An easily overlooked but extreme adaptation in the grey rhebok

@stuartshearer

(Also see https://www.inaturalist.org/posts/53474-a-comparison-of-adaptive-colouration-between-lookalikes-grey-rhebok-and-mountain-reedbuck#)

It has long been recognised that the grey rhebok (Pelea capreolus, https://www.inaturalist.org/taxa/42336-Pelea-capreolus) is 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 (https://www.inaturalist.org/observations/99271720). 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 meters (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 (https://www.inaturalist.org/journal/milewski/39738-contrary-to-field-guide-books-reedbucks-do-not-flag-the-tail-in-alarm#).

And any displays of the white underside of the tail by bushbucks (https://www.zoochat.com/community/media/bushbuck-male.279806/ and https://www.hluhluwegamereserve.com/bushbuck-facts/), kudus and their relatives are less transformative than that of the grey rhebok because these antelopes

  • have complicated rather than plain colouration,
  • have tails that are not consistently raised and do not flash much white, and
  • lack the rocking horse-like running style of the grey rhebok, which gives the tail an extra bobbing motion at a distance.

The grey rhebok economically transforms itself from unusually inconspicuous to unusually conspicuous merely by means of initiating movement and flagging its tail (https://www.inaturalist.org/observations/109537775).

Several species of deer (e.g. https://en.wikipedia.org/wiki/White-tailed_deer) likewise 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 common eland (Taurotragus oryx, https://en.wikipedia.org/wiki/Common_eland), 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.

Note the difference in strategy from e.g. the springbok (Antidorcas marsupialis, https://en.wikipedia.org/wiki/Springbok), which lives in similarly open vegetation but on the nearby plains.

This gazelle is similar in body size to the grey rhebok, but has colouration designed to flaunt its presence (https://www.inaturalist.org/observations/85440835 and https://www.inaturalist.org/observations/58461204 and https://www.inaturalist.org/observations/103583187 and https://www.inaturalist.org/observations/106052235).

Also see https://www.inaturalist.org/journal/milewski/73102-the-possible-adaptive-value-of-high-eyes-and-ears-in-the-grey-rhebok-pelea-capreolus#.

Posted by milewski milewski, 31 August, 2020 02:24

Comments

An incredible piece of work @milewski ! Do you have any more articles on South African antelope that you could share?
You see, I'm currently working on my next article, which is all about tracking hooved mammals in the Southern Cape.

Posted by ludwig_muller 7 months ago (Flag)

I'm going through it now, thanks!

Posted by ludwig_muller 7 months ago (Flag)

A difference between Raphicerus campestris campestris (https://www.inaturalist.org/observations/64700091) and coexisting Raphicerus melanotis is that only the former folds its ear pinnae back and down when 'freezing' in mild alarm, in low vegetation.

The ear pinnae of the two species are similar in size and colouration. However, folding the ears is adaptive in R. campestris is adaptive because this species is diurnally active, and prefers vegetation too low to hide the standing figure.

Posted by milewski 3 months ago (Flag)

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