Adaptive colouration in the bontebok (Damaliscus pygargus pygargus), part 2: infants, juveniles, and adolescents

...continued from https://www.inaturalist.org/journal/milewski/76311-adaptive-colouration-in-the-bontebok-damaliscus-pygargus-pygargus-part-1-adults#

The bontebok is well-known for its extreme colouration. However, what is not generally appreciated is how complex the facial colouration is, as it develops from birth to adulthood.

This complexity has never, as far as I know, been described, let alone explained.

At birth, the colouration of the bontebok is fairly plain (https://www.alamy.com/bontebok-damaliscus-pygargus-dorcas-three-ewes-with-their-calves-dehoop-nature-reserve-western-cape-south-africa-september-image262940011.html?imageid=37BEF09A-E8CD-4363-BA0C-D7A889A5A0DE&p=215617&pn=4&searchId=dca6a568865628209af3d56514ac1f36&searchtype=0).

This is puzzling, given that infants do not hide in this species, instead accompanying their mothers from the start.

However, the puzzle of plain colouration in infancy is eclipsed by a greater puzzle as the animal grows into the juvenile stage.

This is because the facial pattern goes through a series of changes that seem superfluous to the relatively simple conversion of the fawn-coloured rostrum of infants to the white rostrum of adults.

It is almost as if Nature has used the juvenile face of the bontebok as a canvas, on which to paint - merely for their own sake - a series of organised designs, before erasing them (https://www.shutterstock.com/it/image-photo/bontebok-antelope-calf-national-park-south-124626181 and https://www.shutterstock.com/it/image-photo/young-bontebok-waiting-cross-road-national-1012848241 and https://pixels.com/featured/bontebok-tony-camachoscience-photo-library.html).

INFANTS

At birth, the bontebok is fawn-coloured with countershading, plus

• a trace of the ulnar pattern (see the posterior surface of the upper foreleg in https://www.alamy.com/stock-photo-young-bontebok-damaliscus-pygargus-dorcas-in-postberg-west-coast-national-26607506.html?imageid=A6BE5E5E-5F5F-4685-894F-CFAC80895E31&p=18540&pn=3&searchId=6d8a7f95c3b92935b5e8c1d5d07d00ff&searchtype=0),
• an incipient dark tip to the tail, and
• a subtle, but consistent and crisply-defined, pattern on the head.

The pattern on the head may seem negligible at first glance. However, on closer examination it poses a fundamental evolutionary puzzle.

I describe this pattern as follows:

• the rostrum and forehead are fawn-coloured, with a crisp demarcation on the side of the rostrum (https://www.shutterstock.com/it/image-photo/bontebok-calf-national-park-swellendam-western-2090432353),
• the orbits are white, extending towards the corner of the mouth (https://www.inaturalist.org/observations/137997739),
• the upper lips are fawn-coloured, but there is a small triangle of whitish pelage just above the rhinarium (https://en.wikipedia.org/wiki/Rhinarium),
• the cheeks are a pale tone of fawn, grading to whitish medially (https://upload.wikimedia.org/wikipedia/commons/e/e4/Bontebok_%28Damaliscus_pygargus_pygargus%29_young_..._%2832834333045%29.jpg),
• the ventral surface of the mandibles are white, extending to the area between the gape of the mouth and the cheeks (https://www.flickr.com/photos/berniedup/50179826247),
• the base of the ear is whitish, and
• the posterior surface of the ear pinnae is simply fawn-coloured, like most of the animal (https://www.inaturalist.org/observations/83966791).

The facial pattern of infants of the bontebok is inconspicuous, because

• the pale feature on and near the orbits is small-scale, and
• there is no dark pelage, anywhere on the head.

However, what is remarkable is that this pattern is not merely a nebulous or incipient version of the adult colouration, as is the case in hippotragin bovids (https://www.inaturalist.org/journal/milewski/76424-which-ruminant-is-born-with-the-most-conspicuous-facial-pattern-of-colouration-part-1-hippotragin-and-alcelaphin-bovids#).

Instead, the pattern is different from that in adults. It is as if infants and adults are different species.

To be precise, the only parts of the head of infants that already show adult colouration are:

• a small whitish triangle just above the rhinarium, and
• the whitish hairs on the anterior surface of the ear pinnae.

The following show infants close-up:

https://www.inaturalist.org/observations/63400386

https://www.inaturalist.org/observations/115686906

https://www.inaturalist.org/observations/99722303

The following show infants with their mothers:

https://www.dreamstime.com/royalty-free-stock-image-mother-calf-blesbok-antelope-image21744916

https://www.dreamstime.com/stock-photo-bontebok-calf-mother-her-foraging-fynbos-southern-africa-image92719587

https://pixels.com/featured/bontebok-mother-and-calf-peter-chadwickscience-photo-library.html

https://www.dreamstime.com/young-bontebok-adult-fynbos-fawn-coloured-young-bontebok-fynbos-veld-adult-near-cape-point-western-cape-image268185782

https://www.alamy.com/bontebok-damaliscus-pygragus-dorcas-ewe-with-one-day-old-calf-dehoop-nature-reserve-western-cape-south-africa-september-image262940010.html

https://www.alamy.com/stock-photo-bontebok-with-calf-damaliscus-pygargus-pygarus-de-hoop-nature-reserve-59178437.html?imageid=03857B0D-5BE3-4A6C-B22B-16A28380CD06&p=22059&pn=5&searchId=a4908a5258dd3ea4c025bcd202432b49&searchtype=0

https://www.alamy.com/bontebok-damaliscus-pygargus-pygargus-adult-and-calf-sitting-de-hoop-south-africa-december-image263070029.html?imageid=88C59DAE-1AFE-4E31-9DD7-E9102086EA36&p=215536&pn=1&searchId=25869b8bb63848eab5bc67f94e53a679&searchtype=0.

The infantile pattern of colouration persists to the age of three months. At this stage the horn-tips have appeared, and the body mass exceeds a quarter of maternal body mass:

https://www.dreamstime.com/stock-photo-bontebok-calf-mother-her-foraging-fynbos-southern-africa-image92743101

https://www.alamy.com/bontebok-with-young-image1218482.html?imageid=B2DF1491-9EBB-4DA5-8DC2-552475BDDC6F&p=4877&pn=4&searchId=29c543d132619cf1217ce0a23a3945c6&searchtype=0

https://www.alamy.com/bontebok-with-two-youngsters-image1218481.html?imageid=9C191390-7AA0-43F4-9B4F-5038BD5BE66A&p=4877&pn=5&searchId=78e3c36a9917c21e07efdd82f4c4fbc0&searchtype=0

The following series of photos, of an individual infant (1-2 months old), is one of the clearest expositions on the Web of the infantile colouration of the bontebok. The infantile colouration of the bontebok is not as pale as that of the blesbok, but more clearly shows countershading.

Damaliscus pygargus pygargus:

https://www.shutterstock.com/it/image-photo/baby-bontebok-antelope-calf-looks-curiously-744728989

https://www.shutterstock.com/it/image-photo/cute-baby-bontebok-antelope-calf-looks-744728965

https://www.shutterstock.com/it/image-photo/baby-bontebok-antelope-calf-stays-close-744728971

https://www.shutterstock.com/it/image-photo/cute-baby-bontebok-antelope-calf-scratches-744728980

https://www.shutterstock.com/it/image-photo/cute-baby-bontebok-antelope-calf-scratches-744728986

https://www.shutterstock.com/it/image-photo/baby-bontebok-antelope-calf-looks-see-744728992

https://www.shutterstock.com/it/image-photo/two-cute-baby-bontebok-antelope-calves-744728956

JUVENILES

The following series provides one of the clearest illustrations of juvenile colouration in the bontebok:
https://www.shutterstock.com/it/image-photo/bontebok-damaliscus-pygargus-pygarus-south-africa-560325085 and https://www.shutterstock.com/it/image-photo/bontebok-damaliscus-pygargus-pygarus-south-africa-560325091 and https://www.shutterstock.com/it/image-photo/bontebok-damaliscus-pygargus-pygarus-south-africa-560325112.

At the juvenile stage, the colouration on the neck, body, and legs changes directly towards that of the adult.

However, the colouration of the head goes through convoluted changes.

The first change on the head is a darkening of the rostrum (https://www.alamy.com/m2-304d-yearling-bontebok-head-image6516263.html?imageid=6FB7B5E5-32AD-4555-AD1C-BE8C66B0C857&p=1787&pn=6&searchId=d7a105eea18d32dc6155ddf3de390174&searchtype=0).

The following (https://www.shutterstock.com/it/image-photo/bontebok-antelope-calf-national-park-south-124597081 and https://www.shutterstock.com/it/image-photo/bontebok-antelope-calf-earlymorning-light-125978060 and https://www.shutterstock.com/it/image-photo/bontebok-antelope-calf-de-hoop-nature-126157526 and https://www.shutterstock.com/it/image-photo/bontebok-antelope-mother-calf-national-park-124597057 and https://www.dreamstime.com/bontebok-damaliscus-pygargus-jonge-young-image129038812) show that there is a brief stage at which the darkest part of the juvenile figure (apart from the developing tail-tassel) is a particular panel on the front of the face.

