Comparative support for the expensive tissue hypothesis: Big brains are correlated with smaller gut and greater parental investment in Lake Tanganyika cichlids.

Masahito Tsuboi,Arild Husby,Séverine D Buechel,Hanne Løvlie,Alexander Hayward,Josefina Zidar,Alexander Kotrschal,Niclas Kolm

doi:10.1111/evo.12556

Abstract

The brain is one of the most energetically expensive organs in the vertebrate body. Consequently, the energetic requirements of encephalization are suggested to impose considerable constraints on brain size evolution. Three main hypotheses concerning how energetic constraints might affect brain evolution predict covariation between brain investment and (1) investment into other costly tissues, (2) overall metabolic rate, and (3) reproductive investment. To date, these hypotheses have mainly been tested in homeothermic animals and the existing data are inconclusive. However, there are good reasons to believe that energetic limitations might play a role in large-scale patterns of brain size evolution also in ectothermic vertebrates. Here, we test these hypotheses in a group of ectothermic vertebrates, the Lake Tanganyika cichlid fishes. After controlling for the effect of shared ancestry and confounding ecological variables, we find a negative association between brain size and gut size. Furthermore, we find that the evolution of a larger brain is accompanied by increased reproductive investment into egg size and parental care. Our results indicate that the energetic costs of encephalization may be an important general factor involved in the evolution of brain size also in ectothermic vertebrates.

Highlights

Our results indicate that the energetic costs of encephalization may be an important general factor involved in the evolution of brain size in ectothermic vertebrates
We found that the evolution of a larger brain was accompanied by an increase in egg size and prolongation of the parental care period
Our study supports the existence of energetic constraints as important factors influencing across-species patterns of brain size diversification in cichlids

Summary

Introduction

Three main hypotheses concerning how energetic constraints might affect brain evolution predict covariation between brain investment and (1) investment into other costly tissues, (2) overall metabolic rate, and (3) reproductive investment. To date, these hypotheses have mainly been tested in homeothermic animals and the existing data are inconclusive. There are good reasons to believe that energetic limitations might play a role in large-scale patterns of brain size evolution in ectothermic vertebrates. We test these hypotheses in a group of ectothermic vertebrates, the Lake Tanganyika cichlid fishes. The expensive tissue hypothesis (ETH) argues that the cost of encephalization should be compensated by a reduction in the size of other expensive organs

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