The evolution of mammalian brain size

Abstract

Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mammalian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size.

Intensive human contact correlates with smaller brains: differential brain size reduction in cattle types

“…24%) [3], whose brains reduce more than twice as much as those of other domestics [6,[9][10][11][12]. Quantifying these changes has significant implications for assessing differences in information-processing [13,14] and the speed and mode of brain evolution [15][16][17][18], particularly since different sensory systems are variably affected in different domestic taxa [7].…”

The pattern of brain-size change in the early evolution of cetaceans

“…Marino et al [ 7 ] published the first large dataset of EQ estimates (including brain and body masses) for fossil cetaceans. This important body of data, sometimes with slight modifications, has become the basis of numerous analyses [ 8 , 9 , 12 , 21 , 23 , 31 ], again, it is clear that Marino’s initial publications on the subject have inspired sustained scholarship on brain size evolution in cetaceans.…”

“…The purpose of this paper is to build a stronger foundation for the continued study of cetacean brain size evolution. It is our position that the existing data can be improved in three significant ways: 1) the methodology by which brain size is estimated in fossil cetaceans and terrestrial artiodactyls; 2) the manner in which body mass is estimated in fossil cetaceans; 3) and in the scaling factor used to calculate EQs, a point made previously by Boddy et al [ 10 ], and Smaers et al [ 31 ]. In addition, we rectify some erroneous data that have continued to propagate in the literature.…”

“…Although there are alternate methods that seek to observe allometries through time (e.g. Smaers et al [ 31 ] and McCurry et al [ 32 ]), EQ provides a baseline which is independent of the sample under study.…”

“…The implications of this regression’s slope have been much debated [ 5 , 8 , 28 , 66 ], specifically whether the slope is closer to 0.66 or 0.75, which are thought to represent scaling factors inherent to mammalian physiological processes. Although the approach of using a reference population comprising all mammals is appropriate to study broad patterns of brain evolution, specific questions regarding a single group are better answered using regressions based on more taxonomically focused groups [ 11 , 31 , 67 , 68 ], especially in cases where slopes of the regression for the study group deviate significantly from the broader sample that includes all mammals [ 11 , 28 , 31 , 69 ]. Indeed McCurry et al [ 32 ] proposed that constraints on maximum body size are relaxed for fully aquatic taxa and that this biases EQs calculations for fully aquatic cetaceans; although purely terrestrial artiodactyls are known to follow a different allometry than observed for an all mammal dataset [ 10 ].…”