Has the post-withers mane-tuft of Hippotragus evolved in mimicry of oxpeckers (Buphagus)? part 2

...continued from https://www.inaturalist.org/journal/milewski/87946-has-the-post-withers-mane-tuft-of-hippotragus-evolved-in-mimicry-of-oxpeckers-buphagus-part-1#

POSSIBLE EXPLANATORY FRAMEWORK

We can assume a mutualistic relationship between Buphagus and Hippotragus, in which

Buphagus spp. eat, and thus control, parasitic invertebrates, and
Hippotragus spp. not only tolerate, but invite, attendance by the birds.

It is noteworthy that Buphagus africanus is known to remain perched (presumably sleeping) on H. niger, overnight. This suggests a particularly intimate relationship, because the hippotragin is much smaller-bodied than the other ruminants that allow roosting by this oxpecker, namely Giraffa and Syncerus (https://planetofbirds.com/oxpeckers-buphagidae).

The services of oxpeckers may be particularly beneficial for the health of H. equinus and H. niger. This is because these bovids

tend to forage on relatively tall grasses, exposing them to ticks (https://www.inaturalist.org/observations?place_id=56881&taxon_id=52788&view=species) that crawl on to the body from grass-stalks and -leaves,
live in/near woodlands, where tsetse flies (Glossina, https://www.inaturalist.org/observations?place_id=any&taxon_id=67370&view=species) are particularly common,
lack a behavioural repertoire of mutual grooming (https://pubmed.ncbi.nlm.nih.gov/10924201/ and https://pubmed.ncbi.nlm.nih.gov/11193674/and https://www.sciencedirect.com/science/article/abs/pii/S0003347205803197 and https://www.researchgate.net/publication/229520029_Differential_foraging_of_oxpeckers_on_impala_in_comparison_with_sympatric_antelope_species), partly because they are 'touch-averse' as part of a relatively pugnacious temperament,
have manes (with sociosexual functions) that tend to harbour ticks and other parasitic invertebrates,
are among the few tropical ruminants that have lengthened pelage in the dry, relatively cool season, and
are not subject to the open lesions - familiar in Hippopotamus, Diceros, Syncerus, and Giraffa - that Buphagus africanus tends to maintain for a supply of blood and pus as part of its diet (https://planetofbirds.com/oxpeckers-buphagidae).

However, oxpeckers tend to be scarce, overall, in the habitats of H. equinus and H. niger. This is mainly because ungulates in general are scarce in the miombo biome, where soils are nutrient-poor and grass tends to be consumed by combustion instead of herbivory.

Indeed, oxpeckers are - apparently naturally - absent from considerable parts of the distribution of Hippotragus. The most important example is central and eastern Angola, where H. equinus was originally widespread and H. n. variani occurred locally (https://www.inaturalist.org/journal/milewski/54390-why-was-angola-the-empty-quarter-for-african-large-mammals#).

Furthermore, only one species of oxpecker occurs in West and north-central Africa, including the whole range of the widespread subspecies H. e. koba.

Partly on this basis, I hypothesise that there has been an evolutionary pressure on Hippotragus to facilitate the attraction of oxpeckers, by means of a benign form of deception.

It may seem odd that birds as sharp-eyed, and relatively brainy, as oxpeckers could possibly be fooled by the crude resemblance between the post-withers mane-tuft and the silhouette of a perching bird.

However, there are many examples in the literature of avian behaviours stimulated by relatively crude visual cues, beginning with Tinbergen's gulls (https://www.birdnote.org/listen/shows/little-red-spot-gulls-bill#:~:text=Tinbergen%20devised%20experiments%20that%20varied,fed%2C%20and%20thus%20its%20survival. and https://www.sciencedirect.com/science/article/abs/pii/S0003347209000165).

Given that the relationship is mutualistic, there would not necessarily be natural selective pressure to refine the simulation beyond the crude one that we observe.

Oxpeckers are gregarious, demonstrative birds that utilise the bodies of ungulates as a substrate for not only foraging, but also socialising. Within any given area, groups (clans?) of these birds must inevitably split on a repeated basis, because any given individual or small group of Hippotragus - or coexisting Strepsiceros or Phacochoerus - offers a limited surface.

Thus, it is easy to imagine that there is a continual demand for social reunion, and a strong search-image for an indicative silhouette over distances of hundreds of metres.

DISCUSSION

On one hand, the post-withers mane-tuft may seem too sketchy to be a convincing silhouette-mimic for an oxpecker (https://www.inaturalist.org/observations/82258711).

Factors in this sketchiness include the following:

individual variation, and
relatively small size in H. equinus, vs relatively large size in H. niger (https://www.inaturalist.org/observations/107512573 and https://www.inaturalist.org/observations/8693020).

On the other hand, the post-withers mane-tuft

is located in precisely the right anatomical position to be effective, and
appears remarkably early in life, during infancy (https://stock.adobe.com/images/young-sable-antelope-in-its-habitat/606417953?prev_url=detail and https://stock.adobe.com/images/sable-antelope/221707903?prev_url=detail and https://stock.adobe.com/images/sable-antelope-young-in-park/432854759?prev_url=detail).

How else might such an extreme 'aberration' of anatomy and colouration be explained in adaptive terms?

So, I ask:

Is the pattern in both H. equinus and H. niger not consistent and suggestive enough to warrant a hypothesis of mimicry - even if only to stimulate further studies?

Is it possible that we have discovered a new category of phenomenon in biology, namely 'mutualistic mimicry'?

A CHALLENGE TO READERS

How many individuals of Buphagus are apparent on the following specimens?

Hippotragus equinus:

Hippotragus niger: