Marine Megafauna on Coursera

I just signed up for Marine Megafauna: An Introduction to Marine Science and Conservation, and maybe you’d like to sign up too.  Marine Megafauna is a mooc (massive open online course) available to everyone, for free, via coursera.org.  The course website promises that by reading papers published in the open-access scientific journal PLOS ONE, you’ll “explore how marine animals have adapted to the challenges of a cold, dark and deep ocean” and learn, among other things, “how penguins keep warm, how blue whales eat and how everything in the ocean – from the biggest creature to the smallest – is connected.”  Sounds good to me.

Marine Megafauna starts on February 3.  Check it out.

(Or if you prefer your megafauna extinct, there’s an available session of Dino 101: Dinosaur Paleobiology, a course I’ve taken, enjoyed, and mentioned here before.  The current session started on January 6, but fear not! — the entire course is open for you to take at your own pace.)

How Polar Bear Fur Works

Polar bears are tremendously impressive — the very definition of charismatic megafauna.  They’re cute and fluffy but also huge and powerful.

Polar bears in Alaska, courtesy of Alan D. Wilson via wikimedia commons.

Of those four factors, though, it’s not cuteness, size, or strength that impresses most: it’s fluffiness.  Fluffy fur is what lets polar bears survive the frigid temperatures of the Arctic.

Just how does fur work to keep polar bears a balmy 37 degrees Celsius in -40 degree weather?  A new paper by physicist Priscilla Simonis and colleagues illuminates the insulating power of polar bear fur.

As a general rule, insulation limits heat transfer.  Perfect insulation for a polar bear, therefore, would mean that heat transfer between warm polar bear skin and the cold Arctic air is zero.  The polar bear stays 37 degrees without cooling down, and the air remains -40 degrees without warming up.

Often it is presumed that fur works as an insulator primarily by trapping pockets of warm air with very low heat conductivity.  (Until now, some version of this explanation is probably what I would have told my not-quite-three year old had she thought to ask how fur keeps animals warm.)  But Dr. Simonis recognized that this model was too simplistic — the low heat conductivity of air couldn’t fully account for keeping polar bears with 5-inch-long fur a full 77 degrees Celsius higher than the -40 degree background temperatures of the arctic.

Conduction is only one of three methods of heat transfer.  The other two are convection and radiation.  With polar bears, there’s probably not much heat transfer occurring via convection.  This is because convection requires air movement and the air beneath a layer of polar bear fur is generally pretty still.  (This is another insulating benefit of fur.)  That leaves radiation.

Dr. Simonis figured out that, in a universe stripped of several possible confounding variables, polar bears would likely suffer about ten times as much heat loss from radiation as they would from conduction.  So for polar bears to survive in the Arctic, their fur must be countering that radiative effect in some very significant way.

Gradually extrapolating from simple to more complicated mathematical models, Dr. Simonis showed that two elements are necessary to create this type of significant insulating effect against heat loss by radiation:  First, she noted that there is a “rapid decrease in heat transfer rate with [an] increasing number of intermediate absorbers.”  In other words, to achieve insulation against radiated heat, there should be lots of objects — individual strands of fur, say — between polar bear skin and the cold, cold air.  Second, for each of these intermediate objects there should be “small absorption with high reflectances.”  What does this mean?  Well, radiative heat loss occurs via infrared radiation (this is the principle that allows for thermal imaging).  And white surfaces reflect all colors of light, including infrared.  Therefore, to best reflect infrared radiation and insulate a polar bear, its fur should be white.

(It helps that polar bear skin is black and quite able to reabsorb infrared radiation reflected by the white fur.)

The takeaway: polar bears’ white fur serves a dual purpose — it camouflages them in the snow, as you already knew, and it traps their radiated body heat.  Or, to use Dr. Simonis’s words, “The structure of polar bear or snow fox fur is actually multifunctional, providing both visual camouflage and good thermal insulation.”  Impressive.

(So impressive, in fact, that Dr. Simonis proposes using what I’ll call the “polar-bear-fur principle” for improving insulation in such high-tech applications as thermal shields for satellites.)

Reference:

P. Simonis, M. Rattal, E. M. Oualim, A. Mouhse, and J. Vigneron (2014) Radiative contribution to the thermal conductance in animal furs and other wooly insulators. Optics Express, Vol. 22, Issue 2, pp. 1940-1951.  doi: http://dx.doi.org/10.1364/OE.22.001940

New Quetzalcoatlus and Therizinosaurus Figures in 2014

I’ve previously written about some bizarre late Cretaceous creatures.  There are azhdarchid pterosaurs like Quetzalcoatlus — huge beasts with wingspans up to 35 feet that could stand up to 20 feet tall when prowling prehistoric prairies on foot and snacking on sauropod hatchlings.  And there are therizinosaurid dinosaurs like Therizinosaurus — long-necked, small-headed, beak-faced, feathered plodders with truly gigantic claws.

These are some of my favorite ancient oddities.  So today I got to revel in my nerdiness when I learned, courtesy of the Dinosaur Toy Blog, that model-maker CollectA will be introducing new figures of Quetzalcoatlus (complete with baby sauropod snack) and Therizinosaurus this year.  On the off chance that this excites you as much as it does me, I include pictures below for your enjoyment:

CollectA 2014 Quetzalcoatlus, courtesy of Dinosaur Toy Blog.

CollectA 2014 Therizinosaurus, courtesy of Dinosaur Toy Blog.

There are other Quetzalcoatlus and Therizinosaurus figures out there, but none (that I’ve seen) captures the research as well as these new ones do.  Sadly, neither figure appears to be available yet in the United States.  However, I believe Quetzalcoatlus is due out imminently and Therizinosaurus should be out by midyear.

A Beast With Two Names

Back on January 11, I read this on twitter:

Ross Barnett @DeepFriedDNA: It seems that when I wasn’t looking, P atrox has been renamed Naegele’s giant jaguar, thanks to a generous $ donation

Hm?  What was that about?

Panthera atrox was a big cat in the fullest sense of those words.  Significantly larger than the modern lion, it roamed what is now the United States until a global deep freeze killed it off around 11,000 years ago.

For various reasons, P. atrox was classified from the outset as part of the lion family.  But for decades, some scientists pushed back against this leonine designation.  They argued that the beast shared too many jaguar affinities to be classified as any other kind of big cat.  In 2009, a paper by Per Christiansen and John Harris claimed to settle the debate.  Comparatively analyzing the skull morphologies of different species (apparently more or less the same way Christiansen studied clouded leopards), the authors boldly stated that “Panthera atrox was no lion.”  Instead, they proposed, “A possible scenario for evolution of P. atrox is that it formed part of a pantherine lineage that … gave rise to the extant jaguar.”  As P. atrox, in these authors’ estimation, was no longer a lion, but rather something jaguar-ish, it was due a new name.  To honor an ancient-mammal enthusiast and donor to the Natural History Museum of L.A. County, P. atrox was christened “Naegele’s giant jaguar” [pdf].

Because P. atrox was originally classified as a lion, however, it is more likely that you know it as the “American lion.”  And over the decades, this leonine designation has largely resisted any jaguar-ish pushback.  In 2009, a paper by Ross Barnett and colleagues claimed to settle the debate [pdf].  Comparatively analyzing the DNA of different extant and extinct big-cat species, the authors dispassionately stated that “all late Pleistocene lion samples produced sequences that grouped strongly with modern lion data, rejecting any postulated link between atrox and jaguar.”  Indeed, they said, P. atrox was a lion — or at least something lion-ish.  If we accept this classification, then P. atrox may keep and proudly bear the “American lion” moniker.

In my world, DNA analysis trumps skull morphology.  “American lion” wins.

But I’m not the final arbiter of such things.  As long as there’s a dispute, poor P. atrox must suffer from an identity crisis.  For now, it’s still a beast with two names.

Note:  For those interested in more detail on the dueling 2009 P. atrox studies and the quite divergent implications of the two different proposed classifications, the estimable Brian Switek has a much longer read on the topic.

