Adaptive colouration in wildebeests, part 2: facial, caudal and pedal flags

How should we categorise the various conspicuous features of colouration in Connochaetes? (The distributions of forms can be seen in https://www.researchgate.net/figure/Map-showing-the-distribution-of-wildebeest-subspecies-in-Africa-including-some-important_fig2_305317918.)

A bleeze is a feature of dark/pale contrast, large enough to reveal the whole stationary figure at the range relevant to scanning predators. A flag is a relatively small dark, pale or dark-and-pale feature that becomes conspicuous to the predators once it is moved.

There is no clear example of a bleeze in wildebeests (for comparison see Damaliscus pygargus pygargus, https://www.krugerpark.co.za/africa_bontebok.html). This is because a) the conspicuousness of gnou is based on its overall darkness rather than any particular feature, making it convergent with the Cape buffalo Syncerus caffer in this way, b) most of the features under consideration are not simply a matter of pigmentation, but are instead subject to sheen/antisheen, and c) the dark/pale contrast on the face of johnstoni is relatively small-scale and not present in all individuals. However, an effect similar to that of a bleeze is created by the sheen on the rump, in all forms except gnou and taurinus mattosi, in bright sunlight near midday.

Several categories of flags occur. (Evidence for a pedal flag in juveniles will appear in part 5.)

The clearest example is the caudal flag of gnou. The tail-tassel of this species is usually far from white (possibly owing to dust), but it is large enough to be conspicuous (and probably audible) at distance when swished (see https://www.inaturalist.org/observations/9552228 and https://www.inaturalist.org/observations/41123029 and https://www.alamy.com/stock-photo-black-wildebeest-connochaetes-gnou-female-with-calf-on-grass-111520091.html and https://www.gettyimages.com.au/detail/photo/side-view-of-buffalo-standing-on-field-otavi-royalty-free-image/1320030695?adppopup=true). This would be enhanced with backlighting.

There is a clear facial flag in albojubatus (see part 3). In cooksoni it is ambivalent.

The tail is so large that all forms of wildebeest probably qualify for a caudal flag (e.g. see taurinus western form https://www.shutterstock.com/nb/image-photo/this-running-wildebeest-photographed-sunrise-etosha-1667738122). However, the case is strengthened in albojubatus and cooksoni because the blackish of the swishing tail contrasts in certain illuminations with the particularly pale and sheeny hindquarters (see https://www.gettyimages.com.au/detail/photo/white-bearded-wildebeest-royalty-free-image/1308653007?adppopup=true and https://www.gettyimages.com.au/detail/photo/wildebeest-running-in-the-landscape-of-amboseli-royalty-free-image/1174138809?adppopup=true and http://www.birdphotographers.net/forums/showthread.php/29883-Cookson-s-Wildebeest-on-the-run).

The case for a facial flag in johnstoni rests mainly on the whitish bar across the rostrum (see http://www.africahunting.com/hunting-pictures-videos/showphoto.php?photo=10621&title=nyasaland-gnu-in-tanzania&cat=506 and https://realorasafaris.co.za/product/wildebeest-nyasa/) but this is inconsistently present.

Two forms have pale beards, but the beard is not pale enough in mearnsi (see https://www.flickr.com/photos/olegchernyshov/36294975016/ and https://www.fotosearch.com/CLT005/kz5444/) to be conspicuous except when backlit (e.g. see https://www.inaturalist.org/observations/38738619 and https://www.flickr.com/photos/davidbygott/39917897804 and https://www.inaturalist.org/observations/50780031).

Two forms, namely gnou (see part 1) and mearnsi (see https://www.dreamstime.com/stock-photo-lioness-savannah-national-park-kenya-tanzania-masai-mara-serengeti-excellent-illustration-image79105582), are dark enough overall, in most illuminations by day, to be conspicuous without pale accentuation, even while stationary. Both do have ambivalent patterns of pale accentuation: the mane in gnou and the sheen on the rump in mearnsi (see https://www.gettyimages.com.au/detail/news-photo/blue-wildebeest-or-white-bearded-wildebeest-bovidae-news-photo/492753635?adppopup=true). The remaining forms have ambivalent overall colouration when stationary. The figure is darkest in taurinus (with emphasis on face, beard and mane in the western form) and palest in albojubatus and cooksoni.

Facial flags (including pale beards) are further examined in part 3.

The form in which all conspicuous features, other than overall darkness, are most weakly expressed is the eastern form of taurinus (see http://www.joelinnphoto.com/africa/xmrwckt8ri8yocv7kwy1dl7ny0ynu1 and http://www.wildlife-pictures-online.com/wildebeest_knp-0296.html).

Publicado por milewski milewski, 09 de julio de 2021

Comentarios

Yes, indeed, the wildebeest's adaptation in this regard is magnificent; however, how does the dark hue that works so well in so many regards not also work like a heat magnet and hot water storage tank for the sun's rays in a desert that can be blistering hot. When in a sun-soaked desert, I try to avoid wearing (harder to detect) dark shades, especially like in the form of a thick fur coat, and instead opt for (highly visible) light colors that reflect the sun's rays. so I don't overheat or cook myself. Ruth

Publicado por grinnin hace 12 meses (Marca)

Ruth, you have asked an insightful question. It is true that a problem with the overall darkness of the black wildebeest, the western white-bearded wildebeest, and the blue wildebeest is the risk of overheating. There is at least one publication on this: https://pubmed.ncbi.nlm.nih.gov/25436964/. The authors found that the black wildebeest manages to stay out in the sun even at the hottest times, whereas the blue wildebeest seeks shade. They interpret this partly in terms of the thicker, more insulating fur of the black wildebeest. However, I was a bit amused to see that they assume the coat-colour of the black wildebeest to be black, whereas it is actually medium brown. I see this as another example of cognitive dissonance, along the lines that if the name says black then the mind does not even register it when the eyes see medium brown. Putting that paper aside and thinking your question through from scratch: I suggest that a main way in which all the conspicuously dark forms of wildebeest avoid absorbing too much sunlight is by means of sheen/antisheen, i.e. the reflective quality of the hairs resulting from structural features of the hairs as opposed to pigmentation. This is roughly in the same category of phenomena as 'invisibility cloaks'. The light hitting a black hair is absorbed and converted to heat. But the light hitting a wildebeest's hair is largely reflected by a complex physical structure involving a concave cross-section and certain tiny scale-like textures on the surface of each hair. When the reflected light reaches our eyes, we call it sheen, e.g. the car bonnet-like shine on the rump. When the reflected light misses our eyes, we see the surface as blackish, which is what I call antisheen. But the important points are a) that dark wildebeests are not nearly as pigmented as they may seem, and b) they have configured the fur to solve two problems in one solution: the sheen/antisheen both reduces heating and gives these animals the adaptive conspicuousness that suits them for social communication and a 'showoff' approach to anti-predation. And, to round it off, having somewhat dark pigmentation helps them to hide at night out in the open, from the eyes of their predators, to a degree which would not have been possible had they solved their thermoregulatory problems by being depigmented. Does any of this make sense?

Publicado por milewski hace 12 meses (Marca)

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