Latitudinal trends in conspicuous colouration in gazelles, with hints of the significance of ultraviolet

@tonyrebelo @jeremygilmore @koenbetjes @tandala @oviscanadensis_connerties @beartracker @paradoxornithidae @ludwig_muller @davidbygott @capracornelius @hawksthree @chewitt1 @maxallen @aguilita

Dear readers, before you read this Post, please consider the following:

• sunlight varies in intensity and type with latitude, potentially affecting the adaptive colouration of animals;
• dim illumination tends to favour the blue side of the spectrum of visible light (probably including ultraviolet); and
• naturalists are disadvantaged, because humans - unlike many other animals - can hardly see ultraviolet.

See https://www.ucl.ac.uk/news/2011/may/reindeer-use-uv-light-survive-wild#:~:text=Professor%20Jeffery%20and%20his%20team,called%20visible%20spectrum%20of%20colours and https://www.nbcnews.com/science/science-news/color-my-world-cats-dogs-may-see-ultraviolet-n33261.

Just like light visible in human eyes, ultraviolet can be reflected or absorbed by pelage, adapted for this function. Reflection can potentially produce conspicuous brightness, whereas absorption can potentially produce conspicuous darkness.

Thus, in theory, a mammal that seems plain brown in human eyes could possibly look pied in the eyes of ungulates and carnivores.

So, could it be that certain animals living at high latitudes are conspicuous in ultraviolet, and that we humans have been oblivious to this?

FOCUSSING ON THE EXAMPLE OF GAZELLES:

Gazelles tend to live gregariously in open environments, where the benefits of group-cohesion tend to outweigh the benefits of hiding from predators.

When threatened, these already conspicuous ruminants tend to 'stot', rather than immediately to flee. Stotting is a display of individual fitness, helping to ensure that predators choose the least-fit for pursuit. Such display is enhanced by conspicuous colouration.

I have coined the term 'camposematism' for the syndrome comprising conspicuous colouration and stotting (in the loose sense).

However, to the human eye the colouration and anti-predator behaviours of gazelles may seem surprisingly inconsistent among genera and species.

Here, I examine the latitudinal trends in dark/pale contrasts in the colouration of a representative sample of gazelles.
 
I have chosen four species based on as much consistency as possible in gregariousness and a tendency to migrate, or to be nomadic over large distances.

The species chosen are

• Eudorcas thomsoni (which is equatorial),
• Gazella subgutturosa (which lives at temperate latitudes, https://en.wikipedia.org/wiki/Goitered_gazelle#/media/File:Gazella_subgutturosa_map.png),
• Procapra gutturosa (which lives on an extensive, cold plateau), and
• Saiga borealis (which was mainly boreal, although represented today only by subspecies mongolica, which lives at similar latitudes to P. gutturosa).

EUDORCAS:

Eudorcas thomsoni is equatorial in occurrence. Its colouration is probably the most vivid - in human eyes - among all the gazelles, conforming to my specifications for camposematism.

https://es.123rf.com/photo_129486201_gacela-de-thomson-se-encuentra-en-la-hierba-mirando-la-c%C3%A1mara.html?vti=m9uk4wck5twnu00wfy-1-14
https://thumbs.dreamstime.com/x/female-thomson-gazelle-8856646.jpg
 
The following (https://images.robertharding.com/preview/RM/RH/HORIZONTAL/817-127938.jpg)
shows the extreme contrast between the dark flank-band and the ventral white. The latter extends so high on the flank that it is likely to catch the light, rather than to act as countershading.

The dark band contrasts on the dorsal side with pale fawn, which is itself in crisp contrast with the dark fawn of the back.

In addition to this conspicuous ‘lateral double-bleeze’, E. thomsoni possesses extreme dark/pale contrast between the long, black tail and the white of the buttocks, which are themselves slightly accentuated by dark borders (http://www.didiphoto.co.uk/OUTnABOUT/AFRICA/WildlifeGallery/Antelopes-S/AntelopesSGallery/photos/Gazelle_Thomsons_F_J_2007_09__Kenya_Mara_2.jpg
and https://www.dreamstime.com/thomsons-gazelle-eudorcas-thomsonii-thomsons-gazelle-eudorcas-thomsonii-single-adult-young-kenya-september-image161979710 and https://www.agefotostock.com/age/en/details-photo/thomson-s-gazelle-eudorcas-gazella-thomsoni-two-females-tanzania/K77-1209860).
 
