The elephant has a 4.2 kg brain, about 3 times as large as humans or dolphins, and a cortical neuron count of 11,000 million, about the same as humans, and twice that of dolphins or chimps. They build and maintain their brains without regularly consuming seafood, marrow, muscles, or brains of other animals, as do millions of humans around the globe who adhere to plant-based diets for philosophical or cultural reasons: Seventh Day Adventists, Buddhists, Jains, Taoists, and Hindus .
"Millions of individuals around the modern world, including some 2.5% of Americans and 4% of Canadians (American Dietetic Association and Dieticians of Canada, 2003), consume a diet classified as vegan or vegetarian. There are also a number of religious doctrines that emphasize abstention from animal consumption, including Jainism, Buddhism, Hinduism, and Taoism, involving a significant percentage of modern human populations. In the case of these vegetarians, many have maintained such a restricted diet for generations. Neurological impairment under generational deficiency of DHA should result if dietary DHA is essential for neural function. Given that these populations experience normal brain growth and development in the absence of dietary DHA, it seems reasonable to question the nature of our dietary requirements for n-3 fatty acids. If preformed DHA is essential, and only significantly available from aquatic dietary sources, the expected outcome of a vegetarian lifestyle is the failure of neural growth and development. On the other hand, there is no evidence to suggest that the capacity for DHA synthesis in vegetarians is limited (Sanders, 1999). A logical explanation involves the sufficiency of LNA from the dietary intake of plants to provide sufficient DHA for the neural development of these populations."
Here's part 1 of a longer documentary on the Thai elephant orchestra led by two scientists: