The optically puzzling haunch-patch of the topi (Damaliscus jimela)

@botswanabugs @davidbygott @tonyrebelo @jeremygilmore @capracornelius @beartracker @tandala @minivet @matthewinabinett @simontonge

There is a feature of colouration in the topi (Damaliscus jimela) that is hard to explain in terms of either pigmentation or sheen.

I refer to the taken-for-granted 'dark' patch on each haunch of adults and juveniles.

(The haunch is the muscular surface located between buttock and flank, and between rump and upper hindleg.)

This haunch-patch of the topi

• is consistently present in all individuals of both sexes (https://www.inaturalist.org/observations/139783193 and https://www.inaturalist.org/observations/99228531), and
• develops relatively precocially, in juveniles (https://www.inaturalist.org/observations/27634388 and https://www.inaturalist.org/observations/102416167 and https://www.inaturalist.org/observations/71988453 and https://www.inaturalist.org/observations/47491131).

Although uniformly present, it

• looks remarkably variable in tone, from pale to dark, and
• does not seem to function efficiently, in terms of adaptive colouration.

In this Post, I describe the puzzle, and I offer an explanatory hypothesis.

The buttocks of the topi are relatively pale (https://www.inaturalist.org/observations/97049832). This is owing partly to depigmentation, and partly to sheen.

The ostensible function of the haunch-patch is to offset the buttocks (and rump), producing dark/pale contrast analogous to that seen in the closely-related bontebok (Damaliscus pygargus pygargus, https://www.alamy.com/stock-photo-bontebok-damaliscus-pygargus-pygarus-bontebok-national-park-south-59175893.html?imageid=190A38D1-F132-4034-A1D5-89F4685B1E7B&p=22059&pn=1&searchId=25869b8bb63848eab5bc67f94e53a679&searchtype=0).

There is no doubt that, in certain illuminations, the paleness on the buttocks of the topi is conspicuous.

Furthermore, this paleness can extend to the rump - which is not depigmented, but is sheeny in certain illuminations (https://www.inaturalist.org/observations/19669831
and https://www.inaturalist.org/observations/49694092).

A pale feature in the hindquarters is unsurprising. This is because whitish - in various patterns - occurs conspicuously on the buttocks and/or rump of many species of ungulates living gregariously in open environments (https://www.pestdetective.org.nz/image?Type=culprit&ID=142&Parent=170 and https://www.featheredphotography.com/blog/2019/10/31/an-impressive-desert-bighorn-ram/ and https://www.nps.gov/articles/pronghorn.htm and https://www.gettyimages.com.au/detail/photo/pronghorn-antelope-saskatchewan-full-length-of-deer-royalty-free-image/1370773707?phrase=blesbok&adppopup=true and https://www.wlfw.org/how-do-mule-deer-use-sagebrush/ and https://www.canadiannaturephotographer.com/january2014.html).

The problem is that, in the topi,

• the nominally dark haunch-patch does not usually look dark enough to provide dark/pale contrast, and
• it is unclear whether this failure is owing to insufficient pigmentation, or excessive sheen, or some other, unrecognised, optical phenomenon.

The following are typical views (https://www.inaturalist.org/observations/68110748 and https://www.inaturalist.org/observations/66387262 and https://www.inaturalist.org/observations/42292854).

These photos show that the haunch-patch is definite, but the pattern on the hindquarters does not achieve the boldness for which it seems to be designed. The haunch-patch, instead of actually looking dark, comes off as a mere shade of grey.

This 'greying' is only partly owing to depigmentation.

Sheen and gloss are types of reflection so effective that the haunch-patch can look pale when bright sunlight shines directly on to it (https://www.inaturalist.org/observations/98817844
and https://www.inaturalist.org/observations/8066259
and second photo in https://www.inaturalist.org/observations/32196975 and https://www.inaturalist.org/observations/35673621 and https://www.inaturalist.org/observations/19302393 and https://www.inaturalist.org/observations/19299734 and https://www.inaturalist.org/observations/144456049 and https://www.inaturalist.org/observations/139976038).

