but I bet, by volume of carcass elements consumed, the mammals scavenge more, and probably did back then too.

Again, the bottom line is that 1) there is no "obligate" mammalian scavenger (I'll bet even vultures eat live prey too!) and 2) there is no such thing as an unfilled niche.

Ah, now that is a very different question Roman!

The truth is, there really aren't too many bone-crunchers around now or ever. Today, there are really only three species in one lineage. Other bone crunchers (e.g., some canids, Tasmanian devils) do much more flesh eating than bone crunching.

Asking why there aren't bone crunchers in NA is like asking why there aren't sabertooths around today (another recent ResearchGate thread). Morphotypes wax and wane.

Adam, actually my final question was that North American Pleistocene ecosystems didn't support the niche of terrestrial scavenger - I mean that the econiche was missing. Carnivores may have opportunistic behavior, nonetheless some of them evolved adaptations for scavenging and didn't retain generalized opportunistic ecotype. What about scavenging birds, I agree that their impact probably was very modest , because the body mass of the largest scavenging bird is more or less the same as the body mass of the smallest carnivore with scavenging habits, Gulo gulo.

Modern cloud leopard Neofelis nebulosa is regarded as approaching to the ecotype of sabertooth (in a paper of Christiansen, I guess)

I agree with most of you to the fact that there is no search thing as an obligate terrestrial scavenger.

But I think you, Adam, have a misconception about the volume of carcasses consumed by vultures.

In the Masai Mara-Serengeti ecosystem vultures consume 70% of all availiable meat.

see for example : Virani, M., C. Kendall, P. Njoroge, and S. Thomsett. 2010. Major declines in the abundance of vultures and other scavenging raptors in and around the Masai Mara ecosystem, Kenya. Biological Conservation 144 (2011): 746–752. DOI:10.1016/j.biocon.2010.10.024

The mayority of carcasses seems to be consumed by vultures in modern north america as well. This is mentioned in

Neil J. Buckley 1996, Food Finding and the Influence of Information, Local Enhancement, and Communal Roosting on Foraging Success of North American Vultures. The Auk 113 (2): 473-488.

But agree that no durophag terrestrial bone-crushing carnivor with specialized features for this kind of behaviour was present in pleistocene northe america. Although I think the stronger bite of the Canis dirus might have enabled this species to crack bones somewhat more efficiently than modern Canis lupus does.

Jens, thank you for including those references. I am surprised by how much flesh vultures consume. Obviously they have an advantage in searching for carcasses, and they also seem to be much more numerous than terrestrial scavengers. Very cool.

As for the bite of C. dirus, they might have been relatively strong - certainly not dramatically more so than modern wolves - but their teeth indicate that they were equally as hypercarnivorous as modern wolves if not more so (Simpler, E.M., J.M.G. Perry, and A. Hartstone-Rose, Determining diet from the crossectional shape and intercuspid notches of the teeth of the carnivores of Rancho La Brea. Journal of Vertebrate Paleontology, 2011. 31 (Suppl. 3): p. 195A ).

Hi everyone, very interesting question indeed.

I think the only way to answer the question of Roman is to compare ecomorphological disparity of North American carnivorans with that of Eurasiatic-African species. Considering the high number of bears in the North American Pleistocene (in some sites you find co-existence of black bear and Arctodus or grizzly bear and Arctodus) it seems there must have been a kind of compensatory mechanism in North American ecosystems to favour high number of omnivorous carnivorans (bears but also canids: coyote+dire wolf+grey wolf in Rancho La Brea). Consider also that the wolverine is pretty good at cracking bone and this taxon was present in the American Pleistocene too. An example of this approach:

Morphological disparity in Plio-Pleistocene large carnivore guilds from Italian peninsula

http://www.app.pan.pl/article/item/app20100037.html

All the best,

Carlo

The other thing to consider is dental wear. Recent studies by Blaire Van Valkenburgh and others have shown that Pleistocene predators in the La Brea tar pits had a far higher rate of broken teeth than modern predators do, even in "untouched" environments (in scare-quotes because one conclusion of this research is there aren't any truly untouched environments anymore). The broken teeth indicate osteophagy, which probably isn't going to be a common activity until the softer stuff has been at least partially consumed.

