External Measures of Cognition
Among mammals, the most extensive neurobiological data available are overall brain sizes. The size of the brain, relative to body mass, is often. As might be expected, brain size tends to vary weight ratio (or brain-to-body height), which is. The relationship between brain size and intelligence, both amongst brains than humans, and we have about the same brain-to-body mass ratio as mice. eagerly sought to correlate MRI measures of brain volume with IQ.
It is possible that the end of the Ice Age had an effect on the diet, muscle mass, or endocrine system of humans, which decreased brain size. Larger cranial volume is associated with climatic region, the largest averages being found in populations of Siberia and the Arctic.
Brain Size Takes an Evolutionary Backseat to Body Size | BioScience | Oxford Academic
For this reason, Beals et al. There is a general pattern in which neural development peaks in childhood and declines in adolescence, a process known as synaptic pruning. Overall white matter volume does not appear to decline with age, although there is variation among brain regions. From the study Changes in brain weights during the span of human life. The average brain weight in adult males is 1, grams; in adult females, 1, grams.
There was high variability among individuals, however. However, such differences should not be construed as imparting any sort of functional advantage or disadvantage[ citation needed ]; gross structural measures may not reflect functionally relevant factors such as neuronal connectivity and receptor density[ citation needed ].
Brain-to-body mass ratio
Moreover, brain volumes, even in narrowly defined groups e. In later decades, men show greater volume loss in whole brain volume and in the frontal lobesand temporal lobeswhereas in women there is increased volume loss in the hippocampi and parietal lobes.
Race and intelligence In the 19th century, American anthropologist Samuel George Morton reported that whites had the greatest average cranial capacity, followed, in descending order, by Native Americans and blacks. Stephen Jay Gould argued in  and in his subsequent book the Mismeasure of Man that Morton unconsciously misrepresented his data and that when it was properly interpreted, it showed no significant racial differences in cranial capacity.
This claim was criticized in a paper, which concluded that Morton did not manipulate his results, unconsciously or otherwise. Philippe Rushton published multiple studies in the s to s claiming that average brain size was lowest in blacks Negroids and highest in East Asians Mongoloidswith whites Caucasoids in between the two.
In addition, lateral ventricle volume appears to be mainly explained by environmental factors, suggesting such factors also play a role in the surrounding brain tissue.
Early studies yielded suggestive candidate genes.
It is however not clear if the correlation is causal. While consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampus, and the cerebellum, unique variation in these regions account for a relatively small amount of variance in IQ.
Testing this hypothesis requires comparative measures of both neurobiology and exercise capacity. Among mammals, the most extensive neurobiological data available are overall brain sizes. The size of the brain, relative to body mass, is often considered a determining factor in cognitive abilities  — behavioral flexibility and intelligence  — .
Examining the association between exercise and total brain size is therefore ecologically and evolutionarily relevant. However, there are limitations to examining overall brain size, rather than the evolution of specific brain components . Increases in brain size are the result of changes in the size of different brain components, and examinations of total brain size do not take into account the fact that selection on brain components may differ substantially .
Despite this downside to the use of total brain size, it remains the only neurobiological variable for which we have a substantial mammalian database. Additionally, overall brain size may be more appropriate in this study, since we are not concerned with correlations between specific cognitive functions and behavior, but with a more general correlation between physical activity and neurobiology.
In addition to a measure of neurobiology, we require an index of athleticism and exercise frequency across a wide range of mammals.
MMR is the maximum level of oxygen consumed by an individual during exercise before anaerobic metabolism begins supplying energy . Because MMR sets the upper aerobic limit for exercise in organisms, it is related to an individual's capacity for aerobically supported activity and overall aerobic scope and is correlated with an individual's total amount of sub-maximal aerobic activity .
Relationship between Exercise Capacity and Brain Size in Mammals
Thus, relatively high MMRs in mammals are likely tied to relatively high levels of aerobic activity. Based on the relationship between MMR and exercise levels, we predict that relatively high MMRs are associated with relatively large brain sizes.
Support for this prediction would link neurobiology and exercise capacity across mammals for the first time.
Methods Brain masses and MMRs were collected for 29 mammalian species from data compiled by Isler and van Schaik  and Weibel et al. The sample size is necessarily small given the difficulty of collecting MMR data that are reliable across taxa.
- Brain size
- Does brain size matter?
- Relationship between Exercise Capacity and Brain Size in Mammals
For each run in these studies, speed was held constant, and was varied only between runs . MMR was determined as the level of oxygen consumption when a further increase in work output e.
We excluded the calf and pony from Weibel et al. Table 1 Brain mass and MMR data.