This is closely followed by a darkening of the orbits, and the appearance of a complex, pale streak from the cheek, through the temple, to the crown (third photo in https://www.inaturalist.org/observations/121214288 and https://www.shutterstock.com/it/image-photo/bontebok-damaliscus-pygargus-pygarus-south-africa-560325085 and https://animals.sandiegozoo.org/animals/bontebok).

The following shows the dark/pale differentiation on the cheek (https://www.shutterstock.com/it/image-photo/bontebok-antelope-calf-national-park-south-124626181).

The following shows the maximum extent of pale on and near the temples (https://www.shutterstock.com/it/image-photo/baby-bontebok-looking-right-mediumsized-generally-485672296).

The pale on the temples is among the last signs of the juvenile colouration to disappear (https://www.inaturalist.org/observations/137506000).

The whitish ventral surface of the mandibles remains in the juvenile stage (https://www.shutterstock.com/it/image-photo/young-playful-antelope-1327148681). The following shows the discrete pattern that arises at 0.5-1 year old, only to vanish in adulthood (https://www.dreamstime.com/wild-african-animals-bontebok-one-rarest-antelope-world-beautiful-grassland-etosha-national-park-namibia-image183764936).

The following show juveniles nearly 6 months old:

https://www.gettyimages.com.au/detail/photo/bontebok-damaliscus-dorcas-at-bontebok-national-royalty-free-image/523049313?phrase=bontebok&adppopup=true

https://www.inaturalist.org/observations/10843075

https://www.dreamstime.com/stock-images-bontebok-damaliscus-dorcas-image20906614

https://www.alamy.com/stock-photo-herd-of-bontebok-antelope-with-calf-23274728.html?imageid=B8F63BA8-FA72-44CE-80AC-2EF5979BF4BB&p=71732&pn=1&searchId=abde0ec3b06a08943ffb523ff66f0db0&searchtype=0

https://www.inaturalist.org/observations/104575032

The following show juveniles about one year old, when the dark on the rostrum is being gradually replaced by the white hairs of adulthood:

https://www.alamy.com/bontebok-damaliscus-pygargus-pygargus-bontebok-national-park-swellendam-western-cape-south-africa-calf-walking-in-fynbos-image365041507.html?imageid=53762990-B5AF-4A57-AC9A-3B48F49AF21B&p=824482&pn=1&searchId=e1c83791dbb28b2649d55bb669e0fbda&searchtype=0

https://www.inaturalist.org/observations/155658021

https://pixels.com/featured/bontebok-tony-camachoscience-photo-library.html

https://www.inaturalist.org/observations/16478604

https://www.alamy.com/a-vertical-shot-of-bontebok-standing-on-grassland-in-a-park-image499977486.html?imageid=A3EA59CE-7513-4A04-94CC-492D2B267CD8&p=829514&pn=9&searchId=357b330dca08dc7955106e41cd143444&searchtype=0

https://www.alamy.com/extreme-close-up-of-the-head-and-neck-of-a-wild-beautiful-bontebok-antelope-looking-at-the-camera-in-the-wilderness-of-south-africa-shot-on-safari-image435705572.html?imageid=382F9E4B-C34E-4EDB-BCCA-399C10C04B3A&p=576257&pn=1&searchId=abde0ec3b06a08943ffb523ff66f0db0&searchtype=0

https://www.shutterstock.com/it/image-photo/pair-bontebok-grazing-national-park-south-1454575475

https://www.inaturalist.org/observations/137506000

The following (https://www.inaturalist.org/observations/52622577 and https://www.alamy.com/stock-photo-i-see-you-bontebok-the-bontebok-is-a-medium-sized-generally-dark-brown-119828808.html?imageid=262BD3B2-8140-4150-8B1D-BB20F2C8C369&p=260684&pn=4&searchId=29c543d132619cf1217ce0a23a3945c6&searchtype=0 and https://www.dreamstime.com/young-bontebok-grazing-young-bontebok-grazing-cape-point-national-park-cape-town-south-africa-image245392763 and https://www.dreamstime.com/stock-photo-baby-bontebok-medium-sized-generally-dark-brown-antelope-prominent-wide-white-blaze-its-face-image77372211) show particularly clearly the tardiness of the posterior surface of the upper foreleg, above the carpal, in turning dark.

Throughout the juvenile stage, the ventral surface of the neck retains the countershading that will eventually be lost in adulthood (https://www.inaturalist.org/observations/131702545).

