Elongation of the CA1 field of the septal hippocampus in ungulates.

It is widely assumed that the hippocampal formation seen in laboratory rodents and in primates is typical of that seen in other mammals. We have tested this assumption by examining sections of brains of 56 mammals from 20 mammalian orders from images on the brainmuseum.org website. We found wide variation in the form of the hippocampal formation, the most extreme examples of which are seen in ungulates, which possess an unusual elongation of the distal CA1 of the septal hippocampus. This phenomenon has not previously been reported. In individual coronal sections of the brains of seven artiodactyl ungulates, the pyramidal layer of CA1 is four times as long as CA2 + CA3. In a perissodactyl ungulate (Burchell's zebra) the distal end of CA1 is so large that it forms a number of folds. A similar but less pronounced CA1 elongation was seen in the brains of 14 carnivores. A modest elongation of CA1 is also present in some other placental mammals, notably the elephant shrew, hyrax, capybara, beaver, and rabbit. The elongation was not present in brains of primates, marsupials, or monotremes. The distal part of CA1 has been shown to play a role in object integration into the spatial map. We hypothesize that the distal CA1 enlargement could serve to enhance the ability to integrate objects into spatial navigation, which would be an advantage for migrating herds of ungulates. We suggest that the remarkable elongation of Q5 CA1 represents a major evolutionary specialization in the ungulates.

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