Behavioral Ecology Vol. 12 No. 6: 732-739
© 2001 International Society for Behavioral Ecology
Effects of multiple mating and male eye span on female reproductive output in the stalk-eyed fly, Cyrtodiopsis dalmanni
The Galton Laboratory, Department of Biology, 4 Stephenson Way, University College London, London NW1 2HE, UK
Address correspondence to A. Pomiankowski. E-mail: ucbhpom{at}ucl.ac.uk .
Received 1 August 2000; revised 16 February 2001; accepted 26 February 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Females of the stalk-eyed fly, Cyrtodiopsis dalmanni, mate repeatedly during their lifetime and exhibit mating preference for males with large eye span. How these mating decisions affect female fitness is not fully understood. In this study, we examined the effects of multiple mating and male eye span on short-term reproductive output in this species. Experiments that manipulated the number of copulations and partners a female received suggested that obtaining a sufficient sperm supply is an important benefit associated with multiple mating. The average percentage of fertile eggs laid by females increased as a function of mating frequency and ranged from 40% for females mated once, to 80% for females mated continuously. In addition, a high proportion of copulations in this species appeared to be unsuccessful. One-third of all females mated once laid less than 10% fertile eggs. There was no significant difference in reproductive performance between females mated to multiple partners and females mated to a single partner. There was also no indication that females received any short-term reproductive benefits from mating with males with large eye span. In fact, females mated to males with short eye span laid a higher percentage of fertile eggs than females mated to large eye span males.
Key words: diopsid, exaggerated morphology, eye span, multiple mating, sexual selection.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Multiple mating by females (polyandry) occurs in many species. We expect that males are commonly under strong selection to mate multiply. But the reasons for female multiple mating are poorly understood, especially as several male mating characteristics have been shown to be harmful to females (Chapman et al., 1995
;
Fowler and Partridge, 1989;
Rowe et al., 1994). Numerous
hypotheses have been presented to explain the adaptive significance of female
multiple mating and have generally been placed into two
categories—direct benefits and genetic benefits (see
Arnqvist and Nilsson, 2000;
Jennions and Petrie, 2000 for
recent reviews). Common direct benefits associated with multiple mating are
nutritional gains derived from male feeding of females, for instance through
nutrient containing spermatophores (Butlin
et al., 1987; Gwynne,
1984; Lamunyon,
1997; Pardo et al.,
1995), and sperm replenishment
(Ridley, 1988;
Walker, 1980) especially in
species with relatively small ejaculates
(Pitnick, 1993;
Pitnick and Markow, 1994).
Several genetic benefits of multiple mating have been proposed but these have
been more difficult to demonstrate experimentally. A few recent studies have
provided experimental evidence that females benefit from multiple mating by
reducing genetic incompatibility between mates
(Newcomer et al., 1999;
Tregenza and Wedell, 1998) or
creating sperm competition that promotes fertilization by genetically superior
males (Madsen et al., 1992;
Olsson et al., 1996).
Alternatively, multiple matings may have no adaptive value for females
resulting instead from incomplete female control over matings
(Ridley, 1990,
Rowe et al., 1994) or as a
correlated response to multiple mating in males
(Halliday and Arnold,
1987).
Regardless of the number of matings a female receives, choice of a specific
mate will also directly impact on female reproductive performance
(Iwasa and Pomiankowski, 1999;
McLain, 1998;
Savalli and Fox, 1998;
Thornhill, 1976). Females of
numerous species exhibit mate preferences for males with exaggerated
ornamental characters (see Andersson,
1994 for review). Many studies examining the adaptive significance
of mate choice have focused on genetic benefits associated with Fisher's
runaway process or good genes
(Pomiankowski, 1987;
Wilkinson et al., 1998b). But
mate choice may also be related to more immediate changes in female
reproductive success. The expression of ornamental characters is often
dependent on male condition (David et al., 1999; Rowe and Locke, 1996;
Wilkinson and Taper, 1999). If
ejaculate quality also depends on condition, then female preference for males
with exaggerated characters may be associated with improved fertility and
fecundity. A few studies have established a link between female preference and
increased fecundity and fertility. In the seed beetle, Stator
limbatus, females prefer to mate with large males and this mating results
in increased fecundity (Savalli and Fox,
1998). In the stink bug, Nezara viridula, females allowed
to choose their mates laid more fertile eggs during their lifetime than those
assigned males randomly (McLain,
1998). However, neither of these studies examined female choice
with respect to a highly exaggerated male trait.
