June 23, 2021

The colouration, or lack thereof, of the European bison

The European bison (Bison bonasus) is not as photogenic as might be expected for its impressive profile. It is disappointingly plain (e.g. see https://www.inaturalist.org/observations/68748464), lacking even the countershading expected in dull-coloured animals (see https://en.wikipedia.org/wiki/Countershading).

Other large bovines have intriguing patterns. For example, the gaur (Bos gaurus) shows a contrast between the blackish body and the pale horns, ears, lower legs, and rhinarium (see https://www.inaturalist.org/photos/79457298). The banteng (Bos javanicus) has whitish buttocks and lower legs. The water buffalo (Bubalus arnee) has pale markings on the ears, lips, neck and lower legs (see https://www.alamy.com/stock-photo-wild-water-buffalo-bubalus-arnee-adult-male-standing-in-grassland-77481628.html).

However, there is one easily overlooked accentuation in the colouration of the European bison. During summer its medium-tone uniformity blends into the forest, but in winter - when the environment pales and even whitens with snow - it actually darkens, particularly on the side of the face. The resulting contrast makes the species relatively photogenic in the cold season (e.g. see https://wallhere.com/en/wallpaper/686687).

This seasonal change brings the European bison towards conformity with a pattern seen throughout the year in several other species of large bovines. Both the Cape buffalo (Syncerus caffer caffer, see https://www.alamy.com/large-herd-of-african-buffalo-syncerus-caffer-grazing-on-the-open-plain-of-serengeti-national-park-tanzania-image226007961.html and https://www.freepik.com/premium-photo/buffalo-herd-within-volcano-ngorongoro-tanzania_11361020.htm#&position=8) and the river buffalo (Bubalus bubalis, see https://culturecheesemag.com/farm-animal/water-buffalo-cheese/) are so dark that they achieve conspicuousness in typical grassy environments; presumably this reflects the nature of the extinct ancestor of the river buffalo. The same applies to the plains bison (Bison bison bison), the dark, massive head of which stands out from grassland whether green or frost-bleached (see https://wp.nathabblog.com/wp-content/uploads/2012/04/Colin-McNulty-YellowstoneBison1_Web.jpg).

Wherever large bovines congregate in open environments, they seem to adopt large-scale darkness in aid of adaptive conspicuousness. In their self-advertisement they take after many other gregarious species of ruminants from Thomson's gazelle to oryxes, which have similarly forsaken attempts to hide from predators. However, unlike the large bovines, antelopes and deer stand out by virtue of whitish features as much as dark ones. The most photogenic species of all are those achieving pied patterns by means of large patches of whitish and blackish (e.g. bontebok, Damascus pygargus pygargus, see https://blog.nature.org/science/files/2015/07/bontebok.jpg).

Posted on June 23, 2021 09:15 by milewski milewski | 0 comments | Leave a comment

June 22, 2021

Eye-catching highlights above the stifle-fold in gazelles

Gazelles vary in the conspicuousness of their colouration in normal surroundings, some species being adapted to hiding from predators and others being so showy even when stationary that they rely on advertisement of fitness to avoid being selected for the chase (e.g. see https://www.dreamstime.com/lion-springbok-antelope-african-dry-habitat-etocha-np-namibia-mammal-africa-evening-back-light-sunset-safari-image144836274).

A contributing feature is the extension of the ventral whitish of the torso to a level above the stifle-fold. The stifle (https://en.wikipedia.org/wiki/Stifle_joint), at the junction of the hindleg and the flank in ungulates, is marked by a fold of skin. Any whitish above the stifle-fold tends to catch the light conspicuously, because it reaches relatively high on the side of the figure, on a surface made convex by the contours of the belly.

The range of variation can be seen by comparing the springbok with the gerenuk. In the springbok (see https://parody.fandom.com/wiki/Springbok?file=Antidorcas_marsupialis.jpg), pure white extends several centimetres above the stifle-fold, to a position halfway up the lateral profile. By contrast, in the gerenuk (see https://www.sciencephoto.com/media/389677/view/male-gerenuk-gazelle and https://www.123rf.com/photo_11078027_gerenuk-litocranius-walleri-male.html), there is virtually no clearance.

