Reproductive aging and mating: the ticking of the biological clock in female cockroaches

The ecology of ageing in wild societies: linking age structure and social behaviour

The age of individuals has consequences not only for their fitness and behaviour but also for the functioning of the groups they form. Because social behaviour often changes with age, population age structure is expected to shape the social organization, the social environments individuals experience and the operation of social processes within populations. Although research has explored changes in individual social behaviour with age, particularly in controlled settings, there is limited understanding of how age structure governs sociality in wild populations. Here, we synthesize previous research into age-related effects on social processes in natural populations, and discuss the links between age structure, sociality and ecology, specifically focusing on how population age structure might influence social structure and functioning. We highlight the potential for using empirical data from natural populations in combination with social network approaches to uncover pathways linking individual social ageing, population age structure and societal functioning. We discuss the broader implications of these insights for understanding the social impacts of anthropogenic effects on animal population demography and for building a deeper understanding of societal ageing in general.This article is part of the discussion meeting issue 'Understanding age and society using natural populations'.

Seasonal shift in diet affects female reproductive anatomy but not mating behavior

Females experience considerable environmental variability when breeding seasons are long. Adverse nutritional conditions can result in a reduction in mating and reproduction. However, a return to good nutrition may help animals resume high reproductive investment. I tested the silver spoon hypothesis in which females raised under poor conditions are reproductively limited compared to those raised under good conditions regardless of their adult environment. I used a specialist herbivore, Narnia femorata (Hemiptera: Coreidae), that lives on seasonally changing cacti. I provided juveniles and adults with a cactus pad with fruit (good diet), without fruit (restricted diet), or an improved adult diet (no fruit as juveniles, fruit at adulthood) to simulate a seasonal change in their diets near the end of the breeding season. I found that both ovary size and egg presence were reduced for females fed the restricted diet compared to those fed the good diet. Females fed the improved diet grew large ovaries like those fed the good diet, but few produced any eggs. Interestingly, female mating behavior did not change but females were less attractive to males when fed restricted diets. My results support the silver spoon hypothesis for compensatory growth and suggest that tradeoffs may occur between early survival and future reproduction when females experience a poor early life diet.

Flexibility of neural circuits regulating mating behaviors in mice and flies

Mating is essential for the reproduction of animal species. As mating behaviors are high-risk and energy-consuming processes, it is critical for animals to make adaptive mating decisions. This includes not only finding a suitable mate, but also adapting mating behaviors to the animal's needs and environmental conditions. Internal needs include physical states (e.g., hunger) and emotional states (e.g., fear), while external conditions include both social cues (e.g., the existence of predators or rivals) and non-social factors (e.g., food availability). With recent advances in behavioral neuroscience, we are now beginning to understand the neural basis of mating behaviors, particularly in genetic model organisms such as mice and flies. However, how internal and external factors are integrated by the nervous system to enable adaptive mating-related decision-making in a state- and context-dependent manner is less well understood. In this article, we review recent knowledge regarding the neural basis of flexible mating behaviors from studies of flies and mice. By contrasting the knowledge derived from these two evolutionarily distant model organisms, we discuss potential conserved and divergent neural mechanisms involved in the control of flexible mating behaviors in invertebrate and vertebrate brains.

Ovarian apoptosis is regulated by carbohydrate intake but not by protein intake in speckled cockroaches

When the likelihood of reproducing successfully is low, any prior investment in developing oocytes may be wasted. One means of recouping this investment is oosorption - where ova are absorbed and resources salvaged so they can be re-allocated to other traits. Food-limited female speckled cockroaches (Nauphoeta cinerea) appear to use this strategy. However, it is unclear if total food intake or the availability of specific nutrients induces this process. Here, we used the geometric framework of nutrition to determine how protein, carbohydrate and energy intake affect levels of ovarian apoptosis and necrosis (controlled versus uncontrolled cell death) in the terminal oocytes of female N. cinerea. We then compare the effects of nutrient intake on apoptosis (a key step towards oosorption) and offspring production to better understand the relationship between diet, apoptosis and female fitness. We found that even when food was abundant, females experienced high levels of apoptosis if their diet lacked carbohydrate. Necrosis was reduced when energy intake was high, but largely irrespective of nutrient ratio. Offspring production peaked on a low protein, high carbohydrate nutrient ratio (1_P:7.96_C), similar to that which minimized apoptosis (1_P:7.34_C) but not in the region of nutrient space that minimized necrosis. Thus, females consuming an ideal nutrient blend for reproduction can invest heavily in their current brood without needing to salvage nutrients from developing ova. However, offspring production was more dependent on carbohydrate consumption than apoptosis was, suggesting that the importance of carbohydrate in reproduction goes beyond regulating oosorption. This reliance on carbohydrate for female reproduction may reflect the unusual reproductive and nutritional physiology of speckled cockroaches; attributes that make this species an exciting model for understanding how diet regulates reproduction.

The evolution of age-specific choosiness and reproductive isolation in a model with overlapping generations

The strength of mate choice (choosiness) often varies with age, but theory to understand this variation is scarce. Additionally, theory has investigated the evolution of choosiness in speciation scenarios but has ignored that most organisms have overlapping generations. We investigate whether speciation can result in variation of choosiness with age, and whether such variation can in turn affect speciation. We develop a population-genetic model of the evolution of choosiness in organisms with overlapping generations in the context of secondary contact between two divergent populations. We assume that females choose males that match their phenotype, such that choosiness evolves by sexual selection. We demonstrate that speciation can result in the evolution of age-specific choosiness when the mating trait is under divergent ecological selection and age is not used as a mating cue. The cause of this result is that allele frequencies differ between choosy females and males. However, we find that the evolution of age-specific choosiness does not affect the overall level of reproductive isolation compared to a case without age-structure, supporting previous speciation theory. Overall, our results connect life history and speciation theory, and the mechanisms that we highlight have implications for the understanding of the role of sex-specific selection in the evolution of choosiness.

Male age and sexual experience affect male mating behavior in the hawthorn spider mite, Amphitetranychus viennensis

As is reported, in species with first-male sperm precedence, male age and previous sexual experience play crucial roles in male mating behavior. In the hawthorn spider mite, Amphitetranychus viennensis Zacher, previous studies showed that only females that copulated for the first time could achieve fertilization. Based on this, the effects of male age and mating history on male mate choice and male mate competition were investigated. It was confirmed that males could distinguish virgins from fertilized females but they were unable to discriminate between virgins and unfertilized females. Interestingly, the copulation duration of males mated with fertilized females was much shorter than that of males mated with virgins or unfertilized females. Additionally, for male mating choice, males of all ages and more experienced males preferred 5-day-old virgin females, whereas only less experienced males preferred 1-day-old virgin females. In male-male competition, 3-day-old males were more competitive and obtained more copulations compared with others. Copula duration was closely related to male age. Though no significant differences were observed in mating competition between virgin and mated males, copula duration of males in first copulation was the longest and gradually shortened in subsequent copulations. In all, this investigation demonstrated that male age and sexual experience affected male mate choice and male-male competition, leading to further insight into the influences of male age and sexual experience on the reproductive fitness of both sexes.

Fitness costs of female choosiness are low in a socially monogamous songbird

Female mate choice is thought to be responsible for the evolution of many extravagant male ornaments and displays, but the costs of being too selective may hinder the evolution of choosiness. Selection against choosiness may be particularly strong in socially monogamous mating systems, because females may end up without a partner and forego reproduction, especially when many females prefer the same few partners (frequency-dependent selection). Here, we quantify the fitness costs of having mating preferences that are difficult to satisfy, by manipulating the availability of preferred males. We capitalize on the recent discovery that female zebra finches (Taeniopygia guttata) prefer males of familiar song dialect. We measured female fitness in captive breeding colonies in which one-third of females were given ample opportunity to choose a mate of their preferred dialect (two-thirds of all males; "relaxed competition"), while two-thirds of the females had to compete over a limited pool of mates they preferred (one-third of all males; "high competition"). As expected, social pairings were strongly assortative with regard to song dialect. In the high-competition group, 26% of the females remained unpaired, yet they still obtained relatively high fitness by using brood parasitism as an alternative reproductive tactic. Another 31% of high-competition females paired disassortatively for song dialect. These females showed increased levels of extra-pair paternity, mostly with same-dialect males as sires, suggesting that preferences were not abolished after social pairing. However, females that paired disassortatively for song dialect did not have lower reproductive success. Overall, females in the high-competition group reached equal fitness to those that experienced relaxed competition. Our study suggests that alternative reproductive tactics such as egg dumping can help overcome the frequency-dependent costs of being selective in a monogamous mating system, thereby facilitating the evolution of female choosiness.

Males perceive honest information from female released sex pheromone in a moth

There is accumulating evidence that male insects advertise their quality to conspecific females through pheromones. However, most studies of female released sex pheromone assume information transfer regarding merely the species of the female and her mating status. We show that more information is conveyed through the female sex pheromone, positioning it as an honest sexual trait. We demonstrate that females in bad physical conditions (small, starved, or old) lay significantly fewer eggs than females in good conditions (large, fed, or young). The ratio of components in the sex pheromone blend in gland extracts of the female pink bollworm moths provided honest information on most of the phenotypic conditions tested, whereas the pheromone amount in the glands provided an honest signal of quality for extreme phenotypes only. Moreover, males used the information conveyed by the female pheromone to choose their mates, approaching females that signaled higher reproductive potential. In addition, when simulating the female effect, using the synthetic pheromone blend that represents higher quality females (0.6:0.4 ZZ:ZE), more males were attracted to this blend than to the blend representing the population mean (0.5:0.5 ZZ:ZE). Both, female advertisement for males and the male choosiness, suggest that pheromones have evolved as sexual traits under directional, sexual selection. We suggest that the pheromone blend may serve as a multicomponent signal whereby each component adds information concerning the current condition of the female, and all are necessary to elicit a mate searching response.

