Many current theories relating neuroanatomy and neurophysiology to cognition and behavior were developed a generation or more ago, while much significant research has been done in the past several years. It's clear that many assumptions made about animals and humans are no longer valid. For example, functional magnetic resonance imaging (fMRI) gives results in seconds, whereas brain phenomena occur in milliseconds. Thus, attempts to precisely correlate regional brain events with cognitive events are imperfect and "the gap between circuits and behavior is too wide."
In addition, hypotheses about the relationship of brain size and structure to stages of evolution, behavior, and cognitive capacities need to be reevaluated in light of new data and new theories. It should also be noted that because absolute brain size can be a misleading measure researchers often rely on the encephalization quotient (EQ) when making comparisons of brain size among different species. The EQ is "a measure of relative brain size defined as the ratio between actual brain mass and predicted brain mass for an animal of a given size'. Estimates for the EQ's for different species and discussions of what they mean about intelligence and various behavior patterns can be seen here and here.
The birds and the bees and their brains: Size doesn't matter
Sophisticated cognitive abilities have been observed in a wide variety of species and include very unlikely candidates. This general discussion will focus on birds, reptiles, and bees, all of whom display more intelligence than would be explained by a simple comparison of their brains with human brains. Birds perform complex behavior using a very small brain without a neocortex. Songbirds learn songs from mentors and make adjustments through practice and feedback. Finches use strict rules of syntax. New Caledonian crows show the advanced capability of metacognition, as well as counting, making and using tools as well or better than many nonhuman primates (see also), and displaying remarkable memory. Crows also remember specific people, cars, and urban situations and hold grudges with specific people and cars for several years. Some birds show advanced planning and art. Alex, the world renowned African Grey parrot, did arithmetic, mastered same-different relationships, invented words, and the night before he died told his doctor friend that he loved his friend and researcher, Dr. Irene Pepperberg.
Most remarkably, bees with tiny brains use abstract thought and symbolic language. Each day they solve an advanced mathematical problem of how to most efficiently travel between multiple sites. They know when to mix medications for the hive and distinguish complex landscape scenes including types of flowers, shapes and patterns. Bees also learn categories and sequences and adjust them for future rewards. They consider social conditions, locations, time of day, and multiple senses. They are masters of mazes and show short-term and long-term memory, ranging from days to entire life spans.
How can these unexpected observations of complex behavior patterns be explained in terms of brain size structure? Recently, it's been discovered that the brains of all of these animals have unusually complex neurons, neurons that are similar to the types that exist in the human brain, but in different brain structures. In birds a brain center that is correlated with some of their advanced abilities is in a so-called primitive region in the striatum, not the neocortex. When songbirds learn their songs from mentors, specific neurons in the brainstem signal for the singing tutor’s sound and the young bird's sound.
Recently, regions have been found in the brains of birds and turtles that are similar in some ways to the human neocortex levels 4 and 5, but in very different structures (for more on the emotional lives of reptiles please see). In turtles the brain region with these specialized neurons are distributed in one layer. In birds a region called the dorsal ventricular ridge has cells that input data like the human layer 4, and output data in cells like the human layer 5, functioning like the human neocortex. In some ways, the bird’s arrangement is superior for language and cognitive tasks. An example is a specialized region for vocalization, that doesn't exist in the complex multisensory and multi-modal human brain.
The tiny honeybee brain has only around one million neurons and bees "contradict the notion that insect behaviour tends to be relatively inflexible and stereotypical. Indeed, they live in colonies and exhibit complex social, navigational and communication behaviours, as well as a relatively rich cognitive repertoire." As one of us (MB) has noted earlier, Melissa Bateson and her colleagues have shown when honeybees are stressed, they display an increased expectation of bad outcomes. In other words, they become pessimists. When similar behavior is observed in vertebrates it's explained as having an emotional basis. The bees also showed altered levels of neurochemicals (dopamine, serotonin, and octopamine) that are associated with depression.
It's also just been reported that bees use logic to find best flowers and check to see if other bees are making the best choices; they do not copy bees who choose bitter tasting flowers. Bees also outperform computers in solving the traveling salesman problem. People are also taking bird art and bee art seriously. These and other fascinating studies show that we need to be very careful making claims that invertebrates do not have emotional lives or feelings. In fact, there are marked similarities with vertebrates.
Honeybees have forty different kinds of neurons, several comparable to humans. The mushroom body, with 20% of the bee’s brain volume, is a unique integrator of multiple senses, relaying sensory information into value-based information. It functions as a combination of the human hippocampus and cortex. One advanced neuron in the mushroom body has been shown to influence cognitive functions meditating reward-based learning. This neuron is similar to the human dopamine neuron, but is in a totally different place and structure. Bees’ brains also show multiple different places for memories working together. The bee brain also uses the advanced feature of inhibitory neurons much the same way as inhibitory interneurons in humans.
The "cognitive maximization hypothesis"
Current research shows that human-like neurons can occur in many other formats than the one for mammals, allowing for cognitive abilities that are very surprising. Higher-level capacities clearly can occur in animals with totally different brain structures than humans. It is certainly possible that the more we study animal brains additional unusual structures will be found.
One of us (MB) is developing what's called "the cognitive maximization hypothesis" that suggests that perhaps small-brained animals maximize the use of what they have -- their neural endowment -- and perhaps they use the relatively little they have more efficiently then do big-brained animals. Big brains in and of themselves don't really matter to the animals themselves and they do just fine with what they have. Future research will be needed to determine if this is so. The claim that humans use about 10% of their brain is a myth so perhaps the efficiency of processing information is a factor to consider. Regardless, small-brained animals do just fine in their own worlds.
Where to from here? What's so good about big brains?
In an essay titled "Are bigger brains better?" researchers Lars Chittka and Jeremy Niven conclude: "Attempts to relate brain size to behaviour and cognition have rarely integrated information from insects with that from vertebrates. Many insects, however, demonstrate that highly differentiated motor repertoires, extensive social structures and cognition are possible with very small brains, emphasising that we need to understand the neural circuits, not just the size of brain regions, which underlie these feats. Neural network analyses show that cognitive features found in insects, such as numerosity, attention and categorisation-like processes, may require only very limited neuron numbers. Thus, brain size may have less of a relationship with behavioural repertoire and cognitive capacity than generally assumed, prompting the question of what large brains are for. Larger brains are, at least partly, a consequence of larger neurons that are necessary in large animals due to basic biophysical constraints. They also contain greater replication of neuronal circuits, adding precision to sensory processes, detail to perception, more parallel processing and enlarged storage capacity. Yet, these advantages are unlikely to produce the qualitative shifts in behaviour that are often assumed to accompany increased brain size. Instead, modularity and interconnectivity may be more important."
Big brains and high EQ's may be useful for those animals who need them to be card-carrying members of their species, but small-brained animals do very well as long as they can do what they need to do to survive and thrive in their own worlds. The notion that small-brained animals are "less intelligent" than big-brained animals and "suffer less" also needs to be revisited as it's surely a myth.
The rapidly growing fields of comparative cognitive neuroscience and cognitive ethology (the comparative study of animal minds) continue to provide exciting information about the brains and incredibly active minds of the fascinating animals with whom we share our planet. What an exciting future lies ahead.
Note: This essay was written with Jon Lieff M. D., a neuropsychiatrist specializing in the interface of medicine, neurology and psychiatry. He has spent three decades exploring the mind and how it functions in humans, animals, and nature. He is a graduate of Yale College and Harvard Medical School, and a past president of the American Association for Geriatric Psychiatry. His blog is Searching for the Mind, twitter @jonlieffmd, and Facebook, “Searching for the Mind.” We both favor the development of non-invasive research methods.