Introducing the ocular flag, a feature of adaptive colouration epitomised by dik-diks in the Madoqua kirkii/damarensis complex

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It is unsurprising that dik-diks, in the Madoqua kirkii/damarensis complex (https://www.inaturalist.org/taxa/42360-Madoqua-kirkii), have inconspicuous colouration, overall.

However, what is puzzling is that the eyes do not conform to this pattern.

The dark of the eyeball, eyelids, and antorbital gland are accentuated by a ring of pale pelage, making the eyes conspicuous.

https://stock.adobe.com/search?filters%5Bcontent_type%3Aphoto%5D=1&filters%5Bcontent_type%3Aillustration%5D=1&filters%5Bcontent_type%3Azip_vector%5D=1&filters%5Bcontent_type%3Avideo%5D=1&filters%5Bcontent_type%3Atemplate%5D=1&filters%5Bcontent_type%3A3d%5D=1&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=dikdik&order=relevance&safe_search=1&limit=100&search_page=4&search_type=pagination&get_facets=0&asset_id=357094861

https://www.yourafricansafari.com/media/images/photos/2022/09/03/P1090468.JPG

https://www.alamy.com/dikdik-image154260761.html?imageid=3D6DB9B6-DA82-4546-B9D2-310D6D16D008&p=471322&pn=1&searchId=9d24468379105dc9d49ba2ff555a693e&searchtype=0

https://www.kevindowie.com/dik-dik-antelope-tarangire-national-park-tanzania/

https://www.flickr.com/photos/92655181@N07/51360475496

https://zooinstitutes.com/animals/kirks-dik-dik-zoological-center-tel-aviv-ramat-gan-11376.html

https://www.mindenpictures.com/search/naturephotos/silver+dik-dik.html

https://www.mediastorehouse.com/auscape/photographer-galleries/joe-mcdonald/kirks-dik-dik-madoqua-kirkii-20121496.html#openModal

Infant: https://www.youtube.com/watch?v=LNmGqcGd-cI

https://www.mentalfloss.com/article/57795/9-fun-facts-about-dik-dik

How can this anomaly be explained?

I hypothesise that the conspicuous pattern of dark/pale contrast at the eye qualifies as an OCULAR FLAG.

This would function as:

  • an announcement to potential predators that they have been spotted, and
  • a means of intraspecific communication, allowing mates and offspring to maintain visual contact in dim or dappled light.

This flag is hypothetically activated by

  • the utterance of an alarm-whistle, and
  • motion of the head, including while walking routinely during foraging.

COMPARISON AND CONTRAST WITH RAPHICERUS SHARPEI

The Madoqua kirkii/damarensis complex is comparable with Raphicerus sharpei, but the two seem to have functionally opposite patterns of colouration at the eyes.

The distribution of Raphicerus sharpei (https://upload.wikimedia.org/wikipedia/commons/b/bb/Raphicerus_sharpei.png) fills in for the disjunct distribution between M. damarensis damarensis (Namibia and Angola) and the other members of the complex (East Africa, https://upload.wikimedia.org/wikipedia/commons/3/3c/Madoqua_kirkii_map.png).

The similarities include:

  • habitat consisting of 'leafless thickets', tending to be associated with stony ground at the base of rocky outcrops,
  • average body mass falling within the range 5-7.5 kg,
  • colouration that is cryptic overall, partly by virtue of a grizzled effect, and
  • hindquarters that lack any buttock flag or conspicuous tail.

However, a difference is that the eye is accentuated in Madoqua, whereas it is masked in R. sharpei.

The following show the darkness around the eyes in R. sharpei:

https://www.shutterstock.com/image-photo/grysbok-antelope-seen-on-safari-south-1932461939
https://www.inaturalist.org/observations/7702585
https://www.inaturalist.org/observations/652733
https://www.inaturalist.org/observations/61650836
https://www.inaturalist.org/observations/9748595

Please note that both Kingdon (https://www.researchgate.net/figure/fig1_311546836) and Dandelot (https://ibs.bialowieza.pl/wp-content/uploads/2020/09/KERLEY.pdf) have misportrayed this aspect of R. sharpei. They fail to show the typical masking of the eyes in this species.

My explanation of this difference between the Madoqua kirkii/damarensis complex and R. sharpei:

There is greater sociality in the former than in the latter.

In both cases, there is a monogamous pair-bond. However, R. sharpei is one of the most solitary of all antelopes in savanna biomes, whereas dik-diks retain obvious social behavior.

