milewski's Journal

The ostrich (Struthio camelus) combines

• large eyeballs (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
• a small head.

Indeed, the mass of the pair of its eyeballs exceeds that 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/).

In this Post, I record the exact dimensions of the eyeballs and head, which I myself measured.

Subspecies: Struthio camelus massaicus, fully wild (no influence of captivity or domestication)

Location: Wildlife Ranching and Research, later Swara Conservancy, now incorporated into Nairobi National Park

Sample size: n = 3 female adults, measured 9 October, 19 October, and 29 October.

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: 42.1 g (slightly flaccid), 45.2 g, 46.8 g

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: 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

It is easy to show that the ostrich (Struthio camelus) has a small brain relative to like-size ungulates - with which it is ecologically comparable.

What is more subtle is an allometric comparison of brain sizes between the ostrich and trophically comparable but far smaller birds, particularly Phasianidae (https://en.wikipedia.org/wiki/Phasianidae).

In this Post, I suggest that the ostrich is, if anything, rather small-brained for a phasianid of its body size. It seems to be, in a sense, nicely preadapted for domestication.

Source: Hrdlicka A (1907) Brain weights in vertebrates. Smithsonian Miscellaneous Collections XLVIII: 89-112.

'Numida cristata' body mass 467 g, brain mass 3.0 g

Numida data in Crile and Quiring (1940)

Pavo cristatus female adult, body mass 3060 g, brain mass 6.7 g

Lophortyx californicus adults, body mass 151.8 g and 102 g, brain mass 1.22 g and 1.5 g

Callipepla squamata adult, body mass 99 g, brain mass 1.5 g

Bonasa umbellus female adult, body mass 299.3 g, brain mass mean 2.7 g

Colinus virginianus, both sexes, all adults, n = 20, body mass mean 125 g, brain mass mean 1.228 g

Gallus gallus data in Crile and Quiring (1940)

The brain of the ostrich is mean 42.11 grams at say 55% of body mass 123 kg, = 67.65 grams. Its head weighs mean 0.68 kg. at carcass mass 59.9 kg.

Casuariiformes: Dromaius novaehollandiae female adult body mass unrecorded, brain mass 20.3 g

Casuariiformes: Casuarius casuarius sex unknown adult body mass unrecorded, brain mass 31.7 g

My plotting of a regression, using the above data, suggests that a domesticated phasianid, viz. Gallus gallus, has been decephalised by selective breeding.

However, both the ostrich and Casuariiformes have brains smaller than expected, based on the allometry of wild phasianids.

The ostrich (Struthio camelus) is the largest living bird.

It has an uniquely large intestine for a bird, resembling ungulates in this way.

In this series of Posts, I compare the ostrich with ungulates in terms of

• body sizes,
• organ sizes,
• habitats, and
• diet.

This provides a basis for ecological comparison between the ostrich and African ungulates.

I find that the ostrich shares the same body mass as several coexisting ungulates, but not

• other monogastrics, or
• ruminants avoiding a grass diet as the ostrich does.

Major body-parts and organs are similar in size in the ostrich and like-size ungulates, except for head and e.g. spleen.

The gastrointestinal tract of the ostrich resembles that of monogastric hindgut-fermenters. However, its relatively heavy gut-walls are linked to its lack of teeth.

The ostrich prefers dry plains, also inhabited by various ruminants of which like-size spp. tend not to rely on forbs as the bird does.

The diet of the ostrich is qualitatively and even quantitatively similar to those of ruminant concentrate-selectors or 'mixed feeders', particularly coexisting gazelles smaller-bodied than the bird.

I hypothesise that the ostrich is extremely adapted for a combination of

• tolerance to dry heat,
• mobility,
• food selectivity, and possibly
• tolerance of silica-rich dicotyledonous plants, contributing to its ecological separation from ungulates.

The following illustrate juveniles of the eastern white-bearded wildebeest (Connochaetes albojubatus, https://www.inaturalist.org/observations?taxon_id=525438) at an ontogenetic stage corresponding in body mass approximately to adults of the coexisting Maasai ostrich (Struthio camelus massaicus, https://www.inaturalist.org/observations?taxon_id=322201).

Please note that the body mass has just exceeded half of maternal body mass, but the horns - albeit much longer than the ears or mane - are still simple spikes directed dorsally.

https://www.inaturalist.org/observations/7898982

https://www.inaturalist.org/observations/110103601

https://www.inaturalist.org/observations/124808683

https://www.inaturalist.org/observations/102212589

https://www.inaturalist.org/observations/189131122

https://www.inaturalist.org/observations/191952637

Scroll in https://fossilrim.org/animals/common-wildebeest/

Scroll in https://www.zootierliste.de/en/?klasse=1&ordnung=121&familie=12115&art=1160826

Compare with

• younger than the study category, at <1 year old: https://www.inaturalist.org/observations/131034142
• older than the study category, with the horns acquiring a curve at the base: https://www.inaturalist.org/observations/181075800

Sample size large, of both sexes

Location:
Wildlife Ranching and Research, later Swara Plains Conservancy, and now incorporated into Nairobi National Park

Time:
1986-1989

The following are mean values of percentage of body mass, followed by the actual masses in parentheses.

