June 30 , 2016

by Duane Nash

Scavenging gets no respect. There is a reason you don't see hyenas and vultures put on coats of arms or have sports teams named after them. Lions and eagles yes, but never vultures and hyenas - ironic that all of these animals do scavenge (and hunt to various degrees). Doubly ironic is that in carcass disputes it is often vultures and hyenas driving the eagles and lions off of carcasses. It is a pet theory of mine that what makes an animal an exquisite and refined hunter often works against them in carcass disputes. Because the refined hunter depends heavily on preserving the physical armaments that allow it to do its job it is at a disadvantage compared to more generalized opportunistic competitors who can be more reckless in battle. To make the sports analogy it is the difference between the tactical and technical boxer whom excels in the specific and controlled environment of the boxing ring and the unrefined street brawler. The highly skilled and nuanced technical abilities of the boxer will win the day over the street brawler in the ring for sure. But take this tactician out of the ring and into the street against the brawling street fighter - who fights dirty, uses bluff and swagger, and can simply take chances with regards to life and limb that the professional boxer can't - and the outcome swings in favor of the brawler. What makes a good street fighter work well is a different skill set than what a boxer has and what makes a good scavenger work well is a different skill set than what a specialized hunter has.

Taking the theme that I started with on my last post - an over reliance of the import of the "killing claw" in all endeavors dromaeosaurid and refining the dominant RPR model of a "ground hawk" dromaeosaurid - I am going to expand on that in terms of how and why dromaeosaurids were very good, capable, and successful facultative scavengers. The caveat being, I should just explicitly state, that this does not imply a lack to or unwillingness to hunt. But one more thing about that hypertrophied second digit - the "killing claw". It seems several vultures sports such an enlarged second digit - the magnificent cinerous vulture (Aegipius monachus) and the red-headed vulture (Sarcogyps calvus) pictured below.

Enlarging the toe on the second digit serves a useful purpose in pinning meat down which allows the beak and head to get leverage in pulling bites off. The "killing claw' in maniraptorans might have been used just as much if not more in leveraging good bites while scavenging/feeding as opposed to outright "killing". Additionally the line of action on the dental serrations suggests that the denticles on the back end of the tooth - the lingual side - were doing most of the cutting work as the animal pulled back on food items. It really is all about the teeth and jaws anyways...

Ecological practicality of terrestrial scavenging; optimal size; factultative vs. obligate terrestrial scavenging

First things first. The issue of ground based scavenging. We have all heard the argument "you can't have an obligate ground based scavenger because there simply isn't enough carcasses to sustain such an animal". Sounds reasonable enough and this was one of the trump arguments used to disavow the scavenging T. rex hypothesis. Is there a way to test it?

Some researchers recently did just that in a particularly ingenious and creative manner: Body size as a driver of scavenging in theropod dinosaurs (Kane et. al., 2016). Under various conditions of carcass detectability, carcass size, and competition they performed basically a SIMS version of let different sized theropods find dead stuff and see what body size pans out to be the most efficient in terms of cost/benefit analysis. Basically if you are looking for carcasses to supplement your diet the results are congruent with the assertion that only soaring scavengers can make a total living off of scavenging. Both small theropods and large ones had difficulty earning enough calories from facultative scavenging to make the activity worthwhile. If we think about the issues faced by both very small and very large theropods in terms of garnering benefit from scavenging this makes sense. Small scavengers can't cover as much ground and are easily displaced from carcasses. Large theropods - although they can dominate a carcass - have to move all that weight around and by the time that they detect and reach a carcass much of the available calories might already be consumed by smaller and more numerous scavengers.

What the researchers came up with is very interesting in terms of optimal body size for a land based scavenging theropod. There appears to be a mid-sized sweet spot. Small fry like microraptorines didn't do so well and at the other extreme multi ton giants like tyrannosaurids and carcharodontosaurids did not fair too well as facultative scavengers. Facultative scavenging is most optimal in terrestrial theropods in a size range of between (they get pretty specific) 27 and 1,044 kilograms.

Dromaeosaurids slot into that size range very nicely you should note.

While I have read of several people poo-pooing this paper and its conclusions I personally find a lot of merit (with some caveats of course) in this methodology and the conclusions reached. At the lower end of the size scale I would think that Velociraptor and Dromaeosaurus would still fair pretty well even though they are lower than 27 kilograms. Jackals and coyotes do pretty well as factulative scavengers and they are a bit smaller than 27 kilograms. Additionally at the higher size range let us keep some perspective. Imagine just >one< 50 ton sauropod died in an ecosystem with several multi-ton theropods and loads of smaller and immature theropods. Chances are the big theropods could locate, dominate, and feast on that sauropod for a while. Their large size and relatively slower metabolism might allow them to scarf down a load of meat that sated them for several weeks or even months. So the big boys might scavenge less frequently but when they do dominate large carcasses that glut of food might form the bulk of their caloric intake for quite a while.

