Why are giant pandas colored that way? Answer: It’s complicated

Admit it: you’ve wondered, because you’re interested in evolution, why giant pandas are colored that way: “parti-colored”, as they say in the trade. (Their Latin binomial, Ailuropoda melanoleuca, means “cat-foot, black and white”.)

Here’s one, in the extremely unlikely case you’ve forgotten:

900-496813583-giant-panda

No other bear looks anything like that. A new paper in Behavioral Ecology by Tim Caro et al. attempts to explain this pattern, though access isn’t free. (The reference is at the bottom, and if you need a pdf file, judicious inquiry might yield one.)

As you may recall, Caro and his colleagues are the people who have evidence that zebra stripes evolved to ward off biting flies.

In the new paper, the authors first describe the panda’s color pattern in appealing language, and then lay out the hypotheses that have been advanced to explain that pattern. Remember that pandas are the only completely herbivorous bears, subsist on a diet of low-quality bamboo, and live (at least now) almost entirely in Szechuan Province of China, though they previously ranged more widely. They live at fairly high elevations and often encounter snow. This ecology must be taken into account when framing and testing hypotheses.

The color (all indented text comes from the paper):

. . . a small number of mammals do have sharply contrasting black-and-white pelage, the function of which is known for only a very few (Caro 2009; Caro et al. 2014). Perhaps, the most outstanding example is the coat of the giant panda (Ailuropoda melanoleuca) (e.g., Croke 2005). This strangely patterned ursid has black eye markings and black ears set on a white face. The neck and torso are white but the forelimbs, shoulders, and hindlimbs are black (Figure 1a). The species shows little sexual dichromatism. Such a pelage pattern is very rare among mammals and the adaptive functions of the giant panda’s external appearance remain mysterious, despite the species being a familiar charismatic species to people all over the world.

And the four (actually six) hypotheses (my emphasis):

Four ideas have been explicitly proposed to explain giant panda coloration: 1) the species’ pelage is aposematic and advertises pugnacity (Morris and Morris 1966), 2) the white fur of the panda is cryptic against a snowy background (Morris and Morris 1966; Lazell 1974), although its current population is found in snowy habitats for only a third to a quarter of the year, 3) the dark fur is used to retain heat in cold environments (Schaller et al. 1985), and 4) the contrasting markings on the giant panda’s face are used in intraspecific communication (Schaller et al. 1985; Schaller 1993). Additionally, without referring specifically to the species in question, it has been proposed that dark fur around the eyes reduces glare from the sun (Ficken et al. 1971). Sharply contrasting pelage might also be an example of disruptive coloration where internal edges draw the eye away from the true outline of the animal. A theme running through all these suggestions, except the last, is that different regions of the body may have coloration that serves different functions.

I’ll try to be brief in how the authors tested these hypotheses and what they found. They divided up the panda into 7 sections of coloration as in the figure below (it looks like a butcher’s diagram of “panda cuts”!). They used another method as well based on the pelage of other carnivores (“b” in the figure below), and divided the face and head into 12 areas (“c” in the figure below).

They then assessed the coloration of 195 terrestrial carnivore species in each area, getting a “color map” of each species. (Remember, pandas are in the order Carnivora though they eat only bamboo.)

arx00801

(from paper): (a) Regions of the carnivore body used 1 in Method 1. (b) and (c) Regions of the carnivore body used in Method 2.

Then they did correlations of each of these areas with various aspects of the animal’s ecology: where it lives, whether it encounters snow and/or shade, hair depth and length, annual temperature, and aspects of the animals’ social behavior: whether they were “pugnacious” or had noxious anal secretions (which might explain any “warning coloration”), whether they were nocturnal or diurnal, or crepuscular (active at dawn or twilight), whether they lived alone or in social groups, and whether they were territorial. They then tried to correlate pigmentation with each of these variable.

Which hypotheses stood up? Well, none of them explained everything, and most of them were supported only weakly with the data.

Temperature and shade:  There were significant associations between low temperature and white winter coloration, as well as snow and some areas of white coloration (these variables are cross-correlated, of course), so the authors conclude tentatively that the white areas of the panda camouflage them in the snow against predators (they are eaten by dholes [canids], leopards, wolves, and Asiatic black and brown bears). Animals inhabiting shady areas, like pandas, tended to be more darkly marked on the back and legs, though not the head. From this the authors suggest that the black and white body coloration is cryptic in general, hiding pandas in both shade and snow. They don’t address whether the contrast in color might make them, overall, more conspicuous, but I assume they know what they’re talking about.