The following (https://www.istockphoto.com/video/thomsons-gazelles-grazing-gm1163120626-319274294 and https://www.istockphoto.com/video/thomsons-gazelles-in-amboseli-park-kenya-gm473327905-31762028) shows the tail in typical motion. This species compulsively wags its tail (which has little to do with shooing insects), but does not habitually raise it (contrasting with G. subgutturosa, below).

This wagging produces a ‘strobe effect’, even when the animal is not alarmed (https://www.youtube.com/watch?v=jEAyuYOIGnU).

In interim summary:

• There is a total of three dark/pale contrasts on the flank: between the dark fawn and the pale fawn; between the pale fawn and the extremely dark brown; and between the dark brown and the ILLUMINATED white of the lower flanks (which are usually not shaded as the belly is).
• The tail is long and extremely dark. Thus, even when it is not being wagged, it presents extreme dark/pale contrast, with the white bleeze on the buttocks, which is itself edged with dark.

GAZELLA:
  
Gazella subgutturosa occurs in parts of the temperate-zone that are cold enough in winter to warrant seasonal variation in the appearance of the pelage.

https://stonehorsemongolia.com/2020/02/winter-camping-in-mongolias-gobi/dsc01659/
http://isfahan-doe.ir/Dorsapax/Data/Sub_0/File/Gazella%20subgutturosa-Kolahghazi%20National%20Park.jpg
https://www.alamy.com/goitered-gazelle-jeyran-in-field-wildlife-nature-reserve-image419262454.html
https://animalia.bio/goitered-gazelle
https://www.inaturalist.org/observations/77627182
https://www.inaturalist.org/observations/55091268
https://www.inaturalist.org/observations/38097740
https://www.inaturalist.org/observations/32798804
https://www.alamy.com/stock-photo-gazelle-83375938.html?imageid=E8AA7749-5661-4702-9C54-14CB5220ED98&p=258021&pn=1&searchId=1000a8f136721d447e4f61765094ae30&searchtype=0
https://farm8.static.flickr.com/7540/15905445279_aa7300239d.jpg

The following (https://www.alamy.com/gazelle-image211708342.html?imageid=6D21833C-C76B-417D-92FD-A716AB5298A1&p=1760945&pn=1&searchId=5dbe002de2975877518f49e53346538c&searchtype=0) shows that, although the effect is far less vivid, the configuration of colouration is similar in G. subgutturosa to that in E. thomsoni.

The main difference is that the pattern is less vivid in the temperate-zone species. The only stark contrast remaining is that between the blackish tail (which is as long as in E. thomsoni) and the white on the buttocks.
  
The following (http://www.bioacoustica.org/images/gazella_family.jpg and https://www.bing.com/images/search?view=detailV2&ccid=jPlgJrBl&id=935AEA95A80E954A3305E63B8D9442B2E3A5EAA1&thid=OIP.jPlgJrBlGN04SFmFcZYXxQHaF6&mediaurl=https%3a%2f%2fth.bing.com%2fth%2fid%2fR.8cf96026b06518dd38485985719617c5%3frik%3doeql47JClI075g%26riu%3dhttp%253a%252f%252fcokesmithphototravel.com%252fimage%252f66854604.jpg%26ehk%3dha5dj6MZi20x8WPpS8q8lXqFQXZV4n5MJL2x%252b9QIE4Y%253d%26risl%3d%26pid%3dImgRaw%26r%3d0&exph=511&expw=640&q=goitered+gazelle+mongolia&simid=608049889322341696&FORM=IRPRST&ck=7CA20B7712AD093B9AC3816468155E3B&selectedIndex=106&ajaxhist=0&ajaxserp=0) show that, in some cases, the face of G. subgutturosa is conspicuously pale. This is a candidate for a frontal bleeze.

