Journal archives for April 2021

03 April, 2021

The gerenuk (Litocranius walleri) as an unusually striped ungulate

Stripes, whether pale or dark, can make animals hard to tell from their surroundings. However, horizontal stripes along the torso (as opposed to the classical vertical pattern seen in e.g. the tiger, Panthera tigris) are unusual in large mammals.

Why is the gerenuk (Litocranius walleri) - the lankiest of all gazelles and indeed of all antelopes or deer - odd among ungulates in this way?

The gerenuk, in both sexes and at all ages from infancy, has a pale horizontal stripe running from the dorsal base of the neck across the flank to the rump (see and

The colouration is otherwise unusually plain for a gazelle. Presumably the stripe breaks up the figure, helping the gerenuk to hide in the sparsely woody vegetation it inhabits.

The only other gazelles with a similar stripe are

In the latter, the stripe is merely a local variation of what, in most populations, is a pale band.

Horizontal pale stripes occur also in several genera of deer and one genus of bovids, viz.

However, in these animals they tend to be mere details of complex patterns of spotting and striping.

Perhaps the unusual striping of the gerenuk arose partly because its torso is so often viewed in the vertical.

Although various species of deer and antelopes can stand on the hindlegs to reach high on plants, the gerenuk is exceptionally able to remain free-standing for minutes without propping the forefeet on branches. And only the gerenuk rises bipedally so frequently that this seems to be its main posture in foraging.

In the upright stance, the stripe tends to align with the main stems of the tall shrubs and short trees typical of the habitat of the gerenuk (

Two slightly less lanky species of gazelles, the dibatag (Ammodorcas clarkei) and the dama gazelle (Nanger dama), lack any stripe on the torso despite also foraging to some extent bipedally (e.g. see

In the case of the dibatag, the typical habitat is dominated not by 'acacia' (Vachellia species such as tortilis) but instead by sundry tall shrubs in a distinctive vegetation type on sand, called 'gedguwa'. This form of 'open thicket' tends to be more cluttered with foliage at about one metre above ground than is the case in 'acacia scrub', leading to a subtle difference in visibility and thus in adaptive colouration of the gazelles in question.

In the case of the dama gazelle, the habitat is seasonally far more open than that of the gerenuk, and the overall colouration is conspicuous, not camouflaging.

Posted on 03 April, 2021 02:11 by milewski milewski | 0 comments | Leave a comment

05 April, 2021

Blackbuck (Antilope cervicapra) and gerenuk (Litocranius walleri): similar females, extremely different males

Most gazelles (genera Gazella, Eudorcas, Nanger, Litocranius, Antilope, Ammodorcas, Antidorcas and Procapra) are only moderately sexually dimorphic. Adult males are not strikingly larger than adult females but possess horns, or at least larger horns.

However, the blackbuck (Antilope cervicapra, see presents an intriguing comparison with the gerenuk (Litocranius walleri, see

In both, adult females (compare with are hornless and weigh about 30 kg, and female colouration is the least conspicuous among gazelles:

Despite this uniformity of females and juveniles, the mature males could hardly differ more, as follows:

This correlates with the ecological and social differences.

The blackbuck is a specialised grazer which drinks frequently and concentrates in large groups, whereas the gerenuk is a specialised browser which can forgo drinking for years and often appears solitary. Both species are territorial, but in divergent ways.

In the blackbuck, territories are so small, crowded and hectically defended that the competing males show off to each other for most of the time. They alternate this with visual appeasement, because they can forage only by trespassing their way to nearby, untrampled pastures, excusing themselves gesturally along the way there and back.

In the gerenuk, the territories are so large that males seldom even see each other. Not only do they not need to trespass, but they only ever patrol a limited central part of the territory - using smell rather than sight.

What this means is that - despite females being so similar - males have social modes so different that males of the blackbuck make no attempt to hide from predators, whereas those of the gerenuk remain as secretive as possible.

Posted on 05 April, 2021 02:34 by milewski milewski | 1 comment | Leave a comment

06 April, 2021

Why has the gerenuk (Litocranius walleri) become such a focus for photographers?

