Archivos de diario de junio 2021

02 de junio de 2021

The eclipsed life of the western dama gazelle

The western dama gazelle (Nanger dama mhorr), exterminated in the wild half a century ago, is photogenic and unmistakable in colouration. However, several aspects of its biology and conservation are easily overlooked even if the naturalist is familiar with the many hundreds of photos taken in zoos and breeding centres.

All existing photos of this subspecies show the descendants of just one male, which was captured together with three females in Western Sahara. Rescuing the subspecies by means of captive breeding with such a limited founder-population was all the more precarious because reproductive rate of this species may depend on numbers owing to a gregarious and polygynous social system. Perhaps this limitation helps to explain why it has proven difficult to reintroduce the dama gazelle to reserves in Africa.

The full sexual dimorphism of the western dama gazelle is hidden in all current populations by the intense management. Adolescent males are allowed to breed in captivity and reintroduction, but this may be a suboptimal mating system. Masculine brawn tends to keep growing for years after sexual maturity, as revealed by a few photos of three lonely males in Hadj National Wildlife Refuge, before the last of that reintroduced population was killed by corrupt Tunisians, wiping out millions of dollars of investment. One photo in particular, in, shows how proportionately small the head and horns of the mature male can become, suggesting a fully mature body mass almost double that of the adult female. No other species of antilopin bovid is so dimorphic in this way, consistent with a breeding system which would tend to falter in small, remnant populations.

Although its ability to forage with free-standing bipedality (see might suggest a social system similar to that of the barely-gregarious gerenuk (Litocranius walleri), the dama gazelle is more like the springbok (Antidorcas marsupialis) in its migratory hypergregariousness. Even if a reintroduced population is protected in some small reserve in Senegal or elsewhere in its original range, it cannot be truly rewilded unless numbers and movements are restored, which is unlikely. Therefore the subspecies seems condemned to functional extinction as a wild animal - even if it eventually succeeds enough on Texan ranches to be commercially hunted there.

The distinctive and consistent colouration of adults, including fully mature males, hardly prepares the naturalist for the appearance of the infants. There is no subspecies of gazelle or other antilopin bovid in which the ontogenetic change in pattern is so great. In all gazelles, the infant has plainer colouration than the adult, but in the western dama gazelle the difference is extreme, as revealed by several clear photos of newborns in zoos (e.g. and and Can the social biology of the western dama gazelle explain why infants have distinctive colouration whereas fully mature males do not?

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07 de junio de 2021

How does infantile colouration inform subspecies in the dama gazelle?

Any given species of ungulate usually consists of subspecies, living in different parts of the geographical distribution. Subspecies usually differ somewhat in adult colouration, but many naturalists would be surprised to find that the colouration of infants varies accordingly. Newborns can hardly be expected to be subspecifically distinct in colouration given that they rely mainly on lying so low that they are out of sight of both conspecifics and potential predators. Furthermore, there is a notion that 'ontogeny recapitulates phylogeny', suggesting that infants would tend to retain an ancestral pattern of colouration.

Variation within the dama gazelle (Nanger dama) was clinal over a vast, contiguous distribution, any boundaries between subspecies being blurred. This has led to a recommendation in iNaturalist that no attempt be made to identify photos below species-level. Who, then, would have predicted that the infants of the westernmost and easternmost forms of the dama gazelle are easily distinguished by their colouration?

Owing to captive breeding, we now have clear photos of several individual infants of each form, and these show that the western and eastern dama gazelle differ in colouration already at birth. This is all the more surprising because the infantile colouration of the western dama gazelle is so different from the adult colouration that, viewed in isolation, it would not be identified as the same species, let alone subspecies.

In the western dama gazelle, the conspicuous white markings of the adult are absent in the infant, developing only after the horn-tips erupt (see and and . By contrast, in the eastern dama gazelle the hornless infant is already relatively pale overall with white emerging on the face and lower flanks (see and and

While these surprising differences need not, in themselves, validate the subspecies of the dama gazelle, they show the cline to be more profoundly differentiated than previously assumed. Should our scientific names not reflect this differentiation? Given that the populations are now artificially separated (partly because the species is virtually extinct in the wild), my recommendation is to resume the use of the subspecies names mhorr (western), dama (central) and ruficollis (eastern), while acknowledging that these subspecies originally graded into each other.

Ingresado el 07 de junio de 2021 por milewski milewski | 0 comentarios | Deja un comentario

10 de junio de 2021

Beginning a study of the malar stripe in bovids

Many species of bovids, in the antilopin, aepycerotin, caprin and hippotragin tribes, have a feature of colouration called the malar stripe. This is a relatively dark stripe running from the orbit above the eye, diagonally across the side of the face, to near the corner of the mouth. The malar stripe is particularly typical of gazelles, but it also occurs in goats and oryxes, among others.

The adaptive function is deeply ambivalent. On one hand, running through the eye suggests a disruptive effect reducing the conspicuousness of the animal to predators. The eye tends to be one of the most noticeable parts of an animal, and disguising it can be disproportionately effective in helping to hide the whole animal. On the other hand, where the stripe is bold enough, and offset by strikingly pale fur, it can contribute to a flag or even a bleeze, advertising the animal for social purposes. These contrary functions could occur within a given species, according to age or sex.

