The northwestern oribi (Ourebia montana quadriscopa, according to my recent revision of this genus) has adaptively inconspicuous colouration.

The pattern is 'plain', i.e. cryptic (https://en.wikipedia.org/wiki/Crypsis).

In this way, the northwestern oribi conforms with several other ruminants with which it coexists in the savannas of West Africa, viz.

• Sylvicapra grimmia campbelliae (https://en.wikipedia.org/wiki/Common_duiker#/media/File:C%C3%A9phalophe_de_grimm,_crop.jpg),
• Redunca redunca redunca (https://www.alamy.com/stock-photo-bohor-reedbuck-redunca-redunca-female-in-the-bush-pendjari-national-58248297.html?imageid=7C115559-9E07-4EDB-9D6E-8B9C35DFDD71&p=86574&pn=1&searchId=821450ac1fd6e6be4349c0eddb7130c5&searchtype=0), and
• Kobus kob (https://www.alamy.com/stock-photo-buffons-kob-western-kob-kobus-kobus-kob-female-pendjari-national-park-57893965.html and https://www.alamy.com/stock-photo-buffon-kob-or-western-kob-kobus-kob-male-or-buck-grazing-mole-national-82292113.html and http://www.arthurgrosset.com/mammals/photos/kobkob36134.jpg).

Another coexisting ruminant, namely Tragelaphus scriptus (https://www.mindenpictures.com/stock-photo/southern-bushbuck-(tragelaphus-scriptus-sylvaticus)-immature-male-standing/search/detailmodal-0_80126209.html), also has adaptively inconspicuous colouration.

However, in T. scriptus the pattern, instead of being 'plain', is disruptive (https://en.wikipedia.org/wiki/Disruptive_coloration). This is because its intricate markings break up the outline of the figure, against a background of plant stems and dappled shade.

Despite the overall plainness of Ourebia montana quadriscopa, the details of its colouration are noteworthy.

The pale ventral surfaces on the body and inner surfaces of the upper limbs (https://www.inaturalist.org/observations/66999215 and https://www.inaturalist.org/observations/16025242) are consistent with the cryptic principle of countershading (https://en.wikipedia.org/wiki/Countershading).

Minor aspects of the colouration of O. m. quadriscopa, which may have some conspicuous function, are as follows:

• whitish hair-curtains on anterior surface of ear pinnae (in another ssp.: https://www.alamy.com/stock-photo-kenya-masai-mara-game-reserve-oribi-ourebia-ourebi-104129882.html?imageid=7D882469-F59E-427D-BC5D-9D8EF92E5CEC&p=214718&pn=1&searchId=b51320e4aa8f23d8475c9a13e60a54fb&searchtype=0),
• indistinct dark markings on posterior surface of ear pinnae (in other sspp.: https://www.alamy.com/head-of-a-female-oribi-ourebia-ourebi-showing-its-huge-ears-and-eyes-serengeti-national-park-tanzania-image335368004.html?imageid=5BD52F69-0A72-4ABC-9005-5CA746115A8C&p=39397&pn=1&searchId=d592d238c126347ec4d3725c2a7bbfc9&searchtype=0 and https://www.dreamstime.com/female-oribi-ourebia-ourebi-known-as-oorbietjie-afrikaans-serengeti-national-park-tanznia-africa-female-oribi-ourebia-ourebi-image179765613),
• a pale stripe between each eye and the crown/rostrum (https://www.inaturalist.org/observations/17940997 and https://www.inaturalist.org/observations/67005619),
• pale pelage on the upper lips, lower lips, chin, and crook-of-throat (https://www.inaturalist.org/observations/49107479 and https://www.inaturalist.org/observations/8490542),
• slight pale (possibly sheeny) at the base of the ear pinnae (see below for further details),
• dark bare skin in a patch ventral to the base of each ear pinna (https://www.inaturalist.org/observations/67004330 and https://www.inaturalist.org/observations/8490559),
• consistent darkness of the upper surface of the small tail-tassel, which occupies most of the short tail (https://www.inaturalist.org/observations/67005602 and https://www.inaturalist.org/observations/8490541),
• pale pelage on the lower surface of the tail (https://www.inaturalist.org/observations/67004298),
• narrow bracketing of the base of the tail, by pale pelage (https://www.inaturalist.org/observations/49539984),
• dark bare skin on the udder/groin, extending to the junction of hindleg and flank (https://www.inaturalist.org/observations/6441849 and https://www.alamy.com/oriba-ourebia-ourebi-single-mammal-on-grass-uganda-august-2018-image217064210.html?imageid=85B0EE61-8AB6-49DF-A27C-35AADBE48F51&p=2585&pn=1&searchId=b51320e4aa8f23d8475c9a13e60a54fb&searchtype=0),
• dark bare skin around anus/vulva, extending to lower surface of tail (in another ssp.: https://www.alamy.com/oribi-ourebia-ourebi-male-at-a-kopje-serengeti-national-park-tanzania-image462237216.html?imageid=6B66D5F5-0A98-412F-AFA1-061BFAB156DE&p=35179&pn=1&searchId=b51320e4aa8f23d8475c9a13e60a54fb&searchtype=0), and
• pale (and probably sheeny) pelage on the entire surface (including the posterior surface) of the pasterns (https://en.wikipedia.org/wiki/Pastern), both fore and hind.

