Multivariate Meta-Analysis of Brain-Mass Correlations in Eutherian Mammals

Abstract

The general assumption that brain size differences are an adequate proxy for subtler differences in brain organization turned neurobiologists toward the question why some groups of mammals such as primates, elephants, and whales have such remarkably large brains. In this meta-analysis, an extensive sample of eutherian mammals (115 species distributed in 14 orders) provided data about several different biological traits and measures of brain size such as absolute brain mass (AB), relative brain mass (RB; quotient from AB and body mass), and encephalization quotient (EQ). These data were analyzed by established multivariate statistics without taking specific phylogenetic information into account. Species with high AB tend to (1) feed on protein-rich nutrition, (2) have a long lifespan, (3) delayed sexual maturity, and (4) long and rare pregnancies with small litter sizes. Animals with high RB usually have (1) a short life span, (2) reach sexual maturity early, and (3) have short and frequent gestations. Moreover, males of species with high RB also have few potential sexual partners. In contrast, animals with high EQs have (1) a high number of potential sexual partners, (2) delayed sexual maturity, and (3) rare gestations with small litter sizes. Based on these correlations, we conclude that Eutheria with either high AB or high EQ occupy positions at the top of the network of food chains (high trophic levels). Eutheria of low trophic levels can develop a high RB only if they have small body masses.

The brain of the tree pangolin (Manis tricuspis). I. General appearance of the central nervous system

“…The calculated EQs for the individuals studied herein are lower than the average for mammals (average mammalian EQ is 1), with an average EQ of 0.844 being determined (using the regression provided by Manger, 2006). However, it should be noted that the noninnervated scales that cover the dorsal surface of the entire pangolin body make up more than 20% of the total body mass of the animal (Kingdon, 1974 (Steinhausen et al, 2016). Thus, the EQ of the tree pangolin is higher than seen in Xenarthrans, but falls within the lower end of the range of the first standard deviation of carnivores, unless the EQ corrected for scale mass is used, and then they fall well within the range of the first standard deviation for carnivores and well above that seen for Xenarthrans.…”

“…Thus, it appears that the mass of the non‐innervated scales does decrease the EQ below levels considered average for mammals, but when corrected for, the EQ is precisely what one would predict based on regressions determined from other mammals. The EQs for carnivores, based on the brain and body masses of 226 carnivore species provided by Bininda‐Emonds, Gittleman, and Kelly () calculated using the regression provided by Manger (), average 1.32 ( SD : 0.41, range: 0.39–2.64), while that of four species of Xenarthrans ranged from 0.44 to 0.71 (Steinhausen et al, ). Thus, the EQ of the tree pangolin is higher than seen in Xenarthrans, but falls within the lower end of the range of the first standard deviation of carnivores, unless the EQ corrected for scale mass is used, and then they fall well within the range of the first standard deviation for carnivores and well above that seen for Xenarthrans.…”

Cerebrotypes in Cephalopods: Brain Diversity and Its Correlation With Species Habits, Life History, and Physiological Adaptations

“…Many comparative studies on brain evolution in vertebrates utilized multivariate statistics (e.g., van Dongen, 1998 ; Burish et al, 2004 ; Iwaniuk et al, 2004 ; Iwaniuk and Hurd, 2005 ; Lisney and Collin, 2006 ; Gonzalez-Voyer et al, 2009a ; Yopak, 2012 ; Steinhausen et al, 2016 ; Kotrschal et al, 2017 ; Mai and Liao, 2019 ). The approach is useful to investigate brain evolution for two main reasons: (i) there are contingencies among brain regions that result in correlated evolution among some areas; (ii) a multitude of selection pressures and constraints determine the composition and evolution of the brain.…”

An Unparalleled Sexual Dimorphism of Sperm Whale Encephalization

“…Although sex differences in mammalian brain mass or even organization and structure are well known and have been thoroughly discussed (McCarthy, 2016), such disparity in the EQ remains unparalleled in mammals. Numerous and different factors may promote relative and absolute increase in brain size in mammals (DePasquale, Neuberger, Hirrlinger, & Braithwaite, 2016;Dicke & Roth, 2016;Herculano-Houzel, 2015;Manger, 2006;Steinhausen et al, 2016), including sociality (Matějů et al, 2016). To the best of our knowledge there is no study applying to SW that may confirm or exclude the existence of a difference between the two sexes in the number of neurons, their dimensions, or their microcircuits as in some other mammals (Dicke & Roth, 2016;Elston, 2007;Herculano-Houzel, 2015).…”