The following series exemplifies the intermediate positions.

Western dama gazelle (see https://www.dreamstime.com/dama-gazelle-gazella-dama-mhorr-mhorr-gazelle-species-gazelle-dama-gazelle-gazella-dama-mhorr-mhorr-gazelle-image218403317 and https://www.shutterstock.com/ja/image-photo/lovely-mhorr-gazelle-1021810420)

Females of northwestern subspecies of blackbuck (see https://www.dreamstime.com/stock-photo-wild-antelope-doe-south-texas-blackbuck-female-image60390821)

Sand gazelle (see https://www.inaturalist.org/observations/73853863 and https://www.canstockphoto.com/arabian-sand-gazelle-59326840.html)

Dorcas gazelle (see https://www.dreamstime.com/dorcas-gazelle-gazella-dorcas-dorcas-gazelle-gazella-dorcas-also-known-as-ariel-gazelle-small-common-gazelle-image137827563)

Arabian gazelle (see https://www.biolib.cz/en/image/id171992/ and https://www.biolib.cz/en/image/id171991/)

Thomson's gazelle (see https://alchetron.com/Thomson%27s-gazelle)

Bennett's gazelle (see https://www.facebook.com/AlWabraWildlifePreservation/photos/a.551778788259201.1073741855.240937542676662/551778918259188 and https://www.downtoearth.org.in/news/wildlife-biodiversity/18-chinkaras-die-within-a-fortnight-in-jaislamer-60977)

Red-fronted gazelle (see http://awwp.alwabra.com/wp-content/uploads/2014/04/Adult_Male_Redfronted_Gazelle_01.jpg and https://www.flickr.com/photos/24544467@N02/14113291712)

Females of southeastern subspecies of blackbuck (see https://www.enidhi.net/2014/07/maidenahalli-jayamangali-blackbuck.html)

This generally corresponds to a decreasing series in the overall conspicuousness of the figures. The springbok is showy in all perspectives, displaying a posteriolateral bleeze, a lateral bleeze (centred on the dark flank-band) and a facial flag. The dama gazelle is so extensively pale elsewhere on the figure as to subsume the pale above the stifle-fold. By contrast, the only conspicuous dark/pale contrast in the red-fronted gazelle is that between the blackish tail and the white buttocks. And in the gerenuk even the pattern on the hindquarters is hardly noticeable at the distances relevant to scanning predators.

Anomalous in this series is Thomson's gazelle. Its ventral white does not reach high on either the flanks or the rump, i.e. the clearance above the stifle-fold is narrow and the white of the buttocks ends at the root of the tail. Yet this is one of the most conspicuous species of gazelles, rivalling the springbok by means of a shift in emphasis: a particularly dark flank-band juxtaposed with a particularly well-defined pale flank-band above it.

Posted on June 22, 2021 22:21 by milewski milewski | 0 comments | Leave a comment

June 20, 2021

The wapiti: instant megafauna for North America, made in China

Several species of mammals occur naturally in coniferous forests in both Eurasia and North America, such as the wolf (https://en.wikipedia.org/wiki/Wolf#/media/File:Canis_lupus_distribution_(IUCN).png), the brown bear (https://en.wikipedia.org/wiki/Brown_bear#/media/File:Ursus_arctos_range_map.svg), the moose (https://en.wikipedia.org/wiki/Moose#/media/File:Moose_distribution.png), and the least weasel (https://en.wikipedia.org/wiki/Least_weasel#/media/File:Least_Weasel_area.png). This is unsurprising because the two continents - and their coniferous forest biome - were formerly connected across Beringia, the west-east land-bridge now interrupted by the Bering Sea (https://en.wikipedia.org/wiki/Beringia).

What is surprising is that the wapiti (Cervus canadensis) also occurs naturally on both continents (https://en.wikipedia.org/wiki/Elk#/media/File:Wapiti.png). This is a species of relatively low latitudes, which is absent from current Beringia, namely Alaska and far-eastern Siberia. This means that the break in its distribution is far wider than for any other of the shared species of mammals, and it is by far the most disjunctly-distributed of the ungulate species on Earth. The wapiti is so similar in Russia/China and Canada that it is questionable whether it even differs at the level of subspecies.

