Dear Constant Readers,
I've been entertaining the idea of some sort of trivia point system (to be awarded upon meeting in person) for this blog. Maybe I'll appoint someone to an arbitrary position of sorts somewhere along the line. If somebody is able to identify this reference and the non-mammalian amniote rendered a warning via being "corked", those will be some big points. If a screenshot is provided, that is a near-guarantee of becoming an Internet hero.
On an actual topic: after the last post I've realized that trying to do a somewhat comprehensive overview can become rather difficult when the group is seldom written upon. I like to put up information that is fairly uncommon, but also controversial and ergo with enough information to write upon. This particular post will focus on so-called giant mammalian predators and if they are actually just big omnivores.
And no, this will not count as my conclusion to the "Honkin' Big Animals" trilogy (last seen here and here).
The biggest extant land-based mammalian hypercarnivore (a near-exclusive vertebrate predator) is the Siberian tiger Panthera tigris altaica which averages 250-260 kg/551-573 lbs (Wood, 1982) for males (all these sizes are presumably the larger males). The largest cited size is 320 kg/700 lbs (Sorkin 2005, Christiansen & Harris 2005---fig. 5) and reports of larger sizes are probably due to abnormally fat individuals and/or exaggeration (Wood, 1982). The Pleistocene saber-toothed cat Smilodon populator ranged from around 240 to 360 kg (~500 to 800 lbs) and perhaps over 400 kg (~900 lbs +) for "record" specimens (Christiansen & Harris 2005). I should note that the same source states that the American lion Panthera atrox / Panthera leo atrox was a rival for the largest feline, although an earlier paper (Christiansen 1999) gives the weight as 500 kg (1100 lbs). This is even bigger than unnatural ligers and really big claims around that size (see "Nook") are probably of overweight cats. The cat-like ecomorph is of course undoubtedly hypercarnivorous with more than 70% vertebrate prey (Van Valkenburgh, 2007) and, though extreme, I haven't seen any major revisions to the sizes proposed. So those are two giant hypercarnivores definitely not in danger of being reclassified as omnivores. Judging by proportionate changes in energy expended in capturing prey to energy intake, Carbone et al 2007 estimate the largest carnivorous mammals could get up to 1.1 tonnes (2400 lbs). So are there predators that exceed these super-felines? Can they reach the hypothetical size?
To answer the first question, yes, there are extant near-exclusive faunivores that are alive today. Polar bears are some really big animals. Despite their hypercarnivorous diet, the dentition of the bears differs little from their omnivorous relatives (Van Valkenburgh, 2007), but their poorly developed carnassials (for shearing) are explained because of their blubber eating habits (Sorkin, 2006). Polar bears are larger than other bear species and have been weighed at 654 kg (1440 lbs) and estimated up to 800 kg (1760 lbs) for individuals too large to be weighed (Schliebe et al 2006). There have been claims of gargantuan individuals up to a metric tonne (2210 pounds) and standing 11'1"/3.39 m tall (Wood, 1982) (non-obese?) which of course is seen as needing verification (Christiansen 1999). Mentioned by Bjvrn Kurtin were the remains of (Ursus maritimus tyrannus), a sub-species of polar bear "greatly exceeding" the living polar bear in size. A post (in German - poorly translated here) by Markus Bühler has a size comparison of a huge bear 6' (1.82 m) at the shoulder and presumably in the area of a ton (900 kg) or more. This sub-species seems incredibly obscure (not mentioned in any recent .pdf) and I can't help but wonder if somebody downsized it and I didn't notice (unlikely) or if it hadn't adapted towards being a complete faunivore (possible). There is of course the possibility that this was the species that straddled the 1.1 tonne limit hypothesized by Carbone et al and was presumably the largest hypercarnivorous mammal ever. This is all speculation of course until better remains and/or a study is released.
According to good ol' Wikipedia and other popular sources "The Giant Short Faced Bear was arguably in its heyday the largest true terrestrial mammalian predator on Earth" and estimates a size of 900 kg/1 ton and a height of 5.5 feet/1.67 m at the shoulder. Note: the statement of "true terrestrial" which appears to exclude the polar bear in a weaselly sort of way. But still, a predator that sized that lived as recently as 11,000 years ago and presumably interacted with humans is most remarkable indeed! The 900 kg figure is high, according to Christiansen 1999, previous studies pegged it at 470-766 kilograms (1000-1700 lbs), the smaller values being based on body size and the larger figures based on limb bone dimensions. The theory for the bones is that since of course they support the mass of the animal, their dimensions can provide a clue as to how much it weighed. The estimates ranged in the 6-800 kg range (1300-1750 lbs) although it was hypothesized that outsized individuals could weight a tonne (2204 lbs)! As for the anatomy of the animal, Christiansen said that Arctodus had a short and broad cat-like skull, well-developed carnassials, long limbs with a possible "more digitigrade" stance, and presumably a fast running speed. Van Valkenburgh notes that the dentition is not distinguishable from omnivorous bears (like the polar bear), which contradicts the "well-developed carnassials" assertion. The Carbone et al 2007 paper that hypothesized a 1.1 tonne carnivore cited Christiansen's study, but gave a figure of 800 kg - 1000 kg as the size for Arctodus! The abstract also states that Arctodus reached about twice the mass of a polar bear!! A paper discussing controversies as to whether Arctodus was carnivorous or not was dismissed by stating that "ancient bears had morphological similarities to the carnivorous polar bear"!!! I have a dangerous lack of credentials, but these very, um, interesting, statements certainly do need looking in to.
Sorkin's controversial 2006 paper on Arctodus and Agriotherium certainly does have a lot of interesting things to say, and it is such a shame the popular press didn't cover it. What is astounding is that Agriotherium is a convergence upon the Arctodus bauplan (e.g. short skull, long limbs, very large size, cranial and dental features); this particular species seems hardly mentioned in the "largest hypercarnivore" discussions for unclear reasons. Also worthy of note are the alternate theories that the short-faced bears were largely herbivorous or carnivores that scavenged; these theories seem to be ignored in popular media in light of the more exciting "1 ton superpredator" theory. Sorkin re-interprets the size of the bears at 540 kg/1200 lbs for Agriotherium and 570 kg/1250 lbs for Arctodus, a full 40% lower than Christiansen's maximum estimate. But is this plausible? If a polar bear 6' at the shoulder theoretically weighs about a ton, then I suppose it would be reasonable for a short-faced bear 5'6" at the shoulder with long legs to weigh about 3/5 of that. They certainly didn't use my reasoning. Criticisms of Christiansen's method revolve around using a width/length ratio for limb bones of an animal with disproportionately long limbs. While the limbs were longer, the authors reasoned that they were proportionately slimmer than that of a brown bear and probably actually weighed the same in proportion. This assumption of isometry was used to estimate weight using the skull length of other species vs. weight. I will admit their reasoning is confusing to me (more lightly muscled limbs yet wider humerus and femur proportionately?) and I generally find estimations based on the whole body (e.g. models) to be the most convincing. But I still think their estimate is more realistic and not totally unprecedented.
Aside from sheer size, the paper focused a great deal on the, well, ecology and morphology. The relative grinding area (RGA) was calculated by the differing proportion of the molar used for slicing rather than for grinding. Their value is more suggestive of a more carnivorous animal, except that the value is similar to the herbivorous spectacled bear (Tremarctos). Sectorial carnassials and the position of the mandibular condyle were also interpreted as being from a carnivorous animal, but these features are also shared with the spectacled bear. Prominent buccal cusps are shared with the largely herbivorous Kodiak bear (U. a. middendorfi). The most prominent arguments for carnivory are molecular are large amounts of the isotope 15N. However, the amount of this isotope in the animal's bone collagen overlaps with omnivorous brown bears (the 13C did as well) making it apparent that the tests are inadequate to distinguish diets. These findings are fairly neutral, but there are features suggesting it wasn't a predator. The upper canines are proportionately short, the eyes were small and more laterally placed (than felines), limbs were ill-suited to subduing prey, they were unable to accelerate from a crouching position, they had a spine more suggestive of a slower animal, plantigrade feet were present (contra: Christiansen), and other reasons. This is an interesting and I think, fairly convincing, pile of evidence against a giant feline-like bear. I should note that Arctodus lived alongside the giant felines previously mentioned. So what were Arctodus and Agriotherium? The authors criticize the notion that they subsided completely on carrion, but suggested that they also grazed on coarse foliage. As to modern analogies, they suggest instead of a being feline-like, it was hyena like. They specifically linked it to the striped (Hyena) and brown (Parahyena) hyenas which in addition to small prey and carrion often eat large amounts of fruit.
And if you think Sorkin is a habitual fang-corker, a paper he put out later in 2006 written on much the same format proposed that Amphicyon and Ischyrocyon were 550 and 410kg predatory bear-dogs that were the biggest predators in the North American Miocene. Since it is written in much the same format and arrives at opposite conclusions, I think that supports the validity of the short-faced bear paper.
The points raised by the short-faced bear paper are a fascinating new take on this traditional "superpredator", and as far as I can tell the arguments are convincing. Given how people often become attached to the "godzillafication" of animals, I'm sure somebody is going to respond to this. Hopefully there will be some discussions on this, and not strange dismissals. If they are 1 ton superbears, good, if they're not, just as good. The idea of a short-faced bear ecomorph as its own entity is a fascinating concept that I wish I knew more about.
Oh yes, Agriotherium lived in Africa (Arctodus in North America) and Markus Bühler appears to be the first to (half-seriously) connect it to the Nandi Bear. Points for apparently being first to do this (before me anyways).
Exit: Arctodus and Agriotherium
Enter: Andrewsarchus, Megistotherium, and Sarkastodon
Don't worry, the amount of technical literature on these genera are very limited. I'll put them in order from best known to most enigmatic.
