Investigating the levels of polyandry and sperm competition in the New Zealand giraffe weevil (Lasiorhynchus barbicornis)

Abstract

When females mate multiple times (polyandry), they lay the foundations for post-copulatory sexual selection via sperm competition and cryptic female choice. Studying polyandry has great importance for understanding potential post-copulatory selection as it is an evolutionary driver of male behaviour and morphology. Similarly, understanding sperm utilization in relation to sperm precedence and how this affects paternity is a critical part of determining how sexual selection operates. Despite this, surprisingly few studies have documented rates of polyandry in the wild, often instead utilizing laboratory experiments which may underestimate true levels of polyandry. The New Zealand giraffe weevil (Lasiorhynchus barbicornis) is an ideal species for wild observation due to their unique biology and tendency to aggregate in one place. Here I used five polymorphic microsatellite markers in combination with wild field observations of female mating frequency and order to quantify the rates of polyandry and explore sperm utilization in the New Zealand giraffe weevil. Microsatellite markers were used to genotype the sperm stored inside the female’s reproductive organ as a count of the mating frequency (Chapter 3), and the eggs, the mother and candidate fathers to decipher paternity (Chapter 4). Genetic analysis was then compared to mating frequency and mating order observed in the field. Field observations showed 95% of females mated more than once, and genetic analysis showed 90% of females mated more than once. Polyandry was not recorded to increase with female body size in either field observations or genetic analysis, however fecundity attributes including the number of eggs, size of eggs and size of spermatheca, did increase with female body size. From the paternity analysis, the last male to mate sired offspring 50% of the times, however, the males that fertilized the other 50% of eggs did not mate with the female during the mating observations (i.e. they had mated with females prior to observations). This study therefore suggests that a complex pattern of sperm utilization is occurring in this species as evidence for both sperm mixing and last male sperm precedence was presented. This masters thesis has provided critical insight into the mating behaviours and post-copulatory mechanisms acting in this species, suggesting interesting implications for sexually selected traits.