@paradoxornithidae @simontonge @matthewinabinett @katebraunsd @jwidness
I have been closely familiar with Damaliscus pygargus for many decades.
More than half a century ago, I was already sketching both subspecies with as much precision as possible, based on the limited photographic material available at the time.
Furthermore, I have collected what is perhaps the largest dossier of hard-copy photos of the blesbok (https://dewetswild.com/2016/06/23/blesbok/) and the bontebok (https://dewetswild.com/2018/02/02/bontebok/) in the world, clipped from brochures, books, and every other pre-electronic source available.
So, it should be hard to surprise me with any aspect of the appearance of the blesbok and the bontebok.
However, it was only in the last week, when I took the trouble to scrutinise every one of the thousands of relevant photos on the Web, that several surprisingly large differences dawned on me.
In recent Posts, I have covered the
Now, in this Post, I would like to point out a surprising difference in sexual dimorphism between blesbok and bontebok - which, together with the other differences, seems to indicate that these are full species, not merely subspecies.
SEXUAL DIMORPHISM IN BODY MASSES
The blesbok is said to be about 8 kg heavier than the bontebok (https://animaldiversity.org/accounts/Damaliscus_pygargus/). However, this is misleading.
What seems more accurate is that the blesbok and the bontebok are like-size in the case of the more important sex in physiological/metabolic/ecological terms, namely females. It is the fully mature males that seem to differ, owing to the development of extra brawn (particularly on the neck) in males of the blesbok.
The facts of body mass are as follows.
The body mass of the blesbok is given in https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7479364/. The mean was 58 kg, for a sample of eight adult female individuals in good condition.
Huntley (1971, https://www.sasas.co.za/journals/carcass-composition-of-mature-male-blesbok-and-kudu/) reports the mean body mass of mature males of the blesbok as 73.4 kg (sample size was 22 individuals). This varied from 68 kg at the height of the dry season (October, sample size = 5) to 78 kg in the green season (January, sample size = 6).
As reported by Jordaan (https://scholar.sun.ac.za/handle/10019.1/107823), on page 51:
According to Hoffman et al. (2008, https://www.sciencedirect.com/science/article/abs/pii/S0889157508000094), mean body mass for the blesbok was 67.4 kg, for a large sample (65 individuals) of females and males combined.
Smit (2004, Master of Consumer Science thesis, University of Stellenbosch) reports in her Summary that the mean body mass of the blesbok is 60.2 kg for adult females and 67.4 kg for adult males (sample size was collectively 73 individuals).
On page 9 of the same thesis, she states that "The mean live weight for blesbok males is 62.7 to 95.3 kg, and for females is 59.5 to 86.3 kg," I assume that she really means the ranges of individual masses, not the averages.
Other, less helpful references on body masses include the following:
Turning now to the bontebok:
Jordaan (2020, Table 3.1 on page 47) provides definite data for the body mass of adult males of the bontebok. The sample size was 20 individuals, and the mean was 65.8 kg.
The mean body mass of adult males of the bontebok is informally said to be 61 kg (https://www.blueplanetbiomes.org/bontebok.php).
I doubt the following, which seem exaggerated:
SEXUAL DIMORPHISM IN THE SIZE OF THE HEAD
The head, including the horns, is noticeably larger in mature males of the blesbok than in
- mature males of the bontebok, or
- adult females of either blesbok or bontebok.
Van Zyl and Ferreira (2004) state that the mean mass of the head in adult females of the blesbok is about 3.5 kg. This is 6% of body mass.
Jordaan (2020, Table 3.1 on page 47) states that the mean mass of the head in adult males of the bontebok is 4.8 kg. This is 7% of body mass.
I have yet to see corresponding data for fully mature males of the blesbok. However, I suspect that the values exceed 7 kg and 8.5%.
Please compare the proportional sizes of the head in the following figures, standardised by posture:
Blesbok adult male:
https://www.flickr.com/photos/natureshots123/4525625905/in/photolist-7TV1UR-V4E7DZ-2hSfrAk-PKmG8V-YYkeDg-8cdsKb-2KtjvS-8xnKrN-pxJ6q-KxWtKy-7TYfPu-4DP9n2-MU1Bub-daY6w9-UdwFcv-2kYpdWN-7PgBip-dt8gki-2iPJUN6-2jsBqyX-2jg3cmm-84ziXU-oGhA1T-z8RMkU-AFikPb-5DUcDf-5DPWaM-at6mq5-iM3Nm2-6TeDse-29Yu8yw-GQHdeu-QzVdjX-e6MgFY-2gjCi6T-H9VkFH-5oDPzj-ASgX6y-H7BJzh-pi4vHm-eYXy2U-5f8mw4-bFbjtX-YaJrFP-pHAQwC-24MnFrZ-71RcCB-csqwjA-3yg1Bw-4GLgxP
Bontebok adult male:
https://www.inaturalist.org/observations/71049461
Bontebok adult female:
https://www.inaturalist.org/observations/78211094
https://www.inaturalist.org/observations/76594512
DISCUSSION
Adult females, the most important category biologically, seem to be like-size in the bontebok and the blesbok.
In the case of mature males, the available data imply that the bontebok is indeed (as claimed in https://animaldiversity.org/accounts/Damaliscus_pygargus/) smaller-bodied than the blesbok - the mean values being about 66 kg and 75 kg respectively. In full maturity, males of the blesbok have been known to surpass 95 kg.
Based on the above information, I suggest that
- in both the blesbok and the bontebok, mean body mass of adult females is about 60 kg,
- in the blesbok, mean body mass of mature males is about 75 kg, whereas
- in the bontebok, mean body mass of mature males is only about 66 kg.
What seems to have been overlooked by previous authors is that males grow stouter (particularly head and neck) with full maturity in the blesbok than in the bontebok.
In other words, I suggest that it is not so much that the bontebok is smaller than the blesbok; it is more that the former is less sexually dimorphic than the latter.
To get an idea of the scale of the differences involved, we can compare adult male blesbok with adult female bontebok, as follows.
https://upload.wikimedia.org/wikipedia/commons/8/84/Blesbok%2C_Damaliscus_pygargus_phillipsi%2C_at_Krugersdorp_Game_Reserve%2C_Gauteng%2C_South_Africa_%2826872993194%29.jpg
https://www.inaturalist.org/observations/51830740
Dear reader, to appreciate this maximum contrast, please toggle back-and-forth between https://upload.wikimedia.org/wikipedia/commons/8/84/Blesbok%2C_Damaliscus_pygargus_phillipsi%2C_at_Krugersdorp_Game_Reserve%2C_Gauteng%2C_South_Africa_%2826872993194%29.jpg and https://www.inaturalist.org/observations/51830740.
In particular, please note the relative size of the head in each case.
Now, please note the stoutness/stockiness, and proportionately large head, of the following mature male specimen of the blesbok. Has any reader ever seen such proportions in the bontebok?
https://www.shutterstock.com/it/image-photo/bontebok-antelope-looking-food-on-plateau-2156944039
https://www.shutterstock.com/it/image-photo/bontebok-antelope-looking-food-on-plateau-2156181097
The following is fairly typical of mature males of the bontebok. The body-proportions resemble those of females of the blesbok, and the horns are also noticeably smaller than in the blesbok:
https://www.shutterstock.com/it/image-photo/bontebok-antelope-national-park-south-africa-124597060
Overall:
The facts remain somewhat elusive. However, my impression is that the blesbok and the bontebok are about as biologically different from each other as are the tsessebe (Damaliscus lunatus/superstes) and the topi (Damaliscus korrigum/topi/jimela/tiang, https://en.wikipedia.org/wiki/Damaliscus_lunatus).
If these realisations stand up to scrutiny, perhaps we should classify the blesbok and the bontebok as separate (albeit interfertile) species, viz. Damaliscus phillipsi and Damaliscus pygargus?
ADDITIONAL PHOTO-COMPARISONS, STANDARDISED BY POSTURE AND PERSPECTIVE
Blesbok adult male
https://www.alamy.com/stock-photo-bontebok-ram-portrait-south-africa-11776145.html?imageid=AF7DC680-2009-4423-A70C-A4488AE280B8&p=34420&pn=1&searchId=798d07205e132c3bcbdbb01855d0a786&searchtype=0
Blesbok adult male
https://www.istockphoto.com/photo/blesbok-antelope-gm497409595-41786378?phrase=pics%20of%20a%20blesbok
Blesbok adult male
https://es.123rf.com/photo_27663869_a-blesbok-antelope-damaliscus-pygargus-standing-in-grassland-south-africa.html
Blesbok adult male
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=blesbock&order=relevance&price%5B%24%5D=1&safe_search=1&limit=100&search_page=9&search_type=pagination&get_facets=0&asset_id=165284154
Bontebok adult male
https://www.istockphoto.com/photo/bontebok-antelope-gm145835178-5566402?phrase=pics%20of%20a%20blesbokhttps://www.istockphoto.com/photo/blesbok-antelope-gm523614266-92004719?phrase=pics%20of%20a%20blesbok
Bontebok adult male
https://stock.adobe.com/search/images?k=blesbock&asset_id=103184585
Blesbok adult female
http://www.safaribound.co.za/wp-content/uploads/2015/12/Blesbok.jpg
Blesbok adult female
https://www.flickr.com/photos/keith_burton/52595849483/in/photolist-VQ5V7G-21YXkXk-a2fBQc-C9AkZj-2hknz3d-2o8HtD6-aktLVQ-do2Jrp-kDZnDZ-akqZ3V-akqYN8-6XJngW-aktKRb-2g82zPy-2EwKKK-9ZMSZy-2hRmQoU-GSofrF-2jawJah-Hg63fU-bpxo9b-akqXjc-kDZnJt-8UcSMq-Sgocj4-2hdX4hz-2hRvAAH-2nqxXKA-2k7THja-2nFTwAK-NtcqZw-2mKwoFx-K9H7kw-2nYaeMv-2jbEesL-2jbJeMK-NwubQy-FGdmr3-CMZ9sW-2mVjjPf-ThNHJU-21La8WY-8xnKqU-eMntC3-KA9iEt-2kYpdSu-bsgrmu-2jxYMd5-b3zx7T-2hQhV25
Blesbok adult female
https://www.istockphoto.com/photo/blesbok-damaliscus-pygargus-phillipsi-gm487763834-73141255?phrase=pics%20of%20a%20blesbok
Bontebok adult female
https://www.istockphoto.com/photo/bontebok-gm151545556-10145249?phrase=pics%20of%20a%20blesbok
Bontebok adult female
https://www.istockphoto.com/photo/bontebok-antelope-gm182930579-14039835?phrase=pics%20of%20a%20blesbok
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#.
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