Comparative Brain Anatomy in Different Animals -- Some Elementary Observations

From The Evolutionary Layers of the Human Brain at The Brain from 
Top to Bottom, a site of the Canadian Institutes for Health  

It is not uncommon for people to believe that among animals the human brain is functionally unique and is the largest, the most complex, has the most neurons, and has the greatest density of neurons. None of these beliefs is true. A number of animals have larger brains. Many other animals' brains have the same fundamental structure. The number of neurons in the human brain is often overstated, and their density is consistent with other primates.

I thought a couple of images plucked from various biological sites might prove an interesting survey of the field.

From The Brain Engineering Laboratory at Dartmouth University.

The neocortex in mammalian brains likely evolved from the olfactory bulb in reptiles. The most influential model for the evolution of the brain has been the "triune model" which proposes that the mammalian brain has a reptilian core overlain with the limbic system and the neocortex. (An interactive graphic of the triune model can be found here.) In some of its specifics the triune model is outdated (as some elements of the limbic system and neocortex exist in vertebrates as a whole and so predate reptiles), yet it appears to be broadly correct; for example, the neocortex has a six layered structure (usually labeled I to IV) that is unique to mammals.

From the Proceedings of the National Academy of Sciences

According to several studies, the human brain is a scaled up primate brain with the same fundamental structures and neuron density. Indeed, large brains have evolved separately in several different lineages of mammals. The largest brains are found in sperm whales. Both orcas and bottlenose dolphins (among other mammals) have larger and more complex brains than humans.

From the Brain Museum, an excellent comparative anatomy website
with numerous brain images and sections from many animals. Here is a 
similar image showing results by several classes of animal.

Of course, larger animals may be expected to have larger brains, so the ratio of brain weight to body weight may be telling. Such a comparison, however, can be misleading since animals may carry more weight in the form of blubber, thick skin, or fur to respond to environmental conditions, and brain weight alone does not account for complexity, function, or the relative weights of other areas of the brain. Moreover, convergent evolution may result in different brain structures performing similar tasks (as in, for example, tool using birds and mammals).

Another measure is enchephalization quotient (EQ), which measures "the ratio between actual brain mass and predicted brain mass for an animal of a given size." This works out very well for humans, since humans have an extraordinarily large EQ compared to other animals.  There is no universally agreed upon way to measure EQ, however, and the measure has been shown to be unreliable as well as criticized for its assumptions about brain mass correlating with body mass, a correlation which is false for many huge dinosaurs which had tiny brains. Echidnas and some other small rodents, on the other hand, have extraordinarily larger brains for their size. They are not, however, building cities. The fact is that the notion of a unitary measure of "intelligence" is misleading since animals perform many functions and perceptive and processing abilities may function by task, sensory input, and environmental adaptation.

From the Laboratory of Comparative Neuroanatomy

Another problem with EQ is that neurons may be differently sized and less or more dense in an area of a brain. It appears to be a general rule that larger animals not only have larger brains but larger neurons, and the neurons are less densely clustered.

None of this suggests that humans are not extraordinarily gifted in our cognitive abilities compared generally to other animals. What it does suggest is that the notion that humans are unique is wrong and the notion that "intelligence" is a singular clearly defined attribute is wrong.