Do blesbok and bontebok differ enough in sexual dimorphism to be different species?

@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#.

Posted on March 03, 2023 09:32 AM by milewski milewski

Comments

The following nicely illustrates the body-proportions in the bontebok:

https://www.inaturalist.org/observations/124444389

Posted by milewski about 1 year ago

To this day, various websites cite absurd values for the body mass of the blesbok, such as the '194 kg' (up to '270 kg') in this example:

https://omatjete.com/hunting-blesbok/

Posted by milewski about 1 year ago
Posted by milewski 12 months ago

Does any reader have access to https://www.sciencedirect.com/science/article/abs/pii/S0921448803002906

If so, could you please tell me
a) the body mass (live weight) and
b) the head mass,
reported for the blesbok?

Posted by milewski 12 months ago

The body mass stated for the blesbok by Encyclopedia Britannica (https://www.britannica.com/animal/blesbok), viz. 55-80 kg, seems correct.

The values stated in https://animalia.bio/blesbok also seem correct.

Posted by milewski 12 months ago

@kevinatbrakputs

Another surprisingly large difference between the blesbok and the bontebok is in the role of feces in territoriality.

https://www.inaturalist.org/observations/118544787
https://www.inaturalist.org/observations/103296527

In the blesbok, territorial males deposit their feces in collections (https://journals.co.za/doi/10.10520/AJA03794369_2644 and second photo in https://www.inaturalist.org/observations/74507332 and second photo in https://www.inaturalist.org/observations/74507331).

This may be part of the optical and olfactory demarcation of territory (Lynch C D 1971 A behavioural study of blesbok, Damaliscus dorcas phillipsi, with special reference to territoriality. M Sc thesis, University of Pretoria. 102 pages.).

By contrast, neither territorial rivals nor 'bachelors' seem to pay similar attention to feces, in the case of the bontebok. David (1970, https://open.uct.ac.za/handle/11427/18102) inferred that feces are not used to demarcate territory in this form.

Territorial males of the blesbok spend most of their time on or near 'dung-patches'. They frequently lie down on their own feces, impregnating their pelage with their own personal odour. The bontebok is not basically different, because it, too, may lie on its feces. The distinction seems to be the tendency, in the blesbok, to aggregate the feces to the degree that they have a visual function in territoriality.

In iNaturalist, the largest aggregations of feces in the case of the bontebok are https://www.inaturalist.org/observations/10951559 and https://www.inaturalist.org/observations/99547162 and https://www.inaturalist.org/observations/114429951 and https://www.inaturalist.org/observations/113903493 and https://www.inaturalist.org/observations/11058769.

Posted by milewski 12 months ago

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