Brain evolution in Proboscidea (Mammalia, Afrotheria) across the Cenozoic

Julien Benoit,Theodor Obada,Paul Manger,Vladislav Mararescul,Rodolphe Tabuce,Lucas J Legendre

doi:10.1038/s41598-019-45888-4

Abstract

As the largest and among the most behaviourally complex extant terrestrial mammals, proboscideans (elephants and their extinct relatives) are iconic representatives of the modern megafauna. The timing of the evolution of large brain size and above average encephalization quotient remains poorly understood due to the paucity of described endocranial casts. Here we created the most complete dataset on proboscidean endocranial capacity and analysed it using phylogenetic comparative methods and ancestral character states reconstruction using maximum likelihood. Our analyses support that, in general, brain size and body mass co-evolved in proboscideans across the Cenozoic; however, this pattern appears disrupted by two instances of specific increases in relative brain size in the late Oligocene and early Miocene. These increases in encephalization quotients seem to correspond to intervals of important climatic, environmental and faunal changes in Africa that may have positively selected for larger brain size or body mass.

Highlights

Elephants are the largest and among the most behaviourally complex terrestrial mammals
To address whether a large brain in proboscideans evolved as an adaptation or as a consequence of increased body mass, we examined the temporal scaling relationships of brain with body size and with associated environmental changes that have occurred during proboscidean evolutionary history
According to the ancestral character state reconstruction using maximum likelihood (ACSRML), which was performed on the whole dataset (Fig. 2), the EQ in the last common ancestor (LCA) of Moeritherium and Elephantiformes is 0.24

Summary

Introduction

Elephants are the largest and among the most behaviourally complex terrestrial mammals. It has been proposed that evolutionary selective pressures requiring increased intelligence (e.g. cognitive and sensory processing, memory, behavioural flexibility) to cope with a variety of environmental variables (e.g. sociality, gregariousness, diet, habitat) has driven the evolution of large brains in both Primates and ‘ungulates’[14,15,16,17] This is based on the retrospective assignation of implied cognitive needs centred on the apparent behavioural flexibility and intelligence of extant species. This retrospectively applied cognitive explanatory paradigm has been argued to apply to the evolution of a large brain in cetaceans[18,19,20], and elephants[10,12]; alternative explanations of brain size evolution have been forwarded, focussing on features such as the structural laws of form linking body and brain mass over evolutionary time[1,7,21,22].

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Discussion

Conclusion