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

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 ( 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,

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 (

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 ( and and and and and

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 ( and and

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 (
and second photo in and and and and and

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 (

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 ( and
and and and and and

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 ( and and, or would be hardly noticeable were it not for a reddish hue ( - 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 ( and and and and and

In posteriolateral views in which the buttocks and rump appear noticeably pale, the haunch-patch often fails to provide dark/pale contrast ( and

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 ( and and and and

However, such views are hardly typical.

Summarising these findings in more technical terms:

In the topi (,

  • 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 ( and as the pattern on the hindquarters of the bontebok?

For an index to my many Posts about the genus Damaliscus, please see

Posted on April 14, 2023 04:36 PM by milewski milewski


@milewski so pictures need to be taken in the uv spectrum ?

Posted by botswanabugs over 1 year ago


Yes, indeed...

Posted by milewski over 1 year ago
Posted by milewski over 1 year ago


Posted by milewski 7 months ago

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