Body mass 111 kg

Carcass mass = 54.22% (60.184 kg)

Skin = 7.92% (8.788 kg)

Feet = 2.93% (3.253 kg)

Head = 7.29% (8.088 kg)

Brain = 0.267% (0.296 kg)

Eyeballs = 0.0438% (2 X 0.0243 kg)

Tongue = 0.23% (0.259 kg)

Masseter muscle = 0.266% (2 X 0.148 kg)

Heart = 0.726% (0.806 kg)

Lungs = 1.55% (1.726 kg)

Spleen = 0.35% (0.392 kg)

Liver = 1.28% (1.422 kg)

Kidneys = 0.265% (2 X 0.147 kg)

Rumen = 1.67% (1.859 kg)

Reticulum = 0.275% (0.305 kg)

Omasum = 0.43% (0.480 kg)

Abomasum = 0.34% (0.375 kg)

Total intestines (full?) = 4.83% (5.363 kg)

Total stomach = 2.72% (3.019 kg)

DISCUSSION

Elsewhere in this series of Posts, I compare the Maasai ostrich with a coexisting alcelaphin ruminant - namely Alcelaphus cokii (https://www.inaturalist.org/observations?taxon_id=132649) - of similar adult body mass to the bird.

The values presented here - except for brain and skin - exceed those of adults of A. cokii, of similar body mass, in their overall resemblance to the Maasai ostrich.

The ostrich is constituted in some ways like juveniles of a ruminant, with big organs and feet (suggesting emphasis on mobility and foraging and rapid metabolism).

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

https://en.wikipedia.org/wiki/Alcelaphinae

The ostrich does not coexist with any monogastric or ruminant species sharing both its body size and its avoidance of a grass diet.

The diet of the ostrich is qualitatively and even quantitatively similar to those of ruminant concentrate-selectors or 'mixed feeder', particularly coexisting gazelles smaller than the ostrich.

The ostrich seems tolerant of silica-rich forbs, contributing to its ecological separation from ungulates.

The ostrich, in its most extreme habitat, coexists with

• a grazer larger-bodied than itself, viz. Oryx,
• a grazer/browser smaller-bodied than itself, viz. Gazella.

Both are adapted to reduced intakes (ruminants) and have advantages of foraging at night.

The grazer accepts up to 40% of the diet as browse, fruits, tubers, and forbs, largely for their water-content. The grazer/browser accepts up to 30% of the diet as the same, though smaller, items, and probably some insects too. Neither eats faeces, nor relies on forbs. Both avoid competition with the ostrich partly by foraging at times when atmospheric moisture condenses, and partly by resorting to landforms avoided by the ostrich.

The grazer is the arid-zone counterpart of semi-arid-adapted alcelaphins, which are more specialised grazers, partly because they can drink (and ultimately mesic hippltragin and large reduncins).

The grazer/browser is the arid-adapted counterpart of small-bodied reduncins and tragelaphins, because neither grass nor browse will support a specialist.

Where two spp. of gazelles coexist with the ostrich, the smaller-bodied one eats more grass (cannot reach much browse, and does not depend on forbs), and the larger-bodied eats more browse because it can reach it. They have about the same dietary quality, in terms of protein.

The more browsers extend into the arid zone, the ganglier they become (giraffes, gerenuk, dama gazelle). Nanger granti is the last outpost of a 'normal browser' towards dry country, after all the tragelaphins have expired.

Spatial separation and limited bulk demands/food quality are two sides of the same strategy. If a species can survive the shortage in the desert, then the quality is likely to be fair. If physical separation is hard, and coexistence is inevitable, then the animal must eat as little as possible in order to avoid competition and to exploit microspatial separation based on advantages in economy of movement. I.e. do what browsers do, but on the ground floor = go 'down and out'. If the animal can afford to pick and choose, then it can wait to find items others have found too awkward to eat.

The ostrich does not enhance mobility by reducing ingesta mass in body, but rather maximises this (compensating with e.g. reduction of toes) and draws indirect benefits from digestive power and hence reduced bulk demands, allowing it to move instead of having to eat so frequenty.

The ostrich differs from ungulates in the following:

• small head/lack of teeth/small brain
• gastric mill/hindgut fermentation/double caeca/cloaca
• feathers/uric acid/salt gland
• bipedality/air-sacs
• diurnality/high body temperature (1 degree Celsius or less higher than in ruminants)
• omnivory/carnivory/coprophagy
• large clutch/collective breeding/seasonal breeding.

