Some ungulates - particularly Equus, Taurotragus, and Giraffa - have bigger eyeballs than others
Relative to body mass, the following have exceptionally large eyeballs:
- the horse (Equus caballus), and
- the common eland (Taurotragus oryx, https://www.natureinstock.com/search/preview/profile-shot-of-two-young-common-eland-taurotragus-oryx-males-standing-close/0_00018855.html).
This is remarkable for various reasons, e.g.
- the horse shows no attrition of vision in domestication,
- equids reproduce and grow more slowly than do coexisting ruminants, making them vulnerable to predation and helping to explain their visual vigilance,
- the extreme visual sensitivity of the horse demands special management (https://www.sciencedirect.com/science/article/pii/S0168159124000285#:~:text=Blindfolding%20a%20horse%20has%20historically,%2C%20June%204%2C%202021),
- the eland lacks the speed and stamina of existing ungulates, and is instead known to flee early, not allowing approach by potential predators, and
- the eyeballs do not look particularly large in either the horse or the common eland.
The Maasai giraffe (https://craftfineart.com/sink-c-jeffrey-maasai-giraffe-ido-129120) also has notably large eyeballs for an ungulate, relative to its body mass.
Wild, non-bovin bovids in Africa have larger eyeballs than do like-size cervids on other continents, as is apparent if one merely looks at photos of the animals (https://www.istockphoto.com/photo/a-close-up-profile-portrait-of-a-female-black-faced-impala-gm1218530588-356087085).
However, the trend is borne out by the regression below for the red deer, and by information on Axis axis (https://creatures-of-the-world.fandom.com/wiki/Chital_Deer?file=Ftd-axis-deer.jpg) and Odocoileus virginianus (https://www.alamy.com/profile-of-a-white-tailed-deer-image209768621.html and https://pixels.com/featured/whitetail-doe-face-brook-burling.html).
The proportionately small eyeballs of the red deer (https://www.masterfile.com/image/en/700-06758256/portrait-of-a-red-deer-cervus-elaphus-female-bavaria-germany) seem at odds with its unusual orbital prominence, and the fully lateral placement of the eyes (https://stock.adobe.com/images/a-close-up-head-and-shoulder-portrait-of-a-female-red-deer-staring-forward/298586849).
Bovin bovids (https://en.wikipedia.org/wiki/Bovini) have eyeballs smaller than expected for their body mass.
This is particularly noteworthy in the African savanna buffalo (https://www.masterfile.com/image/en/841-06446194/cape-buffalo-syncerus-caffer-with-redbilled and https://www.dreamstime.com/profile-portrait-cape-buffalo-wild-side-view-profile-portrait-cape-buffalo-african-wilderness-image277091409), which scores 20% below par, in contrast to the 50% above par scored by the common eland.
Perhaps the most puzzling of all these findings - despite being well-known - is how small the eyeballs are in the hook-lipped rhino (https://upload.wikimedia.org/wikipedia/commons/6/69/Black_Rhino_at_Working_with_Wildlife.jpg).
It is evident that domestication has led to a diminution of the eyeballs in both
- the domestic sheep (but not necessarily the European bost, https://www.everypixel.com/image-8475227298409467759), and
- the domestic pig.
In the latter case, the resulting eyeballs (https://www.dreamstime.com/profile-view-animal-portrait-big-domestic-pig-big-domestic-pig-profile-view-image186895256) are even smaller, proportionately, than in rhinos, because even wild suids have small eyes.
In the case of the common warthog, there is the same incongruity as in the red deer: the orbits are noticeably prominent (in this case dorsally, not laterally, https://www.dreamstime.com/head-profile-common-warthog-phacochoerus-africanus-image153887595). However, the eyeballs remain small relative to like-size, coexisting bovids (https://www.dreamstime.com/stock-photography-warthog-image2002732).
The following are the quotients, calculated relative to adult body mass from the interspecific regression, in decreasing order of eyeball mass:
Equus caballus +0.5
Taurotragus oryx +0.5
tragelaphin bovids (small sample of two spp., Crile and Quiring 1940) +0.4
Giraffa tippelskirchi +0.3
Equus quagga +0.25
Aepyceros melampus +0.2
alcelaphin bovids including Connochaetes +0.1
reduncin bovids +0.1
gazelles (Eudorcas thomsonii and Nanger granti) +0.05
Oryx (small sample, Crile and Quiring 1940) 0
neotragin bovids (small sample, Crile and Quiring 1940) 0
Bos taurus -0.05
Syncerus caffer (small sample, Crile and Quiring 1940) -0.2
Cervus elaphus -0.2
Ovis aries -0.2
elephantids -0.3
Phacochoerus africanus -0.5
Diceros bicornis -0.7
Sus scrofa domesticus -0.9
Comments
Add a Comment