The megachiropteran pineal organ: a comparative morphological and volumetric investigation with special emphasis on the remarkably large pineal of Dobsonia praedatrix.

This investigation is based upon the pineal organs of 92 specimens of 36 species of the family Pteropodidae (Mammalia, Chiroptera). The size of the megachiropteran pineal correlates well with body size (r = 0.864), confirming the former conclusions that generally larger bodied bats have larger pineals. The range of the pineal size index in 36 megachiropteran species is from 33 to 4393. In most species the pineal organs are small, deeply recessed under the cerebral hemispheres and of Type A (except in Dobsonia and Pteropus, where they are of Type alpha beta C and AB, respectively). Morphological and volumetric data gathered from serially sectioned brains include body and brain weights, pineal type, dimensions, volume and size index for each species. There are distinct dorsal and ventral subdivisions of the pineal in some species and a clear separation of pineal parenchyma into cortical and medullary regions in others. In several species where overlying ependyma is lacking pinealocyte clusters communicate freely with the CSF. Groups of intrapineal neurons are noted in the connective tissue beside blood vessels. The habenular commissure shows much interspecific variation in its course through the pineal. Detailed examination of pineal-brain relationships clearly suggests that, due to the generally deep location of the pineal in relation to cerebral hemispheres, pinealectomies in the species studied may be extremely difficult, it not entirely impossible. The absolutely and relatively largest pineal organ among bats, and relatively perhaps among all vertebrates, has been discovered in the New Guinean naked-backed bat, Dobsonia praedatrix, with pineal size index of 4393, and a volume of 16.3447 mm3, which is 0.56% of the brain. This alpha beta C-type, mushroom-shaped, solid and compact pineal organ measures 5.33 x 4.51 mm. The cortical and medullary parenchyma are divided into lobes by large calibre blood vessels along which numerous intrapineal neurons are observed. A smaller but similarly shaped pineal is noted in the other three Dobsonia. Data on the largest known pineals in ratitae birds, seals and walruses have been compared with that of D. praedatrix and the human pineal. This study supports the hypothesis that pineal development may reflect dependence on habitat and possibly other related factors.