ארכיון יומן של מאי 2021

מאי 12, 2021

Bleeze, flag and semet: necessary new words for describing adaptive colouration in ungulates

All of Biology depends on precise use of words. And much of what makes the thousands of photos of ungulates and other mammals on iNaturalist interesting is the patterns of colouration.

However, words to describe adaptive colouration have been so imprecise and inconsistent that it has been hard to write coherently on this topic. Both the anatomical parts and the features of colouration have been misnamed, or named by misleading analogy.

Describing the carpal joint - which is actually homologous with the human wrist - as the 'knee' is a gross example of this. Confusion between the withers and the shoulders is a subtler but equally important case. However, in anatomy at least the correct word already exists in most instances.

When it comes to important aspects of colouration, we may not progress in our thinking until we fill in the gaps in our lexicon. Here, I introduce three new terms: the bleeze, the flag and the semet.

Various ungulates feature a conspicuous pale patch, informally called a 'blaze'. A typical example is the blesbok (Damaliscus pygargus phillipsi), named after the Afrikaans translation of blaze. The metaphor applies because flame can be eye-catchingly pale, lighting things up. However, should we not avoid ambiguity with combustion? I suggest the new term 'bleeze', applicable only to the biological topic of colouration. We would describe the face of the blesbok as featuring a bleeze, not a blaze. An additional advantage is that bleeze can be defined to include conspicuously dark features, such as that on the face of the eastern white-bearded wildebeest (Connochaetes albojubatus) - or the juvenile of the blesbok itself.

The bleeze is, by its nature, large and prominent enough to be conspicuous to scanning predators even when the animal stands still. However, many large mammals feature pale/dark patterns which are easily overlooked until moved or raised. For these dynamic forms of advertisement, I suggest the term 'flag' instead. The most obvious examples are the caudal flags of the many species of ungulates which raise dark/pale tails in alarm or flight. However, in principle the term can also apply to the ears (e.g. impala, Aepyceros melampus), the feet (e.g. bush duiker, Sylvicapra grimmia), or the buttocks (e.g. steenbok, Raphicerus campestris).

The third word, 'semet', is newly derived from the Greek word for sign (think of semaphore or aposematic). This refers to dark/pale features too small-scale to matter for detection by potential predators, but moved in certain ways for social communication within the species. For example, many species of ungulates have subtle markings around the mouth, or at the base of the ears, which help companions to monitor each other's cud-chewing or listening - facilitating the spread of any alarm within seconds. And then there are many social signals involving colouration, used in rivalry and courtship, which are candidates for also being called semets.

פורסם ב מאי 12, 2021 01:23 לפנה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

Introducing and naming cud-chewing displays

Many ungulates deal with predation partly by being gregarious. Even 'solitary' species may rest in sight of a mate or parent when ruminating, i.e. chewing the cud. A 'strength in numbers' is that the group - whether it numbers two or twenty - functions as a unit in cooperative vigilance by virtue of its collective eyes, ears and noses. The moment one individual becomes suspicious and stops chewing to listen up, the apprehension needs to be communicated efficiently across the 'white noise' of the tooth-grinding of group-members.

The point of mutual monitoring of the routine chewing of companions would be to detect any interruption immediately, and to direct attention accordingly.

Ruminants have a problem which humans do not: they spend much of their lives, both night and day, noisily grinding the cud - while at the same time maintaining constant vigilance by hearing (e.g. see https://www.youtube.com/watch?v=E_Uc0kfNvgg).

The visual system of ungulates is extremely sensitive to slight movements. However, it makes adaptive sense that various species would have evolved patterns of colouration around the mouth to accentuate, or 'amplify', the chewing motion, particularly when the light is dim. And that each species would produce its own 'signature' pattern (e.g. see https://www.shutterstock.com/nb/image-photo/cud-chewing-deer-584459 and https://www.shutterstock.com/nb/image-photo/elk-chewing-his-cud-grasslands-82571272 and https://www.shutterstock.com/nb/image-photo/close-young-bull-elk-537907132 and https://www.inaturalist.org/observations/47408137).

