On the human body, we daily observe that keratin, the substance of horns, can be either pigmented (dark hair) or translucent (fingernails). It should therefore not surprise us to find that bovids show adaptive flexibility in the tone of their horns.
Here I focus on the saiga antelope (Saiga tatarica, https://web.archive.org/web/20160304101617/http://www.science.smith.edu/msi/pdf/i0076-3519-038-01-0001.pdf), the prime example of a bovid species with pale horns, and on the bontebok/blesbok (Damaliscus pygargus), the prime example of a bovid species in which subspecies vary noticeably in the tone of the horns despite the horns remaining similar in size and shape, and showing minimal sexual dimorphism.
The saiga antelope is closely related to gazelles, but unlike any gazelle has horns so depigmented that they are about as clear as human fingernails. The horns accordingly look 'flesh-coloured'.
This is puzzling in terms of evolutionary adaptation. The saiga antelope is extremely peculiar in its nasal anatomy (uniquely proboscis-like, particularly in males, see https://onlinelibrary.wiley.com/doi/10.1111/j.1469-7580.2007.00818.x) and its running gait (camel-like pacing instead of cantering), but it is hard to link these peculiarities rationally to the translucency of its horns.
See
https://en.wikipedia.org/wiki/Saiga_antelope#/media/File:Saiga_Antelope_Skull_and_Taxidermy.jpg
http://www.edgeofexistence.org/species/mongolian-saiga/
https://www.saigaresourcecentre.com/picture/saiga-male-2
https://lightfieldstudios.net/442123620/stock-photo-wild-male-saiga-antelope-saiga.html
https://www.facebook.com/99206759699/photos/a.138830229699/10157302669094700/?type=3
Rhttps://twitter.com/ebubekiryaman/status/1163036189462585345/photo/1
https://external-preview.redd.it/xArAn0TIryi0iwO8EYpeNnWzqGhMSIK4AtPm5Yx4_zs.jpg?auto=webp&s=32277c987de2e4660392f309a26d26d56dd87c32
The bontebok (nominate ssp. pygargus) and the blesbok (ssp. phillipsi) have horns approximately similar in shape to those of the saiga antelope, but in females as well as males. These two subspecies occupied separate parts of South Africa, differing mainly in the boldness of their overall colouration. Who would have predicted that members of a single species would differ so much in the tone of their horns?
As in the saiga antelope, this has yet to be explained in terms of adaptation.
For Damaliscus pygargus pygargus see
https://www.dreamstime.com/stock-photo-head-shot-bontebok-close-up-face-image69834194
https://www.dreamstime.com/stock-photo-bontebok-standing-eating-grass-medium-sized-generally-dark-brown-antelope-prominent-wide-white-blaze-image77512505
https://www.dreamstime.com/royalty-free-stock-images-bontebok-antelope-image12034589
https://naturerules1.fandom.com/wiki/Bontebok?file=121146646_537h.jpg
For Damaliscus pygargus phillipsi see
https://www.dreamstime.com/stock-photography-blesbok-antelope-portrait-damaliscus-pygargus-south-africa-image32804162
https://www.dreamstime.com/stock-images-blesbok-antelope-image1528984
https://www.dreamstime.com/royalty-free-stock-image-blesbok-antelope-image38166886
https://www.dreamstime.com/stock-photo-blesbok-looking-towards-camera-damaliscus-pygargus-phillipsi-near-cradock-south-africa-image72368753
https://www.dreamstime.com/stock-image-two-blesbok-antelopes-image1984331
Comentarios
Wild sheep vary considerably in the tone of their horns, as exemplified by https://www.123rf.com/photo_13964116_dall-sheep-closeup-the-dall-sheep-ovis-dalli-is-a-species-of-sheep-native-to-northwestern-north-amer.html and https://www.shutterstock.com/nb/video/clip-5961233-bighorn-sheep-ovis-canadensis-ram---on and https://www.hunter-ed.com/nevada-sheep-id/studyGuide/What-Is-a-Bighorn-Ewe/211034_89736/.
The chamois (Rupicapra rupicapra) has particularly dark horns: https://www.shutterstock.com/nb/image-photo/portrait-chamois-soft-sunset-light-rupicapra-2018025557 and https://www.shutterstock.com/nb/image-photo/tatra-chamois-rupicapra-tatrica-mountains-autumn-1810434154 and https://www.shutterstock.com/nb/image-photo/curious-tatra-chamois-looking-while-eating-1979435474 and https://www.istockphoto.com/photo/chamois-goat-looking-at-camera-alps-gm1186694233-334916831.
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