Behavioral Ecology Vol. 12 No. 5: 547-552
© 2001 International Society for Behavioral Ecology
Sexual cannibalism and sperm competition in the golden orb-web spider Nephila plumipes (Araneoidea): female and male perspectives
a Department of Population Biology, Zoological Institute, University Mainz, D-55099, Germany b Department of Zoology, University of Melbourne, Parkville, Victoria 3052, Australia
Address correspondence to J.M. Schneider. E-mail: jutta{at}oekologie.biologie.uni-mainz.de .
Received 5 January 2000; revised 29 June 2000; accepted 12 October 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Mating systems are frequently shaped by conflicts over reproductive interests between males and females. Sexual cannibalism can be an especially dramatic manifestation of such conflicts. However, the resolutions of this conflict differ among sexually cannibalistic spider species. Cannibalism may be in the interest of both sexes when females consume males as a foraging decision to improve fecundity and/or males sacrifice their bodies to increase fertilization success. In other species, females exert sequential choice of partner by selectively terminating copulation through cannibalism while males fail to obtain a paternity advantage. Here, we investigate the adaptive value of cannibalism in the orb-web spider Nephila plumipes where 60% of males do not survive copulation. Virgin females in poor condition are more frequently cannibalistic and more likely to kill large males, but the frequency of cannibalism among mated females is not influenced by these factors. Instead, males that mate with mated females increase their fertilization success by being cannibalized. Cannibalized males generally mate for longer, but longer copulations correspond with increased paternity only in mated females. The amount of sperm from particular males that a female stored was not influenced by any of the measured variables. The number of sperm stored was not related to paternity, nor was there any detectable reduction in sperm number after females had reproduced. Our data suggest that the conflict between the sexes differs between virgin and mated females. Females should always cannibalize a male, but males only gain from cannibalism when mating with mated females, not when mating with virgin females. Interestingly, the frequencies of cannibalism are not different in matings with virgin or mated females.
Key words: cannibalism, foraging, mating, paternity advantage.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Mating systems are frequently characterized by conflicts of interest between the sexes, most especially over the frequency of mating. Multiple mating clearly increases the reproductive success of males (Andersson, 1984
;
Bateman, 1948), but several
benefits of multiple mating to females have been recently identified. These
include sequential female choice (e.g.,
Elgar et al., 2000;
Tregenza and Wedell, 1998),
and producing clutches with genetically more variable and/or viable offspring
(e.g., Shykoff and Schmidt-Hempel,
1991; Watson,
1998; see also Møller,
1998). However, polygamy can create a conflict of interest if the
ensuing sperm competition reduces the fertilization success of rival males. An
extraordinarily diversity of male adaptations has evolved in order to prevent
the sperm of rival males from fertilizing the eggs of the female
(Birkhead and Møller,
1998), and these adaptations may act as selective pressures
favoring both male and female counter-adaptations.
Sexual cannibalism, in which the female cannibalizes a potential or actual
mating partner at around the time of copulation
(Elgar, 1992) represents a
particularly dramatic manifestation of sexual conflict. The conflict is
straightforward if cannibalism occurs before insemination; the female may
obtain nutritional benefits, but the male forfeits future reproductive
opportunities (e.g., Elgar and Nash,
1988). However, there are both conflicts and congruences of
interest between the sexes when cannibalism occurs after the male has
inseminated the female (Andrade,
1998; Elgar,
1998). For example, female and male interests may be similar if
his cannibalized soma increases female fecundity, and this paternal investment
compensates for any loss of future reproductive opportunities
(Buskirk et al., 1984).
However, a conflict arises if the female mates with another male and the male
shares paternity with this rival male
(Elgar, 1998).
This conflict appears to be resolved in favor of both sexes in the redback
spider Latrodectus hasselti
(Andrade, 1996). Experiments
involving double matings showed that cannibalized males had a greater
fertilization success than males that survived, in part because cannibalized
males copulated for longer and also because cannibalistic females were less
likely to mate again. Thus, while female L. hasselti may improve
their fecundity by eating their mates, they may lose complete control over the
paternity of their offspring if males can induce females to be unreceptive.
However, this pattern may not be general for all spiders if sexual cannibalism
allows the female to control the duration of copulation; female Argiope
keyserlingi delay cannibalizing relatively smaller males, and these males
copulate for longer and fertilize relatively more of her clutch
(Elgar et al., 2000).
Clearly, any analysis of the adaptive significance of sexual cannibalism
must be made from both the female and male perspective. Sexual cannibalism may
increase female fecundity, and thus females in poor condition may attempt to
cannibalize males more vigorously than females in good condition
(Andrade, 1998;
Newman and Elgar, 1991).
Additionally, larger males may be more attractive prey items than smaller
males, and hence more frequently cannibalized. If sexual cannibalism allows
the female to control the paternity of her clutch, then there should be an
association between cannibalism, copulation duration, and paternity. The
benefits of sexual cannibalism to the male depend upon his future mating
opportunities, which are expected to be low. A paternal investment benefit
requires that either the female mates once only, or that the cannibalized male
fertilizes most of her eggs. Thus, cannibalized males should have a higher
paternity than noncannibalized males. Alternatively, sexual cannibalism may be
against the reproductive interest of the male, and thus he will always attempt
to escape.
The golden orb-web spider Nephila plumipes frequently cannibalizes
males both before and during copulation
(Elgar and Fahey, 1996). This
spider, like others in the genus (Elgar,
1991), is highly size-dimorphic, with males less than 5% of the
body weight of mature, fecund females. The spider is common along the eastern
coastal seaboard of Australia, and usually abundant where it occurs. N.
plumipes are found in large aggregations, with as many as 10 webs that
either share structural threads or are found within 20 cm of another web
(Elgar, 1989). The webs of
adult and sub-adult females may also contain several mature males
(Elgar, 1989;
Elgar and Fahey, 1996;
Robinson and Robinson, 1980).
Here we explore the adaptive significance of sexual cannibalism by conducting
staged mating experiments with males and females of known reproductive
status.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Sub-adult female and both sub-adult and adult male N. plumipes were collected in January and March 1998 from a large single population located in the mangroves at Towra Point, Botany Bay, Australia. Most of the females were housed in separate perspex frames (100 cm x 75 cm x 20 cm), where they built typical orb webs; the remaining females were kept in up-turned plastic cups (1000 ml). The females were watered and fed around 10 bushflies Lucilia cuprina on each of 3 days per week. Females were measured and weighed shortly after they matured and again after they had oviposited for the first time. We used callipers to measure the total body length and the width of the cephalothorax across the dorsal eyes. The female was immobilized by covering her with plastic film. Males were collected as adults from the webs of females or as sub-adults from their own webs, which were mostly found near the trunk of mangrove trees. In the laboratory, males were maintained in individual cups (250 ml) on a diet of Drosophila. Under a dissecting microscope, males were carefully inspected for species-specific traits and the status of their pedipalps; body length of each male was measured to the nearest 0.5 mm. Most of the males were weighed shortly afterwards.
Patterns of paternity were determined using standard double-mating trials
(Parker, 1970); mature males
were randomly assigned to either normal (N) or irradiated (I) treatments;
males in the latter were irradiated with a dosage of 10 krad from a
cobalt-
-emitter. The proportion of developed eggs was then used to
calculate P2, the number of eggs fertilized by the second male.
Females were randomly assigned to one of four categories, which varied the
order and composition of the two kinds of male mating partners. Thus, each
female was provided with either: (a) a normal male and then an irradiated male
(NI); (b) an irradiated male and then a normal male (IN); (c) two irradiated
males (II) (which controls for the sterilization success); or (d) two normal
males (NN) (which controls for the number of undeveloped eggs in a normal
clutch). The second male was always placed with the female on the day that the
first mating had occurred. However, not every female remated the same day, and
it was sometimes necessary to exchange the male or to repeatedly introduce him
to the web for a few days.
Matings were staged by gently placing a male in the lower corner of the
frame, using a small paintbrush. Typically, the male walked up the side of the
frame, eventually encountering one of the support threads of the orb web. He
then traversed the web to the hub, where he would wait on the opposite side of
the female. We noted when the male reached the edge of the web and the hub.
Males rarely moved from this location unless the female captured a prey item
(Elgar and Fahey, 1996), and
so we threw several bushflies into the web. Shortly after the female had
captured a fly, and sometimes while she returned to the hub, the male would
jump onto her body, run over her a few times and then insert his pedipalp. At
that point we started to time the duration of copulation, and checked which
pedipalp was inserted into which opening of the female. We recorded the time
when either the male removed his pedipalp and jumped off the female, or when
the female caught and wrapped him. Some males inserted repeatedly either the
same or the other pedipalp and we stopped and restarted timing the duration of
copulation accordingly. Repeated insertions happened four times. Typically, a
few initial brief insertions precede the one, probably significant,
copulation. Insertions were interrupted by pauses of less than 5 s. Males that
were captured by the female were immediately removed before the female could
feed on them.
