The role of body size in early mating behavior in a simultaneous hermaphrodite, <Emphasis Type="Italic">Chelidonura sandrana</Emphasis>

Intraspecific variation in mating behavior is widespread among simultaneous hermaphrodites but its underlying sources remain largely unexplored. In the sea slug Chelidonura sandrana, most matings are reciprocal. However, despite non-conditional sperm exchange and potential polygamy-mediated benefits, 30% of matings end after unilateral insemination. To resolve this apparent inconsistency, we here investigated the effect of body size on the frequency of reciprocal matings by testing the following two hypotheses. First, sex-allocation theory predicts that the likelihood of reciprocity depends on the size difference between mating partners. Second, if both sex functions temporally differ in reaching maturity, reciprocal matings should be more frequent with increasing absolute body size of the smaller partner. The likelihood of reciprocity increased with body size of the smaller partner. Moreover, smaller individuals acted more often as males among unilateral matings. These findings suggest that the ability to donate sperm develops prior to female functionality in C. sandrana.     

Strategic mating effort in a simultaneous hermaphrodite

Sexual selection theory for simultaneously hermaphroditic animals predicts an overall preference for inseminating partners that have a relatively higher female fecundity. Previous work on the link between male mating decisions and female fecundity has primarily focused on the effect of the partners’ body size using existing variation in this trait within a study population. On the assumption that the body size is positively correlated with female fecundity, sperm donors should preferentially inseminate relatively larger individuals to obtain a higher fitness gain through their male sex function. However, empirical evidence for such size-dependent mate choice in simultaneous hermaphrodites is equivocal, possibly because of confounding variables. We studied the mating behavior of the simultaneously hermaphroditic flatworm Macrostomum lignano and tested for a strategic mating effort in response to the feeding status of the partner. We experimentally manipulated the feeding status of potential mating partners in order to generate variation in female fecundity among them and tested whether this affected the copulation number and the number of sperm that the focal worm managed to store in the partner’s sperm storage organ. We found that the manipulation of the feeding status had a strong effect on the body size of the potential mating partners and that focal worms copulated more frequently with, and stored more sperm in well-fed partners compared to unfed partners. Our results suggest that M. lignano adjusts its mating effort in response to the feeding status of the mating partner.     

Hermaphrodite sex role preferences: the role of partner body size, mating history and female fitness in the sea slug Chelidonura sandrana

Costs and benefits associated with matings and the effects of mating frequency on fitness commonly differ between the sexes. As a result, outcrossing simultaneous hermaphrodites may prefer to copulate in the more rewarding sex role, generating conflicts over sperm donation and sperm receipt between mates. Because recent sex role preference models remain controversial, we contrast here some of their assumptions and predictions in the sea slug Chelidonura sandrana. For this hermaphrodite with sperm storage and internal fertilisation, risk-averse models assume that fitness pay-offs are constantly higher in the female than in the male function in any single mating. Moreover, excluding mutual partner assessment, these models predict male mating behaviour to be independent of receiver traits. The competing gender ratio hypothesis assumes that relative fitness pay-offs, and thus the preferred mating roles, vary and may reverse between matings and predicts that ejaculation strategies co-vary with receiver quality. We found that field mating rates of C. sandrana substantially exceeded what is required to maintain female fertility and fecundity, indicating large variation in direct female benefits between matings. We further demonstrate that male copulation duration adaptively increased with partner body size (i.e. fecundity) but decreased with recent partner promiscuity. These findings are compatible with the gender ratio hypothesis but contradict risk-averse models.     