Van Valkenburgh and his co-authors argued that increased tooth breakage indicated increased numbers of predators (and therefore increased competition), and it's hard to imagine a condition where competition among predators isn't correlated with predators engaging in scavanging. More predators generally means less prey, and frankly it's usually easier to scare off buzzards than to kill something. So there may not have been a lot of ecological room for an obligate or even extensive scavanger.

That said, I don't think we have enough evidence as of yet to differentiate teeth broken via scavanging vs. teeth broken when a predator eats as much of a kill as they can. A broken tooth is a broken tooth, after all.

That could also help explain the omnivorous carnivores, such as bears. If there's a great deal of competition for meat, the simple solution is to find something else to eat--and with increased predation comes decreased prey numbers, and therefore increased plant material. I'm not saying this drove bears to omnivory; I'm just saying that it might be a contributing factor to their success.

Hi James,

I have added also other interpretation on broken teeth in this paper of mine:

Meloro C. (2012) Mandibular shape correlates of tooth fracture in extant Carnivora. Implications to infer feeding behaviour of Rancho La Brea predators. Biological Journal of the Linnean Society 106: 70-80.

http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.2011.01843.x/abstract

Will be pleased to receive comments about it...my idea relates to cold environments. I find most arctic species tend to break their teeth more often probably because of the access to frozen carcasses with thick fur. Osteophagy (in my opinion) is very unlikely for the hypercarnivorous Smilodon.

Cheers

Carlo

Blaire Van Valkenburgh's "Tough Times at La Brea" paper (the study referenced by James) is hardly recent (she published it with Hertel 20 years ago - 1993), but it is ABSOLUTELY a MUST read for anyone interested in carnivory at La Brea. It is an easy and short read (published in Science) for anyone of scientific interest and has been incredibly influential on the research published by many of us. Look it up. You will enjoy it.

I haven't had a chance to read the paper yet, Carlo, but it's certainly an interesting idea. I know that chewing ice is bad for human teeth, and it maeks sense that chewing frozan carcases almost certainly is bad for the teeth of other organisms. I've got some personal experience with how hard frozen meat is, thanks to my mother once testing if some round steak was thawed by whacking me in the head with it ("I thought it was thawed!" "And that makes this better how?"). That stuff is HARD!

The problem I see is that this is going to be that it's of limited applicability, that limit being dictated by climate. Rancho La Brea didn't get cold enough freeze carcases, for example. It'll certainly be applicable in temperate regions, though I think teasing out the tooth breakage due to eating frozen carcases vs. eating bone may prove impossible.

The other thing that springs to mind is Van Valkenburgh's work on polar bears. (located free at http://www.plosone.org/article/info:doi/10.1371/journal.pone.0013870) She did a finite element analysis on the skulls to see, in part, how effective polar bear bites would be against hard food sources. She concluded that they high-grade, eating soft and nutritious bits of the prey, and \would have a lot of trouble against the tougher foodstuffs brown bears and the like regularly eat. In other words, a polar bear is an obligate carnivore (an interpretation Carlo's work supports). If nothing else such a test would be a good way to rule a predator out as a scavanger.

In my new paper "Human Activity as a Possible Factor in the Extinction of Large Mammals in the Late Pleistocene North Eurasia" http://www.e-anthropology.com/English/Catalog/Archaeology/STM_DWL_rTLT_g1lkchYVGMA5.aspx (in Russian) I explained the missing large terrestrial scavengers in North America as a consequence of comparatively low primary ecological production of Late Pleistocene ecosystems.

Arctodus simus was a dominating scavenger (besides vultures) in North America. Its scavenging adaptations based on the skeleton's studies (specific morphological features, analyses of locomotion, bone stress, foraging energetics, etc.) are described in detail in Matheus P., 2003. Locomotor adaptations and ecomprphology of short-faced bear (Arctods simus) in Eastern Beringia.Occasional papers in Earth Sicences, N7.126p. It is so far the best and in depth analyses done so far for the species ecology and adaptations solely based on real material. It should be read by all before trying to touch the subject on scavanging in NA in the Pleistocene, and extinctions.

Apparently, Arctodus simus was not a specialized scavenger because of its extremely weak premolars which could not be used for bone crushing.