A Delawarean Pterosaur: Maybe the Coolest Thing From Delaware Ever

It’s strange now to think how difficult it once was to dig up information.  Growing up, I couldn’t just sit in my family room and browse through scientific papers.  Basic research required a trip to the public library.  Serious research required a trip to the University of Delaware library.  Mostly, though, I relied on a set of c. 1960 World Book Encyclopedias I inherited from my great grandmother.  Now there’s the internet.

This is a long way of saying that there’s a lot about Delaware I can easily learn now that I had essentially no way of knowing then.

For example, I never knew there was a fossiliferous Cretaceous formation accessible about ten miles from the house I grew up in.

And I definitely never knew that pterosaur remains had been recovered from this formation.

Modern rendering of the azhdarchid pterosaur Quetzalcoatlus, courtesy of Witton & Naish 2008.

The Chesapeake and Delaware canal runs across a narrow strip of Delaware and Maryland, connecting the Delaware River to the Chesapeake Bay via the Elk River, a Chesapeake tributary.  In the 1960s and 1970s, the canal’s muddy banks were accessible, and dredged material from the canal was dumped in heaps on the canal’s north side.  The sediments along the banks and in the heaps were rich in fossils (in fact, I just learned that my mom used to go fossil hunting with my grandfather there).

In the early 1970s, a vertebra, humerus, femur, and tibia were recovered from these sediment heaps.  A 1981 paper by Baird and Galton identified these bones as pterosaur remains in part because they were “extremely insubstantial,” with a “paper-thin” outer layer and “pneumatic cavities” inside the bone.  Beyond that, though, the authors declared more specific identification of the specimens “fraught with uncertainty” — their best guess was Pteranodon.

In 1994, Bennett tentatively agreed that the bones were likely Pteranodon: “A short midcervical vertebra of a large pteranodontid (the only large short necked pterosaurs known in the Upper Cretaceous) and other pterosaur fragments are known from the Merchantville Formation (early Campanian) of Delaware (Baird and Galton, 1981). These materials are identified tentatively as Pteranodon.”

But in 2008, Averianov, Arkhangelsky, and Pervushov called the Delaware pterosaur’s identification as Pteranodon into question: “The incomplete cervical vertebra of a pterosaur from the Campanian of Delaware, United States, that was referred to Ornithocheiridae (Baird and Galton, 1981, text-fig. 2), is almost identical to specimen SGU, no. 47/104a and could have been cervical vertebra 3 of an azhdarchid.”  Ornithocheiridae, by the way, is the family that houses Pteranodon.

Now there seems to be an implicit debate: a 2010 paper by Averianov definitively identifies the Delaware remains as those of an azhdarchid, while a 2011 paper by Sullivan and Fowler attributes the remains to the family Ornithocheiridae.  I don’t know enough to take sides, but I’m rooting for an azhdarchid.

Why?

Pteranodon is fine and all — it’s impressive in its way, and certainly iconic.  But the azhdarchids were much more tremendously impressive.  Here’s a quick-hit recap of azhdarchid morphology from Witton and Naish (2008): “All azhdarchids exhibit large skulls …, elongate, cylindrical cervical vertebrae, proportionally short wings …, and elongate hindlimbs. These anatomical features, combined with the large size of some taxa, make azhdarchids one of the most striking and distinctive pterosaur groups.”  Striking and distinctive is perhaps an understatement. Alien-looking azhdarchids could reach approximately 20 feet in height, with  35-foot wingspans.  Wow.

This image just a bout sums up the ornithocheirid versus azhdarchid competition:

Pteranodon, Homo sapiens, and the azhdarchid Hatzegopteryx, courtesy of Mark Witton via Flickr.

And that’s just basic morphology.  Ecology actually bumps azhdarchids up a notch on the impressive scale.

Witton and Naish (2008) have demonstrated convincingly that azhdarchids were probably “terrestrial stalkers,” roaming field and forest on foot while snacking on “small
vertebrates and large invertebrates, possibly supplemented with fruit and carrion.”  Bear in mind that these are 20-foot-tall beasts with beaks as big as entire people — to them, we may well have been snackable “small vertebrates.”

