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 (Antilocapra americana):
CitedAlamy·henrysforkwildlifealliance.org·sdakotabirds.com
The opposite extreme is Myotragus balearicus (Wikipedia), which evolved on small islands, virtually free of predators:
CitedFlickr·Twitter 1·Wikimedia 1·Twitter 2·Wikimedia 2
Turning to the placement of the eyes high on the head:
A peculiar example is Hippopotamus amphibius:
CitedAlamy
The extinct Hippopotamus gorgops was even more specialised:
CitedWikipedia·quoracdn.net
However, there is a similar configuration in warthogs (Phacochoerus spp.), which are the suiforms with the least affinity for water:
Warthogs live on land, but have a short neck and legs
CitedDreamstime
and kneel while foraging:
Thus, they risk not spotting danger above the grass.
CitedDreamstime 1·Shutterstock·Dreamstime 2·Dreamstime 3·Dreamstime 4·Dreamstime 5·Dreamstime 6·Dreamstime 7
In their own way, warthogs are unique in having the eyes placed above the ‘horns’:
CitedDreamstime
Ear pinnae
Among wild spp., an extreme example of low placement is Syncerus, in which the ear pinnae are shaded by the horns:
Citedkidadl.com·mediastorehouse.com.au·Wikimedia
However, the most extreme species is a domestic one, viz. Bos indicus:
Cited123rf.com
In large bovines, the function of the ear pinnae seems to be for thermoregulation as much as hearing (see the following):
CitedRelated post
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 the following:
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. This would seem to make high placement/orientation of the sense-organs redundant.
Furthermore, the grey rhebok partly coexists with Damaliscus pygargus pygargus, 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:
CitedRelated post
This is partly explained by the facts that the grey rhebok is one of the few ungulates that penetrates mature stands of fynbos:
has a body size such that the head of the standing figure corresponds approximately to the height of mature fynbos:
and differs from reedbucks (Redunca arundinum
and Redunca redunca) in not lowering the head, and partly turning the ear pinnae back, when initially alarmed:
Citedstock.adobe.com
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 with R. fulvorufula:
CitedAlamy
However, the eyes are not as high-placed as in Redunca arundinum
and possibly Redunca redunca:
The ear pinnae are far more upright in the grey rhebok than in any species of reedbuck.
Compare grey rhebok
with R. arundinum
and R. redunca:
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.
CitedRelated post
The author added the following in comments beneath the original post.
The following shows the position to which the ear pinnae of the grey rhebok sink in death, before rigor mortis:
Citedafricahunting.com
The upright orientation of the ear pinnae in the grey rhebok has sometimes not been realised by taxidermists:
Citedthetaxidermystore.com·eBay
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:
CitedDreamstime 1·Fine Art America·rlbennettphotography.com·Freepik 1·Freepik 2·Bigstock·agefotostock·Dreamstime 2·Dreamstime 3
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:
CitedDreamstime
Other spp. of cervids have different auricular semets, e.g. Cervus nippon:
CitedGetty Images
Please see also adjacent Posts:
CitedRelated post
Compare Blastocerus dichotomus with Axis axis:
CitedShutterstock
A further observation, cited without comment:
