On the Evolutionary Significance of Encephalization in Some Eutherian Mammals: Effects of Adaptive Radiation, Domestication, and Feralization

Abstract

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.

Is evolution of domestication driven by tameness? A selective review with focus on chickens

“…Absolute as well as relative total brain sizes (proportion of body weight) were on average smaller in the L-birds. This is in accordance with general observations that domesticated variants usually have reduced brain size compared to their wild ancestors (Kruska, 2005;Price, 2002).…”

Flores hominid: New species or microcephalic dwarf?

“…However, it is well established that reduction of body size within a mammal species (including Homo sapiens) is usually associated with only moderate reduction in brain size. Whereas the exponent value for scaling of brain mass size to body mass in comprehensive interspecific comparisons across placental mammals is close to 0.75 (Martin, 1981;Martin et al, 2005), there is a progressive decline with decreasing taxonomic rank and the value for intraspecific scaling among adults of a single species is typically about 0.25 (Martin and Harvey, 1985;Kruska, 2005). One of the best-documented cases is that of the domestic dog, with an exponent value of 0.27 determined for 26 breeds covering a 21-fold range of body sizes generated by artificial selection (Bronson, 1979).…”

Mice selectively bred for high voluntary wheel running have larger midbrains: support for the mosaic model of brain evolution

“…Most, if not all, of these hypotheses suggest that selection is acting on behavior [e.g. behavioral flexibilityenvironmental change hypothesis (Sol et al, 2005;Lefebvre and Sol, 2008;Sol et al, 2008), social brain hypothesis (Byrne and Whiten, 1988; Brothers, 1990; Byrne and Corp, 2004)], rather than directly on brain size per se (see also Kruska, 2005). The primary question in the present study is whether selection on a particular behavioral trait (voluntary exercise), using an experimental evolution paradigm (Garland and Rose, 2009), has resulted in a change in brain size.…”

“…For details on the experimental design, see Swallow et al (Swallow et al, 1998a). Here, it is of interest to note that, although many domesticated mammals show reductions in whole-brain size and/or the size of specific brain regions, domesticated house mice apparently do not (Kruska, 2005). All procedures conducted in this study are in accordance with the UCR Institutional Animal Care and Use Committee and US laws.…”

“…[A previous study that quantified volume of the dentate gyrus of the hippocampus found no statistical difference between the HR and C lines (Rhodes et al, 2003a). ] Additionally, to our knowledge, this is the first time that intraspecific changes in brain mass or volume have been associated with selection for increased locomotor activity in any mammal.In our analyses of brain mass and volume, we used body mass as a covariate because brain size generally scales allometrically with body size, both within and among species of eutherian mammals (see Kruska, 2005), and because the HR mice from three of the four generations sampled had significantly smaller body masses than the control lines (Table1). Lower body mass in HR lines has been a consistent finding in the selection experiment since generation 14 (13.6% smaller at 79days old) (Swallow et al, 1999), and a trend for reduced body mass was observed at generation 10 (e.g.…”