Αρχεία Ημερολογίου για Ιούνιος 2021

Ιούνιος 01, 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 following 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 following 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?

Posted on Ιούνιος 01, 2021 0155 ΠΜ by milewski milewski | 0σχόλια | Αφήστε ένα σχόλιο

Ιούνιος 02, 2021

The eclipsed life of the western dama gazelle (Nanger dama mhorr)

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 https://www.cokesmithphototravel.com/expedition-to-tunisia.html, 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 https://www.dreamstime.com/stock-photo-dama-gazelle-standing-eating-leaves-image42716737) 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. https://www.biolib.cz/en/image/id304240/ and https://www.alamyimages.fr/photo-image-bebe-gazelle-dama-dama-mhorr-nanger-92733857.html and https://www.zooborns.com/zooborns/2017/03/endangered-dama-gazelles-arrive-with-the-spring.html).

Can the social biology of the western dama gazelle explain why infants have distinctive colouration whereas fully mature males do not?

Posted on Ιούνιος 02, 2021 0832 ΠΜ by milewski milewski | 0σχόλια | Αφήστε ένα σχόλιο

Ιούνιος 07, 2021

How does infantile colouration inform subspecies in the dama gazelle (Nanger dama)?

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 https://www.alamyimages.fr/photo-image-bebe-gazelle-dama-dama-mhorr-nanger-92733857.html and https://www.zooborns.com/zooborns/2017/03/endangered-dama-gazelles-arrive-with-the-spring.html and https://www.biolib.cz/en/image/id304238/) .

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 https://www.zooborns.com/zooborns/2008/11/baby-gazelle-at-the-national-zoo.html and https://www.washingtonian.com/2017/09/21/prepare-to-fall-in-love-with-the-new-baby-gazelles-at-the-national-zoo/ and https://www.smithsonianmag.com/smithsonian-institution/dama-gazelle-calf-born-smithsonian-national-zoo-180970587/).

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.

Posted on Ιούνιος 07, 2021 0739 ΠΜ by milewski milewski | 0σχόλια | Αφήστε ένα σχόλιο

Ιούνιος 08, 2021

The peculiar caudal flag of the western dama gazelle (Nanger dama mhorr)

The western dama gazelle (Nanger dama mhorr) has such striking colouration that it is easy to overlook how peculiar the tail is relative to other gazelles.

All species of Gazella, Eudorcas and Ammodorcas have a tail that is large, dark and mobile enough to constitute a caudal flag, particularly in juveniles. The tail is wagged and/or raised in various situations, postures and gaits.

For example, the erect tail of the goitred gazelle (Gazella subgutturosa) is so obvious when the animal flees that an alternative name for the species is 'black-tailed gazelle'.

In Thomson's gazelle (Eudorcas thomsoni) the tail is wagged nervously as the alert or initially alarmed animal walks slowly and indecisively about in the face of possible danger (see https://www.shutterstock.com/ja/video/clip-21099121-thomson's-gazelles-walking-maasai-mara-kenya-africa and https://dissolve.com/video/Thomson-Gazelles-Grazing-Long-Grass-Maasai-Mara-Kenya-rights-managed-stock-video-footage/002-D806-97-026).

The tail skeleton of the western dama gazelle is about as long, proportional to the body, as in other gazelles, but the tassel - proportionately the smallest of any gazelle - looks negligible.

Furthermore, the tail is not demonstrative in adults, being largely inert except when swished to shoo insects. Even when the adult stots to show off its fitness, the tail is kept down.

One way to interpret this is that the whole figure is so conspicuously emblazened, with the rump and buttocks in particular so conspicuously white, that accentuation by the tail is superfluous in this subspecies.

The tail of the infant of the western dama gazelle is small relative to those of the infants of other gazelles, owing to the extreme diminution of the tassel.

However, it does constitute a caudal flag (see https://www.alamy.com/gazelle-in-blijdorp-rotterdam-the-netherlands-image240604149.html). At birth, the tail is pale fawn at the base and tip, but covered in white erectile hairs along the middle of the stalk.