In this study, we examined the effect of variation in mating frequency and
male eye span on short-term female fitness in the stalk eyed fly,
Cyrtodiopsis dalmanni. This species is a useful model system as
females show high rates of multiple mating, typically mating several times
each morning (Wilkinson et al.,
1998a). In addition, male eye span is highly exaggerated and
females prefer to roost and mate with males with large eye span
(Burkhardt and de la Motte,
1988; Hingle et al.,
2001; Wilkinson et al.,
1998a). It has also recently been established that male eye span
is strongly condition dependent, more so than other non-sexual traits, and
there is a genetic component underlying this condition dependence (David et
al., 1998,
2000). Overall, little is
presently known about the fitness consequences of multiple mating or mate
choice in diopsids. Our study examines two aspects of female mating behavior,
one related to mating frequency and one related to mating preference. In the
first part of the study, we manipulated the number of matings a female
received and examined the effects on female fecundity and fertility. Our
experimental design differentiates between the effects of multiple partners
and multiple copulations, similar to the studies of Tregenza and Wedell
(1998) and Newcomer et al.
(1999). This provides a means
for distinguishing direct benefits from genetic benefits. In the second part
of the study, we mated females to males of differing eye span and examined the
effects on fecundity and fertility.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Study animal
Cyrtodiopsis dalmanni is distributed throughout Southeast Asia and is found predominantly at the edge of forest streams. This species forms aggregations at night on roothairs, and males compete with each other for control of these roothairs where the majority of matings occur at dawn and dusk (Wilkinson and Reillo, 1994
). Individuals of C. whitei, a closely related
diopsid species, have been observed to mate over 10 times a day in the field
(Burkhardt et al., 1994;
Lorch et al., 1993) and
similarly high mating rates have been found in C. dalmanni laboratory
populations (Baker et al., unpublished data;
Wilkinson and Reillo, 1994).
In both of these species, copulation duration is relatively short, usually
lasting less than 60 s (Wilkinson and
Reillo, 1994) and sperm transfer is accomplished using a
spermatophore (Kotrba,
1996). The C. dalmanni flies used in the experiments were from a large laboratory colony collected from Gombak, Malaysia in 1993. These flies have been maintained in several population cages of at least 200 individuals per cage with a 1:1 sex ratio. Flies are fed ground corn medium and kept at 25°C, with a 12-h day/night light cycle. Experimental flies were collected as eggs. Emerging adults were segregated by sex and allowed to reach sexual maturity (approximately 3-4 weeks after pupation). All individuals used in this experiment were 6-8 weeks old at the start of each experiment.
The lifespan of C. dalmanni females in the laboratory averages 4 months and can easily reach over 6 months (Reguera P, unpublished data), making it difficult to monitor lifetime fecundity and fertility. Therefore, we chose to examine these variables for a limited period of time, usually 14 days. In addition, because C. dalmanni females have high remating rates, the effect of a single or a few matings is probably most pronounced in the short term.
