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In 80 domestic chickens from 8 breeds, the volumes of 12 brain parts were identified as dimensions in a cluster analysis. Based on Euclidean metrics and the Ward algorithm at least 2 groups were found that are congruent with the breeds ‘White Crested Polish chicken’ and ‘Breda’, although the breed identity was not a variable used in the cluster analysis. Domestication is interpreted as evolution which includes the possibility of speciation. It is hypothesized that White Crested Polish chickens and Bredas are becoming new species in terms of a biospecies concept.

A comparison of brain size and brain composition was made between two uncrested duck breeds and Crested Ducks (CR) and between CR individuals that do possess crests and those that do not have the crest. Domestic ducks of the breed CR have allometrically larger brains than uncrested duck breeds. The crest inserts on a cushion of fat and connective tissue that is partly nourished by brain vessels via small holes in the skull. Through these holes, fat tissue may invade the brain cavity. Because the fat accumulations are sometimes hidden deep between the telencephalon, tectum, and cerebellum, they may be invisible macroscopically and, thus, give the impression of a large brain. The size of the crest, however, is not strictly correlated with fat accumulations in the brain, because 2 among 10 specimens of CR showed no fat body at all, and the investigation of 10 uncrested CR (ducks from the same genetic stock, but without the crests) also revealed fat accumulations in 6 specimens. After subtraction of the volume of the fat body, the brain volume of CR (crested and uncrested) was of equal size to that of "Hochbrutflugenten" and Pommeranian ducks, as was the volumes of most brain structures measured. Significantly smaller in CR were the olfactory bulbs, the prepiriform area, and the cerebellum, which was always situated in close proximity to the fat body in CR.

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