Different allometric intercepts in major Aegus chelifer chelifer stag beetle males from urban and forest habitats

Abstract

Many insects, including stag beetles, possess exaggerated structures and these structures usually grow disproportionately with their body size. Allometry, i.e. the scaling relationship between such traits and other body parts (used as proxies of body size in a species), can provide valuable information about development and evolution. We examined males of Aegus chelifer chelifer, a stag beetle from forest and urban habitats in Thailand to clarify the allometric relationship between weapon (mandible) and body (elytra) size. The relationship between the log-transformed mandible and elytra length was non-linear and best fitted the piecewise linear model. Moreover, this model revealed the existence of dimorphism in males that could be divided into minor and major morphs based on their mandible size, in broad agreement with the morph classification based on mandible shape. Both morphs from the two populations (urban and forest) exhibited positive allometry, and the allometric slope declined in the major morph. Comparison between populations showed the allometric slopes were similar, but the intercepts were significantly different in the major morphs. The genetic or environmental basis should be further explored for the two morphs as well as any behavioural variation.