The tail is raised and wagged demonstratively when the infant stots playfully, as well as while it suckles. Until the horn-tips emerge, the infant has a thoroughly cryptic and camouflaged pattern of colouration except for the raised or moving tail.

What is peculiar about the western dama gazelle is not that the tail is most conspicuous in infancy and when waved.

Instead it is that this conspicuousness is achieved by means of only a limited tract of fur, which is brought to some contrast with the dullness of the rest of the figure by means of piloerection.

The result is that, in this subspecies, the tail is converted from the most noticeable part of the infantile figure to one of the least noticeable parts of the adult figure (see https://www.alamy.com/gazelle-in-blijdorp-rotterdam-the-netherlands-image240604268.html) - with minimal anatomical change as the animal grows.

More than in any other antilopin bovid, the western dama gazelle simply outgrows its tail.

Posted on Ιούνιος 08, 2021 0351 ΠΜ by milewski milewski | 0σχόλια | Αφήστε ένα σχόλιο

Ιούνιος 10, 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 https://www.inaturalist.org/photos/45006655) 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 https://www.inaturalist.org/observations/56238889).

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 https://www.inaturalist.org/observations/74839633). (In the same species, it plausibly disguises the face shortly after birth when the infant relies on lying low; see the fourth photo in https://fossilrim.org/animals/gemsbok).

However, the scimitar-horned oryx (Oryx dammah, see https://www.inaturalist.org/photos/122472718) 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.

Posted on Ιούνιος 10, 2021 1142 ΜΜ by milewski milewski | 0σχόλια | Αφήστε ένα σχόλιο

Ιούνιος 13, 2021

Does Peters' gazelle (Nanger granti petersi) resemble genus Gazella in its differences from Grant's gazelle (Nanger granti granti)?

@variani18 @capracornelius

Peters' gazelle (Nanger granti petersi, https://www.inaturalist.org/observations?taxon_id=563632)

  • occurs only in eastern Kenya, and
  • differs from the other subspecies in several ways, apart from the shape of the horns in males.

The body mass of adult males is probably only about 50 kg (similar to that of males of the common impala, Aepyceros melampus), compared to 60 kg or more in nominate Nanger granti granti.

Adult females seem not to have been weighed. However, my guess is 35kg or less, compared with an average of 45 kg recorded for the nominate subspecies.

Peters' gazelle is the only subspecies lacking a lateral bleeze, in any individual of either sex and any age.

By 'lateral bleeze', I refer to a pattern on the flanks which is conspicuous enough in profile that it makes the whole figure stand out, rather than blending into the environment.

Instead of distinct bands on the flanks, Peters' gazelle retains only faint darkening on the posterior part of the flank (above the stifle-fold, https://www.inaturalist.org/journal/milewski/53569-adaptive-colouration-in-gazelles-eye-catching-highlights-above-the-stifle-fold#).

The latter darkening is extended into a fully dark flank-band in juveniles of the other subspp. (e.g. https://www.inaturalist.org/observations/8088622).

It is also retained, with additional accentuation, by some individual adult females (see https://www.inaturalist.org/observations/26410343) in populations of Nanger granti notata (https://www.inaturalist.org/observations?taxon_id=568581) living in the Laikipia region (https://en.wikipedia.org/wiki/Laikipia_County) of central Kenya.

All Nanger granti, of all ages and both sexes in all subspp., have a conspicuous whitish pattern - which I call an ischiopygal bleeze - on the buttocks and spilling on to the rump.

In Peters' gazelle, the extension of whitish on to the rump resembles the genus Gazella rather than the rest of Nanger, in

  • having minimal extent, and
  • being divided by a fawn mid-line.

Furthermore, in Peters' gazelle the whitish 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

However, Peters' gazelle remains larger-bodied than any form of Gazella.

