Allometries of the brain to body size relationship in eutherian mammals are examined in this study as they can be used for comparative analyses concerning encephalization. In contrast with some modern presentations of this issue, an older concept is revived and expanded through this author’s current study. Three allometries with clearly different slopes are valid and lead to reliable results: interspecific, intraspecific, and ontogenetic allometries. Interspecific allometries follow lines with slope values of 0.56 or 0.63 for larger and smaller species, respectively, and characterize different average encephalization plateaus with rodents and lagomorphs generally more strongly encephalized compared to basal insectivores. Artiodactyls, perissodactyls and carnivores as a whole are again on a higher but rather similar plateau. Several species of carnivores have reached different encephalization levels with respect to their average plateau indicating diverse radiations. A phylogenetic brain size increase from fossil to recent radiations is also evident. Intraspecific allometries have slope values of about 0.25. These are of help in comparing brain sizes of ancestral species with their domesticated relatives. Domestication has generally led to a brain size decrease, but species on higher encephalization plateaus show this trend more strongly than species on a lower level of encephalization. Several brain parts and the sense organs also decrease in size during the domestication process, but vary arbitrarily and to different degrees. Ontogenetic growth allometries are species-specific, but are especially different between altricial and precocial mammals. A very steep 1st phase slope of highly encephalized species is particularly useful for understanding evolutionary and adaptive phenomena. Domesticated mammals that have become feral do not show an increase in brain size despite living many generations in wild habitats.

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The sizes of total brain, the five fundamental brain parts, and certain telencephalic structures were measured in wild mink (Mustela vison energumenos) and ranch mink of a Dark Standard strain of the same species. By means of intraspecific allometric methods for analysing the relationship between brain weight and body weight (net carcass weight), the volumes of the brain parts were compared in both groups. In general, total brain, as well as all the parts measured, were smaller in size in ranch mink independent of body size, age, and sex, indicating that domestication has led to a decrease in size. There were differences in the amount of decrease in various brain parts. These are discussed in connection with domestication time, with comparable results obtained in other species, and with regard to the functional importance of the brain parts.

The brain of a basking shark is described for the first time. Allometrical analysis of brain-body weight relationships indicates that Cetorhinus maximus shows by far the lowest degree of cerebralization of any shark investigated to date, possibly reflecting its plankton-feeding mode of life. Furthermore, some external morphological features of the brain appear to be species-specific. The proportions of brain parts indicate a primitive vertebrate brain organization. The size of the telencephalon, 34% of the total brain, equals that in some other sharks, whereas the cerebellum, 30% of the total brain in the basking shark, is significantly larger than in any other shark investigated. Furthermore, C. maximus shows some peculiarities concerning the nuclear expansion in the telencephalon. A rather large caudal interhemispheric region is especially striking. The relation of brain volume to brain cavity volume is approximately 1:16.

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