<Tr> <Td> Opossum </Td> <Td> 27,000,000 </Td> <Td> Didelphis virginiana </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Brown Rat </Td> <Td> 31,000,000 </Td> <Td> Rattus norvegicus </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Four - toed elephant shrew </Td> <Td> 34,000,000 </Td> <Td> Petrodromus tetradactylus </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Ferret </Td> <Td> 39,000,000 </Td> <Td> Mustela putorius furo </Td> <Td> </Td> <Td> </Td> </Tr>

How many brain cells does a dolphin have