Male crickets adjust ejaculate quality with both risk and intensity of sperm competition

Sperm competition theory predicts that males should increase their expenditure on the ejaculate with increasing risk of sperm competition, but decrease their expenditure with increasing intensity. There is accumulating evidence for sperm competition theory, based on examinations of testes size and/or the numbers of sperm ejaculated. However, recent studies suggest that ejaculate quality can also be subject to selection by sperm competition. We used experimental manipulations of the risk and intensity of sperm competition in the cricket, Teleogryllus oceanicus. We found that males produced ejaculates with a greater percentage of live sperm when they had encountered a rival male prior to mating. However, when mating with a female that presented a high intensity of sperm competition, males did not respond to risk, but produced ejaculates with a reduced percentage of live sperm. Our data suggest that males exhibit a fine-tuned hierarchy of responses to these cues of sperm competition.     

Male Drosophila melanogaster show adaptive mating bias in response to female infection status

Given the non-trivial cost of reproduction for males and substantial variation in female quality, males have been predicted to show mating bias as an evolved strategy. Using a large outbred population of Drosophila melanogaster, we test this prediction and show that males may adaptively bias their mating effort in response to the infection status of females. Given a simultaneous choice between females infected with pathogenic bacteria and sham infected females, males preferentially mated with the latter, who had a higher reproductive output compared to infected females. This may provide evidence for pre-copulatory male mate choice. Assessment of the reproductive behaviour ensured that the observed pattern of mating bias was not due to differences in receptivity between females infected with pathogenic bacteria and sham infected females. Further, there was no evidence for post-copulatory male mate choice measured in terms of copulation duration.     

Influence of female age, sperm senescence and multiple mating on sperm viability in female Drosophila melanogaster

Sperm viability has been associated with the degree of promiscuity across species, as well as the degree of reproductive success within species. Thus, sperm survival within the female reproductive tract likely plays a key role in how mating systems evolve. In the fruit fly, Drosophila melanogaster, however, the extent and cause of sperm death has been the subject of recent debate. Here, we assess sperm death within the female reproductive tract of D. melanogaster following single and multiple matings in order to elucidate the extent of death and its potential mechanisms, including an acute female response to mating, female age and/or sperm senescence. We found no evidence that sperm viability was influenced by an acute female response or female age. We also found that rival ejaculates did not influence viability, supporting recent work in the system. Instead, the majority of death appears to be due to the aging of male gametes within the female, and that at least some dead resident sperm remain in the female after multiple mating. In contrast to earlier in vivo work, we found that overall sperm death was minimal (8.7%), indicating viability should have a negligible influence on female remating rates.     

Size-dependent response to conspecific mating calls by male crickets.

Male sexual displays provide females with information that is crucial to their reproductive decisions. That same information is available to eavesdroppers, with potential consequences for both signaller and receiver. We present empirical evidence for size-dependent responses to intersexual communication by conspecific rivals. Acoustic features of a male house cricket's (Acheta domesticus) mating call are positively associated with its size, with females preferring the calls of larger males. In order to investigate whether conspecific males make use of the information content of mating calls, we examined their phonotactic responses to call recordings that differ in attractiveness to females. Males of all sizes exhibited positive phonotaxis, with smaller males showing a clear preference for female-preferred calls. Smaller males were also less likely to seek contact with the speaker playing their chosen call. We discuss possible explanations for this size-dependent male behaviour.     

Male and female genotype and a genotype-by-genotype interaction mediate the effects of mating on cellular but not humoral immunity in female decorated crickets

Sexually antagonistic coevolution is predicted to lead to the divergence of male and female genotypes related to the effects of substances transferred by males at mating on female physiology. The outcome of mating should thus depend on the specific combination of mating genotypes. Although mating has been shown to influence female immunity in diverse insect taxa, a male–female genotype-by-genotype effect on female immunity post mating remains largely unexplored. Here, we investigate the effects of mating on female decorated cricket baseline immunity and the potential for a male-genotype-by-female-genotype interaction affecting this response. Females from three distinct genotypic backgrounds were left unmated or singly mated in a fully reciprocal design to males from the same three genotypic backgrounds. Hemocytes and hemocyte microaggregations were quantified for female cellular immunity, and phenoloxidase, involved in melanization, and antibacterial activity for humoral immunity. In this system, female cellular immunity was more reactive to mating, and mating effects were genotype-dependent. Specifically, for hemocytes, a genotype-by-mating status interaction mediated the effect of mating per se, and a significant male–female genotype-by-genotype interaction determined hemocyte depletion post mating. Microaggregations were influenced by the female’s genotype or that of her mate. Female humoral immune measures were unaffected, indicating that the propensity for post-mating effects on females is dependent on the component of baseline immunity. The genotype-by-genotype effect on hemocytes supports a role of sexual conflict in post-mating immune suppression, suggesting divergence of male genotypes with respect to modification of female post-mating immunity, and divergence of female genotypes in resistance to these effects.Subject terms: Non-model organisms, Sexual selection, Behavioural ecology, Evolutionary ecology, Genetic interaction     

