Mysteries of the adaptive value of colouration in the bontebok, particularly 'infantile' crypsis and the divulgence of pregnancy

@tonyrebelo @jeremygilmore

I have recently described the colouration of the bontebok (https://www.inaturalist.org/journal/milewski/76311-adaptive-colouration-in-the-bontebok-damaliscus-pygargus-pygargus-part-1-adults# and https://www.inaturalist.org/journal/milewski/76336-adaptive-colouration-in-the-bontebok-damaliscus-pygargus-pygargus-part-2-infants-juveniles-and-adolescents#).

However, my use of the term 'infant' is, in a way, misleading.

This is because the 'infantile' colouration in Damaliscus pygargus actually persists over a trebling of body mass after birth.

The back of neonates only reaches the height of the maternal udder:

https://www.thetablemountainfund.org.za/species-in-focus-the-bontebok/

After three months of growth, the back of 'infants' (really juveniles that have retained infantile colouration as the tips of the horns appear) reaches the height of the maternal anus:

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

This means that the latter are really 'juveniles dressed as infants'.

An important point about adaptive colouration in the bontebok thus emerges:

The ostensibly cryptic 'infantile' colouration, which is countershaded plain fawn (https://en.wikipedia.org/wiki/Crypsis#:~:text=Many%20animals%20have%20evolved%20so,animal%20itself%20(disruptive%20coloration), cannot actually function cryptically.

This is because

  • infants do not hide even in the first few weeks of life, and
  • their accompaniment of the group means that the cryptic potential of their colouration is subverted by the conspicuousness of the adults near them.

Among ruminants, the bontebok is extreme in the dichotomy between the infantile and adult patterns of colouration. These patterns are so different that the only elements in common are:

  • whitish on the ventral surface of the thorax, and on the inner surfaces of the upper hindlegs,
  • whitish on the ulnar surface, on the posterior of the upper foreleg,
  • whitish on the anterior surfaces of the ear pinnae,
  • a small white triangle just above the rhinarium,
  • a demarcation in tone on the side of the rostrum, and
  • darkness at the tip of the tail (the tassel being merely incipient at birth).

These elements are functionally trivial, and cannot outweigh the categorical difference in pattern.

The infantile colouration of the bontebok resembles that of adults of a thoroughly inconspicuous ruminant, the bohor reedbuck (https://www.inaturalist.org/taxa/42304-Redunca-redunca).

Were it the case that infants hide for several weeks, and that the infantile colouration is lost at several weeks old, then the infantile colouration of the bontebok could be interpreted as adaptive. However, such is not the case.

So, how has the evolutionary process resulted in the infantile colouration of the bontebok?

This adds to at least two other unsolved questions about the colouration of the bontebok, viz.

  • how is it adaptive for the bontebok to be so much more conspicuous than other species of Damaliscus - including the blesbok, which ostensibly belongs to the same species?
  • how is it adaptive for the white ventral panel to be configured in such a way that it reveals heavy pregnancy to potential predators, thus accentuating a vulnerability?

The following show how conspicuous pregnancy is in the bontebok, owing to the distension of the pale ventral panel:

https://www.inaturalist.org/observations/97905171

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=b907bfe35d3c66fdb917f996c60ff601&searchtype=0

https://www.bbc.com/news/science-environment-52881831

https://www.dreamstime.com/stock-photo-bontebok-wide-open-field-medium-sized-generally-dark-brown-antelope-prominent-white-blaze-image77512544

https://www.dreamstime.com/stock-photo-bontebok-field-looking-perfect-medium-sized-generally-dark-brown-antelope-prominent-wide-white-image77512668

https://depositphotos.com/598312898/stock-photo-bontebok-damaliscus-pygargus-bushmans-kloof.html

https://www.mediastorehouse.com/ardea-wildlife-pets-environment/animals/mammals-bovidae-bontebok/bonteboks-fynbos-13230782.html

https://www.inaturalist.org/observations/68890585

scroll to 12th photo in https://puppy1952.wordpress.com/category/the-overberg/

https://www.flickr.com/photos/christiansanchez/15277125719

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 marzo 17, 2023 11:20 TARDE por milewski milewski

Comentarios

Please scroll to fourth photo of an illustration of how large the 'infant' of the bontebok becomes, before adopting juvenile colouration:

https://wildlifesafari.info/bontebok.html

Publicado por milewski hace 12 meses

@milewski
Is this a type of neoteny/paedomorphism?

Publicado por paradoxornithidae hace 12 meses

Neoteny (https://en.wikipedia.org/wiki/Neoteny) is the retention of infantile patterns as the organism grows.