Not to mention this paper completely jives with the conclusions of a similar paper that came out in 2011 that has gone a little overlooked... when two independent studies reach similar conclusions that should perk your interest. The 2011 paper is called Intra-guild competition and its implications for one of the biggest terrestrial predators, Tyrannosaurus rex (Carbone et. al. 2011).

Physical evidence suggestive of scavenging

Dave Hone has written a bit on Velociraptor consuming and - in his interpretation - likely scavenging azhdarchid pterosaurs here and here. Make sure to go back and read the commentary it is hardly a close and shut case of scavenging. Hone makes a more compelling case for Velociraptor scavenging a hefty size Protoceratops "fighting dinosaurs", part II. Hone also has some papers on the topics but they are not open access, shucks. The blog posts and commentary get the point across though.

I don't fully agree that the idea of a velociraptor taking down a 2-3 meter wingspan pterosaurs is nonsensical so for me that is equivocal proof of scavenging. However the scraping of flesh off the jaw of the large Protoceratops is a more compelling argument for scavenging.

Additionally you have the famous Yale Deinonychus and Tenontosaurus quarry that might indicate not only scavenging but aggressive intraspecific killing and cannibalism among Deinonychus. And who knows what surprises await us with the new Utahraptor block James Kirkland and co. are working upon...

Roach and Brinkman (2007) advocate a "diapsid like" foraging strategy in Deinonychus and other theropods as the best and most parsimonious null hypothesis. They eschew the notion of cooperative pack hunting posited to bring down herbivores that are several orders of magnitude larger than the theropods but instead highlight scavenging, combat, and cannibalism as the likely culprits that resulted in the taphonomic signal from the Yale quarry. Chief among these is the presence of articulated tails in the quarry the argument being that the bony and tendonous tails were eschewed in favor of more meaty pieces that were hauled off. I do agree with a lot of what they are selling. They do leave the opening for opportunistic group foraging where a congruence of factors might lead to a "mobbing" type foraging scenario. We see this with sharks gathering escorting ailing whales along and literally eating them alive. Or crocodiles gathering at choice feeding situations or latching onto the same prey animal or loosely "cooperating" to shoal prey together. I would also extend this to the manner in which vultures gather around a carcass and use strength in numbers to harass and intimidate other predators off a carcass.

The blog post Raptors: Do They Live up to the Hype (part 2)? Goes a bit further into the Deinonychus saga if you want to go further...

My purpose here is not too weigh too heavily on whether each of the above scenarios are unequivocally evidence of scavenging but merely highlight that there is a body of evidence that points heavily in the direction of scavenging in dromaeosaurids.

Few stories in dinosaur paleontology were as exciting and celebrated as the revelation of an honest to goodness mega-dromaeosaur in Dakotaraptor (DePalma et. al., 2015) that inhabited the latest Cretaceous of North America right alongside good ol' sexy rexy. Met with a flurry of fanfare and fan art I decided to let the dust settle a bit before giving this beast the antediluvian salad treatment.

Dakotaraptor Is Cursorial

Interpretations of Dakotaraptor that highlight the ecology of this animal as a cryptic, stalker of closed habitats - the "ghost of the forest" scenario - are missing the story that the bones are telling us. This animal was leggy, it was cursorial, and in life it would have departed strongly from the more low slung and slower Utahraptor or Deinonychus.

As I discussed in my last post there are certain compromises to be made in terms of raptorial grasping abilities in these animals versus cursorial ability as highlighted by Fowler. To review as grasping ability increases the metatarsals shorten for better leverage and strength but running ability is compromised; conversely as cursorial ability increases grasping strength decreases. Also bear in mind that all dromaeosaurids/maniraptorans were relatively weaker than modern raptors in terms of grasping power of the feet - they were not simply overblown red-tailed hawks.

An Interesting Incongruity

If you followed my articles on Spinosaurus and my argument for underwater punting in that animal you know that I do have a fondness and attraction for anatomical incongruities; two things combined that seem to make no sense but when analyzed in a different light actually offer a more refined animal. In Dakotaraptor we have such an incongruity.

Digit II is hypertrophied. The "killing claw" in this animal lives up to all the hype. Not only is the claw relatively large, the attachment for the muscle and tendons that drive it - the flexor tubercle - is robust. Whatever way in which this animal was using its digit II it is obvious that it has invested heavily in it.

However the incongruity is seen when we move out to digit III and IV where the claws on those digits are not highly recurved nor is there a significant flexor tubercle present. In fact the flexor tubercle is so reduced in those digits to be pretty much non-existent!!

