The possible adaptive value of high eyes and ears in the grey rhebok (Pelea capreolus)

@tandala @capracornelius @oviscanadensis_connerties

The eyes and ear pinnae of ungulates vary in placement, according to habitat and anti-predator strategy.

In the case of the eyes, there is variation in how far to the sides, and how high on the head, they are placed.

In the case of the ear pinnae, there is variation in how high on the head they are placed, and in how they are oriented (upright, sideways, or downwards).

EYES:

An extreme example of lateral placement of the eyes is the pronghorn (Antiocapra americana):

https://www.alamy.com/stock-photo-pronghorn-antilocapra-americana-custer-state-park-south-dakota-usa-28993678.html?imageid=0AC99725-2644-403B-BF25-AEF9B643F1EA&p=37611&pn=1&searchId=ae1a50e771f1a64227bbdc57a0f91ee7&searchtype=0
https://henrysforkwildlifealliance.org/2020/08/yellowstone-pronghorn-restoring-ancient-paths/
https://www.sdakotabirds.com/non_birds/photos/pronghorn_3.jpg

The opposite extreme is Myotragus balearicus (https://en.wikipedia.org/wiki/Myotragus), which evolved on small islands, virtually free of predators:

https://www.flickr.com/photos/_quagga/9347938461
https://twitter.com/rvosa/status/1344341993019891715
https://upload.wikimedia.org/wikipedia/commons/4/48/Myotragus_balearicus-IPMQ.jpg
https://twitter.com/whiterabbit36/status/1261265545640509441
https://upload.wikimedia.org/wikipedia/commons/4/46/Myotragus_craneo.jpg

Turning to the placement of the eyes high on the head:

A peculiar example is Hippopotamus amphibius (https://www.alamy.com/stock-image-large-hippo-submerged-in-the-water-with-eyes-out-of-water-murchison-161607662.html). The extinct Hippopotamus gorgops was even more specialised (https://en.wikipedia.org/wiki/Hippopotamus_gorgops and https://qph.fs.quoracdn.net/main-qimg-ccb534bcef45b220dac7df5f5ad0842a-c).

However, there is a similar configuration in warthogs (Phacochoerus spp., https://www.inaturalist.org/observations/122030537), which are the suiforms with the least affinity for water.

Warthogs live on land, but have a short neck and legs (https://www.dreamstime.com/stock-photos-african-wildlife-warthog-image6861013), and kneel while foraging (https://www.inaturalist.org/observations/69736745). Thus, they risk not spotting danger above the grass.

https://www.dreamstime.com/warthog-phacochoerus-africanus-standing-long-grass-backlit-shades-orange-sunset-kruger-national-park-south-africa-image150292482
https://www.shutterstock.com/image-photo/fat-warthog-standing-dry-grass-looking-173319410
https://www.dreamstime.com/warthog-standing-high-grass-savanna-image228148526
https://www.dreamstime.com/royalty-free-stock-image-warthog-piglet-standing-dry-grass-image16998956
https://www.dreamstime.com/warthog-phacochoerus-aethiopicus-adult-standing-grass-masai-mara-park-kenya-image195794593
https://www.dreamstime.com/warthog-phacohoerus-aethiopicus-mokala-national-park-south-africa-warthog-image245349340
https://www.dreamstime.com/warthog-natural-habitat-south-africa-warthog-phacochoerus-africanus-standing-natural-habitat-south-africa-image114919100
https://www.dreamstime.com/warthog-sturdy-hogs-not-world-s-most-aesthetically-pleasing-animals-image224162902
https://www.inaturalist.org/observations/84424825

In their own way, warthogs are unique in having the eyes placed above the 'horns' (https://www.dreamstime.com/common-warthog-stands-grass-eyeing-camera-image163837674).

EAR PINNAE:

Among wild spp., an extreme example of low placement is Syncerus, in which the ear pinnae are shaded by the horns:

https://kidadl.com/facts/animals/cape-buffalo-facts
https://www.mediastorehouse.com.au/nature-picture-library/2019-december-highlights/cape-buffalo-syncerus-caffer-caffer-portrait-19737490.html
https://upload.wikimedia.org/wikipedia/commons/6/60/African_buffalo_%28Syncerus_caffer_caffer%29_juvenile_head.jpg

However, the most extreme species is a domestic one, viz. Bos indicus:

https://es.123rf.com/photo_13793828_portrait-of-zebu-cow-thailand.html

In large bovines, the function of the ear pinnae seems to be for thermoregulation as much as hearing (see https://www.inaturalist.org/journal/milewski/56414-beneficially-bloodshot-from-birds-to-buffaloes#).

APPLYING THIS FRAMEWORK TO GREY RHEBOK:

The grey rhebok (Pelea capreolus) emerges as having

  • possibly the most upright ear pinnae of any ungulate,
  • unusually - albeit not extremely - high-placed eyes, and
  • eyes as laterally-placed as in any bovid.

Please see https://www.inaturalist.org/observations/141682810 and https://www.inaturalist.org/observations/27859437.

How can the combination of high eyes and extremely upright ear pinnae in the grey rhebok be explained, in terms of adaptation?

It is true that the habitat of the grey rhebok is generally open, with low vegetation (https://www.inaturalist.org/observations/11230875). This would seem to make high placement/orientation of the sense-organs redundant.

Furthermore, the grey rhebok partly coexists with Damaliscus pygargus pygargus (https://www.inaturalist.org/observations/11168935), one of the most conspicuous of bovids, presumably in adaptation to open environments.

However, an apparent paradox is that the grey rhebok - in complete contrast to D. p. pygargus - has extremely inconspicuous (cryptic) colouration (https://www.inaturalist.org/journal/milewski/39938-an-easily-overlooked-but-extreme-adaptation-in-the-grey-rhebok and https://www.inaturalist.org/observations/100092810).

This is partly explained by the facts that the grey rhebok

The combination of features peculiar to the grey rhebok can be summarised as follows, relative to reedbucks.

The eyes are as high on the head, and as laterally placed, as in Redunca fulvorufula.

Compare grey rhebok (https://www.inaturalist.org/observations/113747554 and https://www.inaturalist.org/observations/110031488) with R. fulvorufula (https://www.alamy.com/stock-photo-female-mountain-reedbuck-redunca-fulvorufula-mountain-zebra-national-20458753.html).

However, the eyes are not as high-placed as in Redunca arundinum (https://www.inaturalist.org/observations/1242315 and https://www.inaturalist.org/observations/20172544 and https://www.inaturalist.org/observations/9867856 and https://www.inaturalist.org/observations/132955519 and https://www.inaturalist.org/observations/19794658 and https://www.inaturalist.org/observations/15068371) and possibly Redunca redunca (https://www.inaturalist.org/observations/1747700 and https://www.inaturalist.org/observations/131115200).

The ear pinnae are far more upright in the grey rhebok than in any species of reedbuck.

Compare grey rhebok (https://www.inaturalist.org/observations/115575407 and https://www.inaturalist.org/observations/104752672) with R. arundinum (https://www.inaturalist.org/observations/86379206 and https://www.inaturalist.org/observations/37605492) and R. redunca (https://www.inaturalist.org/observations/14309608 and https://www.inaturalist.org/observations/9944725).

I suggest that this peculiar combination is consistent with the habitats and behaviour peculiar to the grey rhebok. Whereas reedbucks tend to be adapted to the tall grass of seasonal marshes, the grey rhebok is dually adapted to mature fynbos and short vegetation.

Also see https://www.inaturalist.org/posts/53474-a-comparison-of-adaptive-colouration-between-lookalikes-grey-rhebok-and-mountain-reedbuck#

Posted by milewski milewski, December 02, 2022 09:48 PM

Comments

@milewski
The examples highlighted in the post are of unique subtleties, such as the noticeable ear difference in the domestic cattle (Bos taurus differentiating in ear morphology from Bos indicus), i.e. the B. indicus possessing rather "strange" ears. Also, is it the case that the Warthog (Genus Phacochoerus) isn't capable of sentry behavior? I'm assuming they compensate their lesser vigilance by exerting physical strength, differing from other terrestrial ungulates who typically prefer not to defend themselves readily.