Earlier this year, Averianov apparently challenged Witton and Naish’s terrestrial-stalking hypothesis in part because, he said, azhdarchids would have been vulnerable to predation on the ground — these mighty beasts might themselves have been T. rex snacks.  But Witton and Naish quickly responded [pdf] in another 2013 article,* noting that some azhdarchids were actually taller than the largest theropods and possessed intimidating (or weird) enough features that T. rex and its cousins likely would have sought out easier prey.  In fact, even if a tyrannosaur had targeted a large azhdarchid, the pterosaur might well have been able to hold its own.  Witton and Naish note that “azhdarchid like rostra [beaks] are dangerous weapons in some circumstances,” and they compare azhdarchids to some modern storks known “to repeatedly stab human attackers when provoked.”  (Side note: I’ve seen one of the storks referenced — the Jabiru — and it’s an impressive bird indeed.)  Not to mention that if the azhdarchid didn’t feel like fighting back, it could have used its quadrupedal-launch capabilities to flee rapidly into the air.  Again, wow.

All of which is to say that if the Delwarean pterosaur specimen is in fact an azhdarchid, it may well be the coolest thing from Delaware ever.

* If you don’t want to tackle the paper itself, you can read more about these 2013 azhdarchid papers in this post on Darren Naish’s tetrapod zoology blog.  Given their disagreements with Averianov over azhdarchids, I wonder how Witton and Naish would classify the Delaware pterosaur?

References:

D. Baird and P. M. Galton (1981) Pterosaur Bones from the Upper Cretaceous of Delaware.  J. Vertebr. Paleontol. 1 (1), 67–71.

Bennett, C. (1994) Taxonomy and systematics of the Late Cretaceous pterosaur Pteranodon (Pterosauria, Pterodactyloidea): University of Kansas Museum of Natural, Occasional Papers, n. 169, p. 1-70. [Link]

Averianov, A., Arkhangelsky, M., and Pervushov, E. (2008) A New Late Cretaceous Azhdarchid (Pterosauria, Azhdarchidae) from the Volga Region”. Paleontological Journal 42 (6): 634–642. doi:10.1134/S0031030108060099 [Link]

Witton, M. and Naish, D. (2008) A Reappraisal of Azhdarchid Pterosaur Functional Morphology and Paleoecology. PLoS ONE 3(5): e2271. doi:10.1371/journal.pone.0002271.

Averianov, A. (2010) The osteology of Azhdarcho lancicollis Nessov, 1984 (Pterosauria, Azhdarchidae) from the Late Cretaceous of Uzbekistan. Proceedings of the Zoological Institute RAS 314, 264–317 [Link]

Averianov, A. (2013) Reconstruction of the neck of Azhdarcho lancicollis and lifestyle of azhdarchids (Pterosauria, Azhdarchidae). Paleontological Journal 47:203-209.
doi: http://dx.doi.org/10.1134/S0031030113020020

Witton, M. and Naish, D. (2013) Azhdarchid pterosaurs: water-trawling pelican mimics or “terrestrial stalkers”? Acta Palaeontologica Polonica doi: http://dx.doi.org/10.4202/app.00005.2013 [link]

Clouded Leopards: Modern Semi-Sabertooth Cats

When I read Brian Switek’s excellent longform piece “Once and future cats” — about the history of the sabertooth cats — one passage jumped out at me:

While the future course of evolution is unknowable, there is a possibility that we are only in a short lull between sabercats. Long killing fangs have evolved so many times in the past 20 million years that there’s every reason to believe that a newly derived sabercat might evolve again. In fact, Per Christiansen, a zoologist at the University of Aalborg in Denmark, argued in 2012 that the clouded leopard — a mid-sized cat that prowls the tropical forests of Indonesia — has relatively elongated teeth and shows a great deal of similarity to true sabercats. Given a few million years, might the saber-toothed descendants of today’s clouded leopards slash at the throats of mid-sized herbivores of the future?

