Archivos de diario de agosto 2020

23 de agosto de 2020

Puzzles about the geographical distribution of the bush duiker

The bush duiker (Sylvicapra grimmia) is the most widespread and altitude-tolerant species of wild ungulate in Africa, but its distribution and habitat are inconsistent and unpredictable in certain ways. Here are the most important questions.

Firstly, why has the species not occurred in Mediterranean North Africa, and which species - if any - has replaced it ecologically, there? The nominate subspecies grimmia (see https://www.inaturalist.org/observations/64169127) is common in various vegetation types in the mediterranean-type climate of South Africa, suggesting that the species should also occur in the similar climate of rainy winters and dry summers in the other hemisphere. Yet the genus Sylvicapra seems absent even in the fossil record in North Africa.

Secondly, why does subspecies pallidior (see https://www.inaturalist.org/observations/42383809), adapted to the Sahel of the Sudan and Chad, not extend farther west? And why does the similar, but disjunct, subspecies coronata (see https://www.facebook.com/Niokolo.Koba.Expedition/photos/a.1520863891514532/1675677672699819/), occur only in extreme western West Africa? Put together, the puzzle is: why is the bush duiker absent from the Sahel all the way between Senegal and Chad?

Thirdly, why is the bush duiker absent from most of the Horn of Africa (from Eritrea through Djibouti and Somaliland to Somalia and eastern Kenya), given that it is widespread in similarly dry climates in southern Africa (subspecies steinhardti) and the eastern Sahel?

Fourthly, why is it that, to this day, not one photo of the bush duiker has been published from the whole Serengeti Ecosystem, including Ngorongoro Conservation Area and the Maasai Mara National Reserve? The subspecies here is nyansae, which is arguably synonymous with hindei, the subspecies of the Kenyan highlands east of the Great African Rift. And why are there so few photos of the bush duiker - apart from freshly-dead trophies posted on the websites of the hunting industry - from elsewhere in Tanzania, where the subspecies is orbicularis? In contrast to the extreme frequency of photos from e.g. Kruger National Park in South Africa (where the subspecies is caffra), I have yet to see any photo of this species from Manyara, Ruaha, Saadani, Mkomazi, Rukwa, Selous and other reserves in Tanzania. Indeed, there is a puzzling overall scarcity of photos of orbicularis (an exception is https://www.inaturalist.org/observations/4988736), despite its wide distribution from the lower Zambesi River across most of Mozambique and Tanzania to eastern Kenya (where there is also a lack of photos in the Tsavo national parks).

Ingresado el 23 de agosto de 2020 por milewski milewski | 5 comentarios | Deja un comentario

31 de agosto de 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, an aspect of this peculiarity has been 'hiding in plain sight': the unusual simplicity of its colouration.

Ruminants with colouration adapted for inconspicuousness usually have some disruptive markings which help to camouflage the figure. The inconspicuous colouration of the grey rhebok lacks such complications and epitomises countershaded plainness (see https://www.inaturalist.org/observations/10966085 and https://www.inaturalist.org/observations/7026114 and https://www.inaturalist.org/observations/80193150 and https://www.inaturalist.org/observations/11212233). Here we have a prime example of an antelope able to hide - even as a group of individuals - in open vegetation merely by standing still with no attempt to crouch; and this is partly by virtue of the matt effect of its unusually woolly fur, which lacks the slight gloss seen in reedbucks.

When it begins to flee the grey rhebok suddenly becomes conspicuous with equal simplicity by curling its modest-size tail up to reveal a luminescent, handkerchief-sized patch of white which is obvious at ranges up to several hundred metres (see https://www.inaturalist.org/observations/26684308 and https://www.inaturalist.org/observations/55964688 and https://www.inaturalist.org/observations/55964696 and https://www.inaturalist.org/observations/10787618).

Although many field guide-books 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 transformative than that of the grey rhebok because these antelopes have complicated rather than plain colouration; their tails are not consistently raised and do not flash much white; and they lack the rocking horse-like running style of the grey rhebok, which throws up the tail.

The grey rhebok transforms itself from inconspicuous to conspicuous merely by means of a moving figure to which attention is drawn by an economical form of caudal flagging. Several species of deer also advertise flight by means of white caudal flags, but none of them matches the grey rhebok in combining extreme crypsis with extreme simplicity of the tail and its actions.

Perhaps a reason why the grey rhebok is specialised in this way is that it naturally occurs as sparse populations. Even in prime habitat for the grey rhebok in the South African mountains, a predator could patrol for days without encountering it, inevitably limiting the attentiveness of predatory scanning. Furthermore, the grey rhebok, like the eland (Taurotragus oryx), has eyesight even sharper than standard for antelopes of open spaces.