Is it possible that, once the naturalist has a search-image along these lines, the diverse patterns of dark and pale on the lips, chin, cheeks and muzzle of various ruminants may start to look less like random scribblings of capricious Nature and more like functional adaptation?

Here are two examples to start with, illustrating the greater kudu (Tragelaphus strepsiceros, see https://www.storyblocks.com/video/stock/a-female-kudu-chews-cud-in-kruger-national-park-south-africa-e_zttsjf2eillivjey) and common eland (Taurotragus oryx, see https://depositphotos.com/58556159/stock-video-common-eland-head.html and https://www.alamy.com/elenantilope-taurotragus-oryx-nrnberg-bayern-deutschland-image69178906.html).

Note the puzzlingly intricate patterns on the faces. The dark muzzle contrasts with the pale mouth and chin, and there are dark or pale markings on the side of the mandible. Why would such patterns have arisen?

In my last Journal Post I proposed the new term 'semet' for patterns of this small scale, functioning at the close range of social communication but not mattering much to the large-scale conspicuousness or inconspicuousness of the figure to scanning predators. Because this particular category of semet concerns movements of the mouth, I propose calling it a 'buccal semet'. We can expect the buccal semets of the greater kudu and common eland to be dark/pale patterns unique to these species.

Of course the idea is still just hypothetical that ruminants are adapted to spot interruptions in the rumination of companions. And any new technical term may sound awkward at first. But could this open a new line of investigation into the subtle messages hidden in our photos of ungulates?

פורסם ב מאי 12, 2021 11:57 לפנה"צ על־ידי milewski milewski | 3 comments | הוספת תגובה

מאי 13, 2021

Introducing conspicuous patterns about the ears of ungulates

One of the puzzles of adaptive colouration in ungulates is the pale patches which occur at the bases of the ears of certain species, including species with otherwise plain colouration. Among the clearest examples of this are the moose (Alces alces) and the sambar deer (Rusa unicolor). As the ears swivel with changes in attention or emotion, the pale/dark contrasts flick in and out of view, potentially helping onlookers to read the mind, as it were, of the animal in question (e.g. see https://www.fruugoaustralia.com/close-up-view-of-a-cow-moose-and-calf-in-the-chugach-mountains-near-anchorage-southcentral-alaska-autumn-posterprint/p-8952313-19287570?language=en&ac=google&gclid=CjwKCAjwmK6IBhBqEiwAocMc8tCf4ZOTKDNT15CgLk-m4Fxhxrew6BnoyEUl8tunhn7Fkt0C7yAv_BoCfL0QAvD_BwE and https://www.wallpaperflare.com/moose-cow-portrait-close-up-profile-wildlife-landscape-wallpaper-aregm and https://www.canstockphoto.com/close-up-of-head-of-wild-female-moose-40347707.html and https://www.dreamstime.com/stock-photo-female-moose-close-up-grazing-bialowieza-poland-image74453735 and https://www.dreamstime.com/stock-photo-close-up-head-wild-female-moose-elk-alces-alces-largest-extant-species-deer-family-image83773415 and https://www.dreamstime.com/stock-photo-close-up-wild-female-moose-elk-head-eye-image67541853 and https://www.dreamstime.com/stock-photo-close-up-young-moose-found-wandering-around-town-north-idaho-image87320353).

Adaptive reasons for distinct patterns of colouration at the base of the ears include: a) group-members are likely to monitor the movements of each other's ears continually, so that when an individual becomes suspicious its companions can easily detect the direction in which it is listening; and b) in those species which use their ears for facial expressions, the posture of the ears can be emphasised by the 'makeup' of the colouration.

Ungulates may pay attention to each other's ears much as we humans pay attention to each other's eyes. The direction of gaze, and the pattern of movement of the eyes, imply where someone is directing his/her attention; we are extremely perceptive of the whites of the eyes because these accentuate even slight movements of the eyeball (see 'cooperative eye hypothesis' in Wikipedia). Ungulates lack a retinal fovea, and their visual system emphasises the scanning of a wide visual field for movement, rather than focussing on any particular object. This, together with the separation of the eyes on the sides of the head, means that ungulates would learn little from observing each other's eyes even if the whites showed. Instead, it makes sense to monitor the movements of the ears, which would make the pale/dark patterns on or at the base of the ears analogous to human eye-whites.

Please see https://www.youtube.com/watch?v=nEpCz_PMUss and https://www.dreamstime.com/canada-wildlife-close-up-wild-female-moose-close-encounter-beautiful-wild-female-moose-hiking-alberta-image168563330 and https://www.dreamstime.com/moose-head-close-up-profile-view-forest-winter-season-exposing-its-head-ears-eyes-mouth-brown-fur-moose-animal-image165345950 and https://www.dreamstime.com/stock-photo-moose-alces-alces-female-adult-feeding-young-spruce-tree-dense-forest-close-up-face-side-image60098607 and https://www.dreamstime.com/moose-head-close-up-profile-view-forest-winter-season-moose-animal-stock-photos-moose-animal-head-close-up-view-image165345955 for views of the pattern of pale and dark on the ears of the moose, a species which otherwise lacks clearly-defined markings. In the case of the sambar deer, the pale markings at the anterior and posterior bases of the ears are shown in https://www.istockphoto.com/photo/sambar-deer-rest-in-the-forest-gm1223158200-359185051, and https://unsplash.com/photos/84N-_6YXM1E and https://unsplash.com/photos/mJ_R9pxXdn4 are also illustrative.

In my last Post I introduced the concept and name of the buccal semet. In the case of the ears we once again have a semet, i.e. a definite pattern with potential for communication, but one too small-scale to matter for the overall conspicuousness/inconspicuousness of the animal to scanning predators. In this case, I propose the term 'auricular semet'.

פורסם ב מאי 13, 2021 07:26 לפנה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

מאי 06, 2021

A realistic approach to subspecies-identification in the springbok

Naturalists interested in the subspecies of the springbok (Antidorcas marsupialis) should read https://www.zobodat.at/pdf/Zeitschrift-Saeugetierkunde_46_0189-0197.pdf. Although now forty years old, Groves (1981) is still the best reference on a topic which will probably never be resolved satisfactorily.

Zoologists failed to document the nominate subspecies. The Karoo springbok (Antidorcas marsupialis marsupialis) occurred in Namaqualand, the Karoo, Eastern Cape, Free State, Gauteng, and the southern part of Northwest Province, with possible extensions into Mpumalanga, Limpopo, and even Kwazulu-Natal Provinces. Despite the extreme abundance of this form, the museums of the world have virtually no study-skins and only a few skulls. The Karoo springbok was, like the quagga (Equus quagga quagga), taken for granted until it was too late; and far fewer specimens were collected scientifically than for the quagga.

We cannot know exactly what subspecies marsupialis looked like in its original, fully wild condition. It is technically extinct because artificial reintroductions failed to discriminate against subspecies hofmeyri of the Northern Cape, and then there has been nearly a century of advertent and inadvertent selective breeding by farmers, partly to promote mutant genotypes. Body size is too adaptable to mean much, and the facial colouration varies individually in all the subspecies.

South Africa has thus lost its national mammal as a subspecific genetic entity, and more unfortunately we can never define what that entity was in the first place. Our best guess, based on a south-north cline from the northeastern Karoo to Angola (Groves 1981), is that the springbok of the Klein Karoo and Eastern Cape had extremely short, smooth horns in the female, and was not as pale as conspecifics in the Kalahari.

Groves found only 33 study-skins of the whole species of the springbok to examine in the museums of the world, and ironically 39% of these came from Angola and Kaokoland, which remain so remote that iNaturalist still features few photos from these areas. More particularly, we have the odd situation that subspecies angolensis is the best-represented of all the subspecies in terms of study-skins (13 of the 33 specimens examined by Groves), but so poorly represented photographically that to this day no photo shows clearly the darkness of colouration which Groves found relative to subspecies hofmeyri south of the Kunene River, in Kaokoland (also surprisingly well-represented, by 7 study-skins).

The false notion that angolensis extends to northern Namibia, including Etosha, seems to have arisen in Cain et al. (2004) Mammalian Species no. 753 (available free on the Web). Although ostensibly an authoritative summary of the springbok, this misportrayed the subspecies in its distribution map.