Posted by paradoxornithidae about 2 months ago (Flag)
Posted by milewski about 2 months ago (Flag)

The following shows the position to which the ear pinnae of the grey rhebok sink in death, before rigor mortis:

https://www.africahunting.com/threads/grey-rhebok-pictures.33300/

Posted by milewski about 2 months ago (Flag)

The upright orientation of the ear pinnae in the grey rhebok has sometimes not been realised by taxidermists:
https://www.thetaxidermystore.com/african-grey-rhebok-shoulder-taxidermy-mount-14590.html
https://www.ebay.com/itm/161913308361

Posted by milewski about 2 months ago (Flag)

@hhodgson

BUCCAL SEMET:

With respect to https://www.inaturalist.org/observations/141682810, please scroll to the last two of the 8 photos in https://www.ebay.com/itm/161913308361. They show that the buccal semet of the grey rhebok remains after taxidermy.

Posted by milewski about 2 months ago (Flag)

Pelea is undoubtedly the only one of it's kind, such particularised patterns must mean they have only occured in historical times within far Southern Africa (Swaziland, Lesotho, SA)? I must admit I am ignorant in knowledge of that area in general.

Posted by paradoxornithidae about 2 months ago (Flag)

May I ask, what is the auricular semet? I'm sure I already asked this in a previous post, but I can't seem to find it.

Posted by paradoxornithidae about 2 months ago (Flag)

@paradoxornithidae

An auricular semet is a subtle pattern of colouration on or adjacent to the ear pinnae, functioning for intraspecific (social) communication. It is activated by movement, particularly the swivelling of the ear pinnae according to mood/emotion and the direction of attention/intention.

The largest-scale patterns of colouration on/near the ear pinnae may qualify instead as auricular flags, because - once activated by movement - they are capable of making the whole figure conspicuous, including to scanning predators.

An unambivalent example of an auricular semet occurs in Axis axis:
https://www.dreamstime.com/stock-photos-spotted-deer-chital-cheetal-axis-axis-female-grassland-ranthambhore-found-all-forests-areas-indian-image36944653 and https://fineartamerica.com/featured/axis-face-teresa-blanton.html and https://www.rlbennettphotography.com/axis-deer-at-austin-zoo/ and https://www.freepik.com/premium-photo/chital-cheetal-spotted-deer-axis-deer-national-park-thailand_14756481.htm and https://www.freepik.com/premium-photo/chital-is-looking-you_1775714.htm and https://www.bigstockphoto.com/image-415622500/stock-photo-beautiful-axis-deer-standing-close-up-on-nature-fence-background-beauty-antler-or-horn-of-young-bro and https://www.agefotostock.com/age/en/details-photo/spotted-deer-or-axis-deer-axis-axis-bandipur-tiger-reserve-karnataka-india-bandipur-tiger-reserve-karnataka-india-21-02-2019/VIG-6485300 and https://www.dreamstime.com/stock-photo-female-spotted-deer-chital-axis-axis-kanha-national-park-india-image90226102 and https://www.dreamstime.com/stock-photo-female-spotted-deer-chital-axis-axis-kanha-national-park-india-image65483269.

If you focus on the dorsal surface of the anterior base of the ear pinna, you may notice a crisp dark/pale (actually black/white) pattern, on a scale within centimetres.

The configuration changes depending on the position of the ear pinna relative to the head. This means that, as the ear pinnae move in vigilance, attentiveness, or emotion, the movement is accentuated by the pattern of dark/pale contrast.

This pattern cannot be dismissed as functioning mainly for inconspicuousness in conformation with the general spotted pattern of the animal. This is because a) camouflage - which is by definition stationary - cannot explain why the pattern has been configured to change so much with the movements of the ear pinnae, and b) for inconspicuousness, the simplest pattern would be plainness of the whole ear pinna and its base, without any pattern at all (compare https://www.dreamstime.com/female-spotted-deer-peeking-behind-vegetation-selective-focus-axis-also-called-chital-native-to-indian-image220600816).

Other spp. of cervids have different auricular semets, e.g. Cervus nippon: https://www.gettyimages.co.uk/detail/photo/close-up-of-sika-deer-royalty-free-image/1435066223?phrase=aleksey&adppopup=true.

If you like, I could also give examples of auricular flags...

Posted by milewski about 2 months ago (Flag)

@paradoxornithidae

Here is a clear view of both the auricular semet and the buccal semet of Axis axis:

https://www.shutterstock.com/nb/video/clip-1034753474-close-profile-axis-deer-looking-right-chewing

Posted by milewski about 2 months ago (Flag)
Posted by milewski about 2 months ago (Flag)

Add a Comment

Sign In or Sign Up to add comments