Colour-polymorphism in skunks
(writing in progress)
Everyone knows that skunks have warning colouration in the form of dark/pale contrast. However, what is easily overlooked is that they combine this with individual variation in pattern to an extent/degree that makes the colour-polymorphism every bit as remarkable as the aposematism.
To illustrate this I can find no better example than the hooded skunk (Mephitis macroura, https://www.inaturalist.org/taxa/41879-Mephitis-macroura) of Mexico.
The hooded skunk is so variable in appearance that it rivals the remarkable colour-polymorphism of the red fox (Vulpes vulpes, https://www.blackfoxes.co.uk/information.php and https://cdnsciencepub.com/doi/abs/10.1139/z68-083?journalCode=cjz and https://cdnsciencepub.com/doi/10.1139/z68-083).
Indeed, I doubt that there is any better example of colour-polymorphism in any mammal.
This has led me to a new insight: this skunk plays not only with the writing on the road sign (i.e. the precise configuration of tonal contrast signifying warning to any approaching predator), but also with the presence of the road sign in the first place (i.e. some colour-morphs have such ‘faint writing’ on the sign that even the aposematic status is in doubt).
On this basis, I hypothesise a synergistic function in skunks: when a larger predator encounters the skunk, it experiences two emotions simultaneously which amount to something greater than the sum of the two. These two emotions are
- fear (because of the basic principle behind all aposematism: that any animal which flaunts itself so confidently, making no attempt to hide, must surely have a secret weapon), and
- confusion (because skunks not only flaunt themselves, they also disguise their species-identity by adopting a bewildering variety of patterns; and because some individuals of a generally self-advertising species in this case do not even bother to self-advertise by means of obvious tonal contrast).
I reason that any large predator faced with this combination of repulsion (based on suspicion) and attraction (based on curiosity) may experience something approaching a ‘perfect storm’ in its mind, tending to block decisive action. The predator is stupefied by a kind of conflict of interest: it does not know whether to approach (whether to attack or to investigate) or to avoid the bold and unidentifiable apparition. This stalls the predator, giving the skunk time to resolve the confrontation with minimal risk to either party.
The individual variation of colouration in skunks is, indeed, so great that I have quite changed my basic assumption for any future studies of aposematic animals including insects and amphibians. I shall henceforth assume that any animal with warning colouration should also have variable colouration.
In the following photos, I show just how bewildering the appearance of the hooded skunk is. Indeed, no item of literature really does this justice or even summarises it coherently. As a postscript I then show that even the sympatric species of hog-nosed skunk (Conepatus leuconotus), which seems to be a Muellerian mimic of the hooded skunk, has its own variation, albeit of lesser scope. The resulting complex of species and forms must be truly bewildering for any onlooker lacking extensive experience with these mammals.
The following paintings attempt to show two basic colour-morphs of M. macroura. The one at top resembles M. mephitis. The one at bottom resembles Conepatus leuconotus.
https://www.deviantart.com/kailavmp/art/Mephitis-Macroura-Color-Variants-805090626
http://naturalhistory.si.edu/mna/full_image.cfm?image_id=1617
The following sketches attempt to show the same basic dichotomy in the appearance of M. macroura, which as we shall see below is an oversimplification.
https://www.exploringnature.org/graphics/bw_diagram/skunks_hooded_diagram150.jpg
The following shows yet another colour-morph, in which the markings are toned down to the point that one would not classify this animal as aposematic.
http://www.1-costaricalink.com/costa_rica_fauna/wildlife_images/hooded_skunk.jpg
The following repeats the last colour-morph on the right of the painting, and shows yet another colour-morph on the left, in which the pale flank-band is shifted far ventrally, something I’ve never seen in any other species of skunk.
http://www.planet-mammiferes.org/drupal/en/node/38?indice=Mephitis+macroura
The following shows how extremely large the tail of M. macroura is, something that does not really come out in the artwork above.
https://farm2.staticflickr.com/1645/24650720450_8c2c195ddf_b.jpg
The following proves the existence of the colour-morph illustrated in a painting above, in which the main pale feature is a white band on the lower flank.
http://www.saguaro-juniper.com/i_and_i/mammals/stripedNhoodedskunks/09-01-05thomases_hoodedskunk0069.jpg
The following shows the extremely large tail, and this colour-morph combines the extensive white back with a partial version of the white band on the lower flank.
http://www.saguaro-juniper.com/i_and_i/mammals/stripedNhoodedskunks/07-01-01thomases_hoodedskunk0048.jpg
The following shows the same two features, this time both fully expressed. Note the extreme development of the tail, which is white in some colour-morphs and black in others.
https://www.northernjaguarproject.org/wp-content/gallery/other-mammals/hooded-skunk_april-09_los-pavos.jpg
The following proves that the tail can be partly white even in those individuals in which the white on the body is restricted to the lower flanks. Note that this flank-band is far broader and bushier than that painted above.