Concentrate-selecting ungulates differ from roughage grazers in the following morphological features:

• small head and narrow muzzle
• smaller teeth and reduced dental occlusion
• long neck
• long legs
• small stomach (fermentation vat)
• large caecum
• short small intestine

Most bovids have gestation periods of

• about 6 months in the smallest-bodied, fastest-growing spp.,
• about 7 months (relatively small-bodied spp.),
• about 9 months (relatively large-bodied spp.),
• 11 months (Syncerus caffer).

Compare the above values with

• 5 months in Phacochoerus,
• 7 months in Hippopotamus,
• 12 months in Equus quagga,

Most bovids have birth-weight percentages of about 5-10%, exceeding 10% in the most precocial spp. The values for Phacochoerus and Hippopotamus are less than this.

Most bovids reach sexual maturity at

• about 9 months in relatively small-bodied spp.,
• 1.2-1.4 years in Connochaetes,
• 2.5 years in large-bodied, slow-growing spp.,
• 2.75 years in Syncerus caffer.

Compare the above values with

• 1.5 years in Equus quagga and (check) Phacochoerus,
• 4 years in Hippopotamus (exceeding the value for Giraffa)

Crude protein estimation for Grant's gazelle
Values are weighted means (% crude protein X % of diet*).
Harpachne schimperi https://www.inaturalist.org/taxa/343051-Harpachne-schimperi leaves 1220 (8.6%); stems 22.4 (4.2%)
Cynodon dactylon/nlemfuensis leaves 152.7 (4.2%); stems 16.8 (2.3%)
'Harpachne lin' leaves 130.6 (9.2%); stems 12.2 (2.3%)
Microchloa kunthii https://www.inaturalist.org/taxa/165373-Microchloa-kunthii leaves 54.7 (3.85%); stems 12.2 (2.3%)
Themeda triandra leaves 4.2 (0.4%); stems 0.3 (0.1%)
Sida sp. indet. https://www.inaturalist.org/observations?place_id=56881&subview=map&taxon_id=54996&view=species leaves and stems 210.0 (14.0%)
Unidentified leaves 5.5 (0.4%); stems 28.6 (5.4%)
Indigofera leaves and stems 340.2 (14.0%)
Solanum leaves 123.2 (6.2%) fruits 26.2 (1.54%)
Leguminous seeds and pods 127.5 (5.1%)
Asteraceae indet. 76.5 (5.1%)
Balanites aegyptiaca 72.8 (2.6%*)

Total = 1583
Mean = 15.83% crude protein

Sexless means for body composition of ostrich cf warthog. All values of mass are percentages of body mass.

Body mass 111.0 kg 70.7 kg

Dressing percentage 54.2% 52.3%

Head 0.615% 11.605%

Feet 3.8% 1.51%

Hide 5.713% 5.291%

Heart 0.865% 0.379%

Lungs and trachea 1.855% 0.869%

Spleen 0.053% 0.200%

Liver 1.478% 1.353%

Total gastrointestinal tract, empty 8.027% 3.40%

Stomach, empty 3.31% 0.534%

Stomach contents 3.06% 1.007%

Small intestine, empty 1.351% 0.776%

Small intestine contents 1.613% 1.617%

Small intestine length 8.82 m 10.207 m

Caecum length (paired in ostrich) 0.95 m 0.214 m

Large intestine including caecum, empty 2.86% 2.00%

Large intestine including caecum, contents 9.78%(needs checking) 10.3%

Large intestine including caecum, length 11.8 m 7.215 m

Total ingesta 13.94% 12.94%

INTRODUCTION

Ecological separation between the wild ostrich (Struthio camelus) and gazelles is unclear.

The ostrich is sympatric with gazelles, with a similar diet and fermentative digestion of fibre.

There is no obvious separation by foraging height, since the ostrich forages mainly near ground-level, and several gazelles reach to the height of the bird by bipedal standing.

The ostrich and gazelles are both tolerant of dry heat, with no obvious difference in their penetration of arid zones.

The ostrich is diurnal. Gazelles can potentially forage at night. However, this would not per se prevent competition for the same plants.

However, the ostrich exceeds gazelles in stride-length and body size. Furthermore, the bird breeds more synchronously than at least those gazelle spp. coexisting with it in mesic areas.

Offspring of the ostrich are, from the egg stage onwards, left to progressively fewer adult custodians as they develop towards adulthood.

Hence the movements of most adults are possibly not constrained by care of offspring as in gazelles - particularly in view of the potential masculine territoriality of gazelles.

Based on its body size alone, the ostrich should have an advantage of greater daily mobility and greater potential for nomadic movements than those of gazelles. The bird potentially walks faster and more efficiently, with a greater ability to forgo shade and to commute long distances to drink.

The economy of leg-length and bipedality would enhance this. However, such locomotory specialisation would also potentially bring costs of instability relative to quadrupeds.

Hence, the ostrich would be expected to avoid unstable substrates, such as deep, loose sand and rocky slopes.

Together, these considerations would suggest that the ostrich is better-suited than gazelles for exploiting the ephemeral appearance of food in remote areas on flat, firm ground.