Reinforcement and the Proliferation of Species

Adaptive radiations are characterized by the rapid proliferation of species. Explaining how adaptive radiations occur therefore depends, in part, on identifying how populations become reproductively isolated-and ultimately become different species. Such reproductive isolation could arise when populations adapting to novel niches experience selection to avoid interbreeding and, consequently, evolve mating traits that minimize such hybridization via the process of reinforcement. Here, we highlight that a downstream consequence of reinforcement is divergence of conspecific populations, and this further divergence can instigate species proliferation. Moreover, we evaluate when reinforcement will-and will not-promote species proliferation. Finally, we discuss empirical approaches to test what role, if any, reinforcement plays in species proliferation and, consequently, in adaptive radiation. To date, reinforcement's downstream effects on species proliferation remain largely unknown and speculative. Because the ecological and evolutionary contexts in which adaptive radiations occur are conducive to reinforcement and its downstream consequences, adaptive radiations provide an ideal framework in which to evaluate reinforcement's role in diversification.

Reproductive Status Modulates Mortality Rate, Lifespan and Egg Production, but Do Not the Physiological Aging in the Field Cricket Gryllus assimilis (Fabricius, 1775) (Orthoptera: Gryllidae)

The effects of reproduction are variable among insects, as well as the time of mate. In animals, a trade-off is usually observed between reproduction and self-maintenance, mostly in females because of egg producing costs. In the present investigation, manifestations of aging and senescence at population and cellular levels were studied associated with the mating opportunities. The main goal of this study was to evaluate the effect of mating on lifespan and also on the physiological aging in adult Jamaican field cricket females, Gryllus assimilis (Fabricius, 1775). Three experimental groups were set: virgin, normal-mated (14 days old), and late-mated (30 days old) insects. Lifespan, age-specific mortality rate, number of eggs laid, and in situ amount of the age-pigment neurolipofuscin were quantified. Results showed a trade-off between reproductive females and lifespan, with the strongest effects found in late-mated. Age-specific mortality showed a faster increase for late-mated females from 35 to 50 days old, followed by normal-mated females (35 to 60 days old). Virgins had the lowest age-specific mortality of all, increasing just from 65 to 73 days old. Normal-mated had the highest number of eggs laid, followed by late-mated and virgins, respectively. Neurolipofuscin accumulation rate was similar among groups, which was reflected in similar physiological rate of aging. Results indicate that reproductive status did affect the life-history of these field cricket females.

Effects of mating on reproductive performance of Coccophagus japonicus Compere (Hymenoptera: Aphelinidae)

Coccophagus japonicus Compere, an endoparasitoid of Parasaissetia nigra Nietner, has great potential for biological control. To assess the influence of mating on the reproductive performance of this parasitoid, we examined the effects of mating on ovarian development, female longevity and number of eggs laid. The results showed that the egg volume in the ovary of C. japonicus first increased and then decreased with increases in the age of female adults. The peak egg volume in the ovary of mated females occurred 2 days earlier than that of virgin females. Within the female age range of 0–15 days, the numbers of eggs at stages I, II, and III first increased and then decreased with increases in the age of female C. japonicus, whereas the number of eggs at stage IV increased. The duration of the coexistence of females and males significantly influenced the length and width of the female ovaries, and the longest ovary tube and the highest number of eggs were obtained with a coexistence duration of 0 days. C. japonicus female longevity decreased with increases in the number of matings, and the number of eggs laid by females within 15 days decreased with increasing delays in mating. In conclusion, mating can shorten the longevity of C. japonicus females, and selecting newly emerged virgin females for mating can significantly improve the number of eggs laid and the breeding efficiency of the parasitoid.

The evolution of age-specific choosiness when mating

Mate choice is a crucial element of many processes in evolutionary biology. Empirical research has shown that mating preference and choosiness often change with age. Understanding the evolutionary causes of patterns of age-specific choosiness is challenging because different mechanisms can give rise to the same pattern. Instead of focusing on the optimal age-specific choosiness strategy given fitness trade-offs, we approach this question from a more general standpoint and ask how the strength of selection on choosiness changes with the age at which it is expressed. We show that the strength of selection on a modifier of choosiness at a given age depends on the relative contribution of this age class to the pool of offspring but does not depend directly on the strength of selection on fitness components at the age affected by the modifier. We illustrate our results by contrasting two life histories from the literature. We further show how mutation-selection balance at the choosiness locus can shape age-specific choosiness. Our results provide new insights for understanding the evolution of choosiness throughout life, with implications for understanding the evolution of mate choice and reproductive isolation.