Effects of multiple mating
Experiment 1
Three experimental groups were set up to investigate how mating alters
female fecundity and fertility. Females were unmated, mated three times, or
mated repeatedly. In the unmated group, virgin females were housed by
themselves for the duration of the experiment. In the second group, virgin
females were allowed to mate once on three consecutive days with the same
male. Each morning, females were transferred to containers with a single male,
observed until a copulation occurred and then moved back to their original
containers. In the third group, a single virgin female was placed with a
virgin male and the pair were kept together for the remainder of the
experiment. The average eye span of the males used in the experiment was 8.861
± 0.046 (SE) mm. The experiment was divided into three blocks spaced a
week apart with 15 females from each group used in each block. Therefore, a
total of 45 females were assigned to each group.
Females were individually housed in circular 400 ml plastic containers (height = 95 mm, diameter = 75 mm) with a roosting string hanging from the top and moist tissue paper and a food tray at the base. Females were placed in their containers a week prior to the beginning of the experiment in order to acclimatize them to the experimental conditions. Females laid eggs primarily on the tissue paper but occasionally on the food. The tissue and food were removed from the containers three times a week and all the eggs on both substrates were counted. Fecundity was recorded for all groups from the day following the final mating of the second treatment and for the next 14 days. In order to determine the fertility of the eggs, the tissue paper was transferred to a petri dish containing a moist cotton pad and stored at 25°C for 5 days. Eggs of C. dalmanni take 2-3 days to hatch and, following this period, hatching success can be assessed by visual inspection of the eggs at 10x magnification. For fertile eggs that have hatched, only the outer shell of the chorion remains, while unhatched eggs appear full with the embryo still inside the chorion. A few eggs showed clear signs of fertilization and development (i.e., segmental striations and early mouthpart formation) but had not yet hatched. These were recorded as fertile. Eggs collected from females in the stock population cages and kept under identical conditions have a hatching success rate greater than 90%, suggesting that nearly all fertilized eggs hatch and that hatching success provides a close approximation of fertility. Eggs laid on the corn could not be assessed for fertility and were not counted. These eggs, however, represent only 12.6% of the total number laid. A few females died during the experiment. Those that survived at least 7 days were included in the final analysis and the fecundity per day was calculated by dividing the total number of eggs laid by the female's lifespan. No males died during the experiment.
Experiment 2
Three experimental groups were set up to differentiate the effects of
multiple partners from multiple copulations on female reproductive output.
Females were mated once (single mated), mated three times to the same male
(triple mated-1 male) or mated once to three different males (triple mated-3
males). Every male in the experiment was mated to a female from each group
over a 3-week period. So, for example, in week one a given male was mated on
the third day to a single mated female, in week two he was mated once on the
first, second, and third days to a female from the triple mated-1 male group,
and, finally, in week three he was mated once on the first, second and third
days to three different females from the triple mated-3 males group. Groups
were set up in a block design over a 3-week period, with 15 virgin females in
each group being mated in each week. This meant that a total of 45 females
were assigned to each experimental group. Using each male in all three groups
has the advantage of controlling for variation in male effects other than
those caused by the different treatments
(Tregenza and Wedell,
1998).
Females were individually housed in 400 ml containers. Tissue paper and food was collected from the containers three times a week and the number of eggs laid on both substrates was recorded. The hatching success of these eggs was determined as described in the previous experiment. The males used in the experiment were not virgins and had an average eye span of 8.880 ± 0.047 mm. Prior to the start of the experiment, males were housed in large cages (20 x 23 x 33 cm) with females. At the start of the experiment males were moved into individual 400 ml containers. They were kept in these throughout the remainder of the experiment except during the time for their assigned matings.
In both experiment 1 and experiment 2, a large number of females had low hatching success. As the experiment was designed to assess the effect of mating on female egg fertility, it was important to remove sterile females from the data set. To establish whether low hatching success was due to female sterility, females with less than 10% hatching success were further investigated after the main experiment had finished. The 10% hatching success cut-off is an arbitrary designation. As there are occasional errors in scoring hatching, 10% was chosen as a conservative limit to ensure that all sterile individuals were identified. Individual females were housed continuously with two new males in a 400 ml container. Eggs were collected three times over the following 7 days and scored for egg fertility. These females were taken to be sterile if their hatching success remained below 10% under these new conditions of constant access to males. Sterile females were excluded from further analysis.
