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The life cycle of a sexually reproducing species starts from the formation of a zygote by fusion of two gametes. In external fertilisers, the survival of an individual organism from a zygote to an adult is precarious and depends largely on the gametic provisioning of resources by its parents (via the female gamete in species where anisogamy has evolved). In evolutionary ecology, this early survival time has been modelled using a variety of zygote survival functions. In evolutionary modelling applications, the fitness of an individual is measured by the number of its offspring that survive to reproductive maturity, making the survival probability of the offspring a key component in these models. Despite their significance, the existing zygote survival functions remain scattered, with little consensus on their theoretical principles and mathematical tools. Here, our aim was to provide clarification and coherence for future theoretical and empirical research. We achieve this by reconstructing the history of zygote survival modelling, analysing existing zygote survival functions and discussing their theoretical details and problems. Systematising existing survival functions will facilitate future modelling work and will decrease duplication of efforts by concentrating theoretical focus into a unified modelling discourse. Tiivistelmä Suvullisesti lisääntyvän lajin elinkierto alkaa, kun kaksi gameettia yhtyy ja muodostaa zygootin. Ulkoisesti hedelmöittyvillä lajeilla yksilön selviytyminen zygootista aikuiseksi on epävarmaa ja riippuu pitkälti resursseista, jotka zygootti saa vanhemmiltaan sukusolujen kautta (anisogamisilla lajeilla munasoluista). Evoluutioekologiassa selviytymistä tässä elämän varhaisessa vaiheessa on mallinnettu käyttämällä erilaisia zygoottien selviytymisfunktioita. Evolutiivisissa mallinnussovelluksissa yksilön kelpoisuutta mitataan yleisesti sukukypsään aikuisuuteen selviävien jälkeläisten lukumäärällä, mikä tekee jälkeläisten selviytymistodennäköisyydestä oleellisen komponentin näissä malleissa. Tärkeydestään huolimatta olemassa olevat zygoottien selviytymisfunktiot muodostavat hajanaisen joukon vailla yksimielisyyttä niiden teoreettisista periaatteista tai matemaattisista menetelmistä. Tavoitteemme tässä artikkelissa oli selkeyttää ja johdonmukaistaa aihepiirin teoreettista ja empiiristä tutkimusta hahmottelemalla zygoottien selviytymisfunktioiden mallintamisen historiaa, analysoimalla olemassa olevia zygoottien selviytymisfunktioita ja keskustelemalla niiden teoreettisista yksityiskohdista ja ongelmista. Olemassa olevien zygoottien selviytymisfunktioiden systematisointi helpottaa tulevaa mallinnusta ja ehkäisee turhaa työtä keskittämällä teoreettisen painopisteen yhtenäistettyyn mallinnusperinteeseen.

Organism condition plays an important role in sexual selection. Sexual ornaments and displays can be condition‐dependent, reflecting either underlying genetic quality, experience of environmental stressors, or both. As such, the phenotypic expression of such traits, and the resulting patterns of mate choice acting on them, may be shaped by intrinsic genetic quality and the environment. Moreover, condition may also influence the choosing individual in mate choice, influencing their ability to invest in mate discrimination, or changing what traits of the chosen, including resources, are most preferred. Here we consider sexual selection and condition in the seed bug Lygaeus simulans, a species characterised by strong post‐copulatory sexual selection, but rather limited pre‐copulatory discrimination. We manipulated short‐term condition in both males and females by restricting access to water for 24 h. Water is particularly important in these bugs, given their feeding ecology and physiology. We found that water‐deprived males proved less likely to mate, while copulation duration with water‐deprived females was significantly reduced. Given the importance of copulation duration for the successful transfer of sperm by males to females, the data suggest cryptic male choice acting against water‐deprived females. These data add to those suggesting that cryptic male choice for fecund females plays an important role in sexual selection in this species. More generally, our results support the widespread importance of condition in terms of mating dynamics and sexual selection. Organism condition plays an important role in sexual selection. Here, we manipulated condition in the seed bug Lygaeus simulans, by restricting access to water 24 h prior to a mating opportunity. We found that water‐deprived males proved less likely to mate, while copulation duration with water‐deprived females was significantly reduced. Given the importance of copulation duration for the successful transfer of sperm by males to females, the data suggest cryptic male choice acting against water‐deprived females.