Regardless of season:

• the tail tends to be raised while running (https://www.youtube.com/watch?v=mEY7bgCv2XU), something unusual in E. thomsoni, except in stotting (which is seen mainly in juveniles),
• there is a flank-band, but at its darkest it is merely medium-tome, and
• there is a conspicuous contrast between this dark fawn flank-band and the illuminated white of the lower flanks, and this distinction is extended forward across the base of the foreleg to the junction between shoulder and brisket.

The following (http://www.globaltimes.cn/Portals/0//attachment/2015/2015-12-17/0c027b51-3ea0-44ca-b583-89b8815b6501.jpg) shows the overall conspicuousness in winter, against a wintery background. Gazella subgutturosa, although paler than E. thomsoni, seems to remain dark enough in winter to be conspicuously dark, overall, against snow.

Compared with E. thomsoni, G. subgutturosa has de-emphasised the flanks, but has compensated by means of caudal flagging (erection of the dark tail).

Gazella subgutturosa possibly achieves conspicuous colouration, at a whole-body scale, in winter. This species does not use its paleness to blend into pale surroundings in winter. I hypothesise that, viewed in ultraviolet, the winter coat would appear even darker.
 
The tail (https://www.alamy.com/tail-end-of-goitered-gazelle-gazella-subgutturosa-image211285398.html?imageid=3E9767D6-4D8C-4F98-9F53-518E8619D3F9&p=362779&pn=1&searchId=1000a8f136721d447e4f61765094ae30&searchtype=0) is similar to, but, more prominent than, that of E. thomsoni. This is because

• there are few other similarly dark parts of this animal to draw the viewer’s eye, and
• the tail is raised more in G. subgutturosa than in E. thomsoni.

The following suggests that the tail-tassel is larger in winter coat (https://www.bing.com/images/search?view=detailV2&ccid=UZgemHGz&id=2AACDF525FCAED52A6A75FB4DF18957A9E667E03&thid=OIP.UZgemHGzTxIst2NoNh4K1gHaEK&mediaurl=https%3a%2f%2fstatic.dw.com%2fimage%2f19510401_303.jpg&exph=394&expw=700&q=goitered+gazelle+mongolia&simid=608052036815957188&FORM=IRPRST&ck=B7C1C3129913143C7E070E0501ECE6A6&selectedIndex=14&ajaxhist=0&ajaxserp=0) than in summer coat (https://www.alamy.com/goitered-gazelle-jeyran-in-field-wildlife-nature-reserve-image419262454.html).

In interim summary:

• The colouration of G. subgutturosa is ambivalent, with the only consistently conspicuous features being the buttock-bleeze and caudal flag.
• The reduction of the flank-band, from blackish in E. thomsoni to medium brown in G. subgutturosa, means that no lateral bleeze can be seen in the latter, at least in human eyes.
• If the figure is conspicuous in winter, this may be owing to the absorption, not reflection, of ultraviolet light.

PROCAPRA:
 
We now turn to Procapra gutturosa, a species seldom photographed close-up.