Size-assortative mating and non-reciprocal copulation in a hermaphroditic intertidal limpet: test of the mate availability hypothesis

Mating behaviour and mating patterns are affected to a large extent by body size in both hermaphrodites and gonochorists. Detailed research on mating patterns, mate choice, pre- and post-copulatory sexual selection in marine pulmonates is wanting, thus warranting more attention as a study system in the future. The simultaneous hermaphroditic limpet Siphonaria capensis (Pulmonata: Basommatophora) shows size-dependent fecundity, and acts as a suitable organism to test the effects of body size on mating patterns, mating success, gender expression and reciprocity in hermaphroditic marine gastropods. We mainly used bootstrap resampling techniques to estimate the effects of different factors on mating patterns. In the populations studied, a strong size-assortative mating pattern was observed, where small-scale spatial distribution of potential mating partners (the mate availability hypothesis) could explain 65% of this pattern, while mate choice and mating constraints explaining the rest. No significant difference in mating success between limpets with different body size was found, even though in one population, the sperm recipients were larger than the non-copulating limpets. Interestingly, we found that intromission was non-reciprocal during copulations. This may mean that this species does not conform to the common rule of reciprocity predicted for hermaphrodites, unless there is sex-role alternation between individuals in a mating pair. The mating partners consisted of similar sized, acting males and females, thus without any indication of the body size determining the gender expression. The matings took place in early mornings only at spring tides and the animals were observed laying egg masses only during neap tides. Since siphonariid limpets possess both a spermatheca and a seminal vesicle, which may have either a sperm storing and/or digestive function, post-copulatory sexual selection (e.g., sperm competition) cannot be dismissed.     

Mating behavior and sperm competition in the fly,Dryomyza anilis

Summary Mating behavior in D. andis was studied both in nature and in the laboratory. Each mating in nature consisted of several repeated copulation and oviposition bouts during which the male stayed mounted. One copulation bout included an intromission in the beginning followed by several tapping sequences. During one tapping sequence the male touched the external genitalia of the female with his claspers several times. The experimental matings with irradiated sterile males contained one copulation bout after which the female laid her eggs. In these matings the percentage fertilization for the last male to mate with the female increased with increasing number of tapping sequences. The percentage fertilization was affected positively also by the time between the end of the copulation bout and the start of oviposition, but the time spent in the copulation bout was not important. Furthermore, there was a negative correlation between the length of intromission and the percentage fertilization. After the copulation bout, the percentage fertilization was higher among the first eggs laid. This difference between the first and later eggs laid increased with the increasing number of tapping sequences, suggesting that copulating and ovipositing in several bouts further increase the male percentage fertilization. In matings recorded in nature, both the number of tapping sequences and the number of copulation and oviposition bouts varied greatly. Of the variables recorded during these matings, individual size and the number of eggs the female contained in the beginning of a mating affected the number of copulation bouts and the number of tapping sequences. Males also mated differently with females containing immature or mature eggs. The male behavior resulting in sperm precedence and the variability of matings in nature in relation to male sperm competition is discussed.     

The influence of social experience on cooperative behaviour of rats (Rattus norvegicus): direct vs generalised reciprocity

Cooperation among non-kin has been attributed sometimes to reciprocal altruism: Two or more individuals exchange behaviour that benefits the respective partner. According to direct reciprocity, cooperation is based on past behaviour of a known partner. In contrast, in generalised reciprocity, cooperation is based on anonymous social experience where the identity of the partner is irrelevant. In a previous study, female Norway rats (Rattus norvegicus) were found to cooperate according to a generalised reciprocity mechanism. In this study, we tested whether Norway rats would also cooperate as predicted by a direct reciprocity mechanism and whether direct reciprocity would cause a higher propensity to cooperate than generalised reciprocity. Focal animals were experimentally manipulated to receive social experience from known or unknown, helpful or defecting partners in an instrumental cooperative task. Our first experiment shows that rats are more helpful towards a partner from which they had received help before than towards a partner that had not helped (i.e. direct reciprocity). Our second experiment revealed that after receiving help by others, rats were more helpful towards a partner from which they had received help before than towards a new partner (i.e. direct reciprocity generated a higher cooperation propensity than generalised reciprocity). We conclude that in female Norway rats, the tendency to cooperate is influenced by partner-specific information. To our knowledge, this is the first study to demonstrate direct reciprocity in rodents, and it is the first study testing direct vs generalised reciprocity in animals.     

