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Keywords:

  • competition;
  • exaggerated trait;
  • reverse sexual size dimorphism;
  • seasonal abundance

Abstract

Both the sex ratio and the extent of sexual size dimorphism (SSD) can have important implications for the mating system of a species. In particular, the sex that is limited (generally the female) is often the subject of intense competition, which can drive changes in behaviour and morphology to increase the competitive ability of the more abundant sex. The New Zealand giraffe weevil, Lasiorhynchus barbicornis (Fabricius, 1775), is a large and charismatic species found across much of New Zealand, but very little is known about its natural history. We determined the population structure and ecology of L. barbicornis from a single wild population over 3 years. L. barbicornis was found to be highly abundant in the warmer months, between November and May, but showed a peak in abundance in February and March in each year. The sex ratio was consistently male-biased throughout the breeding season (mean sex ratio between seasons 0.59 ± 0.02). Univariate and multivariate analyses showed that L. barbicornis is highly sexually size-dimorphic mostly because of the extreme elongation of the rostrum of the male. All body parts of the males were also found to be more variable in size than those of the females, particularly head length, as shown by higher coefficients of variation. There was, however, no significant seasonal variation in SSD. These data are crucial to gain an understanding of the ecology and of factors driving the mating system in this species. The male-biased sex ratio and SSD are key components of L. barbicornis ecology and are likely to have driven the highly competitive, defence-based mating system observed.