The wapiti is a Eurasian species which was suddenly drawn across Beringia a mere 15 thousand years ago by the rapid extermination of large herbivores in North America. Throughout the Pleistocene, a diverse fauna of ungulates of body mass greater than 200 kg was restricted to North America, and all of these species went extinct abruptly between 15 and eight thousand years ago. The ecological vacuum thus created allowed the marginal populations of the wapiti in what is now far-eastern Siberia to make a crossing of a mosaic of coniferous forest and tundra which had until then been a bridge too far for the species, followed by a rapid colonisation of the more suitable lands to the south (see https://markgelbart.wordpress.com/2014/10/27/did-elk-cervus-elephus-live-in-north-america-prior-to-15200-bp/). Metaphorically, the wapiti was sucked through a narrow aperture from its original continent into a new one, on which it thrived as a partial replacement for the extinct fauna. This doubled the distribution of the species 'overnight' in evolutionary time.

The story for bisons is different, because they have long been part of the Pleistocene fauna of North America. All that happened as a result of the faunal extinction is that a new, relatively small species evolved. Bison bison is in its own way a recent addition to the North American fauna, but the genus is not new there, and the species arose in situ. The wapiti is far more remarkable, because it is truly Eurasian as both genus and species, and merely expanded into North America - later being cut off from its still-existing original populations by a gap as wide as the distance from California to Korea.

See https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2013.2167

Posted on June 20, 2021 21:22 by milewski milewski | 0 comments | Leave a comment

A comparison of adaptive colouration between lookalikes: grey rhebok and mountain reedbuck

The grey rhebok (Pelea capreolus) and the mountain reedbuck (Redunca fulvorufula) are similar in body size, ecologically related, and partly sympatric. Both species are weakly gregarious, and their plain colourations are so similar that naturalists frequently confuse them. Here I point out differences, however subtle, which could be adaptively significant.

The most obvious is that, although both species have a white underside to the tail, this is displayed while fleeing only in the grey rhebok. This difference is so categorical and consistent that it can be used to identify fleeing figures at distances too great for details of the animals to be observed.

Another difference is that, although both species are countershaded with pale on the ventral torso and inner surfaces of the upper hindlegs, white extends just high enough on the belly of the mountain reedbuck to catch the light in the form of a crisply-defined ruff of fur (see https://biggamehuntingadventures.com/wp-content/uploads/2015/05/mountain-reedbuck-shot-placement-broadside.jpg and https://www.dreamstime.com/royalty-free-stock-photo-mountain-reedbuck-south-africa-image11193305 and http://www.waterberg-bioquest.co.za/Mammal%20spp%20pgs/red_fulv.html). Also, the white on the tip of the inert tail extends narrowly up the sides of the tail (see https://www.dreamstime.com/stock-photos-mountain-reedbuck-reedbok-image15546303 and https://fineartamerica.com/featured/female-mountain-reedbuck-peter-chadwickscience-photo-library.html?product=canvas-print and https://www.castledewildt.co.za/mountain-reedbuck-breeding-parcel/). These two white features of the stationary mountain reedbuck, although noticeable only at fairly close quarters, distinguish it from both the grey rhebok (see https://www.inaturalist.org/observations/81306076) and the other two species of reedbucks.

One interpretation is that the white display of the grey rhebok -which tends to be diurnal - is a dynamic, long-distance one communicating mainly with predators, whereas the white display of the mountain reedbuck - which tends to be crepuscular - is a static one communicating secretively within the species. The caudal flag of the grey rhebok discourages pursuit whereas the subtle highlights of the mountain reedbuck maintain group-cohesion as the animals graze in dim light. It makes social sense that the mountain reedbuck - which is more adapted than other reedbucks to exposed grassy slopes - is somewhat more showy than its closest relatives.

A small-scale difference is located about the lips. The grey rhebok has a darkish vertical mark - reminiscent of various species of deer but unlike other antelopes including reedbucks - below the side of the mouth (see https://www.inaturalist.org/observations/16496842 and https://www.inaturalist.org/observations/11867209). In the mountain reedbuck it is the white front of the upper lips that is more noticeable than in the grey rhebok (see https://www.zoochat.com/community/media/mountain-reedbuck-berlin-tierpark-9th-september-2011.162739/). I interpret these patterns as buccal semets, the function of which is to facilitate the mutual monitoring of cud-chewing by group members in a species-specific way.

