Spider sperm competition : the conduit/cul-de-sac hypothesis : a route to understanding or a dead end?
This thesis is an evaluation of the hypothesis that the spennathecae of spiders affects the sperm precedence patterns in a predictable way (Austad 1984). Spermathecae come in two varieties: cul-de-sac and conduit. Cul-de-sac spennathecae, according to the hypothesis, are supposed to lead to second male sperm priority and conduit to first male sperm priority . The hypothesis was evaluated both directly and indirectly. Direct measurements were made of paternity in two species, Pholcus phalangioides and Tetragnatha montana, both of which are cul-de-sac species. It was found that P. phalangioides complies with the predicted precedence pattern and thus does not disprove the hypothesis. This second male priority pattern was despite a much shorter mating time by second mating males. In T. montana no precedence pattern was found, with equal likelihood of first or second mating males of gaining paternity. There was in T. montana a possible influence of the duration of mating affecting the precedence pattern, with longer mating males gaining a higher paternity no matter what order they mated in. It is discussed whether or not this is due to sperm loading or genitalic stimulation (Eberhard 1985). Indirect evaluation of the hypothesis included an analysis of mating behaviour in Zygiella x-notata which is a conduit species and was chosen as a comparison to the two cul-de-sac species. In Z. x-notata it was found that there was no difference between mating duration in first and second mating males. Mating persistence is thus the same in first and second mating males, suggesting that the males cannot detect that the female is a denuded resource to second mating males. Hence first male priority may not be a factor in this species. Other indirect methods of evaluating the hypothesis involved charting the incidence of mate-guarding and mating-plugs. The expected pattern of mate-guarding was for conduit species to pre-mate guard and for cul-de-sac species to post-mate guard, because of the predicted sperm precedence patterns associated with the spermathecae. The predicted pattern was not found. In the case of mating-plugs it was predicted that these should be deployed by cul-de-sac species because it is in these species that second males are able to usurp paternity to a large extent. The opposite pattern was found with mating-plugs of various design being utilized by conduit species. It is postulated that mating-plugs are the mechanism by which first male priorities are established in conduit species, where this pattern is found. The absence of plugs in cul-de-sac species is possibly the reason that second males can cuckold. The additional data collected since 1984 reveal that patterns of paternity found in spiders seem to be more complex than was originally assumed by Austad (1984). Spermathecae are species-specific in character and this may reflect a species specificity in sperm precedence patterns. Thus the conduit I cul-de-sac dichotomy may not reflect a useful prediction of paternity patterns.