@tonyrebelo @jeremygilmore @ludwig_muller @matthewinabinett @nyoni-pete
The largest eye of any land vertebrate is reputed to be that if the ostrich (body weight 63-104 kg, Cramp S, 1977, The birds of the western Palaearctic, vol. 1, Oxford University Press, London).
The ostrich (Struthio camelus) combines
Indeed, the combined mass of the pair of its eyeballs exceeds the mass of its brain (https://www.gulla.net/en/ai/the-curious-case-of-the-ostrichs-eye-and-its-pint-sized-brain/ and https://www.youtube.com/watch?app=desktop&v=THUDef6VyKA and https://www.reddit.com/r/Damnthatsinteresting/comments/t8auso/an_ostrichs_eye_is_bigger_than_its_brain_the/ and https://www.facebook.com/LAZoo/photos/a.304439135272/10165402741960273/?type=3 and https://www.facebook.com/familyfocuseyecare/posts/fun-fact-ostrich-eyes-are-bigger-than-their-brains-/2621767008132812/).
As with many factoids/memes, a risk arises of exaggeration and factual distortion.
In this Post, I record the exact dimensions of the eyeballs and head, which I myself carefully measured.
Subspecies: Struthio camelus massaicus, fully wild (no influence of captivity or domestication)
Location: Wildlife Ranching and Research, later Swara Plains Conservancy, now incorporated into Nairobi National Park
Sample size: n = 3 female adults, measured 9 October, 19 October, and 29 October, 1987.
EYEBALLS
https://www.flickr.com/photos/ceekay/3465110529 and https://www.deviantart.com/frankandcarystock/art/Ostrich-Eye-369635470 and https://www.pixoto.com/images-photography/animals/birds/ostrich-eye-4913795697213440 and https://www.dreamstime.com/stock-image-portrait-ostrich-image16965141
My measurements were similar for the left and right eyeballs.
Eyeball circumference: 151.5 mm, 154 mm, 155 mm
Eyeball diameter: - , 51 mm, 52 mm
Eyeball depth: - , 41.5 mm, 42.5 mm
Eyeball mass (each one, not the pair): 42.1 g (slightly flaccid), 45.2 g, 46.8 g
HEAD
https://unsplash.com/photos/an-ostrich-with-its-mouth-open-and-its-eyes-closed-cHcTA_HyXZ4 and https://www.discovery.com/nature/ostrichlandand https://stock.adobe.com/hu/images/ostrich-eyes-close-up-close-up-portrait-of-an-ostrich-with-big/390285270 and https://unsplash.com/photos/an-ostrichs-face-with-a-blurry-background-DOqXcvrrmSM and https://es.123rf.com/stock-photo/ostrich_eye.html
Head mass: 0.59 kg, 0.92 kg (skull haemorrhaged), 0.70 kg
Head length straight to occipital condyle: 19.4 cm, - , 20 cm
Head dorsal length along contour: 26 mm, 23 mm, 28 mm
Head ventral length along contour: 20 mm, 20 mm, 20 mm
Beak length along contour, tip to gape: 14.7 mm, 14 mm, 15.6 mm
Beak straight length, tip to gape: - , - , 14.6 mm
Beak length along contour, tip to cere (https://www.collinsdictionary.com/dictionary/english/cere): 68 mm, 67 mm, 84 mm
Beak straight length, tip to cere: - , 6.4 cm, 7.6 cm
Beak length along contour, tip to feathers: - , 71 mm, 88 mm
Beak straight length, tip to feathers: 67 mm, 67.5 mm, 81 mm
Beak width at gape: 80 mm, 82 mm, 88 mm
Beak width at cere: 57 mm, 55 mm, 59.5 mm
Head maximum width: 107 mm, 103 mm, 103 mm
Head depth: 93 mm, 90 mm, 92.3 mm
Orbit (bony) width: 60 mm, 58 mm, 64 mm
Tongue length: 20 mm, 20 mm, 12 mm (probably measured differently)
Tongue width: 42 mm, 38 mm, 38.7 mm
Mass of head (excluding horns) in coexisting ungulates:
The following values are means, in which the sexes are combined unless otherwise stated.
Eudorcas thomsonii thomsonii: females 1.1 kg, 1.4 kg (combined: 1.25 kg)
Gazella granti granti: 2.6 kg
Aepyceros melampus: 2.5 kg
Alcelaphus cokii: 5.75 kg
Connochaetes albojubatus: 10.2 kg
Equus equus boehmi (body mass 260.5 kg): 12.25 kg
Taurotragus oryx pattersonianus: 12.95 kg
Sundry notes from the literature:
No bird has eyes approximating the spherical form typically seen in the eyes of mammals.
The eyelids are feathered in the ostrich, rheas, and owls, but not in most birds.
The eyeball of the ostrich has a bulbar axial length of 50 mm (line passing through lens to retina). Please see https://www.perplexity.ai/search/Consider-the-axial-DYkJ039cSnGJ78vdJU4ZEw.
Typically, birds' eyes are so large, relative to the skull, that they meet in the middle of the head, separated by only a thin septum of bone.
Optically, the most important parameters of size are anterior focal length or posterior nodal distance.
