ארכיון יומן של אוגוסט 2020

אוגוסט 23, 2020

five top mysteries about geographical distribution of common duiker

I have just spent six weeks perusing the thousands of photos of Sylvicapra grimmia available on the internet. I have also reviewed the account of this species by Groves and Grubb (2011).

The common duiker is one of the most widespread of ungulates in Africa, but its distribution and habitat are puzzling in certain ways. Here are the five most important puzzles.

Firstly, why has S. grimmia not occurred in Mediterranean North Africa? And which species has replaced it ecologically, there?

Secondly, why does subspecies (treated as a full species by Groves & Grubb 2011) pallidior not occur west of Chad despite being adapted to the Sahel?

Thirdly, why is S. grimmia absent from most of the Horn of Africa, given that it is widespread in similarly dry climates in southern Africa and the eastern Sahel?

Fourthly, why does subspecies (treated as a full species by Groves & Grubb 2011) coronata occur only in extreme western West Africa?

Fifthly, why is it that, to this day, not one photo of the common duiker has been published from the whole Serengeti Ecosystem (including Ngorongoro Conservation Area and the Maasai Mara National Reserve)?

[Footnote: a main reason why Groves & Grubb (2011) gave pallidior species-status is that this form is extremely small and seems distinct despite overlapping in range with subspecies campbelliae. A main reason why these authors gave coronata species-status is that this form has an odd skull-shape. Groves & Grubb (2011) describe this skull as peculiarly narrow but the few photos show me that it also has oddly wide orbits.]

written August 2020

פורסם ב אוגוסט 23, 2020 09:08 אחה"צ על־ידי milewski milewski | 1 comment | הוספת תגובה

אוגוסט 25, 2020

The questionable assumption that reedbucks display the tail in alarm

it is stated repeatedly in the literature that reedbucks (genus Redunca) raise the tail in alarm, this displaying the white underside. I have never observed this in the field for any of the three species and I cannot recall seeing any photo showing this. So I wonder how this apparently misleading information arose.

The white-tailed deer (Odocoileus virginianus) is the wild ruminant most familiar to North Americans, and in summer coat it happens to resemble reedbucks. The tails have similar size and bushiness, and similarly are white underneath. So anyone knowing the white-tailed deer might understandably tend to assume that reedbucks would emulate the deer in displaying the tail.

In fact, the difference could hardly be greater. Firstly, the white-tailed deer not only raises it's tail, it fully erects it and pilo-erects the long white hairs on it's underside, and then waves the upright, fanned tail from side to side as it runs. Secondly and simultaneously, it flares out the white fur on the posteriormost haunch, more than doubling the 'blaze' of extremely conspicuous white on the hindquarters.

Far from having converged evolutionarily with this display, reedbucks do not seem even to possess the anatomical structures needed for it. They do advertise themselves when alarmed, but in a peculiar and poorly-understood way involving the production of popping sounds from hindquarters.

(By the way, the white-tailed deer produces a nasal sound comparable to the whistling of reedbucks, while standing alarmed and watching the intruder. But here again the difference is that the deer flicks its tail up synchronously with each 'sneeze', in this case without any erection of white fur so that the display is one of dramatic motion rather than whiteness.)

To dispel any misconception that reedbucks display the tail in flight, here is a challenge to naturalists. Can anyone show us a video or photo of any reedbuck displaying its tail in any way in alarm? And if not should we begin to write, in field-guides on African wildlife, that a remarkable aspect of reedbucks is the inertness of tails that seem bushy and white enough to be suitable for flagging alarm?

פורסם ב אוגוסט 25, 2020 12:26 לפנה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

אוגוסט 31, 2020

An easily overlooked but extreme adaptation in the grey rhebok

The grey rhebok (Pelea capreolus) has long been recognised as unrelated to other living antelopes despite a superficial resemblance to reedbucks. However, up to now an obvious aspect of this peculiarity has been 'hiding in plain sight'.

The colouration of the grey rhebok is a prime example of crypsis, i.e. plainness designed to blend into the surroundings. Few African antelopes have such uncomplicated markings, so that even a group of six can be overlooked as they stand among the low shrubs of the fynbos.

However, when alarmed the grey rhebok does something not done to the same degree by any other antelope or, indeed, any other bovid. It transforms itself in the simplest but most effective possible way by curling its tail over its rump, producing an unnaturally luminescent, handkerchief-sized patch of white which can be spotted by the naturalist at ranges up to several hundred metres.

Although many field guidebooks state or imply that reedbucks similarly display themselves when fleeing, such is not the case. And any displays of the white underside of the tail by bushbucks, kudus and their relatives are less noteworthy than that of the grey rhebok because the tails are not consistently raised and do not flash much white, and these antelopes have complicated rather than plain colouration in the first place.

The springbok (Antidorcas marsupialis) has an equally spectacular but far more complicated display of white on its hindquarters, produced by unfolding a fur-lined fold of skin on the rump. But this is categorically different from the case of the grey rhebok, because the overall colouration of the springbok (as in gazelles generally) is designed to be conspicuous, not cryptic or camouflaged. So, the springbok when fleeing transforms a flag on its already flag-like body, but does not emulate the grey rhebok in transforming itself from effective invisibility into a white flag.

In this ability to transform its appearance, no bovid and perhaps not any deer rivals the grey rhebok. And this extreme adaptation has been configured in a deceptively simple way, by deploying a tail, in itself similar in its white underside to that of reedbucks and bushbucks, on the plainnest of bodies. All that is needed is for the tail to be raised and held raised in flight.

פורסם ב אוגוסט 31, 2020 02:24 לפנה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

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