Sunday, April 28, 2013

The Many Meals of Microraptor

As a lot of you will already know by the time of reading this, a recent publication in the journal Evolution demonstrates that the four-winged, tiny dromaeosaur Microraptor was at least occasionally in the habit of ingesting fish. Scales and portions of several disarticulated skeletons of the osteoglossiform Jinanichthys were found fossilized in the specimen's gut. The authors do not discount the possibility that the fish were scavenged, but considering other known habits of the four-winged wonder, it seems at least reasonably likely that it caught the fish itself - and unlike previous known gut contents, it couldn't have taken this meal in the trees (though it's hard to know what the nay-sayers will claim next...).

Xing et al 2013 describes fish remains in the gut of a Microraptor specimen

This newest study is another piece in the puzzle of Microraptor's ecology - a puzzle which is looking more complete as time goes by. With over 300 undescribed specimens, at least three perserved meals (all different), a color study, conflicting scleral ring studies, and a myriad of biomechanical research, we now know a tremendous about this animal (at least for being dead 120 million years).

But can we extract a reasonable approximation of its diet and lifestyle from the available information? To answer that, one option is to look towards modern birds. But first, let's look at what we do know.


O'Conner et al 2011 describes a specimen of Microraptor with the entire skeleton of an enantiornithine bird in its gut. Image by Brian Choo, used with permission

We know that Microraptor ate birds, and unlike the recent piscivory paper, the bird skeleton was found whole and partially-articulated in the dromaeosaur's stomach, indicating that it may have been eaten whole. Dead things are not often scavenged whole, so it seems fairly probable that this was a clear predation event (though it may or may not have occurred in the trees - as many people have since pointed out, predation on arboreal animals does not arboreal make).

We also know, from a 2010 SVP abstract by Larsson et al, that a specimen of Microraptor with evidence of a mammal dinner has also been found. This specimen preserves fragments of bone, including pieces of skull and a whole foot, that most closely resemble the semi-cursorial mammals Eomaia or Sinodelphys.

Taken the dietary information that we know so far, it seems that Microraptor was not particularly specialized to be preying on any one sort of prey animal, but was more of a generalist. Xing et al argue that the reduced serrations and mildly forward-pointing teeth on the anterior dentary indicate some minimal adaptations to a piscivorous lifestyle, but it clearly lacks the more pronounced piscivorous adaptations that we can see on animals like spinosaurids and crocodylians, not to mention a myriad of modern piscivorous birds. Microraptor was probably eating a lot of different things, likely just about anything it could get its teeth on.

As the illustrations above demonstrate, Microraptor is also now known with high probability to be a uniform iridescent black, possibly with a glossy blue or green sheen. The authors argue that the glossy black coloration was likely used as a method of communication (either social or sexual).

And here's where it gets interesting. Among modern avialans, we can construct a sort of Venn diagram between birds that are glossy black and birds that are generalist predators. And within the center of this diagram, we get birds that have a few pretty universal traits.

1. Sociality: A glossy black coloration is definitely used as a communication device in modern birds, since it tends to be the best way to spot a member of the same species across a wide distance. Many birds that congregate in large flocks tend to be black, and Microraptor is known from an incredible number of specimens - this could be preservation luck, but it seems more like that there really were just a damned lot of the things. This in itself isn't evidence for sociality, but taken together with the rest, it certainly seems pretty plausible.


Microraptor's glossy coloration may have been a signalling device, suggesting a possible degree of sociality. Image by Jason Brougham, used with permission

2. Omnivory: Perhaps unsurprisingly, the vast majority of social birds that are generalist predators are also semi-omnivorous. If there are a damned lot of you, and you're traveling together, you're going to want to learn to eat just about anything that you find. It makes more sense from the perspective of the ecosystem, too: a large group of an animal that eats only a specific type or size of other animals will easily deplete the prey populations when they pass through. Reduced serrations on the teeth (typically indicative of a predatory lifestyle: the serrations help the animal to "hold on" and tear the flesh of prey) are also consistent with a certain degree of omnivory. I'd posit that amongst those 300 undescribed specimens, at least a couple of them might preserve evidence of past meals of seeds or insects.


Omnivorous behavior in Microraptor seems likely in the context of its ecology

3. Intelligence: Yes, I know, the "hyperintelligent raptor" trope is incredibly old and boring, and it's probably obvious that Microraptor is nowhere near as smart as the modern world's smartest birds - some of which are also glossy black omnivores - like corvids. The fact remains, though, that a combination of social behavior and omnivory often results in (or from) an intelligence level that's greater than that of its ancestors. One working theory of how this tends to go is that a population descended from a largely herbivorous ancestor will begin incorporating more meat into its diet to facilitate the protein needed to aid in brain growth. Omnivory also might somewhat result from greater intelligence, as an animal with more smarts will capitalize on its ability to figure out more things to eat.

So, taken these three factors, what's a good modern analogue for the Microraptor?


Photo by Magnus Manske, Wikimedia Commons

Meet... the great-tailed grackle.

Or, rather, grackles in general, and even more generally, icterids as a family. I personally find them to be GREAT analogues to Microraptor for a variety of reasons.

Icteridae, the blackbird family, contains a variety of passerine birds including new world orioles, grackles, meadowlarks, and blackbirds. They have a very interesting variety of plumage: most meadowlarks and orioles are very brightly-colored, often sporting extravagant yellow and orange plumage. Others, like most grackles, are entirely black and very glossy. Looking over the different groups, an interesting pattern emerges: the more brightly-colored icterids, like most orioles and meadowlarks, tend to be largely insectivorous, supplementing their diets with fruit and nectar. This would make sense, since the carotenoids needed to produce colorful plumage are found almost exclusively in fruits and the insects that eat them. Colorful orioles also tend to be largely solitary during most months, forming seasonal pair-bonds and sometimes migrating in loose flocks.


Those of us who've put oranges out for orioles know how much they love fruit, like this hooded oriole with grape jam. Photo by Linda Tanner, Wikimedia Commons

Grackles, on the other hand - and many other black glossy blackbirds - are much more social animals, foraging in sprawling, noisy flocks that utilize large numbers to detect predators, which they often "mob" by way of defense. Grackles are also highly omnivorous birds, and will eat a variety of seeds and plant matter in addition to frogs, minnows, small fish, eggs, and insects. Grackles are also fairly (though not extraordinarily) intelligent, and can mimic a wide variety of sounds and calls.

This correlation between color, sociality and diet in icterids definitely isn't perfect, but it seems far too strong to be a coincidence. And indeed, if you look to other glossy black birds around the world, you'll likewise find a significant degree of omnnivory and social behavior in those animals: corvids and starlings are other prime examples. Among corvids, the fully glossy black ones (like crows and ravens) tend, on average, to be more omnivorous (and also more intelligent) than the brightly-colored ones (like jays).

I do not know with certainty how universal this correlation is, or indeed, whether it's been explored to any significant degree in the literature. Social recognition and all-black coloration in birds, at least, is a fairly well-established link; the link between a partially frugivorous diet and colorful plumage is as well. As far as Microraptor is concerned, though, an interpretation of the animal as a glossy-black, social, semi-intelligent, opportunistic omnivore seems currently well-supported by the available information.


Microraptor was likely to have gone after insects too, at least occasionally.

What this means, ultimately, is that more Microraptor specimens need to be analyzed, hopefully culminating in an eventual Microraptor monograph! There is still so much to be learned from our beautiful little glossy biplane.

---

Larsson, Hans, Hone, David, Dececchi, T. Alexander, Sullivan, Corwin, Xu, Xing. "The winged non-avian dinosaur Microraptor fed on mammals: the implications for the Jehol biota ecosystem" "Program and Abstracts. 70th Anniversary Meeting Society of Vertebrate Paleontology October 2010" 114A.

Jingmai O'Connor, Zhonghe Zhou, and Xing Xu (2011). "Additional specimen of Microraptor provides unique evidence of dinosaurs preying on birds". Proceedings of the National Academy of Sciences of the United States of America 108 (49): 19662–19665.

Lida Xing et al. (2013). "Piscivory in the feathered dinosaur Microraptor". Evolution (early view). doi:10.1111/evo.12119.

Li, Quanguo (9 March 2012), "Reconstruction of Microraptor and the Evolution of Iridescent Plumage", Science 335: 1215–1219, doi:10.1126/science.1213780


6 comments:

  1. Fascinating post! It really is amazing how much we can infer from Microraptor fossils-- I remember just a few years ago it was taken for granted that the color of dinosaurs could never be known. And your illustrations are lovely.

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    1. Thank you kindly. One of the things I'd really like to see done is an additional color study on a different Microraptor specimen, especially one that fits the M. gui morphotype (assuming the color study specimen with the long retrices fits M. zhaoianus), to see if there's any noticeable difference in color.

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  2. "it seems at least reasonably likely that it caught the fish itself"

    It'd be cool if Microraptor actually caught fish as some have speculated about Archaeopterx ( http://2.bp.blogspot.com/-HSfjkvFVd2E/Tob1WdfRDuI/AAAAAAAAA38/UwxkRGA8qF0/s640/bakker+1986.jpg ) ( http://www.wikidino.com/wp-content/uploads/Archaeopteryx-Mark-Hallett-600x778.jpg ). However, I doubt it for 1 major reason (Please let me know if my reasoning is off): After catching a fish, how would Microraptor get out of the water if it couldn't fly up (like modern aerial fishers) w/those unspecialized shoulder joints or swim away (like modern hoatzin chicks) w/those big hind wings?

    "though it's hard to know what the nay-sayers will claim next..."

    Did you make that? I ask b/c I feel like I've seen it b-4.

    "3. Intelligence"

    Sounds like a good time to share what I’ve learned about dino intelligence: Based on what I've read (E.g. See the 1st 2 quotes), 1) Archaeopteryx were probably as intelligent as domestic chickens (& thus, fell "about mid- range on the intelligence scale of birds": http://books.google.com/books?id=Ct4-qGkuC-kC&pg=PA34&dq=%22about+mid-+range+on+the+intelligence+scale+of+birds%22&hl=en&sa=X&ei=eNN9Ub6MC--70QGFkoCgCQ&ved=0CDYQ6AEwAQ#v=onepage&q=%22about%20mid-%20range%20on%20the%20intelligence%20scale%20of%20birds%22&f=false ), & 2) non-avian maniraptorans were probably either as intelligent as or more intelligent than Archaeopteryx (I'm thinking the latter, given the Bakker quote's emphasis on the difference btwn wild & domestic fowl, intelligence-wise).

    "So, taken these three factors, what's a good modern analogue for the Microraptor?"

    While I thought more-or-less the same thing, I've always hesitated picking a modern analogue for Microraptor: I can't get past those big hind wings & how obviously important they must've been to everything Micraptor did (& thus, how different Microraptor was to anything alive today). Am I over-thinking this? In any case, eudromaeosaurs & unenlagiines are easier to compare to modern species: They're basically ground-running acciptrids & herons w/teeth, respectively.

    -JD-man (See my next comment for said quotes)

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    1. I agree with most of what you've said here. Arguing this point in a comment at DeviantArt, I said: "A dromaeosaur analogy to birds has its own set of problems, largely involving the fact that nearly all behaviors and devices employed by modern birds are in the context of the ability for powered flight." Which is also true regarding Microraptor and its hindwings. There is literally nothing equivalent or even all that similar amongst modern birds (we have feather-footed fancy pigeons, but those are human-bred and the "hind wings" have no functional purpose), so it's very hard to speculate how they'd affect things like selection and ecology.

      I think that a chicken-level of intelligence for stem-birds like Archaeopteryx and Microraptor seems quite appropriate, even a little forgiving. Chickens are actually reasonably intelligent and affectionate animals, with a strong social instinct and a distinct hierarchy, yet are nowhere near the level of the real birdbrain geniuses like corvids and psittacids. Any idea where grackles (and passerines on average, in general) fall on the avian intelligence scale? I'm guessing they're a bit smarter than Microraptor would have been, but not by a lot.

