Old male mating advantage results from sexual conflict in a butterfly

The evolution of sex peptide: sexual conflict, cooperation, and coevolution

A central paradigm in evolutionary biology is that the fundamental divergence in the fitness interests of the sexes (‘sexual conflict’) can lead to both the evolution of sex‐specific traits that reduce fitness for individuals of the opposite sex, and sexually antagonistic coevolution between the sexes. However, clear examples of traits that evolved in this way – where a single trait in one sex demonstrably depresses the fitness of members of the opposite sex, resulting in antagonistic coevolution – are rare. The Drosophila seminal protein ‘sex peptide’ (SP) is perhaps the most widely cited example of a trait that appears to harm females while benefitting males. Transferred in the ejaculate by males during mating, SP triggers profound and wide‐ranging changes in female behaviour and physiology. Early studies reported that the transfer of SP enhances male fitness while depressing female fitness, providing the foundations for the widespread view that SP has evolved to manipulate females for male benefit. Here, we argue that this view is (i) a simplification of a wider body of contradictory empirical research, (ii) narrow with respect to theory describing the origin and maintenance of sexually selected traits, and (iii) hard to reconcile with what we know of the evolutionary history of SP's effects on females. We begin by charting the history of thought regarding SP, both at proximate (its production, function, and mechanism of action) and ultimate (its fitness consequences and evolutionary history) levels, reviewing how studies of SP were central to the development of the field of sexual conflict. We describe a prevailing paradigm for SP's evolution: that SP originated and continues to evolve to manipulate females for male benefit. In contrast to this view, we argue on three grounds that the weight of evidence does not support the view that receipt of SP decreases female fitness: (i) results from studies of SP's impact on female fitness are mixed and more often neutral or positive, with fitness costs emerging only under nutritional extremes; (ii) whether costs from SP are appreciable in wild‐living populations remains untested; and (iii) recently described confounds in genetic manipulations of SP raise the possibility that measures of the costs and benefits of SP have been distorted. Beyond SP's fitness effects, comparative and genetic data are also difficult to square with the idea that females suffer fitness costs from SP. Instead, these data – from functional and evolutionary genetics and the neural circuitry of female responses to SP – suggest an evolutionary history involving the evolution of a dedicated SP‐sensing apparatus in the female reproductive tract that is likely to have evolved because it benefits females, rather than harms them. We end by exploring theory and evidence that SP benefits females by functioning as a signal of male quality or of sperm receipt and storage (or both). The expanded view of the evolution of SP that we outline recognises the context‐dependent and fluctuating roles played by both cooperative and antagonistic selection in the origin and maintenance of reproductive traits.

Disentangling the effects of male age and mating history: Contrasting effects of mating history on precopulatory mating behavior and paternity success

Many studies ask whether young or older males are better at acquiring mates. Even so, how age affects reproductive success is still poorly understood because male age and mating history are confounded in most studies: older males usually have more mating experience. To what extent does mating history rather than age explain variation in male mating success? And how do mating history and male age determine paternity when there is also postcopulatory sexual selection? Here, we experimentally manipulated the mating history of old and young males in the eastern mosquitofish (Gambusia holbrooki). We then recorded male mating behavior and share of paternity (1259 offspring from 232 potential sires) when they competed for mates and fertilizations. Old males, and males with no mating experience, spent significantly more time approaching females, and attempting to mate, than did young males and those with greater mating experience. Male age and mating history interacted to affect paternity: old males benefited from having previous mating experience, but young males did not. Our results highlight that the age-related changes in male reproductive traits and in paternity that have been described in many taxa may be partly attributable to male mating history and not simply to age itself.

Polyphenism of visual and chemical secondary sexually-selected wing traits in the butterfly Bicyclus anynana: How different is the intermediate phenotype?

Polyphenism is a type of phenotypic plasticity supposedly adaptive to drastic and recurrent changes in the environment such as seasonal alternation in temperate and tropical regions. The butterfly Bicyclus anynana shows polyphenism with well-described wet and dry seasonal forms in sub-Saharan Africa, displaying striking morphological, physiological and behavioural differences in response to higher or lower developmental temperatures. During the seasonal transition in the wild, the intermediate phenotype co-occurs with wet and dry phenotypes. In this study, we aimed to characterize the secondary sexually-selected wing traits of the intermediate form to infer its potential fitness compared to wet and dry phenotypes. Among the previously described wing morphological traits, we first showed that the area of the fifth eyespot on the ventral hindwing is the most discriminant trait to identify wet, dry and intermediate phenotypes in both sexes. Second, we characterized the intermediate form for two secondary sexually-selected wing traits: the area and UV reflectance of the dorsal forewing pupil and the composition of the male sex pheromone. We showed that values of these two traits are often between those of the wet and dry phenotypes. Third, we observed increasing male sex pheromone production in ageing dry and wet phenotypes. Our results contrast with previous reports of values for sexually-selected traits in wet and dry seasonal forms, which might be explained by differences in rearing conditions or sample size effects among studies. Wet, dry and intermediate phenotypes display redundant sexually dimorphic traits, including sexually-selected traits that can inform about their developmental temperature in sexual interactions.

A reply to Nieberding and Holveck: beyond experimental design and proximate mechanisms - mate choice in the face of sexual conflict

We summarise our work on male mating behaviour in the tropical butterfly Bicyclus anynana, responding to the commentary provided by Nieberding and Holveck. We acknowledge that our laboratory studies are not free of shortcomings and potential caveats, though we attempted to address or highlight these within each paper. The concerns raised seem to stem mainly from different notions with respect to the proximate basis of old male mating advantage, and specifically the relative importance of male behaviour versus pheromone blend. In our view, our experiments provided compelling evidence for a prominent role of male behaviour, while we were unable to obtain clear evidence for a major role of male sexual pheromones. In addition to the lack of evidence we argue that a preference of females for older males based on pheromone blend is unlikely, as pheromone titres do not seem to indicate male quality and, more importantly, females actually suffer a fitness cost when mating with older males. The latter suggests that old male mating advantage arises from sexual conflict rather than cooperation. We thus highlight the importance of considering both the proximate and the ultimate level for gaining an integrative understanding of complex behavioural patterns.

Impact of male condition on his spermatophore and consequences for female reproductive performance in the Glanville fritillary butterfly

In butterflies, male reproductive success is highly related to the quality and the size of the spermatophore transferred to the female. The spermatophore is a capsule produced by the male during copulation, which in many species contains sperm in addition to a nuptial gift, and which is digested by the female after copulation. The nuptial gift may contribute to egg production and offspring quality, and in some cases also to female body maintenance. The production of the spermatophore, however, represents a cost for the male and, in polyandrous species, ejaculates are sometimes allocated adaptively across matings. Nonetheless, although the ecological factors affecting the reproductive success of female butterflies have been the topic of numerous studies, little information exists on the factors affecting males' contribution to reproduction, and the indirect impacts on female fecundity and fitness. We used the Glanville fritillary butterfly, Melitaea cinxia (Linnaeus, 1758) (Nymphalidae), in order to assess variation in male allocation to matings. In this species, smaller males produce smaller spermatophores, but variation in spermatophore size is not correlated with female reproductive success. We show that spermatophore size increases with male age at first mating, decreases with mating frequency and adult food-deprivation, and is not influenced by developmental food-limitation. The length of copulation period does not influence the spermatophore size nor influences the polyandrous mating behavior in this species. Male contribution to his spermatophore size is clearly influenced by his condition and adult-resource at the time of mating. Despite this variation, spermatophore size does not seem to have a direct impact on female reproductive output or mating behavior.

