The difference in mind between man and the higher animals…is one of degree and not of kind.
—Charles Darwin, 1882


Intelligence Has Evolved Independently Many Times in Animalia

With the 'cognitive toolkit' as our criterion for intelligence, it appears that cephalopod invertebrates demonstrate some degree of intelligence (Finn et al. 2009; Mather 2004). Among the vertebrates, mammals and birds are more intelligent than others. Corvids and parrots are considered the most intelligent birds; primates and cetaceans the most intelligent mammals. Among primates, apes are more intelligent than monkeys and monkeys are more intelligent than prosimians. Among apes, chimpanzees, bonobos and humans appear to be particularly intelligent. Since intelligence has evolved independently in vertebrates and invertebrates, indifferent classes of vertebrates, in different orders of the same class, and in different families of the same order, an orthogenetic view of the evolution of intelligence (i.e., one that places humans at the top of an unilinear scale of ascending intelligences) is effectively ruled out (Roth & Dicke 2005).

Why Did Intelligence Evolve in the First Place?

Intelligence appears to evolve when animals cannot solve the problems posed by their social or ecological environment by innate responses or trial-and-error learning. When an environment can change quickly, hard-wired behaviors such as these can become
maladaptive and flexibility is favored. However, having a brain capable of such flexibility is quite expensive metabolically --the human brain, for example, consumes around 20% of total metabolism (Roth & Dicke 2005). Only environments that are complex and unpredictable could favor the costly strategy of intelligence (Godfrey-smith 2001; Sterelny 2003).

Ecological Diversity, Adversity

Corvids are found throughout the world today (Madge & Burn 1994). The Corvus species (crows and ravens), in particular, survive in some of the harshest environments on earth, from the extreme cold of Alaska and Siberia, to the extreme heat of the Sahara and Mojave deserts. Given their present environmental variability, corvids could easily have faced circumstances similar to those of the great apes in their evolutionary past. After all, birds and mammals both have relatively recent evolutionary histories; the modern form of both creatures appeared around 65 million years ago, just after the cretaceous-tertiary extinction event that marked the end of the Mesozoic era. A single avian species (Archeopteryx) is thought to have survived this event, and so is the presumed progenitor of all modern bird species. The order Passeriformes is the most recently evolved of all, appearing in the fossil record around 37.5 million years ago. Compare this to the first anthropoid primates, which appeared around 40 million years ago. Not only is their timing correlated, but birds and mammals, too, exhibit similar rates of anatomical evolution (Wyles et al. 1983).

The above is a phylogenetic tree of my own making (that is, I added all of the text and filled in the blanks after piecing together several bare trees) showing the phyletic position of the genera within the family Corvidae (far right), branching from superorder Passerimorphae (to save space), branching from class Aves. The relationships depicted our fine for the purposes of this page, but this tree should not be taken as phyletic truth.
Taxonomic information taken from the Tree of Life project and from Taxonomy in Flux by John Boyd III.


In Europe, the oldest corvid fossils are 20-25 million years old (Goodwin 1986), but corvids have been traced to originate in Central Asia and Malaysia (Hope 1989). Jays have become specialized in feeding on nuts and acorns and live in forested environments similar to primitive corvids; they are therefore probably more related to early species. Magpies and True Crows (crows, rooks, and ravens), in contrast, have moved away from forests into more open environments and their diets have shifted from granivory to omnivory, suggesting that these birds are the most recently evolved of the corvids. Interestingly, the most recently evolved genera of corvids (Corvus) and apes (Pan) appeared at approximately the same point in evolutionary time (5-10 million years ago). This period of time, the Late Miocene on into the Pliocene epoch, is known for its environmental and climatic variability and instability which would have significantly impacted the availability of food. These ecological conditions have been implicated in the evolution of great ape cognition (Potts 2004), and it is easy to put forward a similar argument for corvids. Given such variable environmental conditions, more complex and innovative foraging behaviors such the ability to locate spatiotemporally dispersed food and extract food from hard-to-reach places would have been adaptive. These conditions would have also favored an generalist-omnivorous foraging strategy, marked by the dietary inclusion of many new foods including energy-rich meat. As shown on other pages, corvids and apes display highly innovative foraging strategies, and this correlates well with greater relative brain size (Lefebvre et al. 1997; Reader & Laland 2002).

Social Complexity and Cognitive Implications

It is known that the family Corvidae displays a very broad range of social organization (dos Anjos et al. 2009). For example, rooks can nest in colonies of hundred of pairs, and winter roosts can host tens of thousands of individuals (Clayton & Emery 2007). In a recent study on social organization in New Caledonian crows (known for their exceptional tool use; see mechanism), it was determined that, contrary to previous thought, group size is a poor measure of social intelligence and complexity, particularly in birds. Instead Holzhaider et al. (2010) found that social-network size, or the number of individuals having social relationships with each other (even antagonistic ones), seems to be a better predictor. In their study, the social relationships of New Caledonian crows were only of high quality among immediate family. This has been observed in ravens as well (Fraser & Bugnyar 2010). For these reasons, they suggest that high-quality relationships among small family units may have contributed to the cognitive evolution of certain species of genus Corvus, like New Caledonian crows and ravens. This genus is thought to contain the most cognitively advanced of corvid species. It might also go some way toward explaining the impressive tool use in their particular study species, as close familial association allows for vertical transmission of such cultural behavior. The authors summed up the study by saying that "small social networks, extended parental care and high-quality social relationships restricted to immediate family are possible social factors associated with NC crows' impressive tool skills and the evolution of their cognitive abilities. The social system of NC crows thus provides important prerequisites for the cumulative technological evolution of the pandanus tool designs."

Intelligence in corvids and great apes is likely to have arisen through convergent evolution, because the most recent common ancestor of birds and mammals lived around 280 million years ago. Also, the structure of the brain is very different between them
(see mechanism). Foraging and Social factors linked to increased brain size, increased neocortex size, or supposed cognition are: spatiotemporally dispersed food and caching, dietary diversity, innovation, complex foraging and tool use, large group size (or perhaps more importantly, social network size), alliance formation, social facilitation of feeding, social learning of foraging techniques. The table below, from (Seed et al. 2009) compares chimpanzees and corvids in each of these respects. The table and the sources are their own, and are reproduced here without modification.

Note that the authors of the table recognize differences between corvids and chimpanzees in the tool use category because New Caledonian crows are the only species observed that routinely uses tools in the wild. Several other corvid species not observed to use tools in the wild have demonstrated the ability to do so in captivity, which in a way is even more remarkable (see mechanism). The authors of the table conclude that "living in a complex social and physical environment creates both challenges and opportunities, and the pressure to respond to all or some of these may have selected for large brains and complex cognition in both apes and corvids."

While not exclusive and relatively simplified in scope, this comparison with chimpanzees suggests that the evolution of intelligence was highly correlated with the ability to think and act flexibly in a protean environment. In such environments, climate was incredibly variable; food was patchy and disperse, often only ripe for brief periods, or had to be pursued; and increases in the size and complexity of social groups containing many long-lived individuals required the ability to track social relationships. These are all thought to be reasons why complex cognition evolved.


This website was created by Nathaniel Raley for Suzy Renn's Animal Behavior course (BIO 342) at Reed College, Fall 2010