Puzzles in the adaptive colouration of Sarcophilus

(writing in progress)

See additional white markings in various individuals of Sarcophilus shown in https://www.youtube.com/watch?v=I761Bv-Y-gQ .
The following shows something approaching the maximal expression of pale markings in that otherwise blackish species, the Tasmanian devil: http://farm1.static.flickr.com/125/398925241_aabb266df5_o.jpg.
In this specimen, as you can see, the white ‘collar’ is about as broad as it ever gets in S. harrisii, and extends about as far on to the shoulders as it does in any individual. Furthermore, the corresponding mark on the rump is also about as broad and extensive as I’ve seen on any individual.
Another point, seldom remarked in the literature, is that S. harrisii also has pale around the eyes. However, this is not produced by depigmentation of the pelage. It is produced by the pale skin (flesh-coloured) showing through owing to the extreme sparsity of pelage on and near the eyelids. The same applies to the supraorbital tuft of facial vibrissae, although that is not particularly apparent in this photo.
So which other pale markings could any individual of S. harrisii possibly have, in addition to those seen below?
Well, it’s sometimes stated in the literature that individuals may have a white tip to the tail, but Nick Mooney has corrected that notion by pointing out that in all his extensive experience with the species he has never seen pale tipping on the tail. So I think we can cross that one off the list.
The most important pale marking not expressed in the individual below is something I’ve seen in a few individuals of S. harrisii: an isolated white irregular spot/blotch about mid-flank, at the same level as the white band on the shoulders below. Although the white bands on shoulder and rump are never joined in any individual of S. harrisii, the occasional individual hints at such continuousness by having that white mark at ‘mid-point’ along the flanks.
Several photos show individuals with irregular, untidy flecking of white on the body, almost like a faint ‘echo’ of the spotting seen in Dasyurus maculatus. Although such markings are too faint to seem functional in S. harrisii, the interesting thing about them is that they indicate the close relationship of Sarcophilus to quolls (Dasyurus), and then hint at how the bold white markings, illustrated below, have arisen evolutionarily: from a quoll-like ancestral colouration, most spots were lost but some were retained and expanded.
The trouble with the process I’ve just invoked is that no individual of any species of quoll (Dasyurus) that I’ve ever seen had any kind of tonal contrast on its chest. In Dasyurus, the chest is unremarkably coloured (nondescript rather pale pelage) and lacks either a dark ground-colour or any white spotting/blotching. So although Sarcophilus may have arisen from a spotted, quoll-like ancestor its development of tonal contrast on the chest is novel and significant. And, notwithstanding any resemblance to quolls, it remains true that Sarcophilus is the only marsupial worldwide that has converged evolutionarily with a pattern of colouration that crops up again and again among eutherian carnivores: dark pelage with a whitish insignia on the chest or front-of-neck.
The particularly interesting thing about the individual shown above, w.r.t. my comparisons with Gulo, is that this maximal patterning in Sarcophilus extends the evolutionary convergence. If it’s true that merely having white insignia on the chest or front-of-neck shows a general evolutionary convergence in colouration with various bears, mustelids, and even canids, then the extension of the pale areas along the side, to the rump, shows convergence with a DIFFERENT pattern, found unusually in Gulo, in which – quite independent of the pale insignia on the chest and front-of-neck – there is a pale ‘banding’ along the flank, from the shoulders to the root of the tail.
So, the individual above shows two distinct convergences evolutionarily. The pale on its chest shows convergence not only with Gulo but with many other Carnivora including particularly bears. But the pale on its shoulders and rump – despite being so contiguous/congruent with the chest in Sarcophilus – shows a quite separate convergence with a different system of pale pelage in Gulo.
Finally, what about those pale ‘spectacles’ around the eyes in Sarcophilus? Are they convergent with any pattern seen in eutherian carnivores? That’s a question I’ll have to explore further, but offhand I can’t recall anything similar in Carnivora. The general trend in Carnivora is the ‘opposite’ one: various families show a pattern of masking in which the eyes are obscured. Although the eyes are emphasised in some eutherian carnivores as well as some other marsupials, this is usually by pale patches of pelage at the position of the ‘eyebrow’ as seen in the typical appearance of the black-and-tan form of the kelpie breed of domestic dog. In the way it accentuates its eyes, Sarcophilus strikes me as odd compared with Carnivora, and that may warrant further thought.
Why is it that no bear, mustelid, canid, felid, raccoon, mongoose, civet, etc. etc. seems to have pale accentuation of the eyelids? Or have I overlooked some species that does?