One would not expect the gerenuk (Litocranius walleri) to be particularly frequently photographed. It lives in remote areas, its populations are sparse, it is shy, its appearance is rather dull apart from a graceful lankiness, and it is uncommon in zoos.

In the sixties and seventies, few photos of the gerenuk were available. Pierre Dandelot and Helmut Diller, painting the species for the best field guidebooks of the seventies and eighties, erred considerably in their depictions, presumably because they had little material to examine. Yet today, photos on the Web are too many to keep track of. The gerenuk is surpassed only by the springbok (Antidorcas marsupialis) as the most frequently-photographed gazelle in the wild.

The blackbuck (Antilope cervicapra) has, understandably, been photographed extremely frequently. This gregarious and spectacular gazelle still occurs widely in semi-wild conditions in India, where a rising tech-savvy Middle Class, combining the regard for animals of Hinduism with that of the colonial English, has produced more wildlife photographers than expected in a poor country. The blackbuck is the most successful gazelle in zoos worldwide. It is also kept on hunting ranches in the USA and Argentina; there are more photos on the Web of the blackbuck in Texas alone than there are of most species of gazelles in any situation.

Such explanations hardly apply to the gerenuk. Instead, its obvious appeal is anthropomorphic: extreme bipedality for an ungulate while foraging (see and

People love, in animals, not only upright bipedality (penguins) but also other anthropomorphic features such as flattish faces (many primates but not e.g. baboons), binocular placement of the eyes (cats and primates), and apparently smiling mouths (dolphins). Hypothetically any animal combining several of these features would be particularly photogenic, and the popularity of the suricate (Suricata suricatta) can largely be explained by its combination of bipedal standing/sitting, binocular placement of the eyes and a suggestion of a smile (see Penguins feature only bipedality, but they score in that they actually walk bolt-upright, and they also have the attractive dark/pale contrasts of a jacket and shirt.

The gerenuk does not walk bipedally, its eyes are on the sides of its head, and its facial profile is pinched and pointed. However, even this head, when viewed directly from the front, can appear somewhat childlike in perspective with the eyes large relative to the small face, and a hint of a smile (see Thus anthropomorphism, more than the particular biological interest of a 'giraffe-gazelle', may help to explain the proliferation of photos of the gerenuk.

Posted on 06 April, 2021 09:50 by milewski milewski | 4 comments | Leave a comment

07 April, 2021

Why does the colouration of the gerenuk (Litocranius walleri) resemble that of the common impala (Aepyceros melampus)?

For some strange reason, the gerenuk (Litocranius walleri) and the common impala (Aepyceros melampus) have similar colouration (see and

This is true despite the fact that the two species are unrelated phylogenetically and differ ecologically, seldom occurring together.

The gerenuk

  • is specialised to forage with upright bipedality,
  • can forgo drinking for years on end, and
  • tends to be solitary.

By contrast, the common impala

  • has not been observed even to prop its forelegs on a plant stem,
  • must drink nearly daily, and
  • is gregarious.

In the narrow zone where the two species share the same landscapes in Kenya, the gerenuk prefers thorn scrub, while the common impala prefers grassland.

Perhaps the gerenuk has come to mimic the colouration of the common impala for protection against predators.

The common impala tends to be abundant where it occurs, while the gerenuk is everywhere scarce. Based on the likelihood that the gerenuk is not as enduring a runner as the common impala, a naive predator might be misled to turn down hunting opportunities after spotting the gerenuk.

However, the following are obvious problems with this explanation.

For example:

Here is a frontier of understanding in Biology, where new hypotheses are needed.

Please also see

Posted on 07 April, 2021 06:33 by milewski milewski | 0 comments | Leave a comment

08 April, 2021

Detailed similarities and differences in the colouration of gerenuk (Litocranius walleri) and common impala (Aepyceros melampus)

In, I mentioned the oddly convergent patterns of colouration in the gerenuk (Litocranius walleri) and the common impala (Aepyceros melampus).

The similarities are enough that, were it not for the obviously long neck and small face of the gerenuk, it would be hard to tell the two species apart at any distance (see and

In both species

  • dark fawn on the dorsal surface of the torso gives way to paler fawn on the flanks, and then to white on the belly, and
  • the borders between these zones are oddly crisply defined.

The main difference is that, in the gerenuk, the back/flank border is accentuated enough to look like a pale horizontal stripe in its own right (

However, another difference is that only the common impala possesses a dark spot of bare skin at the stifle-fold.

The pattern of double-white on the chest is remarkably similar between the gerenuk and the common impala, while different from other ruminants (see and and and

The face, like the chest, has patterns too similar in detail to have arisen merely by chance.

However, only the common impala possesses a posterior coronal flag ( and

The legs have similar colouration except, for example, that

On the hindquarters, both species give a similar overall impression of inconspicuous vertical bars of whitish near the tail ( and

However, the patterns differ. For example:

  • only the common impala has dark ischial stripes, and
  • only the gerenuk can flare the white pelage on the buttocks.

The tails differ in various inconspicuous ways (see and

For example:

  • in the common impala, the jointed feather-tassel is white, whereas
  • in the gerenuk, the tassel is blackish and small.

Given that the specialised proportions of the gerenuk should make it easy for predators to recognise, what could the adaptive advantages of the similarities in colouration possibly be?

Posted on 08 April, 2021 04:28 by milewski milewski | 1 comment | Leave a comment

10 April, 2021

Quasi-domestication in Gazella

There have been many historical attempts to domesticate both sheep (Ovis) and gazelles (Gazella).

What many naturalists may not realise is that several types of ostensibly wild gazelles seem to have originated, at least partly, by selective breeding in captivity.

The species/subspecies bilkis, dareshurii, erlangeri, farasani, hamishi and muscatensis seem never to have been found in wild populations, although three of these now occur free-range on small islands in the Red Sea or Persian Gulf (e.g. see

I suspect that all are anthropogenically modified variants of the mountain gazelle (Gazella gazella), transported by humans to their locations and at least partly bred in captivity in the past.

Even in its original range (in the Levant), the mountain gazelle has long lived somewhat commensally with tolerant farmers because no truly wild situations have remained in the Biblical Lands for hundreds of years.

The main effects of quasi-domestication in the above gazelles seem to be darkened colouration (e.g. see and and and a reduction in the size of the brain.

Four North African species (cuvieri, dorcas, pelzelni and leptoceros) seem to have been kept in captivity, for example in oases in Tunisia and elsewhere in the Maghreb, for thousands of years. Probably in most cases the animals were caught as wild infants and hand-reared, to be kept as pets but not selectively bred.

However, an odd aspect of Cuvier's gazelle, in addition to a dark colouration in some populations, is that in some individuals there are irregular whitish markings on the face (see and, reminiscent of the asymmetrical colouration so often produced inadvertently by domestication.

Certain traditions in India have long cared for the chinkara (Gazella bennettii), which lives somewhat commensally as well as often being raised as a pet. However, no aspects of colouration suggest that this species has been modified by this relationship.

Why did domestication of gazelles prove so unsuccessful that most naturalists assume them to be purely wild animals? Possibly because all gazelles, unlike all wild sheep, have a territorial social system, which limits their amenability to herding. None of the twelve species of domestic hoofstock originating in Eurasia have territorial wild ancestors.

Which leaves us with an odd thought. Had things turned out differently and Gazella domestica arisen in place of sheep, would the Bible have spoken of gazherds rather than shepherds?

Posted on 10 April, 2021 11:21 by milewski milewski | 1 comment | Leave a comment

11 April, 2021

Why the adaptive radiation of antilopins on the Horn of Africa?

Antilopin bovids range widely in dry climates in Africa and Asia, but their greatest concentration of genera and species occurs in arid to semi-arid northeastern Africa (Somalia, Somaliland, Djibouti, Eritrea and parts of Sudan, Ethiopia and Kenya).

In an area the size of France plus Spain located just north of the equator, seven species of gazelles, the beira (Dorcatragus megalotis, see, and eight species/subspecies of dikdiks (Madoqua) occur. This is more than all the species of bovids, antilocaprids and deer in the whole of the United States of America.

One clue to the reasons for such diversity is the poverty of antilopins in the similarly extensive arid to semi-arid climate in southern Africa, just south of the tropic of Capricorn, where only two species of antilopins occur in the relevant parts of Namibia, Botswana and South Africa.

These are

That is a dozen species versus a couple, in ostensibly similar environments on the same continent.

The antilopins of arid to semi-arid northeastern Africa include some of the most peculiar and specialised of ruminants.

The gerenuk (Litocranius, see has an exceptionally small face, an unrivalled ability to free-stand upright on its hind hooves, and such extreme economy of water that it refuses to drink even when raised in zoos.

A small species of dikdik (see is the smallest of all ungulates, worldwide, that live in semi-desert.

And Speke's gazelle (Gazella spekei) has an oddly inflatable nose even compared to its closest relatives within the same genus (see

Both parts of Africa have similar mean annual rainfall, but a crucial difference is as follows.

Whereas the dry parts of southern Africa have a single rainy season each year (see, those of northeastern Africa tend to have two rainy seasons each year (see owing to the East African Monsoon.

This means that plant growth tends to be more reliable in the northeast than in the southwest of the continent, allowing greater specialisation of body sizes and shapes, diets, and foraging heights, and thus ecological niches.

Whereas in general arid climates mean that 'beggars cannot be choosers' and survival depends on versatility, the antilopins of northeastern Africa include some of the choosiest species of ungulates known with respect to type of terrain, vegetation and diet.

Posted on 11 April, 2021 04:47 by milewski milewski | 0 comments | Leave a comment

12 April, 2021

Locomotory and postural peculiarities of impalas (Aepyceros), part 1

Everyone knows that impalas ( bound in a striking way ( and

However, how many realise that this genus - looking like a normal antelope but with an ancient and distinctive origin - is more aberrant in other aspects of its locomotion and postures?


Impalas walk by ambling, not cross-walking or semi cross-walking.

In this way, impalas differ from

  • all deer (Cervidae), including those most similar in body size and proportions,
  • reduncins (Bovidae: Reduncini), and
  • 'plains game' (e.g. Antilocapridae, Alcelaphini, Hippotragini).

Large gazelles (e.g. Nanger) require further scrutiny in this context.

The walking gait of impalas supports the view of impalas that they are 'plains game gone cover-dependent', in a sense (discussed elsewhere).


Impalas are puzzlingly reluctant to trot.

This standard gait is

One of the few times when impalas trot - and then for only a few steps - is when a courting male approaches a female over a short distance (see

The reluctance of impalas to trot is more odd than their bounding.

This is because an ecological counterpart in India, the blackbuck (Antilope cervicapra,, frequently trots (, in addition to bounding high and far (see and


What truly is distinctive of impalas is a gait that I call kick-stotting ( and

Many types of antelopes and deer stot (e.g. and and in response to the approach of predators. These include

However, the kick-stotting of impalas differs in form and has yet to be explained in function.

As they runs, impalas fling their hind legs high in unison - in some cases so high that they seem to risk somersaulting - while waving the tail high as well (see and

Many naturalists have watched kick-stotting in social play, but few have seen it in serious situations. Since social play is rehearsal, there is presumably a real, life-or-death purpose to stotting in the impala as in other species.

I have noticed that another of the few times when impalas trot is in slowing down to a halt after a bout of playful kick-stotting (see

When charged by most types of predator, impalas do not stot. The limited evidence hints that kick-stotting in earnest may be reserved for the African hunting dog (Lycaon pictus,

M Burton, in an article titled 'Impala behaviour' (Black Lechwe 4(4), pp. 46-48) states: "Impala sometimes use a similar action (to kick-stotting), as when one is chased by a dog. This it soon outdistances, and then it will proceed for a short distance bouncing on stiff legs before resuming the normal method of progression...the conspicuous black and white markings on the rump...are more prominently displayed in moments of excitement".


All bovids and deer can swim.