Furthermore, Peters' gazelle

Posted on Ιούνιος 13, 2021 1202 ΠΜ by milewski milewski | 0σχόλια | Αφήστε ένα σχόλιο

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 (see https://www.inaturalist.org/observations/33111207 and https://www.youtube.com/watch?v=LBLUtuw964I) 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 two forms migrating in opposite directions and the smaller form being unusually dependent on drinking for a gazelle.

The dorcas gazelle (Gazella dorcas, see https://www.gettyimages.com.au/detail/video/dorcas-gazelle-walking-in-arid-desert-stock-video-footage/1070908712) and the dama gazelle (Nanger dama, see https://www.pond5.com/stock-footage/item/111832882-dama-or-mhorr-gazelle-al-ain-zoo-nanger-dama-mhorr) 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 sand gazelle (Gazella marica, see https://www.biolib.cz/en/taxonimage/id171996/?taxonid=509466&type=1) 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 (e.g. see https://www.inaturalist.org/observations/55092041).

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 Ιούνιος 13, 2021 0709 ΜΜ by milewski milewski | 0σχόλια | Αφήστε ένα σχόλιο

Ιούνιος 15, 2021

Why does Roberts' gazelle (Nanger granti robertsi) show perverse change in colouration in males?

There are two dozen species of gazelles, in eight genera (https://en.wikipedia.org/wiki/Gazelle).

Among these, Roberts' gazelle (Nanger granti robertsi, https://www.inaturalist.org/observations?taxon_id=601873), a subspecies restricted to the Serengeti ecosystem, shows a puzzling change in colouration.

I refer to a shift as juvenile males grow into adults.

Juveniles - although individually variable - feature a graphic, dark/pale pattern of complex banding on the flank (https://www.inaturalist.org/observations/15193833 and https://www.inaturalist.org/observations/101600425 and https://www.inaturalist.org/observations/8088622 and https://www.inaturalist.org/observations/185222855 and https://www.inaturalist.org/observations/168801725 and https://www.inaturalist.org/observations/132505883 and https://www.inaturalist.org/observations/67365186 and https://www.inaturalist.org/observations/65556138).

This is suddenly lost, in males, once the horns reach half their mature length (https://www.inaturalist.org/observations/9945347 and https://www.inaturalist.org/observations/175341965 and https://www.inaturalist.org/observations/158741158 and https://www.inaturalist.org/observations/146105752 and https://www.inaturalist.org/observations/33163647).

(Also please see https://www.inaturalist.org/journal/milewski/69402-variation-in-pigmentation-within-genus-nanger#.)

The following shows the colouration, particularly the consistent lack of any dark flank-band, in mature males of Roberts' gazelle (https://www.inaturalist.org/observations/21262806 and https://www.inaturalist.org/observations/193060570 and https://www.inaturalist.org/observations/8507420 and https://www.inaturalist.org/observations/102285295 and https://www.inaturalist.org/observations/102307911 and https://www.inaturalist.org/observations/63674973 and https://www.inaturalist.org/observations/13850159).

(In mature males, the showiest masculine feature - apart from the long dark horns - is instead the sheeny pale of the unusually brawny upper-neck. This is displayed in masculine proud-postures (https://www.inaturalist.org/observations/38785430 and https://www.inaturalist.org/observations/8507420 and https://www.inaturalist.org/observations/122149173). The colouration on the nape is similar in females, but the great enlargement of the nape of mature males makes the sheeny pale conspicuous, https://www.inaturalist.org/observations/38785430.)

At birth, the flank-band is present (https://www.inaturalist.org/observations/9994326), but not as dark as in juveniles.

Roberts' gazelle is also the only form of gazelle which naturally coexisted, throughout its range, with a smaller, far more abundant species of gazelle.

I refer to Thomson's gazelle (Eudorcas thomsoni nasalis, https://www.inaturalist.org/taxa/74321-Eudorcas-thomsonii).

The two species are often seen together in the Serengeti ecosystem (https://www.inaturalist.org/observations/39594586 and https://www.inaturalist.org/observations/78015635 and https://www.inaturalist.org/observations/149765914 and https://www.inaturalist.org/observations/19920657 and https://www.inaturalist.org/observations/20147086 and https://www.inaturalist.org/observations/151393534 and https://www.inaturalist.org/observations/141078112 and https://iago80.files.wordpress.com/2012/11/img_2050.jpg and https://www.inaturalist.org/observations/14282018 and https://www.sciencedirect.com/science/article/abs/pii/S0003347205807845 and https://www.inaturalist.org/observations/180960987 and https://www.inaturalist.org/observations/181794975 and https://www.inaturalist.org/observations/192591863 and https://www.inaturalist.org/observations/177203141 and https://www.inaturalist.org/observations/168727646 and https://www.inaturalist.org/observations/167692596 and https://www.inaturalist.org/observations/163668908 and https://www.inaturalist.org/observations/151676694 and https://www.inaturalist.org/observations/140305597 and https://www.inaturalist.org/observations/9945634).

And, as it happens:

  • there is resemblance in the flank-banding between this coexisting species and juveniles of Roberts' gazelle, and
  • this resemblance in colouration is greatest when the body size of the juveniles approximates that d
    of adults of Thomson's gazelle (https://www.inaturalist.org/observations/44556951).

(In Thomson's gazelle, unlike Roberts' gazelle, the flank-band is fully dark already at birth. Please compare https://www.alamy.com/stock-photo-newborn-baby-grants-gazelle-standing-79192809.html and https://www.alamy.com/stock-photo-grants-gazelle-with-her-newborn-baby-79192811.html with https://www.naturepl.com/stock-photo-grant-s-gazelle-nanger-granti-mother-and-newborn-just-after-birth-image01425894.html and https://www.alamy.com/stock-photo-grants-gazelle-gazella-granti-mother-and-baby-serengeti-national-park-13563596.html.)

The result is that naturalists often misidentify the juveniles. Typical examples of photos subject to misidentification are https://www.inaturalist.org/observations/56957658 and https://www.inaturalist.org/observations/41918928.

This is an embarrassing error, because it confuses not just different species, but different genera, of gazelles.

In summary so far, juveniles of Roberts' gazelle possess a dark flank-band that is subsequently lost in adulthhood. As far as I know, this phenomenon does not occur in any other species of antilopin bovid.

How can this puzzle be explained?

One possible explanation is that the flank-band functions as disruptive colouration, helping to hide the animals in their environment.

This, however, is unlikely, because:

An alternative explanation for the ontogenetic shift invokes deceptiveness in the form of adaptive mimicry, as follows:
Juvenile males appease mature males by resembling the 'signature-pattern' of a neutral companion-species, namely Thomson's gazelle, that evokes no masculine antagonism from mature males of Roberts' gazelle.

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 ('despotic competition', https://www.sciencedirect.com/science/article/abs/pii/016815919190264X) by the territorial male in the vicinity.

If this is a form of interspecific mimicry, it perhaps could be called... ?

Roberts' gazelle differs from Thomson's gazelle in that it is sedentary/resident, not migratory. It remains on the Serengeti Plain in the dry seasons, when/where

  • all green matter becomes scarce,
  • drinking water is absent, and
  • the ostrich (Struthio camelus massaicus) competes for food.

I hypothesise that it is these periods of scarcity that are crucial for Roberts' gazelle. This is because particularly intense competition for resources arises between adult males and juvenile males, for the scattered green dicotyledonous plants that contain crucial moisture.

Under these circumstances, any alleviation of antagonism, by means of passive deceptive mimicry on the part of the juveniles, may have been favoured by selective pressure.

The hypothetical mimicry in question is likely to be all the more effective because juvenile females - which tend naturally to be exempt from bullying - share the dark flank-band.

