Abstract
It has been suggested that mating behaviours require high levels of cognitive ability. However, since investment into mating and the brain both are costly features, their relationship is likely characterized by energetic trade-offs. Empirical data on the subject remains equivocal. We investigated if early sexual maturation was associated with brain development in Atlantic salmon (Salmo salar), in which males can either stay in the river and sexually mature at a small size (sneaker males) or migrate to the sea and delay sexual maturation until they have grown much larger (anadromous males). Specifically, we tested how sexual maturation may induce plastic changes in brain development by rearing juveniles on either natural or ad libitum feeding levels. After their first season we compared brain size and brain region volumes across both types of male mating tactics and females. Body growth increased greatly across both male mating tactics and females during ad libitum feeding as compared to natural feeding levels. However, despite similar relative increases in body size, early maturing sneaker males maintained larger relative brain size during ad libitum feeding levels as compared to anadromous males and females. We also detected several differences in the relative size of separate brain regions across feeding treatments, sexes and mating strategies. For instance, the relative size of the cognitive centre of the brain, the telencephalon, was largest in sneaker males. Our data support that a large relative brain size is maintained in individuals that start reproduction early also during fast body growth. We propose that the cognitive demands during complex mating behaviours maintain a high level of investment into brain development in reproducing individuals.Electronic supplementary materialThe online version of this article (doi:10.1007/s10682-014-9715-x) contains supplementary material, which is available to authorized users.
Highlights
Brain morphology is highly variable at all taxonomic levels among vertebrates (Jerison 1973; Kotrschal et al 2013) and explaining this variation continues to be an important question in modern evolutionary biology (Striedter 2005)
We investigated if early sexual maturation was associated with brain development in Atlantic salmon (Salmo salar), in which males can either stay in the river and sexually mature at a small size or migrate to the sea and delay sexual maturation until they have grown much larger
We found a strong effect of feeding level on the proportion of males that matured early
Summary
Brain morphology is highly variable at all taxonomic levels among vertebrates (Jerison 1973; Kotrschal et al 2013) and explaining this variation continues to be an important question in modern evolutionary biology (Striedter 2005). Theory and empirical data maintain that brain morphology evolves under the balance between positive selection for cognitive ability and the energetic costs of maintaining a larger and more complex brain (Aiello and Wheeler 1995; Darwin 1871; Isler and van Schaik 2006; Jacobs 1996; Jerison 1973; Kotrschal et al 2013). Given the energetically costly nature of the vertebrate brain, trade-offs between investment into the brain and other costly features of an organism’s biology have long been assumed to be important in generating variation in brain morphology (Aiello and Wheeler 1995; Boogert et al 2011; Kotrschal et al 2013; Navarrete et al 2011; Striedter 2005). Despite many decades of interest in the selection pressures for increased brain size and potential trade-offs, experimental data are still scarce regarding the selection pressures that affect brain complexity
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