https://www.inaturalist.org/taxa/42371-Procapra-gutturosa/browse_photos
Scroll in https://www.wildlifeworldwide.com/group-tours/mongolia-cats-of-mountain-and-steppe
https://www.senckenberg.de/en/pressemeldungen/walk-of-life-gazelle-travels-more-than-18000-kilometers/
https://www.tnc.org.hk/en-hk/what-we-do/our-priorities/mongolian-gazelles/
https://wwf.panda.org/wwf_news/?350330/Mongolian-gazelle-migration-must-not-be-stopped
https://www.bbc.co.uk/programmes/p00380tj
https://wwf.panda.org/wwf_news/?341130/Mongolian-gazelle-migrate-across-the-border
https://www.shutterstock.com/image-photo/herd-dzeren-mongolian-gazelle-on-green-2116750568
https://www.shutterstock.com/image-photo/herd-dzeren-mongolian-gazelle-on-green-2075537686
https://www.mindenpictures.com/stock-photo-mongolian-gazelle-procapra-gutturosa-herd-grazing-daurian-nature-naturephotography-image90789421.html
https://www.alamy.com/stock-image-goitered-gazelle-gazella-subgutturosa-herd-in-steppe-mongolia-165825518.html?imageid=75AB6325-88DE-4206-9A0D-7226354DCFE8&p=361048&pn=1&searchId=1000a8f136721d447e4f61765094ae30&searchtype=0
https://parody.fandom.com/wiki/Mongolian_Gazelle?file=Mongolian_Gazelle.jpg
https://www.youtube.com/watch?v=XCuUEvbav9U
https://www.tnc.org.hk/en-hk/what-we-do/our-priorities/mongolian-gazelles/
https://donachyblog.files.wordpress.com/2014/11/gazelle-bull-vocalizing-n.jpg
https://www.alamy.com/goitered-gazelle-gazella-subgutturosa-hilleriana-running-herd-in-menengyn-tal-dornod-mongolia-image229887093.html?imageid=4D371F23-9E9D-4111-9D55-8BEBF1FDB933&p=722001&pn=1&searchId=3c49af6cb8a3a05fe8eb462ea3360571&searchtype=0
https://www.sharperplanet.com/p825617885/h432B0F46#h432b0f46
https://www.bing.com/images/search?view=detailV2&ccid=vZmENVfP&id=0B3F996F486C869B3383137994E70F24FE373BE5&thid=OIP.vZmENVfPM01ZlIoSRf3kCwAAAA&mediaurl=https%3a%2f%2fmongolia.wcs.org%2fPortals%2f110%2fddr-goitered-gazelle.jpg%3fver%3d2018-07-29-185814-990&exph=427&expw=320&q=goitered+gazelle+mongolia&simid=608019871803911234&FORM=IRPRST&ck=A0EDB16A8AD0D59055FC2A06893F72F2&selectedIndex=34&ajaxhist=0&ajaxserp=0

Scroll in https://wwf.ru/en/resources/news/amur/nachalas-sezonnaya-migratsiya-dzerena-iz-mongolii-v-rossiyu/
 
The following (http://media.gettyimages.com/videos/mongolian-gazelle-on-steppe-mongolian-steppe-video-id489298185?s=640x640) shows the plainness of P. gutturosa in summer coat, and the shortness of the tail.

There is no apparent flank-banding, and the head is also fairly plain. The pale ventral surface seems consistent with countershading, i.e. crypsis and inconspicuousness.
 
The following (http://media.gettyimages.com/videos/mongolian-gazelle-herd-runs-on-steppe-mongolian-steppe-video-id489299103?s=640x640 and https://alchetron.com/Mongolian-gazelle and http://cdn2.arkive.org/media/EB/EB8A1971-75A0-478F-8F24-34B71EB275EB/Presentation.Large/Herd-of-Mongolian-gazelle.jpg) show the pale feature around the base of the tail in P. gutturosa, in summer coat.

This pale feature, is too faint to qualify as a bleeze, at least to the human eye. However, the following (https://www.shutterstock.com/video/clip-3512501-mongolian-gazelle and https://www.shutterstock.com/video/clip-3512447-mongolian-gazelle) hints that, if viewed by animals sensitive to ultraviolet, it may indeed qualify as a bleeze.
 
The following view of P. gutturosa in winter coat (http://footage.framepool.com/shotimg/qf/591971318-mongolian-gazelle-steppe-siberia-beauty-in-nature.jpg) again shows

• the heart-shape, pale patch on the hindquarters, which does qualify as a bleeze at this season, even in human eyes,
• the lack of noticeably dark pelage on the tail, and
• the difference between the ‘clamped’ tail, which is inconspicuous, and the lifted tail, which allows the dark peri-anal skin to be seen.

Whereas G. subgutturosa seems conspicuous to the human eye in winter, the same does not seem to apply to P. gutturosa.

The main dark/pale contrast turns out to be one hardly apparent at closer range: i.e. that between the relatively pale haunch and the relatively dark back.

This is a ‘hartebeest-like’ effect relatively rare among gazelles, although common in other groups of bovids. Procapra gutturosa may perhaps qualify as camposematic, but this is nuanced and ambivalent to the human eye, and certainly represents a ‘downgrade’ of the conspicuousness seen in G. subgutturosa and particularly in E. thomsoni.

