But what is the truth of the matter?
Let's start with the actual abstract of the article published in the Proceedings of the National Academy of Sciences of the United States of America:
Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality
by Susanne Shultz and Robin Dunbar
Institute of Cognitive and Evolutionary Anthropology, University of Oxford, United Kingdom
Evolutionary encephalization, or increasing brain size relative to body size, is assumed to be a general phenomenon in mammals. However, despite extensive evidence for variation in both absolute and relative brain size in extant species, there have been no explicit tests of patterns of brain size change over evolutionary time. Instead, allometric relationships between brain size and body size have been used as a proxy for evolutionary change, despite the validity of this approach being widely questioned. Here we relate brain size to appearance time for 511 fossil and extant mammalian species to test for temporal changes in relative brain size over time. We show that there is wide variation across groups in encephalization slopes across groups and that encephalization is not universal in mammals. We also find that temporal changes in brain size are not associated with allometric relationships between brain and body size. Furthermore, encephalization trends are associated with sociality in extant species. These findings test a major underlying assumption about the pattern and process of mammalian brain evolution and highlight the role sociality may play in driving the evolution of large brains.
Now, let's analyze this.
The Oxford University web site clarifies the fact that the research team "examined the growth rates of the brain size relative to body size to see if there were any changes in the proportions over time. The growth rates of each mammal group were compared with other mammal groups to see what patterns emerged."
Thus, the study perpetuates the concept of measuring intelligence by "brain size relative to body size". My question is, how in the world would the relative sizes correspond to actual intelligence? How is that supposed to mean anything? Does it mean that a tall or fat human is not as intelligent as a short or thin human?
(If you want a physical measure to "prove" intelligence, what about the fact that cats have 300 million neurons in their cerebral cortex [the "thinking" part of the brain] and dogs have only 160 million neurons? Not that I'm about to support the idea that actual intelligence derives from such physical measures, but scientists do it all the time, so maybe one of them could try to explain why "brain size relative to body size" could be a better measure of intelligence than the number of actual neurons in the brain.)
This study also tracks relative increases in brain size in various species on an evolutionary time scale. At Care2.com, Jake Richardson makes a good point:
For this study, the focus on physical brain measurements depends upon the assumption that an increase in brain size on a evolutionary time scale indicates dogs are smarter than cats. However, there is another assumption involved — that dogs and cats started off with equal intelligence, and dogs increased. Even if dogs and cats millions of years ago had the same size brains, does that mean they were of equal intelligence? It’s possible cats were smarter then, and still smarter now, but have smaller, more efficient brains. What is more likely given that evolution seems to reward creatures with adaptability and diverse skill sets, is that dog and cat intelligences are different, and comparing them is much more complex than simply measuring physical brain size.
There's another factor that needs to be questioned in this study. The study correlates increase in brain size over centuries with "sociality", which is defined in a somewhat circular manner as 'The state or quality of being sociable'. The trouble here is double: how do you define 'being sociable' in terms of behavior in different species, and how do you objectively measure it?
There is no question that dogs have social interactions in ways that are compatible with human perceptions. Cats behave differently, and people are societally trained to believe that cats are aloof, independent, and loners. But careful observation of cats will reveal that they are very sociable animals. Any veterinarian should be able to tell you that in multi-cat households, the cats form their own societal structure. They can be very affectionate toward each other as well as toward humans. I have observed cats comfort a sick cat, and mourn the death of another cat. I have two cats who will each bring me a toy to initiate a game of fetch or some other game.
No one will ever convince me that cats are not social animals, because that would require me to deny years of experience. Just as no one will ever convince me that intelligence is dependent on brain size.
I'm with Mr. Richardson on this one: each species has its own unique way of expressing its intelligence. Unfortunately, the mass media like the easy headline. It's a shame the easy headline has to originate from an institution of higher learning.