Disruptive and aposematic coloration: The analysis gave no evidence that the patterns serve to break up the body outline (read the paper to see how they did this), nor that the panda’s color was aposematic, since they’re neither pugnacious nor have smelly anal secretions.

Sociality and intraspecific communication: The authors suggest that the black patches on the eyes and ears, but no other aspect of coloration, are there for social communication among individuals. They have no direct evidence for this, but use anecdotal observations to support the idea:

Schaller (1993) noted that a stare represents a threat in giant pandas, and the patches enlarge the giant panda’s eye 10-fold making the stare more potent. To show lack of aggressive intent, a giant panda averts its head, covers the eye patches with its paws, or hides it face. At present, we cannot separate whether giant pandas have exaggerated eye marks to signal aggressive intent to other giant pandas, and possibly predators, or whether they are involved in intraspecific recognition, or both. But we do know that intraspecific signaling is important in this species (Nie et al. 2012; Owen et al. 2016).

As dark ear markings are closely associated with dark eye markings across carnivores, the possibility that dark ears are a form of eye automimicry cannot be discounted. Schaller (1993, p.97) reports “a staring panda often holds its neck low, a position that not only presents the eye patches to an opponent but also outlines the black ears against the white neck, in effect presenting 2 pairs of threatening eyes.”

In the end, as shown in the diagram below, the authors propose a multi-part explanation of the parti-colored pattern. Black and white on the main body are there for camouflage to hide the bears from predators, while black eyes and ears are for communication with other pandas. The authors offer the proper caveats, including that these correlations are based on human vision rather than animal vision, and that correlational analysis in other carnivores may not explain the peculiar situation in pandas.

So here’s the best explanation we have yet, but, due to our inability to go into the wild, dye the fur of pandas, and then see what happens to them, this may be the best we can do:

arx00803

(from paper): Working hypotheses for pelage coloration in the giant panda (drawing by Ricky Patel).

Finally, Caro et al. note that all of this might ultimately come from the panda’s low-quality diet, which prevents it from either hibernating or molting, leaving it stuck with a coat that’s on display year round and can’t be shed:

Ultimately, we suggest that the giant panda’s dual coloration stems from its poor nutritional diet of bamboo and inability to digest plant material efficiently (Schaller et al. 1989; Xue et al. 2015), forcing it to be active throughout the year as it cannot lay down sufficient fat reserves to hibernate. Thus, it encounters several backgrounds and lighting conditions during the course of a year, extremities of which are an alpine snowy habitat and dark tropical forest. We propose that as the giant panda is unable to molt sufficiently rapidly to match each background (although anecdotes of individual black bears changing color between molts have been documented [Rogers 1980]), it has evolved a compromise white and black pelage. This is an alternative evolutionary strategy to smaller carnivores like the ermine and arctic fox that have winter and summer coats.

So, my friends, we’ve reached the end of this post. Panda coloration isn’t nearly as well understood as is the striping of zebras (also black and white), but the authors gave it a game try, and they may well be right! And since you’ve read this far, here’s your reward: a video of baby pandas, perhaps the world’s cutest animal.

h/t: Dom

_________

Caro, T., H. Walker, Z.  Rossman, M. Hendrix and T. Stankowich. 2017. Why is the giant panda black and white?. Behav Ecol 2017 arx008. doi: 10.1093/beheco/arx008

32 Comments

  1. Ann German
    Posted March 2, 2017 at 11:42 am | Permalink

    ??? So that we will fall in love with them and want to save them from extinction!

    • Danny Kodicek
      Posted March 2, 2017 at 11:55 am | Permalink

      I wouldn’t be surprised if this joke answer has at least some element of truth! We’ve done so well in wiping out most of the megafauna, at least one of the reasons pandas survived is their cuteness which made them collectible.

