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
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.