Sexual selection impacts brain anatomy in frogs and toads

Abstract

Natural selection is a major force in the evolution of vertebrate brain size, but the role of sexual selection in brain size evolution remains enigmatic. At least two opposing schools of thought predict a relationship between sexual selection and brain size. Sexual selection should facilitate the evolution of larger brains because better cognitive abilities may aid the competition for mates. However, it may also restrict brain size evolution due to energetic trade-offs between brain tissue and sexually selected traits. Here, we examined the patterns of selection on brain size and brain anatomy in male anurans (frogs and toads), a group where the strength of sexual selection differs markedly among species, using a phylogenetically controlled generalized least-squared (PGLS) regression analyses. The analysis revealed that in 43 Chinese anuran species, neither mating system, nor type of courtship, or testes mass was significantly associated with relative brain size. While none of those factors related to the relative size of olfactory nerves, optic tecta, telencephalon, and cerebellum, the olfactory bulbs were relatively larger in monogamous species and those using calls during courtship. Our findings support the mosaic model of brain evolution and suggest that while the investigated aspects of sexual selection do not seem to play a prominent role in the evolution of brain size of anurans, they do impact their brain anatomy.

Keywords: PGLS; anuran; brain anatomy; brain size evolution; comparative analysis; courtship types; mating system; testes mass.

Figure 1

Dorsal, ventral, and lateral views of frog brain. Shown are the measures (length, width, and height) that were taken from each of the five brain structures (viz. olfactory nerves, olfactory bulbs, telencephalon, optic tectum, and cerebellum)

Figure 2

The phylogenetic tree of the 43 anurans species used in the comparative analysis following Pyron and Wiens (2011). Also see Liao et al. (2015)

Figure 3

Scaling of the total brain size as functions of body size across 43 anuran species

Figure 4

Differences in mean relative bulbus olfactorius size as a function of mating system across 43 anurans species using data corrected for phylogenetic effects. The plotted values refer to residuals from regression of bulbus olfactorius size on body size

Figure 5

The mean relative bulbus olfactorius size in courtship types in 43 anurans species when correcting phylogenetic effects. The plotted values refer to residuals from regression of bulbus olfactorius size on body size