One yet unresolved question in the study of mating system evolution is the occurrence of mating failure, when individuals go through their lives without successfully mating. This includes the failure to produce offspring even following copulation, for instance due to insemination or fertilisation failure. Copulations are costly in a variety of ways, but also a fundamental route to fitness in sexual species, and so we should expect that engaging in copulations that generate no offspring should be strongly selected against. Nonetheless, it has become apparent that mating failure is quite common in nature. Here we consider post‐copulatory sexual selection in Lygaeus simulans seed bugs to test the hypothesis that the high levels of mating failure found in this species (approximately 40%–60%) are caused by cryptic male choice (i.e. males choosing not to inseminate a female during copulation). In our first experiment, we found that mating failure depended on female size, but not male size, with smaller females experiencing mating failure more frequently. Mechanistically this is likely to be due to copulation duration, as shorter copulations were more likely to lead to mating failure. Likewise, copulations with smaller females were shorter. In our second and third experiments, rates of mating failure decreased when pairs were allowed to repeatedly interact with the same partner over longer durations (hours through to days), implying that mating failure is not primarily caused by infertility or chronic mechanical failure. Instead, our results strongly suggest cryptic male choice as the cause of mating failure in this species. We expect cryptic mating failure–the failure to produce offspring due to fertilisation or insemination failure–to be strongly selected against because copulations are costly and are also a fundamental route to fitness. Mating failure is common in nature however, and in the seed bug Lygaeus simulans, rates are high (40%–60%). We performed experiments to investigate the causes of mating failure in L. simulans and found that mating failure is not primarily caused by infertility or chronic mechanical failure, but instead evidence strongly suggests that cryptic male choice is a key driver of mating failure in this species.

ABSTRACT Sexual selection is not a single process. Instead, multiple processes of sexual selection can interact with respect to a given phenotype, in either reinforcing, independent, or conflicting directions. Here we consider how different processes of sexual selection interact in the seed bug Lygaeus simulans. This species is characterised by limited pre‐copulatory sexual selection, but the potential for rather strong post‐copulatory sexual selection. In particular, mating failure is common in this species, with around 40%–60% of copulations failing to result in the successful transfer of sperm. Mating failure is negatively correlated with female size, with smaller females being less likely to end up inseminated. We have recently argued that this pattern is best explained by cryptic male mate choice for large, more fecund females. Males therefore preferentially inseminate larger females. Here we explore how this potential cryptic male choice interacts with another component of post‐copulatory sexual selection: sperm competition. We first manipulated male and female size variation, generating large and small, male and female, size classes. Using a visible mutant marker to assign paternity, we then double‐mated females with males, in all combinations of male and female size. Our results showed that sperm competition outcomes were primarily driven by copulation duration, with longer copulations leading to greater paternity share for a male. We also confirmed that larger females are more likely to produce offspring than smaller females, as predicted by cryptic male choice for large females. This effect was again linked to copulation duration, with longer copulations less likely to lead to mating failure. While larger males tended to be more successful in sperm competition, especially if copulating second, female size had little effect on paternity, suggesting that cryptic male choice and sperm competition are acting relatively independently in this species. Here we consider how different processes of post‐copulatory sexual selection interact in the seed bug Lygaeus simulans. We found that sperm competition outcomes were primarily driven by copulation duration, while larger females were more likely to receive sperm than smaller females, as predicted by cryptic male choice for large females. However, female size had little effect on paternity outcome in our double‐mating experiment, suggesting that cryptic male choice and sperm competition are acting relatively independently in this species.