Of the 120 observations of R. sharpei currently posted in iNaturalist, none shows more than one individual. By contrast, in the case of Madoqua damarensis, the corresponding value is 7% (13 of the 187 observations).

ANTI-PREDATOR REACTION

Dik-diks of the Madoqua kirkii/damarensis complex react to the detection of stalking predators in ways consistent with self-advertisement by means of an ocular flag.

Estes (1991) states: "Dik-diks freeze at the slightest disturbance, often with 1 leg raised. The male moves his head cautiously, trying to identify the danger, while the female remains immobile except for the questing tip of her nose...Once a predator is detected and identified, dik-diks behave in 1 of 2 ways, depending on whether it is a stalker or a courser. If it is a cat, they flee just far enough to be safe, emitting loud, explosive whistles at the first (stotting) bounds. They then proceed to keep watch and sound the alarm - possibly in a duet...The male is the principal caller...When the predator is a hyena, wild dog, or other courser, dik-diks immediately sink to the ground and lie still until the enemy passes. If discovered, they flee without whistling".

This contrasts with R. sharpei.

Skinner and Chimimba (2005) state: "they are difficult to observe and are inclined to lie up very tightly and unless disturbed by close approach remain hidden in the undergrowth. When they do run off they do so crouching low to the ground as they run through the thick underbrush, making them difficult to see. Quite often the disturbance of the undergrowth and a glimpse of a reddish body is all that can be seen of them".

The above differences in anti-predator behaviour between R. sharpei and Madoqua kirkii/damarensis are categorical. This is because there are no circumstances in which the former visually advertises itself to potential predators.

Even in the case of audial advertisement, R. sharpei lacks any snort or whistle of alarm, and its only announcement is the sound of stamping of the feet as it initially flees.

What this means:

Although similar in other ways, R. sharpei and the Madoqua kirkii/damarensis complex are extremely different in their reactions to stalking predators. The former simply hides and then, at the last moment, flees. By contrast, the latter are remarkably demonstrative for such small animals, in

Indeed, I suspect that, of all the many genera and species of bambis on Earth, dik-diks are the most demonstrative towards potential predators in the interval between initial alarm and running.

With respect to social behaviour:

For Madoqua damarensis, Skinner and Chimimba (2005) state: "The pair may be accompanied by up to two offspring... approximately 10% of groups are polygynous, containing two unrelated females...may form temporary aggregations of up to 10 individuals at sites of food abundance...one of the few antelopes in which female territoriality has been documented".

See https://www.alamy.com/stock-photo-dikdiks-or-kirk-dikdik-madoqua-kirkii-adult-animal-stand-dry-grass-146441152.html.

For Raphicerus sharpei, the same authors state: "One usually encounters solitary adults, pairs, or a female with her single offspring...They are shy and secretive and can be overlooked in areas where in reality they are reasonably common".

What emerges is a clear role for an ocular flag in dik-diks, in a syndrome including both anti-predator and social behaviours.

DOES ANY OTHER UNGULATE QUALIFY FOR AN OCULAR FLAG?

If we accept that the pattern of colouration at the eye is the only adaptively conspicuous feature in certain species of dik-dik, then this creates a scientific search-image for ocular flags in other ungulates.

It is well-known that various clades of bovids and cervids contain spp. with pale superciliary (https://www.merriam-webster.com/dictionary/superciliary) or supraorbital (https://www.merriam-webster.com/dictionary/supraorbital) markings.

However, the adaptive function of such markings depends on

  • the relative size of the eye, which depends mainly on body size,
  • the extension of the pale marking to encircle the whole eye (plus antorbital gland, if present), and
  • the presence of other pale markings on the figure, which would distract from the eye.

Body size is important. This is because the larger the body, the proportionately smaller the eye tends to be.

