The western wildebeest as an example of paedomorphic evolution

We have seen that the main differences of the western wildebeest (Connochaetes taurinus mattosi) from the blue wildebeest (Connochaetes taurinus taurinus) are:

the uprightness of the mane, the length of the beard (compare mattosi https://www.agefotostock.com/age/en/details-photo/blue-wildebeest-connochaetes-taurinus-adult-animal-foraging-in-the-high-grass-kgalagadi-transfrontier-park-northern-cape-south-africa-africa/ZI6-3564204 with nominate taurinus https://www.bigstockphoto.com/image-370276231/stock-photo-the-blue-wildebeest-%28connochaetes-taurinus%29%2C-also-called-the-common-wildebeest%2C-white-bearded-wildeb),

and the reduction of brindling (striping) on the body and cheeks (compare mattosi https://www.inaturalist.org/observations/32318148 and https://www.agefotostock.com/age/en/details-photo/blue-widbeest-connochaetes-taurinus-in-close-up-etosha-national-park-namibia/G69-302177 with nominate taurinus https://www.inaturalist.org/observations/37029798 and http://elelur.com/data_images/mammals/wildebeest/wildebeest-08.jpg).

Here I ask whether these differences have arisen paedomorphically, i.e. by the evolutionary retention of juvenile features.

It is well-known that in wildebeests the females in a sense mimic males, by having not only horns but also the kinds of hairy adornments typically associated with masculine aggression and the exaggeration of the male silhouette for the purposes of rivalry (see https://www.sciencedirect.com/science/article/abs/pii/016815919190264X). In all wildebeests, females and males have horns, manes and beards of similar size, which can make it hard to tell the sexes apart in the field.

However, what is easier to overlook is that there is also a kind of precociality in wildebeests, in which the mane and beard are proportionately more prominent in juveniles than in adults.

For example, compare these one year-old nominate taurinus (https://www.inaturalist.org/observations/19266906 and https://www.istockphoto.com/photo/close-up-of-a-wildebeest-standing-on-the-dry-plains-of-the-western-cape-south-africa-gm1193291491-339364166?irgwc=1&cid=IS&utm_medium=affiliate&utm_source=photostockeditor&clickid=xABUceQmnxyOWXNwUx0Mo3kTUkByqxxxtyGw3I0&utm_term=&utm_campaign=&utm_content=258824&irpid=1327686) with this adolescent (two year-old) nominate taurinus (https://www.agefotostock.com/age/en/details-photo/blue-wildebeest-connochaetes-taurinus-eating-grass-kruger-national-park-south-africa/ZI6-1930962, in which the beard has already shrunk proportionately).

For some reason all wildebeests already have visible manes and beards from birth (https://www.bigstockphoto.com/image-381100301/stock-photo-the-blue-wildebeest-%28connochaetes-taurinus%29%2C-also-called-the-common-or-white-bearded-wildebeest-or-b), which means that the growth of hair adornments takes a kind of priority in this genus. Starting ahead in this way means that the hair adornments appear advanced when the animal is still juvenile. (Although the precociality of the growth of the legs makes obvious sense in animals which run for their lives right from birth, the adaptive value of precocial growth of hair adornments is unclear.)

Furthermore, the mane is also still upright in juveniles (nominate taurinus https://www.inaturalist.org/observations/12485101), partly because the hairs are not yet long or spaced-out enough on the nuchal ligament to collapse as seen in adults of the nominate subspecies taurinus (as well as adults of all other forms of wildebeest besides Connochaetes gnou).

Turning to brindling:

the striping on the body and cheeks in wildebeests (see 'Adaptive colouration in wildebeests, part 4: brindling', July 09, 2021 in https://www.inaturalist.org/journal/milewski) is the opposite of precocial. The infant completely lacks brindling, which starts to appear only well after the ground-colour turns from fawn to greyish brown.

What this means is that, at the juvenile stage, the animal combines a prominent mane and beard with minimal brindling. And it is this very combination that persists into adulthood in subspecies mattosi.

In a sense, the appearance of the western wildebeest is that of 'overgrown juveniles' of the eastern, nominate subspecies - except of course for the horns, which do adopt the mature shape.

Now that we have an ontogenetic search-image, can we find evidence in our photos of the western wildebeest that, within adulthood, the really mature individuals are the ones with the most brindling (e.g. see https://www.inaturalist.org/observations/31787361)?

Posted on 17 de julho de 2021, 09:41 PM by milewski milewski

Comentários

One aspect of the precociality of the mane is that it extends farther to the posterior in the infant than in the adult. For example, see the fifteenth photo in https://naturebackin.com/2020/06/04/at-the-waterhole-mkhuze-game-reserves-kumasinga-hide/, which shows Connochaetes taurinus taurinus. In the infant, the already dark mane extends on to the back. However, in adults it has contracted to the withers.

Publicado por milewski mais de 2 anos antes
Publicado por milewski mais de 2 anos antes

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