Beyond brain size: Uncovering the neural correlates of behavioral and cognitive specialization

Abstract

Despite prolonged interest in comparing brain size and behavioral proxies of "intelligence" across taxa, the adaptive and cognitive significance of brain size variation remains elusive. Central to this problem is the continued focus on hominid cognition as a benchmark and the assumption that behavioral complexity has a simple relationship with brain size. Although comparative studies of brain size have been criticized for not reflecting how evolution actually operates, and for producing spurious, inconsistent results, the causes of these limitations have received little discussion. We show how these issues arise from implicit assumptions about what brain size measures and how it correlates with behavioral and cognitive traits. We explore how inconsistencies can arise through heterogeneity in evolutionary trajectories and selection pressures on neuroanatomy or neurophysiology across taxa. We examine how interference from ecological and life history variables complicates interpretations of brain-behavior correlations and point out how this problem is exacerbated by the limitations of brain and cognitive measures. These considerations, and the diversity of brain morphologies and behavioral capacities, suggest that comparative brain-behavior research can make greater progress by focusing on specific neuroanatomical and behavioral traits within relevant ecological and evolutionary contexts. We suggest that a synergistic combination of the "bottom-up" approach of classical neuroethology and the "top-down" approach of comparative biology/psychology within closely related but behaviorally diverse clades can limit the effects of heterogeneity, interference, and noise. We argue that this shift away from broad-scale analyses of superficial phenotypes will provide deeper, more robust insights into brain evolution.

Hominin Brain Evolution: The Only Way Is Up?

“…Reduced shattering is an important domestication trait across numerous crops as it eases harvest and increases yield. Previous work identified selection on OsSH1 and SH4, loci related to shattering in Asian rice, within O. glaberrima [10][11][12], but this signature was not found in the present study. Instead, SH5, a gene that is part of the same shattering pathway, appears to have been targeted in O. glaberrima; however, SH5 does not show signs of selection in Asian rice, suggesting domestication was not entirely in parallel across these crops.…”

Wild and laboratory exposure to cues of predation risk increases relative brain mass in male guppies

“…Some authors have been critical of studies of whole brain size (e.g., Chittka & Niven, ; Healy & Rowe, ; Logan et al., ). We agree that a more granular examination of specific brain regions, and other subtler aspects of neuroanatomy and neural organization, as well as the costs and benefits of brain enlargement, would add essential information to our understanding of neural investment in guppies.…”

“…The majority of studies that examine the evolution of brain size have made use of cross‐species comparisons; however, these analyses can be complicated by phylogenetic relationships and unaccounted for ecological or life‐history factors (Harris, O'Connell, & Hofmann, ; Healy & Rowe, ; Logan et al., ). Intraspecific studies across populations are valuable as they can partially control for some of the potentially confounding variables that inherently complicate the interpretation of interspecies comparisons (Gonda et al., ; Logan et al., ). Leveraging natural variation in ecological conditions among populations represents a powerful approach to the study of brain evolution (Walsh et al., ).…”

Heterochrony in chimpanzee and bonobo spatial memory development

“…However, this line of work primarily uses brain size as a proxy for cognitive skills. Brain size is only a rough index of more specific cognitive abilities (Healy & Rowe, ; Logan et al, ), and there is clear evidence that closely related species may have similar overall brain size but nonetheless exhibit major differences in cognition and behavior (Hare, Wobber, & Wrangam, ; Maclean et al, ; Rilling et al, ; Rosati, ). Consequently, comparisons of cognitive development across species are crucial to test whether life history characteristics shape cognitive development.…”