Environmental enrichment, sexual dimorphism, and brain size in sticklebacks.

Elisavet A Toli,Juha Merilä,Kristina Noreikiene,Jacquelin Defaveri

doi:10.1002/ece3.2717

Elisavet A Toli, Juha Merilä + Show 2 more

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https://doi.org/10.1002/ece3.2717

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Abstract

Evidence for phenotypic plasticity in brain size and the size of different brain parts is widespread, but experimental investigations into this effect remain scarce and are usually conducted using individuals from a single population. As the costs and benefits of plasticity may differ among populations, the extent of brain plasticity may also differ from one population to another. In a common garden experiment conducted with three‐spined sticklebacks (Gasterosteus aculeatus) originating from four different populations, we investigated whether environmental enrichment (aquaria provided with structural complexity) caused an increase in the brain size or size of different brain parts compared to controls (bare aquaria). We found no evidence for a positive effect of environmental enrichment on brain size or size of different brain parts in either of the sexes in any of the populations. However, in all populations, males had larger brains than females, and the degree of sexual size dimorphism (SSD) in relative brain size ranged from 5.1 to 11.6% across the populations. Evidence was also found for genetically based differences in relative brain size among populations, as well as for plasticity in the size of different brain parts, as evidenced by consistent size differences among replicate blocks that differed in their temperature.

Highlights

Given the importance of the central nervous system to organismal performance, and thereby fitness, the size of the brain and its different parts are likely to be traits under strong optimizing selection
In contrast to our expectations, we did not find any evidence for a positive effect of environmental enrichment on the development of brain size or the size of any brain parts in either of the sexes in any of the four populations tested
The same applied to body size, body mass, and condition of the fish, indicating a lack of positive effect on growth and energy balance of the individuals

Summary

Introduction

Given the importance of the central nervous system to organismal performance, and thereby fitness, the size of the brain and its different parts are likely to be traits under strong optimizing selection. Certain brain regions often show an increase in size that is associated with specific ecological conditions likely to select for this growth (Eifert et al, 2015; Gonzalez-Voyer & Kolm, 2010; Krebs, Sherry, Healy, Perry, & Vaccarino, 1989; de Winter & Oxnard, 2001). There appears to be a general consensus that intra- and interspecific variation in brain size and size of different brain parts is, at least to some extent, dictated by variation in the strength of positive natural selection acting on them.

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