Size of the eyeballs relative to the size of the head in the Maasai ostrich (Struthio camelus massaicus)

@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

• large eyeballs (https://birda.org/what-animal-has-the-largest-eyes/ and https://www.iflscience.com/the-largest-bird-on-earth-also-has-the-largest-eyes-of-any-land-animal-73982 and https://www.flickr.com/photos/184806716@N02/48895312468 and https://ostritec.com/blog/the-ostrich-is-the-largest-bird-and-they-have-big-eyes-to-match/ and https://www.flickr.com/photos/bradclinphotography/5233932255 and https://x.com/birda_org/status/1253705044815470601 and https://www.youtube.com/watch?v=BmNcauTcCm4), and
• a small head.

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).

Posted on May 19, 2024 11:15 PM by milewski