Development time mediates the effect of larval diet on ageing and mating success of male antler flies in the wild

High-quality developmental environments often improve individual performance into adulthood, but allocating toward early life traits, such as growth, development rate and reproduction, may lead to trade-offs with late-life performance. It is, therefore, uncertain how a rich developmental environment will affect the ageing process (senescence), particularly in wild insects. To investigate the effects of early life environmental quality on insect life-history traits, including senescence, we reared larval antler flies (Protopiophila litigata) on four diets of varying nutrient concentration, then recorded survival and mating success of adult males released in the wild. Declining diet quality was associated with slower development, but had no effect on other life-history traits once development time was accounted for. Fast-developing males were larger and lived longer, but experienced more rapid senescence in survival and lower average mating rate compared to slow developers. Ultimately, larval diet, development time and body size did not predict lifetime mating success. Thus, a rich environment led to a mixture of apparent benefits and costs, mediated by development time. Our results indicate that 'silver spoon' effects can be complex and that development time mediates the response of adult life-history traits to early life environmental quality.

Neural and molecular mechanisms underlying female mate choice decisions in vertebrates

Female mate choice is a dynamic process that allows individuals to selectively mate with those of the opposite sex that display a preferred set of traits. Because in many species males compete with each other for fertilization opportunities, female mate choice can be a powerful agent of sexual selection, often resulting in highly conspicuous traits in males. Although the evolutionary causes and consequences of the ornamentation and behaviors displayed by males to attract mates have been well studied, embarrassingly little is known about the proximate neural mechanisms through which female choice occurs. In vertebrates, female mate choice is inherently a social behavior, and although much remains to be discovered about this process, recent evidence suggests the neural substrates and circuits underlying other fundamental social behaviors (such as pair bonding, aggression and parental care) are likely similarly recruited during mate choice. Notably, female mate choice is not static, as social and ecological environments can shape the brain and, consequently, behavior in specific ways. In this Review, we discuss how social and/or ecological influences mediate female choice and how this occurs within the brain. We then discuss our current understanding of the neural substrates underlying female mate choice, with a specific focus on those that also play a role in regulating other social behaviors. Finally, we propose several promising avenues for future research by highlighting novel model systems and new methodological approaches, which together will transform our understanding of the causes and consequences of female mate choice.

In humans, only attractive females fulfil their sexually imprinted preferences for eye colour

Early exposure to parental features shapes later sexual preferences in fish, birds, and mammals. Here I report that human males' preferences for a conspicuous trait, colourful eyes, are affected by the eye colour of mothers. Female faces with light (blue or green) eyes were liked better by men whose mother had light eyes; the effect broke down in those who had felt rejected by her as children. These results, garnered on over one thousand men, complete those of a symmetrical study on one thousand women, painting a fuller picture of human sexual imprinting. Both men and women appear to have imprinted on their opposite-sex parents unless these were perceived as cold and unjustly punitive. Birds require strong attachment to sexually imprint-a constraint in place to reduce the perils of acquiring the wrong sort of information. Parents who form no bond with their offspring may fail to be recognised as appropriate parental imprinting objects. Consistent with human females being, as in most of the animal kingdom, the choosier sex, imprinted preferences were displayed by both sexes but translated into real-life partner choices solely in women-attractive women. Apparently, not all of us can afford to follow our own inclinations.

Mating Delay Reduces Reproductive Performance but not Longevity in a Monandrous Moth

Age at mating is one of the most important factors that affect mating success and reproductive fitness in insects. The present study investigated how the age of the two sexes at mating determined mating success, reproductive fitness and longevity in Phauda flammans (Walker) (Lepidoptera: Phaudidae), a serious pest of Ficus spp. trees in South and Southeast Asia. The study may provide basic knowledge for the development of mating disruption programs using sex pheromones to control this pest. The species is monandrous and its adults live for only 4-5 d. We show that delayed mating significantly lowered mating success in both sexes, with males being more severely affected than females. Mating delay also reduced reproductive outputs of both sexes but females were more negatively affected than males. We did not find any effect of delayed mating on longevity of either sex. Our findings suggest that mating disruption with sex pheromones can be an effective method to delay mating in P. flammans, reducing reproductive success and thus limit population growth.

Age at mating and male quality influence female patterns of reproductive investment and survival

The trade-off between the allocation of resources toward somatic maintenance or reproduction is one of the fundamentals of life history theory and predicts that females invest in offspring at the expense of their longevity or vice versa. Mate quality may also affect life history trade-offs through mechanisms of sexual conflict; however, few studies have examined the interaction between mate quality and age at first mating in reproductive decisions. Using house crickets (Acheta domesticus), this study examines how survival and reproductive trade-offs change based on females' age at first reproduction and exposure to males of varying size. Females were exposed to either a large (presumably high-quality) or small male at an early (young), middle (intermediate), or advanced (old) age, and longevity and reproductive investment were subsequently tracked. Females mated at a young age had the largest number of eggs but the shortest total lifespans while females mated at older ages produced fewer eggs but had longer total lifespans. The trade-off between age at first mating and eggs laid appears to be mediated through higher egg-laying rates and shorter postmating lifespans in females mated later in life. Exposure to small males resulted in shorter lifespans and higher egg-laying rates for all females indicating that male manipulation of females, presumably through spermatophore contents, varies with male size in this species. Together, these data strongly support a trade-off between age at first reproduction and lifespan and support the role of sexual conflict in shaping patterns of reproduction.

