<Tr> <Td> horse </Td> <Td> 1: 600 </Td> </Tr> <Tr> <Td> shark </Td> <Td> 1: 2496 </Td> </Tr> <Tr> <Td> hippopotamus </Td> <Td> 1: 2789 </Td> </Tr> <P> Dolphins have the highest brain - to - body weight ratio of all cetaceans . Monitor lizards, tegus and anoles and some tortoise species have the largest among reptiles . Among birds, the highest brain - to - body ratios are found among parrots, crows, magpies, jays and ravens . Among amphibians, the studies are still limited . Either octopuses or jumping spiders have some of the highest for an invertebrate, although some ant species have 14% - 15% of their mass in their brains, the highest value known for any animal . Sharks have one of the highest for fish alongside manta rays (although the electrogenic elephantfish has a ratio nearly 80 times higher - about 1 / 32, which is slightly higher than that for humans). The tiny shrew, which holds about 10% of its body mass in its brain, has one of the highest brain - to - body mass ratios of any vertebrate . </P>

Who has the largest brain to body size