      And yes, I did make the fish-in-a-tree thing (though it was Jon's idea and he found the original image) with a Photoshopped version of my own Microraptor. I posted it on the Hell Creek forum too, maybe you saw it there?

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    2. "Any idea where grackles (and passerines on average, in general) fall on the avian intelligence scale? I'm guessing they're a bit smarter than Microraptor would have been, but not by a lot."

      Most of what I've learned about bird intelligence is based on "Avian Brain & Senses" in general ( http://people.eku.edu/ritchisong/birdbrain.html ) & Lefebvre's work in particular ("Bird IQ Index" is based on this paper: http://www.biology.mcgill.ca/faculty/lefebvre/articles/bbe_2004.pdf ). The latter is heavy reading, but Fig. 1 gives a good general idea of the results b/c, as pointed out in the paper, there's a significant correlation btwn innovation rate & relative whole brain/forebrain size. Basically, you were right in that passerids (including grackles) are more intelligent than galliformes (referred to as craciformes in the paper), but less so than corvids.

      "I posted it on the Hell Creek forum too, maybe you saw it there?"

      I actually don't remember where (which annoys me).

      -JD-man

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  3. Quoting Burish et al. (See "Abstract": http://synapse.princeton.edu/burish_wang04_bbe.pdf ): "Vertebrate brains vary tremendously in size, but differ- ences in form are more subtle. To bring out functional contrasts that are independent of absolute size, we have normalized brain component sizes to whole brain vol- ume. The set of such volume fractions is the cerebrotype of a species. Using this approach in mammals we pre- viously identified specific associations between cerebro- type and behavioral specializations. Among primates, cerebrotypes are linked principally to enlargement of the cerebral cortex and are associated with increases in the complexity of social structure. Here we extend this analy- sis to include a second major vertebrate group, the birds. In birds the telencephalic volume fraction is strongly cor- related with social complexity. This correlation accounts for almost half of the observed variation in telencephalic size, more than any other behavioral specialization ex- amined, including the ability to learn song. A prominent exception to this pattern is owls, which are not social but still have very large forebrains. Interpolating the overall correlation for Archaeopteryx, an ancient bird, suggests that its social complexity was likely to have been on a par with modern domesticated chickens. Telencephalic vol- ume fraction outperforms residuals-based measures of brain size at separating birds by social structure. Telen- cephalic volume fraction may be an anatomical sub- strate for social complexity, and perhaps cognitive abili- ty, that can be generalized across a range of vertebrate brains, including dinosaurs."

    Quoting Walsh (See "Reptiles including nonavian dinosaurs and birds": http://www.academia.edu/1061233/Directions_in_Palaeoneurology ): "It is now clear that while some theropod clades pos-sessed typically ‘reptile’-like brains (Giffin et al. 1988; Gif-fin 1989; Rogers 1999; Brochu 2000; Franzosa and Rowe 2005; Sanders and Smith 2005; Witmer and Ridgely 2009), at least some maniraptoran theropods had surpris-ingly bird-like brains (Kundra´t 2007; Balanoff et al. 2009; Norell et al. 2009). In these taxa, not only is the telen-cephalon significantly enlarged (sometimes to an extent greater than in Archaeopteryx), but flight-related regions such as the cerebellar flocculus, which are especially enlarged in birds, are also very well developed (Kundra´t 2007). This has fuelled the debate about whether these taxa, which often bear feathers, are in fact secondarily flightless birds rather than bird-like theropods (Witmer 2009; Kavanau 2010)."

    Quoting Bakker ( http://blog.hmns.org/2012/06/bakker-blogs-the-kleptomania-continues-with-a-sid-vicious-julieraptor-dino-rustlers-part-ii/ ): "These mini-raptors were big-brained by dinosaur standards — as smart as a wild turkey (not the dumbed-down domestic version). Their eyes were huge — an adaptation for chasing nimble prey, like furry mammals and tree-climbing lizards. The extra-long arms and fingers gave the raptors three-dimensional abilities — they could scramble up trees quadrupedally, grabbing branches with claws on front and back paws. Long feathers on the arms and legs let the raptors glide from branch to branch like dino-flying squirrels."

    -JD-man

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