The retention of spotted colouration in cervids could be interpreted as a form of neoteny.

In cervids, most species are pale-spotted at birth, facilitating the hiding of infants as their mothers forage separately. The spotted pelage is an example of disruptive colouration, i.e. camouflage (https://www.deeranddeerhunting.com/content/articles/deer-news/top-10-fawn-photos-from-ddh-readers).

Because cervid infants hide by lying as close to the ground as possible, the spotting is most important on the dorsal surfaces.

According to this rationale, those cervids that retain strongly spotted colouration into adulthood (even in mature males), such as Axis axis (https://upload.wikimedia.org/wikipedia/commons/a/a7/A_chital_stag_1.JPG), can be interpreted as, in a sense, neotenic.

However, bovids (to which the bontebok belongs) differ basically from deer. This is because there seems to be no trend for spotted camouflage in infants. Few species of bovids have pale spotting at all, and those that do (e.g. Tragelaphus scriptus) show no difference in this respect between infants and adults.

Is it possible that a cryptic pattern of colouration, i.e. plain with countershading, is a basic pattern in bovid infants?

I doubt this, because plainness is in itself not particularly effective in concealment. Countershading is most effective when the figure is standing, because countershaded ventral surfaces tend to be visible in standing postures (https://www.dreamstime.com/eastern-bohor-reedbuck-redunca-redunca-bohor-eastern-bohor-reedbuck-redunca-redunca-bohor-beautiful-shy-antelope-endemic-image223506871 and https://www.jungledragon.com/image/46782/bohor_reedbuck_redunca_redunca.html).

So, I see no clear rational for the cryptic pattern of infants of the bontebok being analogous to the disruptive pattern of cervid infants.

Furthermore, any retention in the bontebok is only for a few months, the cryptic pattern being completely lost by the juvenile stage.

Publicado por milewski hace 12 meses

@milewski
many thanks for the well written explanation.

Publicado por paradoxornithidae hace 12 meses

@paradoxornithidae

One aspect in which a form of neoteny may be involved:

Farmers have bred a 'mutant' strain of the blesbok, in which all the dark features are removed and the figure is now a ghostly pale (https://www.dreamstime.com/stock-photo-white-blesbok-damaliscus-pygargus-phillipsi-blesbuck-south-africa-image41083442).

It occurs to me that, in a way, this is an adult version of the infantile colouration of the blesbok (https://stock.adobe.com/search/images?filters%5Bcontent_type%3Aphoto%5D=1&filters%5Bcontent_type%3Aillustration%5D=1&filters%5Bcontent_type%3Azip_vector%5D=1&filters%5Bcontent_type%3Avideo%5D=0&filters%5Bcontent_type%3Atemplate%5D=0&filters%5Bcontent_type%3A3d%5D=0&filters%5Bcontent_type%3Aaudio%5D=0&filters%5Binclude_stock_enterprise%5D=0&filters%5Bis_editorial%5D=0&filters%5Bfree_collection%5D=0&filters%5Bcontent_type%3Aimage%5D=1&k=blesbok&order=relevance&price%5B%24%5D=1&safe_search=1&limit=100&search_page=3&search_type=pagination&get_facets=0&asset_id=447868924).

What I mean by this is that, in order to acquire this pale adult colouration, farmers have not produced something new, requiring 'fresh' mutation.

Instead, they have reconfigured at least four patterns already existing in the genome/phenome of the blesbok.

Firstly, the overall pallor already exists in infants.

Secondly, the particular pallor of the face already exists in the post-adult expansion of pale pelage, from the rostrum and narrowly-defined forehead on to the muzzle, cheeks, orbits, and broader forehead. (Please note that this is in a sense the opposite of a neotenic origin, because the expansion on the face is a marker of old age.)

Thirdly, the faded appearance of the tail already exists in the 'blond' distal section of the tail-tassel of adults of the blesbok. This has been emphasised in selective breeding, at the expense of the dark part of the tail-tassel.

Fourthly, the pallor of the horns, in females, already exists in that of the horns in males.

So, instead of breeding something new, such as the pied pattern of domestic mammals such as Friesland cows, farmers have selected for infantile plainness and pallor, plus post-adult pallor on the face, plus distal pallor on the tail-tassel, plus masculine pallor on the horns. The result is a colouration unrecognisably different from that of the original blesbok, but not as novel as it may seem.

Undermining my rationale is the observation that infants of the pale 'breed' are themselves inordinately pale (see third photo in https://www.inaturalist.org/observations/152247586).

Your further thoughts?

