Positive genetic correlation between brain size and sexual traits in male guppies artificially selected for brain size.

A Kotrschal,N Kolm,S Immler,A A Maklakov,S Zajitschek,A Corral‐Lopez

doi:10.1111/jeb.12608

A Kotrschal, N Kolm + Show 4 more

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https://doi.org/10.1111/jeb.12608

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Abstract

Brain size is an energetically costly trait to develop and maintain. Investments into other costly aspects of an organism's biology may therefore place important constraints on brain size evolution. Sexual traits are often costly and could therefore be traded off against neural investment. However, brain size may itself be under sexual selection through mate choice on cognitive ability. Here, we use guppy (Poecilia reticulata) lines selected for large and small brain size relative to body size to investigate the relationship between brain size, a large suite of male primary and secondary sexual traits, and body condition index. We found no evidence for trade‐offs between brain size and sexual traits. Instead, larger‐brained males had higher expression of several primary and precopulatory sexual traits – they had longer genitalia, were more colourful and developed longer tails than smaller‐brained males. Larger‐brained males were also in better body condition when housed in single‐sex groups. There was no difference in post‐copulatory sexual traits between males from the large‐ and small‐brained lines. Our data do not support the hypothesis that investment into sexual traits is an important limiting factor to brain size evolution, but instead suggest that brain size and several sexual traits are positively genetically correlated.

Highlights

Brain size is highly variable among animals, but despite over a century of research in this area, our understanding of the evolutionary processes and mechanisms that have generated this variation remains inconclusive
Condition did not differ in pair-housed males (Table 1; Fig. 2a), large-brained males that were kept in larger groups had a higher body condition than small-brained males kept under the same conditions (GLMM: brain size: F1,2 = 7.63, P = 0.010; Fig. 2a)
In our current study we found no evidence for negative associations between brain size and male sexual traits, but a higher expression of several male sexual traits in the larger-brained males

Summary

Introduction

Brain size is highly variable among animals, but despite over a century of research in this area, our understanding of the evolutionary processes and mechanisms that have generated this variation remains inconclusive. The theoretical framework in this field is based on the general idea that relative brain size evolves through a balance between the positive fitness effects of increased cognitive ability and the prohibiting effects of the energetic costs of developing and maintaining a larger brain Empirical evidence for positive effects of increased brain size stems mainly from comparative studies where larger brains have been associated with higher frequencies of cognitively demanding behaviours such as parental care, tool use and social behaviour (Gittleman, 1994; Dunbar, 1998; Gonzalez-Voyer et al, 2009; Brown, 2012; Kotrschal et al, 2013a). Experimental evidence for a causal link between brain size and cognitive ability was provided based on artificial selection on brain size in the guppy (Poecilia reticulata). Using the same brain size selection lines, it was demonstrated that large-brained males were faster at learning to find a potential mate in a spatial maze (Kotrschal et al, 2014)

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