Please note that an individually variable dark spot on the crown of females, which is present in northeastern sspp. of Ourebia montana (https://www.imago-images.com/st/0067825819? and https://www.alamy.com/oribi-ourebia-ourebi-small-antelope-from-african-bushes-and-savannahs-murchison-falls-uganda-image448339248.html and https://www.inaturalist.org/observations/98122948), seems generally absent in O. m. quadriscopa.

Of the above, most can be explained as minor features of disruptive colouration, subsidiary to the main pattern of crypsis.

The main feature that, although small-scale, is unambiguously conspicuous in function, is a pedal flag.

This is because

• it cannot confer any advantage in terms of hiding the stationary figure,
• it can aid intraspecific monitoring without increasing the risk of being noticed by scanning predators, owing to the different distances involved.

The pasterns are pale enough to accentuate the action of walking. I suspect that this paleness is more conspicuous in ultraviolet, and that, to the relevant eyes (i.e. those of the oribi itself), the flag is visible by night. In addition, I suspect that this pedal flag is more conspicuous in ultraviolet than in the spectrum visible to the human eye.

The following show this subtle pedal flag in Ourebia ourebi quadriscopa:

https://www.inaturalist.org/observations/8490596
https://www.inaturalist.org/observations/49107232
https://www.inaturalist.org/observations/8490605
https://www.inaturalist.org/observations/16099463
https://www.inaturalist.org/observations/29326368
https://www.inaturalist.org/observations/91161569
https://www.inaturalist.org/observations/91165825
https://www.inaturalist.org/observations/67044396
https://www.inaturalist.org/observations/67044375
https://www.inaturalist.org/observations/67005624
https://www.inaturalist.org/observations/6441964
https://www.inaturalist.org/observations/67005621
https://www.inaturalist.org/observations/8490600
https://www.inaturalist.org/observations/8490596

Sylvicapra grimmia campbelliae (https://www.inaturalist.org/observations/91165836), which widely coexists with O. m. quadriscopus, is analogous in several ways, as follows:

Firstly, S. g. campbelliae likewise possesses a pedal flag. However, a difference is that in the cephalophin it is the fetlocks that are pale, with the pasterns dark instead.

Secondly, S. g. campbelliae likewise possesses a caudal flag of modest size and conspicuousness.

However, the caudal flags of the two forms differ in both configuration and function. That of O. m. quadriscopus is activated when fleeing from predators, whereas that of S. g. campbelliae is not. In the latter only, the tail is wagged routinely during walking.

to be continued in https://www.inaturalist.org/posts/70101-adaptive-colouration-of-northwestern-oribi-ourebia-montana-quadriscopa-part-2#...

Posted on September 17, 2022 11:05 PM by milewski