Brains vs Brawn: Relative brain size is sexually dimorphic amongst weapon-bearing ruminants.

Here, we investigate the relationship between relative brain size and sexual weapons in ruminants. In most cases, sexual weaponry is heavily male-biased, and costs resulting from growing, maintaining, or wielding weapons are suffered primarily by males. We used phylogenetic comparative analyses to test whether increased investment in sexual weapon size (tusks, antlers, and horns) across four families (Tragulidae, Moschidae, Cervidae, and Bovidae) was associated with decrease in relative brain size and whether the difference in weapon investment relative to conspecific females led to sexual differences in relative brain size. We found no relationship between relative brain size and relative weapon size within males or females, but when we compared males directly to conspecific females, we found that as males invested more in larger weaponry relative to females, they had smaller brain sizes relative to females, regardless of weapon type. Our findings suggest greater investment in some types of elaborate weapons in males, and larger brain sizes in females. We discuss the possibility that larger brain sizes in females could be due to female sociality, lower investment in weaponry, or enhancing female assessment of male traits. Significance statement: We took measurements of antlers, horns, and tusks from skulls of both males and females, as well as brain volume and looked at the relationship between relative weapon size and relative brain size. Our work found support that bearing large, exaggerated sexually selected weapons results in a sexually dimorphic relationship with relative brain size: when males invest more in sexual weaponry, females invest more in relative brain size. Given that most studies largely are focused on tradeoffs solely within one sex, our study compares the relationship between sexes to measure sexual dimorphic investment. The evolution of weaponry in ruminants is one of the most widely studied topics of the last 70 years and this study yields new support for the possible presence of sexual dimorphic trade-offs amongst sexually selected traits. [ABSTRACT FROM AUTHOR]