The eastern subspecies, Peters' gazelle (Nanger granti petersi, https://www.inaturalist.org/observations?taxon_id=563632), 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, https://www.inaturalist.org/observations?taxon_id=568581), found in the Laikipia region of central Kenya, shows the darkening on the flank.

However, the effect is less confusing than in Roberts' gazelle. This is because adult females tend to retain the graphic pattern, rather than losing it as happens in Roberts' gazelle.

Overall, the situation seems to make sense. This is because in the Laikipia region, Thomson's gazelle is at a limit of its distribution, and does not outnumber the larger species of gazelle (https://www.inaturalist.org/observations/180622456).

In this northerly population, the value of deceptive appeasement may be reduced by a moderation of the masculinity; mature males of N. g. notata do not grow as massive as do those of Roberts' gazelle.

Posted on Ιούνιος 15, 2021 0155 ΠΜ by milewski milewski | 41σχόλια | Αφήστε ένα σχόλιο

Ιούνιος 16, 2021

Differences among gazelles in the structure and function of the tail

In this Post, I make preliminary comparisons among Gazella, Nanger, Eudorcas, Antilope, Antidorcas, Litocranius, Ammodorcas, and Procapra, with respect to the tail as an adaptive organ.

I assume that all species swish the tail to shoo insects. 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. However, other demonstrations by means of the tail are surprisingly diverse.

Please note that

  • no standard terms have been established to compare the tails among ungulates;
  • I use 'shaft' to mean the flesh-and-bone structure, and 'tassel' to mean the long hairs concentrated distally; and
  • 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.

Gazella:

In Gazella, the tail is simple in structure but demonstrative in function, as follows.

The tassel is bushy and dark, and covers most of the shaft (see https://www.dreamstime.com/stock-photo-mountain-gazelle-walking-field-israel-image91922790).

In Gazella, the tail is wagged and raised during walking and trotting.

The tail is held in the fully upright position, in non-stotting flight, only in

The latter two species are odd also in possessing, on the sides of the base of the shaft, pale fringes of hair which can be piloerected (see https://www.flickr.com/photos/144908175@N07/40226721515/).

Nanger:

In Nanger, the tail is simple and undemonstrative, even during stotting (https://nationalzoo.si.edu/animals/dama-gazelle).

In this genus, the tassel is small, and in Nanger dama almost absent (see https://pixels.com/featured/1-addra-gazelle-ncz-17-1-robert-michaud.html).

Eudorcas:

In Eudorcas, the tail is proportionately larger than in Gazella, and demonstrative.

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).

Many naturalists may have the impression that Thomson's gazelle (Eudorcas thomsoni) wags the tail while stationary.

However, 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 https://www.youtube.com/watch?v=LPfG028yzpI and https://www.dreamstime.com/gazelle-herd-wagging-their-tails-serengeti-gazelle-herd-wagging-their-tails-serengeti-national-park-tanzania-video216825150).

Antilope:

In Antilope, the fur on the tail tapers to a point excluding any dark tassel.

The tail in Antilope is undemonstrative, apart from erection during some bouts of stotting (https://www.gettyimages.com.au/detail/video/blackbuck-antelope-pronks-on-grassland-velavadar-stock-video-footage/918318364?adppopup=true and https://www.alamy.com/black-bucks-are-resident-species-of-gujarat-india-and-are-found-in-many-places-these-are-found-in-big-groups-and-doing-jumping-and-running-all-day-image359843002.html?imageid=59E8E834-332A-49CE-B096-7063CC504E1F&p=676501&pn=1&searchId=3c49af6cb8a3a05fe8eb462ea3360571&searchtype=0 and https://www.alamy.com/black-bucks-are-resident-species-of-gujarat-india-and-are-found-in-many-places-these-are-found-in-big-groups-and-doing-jumping-and-running-all-day-image359843063.html?imageid=2E159E96-68A4-4276-997E-3EA0F27CAE46&p=676501&pn=1&searchId=3c49af6cb8a3a05fe8eb462ea3360571&searchtype=0).