Mated females were transferred to separate cups, where they were watered
and fed around ten bushflies L. cuprina on each of 3 days per week.
Another unmated female was then placed in the vacant frame. The mated female
laid an egg sac about 35 days later, and this was removed and placed in a
separate sterile plastic container that was closed with cotton wool. The vials
containing the egg sacs were placed in a large open basin of water in a
controlled temperature room (25°C). Eggs hatched 1 month later and were
preserved in alcohol. The hatchlings and undeveloped eggs were subsequently
counted under the microscope. A number of females died either before they had
produced eggs or shortly after they had made their first egg sac (containing
eggs). These females were dissected, and we extracted and counted the sperm
from each spermatheca, following Bukowski and Christenson
(1997). The causes of death are
unknown. Most females were found dead in the early morning and were
immediately preserved. Since this may not be a random sample of the
population, we cannot exclude the possibility of drawing our data from a
biased sample.
Data were visually and statistically inspected for normality and transformed where appropriate. We used nonparametric tests for those data for which we could not obtain normal distributions. The data were analyzed using JMP 3.2.2 (SAS Institute) for the Macintosh. Means are given ± standard error, unless specified otherwise. Sample sizes can be different because not all measurements were taken from all spiders.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We observed a total of 149 matings of N. plumipes, of which 79 were with a virgin female and 70 were the second mating of that female. Males were captured and cannibalized in 56% of the matings with virgin females and in 61% of the matings with mated females (
2 = 0.5, p
>.47).
Cannibalism and body size
Virgin females that cannibalized males were significantly smaller than
females that did not attack males (female body mass, t67 =
3.005, p <.004; female body length, t67 =
2.61, p <.012; female prosoma: t67 = 2.27,
p <.027; Table 1).
However, there was no significant difference in the size of cannibalistic or
noncannibalistic females mating with a second male (female body mass,
t60 = 1.207, p >.2; female body length,
t60 = 0.89, p >.3; female prosoma,
t60 = 0.94, p >.3).
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Females were weighed at or shortly after the day of maturation (median = 1
day, upper and lower quartile: 0-3). Most females were mated with their first
male within the first 2 weeks after maturation (median = 7, upper and lower
quartile: 4-13). The second male was introduced on the same day but not all
mated that day so that the median interval between first and second mating is
1 day (upper and lower quartile: 0-6). The cephalothorax of spiders is a
sclerotized body part that does not change after the final molt. Thus, the
mass of a female relative to her cephalothorax width can be used as an
estimate of her condition when the relationship is linear, which is the case
for our range of data. Here we can only estimate condition at the time of
maturation. Cannibalistic virgin females matured in a poorer condition than
noncannibalistic females (t67 = 2.82, p <.007)
but there was no difference in mated females (t60 = 0.98,
p >.3). Each female received the same quantity of food in regular
intervals and they were also fed during mating trials. Thus, females that
received their second male long after their first male may have improved their
condition such that its effect on cannibalism was masked. Therefore, we
analyzed the relationship between the interval between first and second mating
on the likelihood of cannibalism of the second male (2 = 0.09,
p >.7), and the interval between maturation and first male
encounter on the likelihood of cannibalism of virgin females (Kruskal-Wallis
Test, 2 = 0.06, p >.8). If current condition is
more important than condition at maturity, we would have expected a reduction
in the rate of cannibalism with increasing time.
The size of the male also influenced the outcome of matings. Males that were cannibalized by virgin females were larger than males that avoided cannibalism (male body length, t67 = -3.265, p <.002; male body mass, t39 = -2.415, p <.022; Table 1). However, there was no significant difference in the body size of cannibalized and noncannibalized males that mated with mated females (male body length, t62 = -1.463, p >.15; male body mass, t38 = -0.397, p >.6).
Cannibalism and paternity
The double mating experiments revealed that N. plumipes has a
mixed paternity pattern with a high degree of variation in P2, the
proportion of eggs fertilized by the second male. P2 ranged from
zero to one, with a median of 0.42 and a mean of 0.46 ± 0.05, which was
not significantly different from 0.5 (t = -0.81, p >.4,
n = 33). The distribution was not significantly different from normal
(Shapiro-Wilk Test, W = 0.94, p =.08).
P2 values, analyzed separately for both treatments, revealed a
median of 0.37 (mean = 0.37 ± 0.08) for 16 IN females and a median of
0.43 (mean = 0.54 ± 0.07) for 17 NI females (t31 =
1.55, p >.13). P2 values were corrected for the 72.8%
hatching success in NN controls and 1.17% success in II controls, using the
formula suggested by Boorman and Parker
(1976). Cannibalism by virgin
females had no effect on P2 (F1,31 = 0.56,
p >.4). Males that were killed by virgin females sired a mean
proportion of 0.51 (± 0.07) of the eggs in the first clutch, while
males that survived sired 0.60 (± 0.09) of the eggs
(Figure 1). However,
cannibalism by mated females had a dramatic influence on paternity; males that
were cannibalized by mated females sired 0.56 (± 0.065) of the clutch,
compared with only 0.29 (± 0.081) for males that survived mating
(Figure 1). Two-way analysis of
variance revealed that P2 was significantly influenced by
cannibalism of the second male (F3,29 = 5.60, p
<.025) but not the first male (F3,29 = 0.24, p
>.6). The interaction term was not significant (F3,29 =
0.01, p >.9).
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Although we introduced a second male into the frame the same day after
mating had taken place, the time between first and second matings varied
between 0 and 15 days. This is because not every female immediately remated,
and some females did not remate for several days. Females that cannibalized
their first male were not more reluctant to remate than noncannibalistic
females (Kruskal-Wallis Test, 2 = 0.67, p >.4,
n = 49). However, P2 increased with the time interval
between mating (rs =.44, p <.011). Analysis of
covariance revealed that cannibalism of the second male
(F3,29 = 7.3, p <.012) still explained a
significant proportion of the variance in P2
(r2 =.28) when corrected for the mating interval
(F3,29 = 4.2, p <.049). However, the results
of this multiple analysis may not be reliable because we could not obtain a
normal distribution for the mating interval. Finally, P2 was not
influenced by the size difference between the two males (difference in body
mass, F1,13 = 0.05, p >.8; difference in body
length, F1,32 = 0.86, p >.3).
Copulation duration, paternity, and sperm count
The duration of copulation was measured as the time during which the male
conductor was inserted in the female genital opening. The duration of
copulation ranged between 12 and 298 s (mean = 57.4 ± 7.1, n =
45) for virgin females and 2 and 240 s (mean = 53.0 ± 8.8, n =
38) for mated females. Data for the copulation duration are not normally
distributed so we used log-transformed data for parametric statistics.
Cannibalism prolonged the duration of copulation, independently of whether the female was virgin or mated. Males that were cannibalized by virgin females (n = 26) copulated for a mean of 74 ± 11 s, which was significantly longer than for males that survived (mean = 34.6 ± 3.7 s, n = 19; ANOVA, F1,43 = 18.1, p <.0001). A similar pattern emerged for males that copulated with mated females (cannibalized males, mean = 77.5 ± 17.1 s; noncannibalized males, mean = 33.2 ± 5.1; ANOVA, F1,37 = 6.6, p <.015). P2 was not significantly correlated with the duration of copulation of the first male (rs =.1, n = 26, p >.6), but there was a significant correlation for the second male (rs =.55, n = 21, p <.01, Figure 2). The difference in the duration of copulation for the two matings did not influence P2 (rs =.10, n = 18, p >.6).
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During mating, males usually inserted their pedipalp in only one genital
opening of the female. We were able to relate the number of sperm per
spermatheca to a specific male if only one male used that spermatheca. This
was the case with 16 first males and 13 second males. We could ascribe
unambiguously the content of each spermathecae to either of the two mates for
only 12 females. Data were log-transformed before the use of parametric tests.
There were between 2640 and 48,840 sperm per spermatheca, with no significant
difference in the number of sperm found in the two (paired t test,
t28 = -1.56, p >.13) nor any systematic
difference in the number of sperm in left or right spermathecae
(t56 = -1.2, p >.28). We also found no
difference in the absolute number of sperm stored delivered by first (mean =
14424 ± 2899, n = 16) and second males (mean = 19038 ±
3842, n = 16; Kruskal-Wallis Test 2 = 1.0, p
>.3). However, pair-wise comparisons revealed a significant linear
correlation between the number of sperm stored from the first and second males
(F1,11 = 17.39, p <.002), with consistently
but not significantly more sperm being stored from the second than from the
first male (paired t test: t11 = -2.01,
p =.07).
The number of sperm stored in the spermatheca was not influenced by either
the body mass or the body length of the first male (mass,
F1,8 = 0.66, p >.4; length,
F1,15 = 0.014, p >.9), nor by the mass and
size of the second male (mass, r2 =.21,
F1,9 = 2.1, p >.19; length,
r2 =.38, F1,12 = 3.51, p
<.09). There were no order effects, since there was no difference between
the first and second males in either body mass (paired t82
= -.38, p >.7) or body length (t64 = 0.34,
p >.7). The number of sperm per spermatheca was not significantly
influenced by whether the first (Kruskal-Wallis Test, 2 =
0.63, p >.4) or second (Kruskal-Wallis Test, 2 =
0.34, p >.5) male was cannibalized. Finally, there was no
significant correlation between the number of sperm stored in the spermatheca
and the duration of copulation of the first mating (r2
=.02, F1,14 = 0.005, p >.9) or second mating
(F1,12 = 0.20, p >.6). The difference in the
length of copulation of the first and second male was not significantly
related to the corresponding difference in sperm per spermatheca
(rs =.44, p >.15). The number of sperm per spermatheca
was not influenced by whether the female had produced her clutch of eggs
(Table 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Sexual cannibalism in N. plumipes appears to be a female foraging strategy, and may also represent a male strategy to improve his fertilization success. For polygamous females, the second male sires a larger proportion of the offspring if he is cannibalized. Although cannibalized males copulate for longer than males that escape, they do not necessarily transfer more sperm. Nevertheless, males mating with mated females appeared to transfer more sperm than those mating with virgin females. This suggests that either the male can assess whether a female has mated previously and adjusts the quantity of sperm transferred accordingly, or that the number of sperm stored is under female control.
The evolution of sexual cannibalism should be viewed from the perspective of both males and females, taking into account the possible conflicts and congruences of interests between the sexes. Here we discuss the mating system of Nephila plumipes from first the female and then the male perspective.
Female perspective
Several lines of evidence suggest that virgin females cannibalize males
because of a foraging decision. First, virgin females that mature small and in
poor condition are more likely to cannibalize males than females in good
condition (see also Newman and Elgar,
1991). Second, large males are more often captured than small
males. However, these lines of evidence also raise the question why females
are not always cannibalistic. There are several explanations. Smaller males
may not be attractive prey items. Andrade
(1998) suggested that females
of the redback spider L. hasselti do not always cannibalize their
mates because the nutritional value may not balance the handling costs. This
explanation is unlikely for N. plumipes because Nephila
feeds on a variety of prey items, including those that are small
(Herberstein and Elgar, 1994)
and items that are not immediately ingested are stored in the web for later
consumption (Champion de Crespigny et al.,
2001). Females may spare preferred males, perhaps to encourage
them to mate with them again, or allow them to chase off other suitors. It is
difficult to explain why females would then spare the small males that are
less able to chase off rivals, at least in a related species
(Christenson and Goist, 1979;
Vollrath, 1980). Small males
may be better at evading the female (see also
Elgar and Fahey, 1996),
Alternatively, some females, perhaps the large and fat ones, may be less
rapacious than others; this may be indicated by the connection between
condition at maturation and cannibalistic behavior. Although small females in
poor condition may improve their condition from the day of their last molt to
the day of mating, they may still be more rapacious than females that matured
in a good condition. Size and condition at maturation may reflect habitat
quality and may trigger different foraging strategies.
Interestingly, mated females behave very differently from virgin females.
Mated females cannibalize males with the same frequency as virgin females, but
their success is not related to either their own body parameters or the size
of the male. However, possible effects of body condition may be masked by the
experimental design because females received various prey items during the
mating trials and so they may have been sated by the time the second male was
introduced. This is probably unlikely because Nephila continues
capturing prey indefinitely (see Champion
de Crespigny et al., 2001); if sexual cannibalism was purely
motivated by female hunger state, we would expect the frequency of cannibalism
by mated females to be lower than that of virgins, which is not the case. In
addition, there was no relationship between the likelihood of cannibalism and
the time that had passed between first and second mating. Finally, males
cannibalized by mated females have a higher share of paternity than males that
survived whereas this is not the case for virgin females. Perhaps females
exert sequential choice of partner, although it is not clear which trait forms
the basis of their choice.
Male perspective
The benefit of cannibalism to males depends on the likelihood of sperm
competition. Males that mate with a virgin female and are cannibalized may
increase her fecundity, but the potential for greater male fertilization
success will be reduced if she mates again. Virgin females of N.
plumipes are likely to remate in natural populations because
cannibalistic females are not less likely to remate (cf.