Multiple paternity and individual variation in sperm precedence in the simultaneously hermaphroditic land snail Arianta arbustorum

Intraspecific variation in the proportion of offspring sired by the second mate with a female (P₂) is an aspect of sperm competition that has received little attention. I examined the effects of delay between copulations (range 9–380 days) and size of sperm donor on sperm precedence in double-mated individuals of the simultaneously hermaphroditic land snail Arianta arbustorum. Using shell colour as a genetic marker, paternity was analysed in 132 broods produced by 35 snails that had mated with two partners of different genotype. Sperm precedence (P₂) was influenced by the time between the two matings when the mating delay exceeded 70 days (one reproductive season). P2 averaged 0.34 in the first brood of snails that mated twice within 70 days indicating first mate sperm precedence. In contrast, P₂ averaged 0.76 in broods of snails that remated in the following season, indicating a decreased viability of sperm from the first mate. The size of sperm-donating individuals had no effect on the fertilization success of their sperm in the first brood produced after the second copulation. Analysis of long-term sperm utilization in 23 snails that laid three to nine egg batches over 2 years revealed striking differences among individuals. Five snails (21.7%) exhibited first-mate sperm precedence throughout, eight snails (34.8%) showed second-mate sperm precedence throughout, whereas ten snails (43.5%) exhibited sperm mixing in successive batches. It is suggested that the individual variation in sperm precedence in A. arbustorum may partly be due to differences in the amount of sperm transferred. Paternity analysis in 34 batches laid by 19 wild-caught individuals of A. arbustorum indicated that at least 12 snails (63.2%) used sperm from two or more mates for the fertilization of their eggs. This suggests a high incidence of multiple paternity in broods of A. arbustorum under field conditions.     

Choice in Varroa jacobsoni Oud. between honey bee drone or workerbrood cells for reproduction

Summary The effect of variation in copulation duration on sperm precedence (P₂) of the last male was investigated in a non-territorial libellulid using irradiated-male techniques and observations in a large field enclosure. Copulation duration varied between 6 min and more than 1 h (X = 23.19 min ± 12.74). In the clutch deposited immediately after the copulation, most copulation durations resulted in high sperm precedence for the last male. Several experimentally interrupted short matings still resulted in 100% precedence. However, most interrupted copulations of 10 min or less resulted in a low P₂. After long, uninterrupted copulations, P₂ remained high in clutches that were collected up to 9 days after the mating. After interrupted short copulations, P₂ fluctuated or remained at a low level from the first clutch onwards. This indicates that sperm mixing probably starts within 24 h and that after a long copulation virtually no rival sperm is left in the female's sperm storage organs. As most natural copulations are long, complete removal of rival sperm seems to be common in S. danae. Only 3 f 17 variables investigated explained a significant amount of the variation in copulation duration. Copulations shortened with increasing temperature and for each additional mating of the male. The strongest effect however, was due to time of day: early morning copulations lasted longer than midday copulations. Adjustment of the copulation duration to the decreasing chance of obtaining additional matings was found to be the most likely of five explanatory hypotheses that were evaluated.     

Gender-role alternation in the simultaneously hermaphroditic freshwater snail <Emphasis Type="Italic">Physa acuta</Emphasis>: not with the same partner

In simultaneous hermaphrodites, gender conflicts that arise from two potential mates sharing the same gender preference may be solved through conditional reciprocity (or gamete trading). Conditional reciprocity had initially been considered widespread, but recent studies suggest that its real occurrence may have been overestimated, possibly because most mating observations have been performed on isolated pairs of individuals. Some resulting patterns (e.g., non-random alternation of sexual roles) were indeed compatible with conditional reciprocity but could also have stemmed from the two partners independently executing their own mating strategy and being experimentally enforced to do so with the same partner. Non-random alternation of gender roles was recently documented in the simultaneously hermaphroditic freshwater snail Physa acuta. To distinguish between conditional and unconditional gender alternations, we observed copulations of individually marked snails reared at three contrasted densities. We showed that density affected the overall frequency of copulations during the first 2 days of the experiment with high-density boxes showing more copulations than low density boxes, but it did not affect gender alternation patterns. A change in gender role was observed more often than expected by chance over two successive copulations by the same individual, confirming previous studies. However, gender switches did not preferentially occur with the same partner. We conclude that gender alternation is not due to conditional reciprocity in P. acuta. It may rather stem from each individual having a preference for gender alternation. We finally discuss the mechanisms and the potential extent of this unconditional reciprocity.     