My suggestion for iNaturalist: for photos from Angola, assume angolensis; for photos from the Namib, northern Namibia, Botswana and the Kgalagadi Transfrontier Park, assume hofmeyri; and for all other locations just identify to species-level because the subspecies-status has been irretrievably compromised.

פורסם ב מאי 06, 2021 08:16 אחה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

מאי 07, 2021

At last, clear photos of the Angolan springbok

Rogerio Ferreira has just kindly supplied a good series of photos of the Angolan springbok (Antidorcas marsupialis angolensis), at https://biodiversidadeangola.com/mammals/scientific/9527.

As it turns out, the darkness of the pygal band and other markings, described by Groves (1981) and used as a partial justification to raise this taxon to full species-status by Groves & Grubb (2011), is not striking. The appearance is, after all, similar to that of subspecies hofmeyri, bearing in mind that even within hofmeyri the specimens from southeastern Botswana were paler than those from Namibia (Groves 1981).

In my view, the possibility that the Angolan springbok is a separate species (Groves & Grubb 2011) has not stood up to scrutiny. And even as a subspecies it is not nearly as distinctive as the black-faced impala (Aepyceros melampus petersi) is from the common impala.

If so, how would this change our overall view of subspecific variation in the springbok?

We can accept that the female horns vary in length along a south-north cline culminating in angolensis. However, clinal variation is not the same as subspeciation. If we assume that the geographical variation in body size and the relative length of the legs is phenotypic as much as genotypic in the springbok, what emerges is a species more noteworthy for its uniformity than for its subspeciation. It seems possible, after all, that the original wild springbok of the Eastern Cape, near Addo, was similar enough (except for the shortness of the female horns) that most naturalists would not have noticed any differences in body size or colouration from the Angolan springbok, 2500 km and 20 degrees of latitude distant.

And what this would mean in turn is that, although Science let the nominate subspecies disappear as a 'pure' entity before it was properly described, Nature may have, as it were, let us off lightly for that lapse.

פורסם ב מאי 07, 2021 01:07 אחה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

מאי 14, 2021

A comparison of the goitred gazelle and the springbok

Two gazelles, the goitred gazelle (Gazella subgutturosa, see https://www.istockphoto.com/video/goitered-gazelle-gm473245265-26192520 and https://www.bbc.com/azeri/azerbaijan-44417549 and https://www.pedalverde.net/azerbaijan) of the Iranian region and the springbok (Antidorcas marsupialis) of South Africa, make for an obvious comparison because they live on similar semi-arid plains in opposite hemispheres. Adults can hardly hide in these exposed environments. And a puzzling difference in adaptive colouration is that the goitred gazelle tends to blend into its surroundings whereas the springbok tends to stand out even at a distance.

What makes the springbok so conspicuous is its boldly dark flank-band offset by white highlights on the face, rump, flanks and upper legs (see https://www.inaturalist.org/observations/85319775 and https://www.inaturalist.org/observations/70140445). In the goitred gazelle the basic pattern is similar but toned down to a semblance of camouflage (see https://commons.wikimedia.org/wiki/File:Ceyran.jpg and https://www.dreamstime.com/goitered-gazelle-jeyran-field-wildlife-nature-reserve-goitered-gazelle-jeyran-field-wildlife-nature-reserve-field-image216752540 and video in https://animalia.bio/goitered-gazelle and https://www.shutterstock.com/da/image-photo/gazella-subgutturosa-goitered-gazelle-1155749653).

Both species greet scanning predators with stotting displays, but these accentuate the differences. The goitred gazelle shows the white buttocks as it flees (see https://www.pond5.com/stock-footage/item/130905132-herd-goitered-gazelles-running-desert-001 and https://www.pond5.com/stock-footage/item/130905144-herd-goitered-gazelles-running-desert-004) and stots by bouncing up and down with its black tail erect (see https://www.istockphoto.com/video/running-goitered-gazelle-gm473245311-26196643 and video in https://discover.hubpages.com/education/Goitered-Gazelle-History-Facts-and-Efforts-to-Recover-It and https://www.diana-hunting.com/game/goitered-gazelle). However, the springbok has an extreme performance, bouncing while blazening an uniquely extended white double-crest which is normally folded invisible on the back, rump and buttocks (see https://www.youtube.com/watch?v=jMIiB9DnRXg and https://www.atlasobscura.com/articles/watch-this-gazelle-spring-across-the-african-savanna-like-a-pogo-stick).