However, impalas are among the most inept in the water (see and

This was first noticed in the mission to rescue animals stranded on islands during the filling of Kariba Dam on the Zambezi River ( and

Impalas often live along river banks, where they must risk being chased into the water by predators.

So, it seems odd that gazelles that spend their lives far from rivers can - if needs be - swim more confidently than do impalas (e.g.

The maximum competence of impalas when immersed can be seen in and


Impalas seem unwilling to rise on their hind legs to forage, even in drought when the only remaining food is high on branches.

The blackbuck specialises more on herbaceous plants and is thus less likely than the impala to seek the foliage of shrubs for food. Yet females of the blackbuck sometimes rear up on their hind legs to flail at each other with their hooves, which has not been observed in impalas.


Impalas are reluctant to kneel, whether while drinking or while suckling.

There are many photos on the Web, showing that impalas tend to splay at water's edge, somewhat like giraffes (Giraffa spp.):

The few photos showing kneeling in such situations tend to be where the water is >20 cm below ground level (

Once the suckling juvenile reaches a certain size, it needs either to kneel or to splay its fore legs to reach the teats.

In impalas, the posture adopted is splaying ( - which is unremarkable because various bovids and cervids do the same.

However, this posture undermines the idea that impalas are related to alcelaphins (, which kneel while suckling - in common with hippotragins and e.g. the nilgai (

Finally: even in the case of lying down to chew the cud (, impalas seem odd.

Most other antelopes and deer are easy enough to spot lying down by day.

However, adults of impalas tend to remain standing during its midday rest, reserving their recumbency for the secrecy of night - which they tend to spend in certain open places away from vegetation.

Perhaps this explains why there are few photos in iNaturalist of impalas in a lying position?

to be continued in

Posted on 12 April, 2021 14:03 by milewski milewski | 29 comments | Leave a comment

18 April, 2021

The peculiarly complex tail of impalas: Aepyceros has several caudal flags, but no ischial flag

@variani18 @capracornelius @nyoni-pete @simontonge @tonyrebelo @jeremygilmore @matthewinabinett @dejong @karoopixie @nightjar09 @jwidness @botswanabugs @hfabian @florem @claremateke @christiaan_viljoen @craigpeter @ldacosta


It has long been realised that the impala (Aepyceros melampus, is something of a 'living fossil', unrelated to antelopes of superficially similar appearance (

However, what seems to have been overlooked is the complexity of its tail, relative to other bovids.

What is unsurprising about the tail of the impala is that it is

However, what is surprising about the tail of the impala is that it


I propose the following new terms for the distal structures, composed of long white hairs, on the tails of impalas:

  • jointed feather-tassel in the case of all forms other than petersi, and
  • ?plume-tassel in the case of petersi.

I am aware of the implication that petersi and nominate melampus differ categorically w.r.t. the anatomy of the tail. This needs further investigation.

However, the difference in the tails is great enough to suggest that the black-faced impala (Aepyceros petersi) is a separate species, not merely a subspecies. It differs so much from A. melampus that it might be called 'the fluff-tailed impala' (

Furthermore, the enlargement of its tail is accompanied by depigmentation of the perineal bare skin (

I have previously introduced the term 'kick-stotting' (

There are two different 'folds' in the jointed feather-tassel of Aepyceros:

  • immediately distal to the longitudinal dark stripe, there is a point of flexure, whereby the distal-most long white hairs can be folded back ventrally, and
  • to the left and right of the distal half of the same stripe, there is in each case a line of flexure whereby the long white hairs, arranged laterally, can be piloerected in either of two directions, viz. laterally or ventrally.

When the tail of the impala is flicked as part of the display during kick-stotting, a butcher's cleaver-like shape ( is produced by a combination of ventral foldings as per both the 'joint' described above, and the lateral lines of flexure.

The follwing shows the result ( and

None of the above details seem to have been noticed previously by zoologists.

The following show the distal point of flexure in the tail-tassel of the impala:

The following ( shows lateral piloerection of the tucked tail.


Impalas normally hide the tail, in at least three ways, viz.