This downgrading does seem to be latitudinal in correlation.
  
The final photo of this species (http://soulbuffalo.com/wp-content/uploads/2016/08/Akos.Hivekovics.WorldwideExpeditions.com_.Trip_.524.12.jpg) again shows that the animals do somewhat stand out from their background.

This is not by virtue of a particular paleness or darkness of the ‘ground colour’, but instead by virtue of the darker/paler contrast between the dark fawn of the back and the pale (and sheeny?) fawn of the haunch, accentuated somewhat by the particularly pale area around the base of the tail (and the dark exposed perianally by the raising of the short tail, a punctuation hardly visible at this range).

If this species qualifies as camposematic, this results not so much from pelage pigmentation but more from

• sheen and anti-sheen effect on the pelage, and

• darkness of the bare, shaded skin of the perianal area.
  This does indeed seem consistent with the trend established by a comparison of E. nasalis with G. subgutturosa, in relation to latitude.

As a ‘rump bleeze’, this is relatively poorly developed, and possibly the least developed of any species of gazelle, other than the two spp. of Saiga.

SAIGA:

Finally, we turn to the seldom-photographed and taxonomically unclear Saiga borealis, which survives only as a small population in what may possibly be at the southerly extreme of the former distribution.

http://www.edgeofexistence.org/species/mongolian-saiga/
https://www.saigaresourcecentre.com/literature/two-international-meetings-conservation-mongolian-gazelle-and-saiga-antelope-ulaanbaatar
https://www.edgeofexistence.org/blog/extraordinary-saiga-is-species-of-the-day/
https://mongolia.wcs.org/wildlife/saiga-antelope.aspx
https://www.bing.com/images/search?view=detailV2&ccid=IzYdD7TF&id=D9684D57FDCDD91718E63A9F4CFBD4726236EF31&thid=OIP.IzYdD7TFfnPFUfXLa0SWcwHaE8&mediaurl=https%3a%2f%2fmontsame.mn%2ffiles%2f5d9edde5f0b73.jpeg&exph=600&expw=900&q=goitered+gazelle+mongolia&simid=608028401604122977&FORM=IRPRST&ck=C7699A40EEF32C9CAB1593FE77A1FA5C&selectedIndex=176&ajaxhist=0&ajaxserp=0

Because there are so few photos of S. borealis, I show also the following of Saiga tatarica:
https://www.dreamstime.com/saiga-tatarica-field-yellow-grass-pasture-image248549384
https://www.zoochat.com/community/media/russian-saiga-antelope-saiga-tatarica-tatarica.344372/
http://images.china.cn/attachement/jpg/site1007/20090605/000cf1a48b7f0b92d39010.jpg
https://www.dw.com/en/saiga-antelopes-bounce-back-after-mass-die-off/a-58140835
https://www.pethealthnetwork.com/news-blogs/a-vets-life/85000-rare-antelopes-perish-a-few-weeks-%E2%80%94-edge-extinction
https://www.restorespecies.org/the-saiga-antelope/
https://news.mongabay.com/2017/02/the-spirit-of-the-steppes-saving-central-asias-saiga/
https://www.bbc.com/news/science-environment-57688320
https://www.nationalgeographic.com/animals/article/saiga-antelope-killed-bacteria-2015-mass-die-off-central-asia-spd
https://www.newsweek.com/sudden-death-saiga-antelopes-tied-killer-bacteria-scientists-cant-explain-784478

Saiga seems too plain-coloured to qualify for any bleeze, in any season, in human eyes. However, it remains possible that, in ultraviolet, it features a dark flank-band, a pale flank-band, or both in combination.

If the latter is the case, what this would mean is that the brown pelage of the flanks is adapted to absorb ultraviolet in one band, and to reflect it in another band, notwithstanding the medium pigmentation visible in human eyes. The hypothetical result might be a lateral bleeze.

A bleeze on the rump and haunches is also possible, on the same basis.

The following (http://d.ibtimes.co.uk/en/full/1589498/saiga-antelopes.jpg) shows that the winter coat, although different from the summer coat to a degree not seen in any of the three above species, remains inconspicuously coloured.