https://c1.staticflickr.com/6/5217/5502324403_1a33434431_b.jpg
The following again shows a combination of white back with partial flank-band, this time with a puzzlingly small tail.
https://upload.wikimedia.org/wikipedia/commons/thumb/3/3d/Hooded_Skunk_side_%28Gelsenkirchen%29.jpg/800px-Hooded_Skunk_side_%28Gelsenkirchen%29.jpg
To make things even more bewildering, here is a specimen with an irregular pattern on the body, reminiscent of a domestic animal.
http://cdn1.arkive.org/media/5F/5FCBF78B-2570-42D4-8AFC-B501DC2A1B6E/Presentation.Large/hooded-skunk-colour-morph.jpg
The following shows something close to two of the paintings shown above. Again, this pattern hardly qualifies as aposematic because there is too little white to provide much tonal contrast.
https://emammal.si.edu/system/files/styles/panopoly_image_original/private/picture3_0.png?itok=lw3QpMau
I turn now to the species of Conepatus sympatric with M. macroura, namely C. leuconotus (https://www.inaturalist.org/taxa/41877-Conepatus-leuconotus).
This hog-nosed skunk does not show any fundamental variation in its pattern of tonal contrast. However, it does vary individually in a way that I hesitant to call colour-polymorphism because it is too gradational.
The variation is mainly in the extent of the dorsal white. However, once again this must bewilder large predators lacking repeated experience of skunks. And because hog-nosed skunks are particularly nocturnal, such experience must be hard to come by.
By way of lateral thinking in trying to figure out the function of pale flank-banding in Canis adustus, I surveyed the Carnivora for other species possessing flank-banding. When I noticed that certain skunks possess pale bands on their flanks, as part of their obviously aposematic colouration, I thought it would be a simple matter to establish the species concerned.
In fact, this has proved far harder than I expected, because
- the descriptions of all spp. in the literature are poor,
- most or all spp. show colour-polymorphism, and the less frequently occurring colour-morphs tend not to be mentioned in descriptions,
- some of the material in google images is mislabelled,
- there is bewildering overlap in spp.-ranges, and
- two genera are involved and the spp. within each of these are inconsistently marked.
However, I am fairly sure that the most relevant spp. to this topic are the two spp. occurring in the southern half of South America: Conepatus chinga (https://www.inaturalist.org/taxa/925950-Conepatus-chinga) and Conepatus humboldtii (https://en.wikipedia.org/wiki/Humboldt%27s_hog-nosed_skunk).
In both Conepatus chinga and C. humboldtii, the most common colour-morph is one in which there is a white stripe on the upper flank. In this way these two spp. resemble the side-striped jackal – although analogies do not extend much farther than this.
Although several other spp. of skunks may possess features describable as pale flank-bands, these spp. can be disqualified as follows.
Conepatus semistriatus (https://www.inaturalist.org/taxa/925699-Conepatus-semistriatus), which replaces chinga in the northern half of South America as well as in tropical central America, has the white band shifted dorsally from the flank to the back.
Conepatus leuconotus of southern North America seems to have no banded morph at all. The only pattern I’ve found for this species is one in which there is a single white tract on the dorsal surface, extending from the head to the tail. Other spp. of skunks possess something resembling this pattern only as infrequent colour-morphs, but C. leuconutus has specialised on this particular pattern.
Mephitis mephitis resembles C. semistriatus in having the white bands shifted to the upper flanks, where they have limited relevance.
Mephitis macroura has complex colour-polymorphism in which the most frequent morph resembles C. leuconotus, and where there is a white band on the flanks this is merely subsidiary to other white markings.
There are sundry other skunks on Earth but none of them has a pattern relevant to this topic.
Now that I have established that the most relevant skunks are C. chinga and C. humboldtii, what have we learned that might be relevant to Canis adustus?
The answer seems to be ‘not much’, because even in C. chinga and C. humboldtii the main function of the white flank-bands seems to be aposematic – a concept inapplicable to canids.
However, one point does emerge strongly from my analysis of colouration in skunks. This is that all of the American skunks named above show enough individual variation in colouration to be somewhat bewildering to identify. A possible exception is Conepatus leuconotus, a special case because it seems to be part of a kind of Muellerian mimicry with Mephitis macroura)
In the case of C. chinga and C. humboldtii, certain individuals lack the white bands on the flanks altogether, while others have these bands so narrow that the pelage is largely dark. Furthermore, certain individuals have the black fur converted to brown or even fawn, particularly on the back – a conversion which diminishes the aposematic value because it reduces tonal contrast, but at the same time may compensate for this by making the animal harder to identify.
What does all of this mean?
