Tuesday, March 25, 2014

Food for thought: Omnivore’s dilemma



In case you pay attention to the latest paleontology news, there’s been a lot of publicity towards the ever-growing case for giant flightless birds being herbivores. Isotope analysis done on the Eocene Gastornis and Pleistocene Genyornis suggest diets high in fruits (for Gastornis) and grass (Genyornis), overturning the long-held assumption of them as predators.  http://link.springer.com/article/10.1007%2Fs00114-014-1158-2 So that’s that, it seems; we made a mistake, and now these brilliant scientists have demoted these terrifying runners into placid, docile browsers according to the headlines.  http://link.springer.com/article/10.1007%2Fs00114-014-1158-2


 

Just wait a minute. We’ve seen this before-the oviraptor Citipati, discovered on a bed of eggs, was thought to be a thief of Protoceratops eggs, but the eggs turned to be most likely its own. (Dinosaurs of the Flaming cliffs) The giant bear Arctodus has often been demoted in recent literature from a running predator to a loping browser. http://www.wired.com/wiredscience/2010/02/the-fearsome-short-faced-bear-gets-a-makeover/ The huge piglike entelodonts like Daeodon have left bitemarks on the bones of other ungulates. http://scienceblogs.com/tetrapodzoology/2007/07/11/tet-zoo-picture-of-the-day-23/ There’s even been some speculation that the sharp shearing beaks and teeth of ceratopsians could have been employed to tear through flesh. http://www.flickr.com/photos/markwitton/522293984/  Deinocherius’ huge hands, long thought to be from a terrifying predator, have been now found to be part of a giant ostrichlike browser. http://www.flickr.com/photos/markwitton/522293984/ Is the world being turned upside down? Should we no longer regard these animals in their traditional roles?

Nope, at least, not as dramatically as people have done so in the past.  Humans often pride themselves in uniqueness-we’re the only animal who has emotions, can communicate in languages, and can eat anything we want. Now, all three are false assumptions.  Omnivores, if you look at the world, make up more of the animals of the world than you would expect.

For example, horses eat mostly grass, oats, fruits, and other vegetation. Their teeth are superb for grinding the hard silicaceous fiber into digestible mush. But horses do eat meat. Offer a horse a burger, and they’ll take it. Their teeth and digestive system is not adapted for flesh, but they can and will eat it if available. In the cold dry habitats of Central Asia and the poles, people have successfully fed their horses raw or cooked meat, being able to sustain the animal when no forage is available.  There’s even a book on the terrifying psychological aberration of horses consuming flesh, finding it to their liking, and hunting and killing even humans for it. http://www.amazon.com/Deadly-Equines-Shocking-Meat-Eating-Murderous/dp/1590480031

The opposite is true even for hypercarnivores. Sumatran tigers have been observed eating durian, which is more often the food of orangutans or buffalo (or people with bizarre tastes in fruit) http://www.arkive.org/tiger/panthera-tigris/video-su08b.html . Domestic cats can develop a taste for fruits and vegetables. Mind you, like horses, cats can develop tastes for foods they shouldn’t eat. It’s not hard to find cats and dogs eating vegetables or grains, or even grazing.   Tetrapod zoology once ran an excellent article on the frugivorous behavior of crocodiles and alligators. http://scienceblogs.com/tetrapodzoology/2008/10/03/alligators-eat-fruit/

Of course, these are only the most dramatic examples. Pigs and primates are comfortable eating whatever they can chew. Both true pandas and red pandas eat whatever they can find. You don’t even have to look that far; the sharp incisors and strong teeth of rodents make them excellent omnivores, and rats especially have used their omnivory to become one of the most successful mammals ever.

Can we connect modern analogies to the prehistoric examples? Sure. Let’s start with Deinocheirus. Deinocheirus is an ornithomimid, theropod dinosaurs with small, beaked heads. They’re called ostrich dinosaurs for obvious reasons-the head, neck, torso and legs are very similar, down to the feathers.  Ostriches and other ratites like emus and rheas are omnivores; emus eat whatever plants are available, along with whatever insects they can find. Ostriches are more herbivorous, but again whatever they can fit in their mouth they can eat. The secret is their gizzard- full of stomach stones or gastroliths that do all the chewing for them. Gastroliths have also been found in ornithomimid skeletons, as well as other dinosaurs. This not only allows them to eat fibrous plants, but small bones and eggs. Rheas are more herbivorous but do eat meat, while cassowaries eat much softer fruit and have been known to scavenge as well as eating other plants and small animals. So, using ratites as a model, Deinocheirus could have eaten anything it could fit in its mouth-even its own theropod relatives. http://www.avianweb.com/ratites.html

Daeodon’s only living relative is the hippopotamus, another distant relative of pigs and whales. Hippos eat carrion from time to time as well as the water plants they usually subsist on. One paper mentioned cannibalism and predatory behavior on some occasions. http://www.newscientist.com/article/dn6818-cannibalism-may-have-spread-anthrax-in-hippos.html Whales are entirely carnivorous, ranging from zooplankton and krill to other whales, but they’re aquatic, so that’s not a fair comparison. Entelodonts have often been compared to pigs, only huge and stupid, and pigs have proven themselves excellent omnivores. Pigs and peccaries, either feral or wild, have covered every continent but Antarctica.  Entelodonts were much less successful to say the least, but using hippos as an analogy we could assume these beasties using their large sharp teeth to devour anything they could find.

