NUTRITIONAL EFFECTS ON MORPHOLOGICAL CHARACTERISTICS AND MALE-MALE COMPETITION OF STAG BEETLE Aegus chelifer chelifer MacLeay, 1819

Abstract

Stag beetles have great intraspecific variation in their bodies and weapon sizes, which are strongly influenced by the environment during the larval stage. However, the impacts of environmental factors affect on such variation and the links between environmental factors, morphological characteristics and behaviours are still unclear. This study first surveyed stag beetles discovered in natural habitat of a dry-evergreen forest in Chanthaburi province, Thailand, to roughly examine possible factors relating to growth of stag beetles. The occurrence and numbers of stag beetle larvae found in logs were high in those of a moderate decay class with relatively high nitrogen content and fungal biomass. For laboratory experiments, Aegus chelifer chelifer MacLeay, 1819 was used as the representative species for this study. Allometric relationship between elytra and mandible length of male beetles best fitted to the piecewise linear model and revealed the existence of dimorphism in males of this species. Narrow-sense heritabilities of mandible length and adult body size of stag beetles from captive breeding were not detected. However, comparisons of beetles between Bangkok metropolitan population and Chanthaburi population showed significant differences in body size and allometry. Testing for the effects of diet indicated that diet quantity, nitrogen content and fermentation of sawdust-based diet had significant effects on larval performances and adult body size. Furthermore, rearing of larvae with sawdust infested by wood-decaying fungi showed negative effects on the larval growth. For behavioural study, larger males showed a higher chance to be winners from male-male combat and pronotum width was the best predictor for the fighting outcome. Quantity of diets had more important effects on body size and intraspecific outcomes than diet quality in A. chelifer chelifer, and possibly in other stag beetles or other saproxylic insects with high male-male competition.