Rapid Loss of Resistance to Satyrization in Invasive Mosquitoes and the Effects of Age on Interspecific Mating Frequency

In several areas where Aedes aegypti (L.) (Diptera: Culicidae) and Aedes albopictus (Skuse) (Diptera: Culicidae) have come in contact following successful invasions, Ae. aegypti have been rapidly displaced by Ae. albopictus. Recent work has confirmed that mating interference, in the form of satyrization, is likely a driving factor in these competitive displacements. However, in sites of sympatry, Ae. aegypti females evolve resistance to satyrization, and in the laboratory, satyrization-susceptible Ae. aegypti can evolve resistance within a few generations of cage exposure to Ae. albopictus. The resistance trait may be costly to maintain, as satyrization-resistant females have been shown to invest more time in mate selection. Here, we show that increased satyrization-resistance does not persist in the absence of satyrization pressure, confirming a cost to maintaining this trait. Ae. aegypti lines that had been previously selected for increased satyrization-resistance demonstrated rapid decreases in this trait over eight generations. Support for the hypothesis that condition-dependent mate preferences lead to variation in mating choice among individuals of differing quality within a population has been accumulating. Here we examine how age-related changes in reproductive effort or reproductive value may result in age-dependent mate choice. We hypothesize that older, lower quality individuals may be more likely to accept a subpar mating, in this case an interspecific mating. Our results demonstrate that mosquito age significantly affects interspecific mating rates with older mosquitoes (male and female) engaging in interspecific mating more frequently than younger counterparts (26.32 vs 9.41%). We discuss the possibility of age-related signal variation causing the breakdown of mating barriers with age.

Evidence that fertility trades off with early offspring fitness as males age

Models of ageing predict that sperm function and fertility should decline with age as sperm are exposed to free radical damage and mutation accumulation. However, theory also suggests that mating with older males should be beneficial for females because survival to old age is a demonstration of a male's high genetic and/or phenotypic quality. Consequently, declines in sperm fitness may be offset by indirect fitness benefits exhibited in offspring. While numerous studies have investigated age-based declines in male fertility, none has taken the integrated approach of studying age-based effects on both male fertility and offspring fitness. Here, using a cohort-based longitudinal study of zebrafish (Danio rerio), we report a decline in male mating success and fertility with male age but also compensating indirect benefits. Using in vitro fertilization, we show that offspring from older males exhibit superior early survival compared to those from their youngest counterparts. These findings suggest that the high offspring fitness observed for the subset of males that survive to an old age (approx. 51% in this study) may represent compensating benefits for declining fertility with age, thus challenging widely held views about the fitness costs of mating with older males.

Fitness consequences of delayed mating on reproductive performance of Chilo partellus (Swinhoe)

We investigated the effects of mating among different-aged males and females on reproductive physiology, progeny production, and longevity of Chilo partellus (Swinhoe) adults under laboratory conditions. Present studies involved virgin males (M) and females (F) of three different ages namely, on the day of emergence (1D), next day (2D), and further next day (3D), thus comprising of a total of nine mating treatments (1DM × 1DF, 1DM × 2DF, 1DM × 3DF, 2DM × 1DF, 2DM × 2DF, 2DM × 3DF, 3DM × 1DF, 3DM × 2DF, and 3DM × 3DF). The observations were recorded on fecundity, fertility, hatchability, and longevity of male and female adults. We found that mating among old aged males and females significantly reduce fecundity, fertility, and egg hatchability. Further, the fecundity, fertility, and egg hatching were significantly higher when newly emerged females mated with either of the ages of males as compared with other mating treatments. Conversely, the 3-day-old females, when mated with either of the ages of males, laid more numbers of unfertilized eggs, thus reduction in egg hatchability. Mating among newly emerged adults significantly reduced the longevity of male and female adults. Kaplan-Meier estimation showed that none of the adults across mating treatments survived more than 6 days. The recovery of spermatophores from the reproductive tract of females from 3DM × 1DF were significantly higher than other mating treatments. However, no association was found between spermatophore recovery and longevity of male and female adults. Findings of present studies will help to devise appropriate techniques to disrupt mating and suppress C. partellus population under field conditions.

A Probable Case of Incipient Speciation in Schizocosa Wolf Spiders Driven by Allochrony, Habitat Use, and Female Mate Choice

There is growing evidence that speciation can occur between populations that are not geographically isolated. The emergence of assortative mating is believed to be critical to this process, but how assortative mating arises in diverging populations is poorly understood. The wolf spider genus Schizocosa has become a model system for studying mechanisms of assortative mating. We conducted a series of experiments to identify the factors that control mate pair formation in a Schizocosa population that includes both ornamented and nonornamented males. We show that the population also includes two previously unrecognized female phenotypes. One female phenotype mates mostly or exclusively with ornamented males, and the other mates mostly or exclusively with unornamented males. Assortative mating within these groups is maintained by differences in maturation time, microhabitat use, and female mate preference. We conclude that the population is not a single species, as previously believed, but rather an incipient species pair with multiple overlapping mechanisms of reproductive isolation. The identification of a new incipient species pair in the well-studied and rapidly speciating Schizocosa clade presents new opportunities for the study of speciation without geographic isolation.