Publicado por milewski hace 12 meses

I've seen your post mentioning how Equus zebra may be an ancient hybrid of Equus quagga x Equus grevyi (Equus capensis?). Apparently, there's evidence to suggest Equus capensis may be an ancient form of Equus quagga.

Publicado por paradoxornithidae hace 12 meses

@paradoxornithidae

I follow your point, based on https://en.wikipedia.org/wiki/Equus_capensis and https://www.pnas.org/cai/doi/10.1073/pnas.0903672106.

Earlier literature (https://www.tandfonline.com/doi/abs/10.1080/00359190609519957 and https://core.ac.uk/download/pdf/39675034.pdf) indicated that Equus capensis is the southern counterpart of Equus grevyi, in much the same way as Oryx gazella is the southern counterpart of Oryx beisa.

However, please see https://core.ac.uk/download/pdf/39674984.pdf.

The topic seems still to be in a state of confusion, not so?

Particularly intriguing, for me, is the suggestion that Equus capensis was adapted for foraging in fynbos vegetation (https://www.cambridge.org/core/journals/quaternary-research/article/abs/mixedfeeding-equus-species-from-the-middle-pleistocene-of-south-africa/3D7DF9176B6E9D21C9949868C4C2151E).

Also worth perusing:

https://d1wqtxts1xzle7.cloudfront.net/50967157/THE_BIG_EQUUS_FROM_THE_GEOMETRIC_KEBARAN20161219-18691-eteg76-libre.pdf?1482159492=&response-content-disposition=inline%3B+filename%3DThe_Big_Equus_from_the_Geometric_Kebaran.pdf&Expires=1679720468&Signature=KxZOBip0Q3ZgSk7TSnPaqSeRwPd4~4L7fvDXkXTF~qbnbu2TiNxqBo34Ii9apUrIXfE~BKwbixnIKX9COy3yq5vcaHRSKR1KIGEc636hBqNjVDIJTWtJELaR~7LlnvGcYO1bs7E2lEaJJ7bTBpjbFNE0N~iceGVB3HWrtZG9Dl5XV8rEpdczocnnsEV9KDe7XPJZsXxGOR1rzOnWj6pwAs7r2OYp~YZlBHg0TdsFiRILb1SC01jAd0q3JOC6JY6FJQ4dS0UVGU3BzG8nrzbQfLcbEM0aCVhCOZrYRSWfwpYF9mtWtZO--fj5nHAYHtwwjhBt9OMVPQ3gAfPI84Xb9g__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA

https://www.cambridge.org/core/journals/quaternary-research/article/tracking-the-extinct-giant-cape-zebra-equus-capensis-on-the-cape-south-coast-of-south-africa/D410B4D755781DC0206CB810A35CC74E

https://open.uct.ac.za/handle/11427/32408

Publicado por milewski hace 12 meses

@paradoxornithidae

I see three broad possibilities w.r.t. Equus capensis.

Firstly, it is possible that E. capensis was the southwestern counterpart for E. grevyi. This makes the most sense biogeographically, but seems to have been undermined by anatomy and genetics.

Secondly, it is possible that E. capensis is a form of the Equus quagga complex. This makes little sense biogeographically, mainly because Equus quagga quagga already filled this role (unless, perhaps, E. q. quagga is a smaller descendent of E. capensis).

Thirdly, it is possible that E. capensis was a form of the E. quagga complex specialised for the fynbos biome, with its nutrient-poor soils and fibrous vegetation. If so, there might, in the late Pleistocene, have been a total of five members of this complex in southern Africa, viz. nominate quagga (mainly in Karoo), burchellii (Zululand), 'burchellii' (Namibia), chapmani (mesic, tropical savannas), and capensis (extreme southwestern part of South Africa). (I omit boehmi, which is marginal to southern Africa in southwestern Zambia today, and the Gorongosa form of crawshayi, which occurs in Mozambique.)

Publicado por milewski hace 12 meses

@matthewinabinett @paradoxornithidae

There are remarkably few videos of the bontebok on the Web.

However, the following (https://www.youtube.com/watch?v=_tRs3Kw7_1g) does contain some revealing footage.

This illustrates, inter alia:

a) The nodding display does not seem as pronounced in the bontebok as in the blesbok.

b) The conspicuousness of pale on the back-of-ear in the bontebok, although considerable, does not qualify as an auricular flag.

c) The pale vertical streak on the temples is a previously overlooked feature of the juvenile colouration that persists after the rest of the adult colouration is attained. However, I do not know if it can be used to determine age, because there is individual, and perhaps sexual, variation.

d) Infants are not conspicuously pale in the bontebok.

Publicado por milewski hace 11 meses

Agregar un comentario

Acceder o Crear una cuenta para agregar comentarios.