The following, of juveniles more than one year old (https://www.alamy.com/bontebok-female-with-young-de-hoop-nature-reserve-western-cape-south-africa-africa-damaliscus-dorcas-dorcas-image228117439.html?imageid=373F25EB-E0E9-443A-8DD5-4194A60EEA24&p=205512&pn=1&searchId=abde0ec3b06a08943ffb523ff66f0db0&searchtype=0 and https://www.alamy.com/bontebok-female-and-juvenile-de-hoop-national-park-western-cape-south-africa-damaliscus-dorcas-dorcas-image414952533.html?imageid=304942E1-F18D-4D68-AE40-F31399C02086&p=1420153&pn=4&searchId=29c543d132619cf1217ce0a23a3945c6&searchtype=0), show several aspects particularly clearly, viz.

• the horns are about three-quarters of full length and the body mass is about 60% of maternal body mass,
• the dark pelage on the figure remains paler than that of adult females,
• the face remains proportionately shorter than in adults, limiting the prominence of the whitish (which is not yet fully white) on the face, and
• the dark pelage on the legs remains incomplete.

When the horns reach three-quarters of their full length (https://www.shutterstock.com/it/image-photo/yes-cute-bontebok-mediumsized-generally-dark-484356040 and https://www.shutterstock.com/it/image-photo/baby-bontebok-497894932), the few juvenile features remaining include

• a trace of countershading on the ventral surface of the neck,
• incompleteness of the dark above the carpal, and
• a pale streak on the temple.

ADOLESCENTS

One of the last features to form completely is the dark pelage on the posterior surface of the upper foreleg, just below the white patch on the ulna (https://www.dreamstime.com/stock-photo-bontebok-perfect-pose-medium-sized-generally-dark-brown-antelope-prominent-wide-white-blaze-its-image77512322 and https://www.inaturalist.org/observations/146638259).

As the rostrum rapidly lengthens and the facial colouration approaches completeness, a dark periphery to the pale features on the forehead and rostrum can intensify and linger (https://www.istockphoto.com/photo/bontebok-antelope-gm136712138-18853032?phrase=pics%20of%20a%20blesbok and https://www.alamy.com/stock-photo-portrait-of-an-endangered-bontebok-antelope-damaliscus-pygargus-dorcas-22757989.html?imageid=D0FE2C16-1433-4C7A-9D18-B2AC8938DBEE&p=70019&pn=1&searchId=abde0ec3b06a08943ffb523ff66f0db0&searchtype=0).

The colouration becomes complete before the horns have attained their full length (https://www.dreamstime.com/stock-image-bontebok-antelope-handsome-large-eyes-grass-mouth-image34760281). However, a trace can remain of the last juvenile feature to disappear, namely the pale vertical streak on the temples, even when the horns seem full-length.

DISCUSSION

The complexity of the changes in facial colouration can be contrasted with the simplicity of the changes on the hindquarters (https://www.alamy.com/bontebok-damaliscus-dorcas-adult-and-juvenile-in-a-flower-meadow-south-image8795878.html?imageid=BEFC3720-6ABE-438B-9767-B86189806239&p=33533&pn=11&searchId=909e0db787de943a5f89073993791964&searchtype=0 and https://www.inaturalist.org/observations/47971842).

Unlike the facial bleeze, the ischiopygal bleeze of the bontebok starts to appear at the end of infancy (https://www.inaturalist.org/observations/10980014), and then simply and directly continues to completion within a mere three months.

The ischiopygal bleeze is complete at about six months old (https://www.shutterstock.com/it/image-photo/young-bontebok-grazing-1349950382), when the face is still unrecognisably different from that of adults.

Almost every aspect of the ontogenetic development of colouration in the bontebok is puzzling, from the viewpoint of adaptation and evolution.

The main questions arising from this examination are as follows:

• how is it adaptive for infants to have cryptic colouration, given that they do not hide (this is particularly puzzling because the plain colouration persists despite the body mass being trebled from birth to three months old)?
• why does the facial pattern go through such complex, temporary changes from three months to one year old? and
• how has such disparity arisen between the pygal bleeze and the facial bleeze, with the former developing early and directly, vs the latter being delayed until adolescence, and emerging from an unrecognisably different, transitional pattern?

Also see https://www.inaturalist.org/journal/milewski/75995-adaptive-colouration-in-the-blesbok-damaliscus-pygargus-phillipsi-part-2-infants-juveniles-and-adolescents#.

For an index to my many Posts about the genus Damaliscus, please see https://www.inaturalist.org/journal/milewski/78238-an-index-to-my-posts-on-genus-damaliscus#.

Publicado el 15 de marzo de 2023 03:39 por milewski