The relationship between male head size and harem size in the sexually dimorphic mountain stone weta Hemideina maori

Abstract

Abstract 1. Tree weta are a group of large, flightless orthopterans with pronounced sexual dimorphism. Males have enlarged heads that are used in fighting for possession of cavities in trees or under rocks where females shelter during the day.2. The fieldwork reported here examined the relationship between male head size and mating success in Hemideina maori, an alpine tree weta that shelters under rock slabs that have broken off isolated outcrops or tors.3. The relationship between male head size and harem size in H. maori is not as clear‐cut as thought previously. First, overall body size is a better predictor of male mating success than head size per se. Second, both body size and head size explained a relatively low percentage (19.8%) of the overall variation in mating success. Third, despite the intensity of directional selection being estimated to move the frequency distribution of head size and femur size 0.49 and 0.54 standard deviations from the mean, male heads and femurs were ≈ 2 mm smaller at the main study site than at a second site 100 m higher in elevation. A similar pattern was found for adult females. Additional surveys have indicated that body size in H. maori decreases with decreasing altitude, which is correlated with increasing night‐time temperature.4. Although there are reasons why natural selection might favour weta maturing earlier and at smaller body sizes in warmer environments, relatively large males would still have a mating advantage over smaller males under such conditions. This sexually dimorphic alpine insect might be a good example of the trade‐offs and conflicting demands that sexual selection versus natural selection can place on organisms.