Sexual selection uncouples the evolution of brain and body size in pinnipeds

Abstract

The size of the vertebrate brain is shaped by a variety of selective forces. Although larger brains (correcting for body size) are thought to confer fitness advantages, energetic limitations of this costly organ may lead to trade-offs, for example as recently suggested between sexual traits and neural tissue. Here, we examine the patterns of selection on male and female brain size in pinnipeds, a group where the strength of sexual selection differs markedly among species and between the sexes. Relative brain size was negatively associated with the intensity of sexual selection in males but not females. However, analyses of the rates of body and brain size evolution showed that this apparent trade-off between sexual selection and brain mass is driven by selection for increasing body mass rather than by an actual reduction in male brain size. Our results suggest that sexual selection has important effects on the allometric relationships of neural development.

Comparative analyses of evolutionary rates reveal different pathways to encephalization in bats, carnivorans, and primates

“…Pinnipeds are rarely included in analyses of carnivoran brain evolution. This extensive analysis suggests that the brain-body relationship in pinnipeds is driven much more by body size evolution than in their terrestrial and arboreal relatives, which may also reflect the extreme sexual dimorphism in body size observed in some species (35). Our results suggest that for most carnivorans, changes in relative brain size reflect, in large part, body size evolution rather than selection for neuronal capacity.…”

“…Our results suggest that for most carnivorans, changes in relative brain size reflect, in large part, body size evolution rather than selection for neuronal capacity. It has been similarly suggested that changes in body size may drive life history evolution in carnivorans (35,36), and the framework presented here will allow for rigorous testing of this and other hypotheses concerning the mechanisms underlying macroevolutionary trends. Whether the inclusion of fossils adds valuable information to macroevolutionary studies of brain and body size evolution or not remains a moot point.…”

Increased juvenile predation is not associated with evolved differences in adult brain size in Trinidadian killifish (Rivulus hartii)

“…These same regressions were nearly identical for males between the two food treatments (Figure 4). These regressions also indicate that the significant differences in relative brain size between food levels and the significant “sex × food” interaction are not likely due to increased canalization of brain size versus body size (see Fitzpatrick et al., 2012). The food treatments include fish that exhibit a similar range of variation in size, and the overall differences in body size between high‐ and low‐food levels for males and females are nearly identical (average ln male body size (g): HF = −1.28, LF = −1.52; average ln female body size (g): HF = −0.18, LF = −0.42).…”

The Evolution of Relative Brain Size in Marsupials Is Energetically Constrained but Not Driven by Behavioral Complexity

“…Recent success in using brain components to understand the evolution of brain macromorphology [Sakai et al, 2011;Swanson et al, 2012;Gómez-Robles et al, 2014] and particular behaviors [de Winter and Oxnard, 2001], together with the realization that cell density and connectivity differ substantially among mammals [HerculanoHouzel et al, 2007[HerculanoHouzel et al, , 2011Sarko et al, 2009;Huang et al, 2014], also suggest that overall brain size may be too vague a predictor of cognitive ability. There is mounting evidence that relatively large brains often arise through selection for a smaller body size, rather than larger brain size [Fitzpatrick et al, 2012;Smaers et al, 2012;Swanson et al, 2012], so that in many cases no cognitive selectors of large brain size may exist.…”