Behavioral Ecology Vol. 12 No. 6: 761-767
© 2001 International Society for Behavioral Ecology
Polyandry in grain beetles, Tenebrio molitor, leads to greater reproductive success: material or genetic benefits?
Department of Evolution, Ecology, and Organismal Biology, The Ohio State University, Columbus, OH 43210, USA
Address correspondence to B.D. Worden, Department of Entomology, The Ohio State University, 1735 Neil Avenue, Columbus, OH 43210, USA. E-mail: worden.4{at}osu.edu . P.G. Parker is now at the Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121, USA.
Received 18 April 2000; revised 19 September 2000; accepted 10 March 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSDISCUSSIONREFERENCES |
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Females that mate with more than one male may derive both material and genetic benefits, and differentiating between the two benefits is often difficult. We tested for both material and genetic effects associated with multiple mating in the highly promiscuous yellow mealworm beetle, Tenebrio molitor. Females that mated four times to the same male laid more eggs and produced more larvae than females that mated only once. Whether copulations occurred on the same day or over several days, the result was an immediate increase in the production of eggs by females. Some females were kept on a restricted diet to test whether nutrients in the spermatophore disproportionately benefitted food-deprived females. Although females on poor diets produced fewer and smaller offspring, diet did not significantly affect the proportional benefit of mating treatment on female fecundity. By controlling for male mating history, we were able to separate the effects of mating with different males from the effects of receiving multiple spermatophores from the same male. Females that mated with four different males achieved substantial gains in numbers of eggs produced (32% increase) beyond those of females that mated an identical number of times with the same male. We found no evidence that males allocate fewer sperm to previous mates. Egg hatchability was unaffected by mating behavior, suggesting that genetic incompatibility at that stage is not responsible for the low reproductive success of females mated with a single male. These results suggest that females may delay or reduce oviposition or may be incapable of achieving maximal fecundity until they have gained the material and/or genetic benefits of mating with multiple males.
Key words: beetles, ejaculate pheromones, genetic compatibility, polyandry, Tenebrio molitor.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSDISCUSSIONREFERENCES |
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Females across a wide range of taxa commonly mate with multiple males (Birkhead and Møller, 1998
; Dewsbury,
1984; Ridley,
1988), despite the fact that adaptive advantages of multiple
mating are not as apparent for females as for males
(Williams, 1966). Potential
costs associated with mating include the risk of disease transmission,
increased susceptibility to predators, retaliation or abandonment by social
mates, and energetic costs. In addition, females may suffer from harmful
chemicals in a male's ejaculate (Andersson,
1994; Chapman et al.,
1995). Together, the number and magnitude of potential costs of
multiple mating to females suggest that we might expect to see these costs
balanced by large gains. Only recently has the relationship between female
mating history and fitness begun to be thoroughly investigated in order to
better understand this aspect of female mating behavior. Several hypotheses
have been proposed that provide adaptive explanations for multiple mating by
females. These benefits fall into two main categories, nongenetic (material)
and genetic.
Nongenetic benefits include fertilization assurance
(Walker, 1980), nuptial gifts
or ejaculate nutrients (e.g., Gwynne,
1984; Oberhauser,
1989), ejaculate defensive compounds
(Dussourd et al., 1988;
Eisner and Meinwald, 1987),
and sperm replenishment. In some insect species, spermatophores provide
substantial nutrients to females and incur substantial costs to males (e.g.,
Forsberg and Wiklund, 1989;
Gwynne, 1984). Increased
fecundity may result from ejaculate compounds that stimulate egg maturation or
oviposition in females (e.g., in crickets;
Destephano et al., 1982). In
species where there is considerable interaction between males and females
after copulation, mating with multiple males may provide the female with
benefits if male partners are more cooperative in protecting or rearing
offspring or allowing the female access to their territorial resources (e.g.,
Agelaius phoeniceus; Gray,
1997).
The importance of material benefits, such as ejaculate nutrients, may
depend on the resource status of the female. Females that have access to
abundant food resources to allocate toward reproductive effort would not be
expected to gain as much from ejaculate nutrients as females with fewer
resources. In developing a model for the role of male ejaculate nutrients in
female reproductive success, Boggs
(1990) predicted that as a
female's food consumption increases or becomes more nutritionally complete,
the effect of male-donated nutrients should decline. Compared to well-fed
females, females on poor diets eat more male-donated nutrients in cockroaches
(Schal and Bell, 1982) and
Drosophila (Steele,
1986). Similarly, female bush crickets
(Gwynne, 1990) and seed
beetles (Savalli and Fox,
1999) kept on poor diets mated more often than females on richer
diets. In crickets, egg size increased with multiple mating when females were
kept on a restricted diet (Simmons,
1988).
The genetic benefit hypotheses predict an increase in mean offspring
fitness with multiple mates. Unlike some material benefits in which additional
resources may be acquired through mating multiply with the same male, genetic
benefits result from the acquisition of sperm from two or more males
(Tregenza and Wedell, 1998).
