By Dr Ananya Mandal, MD
The structure of the hippocampus is similar across the various mammalian species, from the echidna through to primates such as humans. Size, however, varies somewhat from mammal to mammal, with the size-to-body-size ratio twice as great in primates than in echidna.
The neocortex-to-body-size-ratio, however, increases at a much greater rate than this meaning the hippocampus takes up a much larger portion of the cortical mantle in a rodent, for example, than it does in human.
The size of the hippocampus is also associated with spatial memory. Generally, spatial memory improves with increasing hippocampal volume when animals of similar species are compared. This also extends to gender differences, with the difference between male and female spatial memory ability being stronger when the between-gender difference in hippocampal volume is greater.
Only mammals have developed a fully functional cortex but the structure it is derived from is present in all vertebrates including the most primitive forms such as the lamprey. This structure is called the pallium and it has three zones; the medial, lateral, and dorsal zones.
It is the medial or central zone from which the hippocampus is derived during development. The medial pallium does not share the seahorse shape of the hippocampus but the structures do have chemical and functional aspects in common.
Research shows that the hippocampus is also involved in spatial cognition among birds, reptiles and fish species. In birds, the association is so widely acknowledged that anatomists even call the medial pallium the "avian hippocampus." Some evidence suggest that birds that catch their food have a larger hippocampus. Fish also have a strong spatial navigation memory and even form "cognitive maps" of the areas they populate.
Reviewed by Sally Robertson, BSc