Posted on June 20, 2021 06:42 by milewski milewski | 3 comments | Leave a comment

June 19, 2021

The white-tailed deer has more than just a caudal flag

Anyone with an interest in ungulates knows that the white-tailed deer (Odocoileus virginianus) raises its tail, showing the white underside. However, what is not widely realised is how expansive, complex and versatile the demonstrations of alarm are in this species.

When the foraging white-tailed deer jerks up its head in routine vigilance, it tends to flick the tail each time before lowering the head. When it suspects the approach of a predator, it shows various combinations of 'foot-stamping', tail-flagging and alarm-sneezing. The forefeet are lifted high but not stamped hard, suggesting that the main message sent is olfactory, from the interdigital glands. The tail may be raised to a rigid horizontal position (see https://www.facebook.com/MPGWildlife/videos/753907188442406), and this seems to be a purely visual signal. Or sometimes the tail is flicked up synchronously with each alarm-sneeze, without any piloerection of white fur.

Variation in the repertoire thus depends on the pattern of movement of the tail and whether white fur is piloerected (flared) or not (https://www.youtube.com/watch?v=Bf1j33Ja1so and https://www.youtube.com/watch?v=zAVb_Jj1CC0 and https://www.youtube.com/watch?v=5fM9qlBsMYE). Sometimes the tail remains down but the long fur on the buttocks is piloerected as the animal stands in initial alarm (http://mtnhp.org/thumbnail/defaultGen.aspx?itemid=391827&maxWidth=1024&maxHeight=768 and https://www.olo-7.top/ProductDetail.aspx?iid=62999139&pr=27.99 and https://www.olo-7.top/ProductDetail.aspx?iid=57741456&pr=27.99 and third photo in https://www.mossyoak.com/our-obsession/blogs/deer/how-to-read-whitetail-body-language). As suspense builds, the animal may flick the tail up and down, without piloerection, while walking stiff-legged before running.

Once fleeing begins, the white fur on the underside of the tail and/or that on the buttocks may be piloerected (see https://www.olo-7.top/ProductDetail.aspx?iid=57741356&pr=22.99 and third photo in https://www.welcomewildlife.com/white-tailed-deer/ and https://wdfw.wa.gov/species-habitats/species/odocoileus-virginianus-leucurus). The tail may be wagged from side to side while erect and flared. Sometimes, the animal stots (https://www.flickr.com/photos/conradkuiper/40834012061 and second photo in https://owenslaterphotography.com/category/north-american-wildlife/mammals/white-tailed-deer/).

Erection of the tail and piloerection of the tail/buttocks tend to be most frequent in females and juveniles (https://www.flickr.com/photos/23326338@N04/8988960173/), and in open vegetation. Adult males sometimes tuck their tails in while fleeing, thus showing no caudal flag (https://fineartamerica.com/featured/high-flying-white-tail-deer-gordon-allen.html ); and this may tend to be when females are absent. The sexual difference in displays is consistent with the tendency for males to have relatively short tails and fur on the buttocks.

Posted on June 19, 2021 09:29 by milewski milewski | 0 comments | Leave a comment

June 17, 2021

A first attempt to identify bleezes in the gazelle genus Nanger

I define the bleeze as any feature of animal colouration showing so much pale/dark contrast, at such large scale, that the whole figure is obvious to scanning predators even when it remains stationary. Such advertisement is interesting adaptively because it defies a basic strategy of prey species in avoiding detection.

The gazelle genus Nanger shows how complex it can be to identify bleezes among the species, subspecies, ages and sexes in ungulates. Once such a classification is made we can begin to weigh the evolutionary costs and benefits of the conspicuous colouration.

All individuals of Nanger granti including standing infants show a combination of whitish buttocks and blackish pygal bands. Its position on the hindquarters means that this pattern qualifies as a bleeze only in posteriolateral view. Therefore Nanger granti can be classified as a species consistently possessing a posteriolateral bleeze. However, this is the only clear case in this genus.

