The ostrich (Struthio camelus) has a small brain, perhaps even relative to phasianid birds
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.
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