It depends what you compare. There is no obligate scavengers in the modern world, and there were probably none in the past. Hyena's million of years specialization towards bone cracking (and scavenging) resulted in its teeth morphology, particularly in last premolars (however its social structure probably has been one of the major factors for the latter). Arctodus didn't have much time, but you cannot expect the distant taxa developing the same teeth morphology towards similar specialization given very different time frame for evolution. However, the Arctodus "managed" to develop some spectacular features of the dentition and skeleton towards scavenging, making tremendous leap from ancestral Tremarctos in its skeletal and body size/proportion changes. It particularly involved enlarging and strengthening premolars (remained small in extinct South Amarica Tremarctine Arctotherium). Its upper 4th premolar developed proportionally large pyramidal shape crown with three, almost same size cones matching lower M1 with enlarged conical para- and metacone (not seen in other Ursidae/Tremactinae), and enlarged (comparatively to other Ursids) triconid on the wide first molar. Other important features were shortening of the "face" that brought carnassials closer to the jaw articulation giving them increased mechanical advantage. The Arctodus skull/dentition specialization evolved towards and at the level of bone crushing, but not (yet) at bone cracking you observe in hyenas ("give him some time" :-), with still many traits (broadened and long lower M1 talonid and wide, long and "multituberculed" m2) of the omnivorous ancestral Tremarctus. Arctodus definitely was not "full-feathered hyena", but had gone a long way of changes from its omnivorous ancestral Tremactin. In other words, the Arctodus was a taxa that developed more scavanging adaptations than any other predatory mammal in the Pleistocene of NA, and had a relatively high (not yet exactly determined) "scavanging ratio" in its flesh/omnivorous diet .

This debate on Arctodus is very interesting and I think it is pretty hard to find a definitive answer. True: there are no obligate scavengers in Carnivora and I think the basic question is to find out if Arctodus was able of consuming bones or not. My limited experience with the mandible shape of this species suggests that this species shared adaptation to omnivory/herbivory typical of Tremarctos. However, I looked only at one anatomical feature. Figueirido et al. (2010). "Demythologizing Arctodus simus, the ‘short-faced’". Journal of Vertebrate Paleontology 30 (1): 262–275 looked also at long bones and suggest that no cursorial adaptations occurred.

Now, consuming bones is currently an adaptation confined to specialised predators such as hyenas (remember spotted hyenas are better predators than lions that scavenge much more by virtue of their body size-that is: if you are large you can steal carcasses in the carnivore world) but also grey wolves, African wild dog, and probably wolverine. A bone cracker is more likely to experience tooth fracture due to masticatory stress and probably the best way to understand if Arctodus was capable of eating bones could be looking at tooth wear and fracture (not done yet I think). Also microwear might help in this however also giant panda (that consume bamboo only) share HSG zig zag bands with bone crackers.

From an ecological point of view, I find hard to believe that a large bear such as Arctodus was a specialised predator especially during Ice Ages. A species that big will need a lot of meat (from metabolic point of view) and that seems unlikely considering that La Brea had plenty of predators. Being a bear, omnivory appears to be a more likely solution physiologically and metabolically. This obviously does not exclude the possibly of Arctodus stealing a carcass to Smilodon or Canis dirus (but note they were probably social and today brown bears rarely win vs pack of wolves).

Having say that: will you include an omnivore in the predatory guild? When wolves were extirpated by Yellowstone brown bear controlled only a little waipiti deer so that the ecosystem suffered of overgrazing. In an ecosystem such as La Brea, Smilodon and Canis dirus had probably a profound impact on controlling prey population and Arctodus was less likely to control prey even if it was an obligate canriovre (it was not obviosuly because lacked typically predatory adaptations).

Scavenging could be occasional and I believe it rarely regulate ecosystem dynamics. This could be a challenge for studies of extant ecosystems.

Hope this might help, and will be happy to continue this discussion.

Carlo

@ Carlo Meloro

I think there is a huge impact of scavenging and related activities (further decomposition of the bodies of dead animals, starting with smaller scale nekrophags like insects) in ecosystem dynamics.

But the group with the most impact would result from the activities scavenging birds like vultures and insects. Of cause you are right: there is no obligate terrestrial scavenger in amniota and there likely never was one in the past.

I agree that Actodus was most likely not a specialised predator. But I wonder what part in an animals diet could be made up of carrion without it beeing a (more than just obligate) scavenger.

