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

Lyuba Zehl,Kerstin Morcinek,Michaela Haas-Rioth,Wolfgang Walkowiak,Charlene Steinhausen,Stefan Huggenberger

doi:10.3389/fnana.2016.00091

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.

Highlights

One of the core questions of neurobiology is how some groups of animals such as primates, elephants, and whales have evolved remarkably large brains (Haug, 1987; Marino, 1998; Roth and Dicke, 2005)
This paper focuses on Eutheria because only few data are available concerning the brain mass of Protheria and Metatheria species
Homo sapiens had a similar relative brain mass (RB) as some rodents and would, for example, be comparable in this regard to the house mouse Mus musculus whose brain makes up 1.9% of the body mass

Summary

Introduction

One of the core questions of neurobiology is how some groups of animals such as primates, elephants, and whales have evolved remarkably large brains (Haug, 1987; Marino, 1998; Roth and Dicke, 2005). Size differences of whole brains were interpreted as an adequate proxy for subtler differences in anatomy and function (Jerison, 1973; Stephan et al, 1988; van Dongen, 1998; Lefebvre et al, 2004, 2007, 2013; Marino et al, 2007). Because larger animals have larger brains (Harvey et al, 1980) but the functional or cognitive capacities of their brains are not necessarily greater, the use of absolute brain mass (AB) to compare different species of varying body size is limited. Meta-Analysis of Brain-Mass Correlations in Mammals than larger animals (van Dongen, 1998). For these allometric reasons, the encephalization quotient (EQ), a parameter indirectly dependent on the size of a body, is an useful metric for comparing brain sizes among mammals of different size (Baron, 2007). The EQ is defined as the ratio of the actual mass of the brain to the expected brain mass given by the body mass (van Dongen, 1998)

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Discussion

Conclusion