For example, Litocranius walleri resembles dik-diks in the pattern of colouration at the eye (https://stock.adobe.com/search?filters%5Bcontent_type%3Aphoto%5D=1&filters%5Bcontent_type%3Aillustration%5D=1&filters%5Bcontent_type%3Azip_vector%5D=1&filters%5Bcontent_type%3Avideo%5D=1&filters%5Bcontent_type%3Atemplate%5D=1&filters%5Bcontent_type%3A3d%5D=1&filters%5Bcontent_type%3Aimage%5D=1&order=relevance&safe_search=1&k=Gerenuk&search_page=1&search_type=usertyped&acp=&aco=Gerenuk&get_facets=0&asset_id=1180611 and https://stock.adobe.com/search?filters%5Bcontent_type%3Aphoto%5D=1&filters%5Bcontent_type%3Aillustration%5D=1&filters%5Bcontent_type%3Azip_vector%5D=1&filters%5Bcontent_type%3Avideo%5D=1&filters%5Bcontent_type%3Atemplate%5D=1&filters%5Bcontent_type%3A3d%5D=1&filters%5Bcontent_type%3Aimage%5D=1&order=relevance&safe_search=1&k=Gerenuk&search_page=1&search_type=usertyped&acp=&aco=Gerenuk&get_facets=0&asset_id=218236488).

However, this is too small, relative to the whole figure, to qualify as a flag (https://www.alamy.com/stock-photo-a-female-gerenuk-litocranius-walleri-15381432.html). Instead, in this case the pattern is more likely to function disruptively, i.e. in aid of camouflage.

In summary, the argument for an ocular flag in dik-diks is based on the following:

  • body size is extremely small (5 kg) for an ungulate, making the eye proportionately large,
  • the eye is relatively large even among the various ungulates of similar, small body size,
  • the rest of the colouration is plain, with even the erectile crest, on the crown of the head, lacking dark/pale contrast,
  • the tail is too small to function as a flag, even if it were dark/pale,
  • the reaction to non-cursorial predators - presumably even by night - is one of self-advertisement, and
  • there is more sociality than expected for a small-bodied, mainly monogamous ruminant.

So, in the spirit of scientific hypothesis-testing, I can make the following prediction.

Among all the ungulates of the world, the only species qualifying for an ocular flag belong to the genus Madoqua.

Which leads to a challenge to readers:

Can anyone find any ungulate, other than dik-diks, that possesses an ocular flag?

Publicado por milewski milewski, 07 de octubre de 2022

Comentarios

Publicado por milewski hace alrededor de 2 meses (Marca)

Excellent illustration of caudal flagging in juvenile Panthera leo just after minute 5:05 in https://www.youtube.com/watch?v=0qV59HdAaiM

Publicado por milewski hace alrededor de 2 meses (Marca)

The following (https://www.inaturalist.org/observations/38458863) shows that some individuals of Raphicerus campestris capricornis have pale around the eyes.

However, a difference from the Madoqua kirkii/damarensis complex is that R. campestris has a major flag on its buttocks, which tends to eclipse the conspicuousness of the eyes.

Publicado por milewski hace alrededor de 2 meses (Marca)

The following show that, even within a species, the eyes are proportionately larger in small-bodied than in large-bodied subspecies/individuals.

Odocoileus virginianus virginianus, adult female body mass about 60 kg:

https://www.gettyimages.in/detail/photo/white-tailed-deer-doe-shenandoah-national-park-royalty-free-image/139814726?phrase=white%20tailed%20deer&adppopup=true

Odocoileus virginianus clavium, adult female body mass about 30 kg:

https://www.gettyimages.com.au/detail/photo/portrait-of-a-key-deer-royalty-free-image/628805524

Publicado por milewski hace alrededor de 2 meses (Marca)

My rationale for an ocular flag depends on a particular degree of sociality in dik-diks.

In support of this, here are relevant excerpts from Cynthia Moss (1975, Portraits in the Wild: Animal Behaviour in East Africa):

The territory "is held exclusively for one male, one female, and up to two of their offspring...the pair bond...lasts for years and perhaps, in some cases, for the life of the animal...The male and female usually walk together in their territory...The male and female use the same dung piles in the territory and will usually go to a dung area together...If there is a dikdik fawn accompanying the parents, it will also use the same dung midden, either before or after its mother...If another dikdik approaches the territory, the doe will stand in a characteristic posture with head held high...if the buck is with her, which is...often the case, he will immediately notice her posture...and will thus be alerted to the presence of the other animal. He will then stand with head high, in...the 'show-off' posture...When... defending...territory or maintaining its boundaries...male and female usually engage in these activities together...When it is time for the mother to suckle (a new) fawn...the...buck and her older fawn, which is just over six months old, may follow her and rest nearby while she attends the fawn...When this youngster is six months old, its mother will have another fawn, and after that it may spend more time with its father"

Publicado por milewski hace alrededor de 1 mes (Marca)

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