It seem reasonable that evolution might select for adaptive behaviors by increasing the relative size of particular brain regions that support those behaviors; for example, bats might have an enlarged auditory cortex since they navigate with echolocation. To some extent this does happen, but such differences are often apparent only after controlling for a much larger source of variance: changes in brain size that correlate with changes in body size - and the implications of this fact are wide-reaching.
As Barbara Finlay and coauthors wrote in this 2001 Behavioral and Brain Sciences article, the order of neurogenesis and ultimate size of various brain regions is highly conserved across all mammals, suggesting natural selection largely works not by selecting for specific "modules" or components in the nervous system as appropriate to a particular organism, but instead "by adjusting the parameters of a 'standard' developmental program."
Finlay et al. first review the history of comparative brain research, citing the well-established fact that brain size scales exponentially with body size. The brain-to-body-mass ratio (also known as the Encephalization Quotient, or EQ) shows some interesting correlations; for example, predators show larger EQs than their prey. Individual brain regions each have a characteristic size "scaling factor" with respect to the size of the rest of the brain; neocortex has the highest scaling factor (also known as "cortical hyperallometry") and the medulla one of the smallest. These relationships appear to explain 95% of the variance in sizes of particular brain regions.
It is thus not surprising that various brain structures tend not to show much variance in size that is related to an animal's behavior. Finlay et al report that most factor analyses reveal two major sources of variance: a general brain size factor and an "olfactory bulb" factor (which is perhaps the only brain region to reliably show behavior-appropriate size variance). On the other hand, it is possible that strong relationships between brain region size and behavior do exist, but are statistically hard to detect.
To investigate this possibility, Finlay et al. used more specific criteria for parcelling up various brain regions (for example, by separating motor, visual and auditory structures). They used measures of 11 brain regions (including cerebellum, hippocampus, neocortex, striatum, olfactory bulb and others) in over 130 mammals, falling into four different taxonomonic groups: 40 insectivores, 43 bats, 21 prosimians, and 27 simians. For every region except for olfactory bulb and medulla, region size was best predicted by a model that included the size of each other region, as compared to models which measures body size, taxonomic group, and all of their interactions.
Finlay et al. also analyzed the duration of neurogenesis and the order of development of each of 51 regions in four species, and discovered that structures with prolonged developmental timelines were the same that showed a disproportionately large scaling factor with respect to other brain regions. Thus not only brain region size, but also developmental timing, appears highly conserved across species.
Although one would expect evolution to select for brain regions that are most directly responsible for adaptive behaviors, it appears that selection operates only at the most gross level in terms of brain size. Later in the article, Finlay et al even argue for "egg before chicken": they suggest that "additional structure preceded enhanced function in hominid brains." Specifically, the authors view neocortex as a "spandrel" which may have evolved merely as a side-effect of other neural adaptations, and was then exapted for its current use.
Stretching a bit, the authors argue further that this could even explain the tens-of-thousands of years that separate the "appearance of anatomically modern people some 100,000 years ago and the advent of unequivocally modern behavior." Sidestepping the somewhat thorny issue of how to define "modern," this work by Finlay et al. goes a long ways towards, this account clearly falls under the "domain-general" perspective of the evolutionary development of higher cognition, including working memory and executive function. How this perspective relates to those advocated by Coolidge & Wynn, for example, will be the subject of future posts.
Don't miss this new paper by Healy and Rowe (doi:10.1098/rspb.2006.3748) in today's Proceedings of the Royal Society B, which is on a very similar topic.
In recent years, there have been over 50 comparative analyses carried out in which social or ecological variables have been used to explain variation in whole brain size, or a part thereof, in a range of vertebrate species. Here, we review this body of work, pointing out that there are a number of substantial problems with some of the assumptions that underpin the hypotheses (e.g. what brain size means), with the data collection and with the ways in which the data are combined in the analyses. These problems are particularly apparent in those analyses in which attempts are made to correlate complex behaviour with parts of the brain that carry out multiple functions. We conclude that now is the time to substantiate these results with data from experimental manipulations.
It's quite an interesting article, and is rather critical of the broad comparisons done between brain size and various behavioral traits. It's also freely accessible to everyone, so take a look.
Hi Chris, have you seen any similar study comparing the ratio (frontal lobes or prefrontal cortex)/ brain volume across species?
Finlay et al's BBS paper has some charts comparing neocortex across species, but I haven't seen anything yet with prefrontal cortex in particular. Based on their argument, prefrontal cortex would change at the same rate as the rest of neocortex, so perhaps that would be a good estimate, at least to start with.
Prefrontal white matter volume is disproportionately larger in humans than in other primates.
* Schoenemann PT,
* Sheehan MJ,
* Glotzer LD.
Department of Anthropology, University of Pennsylvania, 3260 South St., Philadelphia, Pennsylvania 19104-6398, USA. firstname.lastname@example.org
Determining how the human brain differs from nonhuman primate brains is central to understanding human behavioral evolution. There is currently dispute over whether the prefrontal cortex, which mediates evolutionarily interesting behaviors, has increased disproportionately. Using magnetic resonance imaging brain scans from 11 primate species, we measured gray, white and total volumes for both prefrontal and the entire cerebrum on each specimen (n = 46). In relative terms, prefrontal white matter shows the largest difference between human and nonhuman, whereas gray matter shows no significant difference. This suggests that connectional elaboration (as gauged by white matter volume) played a key role in human brain evolution.
here's a nice fat review (pdf) on this subject for free.. i'll try to summarize in a post later.
Amnestic: that's a fascinating paper. The difference may be between a large army composed of drunk generals, lazy gluttons and no discipline/ coordination and a smaller one with great focused strategy and real-time execution.
Looking forward to the summary of the review...
As i recall, the EQ of birds is quite small, considering how smart they really are. Their brain structure differs from ours. There is also evidence in the bird world of brain size to function correlations. For example, male chickadee brains grow larger in the Spring to allow more complicated mating calls. Then, the brains shrink, to conserve energy. Not only does the brain consume energy, but weight makes a big difference for flight.