Coleoid Cephalopods Demonstrate Asocial Path to the Evolution of Big Brains

Abstract

Coleoid cephalopods have large brains, complex nervous systems and show signs of intelligent behavior comparable to birds, cetaceans, and primates. However, contrasting with many large brained vertebrates, many cephalopods live largely solitary, semelparous, and short lives, leaving little to no opportunity for parental care, complex group dynamics, or social learning. Here, we introduce and analyze a comprehensive database of brain size, ecology, behavior, sociality, and life history of 79 diverse species of octopus, squid, and cuttlefish, including an updated phylogeny using Bayesian multilevel linear regression models. In a set of pre-registered statistical models, we find a large effect of habitat, suggesting ecology as a primary selection pressure on brain size in cephalopods. We also find some evidence of a positive relationship between brain size and number of predator groups, and no relationship between brain size and sociality. Taken together, these results are inconsistent with social explanations for brain evolution but consistent with ecological explanations, including the "Asocial Brain Hypothesis"- a regime of predictions from the "Cultural Brain Hypothesis"- and emphasize how these animals, which diverged from humans and other vertebrates over 500 million years ago, offer an insightful comparative study of the evolution of large brains.