Polyandry (defined here as mating with more than one male) may be advantageous
through increasing the genetic diversity of a female's offspring
(Brown, 1997;
Hamilton, 1987;
Liersch and Schmid-Hempel,
1998). Other hypotheses have focused on the potential for sperm
competition or the filtering of sperm within females. If sperm competitiveness
and offspring quality are correlated, then females may increase their fitness
by mating multiply (Madsen et al.,
1992; Watson,
1998). Alternatively, multiple mating may increase the diversity
of sperm within a female's reproductive tract, enabling the screening of sperm
based on its genetic compatibility with the genotype of the egg
(Madsen et al., 1992;
Newcomer et al., 1999;
Zeh, 1997; Zeh and Zeh,
1997a,b;
Tregenza and Wedell, 1998),
leading to increased egg hatchability or offspring fitness.
These hypotheses are not mutually exclusive. In fact, it may be likely that
mating with more than one male will have several effects. For example, if
males provide costly nutrients in their ejaculates, it would be highly
advantageous for males to develop a mechanism to stimulate females to produce
as many young as possible immediately after mating, or else risk losing much
of their nutrient donation to subsequent male competitors. Likewise, polyandry
that is initially driven by genetic benefits would select for males that can
manipulate the immediate use of sperm either through stimulating compounds or
nutrient gifts. Therefore, studies should take care to examine for multiple
effects after one type of benefit has been found. Differentiating between
genetic and nongenetic benefits is difficult, but one important distinction is
that genetic benefits result from copulations with two or more different
males, whereas many types of direct benefits can be gained by multiple
copulations with the same individual. Few studies have attempted to separate
the effects of mate number and multiple copulations in a controlled laboratory
experiment (but see Tregenza and Wedell,
1998; Newcomer et al.,
1999).
In this study, we examined the effects of multiple mating on female
reproductive success in the grain beetle Tenebrio molitor
(Coleoptera, Tenebrionidae). Tenebrio molitor, or yellow mealworm
beetle, is a cosmopolitan pest of stored grains that can be easily reared in
the laboratory. Females and males in the lab will readily mate several times
in a single day. Although this beetle has been widely studied, we are not
aware of any studies examining whether multiple mating is beneficial to female
reproductive success. Second-male sperm precedence has been demonstrated in
T. molitor, but females that mate with two males usually produce
offspring with mixed paternity (Siva-Jothy
et al., 1996), suggesting that genetic benefits are possible. In a
set of three experiments, we examined the relationship between number of mates
and reproductive success and survivorship in females. First, we examined
various aspects of female reproductive success when females mated with one,
two, or five males. In the second and third experiments, we tested several
predictions of material and genetic benefit hypotheses to investigate which
potential advantages of multiple mating might apply to this species. We
measured the magnitude of the interaction of multiple matings with female
condition, examining whether females on poor diets gain relatively greater
benefits from nutritional components of the spermatophore. By controlling for
male mating history, we were able to examine the effects of mating with
multiple males (polyandry) beyond the effects associated with procuring
multiple spermatophores from a single male. We also examined larval weight and
survivorship to determine whether polyandry affects offspring quality. Instead
of focusing on either material or genetic benefits separately, we attempted to
examine the entire range of benefits that may be acquired by polyandrous
females. We provide evidence from controlled experiments that female beetles
gain material and, potentially, genetic benefits and attain the greatest
reproductive benefits by mating multiply with different males.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSDISCUSSIONREFERENCES |
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The T. molitor used in these studies were obtained from stocks maintained by B.D.W. and P. Pappas at Ohio State University (for additional information on maintenance of the beetle stock, see Worden et al., 2000
). We
collected pupae and sexed them by the morphology of the eighth abdominal
segment (Bhattacharya et al.,
1970). Newly emerged adults were placed individually into 60
x 15 mm petri dishes with excess wheat bran and potato. Females were
marked with a single dot of white correction fluid (Liquid Paper, Gillette
Co.) so that we could easily identify the sex of individuals within mating
pairs.
Statistical analysis was performed using parametric ANOVAs after normality
was confirmed by Shapiro-Wilk test. In the cases where data were proportions
(egg hatchability, larval survivorship), we arcsine-transformed the data
before analysis. All p values are two-tailed, and means are provided
with ± 1 SE. We performed statistical analyses using SPSS version 9.0.
Power analysis was performed by SPSS for observed differences and calculated
for hypothetical differences or effects from Zar
(1996).
Experiment 1: effect of multiple mates on female lifetime
reproductive success
We weighed females 24 h before mating and randomly assigned them to a
treatment group: either one (n = 21), two (n = 20), or five
(n = 21) mates. When an adult female was 8 days old, she was placed
with a male and allowed to mate. Males used in the experiment were all
8-day-old virgins. Mating between receptive individuals usually occurs shortly
after initial contact, and copulation duration ranges from 45 s to 2 min. If a
mating failed to occur within 10 min, we replaced the male. If the replacement
male also failed to copulate with the female within 10 min, the female was
removed from the experiment. Mated pairs were separated immediately after
copulation was complete (end of intromission), and we allowed 1 h to pass
between matings so that females that were assigned to the five-matings
treatment completed their final copulation 4.5-5.0 h after their first
pairing.
We collected all of the eggs produced by each female on the third day after mating, and then every 5 days thereafter until the 24th day after mating. Eggs were placed in wheat bran and kept in a 34-36°C incubator for 14 days, after which the bran was sifted and the hatched larvae were counted. In addition, we counted all larvae, but not eggs, produced by a female from the 24th day after mating until her death. We weighed 10 randomly chosen larvae that a female produced 4-8 days after mating. If a female produced fewer than 10 larvae during this time, we weighed all the larvae available.
Results
A total of 7 of 62 (11%) females did not produce any offspring, and these
nonreproductive females were not distributed evenly between mating treatments:
one mate, 5 of 21 (24%); two mates, 2 of 20 (10%); five mates, 0 of 21 (0%). A
greater proportion of singly-mated females did not produce any offspring than
females that mated multiply with either two or five males (2 of 41
multiply-mated females versus 5 of 21 singly-mated females, Fisher's Exact
test, p =.04).
We performed a multiple analysis of covariance (MANCOVA) on the total number of larvae produced (lifetime reproductive success, LRS), hatchability of eggs, and female survivorship. Mating treatment had a significant effect on the number of larvae produced (Figure 1; F2,58 = 5.5, p =.006). Females mated to five males produced significantly more larvae than females mated to one male (p < 0.01, Bonferroni posthoc comparisons), but not significantly more than females mated to two males (p = 0.87, Bonferroni post-hoc comparisons). Although females mated to two males produced nearly twice as many offspring as females mated to one male, this difference was not significant (p =.15, Bonferroni post-hoc comparisons).
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Mating treatment had no significant effect on the proportion of eggs hatching (arcsine-transformed proportions F2,58 = 0.4, P =.65; observed power = 0.14, but power = 0.40-0.50 if groups differ by 10%) or the survivorship of females (F2,58 = 1.4, p =.27; observed power = 0.28, but power = 0.50-0.60 if groups differ by 10%). Overall, egg hatchability was high: one mate, mean = 0.82 ± 0.24; two mates, mean = 0.89 ± 0.08; five mates, mean = 0.84 ± 0.19. Female mass, which was included as a covariate in the model, did not have a significant effect on any of the dependent variables (number of offspring, F1,58 = 2.2, p =.15; arcsine-transformed hatch-ability, F1,58 = 0.8, p =.37; female survivorship, F1,58 = 2.4, p =.13).
To assess whether the effect of mating treatment on egg production varied with time after copulation, we performed a repeated-measures ANOVA on egg production for the five collection periods after copulation. Egg production declined with time since mating (F4,236 = 9.9, p <.001), but there was no significant difference in the patterns of egg production between treatments (treatment x time interaction F8,236 = 1.4, p =.20; observed power = 0.63).
Because some females failed to produce larvae during days 4-8, data on larvae mass were available for only 52 of 62 females. Neither female mass (ANOVA F1,48 = 0.8, p =.36) nor mating treatment (ANOVA F2,48 = 0.9, p =.42) had a significant affect on the average mass of larvae produced 4-8 days after mating.
Experiment 2: the effects of female nutritional status and multiple
mating partners on female reproductive success
Experiment 1 demonstrated that females that mated with five males in a
single day produce significantly more offspring than females that mated
singly; however, it is not clear whether this increase in reproductive success
was a result of multiple copulations or multiple mating partners. Therefore,
we performed a second experiment to differentiate between these two
possibilities as well as to further examine the causes of the effect found in
experiment 1. Females were randomly assigned to either a single copulation
with one male (n = 44), four copulations with one male (n =
43), or four copulations with four different males (n = 43). Each
female began mating when she was 8 days old, and, unlike experiment 1, females
were allowed to copulate only once per day. One possible confounding factor
with this basic design was that females mating with the same male would mate
with nonvirgin males on their second through fourth matings, potentially
receiving smaller successive ejaculates than females that always mated with a
different virgin male. To control for this, we mated the males in the
different-males treatment to nonexperimental females before copulations with
experimental females so that an experimental female's second, third, and
fourth mate had mated on one, two, or three previous occasions,
respectively.
Another potential confounding factor in this experiment is the amount of exposure and social interaction with other conspecifics. Females assigned to the multiple-copulation treatments had more exposure to males as well as more copulations than females in the one copulation treatment. This could be important if some external stimuli (e.g., pheromone) stimulates females to produce more eggs. Therefore, we included an additional treatment group in which four different males were allowed to interact with each female (n = 30), but only the first male placed with the female was allowed to copulate with her. In this last treatment, the second, third, and fourth males had mated with nonexperimental females 5 min before being placed with the experimental female. We used nonvirgin males because males have a roughly 30-min refractory period between copulations (unpublished data), and this enabled us to permit interaction between the beetles without constantly having to prevent copulation. Each non-copulatory interaction lasted 10 min, approximately the same amount of time that females spent with males that copulated with them.
