Subtleties of colouration in Grevy's zebra

(writing in progress)

A basic approach I have followed here is to sort out those features of the colouration that are designed to hide the animal from its predators, and those that are designed to advertise the animal to conspecifics.

Equus grevyi (https://www.gettyimages.com.au/detail/news-photo/kenya-wildlife-environment-zebra-by-helen-vesperini-photo-news-photo/146565397?adppopup=true
and https://photos.willbl.com/explorer/grevys-zebra/#mwl-2477 and https://www.flickr.com/photos/42964440@N08/15764994164 and https://www.flickr.com/photos/42964440@N08/37041005493) has various layers or facets of colouration nested, as it were, within an overall colouration. The stripes have not just one adaptive function but several depending on context, part of the body, illumination, and distance from the observer. Dark/pale contrast will surely be visible to not just human eyes but all the other eyes relevant to the biology of this equid.

Overall, E. grevyi has such narrow stripes, with such intricate and complete striping down to the hooves, that it achieves cryptic colouration by means of what is technically disruptive colouration.

This zebra does not hide in long grass, so it is out of the question that it somehow hides by means of its stripes disrupting the body outline, in the sense that a tiger hides in vegetation. Instead, the overall effect of these narrow stripes is to produce an inconspicuous greyish tone that does indeed, in some views, hide the animal in open vegetation – provided the zebra remains stationary (which is a questionable). So it is indeed possible that most of the stripes on this species of zebra serve to hide the animal. However, my point is that this is not by camouflage, it is by overall tone.

Another point shown by the following photo is that, although E. grevyi has a large tail with a substantial tassel, the tail is inconspicuous. Indeed, it is noteworthy that none of the four species of zebra, and none of the subspecies of plains zebra, have particularly conspicuous tails.

A subtlety about E. grevyi is that the striping is boldest on the neck, a pattern shared with the extinct quagga and shared also in principle with Equus przewalskii, which lacks neck-stripes but has its ground-colour darker on the neck than on the body. However, in this view the boldness of the neck-stripes does not really emerge as a conspicuous feature of E. grevyi.

https://stock.adobe.com/search?filters%5Bcontent_type%3Aphoto%5D=1&filters%5Bcontent_type%3Aillustration%5D=1&filters%5Bcontent_type%3Azip_vector%5D=1&filters%5Bcontent_type%3Avideo%5D=1&filters%5Bcontent_type%3Atemplate%5D=1&filters%5Bcontent_type%3A3d%5D=1&filters%5Bcontent_type%3Aaudio%5D=0&filters%5Binclude_stock_enterprise%5D=0&filters%5Bis_editorial%5D=0&filters%5Bfree_collection%5D=0&filters%5Bcontent_type%3Aimage%5D=1&k=%22grevy%27s+zebra%22&order=relevance&safe_search=1&limit=100&search_page=2&get_facets=0&search_type=pagination&asset_id=403906814

https://www.inaturalist.org/observations/73928153

The following shows that the penis is black, and therefore conspicuous in tone as well as size. However, the point is that usually the male does not show the penis, making it normally hard even to tell male from female in this species, as in all zebras.

http://www.publicdomainpictures.net/pictures/10000/nahled/1-1275995721gA70.jpg

RUMP:

The following shows a clear feature of E. grevyi shared by no other species of zebra although it is shared by the various spp. of Asiatic wild asses: dark/pale contrast on the rump, so obvious that it qualifies as a flag. Grevy’s zebra may be inconspicuous enough in full-body profile but once it turns its back on the predator (its position revealed and the response now being retreat), the zebra advertises itself. It is unlikely that this dark/pale contrast on the rump functions to conceal the animal under any circumstances.

https://www.istockphoto.com/photo/grevys-zebra-equus-grevyi-samburu-park-in-kenya-gm1256989188-368223540

https://www.istockphoto.com/photo/stripy-grevys-zebra-gm147655000-6691970

https://stock.adobe.com/search?k=%22grevy%27s+zebra%22&asset_id=447451709

https://www.gettyimages.com.au/detail/news-photo/kenya-wildlife-environment-zebra-by-helen-vesperini-photo-news-photo/146565371?adppopup=true

https://www.gettyimages.com.au/detail/news-photo/kenya-wildlife-environment-zebra-by-helen-vesperini-photo-news-photo/146565329?adppopup=true

https://www.gettyimages.com.au/detail/news-photo/kenya-wildlife-environment-zebra-by-helen-vesperini-photo-news-photo/146565372?adppopup=true

https://stock.adobe.com/search?k=%22grevy%27s+zebra%22&asset_id=447451675

MANE AND TAIL:

Note also that, although the mane of E. grevyi, is extremely well-developed, there is nothing conspicuous about the colouration of the mane. So the mane and the tail are both surprisingly inconspicuous (in the case of the tail, provided it is not being swished) for such large features.

https://www.dreamstime.com/full-body-side-view-grevy-zebra-walking-bush-samburu-kenya-one-adult-grevy-zebra-walking-samburu-national-image188721775

https://www.dreamstime.com/full-body-side-view-grevy-zebra-walking-bush-samburu-kenya-one-adult-grevy-zebra-walking-samburu-national-image188721775

LOWER FLANKS:

Now we reach a feature that is intriguing in the context of hiding advanced pregnancy, and which is thus important versus predators.

The following photo shows that the striping of E. grevyi, although so painstakingly complete right down to the hooves, just cuts out on the lower flanks, leaving the belly and ventral thorax pure white. One sees a pattern of faint striping on the inner surfaces of the limbs of othe zebras as well as e.g. the striped hyena. Why is there an abrupt end to the stripes in E. grevyi? If one stands back from this photo, the white ventral panels make the animals overall more, not less, conspicuous. This lapse in the striping, a pattern shared with both spp. of mountain zebras but absent in the plains zebra including the extinct quagga.

https://www.inaturalist.org/observations/66641013

The following shows the fallacy of interpreting this stripeless ventral area as a function of countershading: the white area extends so high on the anterior flanks that there is a well-lit triangle, above the body-shadow and posterior to the shoulder, that appears almost like a white flag. Were the stripeless ventral area really just for countershading, then the border between the stripes and the stripeless white tract would follow the shadow-line as seen in this photo, not so?

https://www.freepik.com/premium-photo/grevy-zebra-is-grazing-countryside-samburu-kenya_10093409.htm#query=grevys%20zebra&position=30&from_view=keyword

https://www.freepik.com/premium-photo/grevy-zebra-is-grazing-countryside-samburu-kenya_9791617.htm#query=grevys%20zebra&position=49&from_view=keyword

What I am suggesting, perhaps for the first time, is that the ventral white area (https://stock.adobe.com/search?filters%5Bcontent_type%3Aphoto%5D=1&filters%5Bcontent_type%3Aillustration%5D=1&filters%5Bcontent_type%3Azip_vector%5D=1&filters%5Bcontent_type%3Avideo%5D=1&filters%5Bcontent_type%3Atemplate%5D=1&filters%5Bcontent_type%3A3d%5D=1&filters%5Bcontent_type%3Aaudio%5D=0&filters%5Binclude_stock_enterprise%5D=0&filters%5Bis_editorial%5D=0&filters%5Bfree_collection%5D=0&filters%5Bcontent_type%3Aimage%5D=1&k=%22grevy%27s+zebra%22&order=relevance&safe_search=1&limit=100&search_page=7&get_facets=0&search_type=pagination&asset_id=95508162) actually compromises rather than conforms with the principle of countershading, but that this is worth it because it contrives to confuse any predator scanning the members of the herd for females in heavy pregnancy. I.e. both typical countershading and this particular pattern of ventral stripelessness tend to make the 3-dimensional flanks/belly of the equid look ‘flat’ or 2-dimensional, but this abrupt cut-off of the stripes actually distorts the belly-bulge even once the zebra has been spotted and is being scrutinised. Countershading works only before the prey animal is spotted by the predator; this particular pattern of stripelessness works after the prey animal is spotted, by retarding the process of assessment of body condition.

http://www.ultimateungulate.com/Images/Equus_grevyi/E_grevyi1.jpg

CHEST:

Another oddity of the striping in E. grevyi is how spaced-out the stripes are on the chest, on an animal with otherwise such extremely narrow stripes. The striping is so bold that it manages to look striking even though it is often in the shade of the neck. This is so incongruous, when the animal is viewed full-frontally, that it actually advertises the zebra rather than concealing it – as a ‘chest-flag’ of sorts. Please note how this flagging effect is complemented by the conspicuousness of the anomalously large (and always well-illuminated) stripeless patch on the muzzle. Together, these features tend to make the anterior pole of the animal conspicuous in much the same way as the rump-flag makes the posterior conspicuous.