  2. ThyroidPlanet
    Posted March 2, 2017 at 11:44 am | Permalink

    I must share this movie trivia: the penguin Skipper in the movie “Madagascar” calls Marty the zebra his “monochromatic friend”. I think this is a sign that an animator wrote the dialogue – I don’t think “monochromatic” can mean two different colors, let alone black and white, I think it’s an artist’s designation.

    Thanks.

    • Posted March 2, 2017 at 12:56 pm | Permalink

      Is black a color, or is it a lack of color?

      A monochromatic video monitor only has one color, as the name implies, but each pixel can have a range of intensities ranging from zero to 100%.

      • Posted March 2, 2017 at 5:53 pm | Permalink

        There is no color in nature, just in our brains. Our perception of the color of an object depends on the wavelengths of light it reflects. A white object reflects all wavelengths and a black object reflects none. So if you define color by light wavelength, a black object has no color because it reflects no light.

        • Posted March 3, 2017 at 11:33 am | Permalink

          But you can also “create black” by using the appropriate contrast on other items around it. (This is how CRT TVs work, IIRC.)

  3. TJR
    Posted March 2, 2017 at 12:10 pm | Permalink

    Interesting stuff.

    I’ve always sort of vaguely assumed that their colouring, and their rubbish diet, are probably just genetic drift. Vegan bears – ridiculous.

    You just want to slap them about the face and shout “You’re bears for Dawkins sake, you can eat anything you damn well like!”.

    • Jeremy Rigsby
      Posted March 2, 2017 at 12:39 pm | Permalink

      They’re not above gobbling up large arthropods or even pikas if opportunity presents, which is apparently almost never. In captivity they are usually fed more animal protein, such as eggs, than they would eat in the wild.

  4. rickflick
    Posted March 2, 2017 at 12:23 pm | Permalink

    I can see why scientists would want to never find all the answers here. They’d prefer to just keep making fun pictures and studying the damn things because their cute. (My sympathy for warthogs just skyrocketed).

  5. busterggi
    Posted March 2, 2017 at 12:46 pm | Permalink

    I expect that pandas are colored like that to prevent them from being attacked by zebras.

    Works every time.

  6. flandestiny
    Posted March 2, 2017 at 12:47 pm | Permalink

    Sorry– don’t have time to read the paper. Lots of these adaptationist stories never test the null. Did they?

  7. Mark Sturtevant
    Posted March 2, 2017 at 12:55 pm | Permalink

    I am surprised that all of the ideas are adaptationist. Why not consider that the coloration is for no adaptive reason at all?

    • Dave
      Posted March 2, 2017 at 1:09 pm | Permalink

      Possibly because of the regularity of their coat pattern? There must be some degree of individual variation among pandas, but I don’t recall ever seeing a photo of one that showed any visible deviation from the classic colour scheme shown in the figure here. If pandas were covered in random black and white patches, with large variations between individuals, it would perhaps be easier to argue that it was just caused by some developmental anomaly that had no selective importance or adaptive significance at all.
      Instead, the fact that they all look more or less identical, with a very rigid and regular positioning of black and white, may suggest that it does serve some adaptive purpose, even if it’s extremely difficult to test any of the various hypotheses.

      • Jeremy Rigsby
        Posted March 2, 2017 at 2:30 pm | Permalink

        There is a recently-established and not-universally-validated subspecies of panda, A. m. qinglingensis IIRC, whose subspecific designation was based on some craniodental stuff and mostly on pelage variation, i.e. they’re apparently a sort of dirt brown colour instead of dark brown/black, and their markings are generally less distinct at the brown/white “borders”.

        Even if the subspecific designation is unwarranted, there would seem to be at least one occurring morph that’s much paler and blurrier than we usually think of in pandas.

      • barn owl
        Posted March 2, 2017 at 6:43 pm | Permalink

        I was trying to think of domestic breeds of mammals that have more regular black and white patterning, and though not quite to the panda degree of precision, there are several breeds that have a somewhat regular white belted pattern. I fell down a PubMed (Dutch belted) rabbit hole investigating this, but genes and chromosomal regions associated with belted coat patterns have been identified in Hampshire swine, belted mice, and Dutch Belted and Belted Galloway cattle. This website on coat color genetics in cattle also has a description of a pattern known as “color sided” or lineback, which according to a 2012 Nature paper results from some chromosome acrobatics that affect the KIT gene:

        http://homepage.usask.ca/~schmutz/CowPatterns.html#Color Sided

  8. Posted March 2, 2017 at 1:07 pm | Permalink

    Although the “species shows little sexual dichromatism”, is it possible that sexual selection plays a part at least in the particular pattern of black and white? If the prime reason is disruptive and/or blending coloration, the pattern we see maybe due to pandas finding it the most attractive.

