Ecological not social factors explain brain size in cephalopods

Social factors have been argued to be the main selection pressure for the evolution of large brains and complex behavior, but many cephalopods live largely solitary, semelparous, short lives. This suggests that the large brains found in cephalopods are not the result of social selection pressures. Here, we derive specific, preregistered predictions from the "Asocial Brain Hypothesis" (ABH; an untested extension of the cultural brain hypothesis formal model), and evaluate those predicted associations using a new comparative dataset on brain size alongside social, ecological, and life history factors. Consistent with the ABH and other hypotheses predicting that ecological factors should be the primary selection pressure with larger brains in more calorie-rich complex ecologies, we find that shallower and benthic (seafloor) habitats are associated with larger brain sizes, and that measures of sociality are not. Our findings are not interpreted as causal, but are consistent with ecological hypotheses for brain evolution.