Extreme sexual brain size dimorphism in sticklebacks: a consequence of the cognitive challenges of sex and parenting?

Alexander Kotrschal,Katja Räsänen,Mike Senn,Niclas Kolm,Bjarni K Kristjánsson

doi:10.1371/journal.pone.0030055

Alexander Kotrschal, Katja Räsänen + Show 3 more

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https://doi.org/10.1371/journal.pone.0030055

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Abstract

Selection pressures that act differently on males and females produce numerous differences between the sexes in morphology and behaviour. However, apart from the controversial report that males have slightly heavier brains than females in humans, evidence for substantial sexual dimorphism in brain size is scarce. This apparent sexual uniformity is surprising given that sexually distinct selection pressures are ubiquitous and that brains are one of the most plastic vertebrate organs. Here we demonstrate the highest level of sexual brain size dimorphism ever reported in any vertebrate: male three-spined stickleback of two morphs in an Icelandic lake have 23% heavier brains than females. We suggest that this dramatic sexual size dimorphism is generated by the many cognitively demanding challenges that males are faced in this species, such as an elaborate courtship display, the construction of an ornate nest and a male-only parental care system. However, we consider also alternative explanations for smaller brains in females, such as life-history trade-offs. Our demonstration of unprecedented levels of sexual dimorphism in brain size in the three-spined stickleback implies that behavioural and life-history differences among the sexes can have strong effects also on neural development and proposes new fields of research for understanding brain evolution.

Highlights

Divergent selection pressures between males and females have produced many differences between the sexes in morphology and behaviour [1]
We found that males had significantly heavier brains than females when controlling for the effect of body size, whereas there were no overall differences in brain mass between mud and lava habitats (ANCOVA: body size: F1,118 = 439.50, p,0.0001; sex: F1,118 = 112.56, p,0.0001; habitat: F1,118 = 2.48, p = 0.118; sex6 habitat interaction: F1,118 = 0.79, p = 0.376; figure 1)
Body mass was strongly positively correlated to body size, but did not differ between the sexes or habitats when corrected for body length (ANCOVA: body length: F1,118 = 234.743, p,0.0001; sex: F1,118 = 0.720, p = 0.398; habitat: F1,118 = 0.597, p = 0.442; sex6 habitat interaction: F1,118 = 1.559, p = 0.215)

Summary

Introduction

Divergent selection pressures between males and females have produced many differences between the sexes in morphology and behaviour [1]. One example of sexual dimorphism is the allegedly larger brains of men compared to women [3] These findings remain heavily criticized both for unsuitable statistical methods [4,5] and the inappropriateness of setting up expectations about sexual differences in intelligence. Apart from this questionable example in humans and various species where the sexes differ in their structural architecture of the brain [6,7,8,9], cases of sexual dimorphism in overall brain size are virtually absent. This is surprising considering the generally distinct selection pressures acting on males and females [1], the concurring sex-specific specializations, and the enormous crossspecies brain size variation commonly associated with such specializations [10]

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