Finally, the contribution of male sterility to variation in female hatching success was investigated. To identify sterile males, any male with less than 10% hatching success in both the single mated and triple mated-1 male groups were further investigated after the main experiment had finished. These males were housed continuously with two virgin females in a 400 ml container. Eggs were collected three times over the following 7 days and scored for hatching success. These males were taken to be sterile if their hatching success remained below 10% under these new conditions of constant access to females.
Effects of male eye span
Two experimental groups were set up to investigate whether mating with
males of a particular eye span altered female egg laying rate or egg
fertility. Females were either mated repeatedly to small or large eye span
males. Small eye span males included flies with an eye span less than 7.5 mm
while large eye span males comprised flies with an eye span greater than 8.5
mm. The means of the two groups were 6.642 ± 0.101 mm and 9.023
± 0.035 mm. Female choice experiments have demonstrated that a
difference in eye span of this order (2.381 mm) is sufficient to cause strong
female preference for the large eye span male in binary choice experiments
(Hingle et al., 2001;
Wilkinson et al., 1998a). The
regression coefficient for eye span on body length is 0.969. Therefore, these
two variables represent equivalent effects in this experiment. Eye span was
measured from the outer edge of each eye and body length from the front of the
face to the tip of the wing. Measurements were made using a monocular
microscope connected to a computer with the NIH Image software package
(version 1.55).
Four virgin females were housed with each male in a 1500 ml (height = 164 mm, diameter = 120 mm) container that included a central roosting string hanging from the top and moist tissue paper and a food tray at the base. As in the previous experiments, tissue paper and food were collected three times a week for 2 weeks. All eggs on both paper and food were recorded and hatching success was calculated as described in experiment 1. The sample size was 40 containers for each male type and the experiment was divided into two blocks with 40 containers in each block.
Males and females were placed together a week prior to the collection of eggs and kept together for the duration of the experiment. Males that died during the experiment were excluded from the final analysis and a small male that was unable to engage his genitalia with females was also excluded. Females that died during the experiment were not replaced, and the egg laying variable was standardized by dividing the total number of eggs laid by the four females by the sum of their life spans.
Statistical analysis
For the multiple mating experiments, fecundity, measured as the number of
eggs laid per day, was compared among groups using a square root
transformation of the data. For the male eye span experiment, fecundity had a
normal distribution and was analyzed without transformation. However, due to a
large number of very low hatching success scores in all experiments, this
variable could not be normalized using standard transformations. Therefore,
nonparametric tests were used to examine differences in percent hatching
success among groups. All tests were calculated using the JMP statistical
package (version 3.2.2, SAS Institute Inc.). Identification of significant
pairwise differences between groups was made using either the Tukey-Kramer HSD
test or Dunn's nonparametric test for multiple comparisons
(Zar, 1996). As percentages
are not reliable indicators when the sample size is very small, any female
that did not lay at least ten eggs over the course of the experiment was
excluded from the analysis. Data are presented as mean ± SE unless
otherwise specified.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Effects of multiple mating
Experiment 1
In the first experiment, females were unmated, mated three times, or mated repeatedly. There was a significant difference in fecundity among blocks (F2,122 = 12.291, p <.001), so a block variable was added as an effect in the ANOVAs, but there was no block effect on hatching success (Kruskal-Wallis Test, H corrected for ties = 3.702, p =.157).
The three groups showed heterogeneity in the mean number of eggs laid per day (ANOVA, F2,120 = 32.021, p <.001). Unmated females had the lowest fecundity and females constantly exposed to a male had the highest fecundity (Figure 1; unmated, 1.507 ± 0.228 eggs, n = 39; mated three times, 3.007 ± 0.288 eggs, n = 45; mated repeatedly, 5.590 ± 0.565 eggs, n = 41). Paired comparisons indicated that all treatments were significantly different from each other (unmated versus mated three times, q = 5.209, p <.005; unmated versus mated repeatedly, q = 13.597, p <.001; mated three times versus mated repeatedly, q = 5.462, p <.005).