Aside from the short-faced bear, the non-carnivoran ungulate (mesonychid) Andrewsarchus is often given the title of "largest terrestrial carnivorous mammal known to have ever existed" such as by Wiki P's. You'll see a number of phenomenal claims on that article suggesting it averaged at 1.5 tonnes and got up to 2, preyed on giant brontotheres and ate their "already dead" carcasses, and had the strongest jaws in a land mammal. Yowza. This is of course rampant Internet speculation masquerading as some sort of authority (I, for one, don't hide who I am). The very first article written about it speculates on an omnivorous diet and estimates a body size of 3.82 m/12'6.5" long and a shoulder height of 1.89 m/6'2"...but alas no body weight estimation. The picture of the skull juxtaposed to that of a brown bear is quite stupendous by the way. Following a hot lead on a website supposedly aimed at children (specifically an ambiguous statement by...dun dun duuuun...Darren Naish) I found a paper on maximal body size for free (here). Animal body size is a fascinating subject in general, and hopefully a big name like Jared Diamond will be enough to convince you to read it. Anyways, it gives a weight of 6-900 kg (1300-2000 lbs) from a personal communication and mentions the ambiguous diet. The skull appears to be the only remain known, so much about this animal is just speculation.
Megistotherium is not a carnivoran either, but a related group known as creodonts. It is another candidate of the "world's largest terrestrial predator", and apparently weighs 880 kg/~2000 lbs (Rasmussen 1989). Wiki estimates "at least" 900 kg of course and predatory behavior, although it mentions the possibility of scavenging. This paper provides further anatomical details. The Rasmussen article says it is indeed conventionally regarded as a big predator, but could very well be an omnivore as well. Naish noted that reconstruction was based on an outdated reconstruction, so there definitely is a need to re-examine it. I should mention an awesome drawing of it here.
Sarkastodon is another creodont (an oxyaenid) which Wik's of course portrays it as a 1 ton predator. Where does that information come from? I've never read that in popular literature; it is conventionally portrayed as an omnivore. The only technical article I could find is from the 30's and only discusses skull anatomy. It is short-faced, but reconstructions show otherwise (basically portraying it as a long-tailed bear). Nothing about the size or presumed diet is discussed, so clearly I'm missing something. But even so, I doubt it would have been very recent or comprehensive. So other than this being a rather big creodont, nothing can really be said.
As you can see, all three of these beasties were fairly poorly known. Were they actually hypercarnivores, or just big omnivores? In the current state of knowledge it can't be known with any degree of certainty. I (or better yet, somebody else) could make an educated guess, but that's all it would be, a guess. Popular media gives the impression that scientists have a definite grasp on everything, but that isn't the case. There is confusion, arguments, and it definitely is a growing process. The more I learn, the more I see how big the gaps in knowledge really are. And knowing that there I things I can never truly learn or even hear about is frustrating in a way. But hey, it's a start.
It certainly doesn't matter how large an animal is; but size attracts attention which means there is likely to be more information. I think that, if presented well, there really is no boring group of animals.
Come up next week, Honkin' Big Animals!
I'm not quite certain if that's a joke or not.
P.S. Here is an illustration of Andrewsarchus and Sarkastodon; the latter is looking extremely creepy in the background I might add.
Burness, Gary P. & Diamond, Jared & Flannery, Timothy. 2001. Dinosaurs, dragons, and dwarfs: The evolution of maximal body size. PNAS, 98: 14518-14523. Available for free here.
Carbone, Chris et al. 2007. The costs of carnivory. PLos Biology. Published for free online: Here
Christiansen, Per. 1999. What size were Arctodus simus and Ursus splenaeus (Carnivora: Ursidae)?. Ann. Zoo. Fennici 36: 93-102. Available for free here.
Christiansen, Per & Harris, John M. 2005. Body size of Smilodon. Journal of Morphology: 266, 369-384. Available: Here
Granger, Walter. 1938. A giant oxyaenid from the upper Eocene of Mongolia. American Museum Novitates. Available for free here
Kurtin, Bjvrn. On Evolution and Fossil Mammals. Pages 185-186. Available (partially) for free: Here
Osborn, Henry Fairfield. 1924. Andrewsarchus, giant mesonychid from Mongolia. American Museum Novitates. 146. Available for free here.
Rasmussen, D. et al. 1989. New Specimens of the Giant Creodont Megistotherium (Hyaenodontidae) from Moghara, Egypt. Journal of Mammalology. Vol 70, 442-447. Available: Here
Schliebe, Scott et al. 2006. Range-wide status review of the polar bear (Ursus maritimus). Available for free here
Sorkin, B. 2006. Ecomorphology of the giant short-faced bears Agriotherium and Arctodus. Historical Biology: 18, 1-20. Available: Here
Sorkin, B. 2006a. Ecomorphology of the giant bear-dogs Amphicyon and Ischyrocyon. Historical Biology: 18, 375-388. Available: Here
Van Valkenburgh, Blaire. 2007. Déjà vu the evolution of feeding morphologies in the Carnivora. Integr. Comp. Biol.: 47, 147-163. Available: Here
Wood, Gerald. Guinness book of Animal Facts and Feats. Guinness Superlatives, Middlesex, 1982.