To examine whether ejaculate nutrients are a major contribution to female reproductive success or survivorship, we manipulated the feeding regime so that some females were on a restricted diet and presumably more deprived of resources necessary for egg production. Approximately one-half of the females (n = 72) in each mating treatment were provided excess wheat bran and water. We supplemented the wheat bran with potato every 5 days. The rest of the females (n = 70) were provided water ad libitum, but were starved for 3 days before the beginning of the mating treatments, and then given ad libitum wheat bran thereafter. In addition, we supplemented the food-limited treatment with potato only every 10 days.
Females were placed into a new dish containing wheat bran every 4 days up to the 19th day of the experiment, and then the female remained in a dish until she died. The dishes containing each female's eggs were kept in a 34-36°C humid incubator for 14 days, at which time we counted the number of hatched larvae. We only counted eggs for the first time period after all mating treatments were complete (days 5-9), so egg hatchability data are only for this time period. We randomly selected 10 larvae from each female that were produced during days 5-9. These larvae were weighed and then placed into a dish containing excess wheat bran. Larvae were kept in a 34-36°C humid incubator for 60 days. At this time, we determined survivorship and the mean mass of all surviving larvae.
We performed separate statistical tests for dependent variables that differed in their respective sample sizes. We performed two-way ANOVA on the number of larvae produced and female survivorship to examine the effects of mating treatment and feeding treatment. Because some females did not produce any eggs during the 4-day period that eggs were collected, we performed separate tests of ANOVA on egg hatchability and measures of offspring quality.
Results
Females that interacted with four males but mated only once with one of the
males produced 35.0 ± 6.4 offspring compared to 36.7 ± 6.2
offspring produced by females that mated once with one male without further
social interaction (p >.99, Bonferroni post-hoc comparisons).
Because there was no difference in reproductive success between these two
groups of females that mated only once, we combined them for further
comparisons.
Mating treatment had a significant effect on the number of offspring that a female produced (Figure 2; ANOVA F3,152 = 16.7, p <.001). Females that mated with the same male four times and females that mated with four different males produced 29.2 ± 7.1 and 49.4 ± 7.1 more larvae, respectively, than females that mated once with a single male (p <.001 for each comparison, Bonferroni post-hoc comparisons). Females that mated with four different mates produced 85.4 ± 5.5 larvae compared to 65.2 ± 5.5 larvae produced by females mated four times to a single male, a difference that remained significant after Bonferroni correction (p =.04). The temporal trend in larvae production was similar to that found in experiment 1: Larvae production declined as the time since the last mating increased (Figure 3; repeated-measures ANOVA F3,462 = 57.3, p <.001). Females that mated once, four times to the same male, and four times to different males survived 64.6 ± 13.5, 64.6 ± 9.3, and 63.8 ± 11.4 days, respectively. Overall, we did not detect an effect of mating treatment on female longevity (F2,154 = 0.06, p =.94).
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Feeding treatment had a significant effect on the production of larvae (ANOVA F1,154 = 5.1, p =.03) but not on female survivorship (ANOVA F1,154 = 0.9, p =.33). Females on the richer diet produced more offspring than females on the restricted diet across all mating treatments, but none of the pairwise comparisons among mating treatments was significant (p >.05, Bonferroni post-hoc comparisons). Because one prediction for nutrient donations from males is that females on poor diets should gain greater advantages from multiple mating than females on richer diets, we examined the interaction between female diet and number of mates. There was no significant interaction between the number of mates and diet on either the number of offspring produced (ANOVA F2,154 = 0.46, p =.63; observed power = 0.12, but power > 0.78 if interaction accounts for 10% of observed variation) or female survivorship (ANOVA F2,154 = 0.06, p =.94; observed power = 0.06, but power > 0.70 if interaction accounts for 10% of observed variation).
As in experiment 1, some females did not produce any offspring: 11 of 44 (25%) females mated only once, 2 of 43 (5%) mated to the same male four times, and 0 of 43 (0%) of females mated to four different males. The difference in the number of nonreproductive females was significant between comparisons of females that mated only once and females that mated multiply to the same male (Fisher's Exact test, p =.01) or to different males (Fisher's Exact test, p <.001). However, the proportion of nonreproductive females did not differ significantly between females that mated multiply to the same versus different males (Fisher's Exact test, p =.49). The differences in reproductive success that we found cannot be explained solely on the basis of a difference in the number of nonreproductive females because the effect of the number of mates on female reproductive success remained significant even after removing nonreproductive females from the analysis (ANOVA, F2,137 = 9.0, p <.001).
Sample sizes for egg hatchability data were reduced to 27 in the single mating treatment, 40 in the same-male treatment, and 43 in the different-males treatment. Neither mating treatment nor feeding treatment affected egg hatchability (two-way ANOVA; mating treatment, F3,126 = 0.6, p =.62; feeding treatment, F1,126 = 0.1, p =.78).
We found no evidence that mating treatment affects any of the measures of offspring quality: Mass at hatching and at 60 days old (repeated-measures ANOVA, F2,102 = 2.0, p =.15) and larval survivorship (arcsine-transformed) to 60 days (ANOVA F2,101 = 0.88, p =.42) were unaffected. However, feeding treatment did affect the mass of larvae (repeated-measures ANOVA F1,102 = 10.8, p =.001). Larvae produced by females on a restricted diet weighed 0.81 ± 0.02 mg at hatching and 28.38 ± 0.42 mg at 60 days after hatching compared to 0.89 ± 0.02 mg and 30.67 ± 0.60 mg for larvae produced by females in the high-quality feeding treatment (t test; t114 = -2.6, p =.01 and t106 = -3.1, p =.002, respectively). Survivorship of larvae to an age of 60 days was very high (97.0 ± 5.9%), and, despite the differences in larval mass, survivorship did not differ between feeding treatments (ANOVA on arcsine-transformed survivorship F1,101 = 0.01, p =.92).
Experiment 3: sperm production by males when mating to novel versus
previous mates
If males adjust spermatophore content based on past mating history with a
female, then this could affect the reproductive success of females mated to
the same male compared to females mated to different males. To test for this,
we performed a third experiment to determine whether the number of sperm
allocated to spermatophores differed between males that mated to a new versus
previous mate. Virgin males (n = 39) were assigned to mate with the
same female twice (n = 20) or to two different females (n =
19). As in experiment 2, we controlled for mating history by mating the second
female in the different-female treatment to a nonexperimental male 24 h before
use in this experiment. Therefore, the first female that mated with a male was
always virgin, and the second female had always mated once previously.
Consistent with experiment 2, we allowed 24 h to pass between copulations.
Immediately after a male's second copulation ceased, we decapitated the female
and carefully dissected her reproductive tract. Spermatophores do not begin to
eject their contents until 7-10 min after copulation is complete, and the
spermatophore changes dramatically after discharge
(Gadzama and Happ, 1974);
therefore, the spermatophore that was just produced was quickly removed from
the female's bursa and examined to verify that it was still complete and full
of sperm. The spermatophore was placed on a slide with 20 µL phosphate
buffered saline, pH 7.4. After discharge was observed using a dissecting
microscope, the solution was transferred into a small tube, mixed, and diluted
in a total volume of 100-120 µL saline. Sperm counts of four samples from
each spermatophore were obtained by using an Improved Neubauer hemocytometer
at magnification x 400.
Results
Of the 39 trials in this experiment, 6 were removed because the pair did
not remate within 15 min (3 different-female trials, 3 same-female trials).
All spermatophores (n = 33) contained sperm, but the number of sperm
varied from a low of 1334 to a high of 234,900 (median = 97,200). A male's
previous mating experience with a female did not significantly affect the
number of sperm allocated to a spermatophore (same-female = 116,271 ±
16,106, different-female = 118,233 ± 18,984, t31 =
-0.08, p =.94; observed power = 0.05, and power < 0.30 if means
differ by 10%).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSDISCUSSIONREFERENCES |
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This study provides evidence that by copulating more than once, female grain beetles are able to significantly increase their reproductive success through increased egg production. Whether matings occurred on the same day (experiment 1) or over 4 successive days (experiment 2), females that mated with five or four males, respectively, produced nearly twice as many larvae as females that mated only once. This increase in egg production could be explained by either material or genetic benefits.
Material benefits can be gained from multiple ejaculates, regardless of the genotype of the sperm. We were able to test this hypothesis because it predicts that females that mate with the same male four times should produce more offspring than females mated only once. Our data support this hypothesis because females that mated four times with the same male produced nearly 1.8 times more offspring than females mated only once.
Females may mate with more than one male to negate the risk of mating with
an infertile male (fertility assurance hypothesis;
Walker, 1980). Our data from
both experiments 1 and 2 show that females that mate with more than one male
are less likely to fail to produce offspring altogether. However, our data do
not indicate infertile males, necessarily, because females that mated to the
same male four times, like females that mated to different males, were less
likely to fail to produce any offspring. It appears that if fertility
assurance is an important selective force to female grain beetles, it is
because of "infertile" copulations, not infertile males. There are
two possible explanations for these results: Males sometimes either failed to
pass spermatophores or passed one with inviable sperm (but this was not
supported by sperm count data in experiment 3), or single copulations are
occasionally insufficient to stimulate females to produce eggs. Even after
removing females that did not reproduce from the analysis, the effect of
mating treatment on female reproductive success remained significant.
Therefore, the failure of some males to transfer viable sperm is an
insufficient explanation of the overall relationship between number of
copulations and reproductive success.
We also tested for nutritional benefits. One prediction that arises from
the nutritional benefits hypothesis is that females on poor diets will gain
more from multiple mating than females on richer diets. We did not find
support for this hypothesis because the proportional increase in larvae
production with increasing copulation number was greater, but not
significantly greater, for females kept on the richer diets. This result was
not a result of the food manipulation failing to alter female reproductive
reserves, because females on poor diets produced fewer and less massive
offspring than females on richer diets. Male nutrient donations have been
shown to increase egg size and/or female survivorship in some species (e.g.,
Andersson, 1994;
Boggs, 1990), but in our study,
mating treatment did not have any effect on mass of larvae or female
survivorship even when females were deprived of food. However, males may
supply a special nutrient that is rare in the environment, and our diet
manipulation may not have limited the critical resource
(Eberhard, 1996).
Sperm replenishment is another possible direct benefit attributable to
multiple mating. We did not examine sperm depletion directly; therefore, we
cannot exclude it as an important factor in the mating system of T.
molitor. Ornevich et al.
(2001) found some suggestive
evidence that sperm may be limiting female reproductive success in T.
molitor. However, our finding that multiple matings with the same male
resulted in an immediate (within first 4 days) 57% increase in offspring
production compared to females that mated only once indicates that the benefit
occurs even when sperm is most likely to still be abundant from previous
copulations (Figure 3). The
assumption that sperm is not particularly limiting soon after mating is
supported by the fact that the daily rate of offspring production during days
5-9 (2.6 ± 3.0 larvae/day) did not decline significantly from the rate
during the first 4 days (2.7 ± 2.9 larvae/day) in females that mated to
only one male (paired samples t test, t73 = 0.27,
p =.79).
By controlling for male mating history, we were able to examine the effects
of polyandry compared to multiple copulations with a single male. Our
experiments allowed us to attribute any observed differences in female
fecundity or offspring fitness to the number of mating partners per female.
Females that mated to different males produced 31% more offspring on average
than females mated multiply to the same male. This difference is similar to
the 32% increase in lifetime reproductive success of female pseudoscorpions,
Cordylochernes scorpioides, when mated to two different males versus
the same male (Newcomer et al.,
1999) and to the 29% increase in hatching success found in field
crickets when females mated to four different males
(Tregenza and Wedell, 1998).
Recently, genetic compatibility of gametes has been discussed as an important
factor in female mating strategies (Zeh and Zeh,
(1997a,b).
In studies where genetic compatibility has been carefully tested, it has been
detected by its effect on embryonic survivability
(Newcomer et al., 1999;
Tregenza and Wedell, 1998).
However, unlike the results of these studies, in the present study, the number
of males that mated with a female did not affect egg hatchability in our
population of grain beetles. We also failed to find any significant
differences in posthatching measurements: mass at hatching, survivorship, or
mass at 60 days.
One explanation for the increased reproductive success of polyandrous females is that males adjust their ejaculates based on past mating experience with a female so that novel females receive more sperm. We did not find any evidence that males adjust their ejaculates when copulating with a previous mate. The proportion of pairs that failed to remate was equivalent between same-female and different-female treatments (3 of 17 and 3 of 16 pairs, respectively), and the number of sperm allocated to females did not differ between mating treatments. Although the power of the test was low, owing largely to the huge variance in number of sperm, the observed difference between same-female and different-female treatments was less than 2% (1962 sperm). Although it is possible that males adjust amounts of some seminal product other than sperm, these data indicate that the increase in reproductive success among females that mated to different males is not due to differing male strategies in sperm allocation but is consistent with female plasticity in oviposition rate. If females are responding flexibly to multiple copulations, then the presence of ejaculates from multiple males may trigger increased oviposition in females.
The immediate increase in egg production exhibited by females that mated multiply is consistent with stimulatory ejaculate compounds or other cues. Multiple, direct benefits may exist, but the immediate increase in female reproductive success is most likely the result of some signal that results in oviposition by females. Although high thresholds for ejaculate-triggering cues could also explain the complete lack of reproduction among 25% of singly-mated females, it is equally plausible that multiply-mated females gain the additional benefit of fertility assurance.
Females may delay or suppress oviposition until they have mated several
times to increase their chance of obtaining genetic or material benefits.
Ejaculate-triggering substances or copulation itself could act as the
proximate cue stimulating oviposition. Archer and Elgar
(1999) found that 65% of female
hide beetles that mated only once failed to oviposit, but these females began
ovipositing after mating with a second male. Paternity analysis of
doubly-mated females revealed that at least 70% of first males transferred
viable sperm (Archer and Elgar,
1999). This behavior would be advantageous if increased genetic
variation among offspring were important
(Ridley, 1993), or if
increased sperm competition results in offspring that are more competitive
(Keller and Reeve, 1995;
Madsen et al., 1992). We did
not find any significant differences between mating treatments in the early
life histories of the offspring. However, this does not exclude the
possibility that sperm competition or sibling diversity is correlated with
offspring fitness in ways we did not measure, at later periods in life, or in
more variable environments than we provided in the lab. Female pseudoscorpions
(Zeh et al., 1998) and female
hide beetles, Dermestes maculates
(Archer and Elgar, 1999),
prefer novel males as mates. If T. molitor females can detect
differences between males, females may increase oviposition after mating with
different males. Such behavioral plasticity would explain the observed
difference in production of offspring between females that mated multiply with
the same versus different males.
Alternatively, the patterns of fecundity enhancement in this study support
predictions of interlocus contest evolution (ICE) that result from conflict
between the sexes over female reproductive investments
(Rice and Holland, 1997). Male
ejaculate proteins are under selection to have both a defensive and offensive
function in the face of sperm competition, and ejaculate compounds may
stimulate oviposition in ways that benefit males but are harmful to females
(Chapman et al., 1995).
However, unlike studies on Drosophila, we failed to find an affect of
mate number on female survivorship in T. molitor.
In conclusion, the results reported in this study demonstrate that female
T. molitor produce more eggs after copulating with multiple males
(polyandry). The benefit of multiple copulations exists for matings that occur
within a single day as well as over the course of 4 days. Given this finding,
it is not surprising that female grain beetles mate frequently and often
actively solicit copulations (Worden and Parker, personal observations). As
described by Newcomer et al.
(1999), few studies have
specifically tested for genetic benefits once material, or direct, benefits
have been demonstrated. Not only are material and genetic benefits not
mutually exclusive, but we suggest that the two types of advantages may
commonly co-occur in many polyandrous species. An initial advantage, either
material or genetic, would select for polyandry among females. In turn,
polyandry would select for postcopulatory manipulation and competitiveness in
both sexes. Polyandry expands mate choice from selection of a suitable mate
into the potential ability to select or passively screen gametes
(Eberhard, 1996). With two
recent exceptions (Newcomer et al.,
1999; Tregenza and Wedell,
1998), studies that claim to have demonstrated genetic benefits to
polyandry have not experimentally controlled for confounding factors such as
male mating history, female quality, or benefits attributable to multiple
ejaculates instead of multiple mates. Controlling for these factors, the study
reported here has demonstrated that polyandry is a mating strategy that
enhances female fecundity in the yellow mealworm beetle.
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ACKNOWLEDGEMENTS |
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We thank Bob Grosholz, John Lennon, and Lekshmi Nair for their excellent assistance in collecting the data. We are grateful to Venessa Artman, Jenny Bollmer, Kate Huyvaert, Thomas C. Jones, Rebecca Kimball, Peter Pappas, Jill Soha, Jeanne Zeh, and David Zeh for the helpful comments and discussions on an earlier version of the manuscript. This research was supported by The Ohio State University.
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REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSDISCUSSIONREFERENCES |
|---|
Andersson M, 1994. Sexual selection. Princeton, New Jersey: Princeton University Press.
Archer MS, Elgar MA, 1999. Female preference for multiple partners: sperm competition in the hide beetle, Dermestes maculatus (DeGeer). Anim Behav 58: 669-675.[ISI][Medline]
Bhattacharya AK, Ameel JJ, Waldbauer GP, 1970. A method for sexing living pupal and adult yellow mealworms. Ann Entomol Soc Am 63: 1783.
Birkhead TR, Møller AP, 1998. Sperm competition and sexual selection. London: Academic Press.
Boggs CL, 1990. A general model of the role of male-donated nutrients in female insects' reproduction. Am Nat 136: 598-617.
Brown JL, 1997. A theory of mate choice based on
heterozygosity. Behav Ecol 8:
60-65.
Chapman T, Liddle LF, Kalb JM, Wolfner MF, Partridge L, 1995. Cost of mating in Drosophila melanogaster females is mediated by male accessory gland products. Nature 373: 241-244.[Medline]
Destephano DB, Brady UE, Farr CA, 1982. Factors influencing oviposition behavior in the cricket, Acheta domesticus. Ann Entomol Soc Am 75: 111-114.
Dewsbury DA, 1984. Sperm competition in muroid rodents. In: Sperm competition and the evolution of animal mating systems (Smith R, ed). New York: Academic Press; 547-571.
Dussourd DE, Ubik K, Harvis C, Resch J, Meinwald J, Eisner T,
1988. Biparental defensive endowment of eggs with acquired plant
alkaloid in the moth Utetheisa ornatrix. Proc Natl Acad Sci
USA 85:
5992-5996.
Eberhard WG, 1996. Female control: sexual selection by cryptic female choice. Princeton, New Jersey: Princeton University Press.
Eisner T, Meinwald, J, 1987. Alkaloid derived pheromones and sexual selection in Lepidoptera. In: Pheromone biochemistry (Prestwich GD, Blomquist GJ, eds). Orlando, Florida: Academic Press; 251-269.
Forsberg J, Wiklund C, 1989. Mating in the afternoon: Time-saving in courtship and remating by females of a polyandrous butterfly Pieris napi. Behav Ecol Sociobiol 25: 349-356.
Gadzama NM, Happ GM, 1974. The structure and evacuation of the spermatophore of Tenebrio molitor L. (Coleoptera: Tenebrionidae). Tissue Cell 6: 95-108.[ISI][Medline]
Gray EM 1997. Female red-winged blackbirds accrue material benefits from copulating with extra-pair males. Anim Behav 53: 625-639.
Gwynne DT, 1984. Courtship feeding increases female reproductive success in bush crickets. Nature 307: 361-363.
Gwynne DT, 1990. Testing parental investment and the control of sexual selection in katydids (Orthoptera: Tettigoniidae): the operational sex ratio. Am Nat 136: 474-484.[ISI]
Hamilton WD, 1987. Kinship, recognition, disease, and intelligence: Constraints of social evolution. In: Animal societies: theories and facts (Ito Y, Brown JL, Kikkawa J, eds). Tokyo: Japan Science and Society Press; 81-102.
Keller L, Reeve HK, 1995. Why do females mate with multiple males? The sexually selected sperm hypothesis. Adv Study Behav 24: 291-315.
Liersch S, Schmid-Hempel P, 1998. Genetic variation within social insect colonies reduce parasite load. Proc R Soc Lond B 265: 221-225.
Madsen T, Shine R, Loman J, Häkansson, T, 1992. Why do female adders copulate so frequently? Nature 355: 440-441.
Newcomer SD, Zeh JA, Zeh DW, 1999. Genetic benefits
enhance the reproductive success of polyandrous females. Proc Natl Acad
Sci USA 96:
10236-10241.
Oberhauser K, 1989. Effects of spermatophores on male and female monarch butterfly reproductive success. Behav Ecol Sociobiol 25: 237-246.
Ornevich JM, Papke RS, Rauser CL, Rutowski RL, 2001. Material benefits from multiple mating in female mealworm beetles (Tenebrio molitor L.). J Insect Behav 14: 215-230.
Rice WR, Holland B, 1997. The enemies within: intergenomic conflict, interlocus contest evolution (ICE), and the intraspecific Red Queen. Behav Ecol Sociobiol 41: 1-10.
Ridley M, 1988. Mating frequency and fecundity in insects. Biol Review 63: 509-549.
Ridley M, 1993. Clutch size and mating frequency in parasitic Hymenoptera. Am Naturalist 142: 893-910.[ISI]
Savalli UM, Fox CW, 1999. The effect of male mating history on paternal investment, fecundity and female remating in the seed beetle Callosobruchus maculatus. Funct Ecol 13: 169-177.
Schal C, Bell WJ, 1982. Ecological correlates of
paternal investment of urates in a tropical cockroach. Science
218: 170-173.
Simmons, LW, 1988. The contribution of multiple mating and spermatophore consumption to the lifetime reproductive success of female field crickets (Gryllus bimaculatus). Ecol Entomol 15: 57-69.
Siva-Jothy MT, Blake DE, Thompson J, Ryder JJ, 1996. Short-and long-term sperm precedence in the beetle Tenebrio molitor: A test of the `adaptive sperm removal' hypothesis. Physiol Entomol 21: 313-316.
Steele RH, 1986. Courtship feeding in Drosophila subobscura. I. The nutritional significance of courtship feeding. Anim Behav 34: 1087-1098.
Tregenza T, Wedell N, 1998. Benefits of multiple mates in the cricket Gryllus bimaculatus. Evolution 52: 1726-1730.
Walker WF, 1980. Sperm utilization strategies in non-social insects. Am Nat 115: 780-790.[ISI]
Watson PJ, 1998. Multi-male mating and female choice increase offspring growth in the spider Neriene litigiosa (Linyphiidae). Anim Behav 55: 387-403.[ISI][Medline]
Williams GC, 1966. Adaptation and natural selection: a critique of some current evolutionary thought. Princeton, New Jersey: Princeton University Press.
Worden BD, Parker PG, Pappas PW, 2000. Parasites reduce attractiveness and reproductive success in male grain beetles. Anim Behav 59: 543-550.[ISI][Medline]
Zar JH, 1996. Biostatistical analysis. Upper Saddle River, New Jersey: Prentice Hall.
Zeh JA, 1997. Polyandry and enhanced reproductive success in the harlequin-beetle-riding pseudoscorpion. Behav Ecol Sociobiol 40: 111-118.
Zeh JA, Zeh DW, 1997a. The evolution of polyandry I. Intragenomic conflict and genetic incompatibility. Proc R Soc Lond B 263: 1711-1717.
Zeh JA, Zeh DW, 1997b. The evolution of polyandry II: post-copulatory defence against genetic incompatibility. Proc R Soc Lond B 264: 69-75.
Zeh JA, Newcomer SD, Zeh DW, 1998. Polyandrous females
discriminate against previous mates. Proc Natl Acad Sci USA
95: 13732-13736.
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