Dunbar was an anthropologist, not a neuroanatomy researcher, and Dunbar's Number was an ethnographical observation, not a biological one.
"Your social life has a biological limit: 150. That’s the number—Dunbar’s number, proposed by the British psychologist Robin Dunbar three decades ago—of people with whom you can have meaningful relationships."
#IanBogost, 2021
That's your opening? Not off too a great start there Ian.
#DunbarsNumber