Basava, K. | Search bioRxiv

bioRxiv2024

A phylogeny of extant coleoid cephalopods with brain data

Extant coleoid cephalopods include over 800 species of octopuses, squid, and cuttlefish, which have drawn scientific and public interest for their complex behavior and cognition. Of these, approximately 10% (79) species have adult specimens with recorded measures of central nervous system size distributed across various sources. Here, we use a combination of topological placements from previous phylogenetic studies, along with mitochondrial and nuclear gene sequences obtained from GenBank, to build a composite phylogenetic tree with estimated branch lengths of all species with available brain measurements. This phylogeny is used for analyses in a forthcoming paper on cephalopod brain evolution, and ideally will be of use to other researchers interested in conducting comparative studies of coleoid cephalopod brains.

evolutionary biology↗

bioRxiv2024

Coleoid Cephalopods Demonstrate Asocial Path to the Evolution of Big Brains

Social 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 primates, cetaceans, and birds. 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", a yet untested regime of the Cultural Brain Hypothesis formal model, we compare the relationships between brain size and social, ecological, and other factors in cephalopods. Consistent with the prediction 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--arguably more complex environments than open-ocean (pelagic) habitats--are associated with larger brain sizes, and that measures of sociality are not. Our findings from these highly divergent evolutionary lineages, which diverged from vertebrates over 500 million years ago are not causal, but are consistent with the "Asocial Brain Hypothesis" mechanistic model that describes how ecological selection pressures can also produce large, complex brains.

evolutionary biology↗