The is some information regarding scavenging vs. more plant plant oriteated feeding in

Sorkin, B. 2006, Ecomorphology of the giant short-faced bears Agriotherium and Arctodus, Historical Biology: An International Journal of Paleobiology, 18:1, 1-20

(Link: http://www.tandfonline.com/doi/abs/10.1080/08912960500476366)

Sorkin (2006) states that the brown hyena (Parahyaena brunnea) diet contains as much as 50% of plant material. Jet, it has some adaptations for scavenging. Sorkin theorises that Actodus (and Agriotherium) where not curosial predators but based on overall body-proportions most closely resembled hyenas (esp. brown and striped hyenas) and thus might have had a similar diet.

I personaly doubt that it is usefull to compare animals as large as Arctodus to Hyaena/Parahyaena but the only other model availiable for their possible diet would be brown bears. But it is hard to assess the amount od meat in an omivores diet, especially if the omivore is very flexible (or even opportunistic) like modern bears. Stable isotope values of these species for example would be within a very wide range and could be more influenced by different scouces of meat (fish from different settings, meat of terestrial animals (some of them from migratory species with varying isotopic values around the year), etc.) than by differnt amounts of meat and plant matter in the diet.

A paper regarding possible feeding behaviour of Arctodus is:

Schubert, B.W., Wallace, S.C. 2009, Late Pleistocene giant short-faced bears, mammoths, and large carcass scavenging in the Saltville Valley of virginia, USA. Boreas 38(3): 482-492

(Link: http://onlinelibrary.wiley.com/doi/10.1111/j.1502-3885.2009.00090.x/abstract)

They are very cautious about the feeding behaviour of Arctodus but claim that the hunge individual they found would have been more effective as a scavenger, not only by its size that would have made it more easy to steal carrion, but also by the mechanics of its bite and enlarged m2.

Personally I am very cautinous about the statements in this paper.

Figueirido et al. (2010) proved that the face of Artodus was not much shorted at all, a statement that constrast heavely with the Schuber, Wallace (2009) paper.

Figueirido et al. 2010, Demythologizing Arctodus simus, the ‘Short-Faced’ Long-Legged and Predaceous Bear that Never Was. Journal of Vert...

The argument that Arctodus simus didn't have enough time to evolve specialized characters for scavenging does not sound convincing: the history of open savanna/grassland biome in North America is very long, so, there was enough time for terrestrial large specialized scavengers to evolve. The terrestrial scavenging ecomorphological type is present in the Pleistocene of Old World and missing in North America. Of course, mammals are opportunistic, but it does not rule out the importance of biomechanical and eco-morphological adaptations. Apparently, carrion is always available in any ecosystem, however, the eco-morphological type of specialized terrestrial scavenger may evolve only under certain conditions: a) carrion needs to me easily detectable and available for a scavenger (open landscapes only fit to this condition), and b) the productivity of acosystem needs to be high enough to supply the necessary stable amount of carrion for maintaining a population of a large terrestrial scavenger. It seems, that the grassland biomes of North America did not correspond to one of above mentioned conditions, most probably, because of comparatively low primary ecological productivity. That is what I tried to explain in my recent paper. The low primary ecological productivity of North American grassland biomes from the past was already discussed in the paper of Janis ( "EVOLUTION OF HORNS IN UNGULATES: ECOLOGY AND PALEOECOLOGY", Biological Reviews - Volume 57, Issue 2, pages 261–318, 1982).

Thanks everybody for defining the subject - it is great to see the approach from different angles. It seems that nobody can deny that Arctodus has a lot of predator/scavenger traits in its dental and skeleton morphology, but still having a bit of omnivorous features in its dentition inherited from its ancestors.

The question is still remaining, how much flesh it had in its diet, and what ration of it was coming from scavanging. Stable isotope analyses revealed that the ratio of the protein diet is very high, and the increase was towards the northwest NA habitats that coincides with the trend in body size increase towards the north.

It would be great to put all data of the Arctodus adaptation together, including (not performed) studies, which do not need numerous specimens (not limited to):

1. Brain morphology. CT scan of the Arctodus specimen from the Mammoth Site (not surprisingly) reveals that it has huge olfactory lobes located in a very wide and high rostrum. These are much larger proportionally to the rest of the brain than in brown bears indicating very acute sense of smell (the tendency observed vultures and adult T. rex'es) -another feature supporting scavanging adaptation.

2. Microwear analysis. I agree with Carlo that enamel microwear analyses may not give clear answers, but it will not hurt to perform (would be great project for undegrad/grad students).