Miller (1979) has shown that a large posterior nodal distance is essential if the vertebrate eye is to achieve the maximum theoretical limit of visual resolution. This is because there exists a finite limit on the size, and hence packing density, of the retinal photoreceptors which sample the retinal image.
Clearly if the image is spread over more photoreceptors, then there is an increase in the amount of detail resolved. However, this is limited by diffraction effects and aberrations within the optical system. "Thus, an eye any larger than that of the ostrich...may have little adaptive value, since although the image will be larger, its quality may deteriorate" (G H Martin, page 313, Notes from Form and Function in Birds, 1985, vol. 3, Ed. by A S King and J McLelland, Academic Press, https://books.google.com.au/books/about/Form_and_Function_in_Birds.html?id=gHQXAQAAIAAJ&redir_esc=y).
Comments
https://www.researchgate.net/figure/Anatomical-sections-of-the-ostrich-eye-1-Lens-2-Ciliary-body-3-Ciliary-processes_fig1_310671139
https://www.researchgate.net/figure/Some-anatomical-features-of-the-ostrich-s-eye-a-The-rostral-view-of-the-lens-of-the_fig1_287263780
https://www.birdspot.co.uk/bird-brain/what-is-the-biggest-bird-in-the-world
https://www.scienceofbirds.com/blog/bird-eyes-and-vision
https://sdzwildlifeexplorers.org/animals/ostrich
https://www.ifaw.org/journal/animals-biggest-eyes-world
https://www.researchgate.net/figure/Macrograph-of-the-upper-lower-and-third-eyelids-of-ostrich-a-b-40th-day-of_fig3_304022517
Another individual of Struthio camelus massaicus at the same location, 9 November 1987.
Eyeball mass: left 44.5 g, right 43.0 g
Eyeball circumference, around sclera: left 15.7 cm, right 15.7 cm
Eyeball circumference, across pupil: left 13.0 cm, right 13.1 cm
Eyeball diameter: left 5.1 cm, right 5.1 cm
Head mass 554.7 grams
Head length: 19.6 cm
Head depth: 9.6 cm
Head width: 10.0 cm
Beak length, tip to skin at base on dorsal side: 7.0 cm
Beak length, tip to 'notch' of forehead: 10.9 cm
@paradoxornithidae @botswanabugs @tonyrebelo @jeremygilmore @beartracker @matthewinabinett @nyoni-pete @ptexis @variani18
The mass of the pair of eyeballs in Homo sapiens, relative to an adult body mass of 70 kg, is 0.021%.
The corresponding value for the ostrich is fourfold greater, viz. about 0.082%.
The mean adult body mass of H. sapiens is less than that of the ostrich (https://www.nairaland.com/3411976/kwam-1-poses-giant-ostrich/2 and https://www.gettyimages.com.au/detail/photo/riding-an-ostrich-royalty-free-image/576622486?phrase=man+on+an+ostrich&adppopup=true and https://www.gettyimages.com.au/detail/photo/ostrich-riding-at-farm-royalty-free-image/650527379?phrase=man+on+an+ostrich&adppopup=true and https://www.facebook.com/pinestrawmagazine/photos/a.463123060535227/986121854902009/?type=3 and https://www.tehachapinews.com/news/indian-point-ostrich-ranch-to-close-feb-26/article_cdba8812-ee56-11e6-8ab6-57e98c39e62a.html).
However, even an unusually large individual male of H. sapiens, weighing about as much as the ostrich, has eyeballs only about 20% as heavy as those of the ostrich (https://www.youtube.com/watch?v=pI93e-GPa7Y and https://www.gettyimages.com.au/detail/photo/man-photographing-an-ostrich-royalty-free-image/586012152?phrase=man+on+an+ostrich&adppopup=true and https://mynbc15.com/news/offbeat/ostrich-left-the-nest-for-an-unsupervised-walk-in-missouri-neighborhood).
https://africafreak.com/can-an-ostrich-kill-a-lion
https://birdfact.com/articles/ostrich-size
@milewski thanks for yet another very interesting article. Do you think ostrich winglessness originally may have evolved in a place where there were no large predators ? I wonder if there is a fossil record of eye socket size in ostrich ancestors which can be compared with skull size and the development of two toes. Are ostriches gradually losing their wings with time ? I wonder what the fossil record can tell us. Forgive me if my questions are rather naive as I have no biology education at all. https://americanornithology.org/ostrich-evolution-revealed-fossil-finds-shed-new-light-on-the-evolution-of-the-african-ratite/
@botswanabugs
Many thanks for your questions and the reference you cite. I plan on writing several more Posts on the ostrich, which may offer partial answers...
Both the ostrich (in domesticated form) and the horse (Equus caballus) are sometimes fitted with blinkers, to allay the nervousness of animals with extremely large, motion-sensitive eyes.
The horse has been selectively bred for thousands of years, yet there has been no selective breeding for reduced size and sensitivity of the eyes.
Please note that the largest-bodied breeds of the horse possess the largest eyeballs of all land mammals.
Therefore, it may be true to say that the human species is inadvertently responsible for producing these extremely large eyeballs. The rationale is that a) the wild ancestor of the horse had eyeballs proportionately as large (bearing in mind the allometric relationships involved) as in its like-size domestic derivative, and b) the absolute size of body and eyeballs have been anthropogenically boosted by selective breeding for large breeds of the horse.
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