A posteriolateral bleeze also occurs in Nanger dama, but only in subspecies mhorr and excluding infants even in this subspecies. The whitish on the rump and buttocks is more extensive than in Nanger granti, but the bleeze remains somewhat ambivalent because, in the absence of pygal bands, the darkest adjacent fur is only moderately dark (see second photo in https://www.cbd-habitat.com/en/2019/07/02/the-first-reintroduction-project-for-mhorr-gazelle-into-the-wild/).

Juveniles of Nanger granti granti and Nanger granti notata temporarily develop a blackish flank-band, contrasting with both the whitish ventral torso below and the pale flank-band above (see https://www.zoochat.com/community/media/thomsons-or-grants-gazelle.376153/ and accompanying discussion). This lateral bleeze disappears in all adult males but is retained by some adult female individuals of Nanger granti notata. Nanger dama mhorr may qualify for a different pattern of lateral bleeze, but this is ambivalent because there are no flank-bands.

Nanger soemmerringi lacks any posteriolateral or lateral bleezes because no photo shows sufficient dark on the hindquarters or flanks. However, in at least one subspecies the front of the face in maturity becomes dark enough to contrast with the pale throat and facial stripe (see https://www.istockphoto.com/photo/soemmerrings-gazelle-gm1072202986-286932026 and https://www.biolib.cz/en/image/id359466/ and https://www.flickr.com/photos/timmelling/34068652491). If this pattern is large-scale enough to qualify as a bleeze, it can be called a frontal bleeze.

A frontal bleeze is once again ambivalent in the case of Nanger dama mhorr (see https://www.wikiwand.com/en/Nanger). Although the front of the face becomes whitish in adulthood, the adjacent throat is only moderately dark.

Posted on June 17, 2021 18:50 by milewski milewski | 2 comments | Leave a comment

June 16, 2021

Differences among gazelles in the structure and function of the tail

No standard terms have been established to compare the tails among ungulates. Here, I use 'shaft' to mean the flesh-and-bone structure, and 'tassel' to mean the long hairs concentrated distally. In all gazelles, the ventral surface of the tail is bare skin. I refer only to adults; the tail tends to be most demonstrative in infants. I assume that all species swish the tail to shoo insects.

In Gazella, the tail is simple in structure but demonstrative in function. The tassel is bushy and dark, and covers most of the shaft. The tail is proportionately smallest in Gazella gazella (see https://www.dreamstime.com/stock-photo-mountain-gazelle-walking-field-israel-image91922790) and largest in Gazella subgutturosa. It is wagged and raised during walking and trotting, but reaches the fully upright position in non-stotting flight only in Gazella subgutturosa and Gazella marica. The latter two species are odd also in possessing, on the sides of the base of the shaft, pale fringes of fur which can be piloerected (see https://www.flickr.com/photos/144908175@N07/40226721515/).

In Nanger, the tail is simple and undemonstrative, even during stotting. The tassel is small, and in Nanger dama almost absent (see https://pixels.com/featured/1-addra-gazelle-ncz-17-1-robert-michaud.html).

In Eudorcas, the demonstrative tail is proportionately larger than in Gazella. However, the main demonstration is wagging during nervous, intermittent walking. Once the animal runs the tail tends to be relaxed (see https://www.agefotostock.com/age/en/details-photo/thomson-s-gazelle-male-running-gazella-thomsoni/MEV-10763890).

In Antilope, the fur on the tail tapers to a point excluding any dark tassel. Apart from erection during stotting, the tail is undemonstrative - the anomaly being masculine display, when it is held 'hypererected' to the degree of being turned upside-down.

Antidorcas and Litocranius have similarly tapering shafts ending in similarly dark, small tassels. However, in the former the shaft is whitish (see https://www.istockphoto.com/photo/protective-mom-and-baby-springbok-stare-back-in-the-desert-gm160377111-22791345) and the tassel is piloerected while the tail is inert, whereas in the latter the shaft is dark and the tassel is never piloerected (see https://stock.adobe.com/search?k=gerenuk&asset_id=30423210). The stotting displays are extremely different, but in neither genus is the tail demonstrative; in the stotting springbok the tail is redundant in view of the flared white fur on the rump and buttocks.