(Never seen a white tail tip in a devil.Nick)
We’ve seen that white insignia on the dark chest are typical of bears and certain genera of mustelids.
It’s interesting that these kind of insignia seem quite absent from all of the melanistic morphs of wild felids.
When it comes to canids, there are few melanistic morphs in the wild. However, one species that does possess such a morph is Vulpes vulpes, at least in North America. This is the so-called ‘silver fox’, which as you know is taxonomically identical to the other, coexisting morphs of the same species but has a colouration similar to that of Sarcophilus.
Now, one of the clear differences between melanistic V. vulpes and Sarcophilus is that the former seems always to have a generous white tip to the blackish tail, whereas a white-tipped tail is rare in Sarcophilus and small even where it does occur.
The purpose of this photo-email is to show an interesting convergence between the melanistic morph of V. vulpes and Sarcophilus: in a small percentage of individuals of the melanistic morph of this species of fox, there is indeed an irregular white patch, an echo of what we’ve seen in the coexisting species Ursus americanus but even more intriguing because it is asymmetrical.
I get the impression that this white marking on the chest is associated with juveniles rather than adults, in melanistic V. vulpes.
The following is my tentative interpretation.
When felids go in for melanism, they tend to blacken the entire pelage, including parts normally conspicuous such as the tail tip (e.g. Panthera pardus) and back-of-ear (e.g. Leptailurus serval). I cannot recall ever seeing a melanistic wild felid with a white patch on its chest. My explanation is that felids are so specialised for crypsis and camouflage that white insignia on the chest would compromise their overall colouration too much; and because they are so solitary there is no considerable social imperative for mutual recognition of individuals by sight.
Bears show a quite different approach: they use overall dark colouration for inconspicuousness, but they retain bold pale insignia on the chest for self-advertisement because they do not stalk prey in the way felids do.
The various other families of Carnivora have various intermediates between these extremes. In the case of canids the whole phenomenon of melanism is minor, and the only dark forms are colour-morphs that function as much to confuse enemy taxa (i.e. to delay species-identification) as to hide the canids from their prey.
In the melanistic morph of V. vulpes, I find it significant that, quite unlike melanistic felids and of course also unlike dark bears (which lack noticeable tails) it is the tail tip that is the main glaringly pale ‘insignia’ – albeit not of much use for individual identification. This tail tip can be raised into view even while approaching a conspecific, making it visible at night. Foxes do not eat socially as Sarcophilus does, and so individual insignia matter little to them. So the occasional occurrence of white patches on the chest of the ‘silver fox’ is interesting in the sense that it represents convergent evolution among families and other higher-order taxa but a minor phenomenon in the scheme of things carnivoran. And I would still like to know whether in foxes these insignia are particularly associated with juveniles.
All the following photos show Vulpes vulpes.







(writing in progress)

Publicado el junio 7, 2022 06:44 MAÑANA por milewski milewski


Some have as much white as a saddleback pig. Again they seem to follow the bell curve. One does get the impressipn from seeing hundreds in scores of catchments that a proportion of white is 'standatd' in some catchments perhaps suggezting bottlenecking has occurred there. Farmers often comment that "when we came here the devils had much more white" etc- much the se observation. I go back to my suggestion that devils' small size allows greater abundance  (per food) and thetefore greater liklihood of surviving bottlenecks. If they were larger there would be less and thetefore more local ectinction. Tje npn territoroal nature also allows them to  move if need be, perhaps another mechanism to reduce chances of local ectinctions (through dying out).Nick 

Publicado por milewski hace alrededor de 2 años

Agregar un comentario

Acceder o Crear una cuenta para agregar comentarios.