Few more notes.. Since there were no obligatory scavenging species in mammal history, both Roman's assumptions (a,b, see above) cannot be applied to ecosystems not only of NA or Old World, but to other continents. Then the "terrestrial scavenging ecomorphological type" term possibly could be applied to (?) invertebrates, but not mammals, if the term "type" is addressed.

Of course, the fact that Arctodus "didn't have enough time" to specialize is a hypothesis. If it didn't get extinct almost simultaneously with the largest herbivores in NA, it could have given it the time continuing its specialization.

I'm a bit skeptical towards the argument that the North American environment did not support a bone crushing scavenger because of its lower productivity. Temperate europe supported hyena bone crushers and was probably similarly productive. Conversely tropical savannas in South America, with their higher productivity, did not. I think that an important issue here is the strong phylogenetic signal in extant and extinct bone crushers. As far as I know, hyenas and some of the member of Boraphaginae in N. America are the only two mammalian taxa considered to contain bone crushers (leaving aside the question of the short faced bear). The lack of convergence on this trait suggest it may be relatively hard to evolve.

There would seem to be plenty of food to go around on the major continents - given the diversity of herbivores and carnivores until human arrival. And since much of the spotted hyenas diet is composed primarily of fresh kills, it seems doubtful that food really limited this niche specialization. For instance, one study found that scavenging only provided 5% of the consume biomass for hyenas (Cooper et al, 1999, Afr J. Ecology). This may be on the low end, but it is suggets that selection for better bone crushing may not be strong.

While the paucity of convergence on bone crushing suggests this trait is difficult to evolve, its not entirely clear why that would be. The morphological specialization necessary doesn't appear to be too difficult to evolve - increasing bone, molar and muscle strength. Perhaps the energetic gains are comparatively low for a predator, or selection to be an intermediate bone crusher (bone cracker?) is weak. For whatever reason the selective path to bone crushing may be narrow, occurring only under rare conditions.

Very good comment John. I would like to highlight that bone crushing is not equal to carrion feeding. Most Carnivora need calcium for their metabolism and you like it or not even otters eat bones of the fish they catch. Personally, I do not believe in scavenging as a typical ecomorphological trat. Lion are brilliant scavengers with no adaptation to crak bones in their skull. Bone consumption has to deal more with feeding efficiency than scavenging. African wild dog consume the same amount of bones as hyenas (Van Valkenburgh 1996, J Mammalogy) although they have weak bone crushing adaptation.

Scavenging instead is more about size ad opportunistic behaviours, large carnivores scavenge on smaller ones and others like jackals are just good to wait and scavenge...consider that in La Brea there were also coyotes and they might have been nice scavengers too together with crows (with which wolves have co-evolved) and vultures.

Yes, flexibility in behaviour is and opportunistic behaviours are very important in feeding characteristics of Carnivora. I think that goes beyond oportunities for scavaging.

There is a great deal of flexibility in obtainig food in Carnivora that were present during Pleistocene time but that are still around.

Coyote for instance started to feed much more on carrion after the re-introduction of grey wolfs to the Yellowstone area.

Switalski T.A. 2003, Coyote foraging ecology and vigilance in response to gray wolf reintroduction in Yellowstone National Park. Canadian Journal of Zoology, 2003, 81(6): 985-993

(link: http://www.nrcresearchpress.com/doi/abs/10.1139/z03-080#.UWXTHFfLs3w)

It also appears that coyotes more often hunted elk (Cervus canadensis) calve before the re-introduction of grey wolfs to the yellowstone area. see:

Smith, D.A. et al. 2003, Yellowstone after Wolves. BioScience 53(4):330-340

(link: http://www.bioone.org/doi/full/10.1641/0006-3568%282003%29053%5B0330%3AYAW%5D2.0.CO%3B2)

I agree with Carlo and John. Lions towards feeding behavior towards live pray and carrion are great example of flexibility. And there was a good point that bone cracking/crushing is not equal to carrion feeding. I would add that ability to digest bones ( hyenas are champions) also is not equal of carrion feeding. It is unknown (yet) if Arctodus had such trait.

