Results
The mating duration of the treatment males was significantly affected by
the cues of rival presence to which males were exposed
(X2=15.61, df=2, p=0.00041; Figure 1a). Males exposed
to the full repertoire of rival cues (+ all; p=0.0047) and those that
had been exposed to rivals with the auditory cue removed (+ no sound;
p=0.00078) both significantly extended mating duration relative to males
that had not encountered rivals. This is consistent with previous
research showing that removing one cue signalling the presence of
competitors does not impede a male’s ability to respond by significantly
increasing mating duration.
Latency to mate was significantly predicted by treatment
(X2=5.92, df=2, p=0.052; Figure 1b). Males exposed to
a rival with all cues intact (+ all) took significantly longer to start
mating than males kept alone (- all; p=0.041). Males exposed to a rival
with the auditory cue removed (+ no sound) demonstrated an intermediate
mating latency, which did not significantly differ from either of the
other treatment groups.
Following mating with the treatment males, females were allowed to
oviposit for 24 h before remating. This allowed us to quantify the
reproductive success of the treatment males in the absence of sperm
competition before remating. A Kruskal-Wallis test showed that the
number of offspring produced was significantly affected by rival cues to
which males were exposed (X2=11.00, df=2, p=0.0041).
However, as the offspring count data were zero-inflated, a hurdle model
was then used in which zeroes and non-zeroes were separated and
modelled. Neither the number of zeros (X2=4.50, df=2,
p=0.11), nor the non-zero offspring counts (X2=3.67,
df=2, p=0.16) were significantly predicted by treatment. Contrary to
predictions, the distribution of the data suggested an overall pattern
of males who had not been exposed to rivals (- all) fathering a higher
number of offspring than males exposed to rivals, either with all cues
intact (+ all) or the auditory cue removed (+ no sound; Figure 2a).
Females were given the opportunity to remate to an Sb male 24 h
after their first matings, in order to assess the reproductive success
of the first-mating treatment males under sperm competition. The
proportion of females that remated was low across treatments (+ no
sound: 38%; + all: 28%; - all: 35%), and was not significantly
affected by the rival cues the focal males were exposed to
(X2=2.38, df=2, p=0.30). Neither latency to remate
(X2=2.76, df=2, p=0.25; Figure S3) nor remating
duration (X2=2.23, df=2, p=0.32; Figure S4) were
predicted by the rival cues to which the first males were exposed.
The proportion of offspring produced in the 24 h following the second
mating that were fathered by the first (focal) male was not
significantly affected by the rival exposure treatment
(X2=2.05, df=2, p=0.36; Figure 2b). This is contrary
to the expectation that males exposed to rivals, either with all cues
intact or with the auditory cue removed, would show equivalent increases
in sperm competitiveness, compared to males that had not encountered
rivals.