The causes and evolutionary consequences of variation in female mate choice in insects: the effects of individual state, genotypes and environments

Sexual selection generally involves males evolving secondary sexual characters that satisfy the mating preferences of females. Behavioral ecologists have spent considerable research effort on identifying how variation in sexually-selected traits in insects is maintained among males at the expense of investigating the proximate and ultimate causes of variation in female mating preferences for those male traits. The past decade has witnessed improved effort in redressing this bias in insects with researchers identifying a host of factors intrinsic and extrinsic to the female as mediating flexibility in female mating behavior. Evidence is mounting that a female's social environment, whether experienced during development or as an adult, is key to shaping her mating preferences. Others have extended these observations to show that the genetic identity of the conspecific individuals comprising the social environment can have profound effects on female mating preferences via indirect genetic effects (IGEs), or through interspecific indirect genetic effects (IIGEs) if the genotype of heterospecifics influences plasticity in mating preferences. Considerably more work is needed to not only expand our list of mediating intrinsic and extrinsic factors but also to identify how their interaction influences individual variation in male and female mating preferences.

High fat diet alters Drosophila melanogaster sexual behavior and traits: decreased attractiveness and changes in pheromone profiles

Sexual traits convey information about individual quality to potential mates. Environmental and genetic factors affect sexual trait expression and perception via effects on animal condition and health. High fat diet (HFD) is one environmental factor that adversely affects Drosophila melanogaster health, and its effects on animal health are mediated through conserved metabolic signaling pathways. HFD decreases female attractiveness, resulting in reduced male mating behaviors toward HFD females. HFD also affects the ability of males to judge mate attractiveness and likely alters fly condition and sexual traits to impact mating behavior. Here we show that HFD affects both visual (body size) and non-visual (pheromone profiles) sexual traits, which likely contribute to decreased fly attractiveness. We also demonstrate that adult-specific HFD effects on male mate preference can be rescued by changing metabolic signaling. These results demonstrate that HFD alters Drosophila sexual cues to reflect concurrent effects on condition and that less severe behavioral defects can be reversed by genetic manipulations that rescue fly health. This work expands on current knowledge of the role that metabolic signaling pathways play in linking animal health, sexual traits, and mating behavior, and provides a robust assay in a genetically tractable system to continue examining these processes.

Effects of macronutrient intake on the lifespan and fecundity of the marula fruit fly, Ceratitis cosyra (Tephritidae): Extreme lifespan in a host specialist

In insects, lifespan and reproduction are strongly associated with nutrition. The ratio and amount of nutrients individuals consume affect their life expectancy and reproductive investment. The geometric framework (GF) enables us to explore how animals regulate their intake of multiple nutrients simultaneously and determine how these nutrients interact to affect life-history traits of interest. Studies using the GF on host-generalist tephritid flies have highlighted trade-offs between longevity and reproductive effort in females, mediated by the protein-to-carbohydrate (P:C) ratio that individuals consume. Here, we tested how P and C intake affect lifespan (LS) in both sexes, and female lifetime (LEP), and daily (DEP) egg production, in Ceratitis cosyra, a host-specialist tephritid fly. We then determined the P:C ratio that C. cosyra defends when offered a choice of foods. Female LS was optimized at a 0:1 P:C ratio, whereas to maximize their fecundity, females needed to consume a higher P:C ratio (LEP = 1:6 P:C; DEP = 1:2.5 P:C). In males, LS was also optimized at a low P:C ratio of 1:10. However, when given the opportunity to regulate their intake, both sexes actively defended a 1:3 P:C ratio, which is closer to the target for DEP than either LS or LEP. Our results show that female C. cosyra experienced a moderate trade-off between LS and fecundity. Moreover, the diets that maximized expression of LEP and DEP were of lower P:C ratio than those required for optimal expression of these traits in host-generalist tephritids or other generalist insects.

Larger females are choosier in the gray treefrog (Hyla versicolor)

Individual variation in female mate choice has important implications for sexual trait evolution and the maintenance of phenotypic diversity. In this study we examined several potential drivers of individual variation in female choosiness for the well-studied, energetically expensive courtship signal of male gray treefrogs, Hyla versicolor. Specifically, we investigated the relationship between female choosiness and other female traits (female body size, physical condition, and age) using a costly choice playback experiment where females traveled different simulated distances to reach attractive mates. We found that larger females maintained their preferences for attractive male calls over greater simulated distances (i.e. were choosier) than smaller females. We discuss possible explanations for why larger females may be choosier and suggest several potential avenues of future research.

Subsocial Cockroaches Nauphoeta cinerea Mate Indiscriminately with Kin Despite High Costs of Inbreeding

Many animals have evolved strategies to reduce risks of inbreeding and its deleterious effects on the progeny. In social arthropods, such as the eusocial ants and bees, inbreeding avoidance is typically achieved by the dispersal of breeders from their native colony. However studies in presocial insects suggest that kin discrimination during mate choice may be a more common mechanism in socially simpler species with no reproductive division of labour. Here we examined this possibility in the subsocial cockroach Nauphoeta cinerea, a model species for research in sexual selection, where males establish dominance hierarchies to access females and control breeding territories. When given a binary choice between a sibling male and a non-sibling male that had the opportunity to establish a hierarchy prior to the tests, females mated preferentially with the dominant male, irrespective of kinship or body size. Despite the lack of kin discrimination during mate choice, inbred-mated females incurred significant fitness costs, producing 20% less offspring than outbred-mated females. We discuss how the social mating system of this territorial cockroach may naturally limit the probability of siblings to encounter and reproduce, without the need for evolving active inbreeding avoidance mechanisms, such as kin recognition.

Dietary choice for a balanced nutrient intake increases the mean and reduces the variance in the reproductive performance of male and female cockroaches

Sexual selection may cause dietary requirements for reproduction to diverge across the sexes and promote the evolution of different foraging strategies in males and females. However, our understanding of how the sexes regulate their nutrition and the effects that this has on sex‐specific fitness is limited. We quantified how protein (P) and carbohydrate (C) intakes affect reproductive traits in male (pheromone expression) and female (clutch size and gestation time) cockroaches (Nauphoeta cinerea). We then determined how the sexes regulate their intake of nutrients when restricted to a single diet and when given dietary choice and how this affected expression of these important reproductive traits. Pheromone levels that improve male attractiveness, female clutch size and gestation time all peaked at a high daily intake of P:C in a 1:8 ratio. This is surprising because female insects typically require more P than males to maximize reproduction. The relatively low P requirement of females may reflect the action of cockroach endosymbionts that help recycle stored nitrogen for protein synthesis. When constrained to a single diet, both sexes prioritized regulating their daily intake of P over C, although this prioritization was stronger in females than males. When given the choice between diets, both sexes actively regulated their intake of nutrients at a 1:4.8 P:C ratio. The P:C ratio did not overlap exactly with the intake of nutrients that optimized reproductive trait expression. Despite this, cockroaches of both sexes that were given dietary choice generally improved the mean and reduced the variance in all reproductive traits we measured relative to animals fed a single diet from the diet choice pair. This pattern was not as strong when compared to the single best diet in our geometric array, suggesting that the relationship between nutrient balancing and reproduction is complex in this species.

A conceptual review of mate choice: stochastic demography, within-sex phenotypic plasticity, and individual flexibility

Mate choice hypotheses usually focus on trait variation of chosen individuals. Recently, mate choice studies have increasingly attended to the environmental circumstances affecting variation in choosers' behavior and choosers' traits. We reviewed the literature on phenotypic plasticity in mate choice with the goal of exploring whether phenotypic plasticity can be interpreted as individual flexibility in the context of the switch point theorem, SPT (Gowaty and Hubbell 2009). We found >3000 studies; 198 were empirical studies of within‐sex phenotypic plasticity, and sixteen showed no evidence of mate choice plasticity. Most studies reported changes from choosy to indiscriminate behavior of subjects. Investigators attributed changes to one or more causes including operational sex ratio, adult sex ratio, potential reproductive rate, predation risk, disease risk, chooser's mating experience, chooser's age, chooser's condition, or chooser's resources. The studies together indicate that “choosiness” of potential mates is environmentally and socially labile, that is, induced – not fixed – in “the choosy sex” with results consistent with choosers' intrinsic characteristics or their ecological circumstances mattering more to mate choice than the traits of potential mates. We show that plasticity‐associated variables factor into the simpler SPT variables. We propose that it is time to complete the move from questions about within‐sex plasticity in the choosy sex to between‐ and within‐individual flexibility in reproductive decision‐making of both sexes simultaneously. Currently, unanswered empirical questions are about the force of alternative constraints and opportunities as inducers of individual flexibility in reproductive decision‐making, and the ecological, social, and developmental sources of similarities and differences between individuals. To make progress, we need studies (1) of simultaneous and symmetric attention to individual mate preferences and subsequent behavior in both sexes, (2) controlled for within‐individual variation in choice behavior as demography changes, and which (3) report effects on fitness from movement of individual's switch points.