More than in other ungulates including gazelles and the blackbuck (Antilope cervicapra,, impalas tuck the tassel between the legs ( and and,

This is consistent with the peculiar striped pattern on the buttocks and visible part of the perineum ( and and

The bare skin on the perineum is dark in adults of melampus (, but somewhat depigmented in adults of petersi (

The composite pattern helps to make the whole animal inconspicuous, in the sense of disruptive colouration (roughly equivalent to camouflage,

Many ungulates have either

The following nicely show how inconspicuous the pale pelage on the buttocks is in the impala, compared with Kobus defassa: and

However, the impala is unusual in blending into the surroundings by means of hindquarters marked similarly to the dark stripes associated with carnivores (

The following shows how inconspicuous the hindquarters are in the impala compared with a like-size gazelle (Nanger granti notata, and

The habitual hiding of the white caudal hairs ( and makes sense in this context.

The following reveals the shape of the jointed feather-tassel (folded and 'kinked') when the tail is fully tucked:


The white of the jointed feather-tassel of the impala, in its penicillate form, seems almost luminous ( and and and and

I suspect that this is partly because

The impala displays its tail in sundry behaviours (

In doing so it reveals the caudal anatomy/pelage to be unlike that in any other genus of ruminants.

The long white hairs are piloerected either

It is in the ventral direction that the vertical piloerection of the tassel occurs - unlike the tails of various antelopes, including gazelles, on which any vertically-arranged tomahawk-like hairs (usually black) are on the dorsal side.

The impala reveals the length and whiteness of the jointed feather-tassel when

However, in these cases there is no piloerection in either of the orientations described above.


The size of the tail differs to a surprising degree between the impala and the black-faced impala. I suggest that the shape/anatomy is also significantly different.

Relative to Aepyceros melampus, the tail of the black-faced impala (Aepyceros petersi ( and

Many species of ungulates show subspecific variation in various features. However, it is rare for the tail to vary much within a given species of bovid.

Furthermore, a difference apparently overlooked by everyone in the past is that, in adults, the bare skin of the perineum is somewhat flesh-coloured in A. petersi, as opposed to dark in melampus.


The pattern on the buttocks is not conspicuous enough to qualify as a flag ( and and and and

The colouration of the tail of the impala is as unusual and complex, among ungulates, as that of the buttocks. However, it does qualify as a caudal flag ( and

Caudal flagging in the impala occurs under various circumstances, including

However, it generally does not occur in running, whether the giants galloping or bounding ( and


What is noteworthy about the tail of impalas is that it

I suggest that the three different shapes of the jointed feather-tassel of the impala mean that we can recognise three different caudal flags in this species.

Also see

Posted on 18 April, 2021 11:47 by milewski milewski | 31 comments | Leave a comment

24 April, 2021

The peculiar ordinariness of the larynx of impalas (Aepyceros)

The impala (Aepyceros melampus) is a strange antelope appearing as an ordinary antelope.

This is partly because its peculiarities are small-scale anatomically, such as the nature and arrangement of its fur and the grooming apparatus of its teeth and gums.

Little-known is how odd it is that the impala can roar without obvious modification of the larynx (see and and and and and

Several ruminants are, like the impala, capable of roaring or loud grunting during masculine display, i.e. during the rut (

These are

However, all have obvious modifications of the larynx of the male in season. In the goitred gazelle and red deer the larynx descends so far that it can abut the sternum.

In the goitred and mongolian gazelles the larynx is so enlarged that the very names of the species refer to the swelling. And in all these species, the larynx recoils far down the neck during roaring.

The rutting male impala shows none of these specialisations (see and, yet manages to excel in several ways. It roars more loudly than the goitred gazelle, it roars as loudly while running as deer roar while standing, and it somehow intersperses its roars with loud snorts made non-vocally.

The result is that the impala is one of the loudest of ruminants while retaining a larynx which looks no different in the rutting male from that in the female.

And the female impala has not, as far as I know, been recorded vocalising loudly, although she - like the male - certainly snorts loudly when alarmed by predators (see

Posted on 24 April, 2021 22:26 by milewski milewski | 0 comments | Leave a comment