Facultative sperm storage in response to nutritional status in a female insect

We investigated sperm storage by females of a predatory bug, Nabis rugosus, before and after hibernation and with regard to nutritional status. In the field, females had more sperm available before than after hibernation. Food-deprived females maintained fewer sperm than fed females before but not after hibernation. However, after hibernation food-deprived females suppressed egg production rather than decreasing sperm-storage efficiency. Because mated females did not exhibit increased overwinter survival relative to virgin females there is a low likelihood of direct benefits in the form of ejaculate-derived nutrients. If nutrition-dependent sperm storage by females is widespread in the animal kingdom, our findings may have important implications for the understanding of postcopulatory sexual selection and the evolution of food gifts.     

Male cleaner wrasses adjust punishment of female partners according to the stakes

Punishment is an important deterrent against cheating in cooperative interactions. In humans, the severity of cheating affects the strength of punishment which, in turn, affects the punished individual's future behaviour. Here, we show such flexible adjustments for the first time in a non-human species, the cleaner wrasse (Labroides dimidiatus), where males are known to punish female partners. We exposed pairs of cleaners to a model client offering two types of food, preferred 'prawn' items and less-preferred 'flake' items. Analogous to interactions with real clients, eating a preferred prawn item ('cheating') led to model client removal. We varied the extent to which female cheating caused pay-off reduction to the male and measured the corresponding severity of male punishment. Males punished females more severely when females cheated during interactions with high value, rather than low value, model clients; and when females were similar in size to the male. This pattern may arise because, in this protogynous hermaphrodite, cheating by similar-sized females may reduce size differences to the extent that females change sex and become reproductive competitors. In response to more severe punishment from males, females behaved more cooperatively. Our results show that punishment can be adjusted to circumstances and that such subtleties can have an important bearing on the outcome of cooperative interactions.     

Male dominance determines female egg laying rate in crickets

A key prediction of theories of differential allocation and sexual conflict is that male phenotype will affect resource allocation by females. Females may adaptively increase investment in offspring when mated to high quality males to enhance the quality of their offspring, or males may vary in their ability to manipulate female investment post-mating. Males are known to be able to influence female reproductive investment, but the male traits underlying this ability have been little studied in taxa other than birds. We investigated the relationship between male dominance and female oviposition rate in two separate experiments using the field cricket, Gryllus bimaculatus. In both experiments, females mated to more dominant (but not larger) males laid more eggs. This reveals that either females allocate more effort to reproduction after mating with a dominant male or that dominance status is associated with male ability to manipulate their mates. This is the first evidence that dominance, rather than male attractiveness, has a post-copulatory effect on reproductive investment by females.     

Theoretical influence of female mating status and remating propensity on male sperm allocation patterns

Theoretical models predict that males should allocate more sperm in matings where the immediate risk of sperm competition is high. It has therefore often been argued that males should invest less sperm in matings with virgin females compared with matings with already mated females. However, with relatively polyandrous females, high sperm competition risk will covary with high sperm competition intensity leading to more unpredictable conditions, as high competition intensity should favour smaller ejaculates. With the use of a genetic algorithm, we found that males should allocate more sperm in matings with virgin females when female mating frequency is relatively high, whereas low remating rates will select for higher effort in matings with nonvirgin females. At higher remating rates, first male sperm precedence favours larger ejaculates in matings with virgin females and second male precedence favours the reverse. These results shed some light on several findings that have been difficult to explain adaptively by the hitherto developed theory on sperm allocation.     

A novel method of comparing mating success and survival reveals similar sexual and viability selection for mobility traits in female tree crickets

The relationship between sexual and viability selection in females is necessarily different than that in males, as investment in sexual traits potentially comes at the expense of both fecundity and survival. Accordingly, females do not usually invest in sexually selected traits. However, direct benefits obtained from mating, such as nuptial gifts, may encourage competition among females and subsidize investment into sexually selected traits. We compared sexual and viability selection on female tree crickets Oecanthus nigricornis, a species where females mate frequently to obtain nuptial gifts and sexual selection on females is likely. If male choice determines female mating success in this species, we expect sexual selection for fecundity traits, as males of many species prefer more fecund females. Alternatively, intrasexual scramble or combat competition on females may select for larger jumping legs or wider heads (respectively). We estimated mating success in wild caught crickets using microsatellite analysis of stored sperm and estimated relative viability by comparing surviving female O. nigricornis to those captured by a common wasp predator. In support of the scramble competition hypothesis, we found sexual selection for females with larger hind legs and narrower heads. We also found stabilizing viability selection for intermediate head width and hind leg size. As predicted, traits under viability and sexual selection were very similar, and the direction of that selection was not opposing. However, because the shape of sexual and viability selection differs, these episodes of selection may favour slightly different trait sizes.     

