Brains

Mammals have large brains for their body size--larger than most members of other vertebrate groups.

If the brains of elephants living today are a reliable guide, extinct mammoths' brains probably weighed more than 11 pounds.
AMNH/D. Finnin

Just as mammals come in all sizes, so do their brains. But it's the size of the brain relative to the size of an animal's body that really matters.
 
The aye-aye stands less than a foot-and-a-half tall with a brain the volume of a golf ball. The extinct Columbian mammoth had a brain bigger than a cabbage that weighed about 11 pounds. It turns out that the aye-aye trumps the massive mammoth because, for their body size, an aye-aye's brain is more enlarged.
 
Humans claim the largest brain relative to body size at more than seven times the predicted size ratio. But the same relationship does not hold true for all individual parts of our brain. Our olfactory bulb - the area of the brain that processes smells - is smallest in relative size when compared to the opossum, the wolf and the platypus. The winner? The opossum.

The foot-and-a-half tall aye-aye has a brain with a volume comparable to a golf ball. But, when considering brain size relative to body size, the aye-aye trumps even the extinct Columbian mammoth – despite it having an 11-pound brain.
David Haring

Human brains are about seven times more than the predicted ratio of body size vs. brain size. That gives us one of the largest relative brains of all the vertebrates.
Shutterstock

At 3.5 inches tall, the tarsier has the largest eyes of all the mammals, relative to body size.
Biosphoto/Fischer Berndt/Peter Arnold, Inc.

Relative to body size, the Virginia opossum has a larger olfactory bulb – the area of the brain that controls smell – than a platypus, a wolf, or even a human!
Purestock/AGE Fotostock

Fun Facts

• The sperm whale has the biggest brain of any animal species, weighing up to 20 pounds (7 to 9 kilograms).
• Larger brains don't necessarily make a smarter mammal. But as mammals evolved, many groups, from Primates to Carnivora, have shown independent increases in brain size. The fact that unusually big brains show up again and again in mammalian evolution suggests that bigger brains must provide some major benefits.
• The nocturnal tarsier is only 3.5 inches tall, but it has the largest eyes relative to body size of any mammal. The eyes have the same volume as the animal's entire brain, in order to capture light on dark nights.