Archivos de diario de abril 2021

03 de abril de 2021

The gerenuk 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 the female and juvenile blackbuck (Antilope cervicapra, see and some populations of the goitred gazelle (Gazella subgutturosa), typically in Azerbaijan (see 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 (Dama, see; Cervus, see; Axis, see; and Rangifer, see and in several species of the antelope genus Tragelaphus (see and 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.

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05 de abril de 2021

Blackbuck and gerenuk: 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. The dark flank-band typical of gazelles has been lost; there is a pale horizontal stripe along the upper flank; there is pale around the eye but the pale facial stripe and dark malar stripe, both typical of gazelles, are minimal; and on the hindquarters the bold effect of most gazelles is lost because the pygal band is minimal, and the white on the buttocks is restricted (compare or with

Despite this uniformity of females and juveniles, the mature males could hardly differ more. In the gerenuk, males have modest horns and feminine colouration except on the crown of the head (see In the blackbuck males grow extremely long, corkscrew horns; and their colouration is so converted into a whole-body dark-and-pale 'beacon' (see that little remains of the pattern of gazelles. Even the face becomes so showily dark-and-pale (see that it seems unrelated to the gerenuk and most gazelles.

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.

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26 de abril de 2021

What makes the impala tick? Some initial thoughts...

The impala (Aepyceros melampus) is unusual in its oral allogrooming (mutual grooming by different individuals of the fur). It is well-equipped to self-groom most of its fur by means of specialised, comb-like front teeth plus an additional tooth-like bulge of the gums. The difficulty of reaching its own head and neck helps to explain the allogrooming.

Several other extremes in the nature of the impala may be relevant here.

Firstly, the impala is unusually gregarious for a cover-dependent species with camouflage-like colouration. This helps to explain why the impala is seldom kept in zoos, which are too cramped for a large group. Likewise it has long been known that the impala cannot be reintroduced to game ranches as a few founders in the normal way. Instead, a group-size at least in the dozens is needed right from the start.

Secondly, the impala is the smallest species of host attractive to oxpeckers (Buphagus), which comb through the fur in search of ticks and other parasites, as well as - on far larger hosts such as giraffes and the African buffalo (Syncerus caffer) - blood and pus oozing from wounds.

These three extremes, in allogrooming, gregariousness and complementary grooming by birds, may fit together. We may never fully distinguish cause and effect in the full context of the predator- and parasite-rich habitat of the impala. However, let me start to put together the pieces of the puzzle by pointing out a fourth extreme, in the form of the fur itself.

Some African ungulates have hairs which are simply circular or oval in cross-section. However, many have hairs which are kidney-shaped in cross-section, which may alter the colouration by means of complex sheen-like effects. For example, wildebeests (Connochaetes) are brown up-close but can look black, bluish or car-bonnet silver at a distance, because of how the light plays on the micro-texture of the fur.

Oddly, the hairs of the impala have a triangular cross-section, resembling that of a cricket bat.

Possibly its odd fur helps the impala to blend in as it forages among the bushes by day and rests in patches of open ground by night. But a downside of its hair-shape may be that ticks find it particularly easy to cling to the fur, requiring the extreme compensation by various forms of grooming that we see in the impala.

If so, the various oddities might begin to add up to a coherent adaptive strategy.

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11 de abril de 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 the springbok (Antidorcas marsupialis, which is a type of gazelle, see and the steenbok (Raphicerus campestris, which is intermediate in size between dikdiks and the beira, see

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 this. 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.

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10 de abril de 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?

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18 de abril de 2021

The peculiarly versatile tail of the impala

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 versatility of the tail relative to other bovids.

The impala normally hides its tail, tucking the tassel between the legs more than occurs in other ungulates including gazelles and the blackbuck (Antilope cervicapra). This is consistent with the peculiar striped pattern on the posterior of the haunches (see, which helps to make the whole animal inconspicuous in the sense of disruptive colouration (roughly equivalent to camouflage). Many ungulates have boldly-marked hindquarters, conspicuous from a distance, and many others have plain hindquarters which blend into the surroundings, but the impala is unusual in blending into the surroundings by means of hindquarters marked similarly to the stripes of the tiger (Panthera tigris). The habitual hiding of the white tassel makes sense in this context.