Consider the different regimes of natural predation in the Iranian region and South Africa. Here, the counterintuitive principle is: the more intense the predation, the more conspicuous certain ungulates evolve to be.

In both regions, the natural predators included large cursorial canids (wolf Canis lupus vs painted hunting dog Lycaon pictus) which scanned the groups of gazelles by day to choose the least-fit member before starting a chase of endurance. This preliminary scrutiny means a selective pressure for self-advertisement ('showing off') instead of keeping a low profile.

A crucial difference was that the northern species was the only wild ruminant in most of its habitat, whereas the southern species coexisted with the red hartebeest (Alcelaphus caama), the blesbok (Damaliscus pygargus phillipsi) and/or the black wildebeest (Connochaetes gnou). The greater reliability of prey in South Africa is likely to have maintained a denser population of predators, thus boosting the risk of any particular group of the springbok being encountered by the painted hunting dog. Imagine an evolutionary tipping point, at which the adaptive colouration of adults of the springbok lost any concealing features and became thoroughly revealing.

The best chance for the individual springbok is not to self-efface (either in the landscape or in the group) but to announce itself as vitally as possible, disqualifying itself from being worth chasing. In the case of the goitred gazelle as for most other antelopes, encounters with predators remained infrequent enough that it made sense to bet-hedge rather than committing to conspicuousness. Red hartebeest (see https://www.picfair.com/pics/06662183-red-hartebeest-alcelaphus-buselaphus), blesbok (see https://www.123rf.com/photo_19680505_a-blesbok-antelope-damaliscus-pygargus-south-africa.html?vti=mcypwdomf20ai7hkf7-1-5) and black wildebeest (see https://rangerdiaries.com/diary/black-wildebeest-2/) also have dark/pale features obvious to predators in certain settings, but their overall colouration remains ambivalent. This may be because all are larger and more enduring in flight than the springbok, and thus less vulnerable to the painted hunting dog.

פורסם ב מאי 14, 2021 06:16 לפנה"צ על־ידי milewski milewski | 3 comments | הוספת תגובה

מאי 29, 2021

Surprising variation in infantile colouration in gazelles

All gazelles and other antilopin bovids hide their newborns for at least one week, during which the infant lies as low as possible. Because these antelopes live in relatively open vegetation, concealment of infants depends partly on inconspicuous colouration.

In the gerenuk (Litocranius walleri), the gazelle most likely to hide by means of inconspicuous colouration, it is unsurprising that the infant has virtually identical colouration to the adult. However, those species which have conspicuous colouration in adults vary surprisingly in the patterns of infants. Take, for example, Thomson's gazelle (Eudorcas thomsoni, see https://www.shadowsofafrica.com/thomson-s-gazelle-rufifrons) and the western dama gazelle (Nanger dama mhorr, see https://www.cbd-habitat.com/en/2019/07/02/the-first-reintroduction-project-for-mhorr-gazelle-into-the-wild/). The former has a striking contrast between the dark flank-band and tail and the adjacent pale surfaces, while the latter has conspicuous white on the rump, lower flanks, upper legs and face.

The infant of Thomson's gazelle (see https://www.alamy.com/stock-photo-newborn-baby-grants-gazelle-79192809.html and https://www.flickr.com/photos/davidbygott/7842769892) has colouration similar to that of the adult, the conspicuous markings being suppressed to some extent by posture, body-proportions and the relative length of the fur. By contrast, the infant of the western dama gazelle (see https://www.biolib.cz/en/image/id304240/) is so different that, were it observed in isolation, one could scarcely tell which species - or even genus - it belongs to.

The adults of neither species make much attempt to hide. What differs is that the conspicuous features are already present at birth in Thomson's gazelle whereas they are not in the western dama gazelle. Particularly odd is that the infant of the western dama gazelle lacks most of the pale on the hindquarters, in contrast to Thomson's and other gazelles, in which erectile white fur on the buttocks is precocial. In both species, the tail is most conspicuous in the infant, and frequently erected in excitement. However, the difference is that in the infant of the western dama gazelle the only conspicuous feature on the whole figure is the white mid-section of the tail. White becomes conspicuous on its buttocks only when the infant grows into a juvenile.

