Sex and conflict in the New Zealand giraffe weevil (Lasiorhynchus barbicornis)

Abstract

Sexual selection is responsible for driving a diverse range of exaggerated traits across the animal kingdom, increasing the competitive ability of males that possess them, and consequently improving their reproductive success. Alternative reproductive tactics (ARTs) are expected to evolve when there is the potential for large disparity in reproductive success between groups of males, and when there is intense sexual selection due to competition between males for access to females. This study focused on the ecology and mating system of the New Zealand giraffe weevil, Lasiorhynchus barbicornis (Coleoptera: Brentidae), a species whose ecology and behaviour was largely unknown prior to the research presented herein. The main aim of this study was to determine the factors leading to the evolution of weaponry and alternative reproductive tactics in this species. To address this aim I used a combination of behavioural observations in the field, intensive morphometric sampling at a single site over three years, wider-scale sampling across the North Island of New Zealand, and a range statistical modeling techniques. Firstly, I characterised the sex ratio, seasonal abundance, size variation and sexual dimorphism of L. barbicornis for a single population over three years. The population was found to be consistently male-biased in sex ratio, and both sexes showed high variability in size. There was a high level of sexual dimorphism, mostly due to the overall elongation of the male body, particularly the exaggeration of the male rostrum used as a weapon during male-male contests. A comparison of linear and non-linear regression models showed that the scaling relationship between body size and rostrum length in males was steeply, positively allometric, and was best described by a Weibull growth function. The non-linearity was due to an asymptote in the slope towards the upper end of the distribution, suggesting that rostrum length for the very largest males is under constraint. Importantly, I found no evidence of dimorphism from the scaling relationship, and rostrum length was unimodal in distribution. In comparison, rostrum length in females was negatively allometric and linear in shape. I found evidence of condition-dependent alternative reproductive tactics (ARTs) by conducting focal-male observations in the field. In addition to guarding and fighting behaviour, males less than 40 mm also used a sneaking tactic to gain access to females. Small males were more likely to sneak if the opponent male was larger than them, but would aggressively defend females if the opponent male was the same size or smaller than them. Mating frequency and duration was equal across all male body sizes, suggesting that the ability of small males to adopt a highly flexible set of tactics has allowed them to be as successful as larger males. Furthermore, there was no evidence of sexual selection on body size or rostrum length in males. Focal observations determined the role of the rostrum as a weapon during male-male contests. I characterised the structure of antagonistic contests and correlated contest duration with the absolute and relative size of opponents to determine the competitive assessment strategy of L. barbicornis. Fighting was best described by the sequential assessment model (SAM), indicating that males use mutual assessment of their opponents to determine persistence during contests. Finally, I determined geographical and temporal variation in mean body and weapon (rostrum) size by first measuring variation between 11 populations of L. barbicornis across the North Island of New Zealand, and then focusing on variation within a single population between and within three breeding seasons. The main finding from these studies was that there was significant variation in mean body size, rostrum size, and weapon allometry both within and between populations. Most importantly, I found that while mean body size increased with latitude in accordance with Bergmann‟s rule, weapon allometry in males decreased with latitude. To my knowledge this is the first time weapon allometry has been shown to vary with latitude, and suggests that rostrum length is traded-off with increasing body size across latitude. I also speculate that temporal fluctuations in body and weapon size are due to developmental phenotypic plasticity in response to resource stress and environmental variation. This study has contributed novel findings to the collection of literature on alternative reproductive tactics, competitive assessment behaviour and weapon evolution and variation. Furthermore, I have highlighted the giraffe weevil as a useful model species for behavioural ecological study. This thesis comprised the first major study of L. barbicornis‟ life history, and it provides a strong framework from which to base future single-species or comparative studies of the Brentidae.