The impala does frequently display its tail in certain behaviours, but in doing so reveals the caudal anatomy to be unlike that in any other ruminant. Firstly, the long white hairs are piloerected either laterally (as in masculine displays in which the tail looks like a white fan, see or vertically (as when the tail is flicked up synchronously with the kicking of the hind legs in kick-stotting, see and and Secondly, it is the ventral surface of the tail on which the vertical piloerection 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 also shows the white tassel when shooing insects and when passing urine or faeces, but in these cases there is no piloerection in either of the orientations described above (e.g. see

Surprisingly, the length of the tassel differs between the two main subspecies of the impala. Many species of ungulates show subspecific variation in various features, but it is rare for the tails to vary much within a given species. In the black-faced impala (Aepyceros melampus petersi) the tail is so large (eg. see and that it seems to possess an additional curve in its shape as seen in kick-stotting. When tucked between the legs, the tail-tip reaches as far as the prepuce, instead of merely the scrotum.

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29 de abril de 2021

Predators are dark-spotted but their ungulate prey are pale-spotted. Why?

Spotting is a form of camouflage for both predators and their prey. However, what remains to be explained is why the the pattern is inverted in the two categories in mammals.

The spots of cats (Felidae, see, hyenas (Hyaenidae, see, civets (Viverridae, see and other Carnivora are dark against a relatively pale ground-colour. By contrast, the spots of deer (Cervidae, see, tragelaphin Bovidae (see and certain relatively large Neotropical rodents (see are pale against a relatively dark background.

One way to approach this puzzle is to examine any exceptions, but this turns out not to be particularly enlightening.

In the case of Carnivora, I cannot think of any real exceptions. Broadening the search, an exception can be found in an Australian carnivorous marsupial. The spotted-tailed quoll (Dasyurus maculatus) certainly has pale spots on a relatively dark background (see

In the case of ungulates, an obvious exception is giraffes (Giraffa, see However, these are also by far the largest land-animals with camouflage colouration, complicating any comparisons. Why giraffes are spotted in the first place is a question unto itself.

One reason why the nearly categorical difference between predators and prey is surprising is that all of the spotted Carnivora are themselves vulnerable to their largest local relatives. The lion (Panthera leo), after all, readily kills all other Carnivora regardless of whether it finds them acceptable as food. This makes it hard to know whether the spotting of the cheetah (Acinonyx jubatus), for example, serves more to hide it from its prey, or from its own predators, namely the lion, the leopard (Panthera pardus) and the spotted hyena (Crocuta crocuta). Even in the case of the spotted hyena, second only to the lion in the African hierarchy, the spotting is puzzling because this species hunts by pursuit to the apparent exclusion of camouflage-dependent stalking.

Any naturalist who has spent time pondering adaptive colouration will know how enigmatic this field of biology can be. The patterns of colouration in animals seem to defy generalisation and prediction, discouraging further enquiry. However, in the riddle of the dark-spotted predators versus the pale-spotted prey we at least have an unusually clear-cut question. Are we up to the challenge of solving at least this major puzzle?

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06 de abril de 2021

Why has the gerenuk 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.

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24 de abril de 2021

The peculiar ordinariness of the larynx of the impala

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 the goitred (Gazella subgutturosa) and mongolian (Procapra gutturosa, see and gazelles and the red (Cervus elaphus, see and and fallow (Dama dama, see and deer.

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

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08 de abril de 2021

Detailed similarities and differences in the colouration of gerenuk and impala

In my last Post I mentioned the oddly convergent patterns of colouration in the gerenuk and the impala. The following list shows that 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

Both species have dark fawn on the dorsal surface of the torso, giving way to paler fawn on the flanks and then white on the belly, the borders between the zones being 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 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 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, the back of the head differs in that only the impala possesses dark posterior ear-tips and a sheeny-haired crown which switches from fawn to silvery in some lights (

The legs have similar colouration except, for example, that the dark gland-tuft is on the foreleg of the gerenuk, vs the hindleg of the impala. Only the impala has white pasterns, visible when the animal stands on bare ground or close-cropped lawn ( and

As for the hindquarters, the patterns are different in detail (e.g. only the impala has dark pygal stripes) but give a similar overall impression of inconspicuous vertical bars of whitish near the tail (see and

The tails differ in various inconspicuous ways (see and Whereas the terminal, insect-swishing tassel is white and relatively large in the impala vs blackish and small in the gerenuk, only the gerenuk can flare the white fur on the posterior haunch.

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?

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