פורסם ב מאי 29, 2021 06:45 לפנה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

מאי 21, 2021

The peculiar showiness of the subauricular gland of the pronghorn

The pronghorn (Antilocapra americana) is the only ungulate possessing a subauricular gland, located between the base of the ear and the crook of the throat (see https://www.animalspot.net/wp-content/uploads/2015/05/Pronghorn-Male.jpg). It is also the only ungulate in which the most conspicuous dark feature in the colouration of the whole body corresponds to the location of any gland (see https://www.inaturalist.org/observations/45924357).

The subauricular gland occurs only in the male of the pronghorn, and the dark patch of fur covering this gland is also restricted to the male. The gland has a well-known function in courtship: the male presents the side of the face to the female in an obvious posture (see https://sonoranimages.wordpress.com/2017/09/27/pronghorn-unrequited-love/). The presentation has both an olfactory and a visual component, because the location of the gland - which becomes surprisingly large when in full use - is accentuated by a large patch of blackish in contrast with the white cheek. And this bold pattern, on the side of the face of the male, is the most clear-cut and consistent feature of dark/pale in the entire colouration of the species.

The overall colouration of the front of the pronghorn, in both sexes and at all ages, tends to conceal the animal by disrupting the outline of the neck and head (e.g. see https://www.inaturalist.org/observations/32645239). This is because the front of the neck and the front of the face have a complex series of chevron-like markings (see https://www.inaturalist.org/observations/63215153 and https://www.inaturalist.org/observations/15710472), which distract rather than attracting the eye of the onlooker at the distances relevant to scanning predators. In the female the face becomes hardly more conspicuous when it is turned to profile (see https://www.inaturalist.org/observations/76386666 and https://www.inaturalist.org/observations/34721203). But in the male the facial profile reveals a pattern so bold as to form a flag rather than camouflage (see https://www.inaturalist.org/observations/31304964).

It is unsurprising for a subcutaneous gland to be sexually dimorphic, and to have some visual accentuation. What is surprising is that the subauricular visual accentuation is developed to the point that it is noticeable even at a distance, revealing the animal to predators in what seems like an unnecessary way.

פורסם ב מאי 21, 2021 05:13 אחה"צ על־ידי milewski milewski | 2 comments | הוספת תגובה

מאי 20, 2021

A little-known aspect of the erectile bleeze of the pronghorn

The pronghorn (Antilocapra americana) is a ruminant living in open vegetation. The large patch of whitish on its hindquarters, and broadly whitish flanks, make it conspicuous to scanning predators even at a distance (see https://www.worldwildlife.org/photos/pronghorn-antilocapra-americana--2 and https://allthingsforthewin.com/nature-all-around-us/). Upon alarm the fur on the hindquarters is erected to form a disc-like shape, revealing a white so pure that it seems almost luminous (see https://nhpbs.org/natureworks/pronghorn.htm).

What is not well-known is that the neck also contributes to this display. Under certain conditions, a dark patch appears on the back of the neck in conjunction with the piloerection on the hindquarters. The term 'mane' fails to do justice to the complexity of the structures (e.g. see https://www.inaturalist.org/observations/36998216, and my comment on that photo).

The mane of the pronghorn is unlike that of any other ungulate. Its peculiarities remain to be fully investigated, but three are relevant here. Firstly, it is more erectile than expected in ruminants, and virtually disappears when fully folded (see https://www.flickr.com/photos/33403047@N00/4190974216/ and https://www.alamy.com/stock-photo-pronghorn-antelope-young-antilocapra-americana-south-dakota-usa-twins-32554470.html). Secondly, it is proportionately larger in juveniles and females (see https://www.alamy.com/a-young-pronghorn-antelope-antilocapra-americana-buck-with-mane-and-rump-hair-raised-image212343642.html) than in adult males (see https://twitter.com/ironc0re/status/1188562672490438657). And thirdly, it seems to be activated in two different ways, and their intermediates: vertically erected to a tomahawk-shape (see https://www.inaturalist.org/observations/18386792 and https://gramho.com/media/2582497469476833997) or spread sideways into a broad dark patch.

