Whatever You Want: Inconsistent Results Is The Rule, Not The Exception, In The Study Of Primate Brain Evolution (original) (raw)
Related papers
PLOS ONE, 2019
Primate brains differ in size and architecture. Hypotheses to explain this variation are numerous and many tests have been carried out. However, after body size has been accounted for there is little left to explain. The proposed explanatory variables for the residual variation are many and covary, both with each other and with body size. Further, the data sets used in analyses have been small, especially in light of the many proposed predictors. Here we report the complete list of models that results from exhaustively combining six commonly used predictors of brain and neocortex size. This provides an overview of how the output from standard statistical analyses changes when the inclusion of different predictors is altered. By using both the most commonly tested brain data set and the inclusion of new data we show that the choice of included variables fundamentally changes the conclusions as to what drives primate brain evolution. Our analyses thus reveal why studies have had troubles replicating earlier results and instead have come to such different conclusions. Although our results are somewhat disheartening, they highlight the importance of scientific rigor when trying to answer difficult questions. It is our position that there is currently no empirical justification to highlight any particular hypotheses, of those adaptive hypotheses we have examined here, as the main determinant of primate brain evolution.
Understanding primate brain evolution
Philosophical Transactions of The Royal Society B: Biological Sciences, 2007
We present a detailed reanalysis of the comparative brain data for primates, and develop a model using path analysis that seeks to present the coevolution of primate brain (neocortex) and sociality within a broader ecological and life-history framework. We show that body size, basal metabolic rate and life history act as constraints on brain evolution and through this influence the coevolution of neocortex size and group size. However, they do not determine either of these variables, which appear to be locked in a tight coevolutionary system. We show that, within primates, this relationship is specific to the neocortex. Nonetheless, there are important constraints on brain evolution; we use path analysis to show that, in order to evolve a large neocortex, a species must first evolve a large brain to support that neocortex and this in turn requires adjustments in diet (to provide the energy needed) and life history (to allow sufficient time both for brain growth and for 'software' programming). We review a wider literature demonstrating a tight coevolutionary relationship between brain size and sociality in a range of mammalian taxa, but emphasize that the social brain hypothesis is not about the relationship between brain/neocortex size and group size per se; rather, it is about social complexity and we adduce evidence to support this. Finally, we consider the wider issue of how mammalian (and primate) brains evolve in order to localize the social effects.
Significance of Evolutionary Lags in the Primate Brain Size/Body Size Relationship
bioRxiv (Cold Spring Harbor Laboratory), 2024
Although brain size and body size co-evolves in primates, the correlation is far from perfect. This was originally interpreted as implying that evolutionary changes in brain size lag behind evolutionary changes in body size. Subsequent tests of the hypothesis, however, concluded that there is no meaningful lag. I reanalyse the original data taking socio-cognitive grades into account and show that there is, in fact, a very strong lag effect, but that the original "catch-up" hypothesis is not the explanation. Rather, the "lag" is part of an adaptive response to predation risk in which species initially respond by increasing body size, but later switch to increasing group size (with the latter made possible by a correlated increase in brain size). This adaptive response takes between 2 and 8 million years to fully implement, and is dependent on a switch to a more energyrich diet. This trajectory can be clearly documented in the evolutionary history of fossil hominins over the past 5 My.
Evolution of brain size and juvenile periods in primates
Journal of human …, 2006
This paper assesses selective pressures that shaped primate life histories, with particular attention to the evolution of longer juvenile periods and increased brain sizes. We evaluate the effects of social complexity (as indexed by group size) and foraging complexity (as indexed by percent fruit and seeds in the diet) on the length of the juvenile period, brain size, and brain ratios (neocortex and executive brain ratios) while controlling for positive covariance among body size, life span, and home range. Results support strong components of diet, life span, and population density acting on juvenile periods and of home range acting on relative brain sizes. Social-complexity arguments for the evolution of primate intelligence are compelling given strong positive correlations between brain ratios and group size while controlling for potential confounding variables. We conclude that both social and ecological components acting at variable intensities in different primate clades are important for understanding variation in primate life histories.
Brain, Behavior and Evolution
Both absolute and relative brain sizes vary greatly among and within the major vertebrate lineages. Scientists have long debated how larger brains in primates and hominins translate into greater cognitive performance, and in particular how to control for the relationship between the noncognitive functions of the brain and body size. One solution to this problem is to establish the slope of cognitive equivalence, i.e., the line connecting organisms with an identical bauplan but different body sizes. The original approach to estimate this slope through intraspecific regressions was abandoned after it became clear that it generated slopes that were too low by an unknown margin due to estimation error. Here, we revisit this method. We control for the error problem by focusing on highly dimorphic primate species with large sample sizes and fitting a line through the mean values for adult females and males. We obtain the best estimate for the slope of circa 0.27, a value much lower than t...
Brain, behavior and …, 2007
For over a century, various neuroanatomical measures have been employed as assays of cognitive ability in comparative studies. Nevertheless, it is still unclear whether these measures actually correspond to cognitive ability. A recent metaanalysis of cognitive performance of a broad set of primate species has made it possible to provide a quantitative estimate of general cognitive ability across primates. We find that this estimate is not strongly correlated with neuroanatomical measures that statistically control for a possible effect of body size, such as encephalization quotient or brain size residuals. Instead, absolute brain size measures were the best predictors of primate cognitive ability. Moreover, there was no indication that neocortex-based measures were superior to measures based on the whole brain. The results of previous comparative studies on the evolution of intelligence must be reviewed with this conclusion in mind.
Re-evaluating the link between brain size and behavioural ecology in primates
Proceedings of The Royal Society B: Biological Sciences, 2017
Comparative studies have identified a wide range of behavioural and ecological correlates of relative brain size, with results differing between taxonomic groups, and even within them. In primates for example, recent studies contradict one another over whether social or ecological factors are critical. A basic assumption of such studies is that with sufficiently large samples and appropriate analysis, robust correlations indicative of selection pressures on cognition will emerge. We carried out a comprehensive re-examination of correlates of primate brain size using two large comparative datasets and phylogenetic comparative methods. We found evidence in both datasets for associations between brain size and ecological variables (home range size, diet and activity period), but little evidence for an effect of social group size, a correlation which has previously formed the empirical basis of the Social Brain Hypothesis. However, reflecting divergent results in the literature, our results exhibited instability across datasets, even when they were matched for species composition and predictor variables. We identify several potential empirical and theoretical difficulties underlying this instability and suggest that these issues raise doubts about inferring cognitive selection pressures from behavioural correlates of brain size.
Why big brains? A comparison of models for both primate and carnivore brain size evolution
PLoS ONE, 2021
Despite decades of research, much uncertainty remains regarding the selection pressures responsible for brain size variation. Whilst the influential social brain hypothesis once garnered extensive support, more recent studies have failed to find support for a link between brain size and sociality. Instead, it appears there is now substantial evidence suggesting ecology better predicts brain size in both primates and carnivores. Here, different models of brain evolution were tested, and the relative importance of social, ecological, and life-history traits were assessed on both overall encephalisation and specific brain regions. In primates, evidence is found for consistent associations between brain size and ecological factors, particularly diet; however, evidence was also found advocating sociality as a selection pressure driving brain size. In carnivores, evidence suggests ecological variables, most notably home range size, are influencing brain size; whereas, no support is found ...