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There was also a significant difference between the two mated groups in hatching success (Mann-Whitney U test, U = 423, p <.001), with the group mated repeatedly laying a higher percentage of fertile eggs than the group mated three times (Figure 1; mated three times, 61.9 ± 5.0%, n = 40; mated repeatedly, 81.3 ± 3.2%, n = 39).
Experiment 2
In this experiment the three experimental groups differed with respect to
the number of partners as well as the number of copulations. Females were
mated once (single mated), mated three times to the same male (triple mated-1
male) or mated three times to three different males (triple mated-3 males).
There was a block effect on fecundity (F2,128 = 11.737,
p <.001), so we included block as an effect in the ANOVAs, but
there was no block effect on hatching success (Kruskal-Wallis Test, H
corrected for ties = 5.530, p =.063).
Unlike the previous experiment, there was no difference among the three groups in fecundity (Figure 2; ANOVA, F2,126 = 0.938, p =.394; single mated, 3.121 ± 0.329 eggs, n = 44, triple mated-1 male, 3.007 ± 0.228 eggs, n = 45, triple mated-3 males, 2.594 ± 0.259 eggs, n = 42).
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The hatching success scores for the experiment were characterized by a substantial number of infertile matings. Of the 110 females that laid at least 10 eggs, 19 had hatching success scores less than 10%. To determine if any of these scores were due to female sterility, we placed each of these females with three new males after the experiment had finished. The hatching success of their eggs was then scored for 7 days. By this method, only two out of the 19 females were considered sterile (i.e., continued to produce less than 10% fertile eggs) and were excluded from further analysis.
The experiment revealed that the number of matings a female received affected hatching success (Kruskal-Wallis Test, H corrected for ties = 18.318, p <.001; single mated, 40.0 ± 5.5%, n = 38, triple mated-1 male, 61.9 ± 5.0%, n = 40, triple mated-3 males, 73.4 ± 4.0 %, n = 32). Females that mated only once had a significantly lower percentage of fertile eggs than the two groups of females mated three times, but these multiply mated groups did not differ from each other (Figure 2; single mated versus triple mated-1 male, Q = 2.867, p <.02; single mated versus triple mated-3 males, Q = 4.169, p <.001; mated triple mated-1 male versus triple mated-3 males, Q = 1.479, p >.5). While not significant, the difference in hatching success between the two multiply mated groups (61.9% versus 73.4%) is large enough to suggest a significant effect might be detected if sample sizes were increased. This 11.5% difference, however, is probably higher than the real difference for the flies in our population. The difference between the two groups is only 4% if the number of eggs and number of hatched eggs for each female in a group are summed together prior to calculating hatching success (67.6% versus 72.0%) or if the females laying less than 10 eggs are included in the analysis (58.5% versus 62.4%).
Overall, the pattern of hatching success scores suggested that individual copulations did not provide an adequate supply of sperm for females. The mean hatching success for single mated females was only 40% and the distribution of hatching success for each of the groups (Figure 3) indicated that over one-third (34.2%) of single matings were unsuccessful (i.e., hatching success scores less than 10%). When the number of matings increased to three the number of infertile scores dropped dramatically (12.5% for triple mated-1 male and 3% for triple mated-3 males).
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To examine whether the low female hatching success scores were caused by a large number of sterile males, experimental males with low hatching success (less than 10% in both the single mated and triple mated-1 male groups) were given continuous access to two virgin females and scored for hatching success. Despite the high occurrence of low hatching success among the experimental females, only one male was shown to be sterile (i.e., continued to exhibit hatching success less than 10%). This suggests that the lack of fertility among experimental females arose from unsuccessful copulations rather than male sterility.
