Ecological not social factors explain brain size in cephalopods

AbstractSocial factors have been argued to be the main selection pressure for the evolution of large brains and complex behavior on the basis of data from mammals and birds. Coleoid cephalopods (octopuses, squid, and cuttlefish) have large brains, complex nervous systems and show signs of intelligent behavior comparable to that of birds, cetaceans, and primates. However, many cephalopods live largely solitary, semelparous, short lives, and many are cannibalistic, leaving little to no opportunity for parental care, complex group dynamics, or social learning. This suggests that the large brains found in cephalopods are not the result of social selection pressures. Here, motivated by the predictions of the “Asocial Brain Hypothesis” formal model, we compare the relationships between brain size and social, ecological, and other factors in cephalopods, predicting that ecological factors should be the primary selection pressure. Consistent with these predictions, we find that benthic (seafloor) habitats—arguably more complex environments than open-ocean (pelagic) habitats—are strongly associated with larger brain sizes, whereas measures of sociality are not, therefore suggesting ecology as a primary selection pressure on brain size in cephalopods. These results align with the “Asocial Brain Hypothesis,” which predicts that ecological complexity rather than sociality drives the evolution of large brains in asocial animals. Our findings challenge the long-standing emphasis on social drivers of brain expansion and show that ecological selection pressures can produce complex brains in highly divergent evolutionary lineages, which diverged from vertebrates over 500 million years ago. This result highlights the need for more general frameworks that capture different pathways to large brains and intelligence—both social and asocial—and underscores the power of modeling and comparative approaches to unify disparate findings in the study of brain evolution.