I agree that bone crushing does not equal scavenging, and that many (most?) predators are also good scavengers. Still, I think its interesting that bone crushing of marrow rich long bones or skulls (getting at fat rather than calcium) was apparently quite rare over macroevolutionary scales. The only tradeoff I can think of when evolving bone crushing is that carnassial teeth are too delicate. This would eliminate most cats. Strength is partly a function of body size, so this would eliminate small predators. This leaves us with large canids, ursids, and hyenids, limiting the pool for bone crushing evolution.

A branch of hyenids evolved the bone crushing trait and are still with us today. A dog lineage (borophaginae) did as well, but disappeared a few million years ago and apparently weren't replaced, (unless arctodus filled this role). The question then was, was this trait/niche truly unoccupied for several million years, and if so, why? Perhaps there is little reward for intermediate bone-cracking, although cracking a skull open would seem to offer a nice reward. The risk of cracking a tooth would also mitigate against biting hard things for the heck of it.

There is some evidence that an alaskan gray wolf morphotype evolved bone cracking abilities: http://www.sciencedirect.com/science/article/pii/S0960982207015059. One could speculate that selection for dealing with frozen carcasses might favor strong jaw muscles, shorter snouts, and tougher teeth - good preadaptations for bone crushing. At the very least, it suggests that perhaps this trait was (somewhat) more common than people realize.

Hi John,

you can find more arguments about tooth fracture and frozen carcasses here:

Meloro 2012. Mandibular shape correlates of tooth fracture in extant Carnivora: implications to inferring feeding behaviour of Pleistocene predators. Biol J Linn Soc Volume 106, Issue 1, pages 70–80.

http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.2011.01843.x/abstract

Hope you might like it.

Carlo

Carlo, I meant to thank you for suggesting the paper. One point I find intriguing is that elevated tooth fracture rates are seen in arctic foxes and other scavengers of frozen carcasses. With the end Pleistocene extinction of most cold adapted megafauna, scavenging on frozen meat is probably far rarer today. This would account for the relative rarity of tooth fractures today compared to relatives at la Brea and other sites.

John, do you really think the pleistocene climate at La Brea was sufficiently cold to explain the higher number of tooth fractures by scavenging on frozen carcasses? I think that would work for some other, more northern localities but I am sceptical to apply that theory at La Brea.

What can we say about the climate of that area through the plestocene series.

Jens,

Yes I may have been too quick to assume that winter temperatures regularly fell below 0 C, a precondition for my conjecture. I'm no climate expert (though I know some people I could ask), so perhaps this is untrue in La Brea. Its an interesting question, because it gets to the issue of whether high fracture rates reflect climatic or biotic causes. That is, were Pleistocene carnivore communities fundamentally similar to intact holocene communities? After a quick search, I found one paper indicating a mean annual temp of 5 and 11 ºC for two southern California sites (36º N) (Heusser, 1998, Paleoceanography). The question becomes, which they don't answer, what was the mean winter temperature? If a mean annual of 5 C were true for La Brea, the I'd assume mean winter was

The LaBrea tarpits contain a large scavenging bird, Teratornis, as well several members of the vulture family. Perhaps these were the principal scavengers of Pleistocene North America, as depicted in the famous Charles R. Knight painting of La Brea. But the large carnivorous mammals, like the dire wolf or short-faced bear, probably did their part. Perhaps even Smilodon did not refuse a freshly dead carcass. Large carnivore /scavengers like the birds and wolves were widely distributed across North America in the Late Pleistocene and probably did their fair share of scavenging.

In regards to scavenging, the Old World really doesn't contain much in the way of obligate scavenging mammals in the first place. Of the four living species of hyenas, Proteles is a termite specialist and Crocuta is an active hunter. Only the two species of Hyena can really be characterized as scavengers, and the striped hyena may be more of an opportunist than anything else. At the same time, obligate scavengers seem to be really rare in the fossil record; a number of inferred obligate scavenging species like percrocutids have turned out to be active hunters (https://www.researchgate.net/publication/225812176_Osteological_evidence_for_predatory_behavior_of_the_giant_percrocutid_(Dinocrocuta_gigantea)_as_an_active_hunter).

On the subject of Arctodus, the emerging consensus seems to be that rather than a super-specialized predator, Arctodus pretty much just ate anything it wanted, like a modern bear. The diet appears to have varied from region to region, with individuals in areas like Berengia primarily consuming meat and individuals in southern regions where berries and other edible vegetations was more prevalent having a diet similar to modern bears.

Article Osteological evidence for predatory behavior of the giant pe...