In Ammodorcas, the dark tail is particularly noticeable because it is longer than in any other gazelle, and erected during flight. This genus is ecologically similar to Litocranius but surprisingly different in the form and function of the tail.

As far as I know no gazelle in stationary alarm either holds the tail erect (as seen in certain species of deer) or wags the tail nervously. Thomson's gazelle (Eudorcas thomsoni) has a reputation for doing the latter, but careful observation shows that the tail is activated only once a leg moves (see video in https://dissolve.com/video/Thomson-Gazelles-Blue-Wildebeest-Mara-River-Maasai-Mara-rights-managed-stock-video-footage/002-D806-133-014 and http://www.blaircostelloe.com/research/thomsons-gazelles/preventing-predation-of-neonates/).

Posted on June 16, 2021 13:36 by milewski milewski | 1 comment | Leave a comment

June 15, 2021

Why does the southern Grant's gazelle show such extreme change in colouration in males?

Among the two dozen species of gazelles, in eight genera, the southern Grant's gazelle (Nanger granti granti) shows the most puzzling change in colouration from juvenile males to adult males. Juveniles develop a graphic dark/pale pattern of complex banding on the flank (see https://www.inaturalist.org/observations/8088622), which is suddenly lost when the horns reach half their mature length. In mature males the showiest feature, apart from the long dark horns, is instead the sheeny pale of the unusually brawny upper-neck, which is displayed in masculine proud-postures (see https://www.inaturalist.org/observations/38785430 and https://www.inaturalist.org/observations/8507420).

The southern Grant's gazelle is also the only form of gazelle which naturally coexisted throughout its range with a smaller, far more abundant species of gazelle - the groups readily mixing (see https://iago80.files.wordpress.com/2012/11/img_2050.jpg). And as it happens there is such resemblance in the flank-banding between juveniles of the southern Grant's gazelle and this coexisting species, namely Thomson's gazelle (Eudorcas thomsoni), that naturalists often misidentify the juveniles - an embarrassing error because it actually confuses different genera of gazelles.

One explanation for this deceptiveness is in adaptive mimicry: juvenile males appease mature males by resembling the 'signature-pattern' of a harmless companion-species. This appeasement hypothetically allows the juveniles to remain secure with their mothers for longer than would otherwise be possible in a society where any hint of masculine precociality is likely to attract bullying by the territorial male in the vicinity. The mimicry is all the more effective because juvenile females - naturally exempt from bullying - share the dark flank-band.

The eastern subspecies, Peters' gazelle (Nanger granti petersi), shows little darkening on the flank, even in juveniles. This is possibly because its range is beyond that of Thomson's gazelle. The northerly subspecies (Nanger granti notata), found in the Laikipia region of central Kenya, shows the darkening on the flank but with a less confusing effect because adult females tend to retain the graphic pattern rather than losing it as happens in the southern Grant's gazelle. This perhaps makes sense because Thomson's gazelle is at a limit of its distribution, and does not outnumber the larger species of gazelle, in the Laikipia region. In this population the value of deceptive appeasement may be reduced by a moderation of the masculinity; mature males of the northerly subspecies do not grow as massive as in the southern Grant's gazelle.

Posted on June 15, 2021 01:55 by milewski milewski | 0 comments | Leave a comment

June 13, 2021

Coexistence and adaptive shifts in gazelles

Gazelles are antilopin bovids belonging to the genera Gazella, Eudorcas, Nanger, Litocranius, Ammodorcas, Antidorcas, Antilope and Procapra. In general, only one species of gazelle occurs in any given habitat. Where coexistence is achieved, this tends to be by means of regional shifts in body-size and -shape which correspond partly to differentiation into subspecies.

The best-known example is in the Serengeti ecosystem. Thomson's gazelle (Eudorcas thomsoni nasalis) and Grant's gazelle (Nanger granti granti) coexist here by means of divergent body masses, adults of the former subspecies weighing less than half the average for the latter subspecies in the case of females, and about a third in the case of males. The local form of Thomson's gazelle is the smallest of its genus while the local form of Grant's gazelle is the largest of its species. There is also divergence in water-dependence and movement patterns, with the larger form sedentary and the smaller form migratory and dependent on drinking.