The result: the grey rhebok tends to rely on being overlooked right out in the open, partly because it can spot the predator coming and stand still until it has passed. This is the antithesis of the strategy of e.g. the springbok (Antidorcas marsupialis) in similarly open vegetation on the nearby plains. This gazelle is similar in body size, but has colouration designed to flaunt its presence (see https://www.inaturalist.org/observations/58461204).

Ingresado el 31 de agosto de 2020 por milewski milewski | 0 comentarios | Deja un comentario

25 de agosto de 2020

Contrary to field guide-books, reedbucks do not flag the tail in alarm

It is stated repeatedly in field guide-books that reedbucks (genus Redunca, https://en.wikipedia.org/wiki/Reedbuck) raise the tail in alarm, thus displaying the white underside.

There is little evidence of such flagging, during either standing or running (see https://www.dreamstime.com/female-bohor-reedbuck-running-female-bohor-reedbuck-redunca-redunca-endemic-to-ethiopia-running-prairei-dinsho-wetland-image145413945 and https://www.mindenpictures.com/stock-photo/southern-reedbuck-(redunca-arundinum)-male-running-rietvlei-nature-reserve/gallery-132-2442-1961-0/detailmodal-0_00559122.html and https://www.gettyimages.com.au/detail/video/shot-0f-reedbuck-male-running-at-okavangoforest-stock-video-footage/477433085 and https://www.dreamstime.com/female-bohor-reedbuck-running-female-bohor-reedbuck-redunca-redunca-endemic-to-ethiopia-running-prairei-dinsho-wetland-image145414156 and https://kenya.hsmagazine.digital/reedbuck-the-african-antelope-article-by-gareth-jones-3/ and https://www.shutterstock.com/image-photo/southern-mountain-reedbuck-redunca-fulvorufula-running-1024727896 and https://www.agefotostock.com/age/en/details-photo/jumping-reedbuck-masai-mara-national-reserve-kenya/WE109495/1 and https://www.flickr.com/photos/timmelling/33417983882 and https://www.artranked.com/topic/Reedbuck#&gid=1&pid=17 and https://www.artranked.com/topic/Reedbuck#&gid=1&pid=6).

Is this an example of an error, once published, being mindlessly passed from one author to another like a meme, decade after decade?

(Incidentally, among those who have been misled are taxidermists: see https://www.zululandtaxidermy.com/mountain-reedbuck and https://africanwildlifeartistry.co.za/mountain-reedbuck-floor-base-full-mount/).

Instead, one of the most interesting aspects of reedbucks is how little they raise the tail in alarm, given that the structure and colouration of the tail seem suited to caudal flagging, and given that at least two of the three species of reedbucks stot in alarm. Several ruminants which stot raise the tail or pilo-erect the fur on the hindquarters as part of the show, but I have never seen evidence that reedbucks do either.

The way that two of the three species do advertise themselves when alarmed is by loud whistling and the production of popping sounds from the hindquarters, presumably from the inguinal glands. This suggests that reedbucks rely on communicating with their predators by sound and smell rather than visually.

So, is olfactory communication in reedbucks related to the fact that waterbucks, unusually among ruminants, discourage predation by means of disgusting substances on the skin and fur? Waterbucks (Kobus ellipsiprymnus and K. defassa) belong to the same bovid tribe, namely Reduncini, as reedbucks.

The mountain reedbuck (Redunca fulvorufula) is particularly puzzling in this way. I have certainly heard it whistle loudly (https://sounds.bl.uk/Environment/Listen-to-Nature/022M-LISTNAT00076-0001V0), but in which ways does it display to predators? It does not seem to have been recorded flagging its tail in alarm or flight, or stotting, or popping its inguinal glands.

Thinking about this more deeply: it seems to me that an overarching oddity of reduncin bovids is that they have largely transferred their glandular communications from the social realm, normal for other antelopes, to the anti-predator realm. Unlike many antelopes, reduncins do not mark socially by means of preorbital or interdigital glands. They do possess well-developed glands - in the case of reedbucks including a bare patch below the ear-base (see https://www.inaturalist.org/observations/65592013) - but it seems possible that the smells produced are more for communication with predators than for intraspecific communication.

If so, would this help to explain the apparent redundancy of caudal flags in reedbucks?

Ingresado el 25 de agosto de 2020 por milewski milewski | 6 comentarios | Deja un comentario