Skunks fall into a different category of colouration from the side-striped jackal, because all of them are aposematic. However, the common theme is that in skunks as well as Canis, the stripe is individually variable to the point of absence.
This means that a common theme emerging in all Carnivora with flank-bands is:
individual variation so great that it confuses any potential predator trying to identify the species.
(This includes even Canis mesomelas, despite the fact that that species usually has a dark, not pale, flank-band.)
Putting this a different way:
All Carnivora with a pale flank-band use this marking as a kind of distraction, whereby species-identification is delayed and any potential predator is likely to be so confused that the banded carnivore can buy some time for escape.
This applies to C. adustus because in that jackal the flank-band is inconsistent among individuals in various ways and at various levels. In the case of the skunks, this scrambling of the search-images in the minds of potential predators is always associated with warning colouration. However, in two spp. of skunks living in a region with limited incidence of large predators (i.e. relatively open vegetation in the southern half of South America) the aposematic aspect of the colouration is somewhat downplayed in favour of the confusion function.
The skunks in question (about 3 kg) are smaller-bodied than the side-striped jackal (10 kg). However, in both cases the pale side-stripe is used as a way of denying any potential predator the benefit of previous experience with the species concerned.
There is, on reflection, another way in which a kind of analogy can be drawn between C. adustus and these skunks: the use of the tail as a showy feature.
When alarmed, both the side-striped jackal and these various skunks can deploy the tail by virtue of its sheer size as well as its dark/pale contrast. In the case of the skunks the function is mainly to make the whole animal seem even more outlandishly self-advertising in the interests of warning colouration. In the case of the side-striped jackal the function is, presumably, more one of target-deflection in any close chase by a larger predator (a tactic usually deployed by foxes rather than Canis).
However, in a sense we can say that in both C. adustus and skunks the bushy, showy tail forms part of the deception. And although the genus Conepatus has a less bushy tail than that of Mephitis, this principle does still apply to the two spp. on which I focus in this Post.
Of hundreds of photos of American skunks of various spp., the following is perhaps the best in showing a particular colour-morph of a particular species which exemplifies the term ‘pale flank-band’. In most skunks the white band on the flanks, even if present, is far less graphic than this, for various reasons.
One might be able to claim that Conepatus chinga is THE pre-eminent example, in all the Carnivora, of a pale flank-band, were it not the case that
- other individuals of the same species do not look like this, and
- the fact that there is a white band on the flanks is less important than the fact of the extreme tonal contrast – in which the exact placement of the pale parts hardly matters as long as maximum visibility and self-advertisement is achieved.
Most taxa of skunks, worldwide, have the white bands/stripes placed more dorsally, and even within this genus, Conepatus, this is true. So this photo, while seeming graphic and unambiguous enough, is actually more like ‘a grain of truth’ than ‘the whole picture’.
Conepatus chinga:
https://c1.staticflickr.com/8/7034/6402633859_432a3b150e_z.jpg
In the following specimen of the same species, the white flank-band has been narrowed to the degree that it has lost most of its value in providing a whole-animal tonal contrast sufficient for aposematism. This relaxation, in some individuals of this southern South American species, can be explained by the relative paucity of large predators in the environment. So, of hundreds of photos of scores of spp. of Carnivora surveyed by me over the last few weeks, this photo is possibly the ‘closest thing’ I’ve ever seen to Canis adustus in another carnivore, in terms of colouration. However, it is a case of ‘so near and yet so far’, because the difference remains that this skunk is a noisy forager (relying on snuffling and fossicking or actually digging), and that it defends itself not mainly by fleeing but mainly by ballistic use of its anal glands. The aposematic colouration may be downplayed here but it remains true that this skunk lacks any countershading, implying that it there is any aspect of its colouration that has been modified make the animal inconspicuous, this is too ambivalent to be sure about. This colour-morph remains aposematic; it’s just not as extremely aposematic as one expects in a skunk.
Conepatus chinga:
https://www.flickr.com/photos/dalsphotography/19818540965/in/photolist-gFniYd-dMFQfh-omGZAo-NZeQUC-EFnLQ7-8Ni59Y-4YSmdW-99ZCYh-4YN5eX-9MQW8e-5C8WPD-5NL7ms-c5vamL-7pWaau-Eztjyt-6XSCUS-9KUwuD-9KUCJc-9MQXfV-4DTKDv-5LLAp1-g9g8bF-5Vft6C-9KUue6-dZGXx4-ddZS8T-sohkbY-5wekzS-kiFdY6-gr3iEE-9mTpP1-5w9ZYn-rAinCB-dMAgUR-82FVYa-g9gx8X-xpzS5h-xw2iRx-tkmfAe-soh9CJ-uwm1FL-soh3Z9-ysKY3Z-uTPJg8-roBGvY-xdPbEo-t3Gqub-wypSXY-t3Grcy-wcicLa
(writing in progress)
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