The dorcas gazelle (Gazella dorcas) and the dama gazelle (Nanger dama) coexisted until recently on the southern and western fringes of the Sahara. This was achieved mainly by means of an extreme difference in body sizes and the maximum height of foraging. The former species is the smallest gazelle In Africa while the latter is the largest of all antilopin bovids. In Arabia, the particularly diminutive local form of the dorcas gazelle (Gazella saudiya) has been hunted to extinction while the Arabian sand gazelle (Gazella marica) has survived. These forms differ little in body sizes, suggesting that the Saudi gazelle was more restricted in substrate type and vegetation type than was the case in North Africa, limiting its population even before persecution by humans.

Bearing these patterns in mind, it is interesting to consider how coexistence has been achieved among certain poorly-known gazelles on the Horn of Africa. The gerenuk (Litocranius walleri) coexists as a large northern subspecies with Soemmerring's gazelle (Nanger soemmerringi) and as a small southern subspecies with Grant's gazelle (see https://www.inaturalist.org/observations/44453752). Notwithstanding the greater specialisation of the gerenuk for bipedal foraging, it is puzzling that, in e.g. Tsavo National Park, Peter's subspecies of Grant's gazelle (Nanger granti petersi) has similar body mass (a possible average of 30-35 kg for adult females) to the local form of the gerenuk. A similar puzzle, at larger body sizes, may apply to Soemmerring's gazelle and the gerenuk in Somaliland, a destination for adventurous naturalists.

Posted on June 13, 2021 19:09 by milewski milewski | 0 comments | Leave a comment

Peters' gazelle differs from Grant's gazelle in ways resembling genus Gazella

Peters' gazelle (Nanger granti petersi) occurs only in eastern Kenya, and differs from the other subspecies in several ways apart from the shape of the horns in the male.

The body mass of adult males is probably only about 50 kg (similar to that of the male impala, Aepyceros melampus), compared to 60 kg or more in nominate Nanger granti granti. Adult females seem not to have been weighed, but my guess is 35kg or less, compared with an average of 40 kg recorded for the nominate subspecies.

Peters' gazelle is the only subspecies lacking a lateral bleeze - i.e. a pattern on the flanks which is conspicuous enough in profile that it makes the figure stand out rather than blending into the environment - in any individual of either sex and any age. This is because Peters' gazelle retains only the faintest darkening on the posterior part of the flank (above the stifle-fold). This feature is extended into a fully dark flank-band in juveniles of the other subspecies (e.g. see https://www.inaturalist.org/observations/8088622) and retained with additional accentuation by some individual adult females (see https://www.inaturalist.org/observations/26410343) in populations of Nanger granti notata living in the Laikipia region of central Kenya.

All Nanger granti, of all ages and both sexes in all subspecies, have a conspicuous whitish pattern - which I call a posteriolateral bleeze - on the buttocks and spilling on to the rump. In Peters' gazelle, the extension on to the rump is minimal and divided by a fawn mid-line, thus resembling the genus Gazella rather than the rest of Nanger. Furthermore, in Peters' gazelle the white extends minimally on to the tail-stalk, leaving the tail mainly dark.

Other details: in the facial colouration, the dark rostral spot is minimal in all individuals of Peters' gazelle; and whitish wraps so narrowly on to the front of the upper hindleg that it is hardly visible in profile.

Peters' gazelle is the form of genus Nanger most closely resembling Gazella in colouration (e.g. see https://www.facebook.com/NikonIndia/posts/indian-gazelle-one-of-the-finest-posers-in-nature-throughyourlensimage-courtesy-/3607500265928157/), and can be regarded as a replacement for that genus beyond its southernmost limit on the Horn of Africa. However, Peters' gazelle remains larger than any form of Gazella. It emphasises the pygal band (i.e. the dark vertical feature between the haunch and the whitish buttock) and de-emphasises the dark flank-band to a degree unknown in Gazella. The tail is slimmer and less demonstrative in its movements than in any species of Gazella. And the malar stripe forms a dark accentuation of the eye to a degree not seen in Gazella.

Posted on June 13, 2021 00:02 by milewski milewski | 0 comments | Leave a comment