    Evidence for this may be that their appearance also appeals to us. That is subjective but we also like colorful birds even though they did not set out to impress us potential predators. Perhaps the panda eye (and ear) patches are purely aesthetic, serving the same function as kohl/mascara in humans. Big eyes are more appealing to us and maybe also to pandas.

    Both sexes would exhibit the same feature because dual coloration in the first place is driven by non-sexual selective forces. (If indeed the appeal of the pattern only matters to the female.) Unlike the case of birds where colorful plumage is purely for sexual selection and is not expressed in females as only the more expendable males can risk catching predators’ eyes. (And even then, the plumage is often seasonal or only visible when deployed during courtship to minimise the risk.)

    Incidentally, in appealing unwittingly to humans, pandas have hit the evolutionary jackpot. They probably would be extinct by now but for us, having followed the very dangerous path of adopting such a “poor nutritional diet of bamboo and inability to digest plant material efficiently”. Not for nothing it was chosen as WWF’s logo.

    Which (appeal to other species), finally, raises another question: is it possible pandas use paedomorphism (big head, large eyes, teddy bear proportions, general cuddliness, the coloration even) as a defensive measure?

  9. Taskin
    Posted March 2, 2017 at 1:44 pm | Permalink

    Interesting post, thanks!

  10. kelskye
    Posted March 2, 2017 at 2:33 pm | Permalink

    I suppose they could test the social hypothesis of the eyes (and ears) by reducing or enlarging the area with dye and observing how it changes behaviour. But with an endangered species, would such experiments be unethical?

  11. David Andrews
    Posted March 2, 2017 at 3:47 pm | Permalink

    Geez, Jerry, Pandas look like that because God made them that way! What’s your problem?

  12. Posted March 2, 2017 at 3:49 pm | Permalink

    Reblogged this on The Logical Place.

  13. Charles Morrow
    Posted March 2, 2017 at 6:41 pm | Permalink

    Can you comment on why Pandas are herbivores? I’ve read that they evolved from carnivore ancestors and that their digestive tract is still more suited for a meat diet. If that is true, then why do they exist on a 100% vegetarian diet? And if so, can they digest meat if presented with that opportunity?

  14. Posted March 2, 2017 at 8:38 pm | Permalink

    Such a fascinating paper, qualitatively and methodologically. Thank you, Jerry, for highlighting this! I started working my way through it this afternoon, but it’s going to take a couple days for me to properly scrutinize the methodology. Fun times for me!

  15. jeremy pereira
    Posted March 3, 2017 at 4:30 am | Permalink

    Maybe it’s just so that one panda can easily spot another panda at a distance.

    During and after D-Day it was deemed more important that allied forces be able to spot allied aircraft than that the aircraft be hidden from the enemy, so all the planes were painted with distinctive black and white stripes.

    I’m not suggesting that pandas might be in danger of shooting each other down, but, if the advantage of being able to spot fellow members of your species at a distance outweighed the disadvantage of being easy to spot by predators, wouldn’t you expect bold distinctive patterns to evolve?

    • Dave
      Posted March 3, 2017 at 6:42 am | Permalink

      That’s an interesting idea. Pandas have pretty small eyes, which might indicate that their eyesight isn’t great, so perhaps a bold visual signal is what they need to recognize a conspecific.

      But on the other hand….isn’t their normal habitat dense bamboo forests, in which case there’s very little chance of seeing anything at a distance, or at least not until long after you’ve detected it by scent?

  16. Dominic
    Posted March 3, 2017 at 5:44 am | Permalink

    Complicated indeed 🙂

  17. Posted March 3, 2017 at 11:35 am | Permalink

    I remember reading as a child that pandas were not bears, but raccoons. Was that wrong?

  18. Posted March 3, 2017 at 6:45 pm | Permalink

    Maybe it’s sexual selection? Both sexes just prefer that look.

    What i wanna know about are those thumbs, eh?


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