There’s something called a “clouded leopard” that’s alive right now but very similar to sabercats?  Intrigued, I did some homework.  Then I kept reading.

Let’s start at the very beginning — what is a clouded leopard?  The name “clouded leopard” actually refers to two “somethings,” separate species of the genus Neofelis.  Both are midsized, southeast Asian cats as Switek says, both are listed as “vulnerable” — a step short of endangered — and both are covered with gorgeous cloud-shaped markings.

But it’s not the markings that really impress — it’s the teeth.

The formidable teeth of the clouded leopard, courtesy of Eric Kilby via flickr.

Modern “big cats” — lions, tigers, jaguars &c. — generally have upper canines that measure less than 20% of the length of their skulls.  But not the clouded leopard.  Neofelis nebulosa has an average ratio of 23%, with some individuals possessing upper canines that measure a full 25% of the length of their skull.  This ratio is an “outlier” among modern cats — an indicator of very long teeth indeed.  It doesn’t approach the famed sabertooth cat Smilodon, whose impressive canines were a full 50% of the length of their skull, but it’s impressive nonetheless.

(As Smilodon enters the conversation, it should be noted that felids are generally broken down into three major “subfamilies”: the true sabertooth cats like Smilodon, all classified as machairodontines, are all extinct; the clouded leopard is a pantherine like many big cats; while housecats and some big cats like mountain lions and cheetahs are felines.  Now back to the narrative.)

The clouded leopard’s impressive teeth led the previously mentioned cat-skull specialist Per Christiansen to publish a paper back in 2006 titled “Sabertooth Characters in the Clouded Leopard (Neofelis nebulosa Griffiths 1821).”  Christiansen found that long teeth weren’t the only morphological outlier among the clouded leopard’s skull dimensions.  For one thing, the clouded leopard’s face slopes back more like Smilodon‘s than a lion’s, allowing for a wider bite.  And that wider bite showed up even more clearly when Christiansen looked at just how far a clouded leopard can open its jaw: he found that the clouded leopard is capable of achieving a “maximum gape” of almost 90 degrees.  This is not only “the largest gape of any extant carnivoran” but also “a value normally considered feasible in extinct sabertooths only.”

In sum, the clouded leopard is not just “divergent and peculiar,” Christiansen said.  Instead, his “analysis demonstrates that several of its peculiar features are actually characters present in, and in some cases considered characteristic of, sabertooth predators exclusively, and thus simply assumed to be absent in extant animals.”

The natural question is why — why does the clouded leopard alone among modern cats possess the peculiar combination of extra-long teeth and extra-wide gape?  It’s hard to know for sure, given how little we know about the ecology of clouded leopards.  Here’s what Christiansen proposed (edited lightly):

[The clouded leopard] is known to feed on a variety of arboreal mammals, such as monkeys and lorises, but also kills much larger prey, such as bearded pigs, hog deer, and muntjak, which either rival or exceed the body mass of Neofelis, demonstrating its ability to subdue large prey. There is one potential difference between the killing mode of Neofelis and other large felids, however. Large felids, such as the puma and the pantherines, often kill small prey with a powerful nape bite, but usually subdue large prey with a suffocating throat bite.  In contrast, available evidence suggests that Neofelis kills even large prey and each other with a powerful nape bite. It may be that its enlarged gape and hypertrophied canines are an adaptation for nape killing of large prey, but this is, at present, speculation.

This “powerful nape bite” has been proposed as the evolutionary driver of “enlarged gape and hypertrophied canines” in past creatures like sabertooth cats.  When competition is fierce, a predator’s ability to kill quickly — for example, by using huge teeth to puncture or tear out a throat as opposed to slowly strangling prey with a vise-like bite — provides a clear advantage.

That said, the clouded leopard does not appear to live in a particularly competitive environment for predators.  Why the long teeth, then?

