Category Archives: Mammals

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.)

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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

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.

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.

The “Highland Tiger”: A Critically Endangered Subspecies

I’ve already pronounced myself a sucker for wild cats; I’m also part-Scottish (clans Lockhart and Arbuthnot).  So imagine my delight when I learned recently that there exists such a thing as the Scottish wildcat, also called the highland tiger!  Unfortunately, this unique beast is almost extinct.

Wildcat at British Wildlife Centre
A Scottish wildcat, courtesy of Peter Trimming via Wikimedia Commons.

Felis silvestra grampia is a subspecies of the European wildcat restricted only to Scotland.  Though the full species is in no danger of extinction, the highland tiger is.  According to University of Chester biologist Paul O’Donoghue (as quoted in this May 2013 BBC article): “The plight of the wildcat is now so serious that unless urgent and targeted conservation activities take place, its extinction due to hybridisation is a certainty.  Recent estimates suggest that fewer than 100 remain, making it one of the rarest animals in the world.”  Dr. O’Donoghue continued: “Unless decisive action is taken, the wildcat could be declared extinct with the next 12 to 24 months.”

So things are not looking good for the highland tiger.  What will the world lose if this creature goes extinct?  Let’s hear the Scottish Wildcat Association tell it:

Surviving human persecution for five hundred more years than the British wolf and over a thousand more years than the British lynx or bear, they inspired and terrified the same Highland clans that defied the Roman and English empires. … Although wildcats look similar to domestic cats, these are no feral or farm cats run wild; they’re Britain’s only remaining large wild predator and have walked this land for millions of years before mankind arrived or domestic cats appeared. Every inch a cat in every sense of the word the Scottish wildcat epitomises the independent, mysterious and wild spirit of the Highlands like no other creature.

Unsurprisingly, the Scottish Wildcat Association has a rather colorful description of the highland tiger’s appearance:

By appearance the Scottish wildcat resembles a very muscular domestic tabby, the coat is made up of well defined brown and black stripes and usually has a ruffled appearance due to its thickness. The gait is more like that of a big cat and the face and jaw are wider and more heavy set than the domestic cat. Most apparent is the beautiful tail; thick and ringed with perfect bands of black and brown ending in a blunt black tip. … Their body is an evolutionary perfection; eighteen razor sharp retractable claws and rotating wrists for gripping prey and climbing trees, immensely powerful thigh muscles for 30mph sprinting, the ability to fall from the highest pine tree, land on its feet and walk away unscathed, incredible stealth, balance and agility all wrapped in a thick, camoflaged and religiously cleaned coat with one downy layer to keep in the warm and another outer layer to keep out the rain and cold.

Lucky for all of us, some organizations are working to keep the highland tiger alive.  While the Scottish Wildcat Association (the source of the text above) appears to be winding down, check out the organization Highland Tiger.

An Ocelot Loss

I’m a sucker for wild cats, especially those native to North America.  One of the most exciting moments of my life was when my wife and I encountered a margay and jaguar while hiking in a Belizean jungle.  But a moment when my daughter, then not even two years old, tried to coax conversation from an interested ocelot at the Oregon Zoo (“Come on, ocelot!”) has especially endeared that mid-sized cat to me.

Ocelot at the Oregon Zoo (“Come on, ocelot!”) courtesy of Kasey Myers.

I was surprised some years ago to learn that there still seem to be a few wild jaguars in the United States. And I was surprised today to learn that there’s a small ocelot population in Texas too.  Unfortunately, I learned this because one of these ocelots was struck and killed by a car.

National Geographic provides a nice description of ocelots and mentions the Texas population:

Twice the size of the average house cat, the ocelot is a sleek animal with a gorgeous dappled coat. These largely nocturnal cats use keen sight and hearing to hunt rabbits, rodents, iguanas, fish, and frogs. They also take to the trees and stalk monkeys or birds. Unlike many cats, they do not avoid water and can swim well.

Ocelots’ fine fur has made them the target of countless hunters, and in many areas they are quite rare, including Texas, where they are endangered.

It’s a shame that one of these rare and beautiful creatures, already threatened by hunting and habitat loss, should have fallen to a car.

Harvard Museum of Natural History

There’s nothing quite like a visit to a natural history museum to remind you of the wondrous diversity of animal life past and present. Today I took my kids to the Harvard Museum of Natural History, and we marveled together at some of the creatures on display there:

The “terrible claw” of aptly-named Deinonychus:
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The strange forms and bright colors of various insects:
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The possibly unrivaled peculiarity of the platypus among modern animals:
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The dense osteodermal armor of an ancient glyptodont:
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And — what most impressed my two-going-on-three-year-old — the massive, horned skull of Triceratops horridus:
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