Directional selection on body size but no apparent survival cost to being large in New Zealand giraffe weevils

Rebecca Le Grice *1, Gabriela Tezanos-Pinto 2, Pierre de Villemereuil 1, Greg Holwell 1, Christina Painting 1

1 The University of Auckland
2 Massey University

Eligible for student prize

To reproduce is the ultimate aim for an individual during their lifespan. When an individual’s reproductive success relies on winning fights to secure mating opportunities, larger body size and weaponry are typically advantageous. However, sexual selection can be extremely complex, and over an animal’s life history the opportunity to reproduce is influenced by many different elements including environmental conditions, competition, and lifespan. In this study we investigated a wild population of giraffe weevils (Lasiorhynchus barbicornis) which exhibit enormous intra and intersexual size variation. In addition, males bear an elongated rostrum used as a weapon to fight other males for access to females. However, males also employ alternative reproductive tactics where smaller males will choose to try and mate with females using sneaking behaviour rather than fighting. We investigated sexual selection in a wild population by tracking individual males and females daily over two 30-day periods to measure long-term mating success. Using capture-mark-recapture analyses we also assessed how both survival and recapture probabilities vary with sex and body size for giraffe weevils using longitudinal datasets collected over three breeding seasons at coarse (weekly) and fine (daily) sampling intervals. Finally, we considered whether there was any evidence for size assortative mating. Our overall findings provide evidence for direction selection on body size in both sexes. Most interestingly, we found no apparent survival trade-off to greater body size. Larger males mate more often and have a higher survival probability, suggesting an accumulation of mating success benefits to bigger individuals. Finally, we found evidence of size assortative mating. All males choose to mate with bigger and probably more fecund females, but larger and more competitive males mate with larger females more often, furthering their potential reproductive success.

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