Effects of male eye span
In the final experiment, females were mated repeatedly to males with either
small or large eye span. Similar to the previous experiments, there was a
block effect on fecundity (t test: t = 5.579, p <.001),
but not hatching success (Mann-Whitney U, U = 679.5, p
=.9612), so we included block as an effect in the fecundity ANOVA. Male eye
span did not affect female fecundity
(Figure 4; ANOVA,
F1,72 = 0.315, p =.577). There was no significant
difference between females mated to small and large eye span males in the
number of eggs laid per day (small, 4.999 ± 0.330 eggs, n =
34; large, 4.867 ± 0.334 eggs, n = 41). However, male eye span
did affect female hatching success (Figure
4; Mann-Whitney U, U = 474.5, p =.0258). Females
mated to small males had a higher percent hatching success than females mated
to large males (small, 83.3 ± 4.0 %, n = 34; large, 69.1
± 5.4 %, n = 40).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Effects of multiple mating
In this study we showed that additional matings increased the egg laying rate of C. dalmanni. This effect was only evident when there was a substantial difference in the number of matings females received. Females that were allowed unlimited matings produced more eggs than females mated three times and both of these groups laid more eggs than did virgins (Figure 1). However, females mated once had the same fecundity as females mated three times (Figure 2).
While numerous studies have demonstrated that fecundity rates are increased
by male-derived nutrients transferred during copulation
(Butlin et al., 1987;
Gwynne, 1984;
Lamunyon, 1997;
Pardo et al., 1995), the
mating conditions in C. dalmanni are not generally similar to these
other species. The majority of insects that provide nutritional gifts produce
large spermatophores that comprise a substantial proportion of a male's body
weight (Gwynne, 1984;
Savalli and Fox, 1998).
Copulation durations for these insects are usually greater than 30 min and
often take hours (Butlin et al.,
1987; Rutowski et al.,
1987; Taylor et al.,
1998; Taylor and Yuval,
1999). In contrast, C. dalmanni produces a spermatophore
that is smaller than most other diopsids
(Kotrba, 1996) and has
copulations that generally last less than 60 s, suggesting little nutritional
substance is transferred. If C. dalamanni males do provide small
scale nutrient transfers during mating, they could still be important as
females mate many times per day. Such an interpretation is consistent with the
observation of no fecundity difference between females mated once and three
times, but a significant increase in fecundity between females mated three
times and those mated repeatedly (Figure
2).
An alternative interpretation of our fecundity results is that the egg
laying rate increased as a response to chemical or physical stimuli received
during mating. In Drosophila melanogaster, as well as some other
Diptera (see Chapman et al.,
1998 for review), accessory proteins, such as the "sex
peptide," stimulate egg production and increase laying rate
(Chen et al., 1988). Whether
this is happening in C. dalamanni will require further experiments
manipulating the levels of accessory gland products given to female.
The fertility results from our study provided no evidence for genetic
benefits that have been reported in other species
(Newcomer et al., 1999;
Tregenza and Wedell, 1998).
The second experiment was designed to separate the effects of multiple
partners from multiple copulations on female reproduction. Of the three
treatments, one pair differed with respect to the number of copulations (one
versus three) but not the number of partners, while another pair differed with
respect to the number of partners (one versus three) but not the number of
copulations. Genetic effects are expected to occur as a result of an increased
number of partners. Contrary to this prediction, hatching success did not
differ between females mated three times to a single male and females mated
three times to three different males. The significant difference in this
experiment was between the single mated females and the multiply mated females
(both those mated to one male and those mated to three males). It is important
to note, however, that the hatching success measure used in this study
provided only a partial indication of offspring fitness. It is possible that
genetic effects will be evident when larval mortality and pupation success are
also examined.
Our results suggest obtaining a sufficient sperm supply is the primary
reason for the high frequency of female mating in C. dalmanni.
Females in constant contact with a male had significantly higher hatching
success than females mated three times, and in turn females mated three times
had significantly higher hatching success than females mated only once.