Here I illustrate a few examples of how subtle the effects remain, even once the dichotomy is understood between reducing conspicuousness and boosting conspicuousness.

In gazelles, the malar stripe tends in most species to be effacing. However, in Soemmerring's gazelle (Nanger soemmerringi, see it has been drafted to form one border of a bold black-and-white pattern on the front of the face. This species accordingly possesses a facial flag or even a frontal bleeze. In the case of the springbok (Antidorcas marsupialis), which has a showily whitish face, the malar stripe seems perverse. It is not bold enough to offset the pale when viewed at distance, and by crossing out the eye seems to detract from the facial advertisement (see

The malar stripe of the gemsbok (Oryx gazella) is part of a conspicuous pattern of dark and pale on the face of the adult (see (In the same species, it plausibly disguises the face shortly after birth when the infant relies on lying low; see the fourth photo in However, the scimitar-horned oryx (Oryx dammah, see is analogous to the springbok in being, overall, a species with conspicuously pale colouration. And, as in the case of the springbok, the malar stripe seems to detract from the showiness of the face, which leaves us puzzled as to its adaptive value.

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13 de junio de 2021

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 and retained with additional accentuation by some individual adult females (see 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, 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.

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16 de junio de 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 (see The tail is wagged and raised during walking and trotting, but is held in 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

In Nanger, the tail is simple and undemonstrative, even during stotting. The tassel is small, and in Nanger dama almost absent (see

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

In Antilope, the fur on the tail tapers to a point excluding any dark tassel. Apart from erection during some bouts of stotting (see, 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 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 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 and

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17 de junio de 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 (see and 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

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 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 and and and 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 Although the front of the face becomes whitish in adulthood, the adjacent throat is only moderately dark.

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22 de junio de 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

A contributing feature is the extension of the ventral whitish of the torso to a level above the stifle-fold. The stifle (, 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, pure white extends several centimetres above the stifle-fold, to a position halfway up the lateral profile. By contrast, in the gerenuk (see and, there is virtually no clearance.

The following series exemplifies the intermediate positions.

Western dama gazelle (see and

Females of northwestern subspecies of blackbuck (see

Sand gazelle (see and

Dorcas gazelle (see

Arabian gazelle (see and

Thomson's gazelle (see and

Indian gazelle (see and

Red-fronted gazelle (see and

Females of southeastern subspecies of blackbuck (see

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 not particularly great and the bleeze on 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.

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01 de junio de 2021

Regional interspecies mimicry in conspicuous colouration of ruminants?

All three of the largest ruminant species in the Saharan region have extremely pallid colouration, unmatched by desert ungulates elsewhere. Why have the scimitar-horned oryx (Oryx dammah,, addax (Addax nasomaculatus, and dama gazelle (Nanger dama, converged in this way?

Likewise, four sympatric species of ruminants in western North America resemble each other in pale patterns, in this case a bold whitish patch on the hindquarters. Why have the wapiti (Cervus canadensis,, Rocky Mountain mule deer (Odocoileus hemionus hemionus, and, bighorn sheep (Ovis canadensis, and pronghorn (Antilocapra americana, converged in this way?

In both cases, the regional convergences have arisen independently in unrelated clades, as if to suggest some sort of interspecies mimicry.

In Muellerian mimicry, various toxic or venomous species converge on a single pattern of warning colouration, apparently to reinforce a shared message to potential predators. In the case of the above ungulates, there is no question of warning colouration in this strict sense, because there is scant ability to harm predators in self-defence. Instead the relevant anti-predator tactic, in addition to gregarious vigilance and fleeing, is of self-advertisement of individual fitness and vigour. This show-off pattern, accentuated by stotting gaits, persuades the scanning predator that the individual has no debility that would make it vulnerable. Could something similar to Muellerian mimicry apply here?

The relevant message to the potential predator is 'before you attack me, here is some honest information suggesting that it may be too costly to you'. In the case of Muellerian mimicry as strictly defined, the information is about chemical harm that the prey animal could inflict on the predator. But if we broaden the concept, then information about fitness could be similarly useful inasmuch as a futile chase costs the predator energy and opportunity while at the same time risking accidental injury and the attracting of unwanted attention from the predator's own predators.

If this working hypothesis makes enough sense to deserve testing, which new name should we adopt for this type of interspecies resemblance?

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19 de junio de 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, 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 ( and and Sometimes the tail remains down but the long fur on the buttocks is piloerected as the animal stands in initial alarm ( and and and third photo in 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 and third photo in and The tail may be wagged from side to side while erect and flared. Sometimes, the animal stots ( and second photo in

Erection of the tail and piloerection of the tail/buttocks tend to be most frequent in females and juveniles (, and in open vegetation. Adult males sometimes tuck their tails in while fleeing, thus showing no caudal flag ( ); 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.

Ingresado el 19 de junio de 2021 por milewski milewski | 0 comentarios | Deja un comentario

20 de junio de 2021

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 and and Also, the white on the tip of the inert tail extends narrowly up the sides of the tail (see and and These two white features of the stationary mountain reedbuck, although noticeable only at fairly close quarters, distinguish it from both the grey rhebok (see 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 and In the mountain reedbuck it is the white front of the upper lips that is more noticeable than in the grey rhebok (see and 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.

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