MUZZLE:

The following shows that, even in the infant, the chest-flag is obvious. How can one explain this remarkable difference between the boldness of the striping on the chest, and the almost excessive intricacy of the striping on the legs, without invoking some sort of flagging effect? Grevy’s zebra certainly does not have a leg-flagging effect but it may well have a chest-flagging effect, not so?

http://www.zooborns.com/.a/6a010535647bf3970b01347fd80326970c-pi

EAR PINNAE

In E. grevyi, the basic pattern of colouration of the back-of-ear (https://stock.adobe.com/search?filters%5Bcontent_type%3Aphoto%5D=1&filters%5Bcontent_type%3Aillustration%5D=1&filters%5Bcontent_type%3Azip_vector%5D=1&filters%5Bcontent_type%3Avideo%5D=1&filters%5Bcontent_type%3Atemplate%5D=1&filters%5Bcontent_type%3A3d%5D=1&filters%5Bcontent_type%3Aaudio%5D=0&filters%5Binclude_stock_enterprise%5D=0&filters%5Bis_editorial%5D=0&filters%5Bfree_collection%5D=0&filters%5Bcontent_type%3Aimage%5D=1&k=%22grevy%27s+zebra%22&order=relevance&safe_search=1&limit=100&search_page=7&get_facets=0&search_type=pagination&asset_id=372077295) resembles that in the plains zebra E. quagga. In both species, the tip is white, there is a broad simple dark stripe distally, and there is a second (complex) dark stripe more proximally, which tends to be simpler on the medial side and more complex (broken into sub-stripes) on the lateral side. But the point of the following photo is that, unlike most subspecies of the plains zebra, E. grevyi lacks a back-of-ear flag consisting of lapsed striping and a consequent white panel. Instead, the back-of-ear of E. grevyi is more or less fully striped, and any conspicuousness of the ear (when held still) would result more from the sheer size of the ear pinna than from a stripeless white patch on this ear.

http://www.educationalresource.info/animals/wild-animals/grevys-zebra-2.jpg

The following shows the back-of-ear as seen from the front, when the ear is turned backwards. As you can see, the basic pattern is similar to that in Equus quagga. It’s true that the stripes on the ear are so bold as to be conspicuous, but one has to consider that the ears tend to move anyway, thus appearing conspicuous simply on account of their movement. So although there is certainly an element of conspicuousness in the ears of E. grevyi, what is different between E. grevyi and E. quagga is the lack of a stripeless back-of-ear flag in the former species.

http://www.konicaminolta.com/kids/endangered_animals/library/field/img/g-zebra_img01-l.jpg

The following shows that the white tips of the ear pinnae of E. grevyi tend to be conspicuous rather than to hide the animal. As long as the ears are kept still, the front-of-ear in E. quagga, E. zebra and E. hartmannae is remarkably inconspicuous. However, in E. grevyi the front-of-ear is more conspicuous and this is not just because of the sheer size of the ear, it’s also because of this conspicuous white tipping.

http://thumbs.dreamstime.com/z/grevy-zebra-1955866.jpg

Ears are used for facial expression, socially/intraspecifically, in all equids. In the case of E. grevyi this is accentuated by the markings. The following photo shows the full back-of-ear pattern in infant E. grevyi. This pattern is similar to the fully expressed back-of-ear pattern of E. quagga, i.e. in individuals lacking the back-of-ear flag on the lateral side.

http://www.zooborns.com/.a/6a010535647bf3970b0120a7c31aa4970b-800wi

CHESTNUT:

The following shows that the chestnut on the inner foreleg is so small as to be negligible in E. grevyi. Indeed, it is this species that has the smallest chestnut of any living species of equid. The chestnut can be completely disregarded w.r.t. the conspicuous aspects of colouration of E. grevyi in contrast with E. quagga, particularly E. q. quagga. Furthermore, although E. grevyi resembles Asiatic wild asses in its dorsal stripe and rump flag, it could hardly be more different from them in its leg colouration, because not only do Asiatic wild asses lack stripes on the legs, but they also have a prominent chestnut. As a side-note: once again please note how remarkably inconspicuous the tail is in E. grevyi (except of course by virtue of its movement).

https://s3.amazonaws.com/photos.safaribookings.com/library/kenya/xxl/Lewa_Wildlife_Conservancy_008.jpg

(writing in progress)

Publicado el junio 20, 2022 03:07 MAÑANA por

milewski

Subtleties of colouration in Grevy's zebra

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