ABSTRACT Sperm morphology can differ dramatically among closely related species. Within songbirds, the typical filiform sperm has a slender, corkscrew‐shaped head and an elongated midpiece coiled around the flagellum. However, three songbird species are known to have an unusual tadpole‐like sperm morphology with a round or ellipsoid head and short, uncoiled midpiece, which may arise developmentally via neoteny. Here, we describe tadpole‐like sperm phenotypes from two additional songbird species, the white‐breasted nuthatch (Sitta carolinensis) and the wrentit (Chamaea fasciata). These five species with tadpole‐like sperm share several ecological characteristics that can inform hypotheses for the evolution of this unusual phenotype: They are largely non‐migratory, form long‐lasting pair bonds, and have high paternal investment, small testes (no data for wrentit), and short and highly variable sperm lengths. These characteristics could indicate particular natural selection pressures driving physiological states such as low testicular testosterone levels. Additionally, though direct measures of female promiscuity are lacking in these species, these characteristics are consistent with weak post‐copulatory sexual selection. Further study of these similar yet independent evolutionary events, across all levels of analysis, would be valuable for understanding how such dramatic shifts in phenotype evolve. We describe two dramatic evolutionary shifts in sperm morphology in songbirds, which are independent of but similar to two previously known shifts. Species with this atypical, “tadpole‐like” sperm morphology appear to share sedentary life histories with long‐term pair bonds, high paternal investment, and likely low post‐copulatory sexual selection.

In internal fertilisers, the precise timing of ovulation with the arrival of sperm at the site of fertilisation is essential for fertilisation success. In birds, mating is often not synchronised with ovulation, but instead females utilise specialised sperm storage tubules (SSTs) in the reproductive tract, which can ensure sperm are always available for fertilisation at the time of ovulation, whilst simultaneously providing a mechanism of post‐copulatory sexual selection. Despite the clear importance of SSTs for fertilisation success, we know little about the mechanisms involved in sperm acceptance, storage, and release. Furthermore, most research has been conducted on only a small number of species, based on which SSTs are usually assumed to look and function in the same way across all species. Here, we conduct a comparative exploration of SST morphology across 26 species of Galliformes. We show that SSTs, and the surrounding tissue, can vary significantly in morphology across species. We provide observational evidence that Galliformes exhibit at least 5 distinct categories of tubule types, including distinctive coiled and multi‐branched tubules, and describe 2 additional features of the surrounding tissue. We suggest functional explanations for variation in tubule morphology and propose next steps for future research. Our findings indicate that SSTs are likely to be far more variable than has previously been assumed, with potentially important consequences for our understanding of sperm storage in birds and post‐copulatory sexual selection in general. Female birds utilise specialised sperm storage tubules in the reproductive tract, which ensure sperm are always available for fertilisation, whilst also potentially facilitating female sperm selection . Here, we show that sperm storage tubules, and the surrounding tissue, can vary substantially in morphology across species of Galliformes. We present 5 distinct categories of tubule and describe 2 additional features of the surrounding tissue. These findings have important implications for our understanding of sperm storage in birds and post‐copulatory sexual selection in general.

Polyandrous mating systems result in females mating with multiple males, generating opportunities for strong pre‐mating and post‐mating sexual selection. Polyandry also creates the potential for unintended matings and subsequent sperm competition with hybridizing species. Cryptic female choice allows females to bias paternity towards preferred males under sperm competition and may include conspecific sperm preference when under hybridization risk. The potential for hybridization becomes particularly important in context of invasive species that can novelly hybridize with natives, and by definition, have evolved allopatrically. We provide the first examination of conspecific sperm preference in a system of three species with the potential to hybridize: North American native Atlantic salmon (Salmo salar) and brook char (Salvelinus fontinalis), and invasive brown trout (Salmo trutta) from Europe. Using naturalized populations on the island of Newfoundland, we measured changes in sperm swimming performance, a known predictor of paternity, to determine the degree of modification in sperm swimming to female cues related to conspecific sperm preference. Compared to water alone, female ovarian fluid in general had a pronounced effect and changed sperm motility (by a mean of 53%) and swimming velocity (mean 30%), but not linearity (mean 6%). However, patterns in the degree of modification suggest there is no conspecific sperm preference in the North American populations. Furthermore, female cues from both native species tended to boost the sperm of invasive males more than their own. We conclude that cryptic female choice via ovarian fluid mediated sperm swimming modification is too weak in this system to prevent invasive hybridization and is likely insufficient to promote or maintain reproductive isolation between the native North American species.

Extra-pair paternity may drive selection on spermatozoa and ejaculate characteristics through sperm competition and cryptic female choice. Here, we examine sperm morphology in the black-throated blue warbler (Setophaga caerulescens), an ecological model species where extra-pair paternity is frequent and is linked with male age. We test whether sperm morphology relates to several aspects of male phenotype known or suspected to affect extra-pair paternity success. Sperm morphology did not correlate with the size of the white wing spot, a social status signal, nor with the volume of the cloacal protuberance. However, older males tended to have longer sperm cells. Although the sample size was limited, this pattern is intriguing, as longer cells may be advantageous in post-copulatory sexual selection and older males have larger testes and higher extra-pair paternity success in this species. Changes in sperm morphology with age are not observed in other birds, though they have been observed in insects and fishes. More research on sperm morphology is needed to clarify its role in extra-pair fertilizations in this well-studied species.

1. Polyandry is common in insects. Nevertheless, the evolutionary causes and consequences of this phenomenon remain contentious, in part because of a lack of information about natural mating rates and the fact that most post-copulatory processes are hidden from view within female reproductive tracts. 2. We captured wild female yellow dung flies (Scathophaga stercoraria) over the whole spring season and genotyped the sperm from their spermathecae to obtain information on sperm transfer, sperm storage and natural levels of polyandry for this model species of post-copulatory sexual selection research. 3. On average, females stored sperm from a minimum of 2-47 males (based on the most conservative estimate). Incorporating knowledge of population allele frequencies yielded a slightly higher estimate of 3-33 mates per female. 4. Sperm storage and therefore sperm competition intensity showed high temporal variation. The proportion of multiply mated females (i.e. females with sperm from ≥ 2 males within their sperm stores) and the absolute number of ejaculates detected within females increased strongly over the spring season before sharply decreasing as midsummer approached. 5. Interestingly, we detected a positive relationship between the number of stored ejaculates and females' wing injuries, suggesting that mating not only causes measurable cumulative damage to wild females but also provides a potential mechanism by which males may be able to assess the intensity of sperm competition within a female. 6. Our study found no evidence for intraejaculate sperm sorting, but importantly, the number of ejaculates in storage differed amongst the three sperm storage organs (spermathecae) of female yellow dung flies. Different sperm mixtures across the spermathecae could enable females to bias paternity towards certain males if females can selectively use sperm from a certain spermatheca at the time of fertilization.

Mate choice can continue after mating via chemical communication between the female reproductive system and sperm. While there is a growing appreciation that females can bias sperm use and paternity by exerting cryptic female choice for preferred males, we know surprisingly little about the mechanisms underlying these post-mating choices. In particular, whether chemical signals released from eggs (chemoattractants) allow females to exert cryptic female choice to favour sperm from specific males remains an open question, particularly in species (including humans) where adults exercise pre-mating mate choice. Here, we adapt a classic dichotomous mate choice assay to the microscopic scale to assess gamete-mediated mate choice in humans. We examined how sperm respond to follicular fluid, a source of human sperm chemoattractants, from either their partner or a non-partner female when experiencing a simultaneous or non-simultaneous choice between follicular fluids. We report robust evidence under these two distinct experimental conditions that follicular fluid from different females consistently and differentially attracts sperm from specific males. This chemoattractant-moderated choice of sperm offers eggs an avenue to exercise independent mate preference. Indeed, gamete-mediated mate choice did not reinforce pre-mating human mate choice decisions. Our results demonstrate that chemoattractants facilitate gamete-mediated mate choice in humans, which offers females the opportunity to exert cryptic female choice for sperm from specific males.