My impression is that the mane of the pronghorn is displayed in different ways by adult males vs by juveniles/females. In the former, it is vertically erected (to a shape most visible in profile) as a minor component of display in certain postures of masculine rivalry and courtship. In juveniles/females, the circumstances of its vertical erection are poorly understood. Instead, the point of particular interest is that it seems to be spread laterally, to a shape most visible from behind. The posteriolateral bleeze is relevant to predators, and the spread mane adds a dark component for contrast (see https://www.nogalesinternational.com/news/41-pronghorn-antelope-have-new-home-in-santa-cruz-county/article_bec4033e-7ebe-11e3-82db-0019bb2963f4.html). This accentuation is consistent with the fact that stotting behaviour is shown more by juveniles than by adults.

If this interpretation is correct, then I would call the pattern seen in stotting juveniles a bimodal erectile posteriolateral bleeze. What are added by the erections/activations are the extension of white on the rump and haunches, and extensive darkness on the back of the neck.

How can a mane take such diverse forms? Is there a skin-fold on the nape of the pronghorn, similar to that on the rump of the springbok (Antidorcas marsupialis) albeit smaller and less noticeable to naturalists?

פורסם ב מאי 20, 2021 12:54 אחה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

מאי 22, 2021

Why the pronghorn has a striped neck

The colouration of the pronghorn (Antilocapra americana) is strangely incongruous between the hindquarters and the forequarters. On the former, the pattern is simple and identical for both sexes and all individuals. On the latter (see https://www.jasonsavagephotography.com/tag/pronghorn and https://www.inaturalist.org/observations/57653446), there is a complex pattern varying individually and with age and sex.

Furthermore, the horizontal striping on the front of the neck seems incongruous with the habitat. The pronghorn is a gregarious animal of open vegetation, which relies on speed and endurance, rather than hiding, to evade predation. Inasmuch as blending into the grassland remains adaptive in animals subject to inevitable exposure, one might expect the colouration to be plain: a uniform fawn or grey on the forequarters, exemplified by reedbucks (Redunca) and the grey rhebok (Pelea capreolus, see https://animalia.us-east-1.linodeobjects.com/animals/photos/full/original/grey-rhebok-pelea-capreolus-male-.jpg).

Although antelopes and deer can blend into their environments by means of either plain colouration or a complex pattern of stripes and spots, one generally expects the former in open vegetation and the latter in dense vegetation. The colouration on the front of the pronghorn would seem more appropriate in a stalking carnivore (e.g. compare https://depositphotos.com/243807632/stock-photo-close-rare-endangered-ethiopian-wolf.html with https://www.shutterstock.com/ja/image-photo/female-pronghorn-open-meadow-grand-teton-1839926248).

A possible explanation for these incongruities is based on the extreme eyesight of the pronghorn.

Ungulates vary in the size and placement of the eyeballs, with most deer (Cervidae) and cover-dependent bovids having relatively small eyes, placed somewhat forward on the face. Most antelopes of open vegetation, such as gazelles, alcelaphin bovids, and wild sheep and goats, have relatively large eyes, placed on the side of the face with some degree of rear-vision. However, the pronghorn exceeds even the antelopes in its visual specialisation: extremely large eyes, placed so far on the side of the head that they can be clearly seen if the head is viewed from directly behind (see https://www.inaturalist.org/observations/16478652 and photo and discussion in https://forums.bowsite.com/tf/bgforums/thread.cfm?threadid=407206&forum=8).

What this suggests is that, at distances dimming the vision of predators, the pronghorn can still see members of its own species clearly enough to discern the striped pattern on the front of the neck and head, as a kind of bar-code to individual identity. However, this explanation depends on the social system. If, as in the springbok (Antidorcas marsupialis), gregariousness is so indiscriminate that there is no attempt to recognise and monitor the whereabouts of kin and long-term companions, then my explanation would be undermined.

פורסם ב מאי 22, 2021 04:40 לפנה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה