Background: Phenotypic integration among different anatomical parts of the head is a common phenomenon across vertebrates. Interestingly, despite centuries of research into the factors that contribute to the existing variation in brain size among vertebrates, little is known about the role of phenotypic integration in brain size diversification. Here we used geometric morphometrics on the morphologically diverse Tanganyikan cichlids to investigate phenotypic integration across key morphological aspects of the head. Then, while taking the effect of shared ancestry into account, we tested if head shape was associated with brain size while controlling for the potentially confounding effect of feeding strategy. Results: The shapes of the anterior and posterior parts of the head were strongly correlated, indicating that the head represents an integrated morphological unit in Lake Tanganyika cichlids. After controlling for phylogenetic non-independence, we also found evolutionary associations between head shape, brain size and feeding ecology. Conclusions: Geometric morphometrics and phylogenetic comparative analyses revealed that the anterior and posterior parts of the head are integrated, and that head morphology is associated with brain size and feeding ecology in Tanganyikan cichlid fishes. In light of previous results on mammals, our results suggest that the influence of phenotypic integration on brain diversification is a general process.

Brain size is strongly associated with body size at all taxonomic levels. This relationship has been hypothesized to be an important constraint on adaptive brain size evolution. The essential assumption of this idea is that allometry has a limited ability to evolve, and that evolution of relative brain size is therefore constrained to occur along the direction of static (i.e. within species) allometry. However, recent studies have reported mixed support for this view. Here, we examine if static allometry has affected the rate of relative brain size evolution in Lake Tanganyika cichlids. The evolution of brain-body allometry showed a recent rapid divergence whereas brain size evolution represented a more gradual phenotypic divergence across the history of diversification. Accordingly, we found no support for that static allometry affected the rate of absolute or relative brain size evolution in this group. Instead, we detected low, but existing evolvability of static allometry. Moreover, static allometry evolved faster in species with relatively small and large brains than in species with medium brain size. We propose that a combination of allometric constraints and partial evolvability of static allometry have allowed for independent evolution of brain size in Lake Tanganyika cichlids. Overall, our results demonstrate a complex, yet important, role of brain-body allometry in brain size evolution. 

The brain is one of the most energetically expensive organs in the vertebrate body. Consequently, the energetic requirements of encephalization are suggested to impose considerable constraints on brain size evolution. Three main hypotheses concerning how energetic constraints might affect brain evolution predict covariation between brain investment and (1) investment into other costly tissues, (2) overall metabolic rate, and (3) reproductive investment. To date, these hypotheses have mainly been tested in homeothermic animals and the existing data are inconclusive. However, there are good reasons to believe that energetic limitations might play a role in large-scale patterns of brain size evolution also in ectothermic vertebrates. Here, we test these hypotheses in a group of ectothermic vertebrates, the Lake Tanganyika cichlid fishes. After controlling for the effect of shared ancestry and confounding ecological variables, we find a negative association between brain size and gut size. Furthermore, we find that the evolution of a larger brain is accompanied by increased reproductive investment into egg size and parental care. Our results indicate that the energetic costs of encephalization may be an important general factor involved in the evolution of brain size also in ectothermic vertebrates.

The brain is one of the most energetically expensive organs in the vertebrate body. Consequently, the high cost of brain development and maintenance is predicted to constrain adaptive brain size evolution (the expensive tissue hypothesis, ETH). Here, we test the ETH in a teleost fish with predominant female mating competition (reversed sex-roles) and male pregnancy, the pacific seaweed pipefish Syngnathus schlegeli. The relative size of the brain and other energetically expensive organs (kidney, liver, heart, gut, visceral fat, ovary/testis) was compared among three groups: pregnant males, non-pregnant males and egg producing females. Brood size in pregnant males was unrelated to brain size or the size of any other organ, whereas positive relationships were found between ovary size, kidney size and liver size in females. Moreover, we found that the size a suite of energetically expensive organs (brain, heart, gut, kidney, liver) as well as the amount of visceral fat did not differ between pregnant and non-pregnant males. However, we found marked differences in relative size of the expensive organs between sexes. Females had larger liver and kidney than males, whereas males stored more visceral fat than females. Furthermore, in females we found a negative correlation between brain size and the amount of visceral fat, whereas in males a positive trend between brain size and both liver and heart size was found. These results suggest that, while the majority of variation in the size of various expensive organs in this species likely reflects that individuals in good condition can afford to allocate resources to several organs, the cost of the expensive brain was visible in the visceral fat content of females, possibly due to the high costs associated with female egg production. 

Brain size varies greatly at all taxonomic levels. Feeding ecology, life history and sexual selection have been proposed as key components in generating the existing contemporary diversity in brain size across vertebrates. Analyses of brain size evolution have, however, been limited to lineages where males predominantly compete for mating and females choose mates. Here, we present the first original data set of brain sizes in pipefishes and seahorses (Syngnathidae). In this group, intense female mating competition occurs in many species (i.e. reversed sex-roles), and mating patterns include monogamy, polygynandry and polyandry. After controlling for the effect of shared ancestry and overall body size, relatively larger brains were positively correlated with relatively longer snout length, which is related to the propensity for feeding on motile and evasive prey items in Syngnathidae, and larger egg size. Furthermore, we found that females, on average, had 4.3% heavier brains than males and that polyandrous species tended to demonstrate female-favored brain size dimorphism. Our results suggest that adaptations for feeding on motile prey items, energetic constraints associated with production of large-brained juveniles and sexual selection in females are important factors in brain size evolution of pipefishes and seahorses.