There is a slight pattern on the hindquarters, in human eyes. As in P. gutturosa, the pale patch is more extensive in winter than in summer coat.

An intriguing detail is that the white posterior surface of the upper fore leg is conserved, even in S. borealis.

The following of Saiga tatarica (https://i1.wp.com/www.mammalwatching.com/wp-content/uploads/AAAMammalwatching/Images/Palearctic/Russia/Saiga-herd.jpg?resize=1024%2C297) shows Saiga galloping, in summer coat.

The point of interest is that, despite the plainness of the colouration in this genus, there is indeed some dark/pale contrast on the rump, particularly between the dark midline of the rump and the whitish pelage near the base of the tail.

I get the impression that this display is more prominent in this view of a fleeing herd at some distance than it would be close-up, suggesting some sort of sheen and anti-sheen effect, as opposed to pigmentation and depigmentation alone.

The latitudinal trend from a vividly conspicuous equatorial species of gazelle culminates, in the boreal zone, with an extremely plainly coloured species. However, ultraviolet needs to be investigated.
 
STOTTING
 
As readers know, ‘stotting’ behaviours in mammals aid both the prey species and their cursorial predators. This is because they honestly demonstrate fitness, and help to ensure that the individuals targeted for predation are the least fit in the herd.

The point of this, w.r.t. colouration, is that those species with well-developed ‘stotting’ displays can be expected, logically, also to have generally self-advertising colouration, rather than colouration designed to hide the animals in their normal environments. I.e. there should be a correlation between ‘stotting’ and camposematism.
 
I am familiar with Eudorcas thomsoni from personal experience. Photos on the Web indicate that G. subgutturosa behaves similarly, despite the obvious difference in tail posture during non-stotting locomotion between the two spp. (https://www.youtube.com/watch?v=mEY7bgCv2XU).
  
The following of Saiga tatarica (scroll in https://www.businessinsider.com/climate-change-infection-killing-saiga-antelope-2015-9#saiga-are-grass-eating-antelope-that-have-an-unusual-humped-nose-that-drapes-over-their-mouths-1) suggests ‘stotting’ behaviour in Saiga, in the form of a ‘style trot’.

According to documentary footage that I have watched, the normal commuting gait of Saiga is a pace (not a trot).

The gait shown above is certainly a trot, suggesting to me that Saiga reserves the trot for fitness-display to potential predators. The raised tail supports this interpretation.

I thus hypothesise that Saiga does not stot like a typical gazelle, but instead style-trots to display individual fitness anti-predation. If so, one would expect some sort of dark/pale contrast to accentuate or emphasise this gait. However, such contrast is not apparent to the human eye.

The following of Saiga tatarica (https://i1.wp.com/www.mammalwatching.com/wp-content/uploads/AAAMammalwatching/Images/Palearctic/Russia/Saiga-2.jpg?resize=708%2C269) shows what I take to be the normal pacing gait of Saiga when it is commuting at a run rather than a walk.

However, note the suggestion of raised tails, which is confusing.

Stotting occurs in Procapra gutturosa, at least in juvenile play. The animal bounds relatively high, bounces somewhat, and style-trots, all these gaits being accompanied by the raising of the tail to the horizontal. The shooting behaviour seems generalised and rather low-key.

DISCUSSION:

Please see https://www.jstor.org/stable/24363696 and https://journals.biologists.com/jeb/article/214/12/2014/10274/Arctic-reindeer-extend-their-visual-range-into-the#aff-1.

Could it possibly be that certain ungulates possess camposematic colouration in ultraviolet, largely invisible in human eyes?
 
Several principles to bear in mind:

In the eyes of the animals concerned, dark/pale contrast can be produced by

• pigmentation/depigmentation, which is most effective at relatively close range,
• anti-sheen/sheen effects, which are most effective at a distance, and
• ultraviolet reflection.

Although poorly visible in human eyes, ultraviolet is a potential way of producing conspicuous colouration, including at a whole-body scale. Whether this should be considered a form of paleness, or a form of bright hue (?blue in the eyes of the animals concerned) is moot.
 
