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05.07.2022: A recently published study conducted by Vetmeduni’s Konrad Lorenz Institute of Ethology tests several common hypotheses on the evolutionary benefits of large brains. The study shows that the lifespan of fish with large brains is shorter than that of fish with small brains. Moreover, fish with smaller brains are more likely to invest in parental care than those with large brains. Both findings contrast sharply with results from studies of birds and mammals. Against the background, the researchers stress the importance of testing such hypotheses with more comprehensive datasets that are not limited to just a few animal species.

Brain size varies considerably within the animal kingdom. As the evolution of a large brain involves considerable cost, the question arises from an evolutionary perspective why some organisms invest more in their brains than others. Typically, the differences in brain size are explained through trade-offs: the benefits of a larger brain, such as enhanced cognitive abilities, are balanced against potential costs, such as increased energetic demands. Several hypotheses have been formulated in this framework, with different emphasis on ecological, behavioural or physiological aspects of trade-offs in brain size evolution. These hypotheses largely relate to mammals and birds, leaving some uncertainty as to the universal validity of the respective arguments.

Current hypotheses only in part universally valid

In their study, which was recently published in the Journal of Evolutionary Biology, the two researchers tested three of the most prominent of these hypotheses – the expensive tissue hypothesis, the social brain hypothesis and the cognitive buffer hypothesis – in fish. The analysis was based on a large dataset derived from a publicly available resource (FishBase). Consistent with predictions from the expensive tissue and social brain hypotheses, larger brains are associated with reduced fecundity and increased sociality in at least some fish groups species.

The study failed to verify other hypotheses, however, as first author Stefan Fischer from Vetmeduni’s Konrad Lorenz Institute of Ethology explains: “Contrary to expectations, lifespan is reduced in large-brained fishes and there is a tendency for species that perform parental care to have smaller brains.” This means that parental care in fish is indirectly proportional to brain size – a surprising finding, according to the researchers.

Basic hypotheses should be tested using a variety of taxa

In summary, some potential costs (reduced fecundity) and benefits (increased sociality) of large brains appear to be almost universal for vertebrates, while others have more lineage-specific effects. According to final author Arne Jungwirth from Vetmeduni’s Konrad Lorenz Institute of Ethology, the research highlights the need for a taxonomically diverse approach to all fundamental questions in evolutionary biology: “Our work clearly shows that it is necessary to test even supposedly well-established hypotheses using as many different taxa, or groups as possible – life is always more diverse and fascinating than our theories give it credit for.”

The article “The costs and benefits of larger brains in fishes” by Stefan Fischer and Arne Jungwirth was published in Journal of Evolutionary Biology.

Scientific Paper