Andrade, 1996). Field surveys
showed that the webs of adult females typically had more than one male in
attendance with some exceeding five males (see also
Elgar, 1989;
Elgar and Fahey, 1996). The
absence of any reproductive benefits strongly suggests that there is no male
complicity in sexual cannibalism by virgin females of N. plumipes.
Instead, sexual cannibalism by virgin females creates an extreme conflict of
interest; a male mating with a virgin female should always attempt to avoid
cannibalism, since this will allow him to either obtain further copulations or
guard her from rival males. The roughly 50% frequency of sexual cannibalism by
virgin females suggests that the conflict is not resolved in favor of either
sex. Perhaps larger males are more reluctant to mate with smaller, virgin
females.
Sexual cannibalism by mated females has a clear effect on male fertilization success. Males that survive copulating with mated females cannot expect to sire more than 30% of the clutch, but this value is doubled if he is captured and eaten. However, the true value of this increase has to be balanced against the probability that he encounters and inseminates other virgin or mated females. It is not possible to comment on the true benefits of cannibalism without these data.
There is no obvious mechanism determining patterns of paternity in N.
plumipes. In the closely related orb-web spider N. edulis,
paternity is strongly correlated with the duration of copulation
(Schneider et al., 2000).
However, while the duration of copulation of cannibalized males was generally
longer than that of males that escaped, this translated into greater paternity
only for males mating with mated females and increased copulation duration did
not result in larger numbers of sperm stored by the female. Furthermore, males
mating with nonvirgins either transferred more sperm or their mates stored
more of their sperm than that of their predecessor. However, greater sperm
numbers did not appear to translate directly into higher paternity. The result
is curious and suggests that either the second male manipulates his rival's
sperm while he is being cannibalized or that the female exerts some form of
sperm choice.
Why are males of N. plumipes unable to influence paternity through
mating for a longer time or depositing more sperm? One explanation is that the
two spermathecae of the female are, in many cases (16 of 27 cases where the
side of insertion of both males was observed), filled separately by each male.
Thus, the sperm of two rival males may not necessarily mix, and hence may not
always compete directly. In this situation, the raffle model of sperm
competition (Parker, 1998) may
not apply. However, the pay-off for male mating strategies may change
dramatically if the female mates with more than two males. In this situation,
the third male will usually deposit sperm in a spermatheca that already has
sperm, and thus copulation duration and numbers of sperm transferred may be
important (see Schneider et al.,
2000). Sperm numbers may not be important for fertilization
success, which is indicated by the quantity of sperm in the spermathecae of
females that had produced a clutch (and thus had used sperm) and those that
had not oviposited. However, it is possible that our data of sperm numbers may
be biased because we only sampled females that died, rather than obtain a
random sample of the population.
Brown (1985) suggested that
sperm in the spermatheca require a number of days to become flagellate and
capable of fertilizing eggs. For example, females of Nephila clavipes
that mated shortly after maturation oviposited sooner than females that mated
10 or more days after maturation. One explanation is that males that mate with
a mated female shortly before oviposition have a small chance of paternity
because their sperm have not had time to capacitate
(Brown, 1985). Our results
contradict this prediction since the paternity of the second male of N.
plumipes increased with the time interval between first and second
copulation.
Sexual cannibalism may have influenced the dramatic sexual dimorphism in
N. plumipes. Males that are not captured by females during mating are
generally smaller, and these males may also be more likely to avoid
cannibalism before mating (see Elgar and
Fahey, 1996). In addition, cannibalism quickly reduces the number
of males ready to mate and may relax male-male competition. As a consequence,
the commonly presumed large size advantage in male-male competition may lose
importance (see also Vollrath and Parker,
1992). However, sexual cannibalism cannot be the only factor
determining size dimorphism, since N. edulis is similarly dimorphic
but sexual cannibalism is rare in comparison to N. plumipes
(Schneider et al., 2000;
Uhl and Vollrath, 1998).
The interests of male and female N. plumipes differ over sexual cannibalism. Females should always consume a mating partner, whereas males should escape, especially after copulating with a virgin female. The conflict changes for copulations with mated females, who may prefer to exert sequential choice, while males may have an advantage through cannibalism. Interestingly, the observed frequency of cannibalism is similar, even though the nature of the conflict may differ.
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ACKNOWLEDGEMENTS |
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We are very grateful to Melissa Thomas for her invaluable help with spider husbandry and data collection and to Nina Wedell for teaching us to count sperm. We wish to thank Mariella Herberstein, Jennifer Maupin, Tom Tregenza, Nina Wedell, and several anonymous referees for discussions and comments on the manuscript.
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REFERENCES |
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