Well, Christiansen has an answer to that too: “potentially the Neofelis lineage may have evolved a number of primitive sabertooth morphological adaptations soon after the split from the pantherine lineage, but never became specialized owing to a lack of competition from other carnivores in the dense forest habitats.”  In other words, if I have this right, several million years ago the clouded leopard diverged from the other big cats.  At this point, there was intense competition that drove the clouded leopard into rapid sabertooth-ification.  Then, when the clouded leopard came to fill a sufficiently unique ecological niche, competition died down and evolutionary change slowed down.  The clouded leopard simply stayed semi-sabertoothed and carried on to today.

References:

Christiansen P (2006) Sabertooth characters in the clouded leopard (Neofelis nebulosa Griffiths, 1821). J Morphol 267: 1186–1198. doi: 10.1002/jmor.10468.

Christiansen P (2008) Evolution of Skull and Mandible Shape in Cats (Carnivora: Felidae). PLoS ONE 3(7): e2807. doi: 10.1371/journal.pone.0002807.

Christiansen, P., Kitchener, A.C. (2010) A neotype of the clouded leopard (Neofelis nebulosa Griffith 1821). Mammal. Biol. doi: 10.1016/j.mambio.2010.05.002.

King, Leigha M. (2012) Phylogeny of Panthera, Including P. atrox, Based on Cranialmandibular Characters. Electronic Theses and Dissertations. Paper 1444. http://dc.etsu.edu/etd/1444.

Orb-weaving Spiders Eat Plant Pollen (And Their Own Webs)

We all know spiders as crafty predators, building sticky webs to snare unsuspecting insects as well as the occasional frog, mouse, lizard, or bird.  Then, as I learned it, they’ll wrap up their captured prey, inject it with digestive enzymes, and slurp up the resulting product.

Just look at this spider: creepy, and definitely bloodthirsty, right?

Araneus diadematus, courtesy of Frode Inge Helland via wikimedia commons.

Well, that’s what I always thought, anyway.  It turns out, though, that some spiders (like the orb-weaver A. diadematus pictured above, for one) eat more than just other animals.

Herbivory in Spiders: The Importance of Pollen for Orb-Weavers,” an evocatively named article recently published in PLOS ONE, explains that some spiders capture pollen in their webs, digest it externally, and slurp it up to the tune of 25% of their diets.  A side salad with every slab of steak, so to speak.  How genteel.  Anyway, the article shows convincingly that spiders’ pollen consumption is not just a fluke — through an array of experiments, its authors provide “direct proof that orb-weaving spiders (Araneidae) indeed feed on pollen captured in the sticky spirals of their webs and incorporate this into their body tissue, even when prey is available.”

The article is good reading.  In fact, I can’t resist sharing another fascinating detail of spider diets: “Orb-weaving spiders (Araneidae) take down and eat their webs at regular intervals, which enables them to recycle the silk proteins efficiently.”  I am happy I now know this, and I hope you are too.

I’ll close with the article’s abstract and leave it to you to click through and read for yourself if you like:

Orb-weaving spiders (Araneidae) are commonly regarded as generalist insect predators but resources provided by plants such as pollen may be an important dietary supplementation. Their webs snare insect prey, but can also trap aerial plankton like pollen and fungal spores. When recycling their orb webs, the spiders may therefore also feed on adhering pollen grains or fungal spores via extraoral digestion. In this study we measured stable isotope ratios in the bodies of two araneid species (Aculepeira ceropegia and Araneus diadematus), their potential prey and pollen to determine the relative contribution of pollen to their diet. We found that about 25% of juvenile orb-weaving spiders’ diet consisted of pollen, the other 75% of flying insects, mainly small dipterans and hymenopterans. The pollen grains in our study were too large to be taken up accidentally by the spiders and had first to be digested extraorally by enzymes in an active act of consumption. Therefore, pollen can be seen as a substantial component of the spiders’ diet. This finding suggests that these spiders need to be classified as omnivores rather than pure carnivores.

Reference:

Eggs B, Sanders D (2013) Herbivory in Spiders: The Importance of Pollen for Orb-Weavers. PLoS ONE 8(11): e82637. doi:10.1371/journal.pone.0082637