Variation in copulation duration and the resultant degree of sperm removal inOrthetrum cancellatum (L.) (Libellulidae: Odonata)

Summary Orthetrum cancellatum showed two distinct copulation durations at a study site in the south of France. These different durations are correlated with different degrees of sperm displacement from the female sperm stores; long copulations (894±142 s) resulting in almost 100% sperm removal, and short copulations (21.0±13.5 s) resulting in 10–15% removal. The difference in copulation duration is also related to the site of copulation and to the relative age of the copulating males. At oviposition sites males who gained copulations were relatively young and copulated for long periods. The difference in the duration of copulation and the degree of sperm removal is discussed with reference to the female habit of ovipositing before remating during oviposition episodes, and the possibility of differences in the rate at which males encounter receptive females at the two sites.     

Determinants of paternity success in the spider Pholcus phalangioides (Pholcidae: Araneae): the role of male and female mating behaviour

In double mating experiments, we examined whether and to what extent various male and female behavioural traits influence the course of mating and fertilization success in the cellar spider. In males, we focussed on pre-copulatory behaviour and on the rhythmic twisting movements that the male performs with his pedipalps during copulation. In females, we investigated remating decisions and the effect of female termination of copulation. Second males fertilized a high proportion of the eggs (P ₂: median 89%) despite much shorter second matings, with high variation in relative paternity success. The number of pedipalp movements (PPMs) of either male was a better predictor of paternity than copulation duration. Our results suggest that in second matings, PPMs help to remove sperm from previous males, whereas in first matings a high number of PPMs enhances fertilization success, either due to numerical sperm competition or cryptic female choice. Furthermore, we found a negative male age effect on paternity in second matings, implying that age-related deterioration of spermatozoa may promote variation in fertilization success. Female receptivity decreased significantly in second matings; only 70% of the females remated. Females that accepted a second copulation were found to terminate these much earlier and with higher probability than first matings. This suggests that the intensity of conflict between the sexes is higher in second matings. Increased intensity of sexual conflict may be responsible for stronger selection on male traits, as pre-copulatory behaviour and age only affected male copulatory performance and paternity in second matings. Electronic Publication     

Fertilisation success in the fly Dryomyza anilis (Dryomyzidae): effects of male size and the mating situation

Male fertilisation success in relation to male size and the mating situation (ordinary pair formation with a single, nonvirgin female vs. take overs) was examined in the fly Dryomyza anilis. In ordinary matings, large males achieved higher fertilisation success than small ones when they were the second to mate with the female. Take overs differ from ordinary pair formation in that the second male experiences intensified sperm competition. This is because in take overs the female is not able to discharge any of the sperm inseminated by the first male as she usually does before a new mating. Compared with ordinary matings, take overs reduced the fertilisation success of the second male by 8–10%, whereas that of the first male was 7–14% higher in take overs. Even though the intruder was always larger than the paired male his superior fertilisation success did not compensate for the effect of the sperm already present in the female. In D. anilis, males can increase their fertilisation success by tapping the female's external genitalia with their claspers or having several copulation bouts per mating. Thus, in a take over, the intruder could respond to the intensified sperm competition by performing more tapping sequences per copulation bout or more copulation bouts per mating. In matings observed in the wild, males performed more tapping sequences after a take over than after pair formation with a single female, although the difference was not significant. The results show that there are differences in fertilisation success between males of different size. In addition, different mating situations can result in considerable variation in the fertilisation success of an individual male. Higher fertilisation success for the first male after a take over may be significant, in particular, for the reproductive success of small males, which frequently lose their females to large males.     

Prolonged copulation: A male ‘postcopulatory’ strategy in a promiscuous species,Lygaeus equestris (Heteroptera: Lygaeidae)

Summary Copulation in Lygaeus equestris L. (Heteroptera, Lygaeidae) is known to last 0.5–24 h. Variations in copula duration of field-collected insects were studied in the laboratory, and different hypotheses concerning the significance of prolonged copulations were tested.Through reciprocal matings with normal and sterile (irradiated) males, sperm displacement was estimated at about 90%. A male could thus increase his fitness by preventing subsequent matings of an inseminated female.Copulations with virgin insects, observed over 15 h, were classified in two categories: short (0.5–8.0 h) and long (> 15 h) duration (Fig. 1). No difference in the number of fertilized eggs was found between long and short copulations, and the insemination rate was highest during the first hour of a couplation (Fig. 2). Long-lasting couplations did not give rise to increased sperm displacement (Table 2). These results indicate that there is no difference in the amount of sperm transferred during short and long copulations and that no insemination takes place during the latter part of a prolonged copulation.Longer lasting copulations occurred when the sex ratio was male-biased than when it was female-biased (Fig. 1). The frequency of prolonged copulations was higher when the female was gravid than when she contained no eggs (virgin) (Figs. 3 and 4). Support is thus given to the hypothesis that prolonged copulation is a male postinsemination strategy to prevent subsequent matings of a female.Females subjected to male competition over a longer period, laid fewer eggs and died earlier than females that were mated but subsequently isolated from males (Fig. 5). Egg batches from females living with males were larger than batches from mated, isolated females (Table 3). This is probably due to a combination of many matings with short intermissions and prolonged copulations, both of which postpone oviposition.Comparisons between copulations of males with either gravid or virgin females during these experiments, led to the conclusion that short copulations with virgin females result from a male decision to terminate copulation after insemination is completed, whereas copula duration with gravid females is more likely to depend on a combination of male and female behavior.     

Post-copulatory aggression toward their mates by males of the rove beetle Leistotrophus versicolor (Coleoptera: Staphylinidae)

Summary Rove beetles (Leistotrophus versicolor) forage and mate at dung and carrion in the riparian forest of northwestern Costa Rica. After copulating, males often launch a post-copulatory attack against their recent partner. We test four hypotheses on the adaptive value of male behavior: (1) The sperm competition hypothesis proposes that the behavior may be the functional equivalent of mate-guarding, (2) the sperm-transfer signal hypothesis states that males bite their mates after copulating to signal that they have successfully passed sperm, (3) the feeding competition hypothesis argues that male aggression toward mates occurs to drive away competitors for fly prey, and (4) the redirected aggression hypothesis is that male attacks after mating occur when threatened males redirect their aggression onto their partners. Only the sperm competition hypothesis withstands testing. As required by this hypothesis, females are usually receptive while at dung, and will mate with more than one male in a morning. In addition, males are more likely to attack a mate when they have fought earlier in the day with other males, an indicator of the presence of rival males and the risk of sperm competition. Contrary to the sperm-transfer signal hypothesis (2), biting of mates does not occur after nearly 40% of all copulations; it seems unlikely that mating males so often fail to transfer sperm. Whether males have fed or not prior to mating has no effect on the probability of post-copulatory attack, a result that contradicts the food competition hypothesis (3). Finally, the occurrence of attacks by males on females in the absence of an immediate threat from a rival argues against the redirected aggression hypothesis (4).     

Prolonged mating in the milkweed leaf beetle Labidomera clivicollis clivicollis (Coleoptera: Chrysomelidae): a test of the “sperm-loading” hypothesis

Summary Female milkweed leaf beetles (Labidomera clivicollis clivicollis) frequently mate with more than one male, and pairs form mating associations which last for up to 42 h in the field. I tested the hypothesis that males remaining with females for long periods of time benefit by numerically overwhelming the sperm of their competitors. Male L.c.clivicollis copulated intermittently with females throughout an 11 hour period in the laboratory. When virgin females were allowed a single copulation, 94.3% of the sperm they received were located in the spermatheca immediately afterward. Males were not sperm-depleted, for they had large numbers of sperm available after one copulation (mean=230,000±43,200); the maximal number of sperm a male transferred to a female in 24 h was 30,500. There was a positive linear relationship between the number of sperm transferred and time up to 24 h after mounting (r ²=0.178, P<0.003). These data suggest that males transfer increasing numbers of sperm throughout a 24-h-period. Mating duration was the most important determinant of paternity when females were placed with one male for 24 h and another male for 6 hours. Females whose first matings were longer showed first male sperm predominance (as determined by starch-gel electrophoresis), while females whose second matings were longer showed last male sperm predominance. In view of these data, it is puzzling that males do not inseminate with large numbers of sperm immediately after mounting the female. It is possible that female refractory behaviors make insemination difficult and favor prolonged mating by male milkweed leaf beetles.     

Size-dependent mating and gender choice in a simultaneous hermaphrodite, Bulla gouldiana

Simultaneous hermaphrodites are predicted to optimally divide resources between male and female function, which can result in both size-dependent mating behaviors and conflict between potential mates. Predicted strategies include size-assortative mating, conditional exchange of gametes, and mating patterns where relative size affects investment in each sexual role. This study investigated the effect of body size on the mating strategies of a hermaphroditic opisthobranch, Bulla gouldiana. Although individuals were spatially aggregated in the field with high levels of movement and size variation, there was little evidence for predictions. Laboratory experiments, however, revealed complicated effects of mass on the probability and duration of mating, as well as gender choice. Pairs were more likely to mate if they included at least one large animal, with the larger animal typically inseminating the smaller. When both individuals were large, they were more likely to each mate in both sexual roles by switching roles once. Although B. gouldiana did not usually alternate between sexual roles multiple times within mating events, paired individuals behaved similarly (neither or both mating as sperm donors) more often than expected by chance. This suggests some level of reciprocity, which is unlikely to be conditional given rates of unilateral mating. When the larger member of the mating pair inseminated the smaller, the duration of insemination increased with the size of the smaller sperm recipient. Copulations lasted longer in pairs that switched sexual roles than in those that did not switch roles. This study suggests that variation in body size can lead to size-dependent mating patterns, but only some of the patterns in B. gouldiana support theoretical predictions. We review other studies that have addressed similar issues, providing inconsistent mating patterns in sperm-storing hermaphrodites.     

Large donors transfer more sperm,but depletion is faster in a promiscuous hermaphrodite

Males are limited in sperm and seminal fluid, which can lead to prudent sperm allocation across consecutive matings. While sperm depletion is usually considered a population characteristic, we investigated an individual size-related variation in sperm depletion in the promiscuous sea slug Chelidonura sandrana. We found that sperm counts substantially declined across four successive copulations, indicating sperm depletion. Sperm depletion occurred at a disproportionally fast rate in larger sperm donors. Based on our finding of body size-dependent sperm depletion, we further investigated whether this translates into prudent sperm allocation across ejaculates. We found no evidence that slugs adjusted the number of delivered sperm in response to partner body weight, a tight proxy of fecundity. Instead, sperm counts increased with sperm donor body weight. Sperm donation independent of partner body size and—as found in previous work—partner novelty leaves scope for currently unidentified traits like the sperm competition environment to affect male sperm allocation. Alternatively, we propose that, in C. sandrana, a low optimal male mating rate close to the female optimum as indicated by our data may ultimately render indiscriminate sperm allocation the most beneficial male mating strategy.     