Discussing the species in order of geographical latitude:

E. thomsoni

• is perhaps the most graphically marked of all gazelles,
• features lateral and buttock-bleezes, and
• qualifies as camposematic.

G. subgutturosa is has lost the dark flank-band and facial markings of E. thomsoni. However, it retains several facets of dark/pale contrast, making it conspicuous at a whole-body scale, by virtue of

• a buttock-bleeze (and caudal flag), consistently present in all individuals in all seasons
• a tendency towards a pale frontal bleeze in some individuals, and
• an arguably light-catching pale border on the lower flanks.

An intriguing aspect of this species is that, in winter, it seems conspicuously dark, despite being among the paler of Gazella spp.

Both E. thomsoni and G. subgutturosa have caudal flags. However, in the former the tail is wagged across the buttock-bleeze while the animals mill around. In the latter, the tail is raised to the vertical while the animals flee.

Comparing G. subgutturosa with E. thomsoni, there is a shift of emphasis as well as an overall reduction of conspicuousness. This may possibly make sense in terms of the different illuminations at different latitudes. E. thomsoni epitomises the concept of camposematism, but G. subgutturosa may also qualify as camposematic in human eyes, supported by its stotting behaviour.

I have no hypothesis about ultraviolet in the above two spp.
 
Proceeding now to Procapra:
 
P. gutturosa seems remarkably plain for a gazelle. However, it retains a noticeable pattern of dark/pale contrast on the hindquarters (in winter coat), which is two-tiered, as it were:

• a patch of whitish around the tail, plus a small patch of dark on the bare peri-anal skin, exposed when the tail is lifted.
• a pale, somewhat sheeny, fawn patch on the haunch, contrasting somewhat with the darker fawn on the dorsal part of the rump, as well as the upper flanks and back.

In the case of P. gutturosa, I predict that, if viewed in ultraviolet, there is a bleeze in all individuals in all seasons.

This leads me to suspect that P. gutturosa is equally conspicuous in ultraviolet in winter as in summer, and that, in both seasons, there is a two-scale bleeze on the hindquarters.

The lack of a lateral bleeze in P. gutturosa is food for thought, and may perhaps be explained latitudinally.

The conspicuous pattern I hypothesise for P. gutturosa is far less complex than that of E. thomsoni, but not necessarily any the less striking at a whole-body scale.
 
Proceeding now to the last species in our four-species comparison, namely S. borealis:
 
Saiga is intriguing in terms of colouration. It gives an initial impression of plainness, but then proceeds to divulge several nuances, as more photos are scrutinised.

However, my impression is that the human eye, particularly through photography and film, can detect enough of a pattern to lead to a suspicion of a kind of ‘tip of the iceberg’ effect.

I interpret what I have seen as a hint of a pattern which, in ultraviolet, would be vivid.

I predict that biologists will be surprised to see what Saiga borealis looks like in ultraviolet: as conspicuous at a whole-body scale as E. thomsoni, including a flank-band, albeit a ‘pale’ flank-band rather than the three-layered, dark-centred one in E. thomsoni.
 
In summary, my scrutiny of these four spp. has led me to ‘double down’ on my prediction that ultraviolet plays a significant role in the colouration of Saiga and also, to some extent, Procapra.

I have yet to think about sheen and anti-sheen effects clearly in the case of Saiga. However, I see less need to invoke them than to invoke ultraviolet.

And then, of course, there remains the possibility that even just in ultraviolet there may be ultraviolet-sheen and ultraviolet-antisheen effects at play in gazelles.
 
My prediction can be seen this way: with the increase in geographical latitude and therefore the prevailing conditions of illumination, depigmentation visible to the human eye tends to be replaced by depigmentation invisible to the human eye.

I do not rule out the possibility of a role for ultraviolet in E. thomsoni. However, the pattern functions so well, in the wavelengths visible through the human eye-lens, that there seems no need to invoke ultraviolet. This makes sense at the equator, where visible light is intense and the blue side of the spectrum is not particularly prominent.

Posted on August 26, 2022 11:42 PM by milewski