Reversed brain size sexual dimorphism accompanies loss of parental care in white sticklebacks.

Dolph Schluter,Davis Iritani,Kieran Samuk

doi:10.1002/ece3.1175

Abstract

Uncovering factors that shape variation in brain morphology remains a major challenge in evolutionary biology. Recently, it has been shown that brain size is positively associated with level of parental care behavior in various taxa. One explanation for this pattern is that the cognitive demands of performing complex parental care may require increased brain size. This idea is known as the parental brain hypothesis (PBH). We set out to test the predictions of this hypothesis in wild populations of threespine stickleback (Gasterosteus aculeatus). These fish are commonly known to exhibit (1) uniparental male care and (2) sexual dimorphism in brain size (males>females). To test the PBH, we took advantage of the existence of closely related populations of stickleback that display variation in parental care behavior: common marine threespine sticklebacks (uniparental male care) and white threespine sticklebacks (no care). To begin, we quantified genetic differentiation among two common populations and three white populations from Nova Scotia. We found overall low differentiation among populations, although FST was increased in between-type comparisons. We then measured the brain weights of males and females from all five populations along with two additional common populations from British Columbia. We found that sexual dimorphism in brain size is reversed in white stickleback populations: males have smaller brains than females. Thus, while several alternatives need to be ruled out, the PBH appears to be a reasonable explanation for sexual dimorphism in brain size in threespine sticklebacks.

Highlights

Vertebrates have long been known to display impressive levels of variation in the size and shape of their brains
As predicted by the parental brain hypothesis (PBH), we found that sexual dimorphism in brain size was altered in white sticklebacks (Table 1, Sex:Type, F1,239=13.46, P = 0.0007)
Our results show that male “white” threespine sticklebacks lacking parental care show significantly reduced male brain mass compared with females, the opposite of the pattern found in common stickleback populations

Summary

Introduction

Vertebrates have long been known to display impressive levels of variation in the size and shape of their brains. The evolutionary and proximate forces that shape this variation remain poorly understood. Brain morphology (overall size and the size of individual structures) has been shown to correlate with diverse behavioral, sensory, and ecological variables across a wide variety of taxa (Gittleman 1994; Lefebvre et al 1997; Farris 2005; Gonda et al 2009; Smith et al 2010). Establishing specific associations between neurological traits and other organismal traits remains a major challenge for evolutionary biologists. One specific behavioral trait recently suggested to shape variation in brain morphology is parental care (GonzalezVoyer et al 2009a). Parental care often involves a wide variety of complex and novel behavioral interactions between parent and offspring (Clutton Brock 1991)

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Discussion

Conclusion