By the way, dear Reader, while perusing the illustrations in this Post, please continually compare the darkness of the haunch-patch with that of the upper foreleg, i.e. just above the carpal joint (https://afs.ca.uky.edu/livestock/beef/skeletal/carpal-bones).

What is consistent is that the dark on the foreleg remains far darker than that of the haunch-patch, in virtually all illuminations and perspectives (https://www.inaturalist.org/observations/32009259 and https://www.inaturalist.org/observations/145979173
and https://www.inaturalist.org/observations/30840859 and https://www.inaturalist.org/observations/27272205 and https://www.inaturalist.org/observations/32009255 and https://www.inaturalist.org/observations/41921165 and https://www.inaturalist.org/observations/76540065).

I do not know the reason for this difference, but it is probably some combination of intense pigmentation and minimal sheen on the upper foreleg.

Returning to my description of the main puzzle:

In some views, the pattern on the hindquarters is hardly noticeable (https://es.123rf.com/photo_107357682_topi-damaliscus-korrigum-single-mammal-on-grass-uganda-august-2018.html and https://www.inaturalist.org/observations/106521912 and https://www.inaturalist.org/observations/106859171), or would be hardly noticeable were it not for a reddish hue (https://www.inaturalist.org/observations/46680897) - which may not be perceptible in the visual system of ungulates and carnivores.

In many other views, the haunch-patch does look somewhat dark. However, in such cases the buttocks are usually not particularly 'lit up' by sheen - so that any dark/pale contrast remains limited (https://www.inaturalist.org/observations/29101210 and https://www.inaturalist.org/observations/8009791 and https://www.inaturalist.org/observations/154888670 and https://www.inaturalist.org/observations/62025952 and https://www.inaturalist.org/observations/56613349 and https://www.dreamstime.com/royalty-free-stock-photos-antelope-topis-damaliscus-korrigum-image22077428).

In posteriolateral views in which the buttocks and rump appear noticeably pale, the haunch-patch often fails to provide dark/pale contrast (https://www.inaturalist.org/observations/104585211 and https://www.inaturalist.org/observations/9883466).

In a few photos on the Web, it all seems to come together:
there is dark/pale contrast bold enough to make the whole figure conspicuous (https://www.inaturalist.org/observations/102025376 and https://www.inaturalist.org/observations/28289363 and https://www.inaturalist.org/observations/74563951 and https://www.gettyimages.ie/detail/photo/topi-herd-damaliscus-korrigum-on-savanna-tanzania-royalty-free-image/128139798?phrase=damaliscus%20korrigum&adppopup=true and https://www.naturepl.com/stock-photo-topi-damaliscus-korrigum-females-and-youngs-masai-mara-game-reserve-image01427626.html).

However, such views are hardly typical.

Summarising these findings in more technical terms:

In the topi (https://www.inaturalist.org/observations/35622892),

• the ischial flag is conspicuous owing mainly owing to a pale surface, whereas the ulnar flag is conspicuous owing mainly to a dark surface, and
• the paleness of the ischial (and pygal) surface in question is owing mainly to sheen, whereas the darkness of the ulnar surface in question is owing mainly to pigmentation.

I hypothesise that the puzzle of an indifferently greyish haunch-patch in the topi might be explained by ultraviolet, invisible to the human eye but visible to the eyes of ungulates and carnivores.

I predict that, when viewed through suitable optical equipment, the pelage of the topi will be found to have a distinct reflective pattern.

More particularly, I predict that the buttocks will prove to be reflective of ultraviolet, whereas the haunch-patch, absorbing these short wavelengths, will seem black in ultraviolet.

What this would mean, if I am right:

what our human eye sees as an indifferent pattern of 'shades of grey' - partly owing to mutually-cancelling sheen effects - is actually a vivid pattern in the eyes of the animals concerned...

perhaps even as vivid (https://www.inaturalist.org/observations/115550547 and https://www.dreamstime.com/stock-photo-bontebok-looking-stoned-medium-sized-generally-dark-brown-antelope-prominent-wide-white-blaze-its-face-image77511451) as the pattern on the hindquarters of the bontebok?

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 14 April, 2023 16:36 by milewski