Biting off more than you can chew: sexual selection on the free amino acid composition of the spermatophylax in decorated crickets

Nuptial food gifts function to enhance male fertilization success, but their consumption is not always beneficial to females. In decorated crickets, the spermatophore transferred at mating includes a gelatinous mass, the spermatophylax, which is consumed by females after mating. However, females often discard spermatophylaxes shortly after mating, whereupon they terminate sperm transfer. We hypothesized that females discard gifts based on their assessment of the gift itself, and specifically the composition of free amino acids. We tested this hypothesis by comparing spermatophylaxes discarded by females after mating with those that were destined to be fully consumed, and employed multivariate selection analysis to quantify the strength and form of multivariate sexual selection operating on the free amino acid composition of gifts. The analysis yielded a saddle-shaped fitness surface with two local peaks. Different amino acid profiles appear to elicit continued feeding on the spermatophylax either because they offer the same level of gustatory appeal, or because they differentially affect both the gustatory appeal and texture of the spermatophylax. We conclude that the gustatory response of females to males' nuptial food gifts represents an important avenue of post-copulatory mate choice, imposing significant sexual selection on the free amino acid composition of the spermatophylax.     

Food fight: sexual conflict over free amino acids in the nuptial gifts of male decorated crickets

In decorated crickets, Gryllodes sigillatus, the spermatophore that a male transfers at mating includes a gelatinous spermatophylax that the female consumes, delaying her removal of the sperm‐filled ampulla. Male fertilization success increases with the length of time females spend feeding on the spermatophylax, while females may benefit by prematurely discarding the spermatophylaxes of undesirable males. This sexual conflict should favour males that produce increasingly appealing spermatophylaxes, and females that resist this manipulation. To determine the genetic basis of female spermatophylax feeding behaviour, we fed spermatophylaxes to females of nine inbred lines and found that female genotype had a major influence on spermatophylax feeding duration. The amino acid composition of the spermatophylax was also significantly heritable. There was a positive genetic correlation between spermatophylax feeding duration and the gustatory appeal of the spermatophylax. This correlation suggests that genes expressed in males that produce more manipulative spermatophylaxes are positively linked to genes expressed in females that make them more vulnerable to manipulation. Outbred females spent less time feeding on spermatophylaxes than inbred females, and thus showed greater resistance to male manipulation. Further, in a nonspermatophylax producing cricket (Acheta domesticus), females were significantly more prone to feeding on spermatophylaxes than outbred female Gryllodes. Collectively, these results suggest a history of sexually antagonistic coevolution over the consumption of nuptial food gifts.     

Repeated matings offset costs of reproduction in female crickets

Summary Courtship food gifts can be a significant source of nutrition to females and costly for males to produce; hence, costs of reproduction should be reduced for multiple-mating females and increased for multiplemating males in a gift-giving species. We tested this hypothesis by experimentally manipulating mating opportunities of males and females of two cricket species,Gryllodes sigillatus, a gift-giving species andGryllus veletis, a non-gift-giving species. Females of both species consume the externally attached spermatophore after mating, but inG. sigillatus, the sperm-containing ampulla is accompanied by a large gelatinous spermatophylax. In both species, survival of mated females given limited access to males was reduced relative to virgin females, thus suggesting a cost of reproduction to females. However, females given unlimited mating opportunities lived as long as virgins and also produced significantly more offspring than limited-access females. These results suggest that benefits of repeated matings, particularly those arising through spermatophore consumption, offset costs of reproduction in females. Lack of a treatment by species interaction suggests that females of both species derive nutritional benefits through spermatophore consumption, and that any additional advantage to the consumption of the spermatophylax inG. sigillatus is offset by more frequent mating byG. veletis females. In contrast to females, varying mating opportunities had no effect on male survival, suggesting that mating effort is not very costly to males. Male survival increased linearly with body mass but only when males were food-deprived, suggesting that larger males possess greater initial energy reserves to sustain their longevity when food-stressed.     

