Sexual size dimorphism and male reproductive traits vary across populations of a tropical rainforest dung beetle species (Onthophagus babirussa)

Abstract

Sexual size dimorphism (SSD) arises when natural selection and sexual selection act differently on males and females. Male‐biased SSD is rarer in insects and usually indicates strong sexual selection pressure on male body size in a species. Patterns of SSD can also vary between populations of species that are exposed to different environmental conditions, such as differing resource availability and diversity. Here, we investigate intraspecific variation in SSD as well as relative investment in precopulatory (horn length) and postcopulatory traits (sperm length and testes weight) in a tropical rainforest dung beetle Onthophagus babirussa across Singapore and Peninsular Malaysia. Overall, three out of four populations displayed significant male‐biased SSD, and SSD was greater in populations with smaller overall body size. Average male body size was similar across all populations while female body size was significantly smaller in Singapore, suggesting that the pronounced SSD may also be due to stronger sexual selection on male body size in Singapore populations. All populations showed significant investment in horns as a weapon likely used in male‐male competition, while postcopulatory traits showed no clear scaling relationship with body size, suggesting a higher priority on precopulatory sexual traits in the mating system of this species.

Evolutionary trade-offs between testes size and parenting in Neotropical glassfrogs

“…Whereas the relationship between male-biased SSD and testes size shifts from positive to negative across taxa with an increasing prevalence of female monopolization [ 2 ], paternal care and the expression of sexual weaponry have been positively associated within some taxa (e.g. anurans [ 63 ], fishes [ 64 ], insects [ 65 ]). Hence, we also tested if testes size, paternal care and humeral spine in glassfrogs were associated with SSD, another trait often responding to pre-mating sexual selection.…”