• The evolution of large testes: Sperm competition or male mating rate?

      Vahed, Karim; Parker, Darren James; University of Derby (Wiley Blackwell, 2011-12)
      A positive relationship across species between the extent to which females mate with more than one male and relative testes mass has been demonstrated in a wide range of vertebrate taxa and certain insects. At least two hypotheses, which are not necessarily mutually exclusive, could account for this pattern: (1) the numerical sperm competition hypothesis, which assumes that larger testes enable the male to transfer more sperm to each female, giving the male an advantage in sperm competition and (2) the male mating rate hypothesis, which proposes that larger testes allow the male to produce a greater number of (potentially smaller) ejaculates to engage in frequent copulations with different females. Of these hypotheses, the former has won broad acceptance, while the latter has tended to be dismissed. Here, we argue that the lines of evidence commonly used to support the numerical sperm competition hypothesis in favour of the male mating rate hypothesis are not as clear cut or generally applicable as they are purported to be and that, consequently, the male mating rate hypothesis cannot be excluded with confidence on the basis of the current evidence. Furthermore, some evidence, such as the finding that ejaculate mass and/or sperm number is negatively correlated with testes mass across species in some insects, and that larger testes in Drosophila can evolve in response to an increase in the number of females available for mating in the laboratory, provides support for the male mating rate hypothesis. Further work is needed to disentangle the relative effects of these selective pressures on the evolution of testes size.
    • The intensity of pre- and post-copulatory mate guarding in relation to spermatophore transfer in the cricket Gryllus bimaculatus

      Parker, Darren James; Vahed, Karim; University of Derby (Springer, 2009-07-22)
      While post-copulatory mate guarding has been well documented in field crickets (Orthoptera: Gryllidae), the occurrence of pre-copulatory mate guarding in this family has been largely overlooked. We examined the relationship between the intensity of two components of mate guarding (body judders and antennal whips) and the time before and after copulation. We found that when male Gryllus bimaculatus encounter a female but do not have a spermatophore ready to transfer, they engage in pre-copulatory mate guarding that is very similar to post-copulatory mate guarding. The intensity of pre-copulatory mate guarding increased up to the point at which the male was ready to transfer his spermatophore. Following copulation, the intensity of mate guarding initially remained high before declining, after which it began to increase again just before the male resumed courtship stridulation. We interpret this pattern of post-copulatory mate guarding as being consistent with both the ejaculate-protection and spermatophore-renewal hypotheses for the function of mate guarding. We found no significant relationship between mate guarding intensity and male body mass.
    • Larger testes are associated with a higher level of polyandry, but a smaller ejaculate volume, across bushcricket species (Tettigoniidae)

      Vahed, Karim; Parker, Darren James; Gilbert, James D. J.; University of Derby (The Royal Society, 2010-11-10)
      While early models of ejaculate allocation predicted that both relative testes and ejaculate size should increase with sperm competition intensity across species, recent models predict that ejaculate size may actually decrease as testes size and sperm competition intensity increase, owing to the confounding effect of potential male mating rate. A recent study demonstrated that ejaculate volume decreased in relation to increased polyandry across bushcricket species, but testes mass was not measured. Here, we recorded testis mass for 21 bushcricket species, while ejaculate ( ampulla) mass, nuptial gift mass, sperm number and polyandry data were largely obtained from the literature. Using phylogenetic-comparative analyses, we found that testis mass increased with the degree of polyandry, but decreased with increasing ejaculate mass. We found no significant relationship between testis mass and either sperm number or nuptial gift mass. While these results are consistent with recent models of ejaculate allocation, they could alternatively be driven by substances in the ejaculate that affect the degree of polyandry and/or by a trade-off between resources spent on testes mass versus non-sperm components of the ejaculate.
    • Paternity analysis of wild-caught females shows that sperm package size and placement influence fertilization success in the bushcricket Pholidoptera griseoaptera

      Parker, Darren James; Zaborowska, Julia; Ritchie, Michael Gordon; Vahed, Karim; University of Lausanne; University of St Andrews; University of Derby; Centre for Biological Diversity; University of St Andrews; St Andrews KY16 9TH UK; Centre for Biological Diversity; University of St Andrews; St Andrews KY16 9TH UK; Centre for Biological Diversity; University of St Andrews; St Andrews KY16 9TH UK; et al. (Wiley, 2017-04-07)
      In species where females store sperm, males may try to influence paternity by the strategic placement of sperm within the female’s sperm storage organ. Sperm may be mixed or layered in storage organs and this can influence sperm use beyond a ‘fair raffle’. In some insects, sperm from different matings is packaged into discrete packets (spermatodoses) which retain their integrity in the female’s sperm storage organ (spermatheca), but little is known about how these may influence patterns of sperm use under natural mating conditions in wild populations. We examined the effect of the size and position of spermatodoses within the spermatheca and number of competing ejaculates on sperm use in female Dark bushcrickets (Pholidoptera griseoaptera) that had mated under unmanipulated field conditions. Females were collected near the end of the mating season and seven hypervariable microsatellite loci were used to assign paternity of eggs laid in the laboratory. Females contained a median of 3 spermatodoses (range 1-6) and only 6 of the 36 females contained more than one spermatodose of the same genotype. Both the size and relative placement of the spermatodoses within the spermatheca had a significant effect on paternity, with a bias against smaller spermatodoses and those further from the single entrance/exit of the spermatheca. A higher number of competing males reduced the chances of siring offspring for each male. Hence both spermatodose size and relative placement in the spermatheca influence paternity.