Following the work of Bateman
(1948), evolutionary biologists
have generally focused on identifying factors other than sperm-limitation to
explain the evolution of multiple mating in females
(Butlin et al., 1987;
Gwynne, 1984;
Lamunyon, 1997;
Madsen et al., 1992;
Newcomer et al., 1999;
Olsson et al., 1996;
Pardo et al., 1995;
Tregenza and Wedell, 1998).
However, Eberhard (1996) lists
over 25 insect species in which male ejaculate size is smaller than the
storage capacity of females and comprehensive surveys of insects have found
that a single copulation is rarely sufficient to maximize female fecundity
(Ridley, 1988;
Arnqvist and Nilsson, 2000).
C. dalmanni appears to be another example. The exact number of sperm
transferred by males of C. dalmanni is unknown but it is likely to be
small. Its sister taxa, C. whitei, has been estimated to transfer an
average of 90 sperm per mating (Lorch et
al., 1993) and the spermatophores of these two species are of
similar size (Kotrba, 1996).
In any case, it is clear that a single mating rarely results in maximum
fertility. Only 18% of the females mated once had hatching success scores
above 80% (Figure 3), a value
which is the average hatching success for females mated repeatedly to a single
male (Figure 1).
The mating system of C. dalmanni is similar to that found in some
Drosophila species that are distinguished by frequent matings and
small ejaculates (Markow,
1985; Pitnick,
1993; Pitnick and Markow,
1994). One prediction of sperm competition theory
(Parker, 1990) is that
ejaculate size should increase as a function of mating frequency because
larger ejaculates ensure greater fertilization success when fertilization is a
raffle process among sperm from several males. This model, however, assumes
female mating frequency is driving the evolution of ejaculate size and not
vice versa. Mating systems that provide the opportunities for numerous
copulations by a single male may select for males that partition their
ejaculate among many females (Pitnick and Markow, 1984). In C.
dalmanni, the aggregation behavior exhibited by females may have promoted
an evolutionary transition in males toward higher mating frequency with
reduced sperm transfer. Monomorphic species that are phylogenetically
ancestral to C. dalmanni and which do not form mating aggregations,
such as C. quinqueguttata and Teleopsis quadriguttata
(Presgraves et al., 1999),
produce a larger spermatophore (Kotrba,
1996), mate less often (Wilkinson et al., 1998) and for a longer
period of time (Kotrba, 1996)
than C. dalmanni.
Not only do males transfer small amounts of sperm in a given mating, but a
significant proportion of copulations in this species are unsuccessful.
One-third of the females mated once had hatching success scores less than 10%.
Mating systems with a substantial number of unsuccessful copulations have been
found in several other species (Hoogland,
1998; Lamunyon,
2000; Petersson,
1990; Whittier and Shelly,
1993). In C. dalmanni, this may occur for several reasons
including improper spermatophore construction or orientation, female
spermatophore rejection, or extremely low levels of sperm transfer. A study on
C. whitei indicated male mating duration had a bimodal distribution
with only matings longer than 35 s resulting in sperm transfer
(Lorch et al., 1993). All of
the matings in our experiment, however, were longer than 35 s. In the future,
it will be important to determine the proportion of long copulations that
result in proper spermatophore construction and whether certain males are
generally more successful at constructing spermatophores.
The results from this study clearly indicate that, as a result of the combination of small ejaculates and unsuccessful copulations, numerous matings are required by females to maximize fertility. Unfortunately, the mating frequency of females in the field has not been determined so it is unclear if females mate more or less often than is necessary to ensure high fertility rates. When provided with continual access to several males in the lab, females will, on average, mate over four times in a 90 min dawn period (unpublished data). This high mating rate suggests females may copulate beyond the level needed for fertility assurances and additional research, in both the field and the lab, is needed to determine if and when factors other than sperm limitation become important in determining female mating rate.