Going back to my earlier bit on the cursorial aspects of this animal and resolving its place in the ecology of the environment in which it lived proves useful I contend. Several inferences are useful in giving better precision as to how this animal likely operated.

Inference 1 Dakotaraptor was highly cursorial.

Inference 2 Dakotaraptor - at least in the adult morph - was fairly limited in arboreal capacity. At best they were probably very clumsy in trees due to large size, long legs, and limited foot strength.

Inference 3 Lesser dromaeosaurids, azhdarchids, and tyrant lizards - especially juvenile and rapidly growing teenage rexes - created an especially quarellsome predatory and scavenger ecology. All of these animals would have been usurping, competing, and fighting over carcasses. Azhdarchids, juveniles rexes, dromaeosaurids, and Dakotaraptor all slot into the ideal size niche to be successful facultative scavengers.

Inference 4 Dakotaraptor could dominate pterosaurs, dromaeosaurids, and smaller rexes. However because it could not dominate larger immature rexes - which were still very cursorial - Dakotaraptor itself needed both speed and maneuverability to evade these animals as it was likely not highly arboreal.

Inference 5 The enlarged and robust ungual and claw in digit II primarily aided in combat - especially in competitive skirmishes over carcasses - and assisted in pinning meat down as the teeth, jaws, and neck pulled bites away from the animal (alive or dead). The denticles on the teeth of Dakotaraptor are most defined on the rear (lingual) side as they are in other dromaeosaurids supporting this notion, The RPR restraint model appears less important in this animal relative to other dromaeosaurids in prey capture. Instead digit II could primarily be used in combative endeavors while wing pummeling and the teeth and jaws did the actual killing.

Wings Suggested as Dominance Display

As in my last post on wing pummeling I will get behind another spotential behavior for dromaeosaurid wings that gets surprisingly scant attention - dominance display.

Such a shame that vultures do not get the recognition and respect they deserve for their role in modern competitive ecosystems; that they are spiralling into extinction; that they are not posited as the go to analogy for how extinct theropods looked, behaved, and operated when rendering carcasses - despite the fact that, quite simply, they are the modern theropod that does such things most often. Instead grumble, grumble flipping venomous lizards for crying out loud. Venomous lizards.

When you are watching a scrum of vultures feeding you are witnessing a direct portal into to the feeding activities of dromaeosaurids and other theropods in the Mesozoic's past. The theatrical displays; the spread wings; the fast jerky bites; the constant fight for dominance and hierarchy; and, most importantly, the constant and non-stop flurry of motion, action, and violence. Such scenes would be enacted on a much larger and grander scale in the Mesozoic. A difference of degree but not type. A sight both startling and jaw dropping.

References

Carbone, C., Turvey, S.T., Bielby, J. (2011) Intraguild competition and its implications for one of the biggest terrestrial predators. Proceeding of Biological Sciences. Sep 7(278) 1718 2682-2690. online here

Currie, P., & Jacobsen, A. (1995). An azhdarchid pterosaur eaten by a velociraptorine theropod Canadian Journal of Earth Sciences, 32 (7), 922-925 DOI: 10.1139/e95-077

DePalma, Robert A.; Burnham, David A.; Martin, Larry D.; Larson, Peter L.; Bakker, Robert T. (2015). "The First Giant Raptor (Theropoda: Dromaeosauridae) from the Hell Creek Formation.". Paleontological Contributions (14).
Fowler, D., Freedman, E., Scannella, J., & Kambic, R. (2011). The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds PLoS ONE, 6 (12) DOI: 10.1371/journal.pone.0028964
Hone, D., Choiniere, J., Sullivan, C., Xu, X., Pittman, M., & Tan, Q. (2010). New evidence for a trophic relationship between the dinosaurs Velociraptor and Protoceratops Palaeogeography, Palaeoclimatology, Palaeoecology, 291 (3-4), 488-492 DOI: 10.1016/j.palaeo.2010.03.028
Hone, D., Tsuihiji, T., Watabe, M., Tsogtbaatr, K. (2012). Pterosaurs as a food source for small dromaeosaurs Palaeogeography, Palaeoclimatology, Palaeoecology : 10.1016/j.palaeo.2012.02.021
Kane, A., Healy, K., Ruxton, G.D., Jackson, A.L., (2016) Body size as a driver of scavenging in theropod dinosaurs. American Naturalist June 2016, V.187, No. 6

Roach, B.T., Brinkman, D.L. (2006) A reevalutation of cooperative pack hunting in Deinonychus antirrhopus and other non-avian theropod dinosaurs. Bulletin of the Peabody Museum of Natural History 48(1): 103-138

http://antediluviansalad.blogspot.ru/2016/06/making-dromaeosaurids-nasty-again-part_24.html