Scalable architecture in mammalian brains

Abstract

Comparison of mammalian brain parts has often focused on differences in absolute size, revealing only a general tendency for all parts to grow together. Attempts to find size-independent effects using body weight as a reference variable obscure size relationships owing to independent variation of body size and give phylogenies of questionable significance. Here we use the brain itself as a size reference to define the cerebrotype, a species-by-species measure of brain composition. With this measure, across many mammalian taxa the cerebellum occupies a constant fraction of the total brain volume (0.13 +/- 0.02), arguing against the hypothesis that the cerebellum acts as a computational engine principally serving the neocortex. Mammalian taxa can be well separated by cerebrotype, thus allowing the use of quantitative neuroanatomical data to test evolutionary relationships. Primate cerebrotypes have progressively shifted and neocortical volume fractions have become successively larger in lemurs and lorises, New World monkeys, Old World monkeys, and hominoids, lending support to the idea that primate brain architecture has been driven by directed selection pressure. At the same time, absolute brain size can vary over 100-fold within a taxon, while maintaining a relatively uniform cerebrotype. Brains therefore constitute a scalable architecture.

The evolution of the cortico-cerebellar complex in primates: anatomical connections predict patterns of correlated evolution

“…The first part expands on studies showing that the cerebellum has undergone correlated evolutionary size changes with the neocortex 7,13,14 . The evolution of the cerebellum and the neocortex is investigated in more detail here to determine which particular areas of the cerebellum have shown correlated evolutionary changes with the neocortex.…”

The evolutionary roles of nutrition selection and dietary quality in the human brain size and encephalization

“…In small animals, the volume of the neocortex increased from only 16% of the volume of the whole brain, to 74% in Hominoidea. In contrast, the relative volume of the cerebellum remained constant at 13% of whole brain volume, regardless of the absolute size of the brain [11,12].…”

Social fishes and single mothers: brain evolution in African cichlids

“…Because increasing demands on cognitive ability predominantly alter the size of neural structures rather than their connectivity (Kotrschal & Junger 1988;Huber & Rylander 1992), interspecific variance in relative brain size, or the size of neural structures, should reflect differences in the cognitive challenges that have shaped brain evolution (Clark et al 2001;de Winter & Oxnard 2001). Phylogenetic comparative analyses have proved a useful tool to identify the correlates of brain size across a wide range of taxa.…”

“…Healy & Rowe (2007) also questioned whether significant changes to one, or several parts of the brain, could be detected by measuring whole brain size, when it is on the size of these separate components that natural selection probably acts. However, neural structures within the brain evolve in concert in response to specific cognitive challenges as shown by the presence of 'cerebrotypes' that highlight convergent response of brain architecture to specific selection pressures (Clark et al 2001;de Winter & Oxnard 2001;Iwaniuk & Hurd 2005), such changes are expected to be reflected in total brain size. As an example of this, in Tanganyikan cichlids, whole brain size explained 50-76 per cent of the variation in all brain structures, except for the dorsal medulla where the variance explained was 18-32 per cent (Pollen et al 2007).…”