Hydration benefits to courtship feeding in crickets

The spermatophore transferred by male decorated crickets (Gryllodes sigillatus) at mating includes a large gelatinous spermatophylax that the female consumes after copulation. Although previous studies have shown that G. sigillatus females gain no nutritional benefits from consuming food gifts, there may be other benefits to their consumption. We examined potential hydration benefits to females by experimentally manipulating both the availability of water and the number of food gifts that females consumed, and by measuring their effect on female fitness. Analysis of the number of nymphs produced by females revealed a significant interaction between the number of spermatophylaxes consumed and water availability. When spermatophylaxes were not provided, females given water ad libitum produced significantly more nymphs than females subjected to water stress. Female longevity was significantly affected by water availability, with an increase in the availability of water corresponding to a significant increase in female longevity. These data suggest that female G. sigillatus accrue fitness benefits by consuming spermatophylaxes when alternative sources of water are unavailable. In addition, females appear to allocate water contained in spermatophylaxes towards reproduction as opposed to survival.     

RESOURCE INFLUENCED SEX ROLES OF ZAPROCHILINE TETTIGONIIDS (ORTHOPTERA: TETTIGONIIDAE)

Male tettigoniids donate nutrients to females at mating in the form of a spermatophylax. Male-donated nutrients function as paternal investment leading to a reversal in the sex roles of males and females. Reversal in the behavioral sex roles of a zaphrochiline tettigoniid was found to be directly related to the current availability of food resources in the environment. When resource availability was low, females were less fecund and males had lower and more variable accessory gland weights (the spermatophylax producing gland) than when resource availability was high. When resources were scarce, larger individuals had a reproductive advantage having more eggs or heavier accessory glands. All individuals were equally fecund or had equal accessory gland weights when resources were plentiful. During low resource availability males that had a spermatophylax to offer were choosy of their mates, and females were competitive. When resources were plentiful, males were less discriminative and females showed signs of discrimination. There was evidence that female sexual motivation decreased when environmental resources were plentiful (as indicated by mating frequency) and that only females of low nutritional status continued to mate. When resources were scarce, females achieved fecundities equivalent to those achieved during high resource availability through spermatophylax consumption. These data support the hypothesis that females continue to mate to compensate for low resource availability and that male parental investment may be important only in poor quality habitats. When resources are plentiful females do not need to remate. Thus resource availability may directly influence the number of sexually active males and females (operational sex ratio) and the form of sexual selection.     

Virgin male mating advantage in a primitive acoustic insect (Orthoptera: Haglidae)

Male reproductive behavior in the relict flightless haglid, Cyphoderris strepitans,entails the generation of sound signals and the provision of nuptial gifts to mates. These food gifts take two forms: (1) a gelatinous mass (spermatophylax) augmenting the spermatophore and (2) fleshy metathoracic wings adapted to be eaten. The female consumes a portion of the male's underwings during courtship and copulation and the spermatophylax afterward. The incidence of wing-feeding wounds can be used to monitor the mating success of field-caught males. If, when a male mates, he compromises his ability to provide subsequent nutritive gifts, females would benefit by mating with virgin rather than nonvirgin males. To test this, mating success of virgin and nonvirgin males was compared in a field population. Virgins were found to obtain more matings than explained by their numbers relative to nonvirgins in the population. We conclude that, having mated, a male is at a disadvantage, relative to his virgin competitors, in securing further matings.     

Female Remating Behavior in a Lekking Moth

In polyandrous species, male reproductive success will at least partly be determined by males' success in sperm competition. To understand the potential for post‐mating sexual selection, it is therefore important to assess the extent of female remating. In the lekking moth Achroia grisella, male mating success is strongly determined by female choice based on the attractiveness of male ultrasonic songs. Although observations have indicated that some females will remate, only little is known about the level of sperm competition. In many species, females are more likely to remate if their first mating involved an already mated male than if the first male was virgin. Potentially, this is because mated males are less well able to provide an adequate sperm supply, nutrients, or substances inhibiting female remating. This phenomenon will effectively reduce the strength of pre‐copulatory sexual selection because attractive males with high mating success will be more susceptible to sperm competition. We therefore performed an experiment designed both to provide a more precise estimate of female remating probability and simultaneously to test the hypothesis that female remating is influenced by male mating history. Overall, approximately one of five females remated with a second male. Yet, although females mated to non‐virgin males were somewhat more prone to remate, the effect of male mating history was not significant. The results revealed, however, that heavier females were more likely to remate. Furthermore, we found that females' second copulations were longer, suggesting that, in accordance with theory, males may invest more sperm in situations with an elevated risk of sperm competition.     

Sexual conflict in Sepsis cynipsea: female reluctance, fertility and mate choice

Sexual conflict can elevate mating costs via male inflicted damage to females. Possible selective advantages to males include decreasing the likelihood that females remate and/or increasing females' current reproductive investment in a manner analogous to terminal reproductive investment. We investigated female mating behaviour relative to their number of previous copulations in the fly Sepsis cynipsea, and whether males accepted as first mates were more likely to be accepted again. Females were more likely to remate with new rather than original males, although there was no associated fitness benefit, and in contrast to theoretical predictions, females became less reluctant to remate as the number of previous copulations increased. Additionally, females did not increase reproductive investment as would be expected if they were ensuring their final reproductive efforts were maximized by remating. This suggests that damaging females is a pleiotropic effect which inadvertently leads to increased, not decreased, polyandry.     

SPERMATOPHORE SIZE IN BUSHCRICKETS: COMPARATIVE EVIDENCE FOR NUPTIAL GIFTS AS A SPERM PROTECTION DEVICE

During courtship and copulation, males of many insect species provide the female with a nuptial gift of a prey item or synthesized material. These gifts may be explained as a form of paternal investment by increasing female reproductive output, or in terms of mating effort by increasing male fertilization success. These explanations, while not mutually exclusive, are controversial. While experimental studies examine the maintenance of nuptial gifts in single species, comparative studies are required to indicate more general evolutionary trends. Male bushcrickets provide females with a nuptial gift, a spermatophylax, which is transferred to females at mating along with the sperm-containing ampulla. Analysis of comparative data of 28 species of bushcrickets (Orthoptera: Tettigoniidae), reveals that male spermatophore size (spermatophylax and ampulla weight) is positively correlated with female refractory period, which, in turn, correlates with male fertilization success. Moreover, gift size (the spermatophylax) covaries with ejaculate size (the ampulla), which is consistent with the hypothesis that it serves as a sperm protection device. In contrast, there is no significant correlation between any measure of female fecundity and male spermatophylax size. This indicates that the variation in spermatophore size among bushcrickets is better explained by a mating-effort function than a paternal investment function.     

Nuptial gift consumption influences female remating in a scorpionfly: male or female control of mating rate?

In the scorpionfly Panorpa cognata, males provide females with saliva secretions as nuptial food gifts. Consequently, females derive material benefits and possibly also genetic benefits from multiple matings. Females therefore generally should have a high motivation to remate. Males, on the other hand, do not share this interest, which will generate a sexual conflict over remating interval, possibly leading to male adaptations that prevent females from remating with other males. In this study, I found that mated females were less prone to copulate than virgin females, despite female benefits of multiple matings. Further, I found that the remating interval was significantly longer if the first copulation was long compared to shorter matings. This effect does not entirely depend on copulation duration per se, but on the amount of saliva, that a female is consuming during copulation. These results suggest a mating-induced refractory period and can be interpreted as male manipulation of female remating behaviour mediated through substances in the nuptial gift. Alternatively, receiving large nuptial gifts may decrease the prospective direct fitness benefits from further copulations, and thus change optimal female remating rate. Furthermore, gift size has been shown to correlate with male nutritional condition, which may be an indicator of male genetic quality. Females may therefore benefit indirectly by not remating following copulations involving large saliva gifts. In this scenario, female remating interval would be an effect of cryptic female choice.     

Paternity of offspring in multiply-mated,female crickets: the effect of nuptial food gifts and the advantage of mating first

The spermatophore transferred by male decorated crickets (Gryllodes sigillatus) includes a large gelatinous mass, the spermatophylax, that is consumed by the female after mating. This nuptial gift preoccupies the female while sperm are discharged from the remaining portion of the spermatophore, the sperm ampulla, into her reproductive tract. There is considerable variation in the mass of the spermatophylax, and about half of all males produce spermatophylaxes that are too small to ensure complete sperm transfer. We tested two hypotheses concerning the maintenance of this variation: (i) males trade-off investment in spermatophylaxes against copulation frequency; and (ii) males synthesize the largest spermatophylaxes of which they are physiologically capable. Males synthesizing large and small food gifts were permitted multiple mating opportunities with the same females, and allozyme markers were used to establish the paternity of offspring. There was a significant advantage to those males that mated first irrespective of gift size. This advantage probably arose, in part, because the sperm of first males would have had exclusive access to females'' eggs during the first 24 hours of oviposition, and underscores the benefits of matings with virgin females. The paternity of ''small-gift'' males increased with gift mass, but there was no such increase in ''large-gift'' males. This difference probably stems from the relationship between gift mass and sperm transfer: most of the gifts of the large-gift males would have been above the threshold needed to achieve complete inseminations, whereas those of small-gift males would have been below the threshold. Within mating-order positions, there was no significant difference in the paternity of large-gift and small-gift males, a result seemingly consistent with the ''trade-off'' hypothesis. However, there was no correlation between spermatophylax mass and male mating frequency, so that the mechanism by which small-gift males offset their fertilization disadvantage remains unknown.     

No genetic correlation between the sexes in mating frequency in the bean beetle, Callosobruchus chinensis

Female multiple mating, which is common in animals, may have evolved not in response to fitness advantages to females but as a genetic corollary to selection on males to mate frequently. This nonadaptive hypothesis assumes a genetic correlation between females and males in mating frequency, which has received a few empirical investigations. We tested this hypothesis by observing the correlated response in male mating frequency in the adzuki bean beetle, Callosobruchus chinensis to artificial selection on female propensity to remate. Compared to control females, females from lines selected for increased or decreased female propensity to remate had, respectively, higher or lower mating frequency measured by the number of mating within a given period. This indicates that female receptivity to remating is genetically correlated with female mating frequency, and thus the artificial selection for female propensity to remate influenced female mating frequency. In contrast, males from the selected lines that diverged in female mating frequency did not vary significantly in their mating frequency. These results indicate that there is no genetic correlation between the sexes in mating frequency in C. chinensis. This study shows that the reason why females in C. chinensis remate despite suffering fitness costs cannot be explained by indirect selection resulting from selection on males to mate multiple times.     

Sex in the morning or in the evening? Females adjust daily mating patterns to the intensity of sexual harassment

Selection on males to mate at a higher rate than females often results in male harassment of females and counteracting female responses. When the reproductive value of copulation changes over time, these mating strategies are expected to be time dependent. Here, we demonstrate that variation in the intensity of male harassment leads to drastic changes in female daily mating patterns. In feral populations of fowl Gallus gallus domesticus, male harassment is intense, particularly in the evening when inseminations are most likely to result in fertilization. We experimentally manipulated the intensity of male harassment through similar-sized groups of different sex ratios. Male mating propensity was always higher than females', particularly in male-biased groups and in the evening, when males were closer to and more likely to approach females. Females counteracted male harassment by escalating resistance to mating and--crucially--by shifting their daily mating pattern: in strongly female-biased groups with relaxed sexual harassment, females solicited sex in the evening, while in male-biased groups, they solicited sex in the morning, thus avoiding harassment in the evening. Together, these results indicate that intersexual conflict may occur not only over mating rates but also over when in the day to copulate.     

Weighing costs and benefits of mating in bushcrickets (Insecta: Orthoptera: Tettigoniidae), with an emphasis on nuptial gifts,protandry and mate density

ABSTRACT: Sexual selection is a major force driving evolution and is intertwined with ecological factors. Differential allocation of limited resources has a central role in the cost of reproduction. In this paper, I review the costs and benefits of mating in tettigoniids, focussing on nuptial gifts, their trade-off with male calling songs, protandry and how mate density influences mate choice. Tettigoniids have been widely used as model systems for studies of mating costs and benefits; they can provide useful general insights. The production and exchange of large nuptial gifts by males for mating is an important reproductive strategy in tettigoniids. As predicted by sexual selection theory spermatophylax size is condition dependent and is constrained by the need to invest in calling to attract mates also. Under some circumstances, females benefit directly from the nuptial gifts by an increase in reproductive output. However, compounds in the nuptial gift can also benefit the male by prolonging the period before the female remates. There is also a trade-off between adult male maturation and mating success. Where males mature before females (protandry) the level of protandry varies in the direction predicted by sperm competition theory; namely, early male maturation is correlated with a high level of first inseminations being reproductively successful. Lastly, mate density in bushcrickets is an important environmental factor influencing the behavioural decisions of individuals. Where mates are abundant, individuals are more choosey of mates; when they are scarce, individuals are less choosey. This review reinforces the view that tettigoniids provide excellent models to test and understand the economics of matings in both sexes.     

Courtship role reversal in bush crickets: another role for parasites?

The last decade has seen an increasing body of evidence in supportof the idea that parasites can play a role in sexual selection.Parasites have been shown to influence mate choice and matingcompetition. Here I demonstrate a further role for parasitesin that they can determine the courtship roles of males andfemales and thus the sex on which sexual selection acts. Malebush crickets, Requena verticalis, feed their mates during inseminationwith a nutritious meal, the spermatophylax, that increases femalefecundity. I show how the ability of males to donate nutrientswas reduced by increased intensities of infection of a protozoangut parasite. Further, increased intensity of infection reducedfemale fecundity. Mating trials showed that when females wereuninfected the typical courtship roles prevailed; females werethe coy, discriminating sex. However, when infected with gutparasites, females attempted to mate more often and males adoptedthe coy discriminative role in courtship. Thus it appeared thatmale donations were of greater importance for reproductivelyconstrained, infected females. The infection status of the malehad no influence on courtship role reversal supporting the ideathat proximate cues in female behavior determine the courtshiprole adopted by males. It is argued that parasites will be animportant determinant f courtship role reversal in any specieswhere male gifts influence female fecundity, and parasites constrainhost reproduction, because of the opposing way in which reproductiveconstraints will influence male and female mating frequency.     

The secondary copulatory organ in female ground weta (Hemiandrus pallitarsis,Orthoptera: Anostostomatidae): a sexually selected device in females?

Secondary sexual devices in female insects, primarily abdominal modifications, appear to function as a means of thwarting coercive mating attempts by males or are, in rare cases, sexually selected adaptations. Female ground weta, Hemiandrus pallitarsis (Orthoptera: Anostostomatidae), have an elaborate elbowed device on the underside of the mid-abdomen. Experimental removal of this accessory organ shows that it does not increase the probability of mating as predicted by the thwart-copulation hypothesis. Instead, removal prevents copulation, thus demonstrating that it is a secondary copulatory device. The male attaches to the organ both at the beginning of copulation and at the end, when he positions himself to adhere a spermatophylax food gift onto the mid-ventral region of his mate. The female accessory organ does not function to manipulate eggs or larvae (females provide care to their single clutch of offspring) and is unlikely to be a copulatory structure that prevents hybridization. The great extent of the modification of the ventral abdominal segments of H. pallitarsis females compared to other Hemiandrus species is consistent with a history of sexual selection on the accessory organ. Taken together, these results and the finding that the length of the accessory organ of H. pallitarsis correlates with female fecundity, suggest that this structure evolved under sexual selection to acquire nuptial gifts from males.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 463–469.     

The function of nuptial feeding in insects: a review of empirical studies

Nuptial feeding encompasses any form of nutrient transfer from the male to the female during or directly after courtship and/or copulation. In insects, nuptial gifts may take the form of food captured or collected by the male, parts, or even the whole of the male's body, or glandular products of the male such as salivary secretions, external glandular secretions, the spermatophore and substances in the ejaculate. Over the past decade, there has been considerable debate over the current function of nuptial feeding in insects. This debate has centred on the issue of whether nuptial gifts function as paternal investment (i.e. function to increase the fitness and/or number of the gift-giving male's own offspring) or as mating effort (i.e. function to attract females, facilitate coupling, and/or to maximize ejaculate transfer), although the two hypotheses are not mutually exclusive. In the present article, evidence for the potential of nuptial gifts to function as either paternal investment, mating effort, or both is reviewed for each form of nuptial feeding in each insect taxon for which sufficient data are available. Empirical evidence suggests that many diverse forms of nuptial feeding in different insect taxa function, at least in part, as mating effort. For example, nuptial prey and salivary masses in the Mecoptera, regurgitated food in Drosophila (Diptera), hind-wing feeding in Cyphoderris (Orthoptera) and the secretion of the male's cephalic gland in Neopyrochroa (Coleoptera) and Zorotypus (Zoraptera) appear to function to entice females to copulate and/or to facilitate coupling. Nuptial prey and salivary masses in the Mecoptera also appear to function to maximize ejaculate transfer (which is also a form of mating effort), as do nuptial prey in Empis (Diptera), external glandular secretions in Oecanthus and Allonemobius (Orthoptera) and the spermatophylax in gryllids and tettigoniids (Orthoptera). Large spermatophores in, for example, the Lepidoptera and Coleoptera, also appear to be maintained by selection on the male to maximize ejaculate transfer and thereby counter the effects of sperm competition. In contrast to the large amount of evidence in support of the mating effort hypothesis, there is a relative lack of good evidence to support the paternal investment hypothesis. Certain studies have demonstrated an increase in the weight and/or number of eggs laid as a result of the receipt of larger gifts, or a greater number of gifts, in tettigoniids, gryllids, acridids, mantids, bruchid beetles, drosophilids and lepidopterans. However, virtually all of these studies (with the possible exception of studies of the spermatophylax in tettigoniids) have failed to control adequately for hormonal substances in the ejaculate that are known to affect female reproductive output. Furthermore, in at least four tettigoniids (but not in the case of two species), three lepidopterans, a drosophilid and probably also bruchid beetles and bittacids, evidence suggests that the male has a low probability of fertilising the eggs that stand to benefit from his nuptial gift nutrients. Therefore, the hypothesis that paternal investment might account for the function of nuptial gifts in general is not supported.     

Female nursery web spiders (Pisaurina mira) benefit from consuming their mate

The often coincidental involvement of cooperation and conflict in animal reproduction is epitomized by sexual cannibalism, which can lead to obvious male costs while simultaneously providing direct benefits to developing offspring. Male nursery web spiders (Pisaurina mira) avoid postcopulatory sexual cannibalism by wrapping females with silk. Here, we test the hypothesis that this silk wrapping results in a loss of consumption cost for females. In specific, we hypothesize that females lose out on potential fitness benefits associated with cannibalizing their mating partners. To test this, we paired females with males that were experimentally manipulated to prevent the silk wrapping of females, thereby increasing the likelihood of sexual cannibalism. Females either did not kill their mate, and thus consumed nothing, or did kill their mate. If females killed their mating partner, we allowed them to consume the male, consume nothing, or consume a cricket. We found no effect of female or male body sizes on the likelihood of females killing their mate. While our treatments did not affect the number of offspring females produced, females that consumed either a male or a cricket produced egg sacs faster than females that consumed nothing, suggesting a benefit of increased postcopulatory food consumption. Further, only females that ate a male had heavier and longer lived offspring, suggesting a benefit of sexual cannibalism specifically. Our results support the hypothesis that females can receive fitness benefits associated with sexual cannibalism.     

Neither Mating Rate Nor Spermatophore Feeding Influences Longevity in a Ladybird Beetle

Females of many species experience costs associated with mating. Seminal products, including nuptial gifts, may mitigate these mating costs or exacerbate them. For example, nuptial gifts derived from male accessory glands may transfer nutrition or potentially harmful seminal proteins to females. In this study, we assay the costs of multiple mating and the consumption of seminal products in a ladybird beetle. We compared longevity in females mated singly or multiply, while allowing or preventing spermatophore consumption at each mating. In order to distinguish a cost of mating per se from a cost of elevated reproduction, we prevented reproduction by using nutrient‐stressed females. Mating singly or multiply had no effect on female longevity, nor did spermatophore feeding influence longevity. The results imply, first, that intermediate mating rates do not directly harm females, though females may experience other indirect costs of mating (e.g. reduced foraging efficiency) or costs of reproduction; and second, that spermatophores transfer neither food nor directly harmful substances to female ladybirds.