Male genital traits and mating interval affect male fertilization success in the water strider Gerris lacustris

Parker's seminal work brought attention to the possibility of postmating sexual selection by non-random fertilization success. Mechanisms for these processes are still only partly understood and there is clearly a need for more studies of intraspecific variation in sperm precedence. Here, we report results from an experimental study of the variation in fertilization success between males of the water strider Gerris lacustris. Genital morphology, male body size, and copulation duration were examined as possible correlates of paternity. The significance of guarding duration was also analysed. Only male genital morphology was correlated to fertilization success. This is one of the first studies showing a relationship between male genital traits and fertilization success, supporting the view that sexual selection may be responsible for the rapid and divergent evolution of genital structures in animals with internal fertilization. The fertilization success of last males varied considerably after double matings with a short mating interval (10 min). Last-male priority ranged from 0 to 100% and usually one of the males involved fertilized almost all the eggs. After double matings with a short mating interval, the proportion of eggs fertilized by the last male averaged 0.68 and was greater than 0.5. In contrast, the average fertilization success was biased towards the first male when the matings were more spread out over time (24 h). These results do not support earlier suggestions of a widespread last-male sperm priority in water striders. Received: 28 July 1998 / Received in revised form: 15 March 1999 / Accepted: 28 March 1999     

Sperm removal and ejaculate size correlate with chelae asymmetry in a freshwater crayfish species

Asymmetry in traits of sexual relevance may impair copulation behaviour and sexual performance of males, ultimately resulting in a fitness cost. Freshwater crayfish males use chelae, a sexually selected trait, to secure and position the female prior to and during mating. Thus, a relatively large chelae asymmetry, resulting from accidental loss and regeneration of one cheliped after autotomy, could have great consequences for male sexual behaviour. We studied copulatory behaviour and sperm expenditure of males paired to a mated female in Austropotamobius italicus, a freshwater crayfish species where both male and female mate multiply and where last-mating males are able to actively remove previously deposited sperm. We aimed at assessing whether male sperm removal and expenditure varied according to sperm allocated by first-mating males, and according to copulation behaviour and phenotypic traits (carapace length, chelae length and relative chelae asymmetry) of second-mating males. Second-mating males did not adjust their ejaculate size in relation to first-mating male ejaculate, nor to the first-mating male’s sperm removed. Moreover, the amount of sperm removed by second-mating males increased with increasing first-mating males ejaculate size, and first-mating male sperm remaining after removal did not correlate with the original first-mating male ejaculate size. Interestingly, the amount of sperm removed by second-mating males decreased with increasing relative chelae asymmetry, while increasing with male body size. However, second-mating (but not first-mating) asymmetric-clawed males produced larger ejaculates than symmetric-clawed ones. Importantly, the proportion of second-mating male sperm remaining after the two matings did not vary with relative chelae asymmetry nor with body size of second-mating males. Thus, small, asymmetric-clawed crayfish males appear to adopt sperm allocation tactics that allow them to fully compensate for their inferior sperm removal ability.     

Female benefit,male risk: Polyandry in the true armyworm Pseudaletia unipuncta

In Lepidoptera polyandry is common and females may increase their lifetime reproductive output through repeated matings if they acquire essential resources from male ejaculates. However, the paternity of males mating with previously-mated females is far from assured unless sperm precedence is absolute. In this study on the polyandrous armyworm, Pseudaletia unipuncta, we used two strains of male (the black-eyed wild type and a red-eyed homozygous, recessive mutant), mated with red-eyed females, to determine (i) whether male investment has any impact on female reproductive output, and (ii) if females do benefit from multiple matings, to what extent males fertilize the eggs to which they contributed. Multiple mating resulted in a significant increase in both the fecundity and longevity of females. However, the degree of sperm precedence (those eggs fertilized by the second male) varied from 0–100%, but was not affected by either male size or age, or by the duration of copulation. In cases where sperm precedence was <50% (x = 12%) females produced significantly more eggs (1384 versus 940) prior to the second mating than females where sperm precedence was >50% (x = 89%), indicating that the quality of the first mating influenced the fertilization success of the female's second mate.