Finally, another factor that may be important in the evolution of multiple
mating in C. dalmanni is meiotic drive. An X-linked meiotic drive
system that produces a female-biased sex ratio has been found in C.
dalmanni (Presgraves et al.,
1997). Wilkinson et al.
(1998a) point out that, among
three Cyrtodiopsis species, mating rate correlates with the frequency
of the driving X chromosomes. Meiotic drive has been suggested as a factor
promoting the evolution of polyandry, as females can reduce the proportion of
offspring fertilized by sperm carrying the drive locus by mating with several
males (Haig and Bergstrom,
1995). The laboratory population used in our experiment, however,
shows no evidence of biased sex ratios and probably lacks meiotic drive. So
the suggested involvement of meiotic drive as a major component favoring
multiple mating remains to be investigated.
Effects of male eye span
In most insect species there is a positive correlation between body size
and sexually selected fitness components
(Partridge, 1988), and the
mating advantage of males with large eye span in sexually dimorphic diopsid
species has been well documented (Burkhardt and de la Motte,
1988,
1994;
Hingle et al., 2001;
Lorch et al., 1993;
Panhuis and Wilkinson, 1999;
Wilkinson and Reillo, 1994;
Wilkinson et al., 1998a). In
C. dalmanni, females show strong preference for large eye span males
(Hingle et al., 2001;
Wilkinson et al., 1998a).
Recent experiments have shown that male eye span is highly
condition-dependent, and that there is genetic variation underlying this
response as predicted by handicap models of sexual selection (David et al.,
1998,
2000). Selection experiments
indicate that large eye span also provides a signal of male quality by
indicating resistance to meiotic drive
(Wilkinson et al., 1998b). The
possibility of more immediate female fecundity and fertility benefits from
mating with large eye span males has not been investigated before. Results
from this study provide no indication of such benefits. In fact, the small
males used in this study outperformed large males with respect to their
ability to enhance female fertility.
Other studies have found that small males outperform large males in some
situations. In both Drosophila melanogaster
(Pitnick, 1991) and the water
strider, Gerris incognitus,
(Arnqvist et al., 1997) small
males had a higher hatching success than large males and it was suggested that
this results from large males, who have more opportunities for matings,
allocating less sperm per ejaculate. This is unlikely to be a factor in our
study because males were allowed constant contact with females. All of the
males in our experiment were observed to mate but the exact number of matings
during the experiment was not recorded. Under similar experimental conditions
(i.e., one male housed with four females), males have been observed to mate,
on average, nearly four times in a 1.5-h period each morning (unpublished
data). Therefore, over the course of the experiment (including the week long
acclimation period) each female would have received, on average, at least 20
matings. With such a high number of matings, effects due to variation in
ejaculate size are likely to be small relative to the effects of overall sperm
quality or proper spermatophore construction. Studies that control for the
number of matings and directly measure the ejaculate size produced by small
and large males could help identify whether fertility or fecundity differences
exist between these types of males.
It has been suggested that selection for increased competitive ability of
sperm may create characteristics that cause weaker fertilization ability in
the absence of competition (Eberhard,
1996). Burkhardt and de la Motte
(1994) found that large males
of C. whitei had higher fertilization success than small males when
they both mated the same female. Therefore, the sperm of large males may have
superior competitive ability, but not superior fertilization ability, than
that of small males. The advantage found for small males in this experiment
occurred in isolation from other males and may not persist when more
interaction among males is allowed. It will be important for future studies to
examine the performance of sperm from large and small males of C.
dalmanni in various competitive and non-competitive situations.
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ACKNOWLEDGEMENTS |
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We thank Matthew Denniff for invaluable assistance maintaining cultures, measuring flies, and counting eggs. Jeremy Field kindly provided helpful advice on the data analysis. Funding for this project was provided by NERC (grant to T.C., K.F., and A.P. for support of R.B.), and the Royal Society (summer research studentship to T.R.).
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