Environmental and Social Effects on the Neuroanatomy of Cichlid Fishes

Abstract

The evolution of the brain and the factors that affect variation in brain size are pivotal areas of investigation in evolutionary biology, providing insight into how animals adapt cognitively to their environments. This dissertation explores factors driving brain evolution, particularly focusing on ecological and social influences, with the goal of contributing intraspecies evidence and new perspectives to the field. This dissertation unfolds across three main chapters: the ecological impact on brain size, the role of social stimuli in driving brain size changes through plasticity, and an in-depth examination of the social brain hypothesis at the intraspecies level. Together, these chapters assess the contribution of both ecological and social influences on brain morphology, thereby advancing our understanding of how environmental and social complexities shape brain evolution. In Chapter 1, I explore the relationship between habitat complexity and brain size across populations residing in structurally diverse environments. Results reveal a negative correlation between habitat complexity and brain size, with fish in simpler habitats exhibiting significantly larger brains. Specifically, cerebellar enlargement was prominent in lowcomplexity environments, suggesting adaptations for enhanced spatial navigation under greater predation pressure and limited refuge availability. These findings challenge the Clever Foraging Hypothesis (CFH), which traditionally posits that more complex environments drive brain enlargement. Chapter 2 examines the effects of social stimuli on brain plasticity by comparing wild and captive populations of the same lineages. Captive fish exposed to reduced ecological pressures but heightened social interaction had increased brain sizes, particularly in species not naturally exposed to high-density social environments. These results support the Social Brain Hypothesis (SBH), which suggests that brain enlargement is driven by social demands. Species-specific brain region alterations, particularly in the telencephalon and hypothalamus, highlight the brain's adaptive response to social stimuli, while reductions in the optic tectum might suggest lower visual processing demands in captive environments. In Chapter 3, we test the SBH in the wild, examining the relationship between social complexity and brain size at the intraspecies level. A positive correlation between wild group size and brain volume was found, and controlled experiments showed that larger-brained individuals benefit from social integration but not territorial competition, suggesting that brain enlargement is correlated social competence. These findings provide robust intraspecies evidence for SBH, demonstrating that social complexity is directly linked to brain size in the absence of confounding ecological factors. This dissertation highlights the remarkable plasticity of the fish brain and demonstrates the unique advantages of using cichlid fish as a model for studying brain evolution. The findings emphasize that established hypotheses cannot be universally applied across all species or contexts. Instead, each hypothesis may hold different relevance depending on the specific species and environmental conditions. This underscores the importance of adopting a comprehensive, integrative approach to studying brain evolution. Future research should aim to unravel the underlying mechanisms through which ecological and social factors influence brain size, offering deeper insights into the dynamic interplay between environmental pressures and brain evolution.