Arctodus, the short faced bear, is closely related to the spectacled bear of South America, a very adaptable and mostly herbivorous species. Closer in dimensions and environment is the brown bear or grizzly, and indeed their teeth are very similar to primates and pigs as well as their diet. Isotope studies on Arctodus have suggested an almost entirely carnivorous diet, while morphological tests have argued for omnivory. Again, their craniodental similarity to brown bears argues for an omnivorous diet.  Brown bears, like pigs, have been extremely successful, and have been found all over each of the northern continents.  Bears have been observed eating any kind of animal or vegetable they could try, using their powerful paws and strong jaws to acquire almost anything they want. Being relatively slow hampers their ability to hunt down prey, so they eat mostly vegetation, but they do kill and eat anything from rodents to bison if they can catch it. The short-faced bear is different in proportions, but it’s very likely it had a very similar lifestyle.

Oviraptorans are much less straightforward. Oviraptoran skulls have large beaks and very developed upper and lower jaws. Oviraptor itself has been found with a lizard in its stomach, while Citipati’s famous nest also contained skulls of Byronosaurus (a Troodont) chicks, possible prey for either mother or brood.  Caudipteryx, on the other hand, an earlier genus, has been found with gastroliths suggesting that it could have easily eaten plants too. Early species have tiny teeth at the front of the mouth (with the exception of the bucktoothed Incisivosaurus), while the late Creteaceous forms like Oviraptor and Citipati lacked teeth entirely. Their jaws could easily crush wood, bone, or shell, or cut through fiber or flesh, using turtles and parrots as examples.  https://www.duo.uio.no/bitstream/handle/10852/11785/Jansenx2008x-xBeakxmorphologyxinxoviraptoridsxxbasedxonxextantxbirdsxandxturtlesxxCompressedxpicturesx.pdf?sequence=1 Most parrots are herbivorous, but there are several omnivorous species, while turtles vary in omnivory from the mostly herbivorous green sea turtle to the more carnivorous leatherback.  The same is true for Ceratopsians; New Zealand’s kea, as their normal forage is quickly exploited by sheep, have in turn turned to sheep as a food source, using their sharp beaks to “graze” and “browse” on the sleeping sheep, sometimes killing them. http://www.nature.com/nature/journal/v52/n1356/abs/052629b0.html

And that brings us back to Gastornis. There are many modern herbivorous birds of similar proportions http://gwawinapterus.wordpress.com/2013/12/15/herbivorous-flightless-birds-and-their-big-heads/, but that doesn’t make it a shut in case. First, there’s the question of scale-none of the birds mentioned were six feet tall, and proponents of a purely herbivorous Gastornis have never mentioned what local plant species around could have provided the giant fruits and nuts that the overbuilt head could have fed on.  Sylvornis, the gigantic megapode of New Caledonia that was hunted to death by humans, is given as an example, but while insular gigantism applies in this case, it does not in the case of Gastornis. You see, the falling sea level converted the European islands of the Cretaceous into a more recognizable form, while Gastornis’ presence in the uninterrupted forests of Asia and North America are as far as insular gigantism as you can get.  Furthermore, like Gasotrnis, Sylvornis is extinct and its living relatives are comparatively smaller heads and beaks. I’ll be more apt to take the comparison seriously if I was shown the complete skull and skeleton of the bird. The article also mentions Aepyornis and Pachyornis, but the resemblance in skull shape and proportions is extremely fleeting.

Compare: 




And it’s not as if large billed ground birds can’t be carnivores, either. Ground hornbills have proportionally bigger heads than the examples of herbivorous birds used, and they’re strictly carnivores.

So where does this leave us? Well, I’m not an ornithologist, so I can’t think of a suitable analogy for Gastornis, nor am I a palaeobotanist, so I can’t possibly name what plants existed in the early Paleogene. I put the onus on those arguing for a herbivorous lifestyle on the basis on beak morphology, and for those using isotopes, only one of many specimens was analyzed and it was not compared to herbivorous birds, only the carnivorous ones.

Yes, I concede that Gastornis did eat plant material, although with that kind of head, it could have eaten whatever it wanted.  Writers claiming Gastornis was “gentle”, “harmless” are applying ridiculous stereotypes about herbivores, making leaps in logic, and sensationalizing the discovery to get more readers. I don’t blame scientists; I blame journalists, and that’s a rant for another day.

There is a spectrum here between the two extremes-


I’ll end this article with a quote by Sir Arthur Conan Doyle’s character Professor George Edward Challenger-
"It proves," he roared, with a sudden blast of fury, "that you are the damnedest imposter in London--a vile, crawling journalist, who has no more science than he has decency in his composition!"



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