The anomaly in Antilope is masculine display, in which the tail is held 'hypererected', to the degree of being turned 'upside-down' (https://www.inaturalist.org/posts/47421-surprising-differences-in-displays-of-the-tail-between-the-blackbuck-and-other-gazelles#).

Antidorcas and Litocranius:

Antidorcas and Litocranius resemble each other in having similarly tapering shafts, ending in similarly dark, small tassels.

However,

The stotting displays of Antidorcas and Litocranius are extremely different.

However, in neither case is the tail demonstrative. When Antidorcas stots (https://www.youtube.com/watch?v=jMIiB9DnRXg), the tail seems redundant, in view of the flared white pelage on the rump and buttocks.

Ammodorcas:

Ammodorcas is ecologically similar to Litocranius, but surprisingly different in the form and function of the tail.

The dark tail is particularly noticeable (https://uk.inaturalist.org/taxa/42272-Ammodorcas-clarkei). This is because it is

  • longer than in any other gazelle, and
  • erected during fleeing.

Procapra:

Finally, we have the genus Procapra (see https://www.inaturalist.org/journal/milewski/69383-adaptive-colouration-in-procapra#), which is anatomically at the other extreme from Ammodorcas in terms of the length of the tail.

The tail is too short in Procapra to warrant coverage in this Post.

Posted on Ιούνιος 16, 2021 0136 ΜΜ by milewski milewski | 3σχόλια | Αφήστε ένα σχόλιο

Ιούνιος 17, 2021

A first attempt to identify bleezes in the gazelle genus Nanger

I define a bleeze as follows:
any feature of animal colouration that shows so much pale/dark contrast, at such large scale, that it renders the whole figure obvious to scanning predators - even when the figure remains stationary.

Such advertisement is interesting adaptively, because it defies a basic strategy of prey species in avoiding detection.

The gazelle genus Nanger (https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=71873&view=species) exemplifies the complex variation in the incidence/development of bleezes among the species, subspecies, ages, and sexes of ungulates.

Once bleezes are identified, we can begin to weigh the evolutionary costs and benefits of the conspicuous colouration.

All individuals of Nanger granti (https://www.inaturalist.org/taxa/74724-Nanger-granti), including standing infants, show a combination of

  • whitish buttocks, and
  • blackish pygal bands.

I call this an ischiopygal bleeze.

The position of this pattern on the hindquarters means that it is most conspicuous in posteriolateral view (see https://shootplanet.photoshelter.com/image/I0000WGHanq6qvr8 and https://destinationuganda.com/travel-guide/mammals/grants-gazelle/).

Therefore, Nanger granti can be classified as a species consistently possessing a bleeze on the hindquarters. However, this is the only clear case in this genus.

A distinct bleeze also occurs in Nanger dama (https://www.inaturalist.org/taxa/74723-Nanger-dama). However, this refers only to subspecies mhorr, and excludes infants even in this subspecies.

The whitish on the rump and buttocks is more extensive in N. dama than in Nanger granti. However, any bleeze remains somewhat ambivalent. This is 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 (https://www.inaturalist.org/observations?taxon_id=601873) and Nanger granti notata (https://www.inaturalist.org/observations?taxon_id=568581) temporarily develop a blackish flank-band, contrasting with both

This lateral bleeze disappears in all adult males. However, it is retained by some adult female individuals of Nanger granti notata.

Nanger dama mhorr may qualify for a different pattern of lateral bleeze. However, this is ambivalent, because there are no flank-bands.

Nanger soemmerringi (https://www.inaturalist.org/taxa/74725-Nanger-soemmerringii) lacks any posteriolateral or lateral bleezes. This is because no photo shows sufficient dark on the hindquarters or flanks.

However, in at least one subspecies of N. soemmerringi, 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 and http://www.arthurgrosset.com/mammals/photos/nansoe46234.jpg).

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 Ιούνιος 17, 2021 0650 ΜΜ by milewski milewski | 2σχόλια | Αφήστε ένα σχόλιο