The impact of ageing on male reproductive success in Drosophila melanogaster

Male reproductive ageing has been mainly explained by a reduction in sperm quality with negative effects on offspring development and quality. In addition to sperm, males transfer seminal fluid proteins (Sfps) at mating; Sfps are important determinants of male reproductive success. Receipt of Sfps leads to female post-mating changes including physiological changes, and affects sperm competition dynamics. Using the fruit fly Drosophila melanogaster we studied ageing males' ability to induce female post-mating responses and determined the consequences of male ageing on their reproductive success. We aged males for up to 7weeks and assayed their ability to: i) gain a mating, ii) induce egg-laying and produce offspring, iii) prevent females from remating and iv) transfer sperm and elicit storage after a single mating. We found that with increasing age, males were less able to induce post-mating responses in their mates; moreover ageing had negative consequences for male success in competitive situations. Our findings indicate that with advancing age male flies transferred less effective ejaculates and that Sfp composition might change over a male's lifetime in quantity and/or quality, significantly affecting his reproductive success.

Effects of age on the courtship, copulation, and fecundity of Pardosa pseudoannulata (Araneae: Lycosidae)

According to sexual selection theory, age affects the preference of mate choice, and this preference ultimately influences the fecundity of the female. Pardosa pseudoannulata (Araneae: Lycosidae) is a valued predator in many cropping systems. By determining oviposition rate, egg hatching rate, and also the number and carapace width of the 2nd instar spiderlings of the F₁ generation, we explored the effects of age on fecundity of the female spider. There were no significant effects of age on courtship duration, sexual cannibalism rate, mating rate, oviposition rate, egg hatching rate, or the number and carapace width of 2nd instar spiderings of P. pseudoannulata. However, age had a significant effect on courtship latency, courtship intensity, and mating duration of the spider. Courtship latency decreased significantly with an increase in the age of the male, and courtship intensity of the low-age male increased with increasing female age. Increasing age of male and female spiders was associated with significantly prolonged mating duration. The results indicated that low-age male spiders were more inclined to mate with high-age females, and age had no significant effect on sexual cannibalism rate or the fecundity of the female.

Young male mating success is associated with sperm number but not with male sex pheromone titres

Background

Intraspecific communication is of crucial importance throughout the animal kingdom and may involve a combination of visual, gustatory, olfactory and acoustic cues. Variation in male sex pheromone amount and composition may convey important information to female conspecifics, for instance on species identity or age. However, whether increased male pheromone titres are associated with fitness benefits for the female, thus indicating a role as an honest signal, is under debate.

Results

Against this background, we tested in the butterfly Bicyclus anynana (1) whether young males being successful or unsuccessful in gaining a mating differed in sex pheromone titres and (2) for associations between male pheromone titres and spermatophore mass, eupyrene sperm number, and a variety of female and offspring life-history traits. Successful and unsuccessful males did not differ in pheromone titres, however eupyrene sperm number was much higher in successful males. Pheromone titres were not associated with any fitness-related female or offspring trait measured in our study, though correlation analyses yielded evidence for trade-offs among specific traits. Patterns did not differ among control and olfaction-blocked females.

Conclusion

Therefore, we suggest that in young B. anynana pheromone titres do not indicate male quality.

Old-male paternity advantage is a function of accumulating sperm and last-male precedence in a butterfly

Old-male mating advantage has been convincingly demonstrated in Bicyclus anynana butterflies. This intriguing pattern may be explained by two alternative hypotheses: (i) an increased aggressiveness and persistence of older males during courtship, being caused by the older males' low residual reproductive value; and (ii) an active preference of females towards older males what reflects a good genes hypothesis. Against this background, we here investigate postcopulatory sexual selection by double-mating Bicyclus anynana females to older and younger males, thus allowing for sperm competition and cryptic mate choice, and by genotyping the resulting offspring. Virgin females were mated with a younger virgin (2-3 days old) and afterwards an older virgin male (12-13 days old) or vice versa. Older males had a higher paternity success than younger ones, but only when being the second (=last) mating partner, while paternity success was equal among older and younger males when older males were the first mating partner. Older males produced larger spermatophores with much higher numbers of fertile sperm than younger males. Thus, we found no evidence for cryptic female mate choice. Rather, the findings reported here seem to result from a combination of last-male precedence and the number of sperm transferred upon mating, both increasing paternity success.