Skull-optics in ungulates, part 1
Ungulates look out for danger, the eyes being as important as ears and noses for detecting predators. Because there is regional variation in the incidence of carnivores, we can expect variation among species in eye-size proportional to the size of the body.
So far, we expect big-eyed ungulates in intensely predatory environments such as African savannas, vs small-eyed ungulates in safe environments such as islands.
Seeing in the dark takes a bigger retina than seeing in sunlight. So we also expect nocturnally active ungulates to be bigger-eyed than diurnally active ungulates, other factors being equal.
In open environments, ungulates scan a wide horizon for danger, whereas in dense vegetation the visibility is so limited that looking is less vigilant than listening. So we expect ungulates of treeless grassland to have the widest-set eyes, achieving some rear-vision by means of projecting orbits.
Altogether, we expect the proportionately biggest, widest-set eyes to occur in ungulates of treeless grassland in intensely predatory regions, particularly if these ungulates are active by night as well as by day. By contrast, we expect the proportionately smallest, most narrowly-set eyes in diurnal ungulates of forest and thicket in minimally predatory regions.
Two ungulates come immediately to mind for extremes of eye-size and -setting: the pronghorn and the Balearic Islands cave goat. How do these conform to predictions?
The pronghorn (Antilocapra americana) is diurnal in treeless grassland, but its predatory regime at the time of European arrival was not particularly intense. So its extremely big and wide-set eyes, like its extreme speed and endurance in flight, seem somewhat superfluous.
The following should be self-explanatory enough:
http://justfunfacts.com/wp-content/uploads/2018/02/pronghorn-5.jpg and https://fineartamerica.com/featured/pronghorn-antelope-doe-in-soft-light-max-allen.html?product=greeting-card
https://www.acornnaturalists.com/antelope-pronghorn-skull-replica.html and https://www.deviantart.com/minotaur-queen/art/Doe-Pronghorn-Antelope-Skull-2-171590387 and https://fineartamerica.com/featured/1-pronghorn-antelope-skull-millard-h-sharp.html and https://www.deviantart.com/minotaur-queen/art/Doe-Pronghorn-Antelope-Skull-171589759
The following contains a photo showing that the eyes of the pronghorn can be seen even from directly behind the animal: https://forums.bowsite.com/tf/bgforums/thread.cfm?threadid=407206&forum=8.
The extinct Balearic Islands cave goat (Myotragus balearicus) evolved on carnivore-free islands. It was probably diurnal and the vegetation was probably open. The eyes were more forward-facing than in any other ungulate, and were provably small as well.
The following should be self-explanatory enough:
https://en.wikipedia.org/wiki/Myotragus and https://prehistoric-fauna.com/Cave-goat
https://prehistoric-fauna.com/Myotragus-balearicus
https://www.flickr.com/photos/_quagga/9347938461 and https://www.zoochat.com/community/media/skull-of-balearic-island-cave-goat-myotragus-balearicus-at-national-history-museum-london.431640/ and https://pixels.com/featured/myotragus-antelope-skull-natural-history-museum-london.html and https://twitter.com/whiterabbit36/status/1261265545640509441
Comparison with chamois (Rupicapra rupicapra): https://www.researchgate.net/figure/Skulls-of-Late-Pleistocene-Myotragus-balearicus-Majorca-Cova-de-Moleta-and-extant_fig5_8916341 and https://www1.flightrising.com/forums/gend/2305041/8
https://www.researchgate.net/figure/Left-Reconstruction-of-a-skeleton-of-Myotragus-balearicus-ICP-Right-Crania-of_fig3_236979422 and https://en.wikipedia.org/wiki/Myotragus#/media/File:Chevre_global.jpg
https://www.flickr.com/photos/_quagga/9570138262 and https://twitter.com/rvosa/status/1344341993019891715 and https://farm1.static.flickr.com/201/454066801_f122a4382a.jpg
https://www.karger.com/Article/Pdf/76239.
These two initial species seem rather contradictory. The extreme reduction of visual vigilance in the Balearic Islands cave goat seems to reflect freedom from carnivores. On the other hand, a reduction in the carnivore community in North America owing to megafaunal extinctions has apparently not led to any such modification in the pronghorn, which remains bigger- and wider-eyed than any African ungulate, proportional to body size.
Is this just a matter of evolutionary time, given that Myotragus had millions of years to adapt fully to isolation, whereas the pronghorn has enjoyed relative safety for only ten thousand years?
to be continued...
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