Male mating behaviour and sperm production characteristics under varying sperm competition risk in guppies

Since natural populations of guppies, Poecilia reticulata, often differ from one another in social structure, the intensity of sperm competition is likely to vary between localities. Guppies are promiscuous, with female choice for colourful males playing a central role in the mating system. In addition, male guppies use forced copulations to circumvent female choice. Both methods of copulation are used interchangeably by individual males, but the degree to which either is used may depend on the social environment into which males are born. Here we show that male mating behaviour varies according to the rearing sex ratio: when reared in male-biased groups, males performed more forced copulations and fewer courtship displays but showed the opposite pattern of behaviour when reared in female-biased groups. Our prediction, based on sperm competition theory, that stripped sperm number would reflect social structure was not supported by our results. Instead, the overall level of sexual activity (gonopodial thrusts+sigmoid displays) was a better predictor of sperm number in the different groups of males. Rearing density, where sex ratio was controlled, did not significantly affect male mating behaviour or sperm traits. Males reared under the different sex ratios continued to show their characteristic behaviour patterns when placed in equal sex ratio tanks. We conclude, therefore, that males adopt mating strategies to suit their social environment, and that these strategies remain fixed, for short periods at least, if population structure changes. Copyright 1999 The Association for the Study of Animal Behaviour.     

Male competition and female choice interact to determine mating success in the bluefin killifish

Whether male competition and female choice act in concert, independently,or in opposition is a critical issue for understanding sexualselection. In complex social systems, the outcomes of pairwiseinteractions may not be accurate indicators of how sexual selectionemerges. We investigated how female choice and male competitioninteract in the bluefin killifish, Lucania goodei, in a 3-stagedexperiment where 1) females could choose between 2 males, 2)those males could interact in the presence of that female, and3) females and males could freely interact and spawn. In thepairwise stages (1 and 2), females displayed pronounced preferencesbetween males and male competition produced a distinctly dominantindividual. None of the morphological traits, including color,measured in males were associated with either female preferenceor male dominance. When all 3 fish interacted (stage 3), maleactivity level was the sole predictor of spawning success. Maleswith elevated activity levels were more aggressive toward malesand females, exhibited intensified courtship, and obtained morespawns. Female preference did not predict the number of spawnswith a male, but it did predict her latency to spawn; femalesspawned more quickly with preferred males. Thus, male competitionand female choice interact to determine reproductive success,but there is evidence for conflict and a cost to females ofassociating with dominant males. Reproductive success in thisspecies is not easily predicted from simple measures of morphologyor female preference and is influenced by complex social interactions,both between males, and between males and females.     

Male fish use prior knowledge about rivals to adjust their mate choice

Mate choice as one element of sexual selection can be sensitive to public information from neighbouring individuals. Here, we demonstrate that males of the livebearing fish Poecilia mexicana gather complex social information when given a chance to familiarize themselves with rivals prior to mate choice. Focal males ceased to show mating preferences when being observed by a rival (which prevents rivals from copying mating decisions), but this effect was only seen when focal males have perceived rivals as sexually active. In addition, focal males that were observed by a familiar, sexually active rival showed a stronger behavioural response when rivals were larger and thus, more attractive to females. Our study illustrates an unparalleled adjustment in the expression of mating preferences based on social cues, and suggests that male fish are able to remember and strategically exploit information about rivals when performing mate choice.     

Male dominance status,female choice and mating success in the brown lemming, Lemmus trimucronatus

The objective of this study was to determine whether female brown lemmings would selectively mate with dominant conspecific males. In dyadic encounters, oestrous females paired with dominant males engaged in more frequent lordosis and contact social behaviours than those paired with subordinate males. However, the mount and thrust scores of dominant and subordinate males did not differ significantly. When females had simultaneous access to tethered dominant and subordinate males, they spent more time in close proximity to dominant males and avoided contact with subordinate males. Copulation scores were higher with dominant males. In triadic encounters with free-moving dominant and subordinate males, oestrous females spent more time near, and engaged in more sexual behaviour with, dominant males. Intermale agonistic interactions resulted in ‘privileged access’ to females by dominant males. Sexual behaviour scores were lower and there was a 10-fold increase in intermale aggression in triadic encounters between females and unfamiliar dominant and subordinate males. Both intermale agonistic interactions and female ‘choice’ behaviour accounted for the differential copulatory success of dominant males.