Genetic variation but weak genetic covariation between pre- and post-copulatory episodes of sexual selection in Drosophila melanogaster

When females mate polyandrously, male reproductive success depends both on the male's ability to attain matings and on his ability to outcompete rival males in the fertilization of ova post-copulation. Increased investment in  ejaculate components may trade off with investment in precopulatory traits due to resource allocation. Alternatively, pre- and post-copulatory traits could be positively related if individuals can afford to invest heavily in traits advantageous at both episodes of selection. There is empirical evidence for both positive and negative associations between pre- and post-copulatory episodes, but little is known about the genetic basis of these correlations. In this study, we measured morphological, chemical and behavioural precopulatory male traits and investigated their relationship with measures of male fitness (male mating success, remating inhibition and offensive sperm competitiveness) across 40 isofemale lines of Drosophila melanogaster. We found significant variation among isofemale lines, indicating a genetic basis for most of the traits investigated. However, we found weak evidence for genetic correlations between precopulatory traits and our indices of male fitness. Moreover, pre- and post-copulatory episodes of selection were uncorrelated, suggesting selection may act independently at the different episodes to maximize male reproductive success.

Female choosiness and mating opportunities in the blood-sucking bug Rhodnius prolixus

We investigated the relationship between female choosiness and mating opportunities in the blood sucking bug Rhodnius prolixus. Rhodnius prolixus females exhibit active discrimination behaviour to male mating attempts which delays or prevents copulation. Female rejection behaviours can be beneficial if mating opportunities are readily available and the pool of males varies in their quality. Thus, the benefits of finding a better quality male may override the cost of rejecting a mating opportunity. Since the availability of mating opportunities is affected by the ratio of sexually active males to females, we randomly assigned focal pairs to arenas with a sex ratio biased toward males, females or without other individuals. More females exhibited rejection behaviour when conspecifics were present, however, no differences were found when the sex ratio was biased toward either males or females. We discuss possible explanations for these results and hypothesize about the adaptive function of female rejection behaviour.

COHORT MARRIAGE KINETICS IN THE CONTEXT OF MIGRATION, WITH A CASE STUDY OF JAPAN, 1920-1940

The concept of marriage squeeze expects a positive association between marriage formation and the availability of preferred mates. Previous research to test the hypothesis has had mixed results owing to inconsistent marriage measures, inconsistent age focuses and insufficient attention to migration. This study derives kinetics equations of marriage formation to link cohort age-specific mate availability to migration-adjusted marriage rate/incidence, a measure in contrast to nominal marriage rate. On testing the equations with Japanese census data for 1920-1940, it is found that, in female cohorts, mate availability impacts first marriage rate at the life-course stage from 15-19 to 20-24 years, but not at later stages. Among young females, the decline in mate availability accounted for about 21% of the decline in first marriage rate over the period 1920-1940, when there was a trend towards later but not less marriage in Japan. The study suggests that the flexibility of mate/spouse choice in females varies along the marriageable life course and is more manifest at older ages. At young ages, however, the marriage squeeze hypothesis could hold, presumably because young women are evolutionarily shaped to be choosier, perhaps postponing their marriages when preferred mates are in short supply.

Mate preference for dominant vs. subordinate males in young female Syrian hamsters (Mesocricetus auratus) following chemically-accelerated ovarian follicle depletion

Life history theory predicts that selectivity for mates generally declines as females age. We previously demonstrated this phenomenon in Syrian hamsters (Mesocricetus auratus), in that older females showed reduced preference for dominant over subordinate males. To test the hypothesis that decreased reproductive quality due to aging reduces mate preference, we decoupled reproductive and chronological age by treating young female hamsters with 4-vinylcyclohexene diepoxide (VCD), which destroys ovarian follicles and functionally accelerates ovarian follicle depletion without compromising the general health of rodents. In this study, VCD effectively reduced follicle numbers in young Syrian hamsters. VCD-treated and control females were allowed to choose between a dominant and a subordinate male in a Y-maze on the day of proestrus. Both VCD-treated and control females demonstrated preference for the dominant male by leaving a greater proportion of vaginal scent marks near him, which is a behavior that females display when soliciting prospective mates. However, there was no effect of treatment on the proportion of vaginal scent marks left for the dominant male. Furthermore, ovarian follicle numbers were not significantly correlated with any behaviors in either group. We conclude that accelerated ovarian follicle depletion does not reduce mate preference in young female hamsters.

Live fast, die old: no evidence of reproductive senescence or costs of mating in a damselfly (Odonata: Zygoptera)

Recent examples of actuarial senescence in wild insect populations have challenged the long-held assumption that the brevity of wild insect life spans precludes senescence. We investigate age-related patterns in mating behaviour in adults of a short-lived damselfly, Coenagrion puella and the implications of this mating. Using capture histories for 1033 individuals over two field seasons, we conduct both pooled and stratified analyses of variations in breeding activity. Pooled analyses suggest that there is strong age-related variation in the probability of being present at the mating rendezvous. However, no age-related variation was observed in the probability of mating. Stratified approaches confirmed a general pattern of age-related declines in survival probability, but provided only equivocal evidence of an effect of age on transition between temporary breeding states. Mating males and females showed greater survival than non-mating individuals, possibly as a consequence of higher body condition. Older males that were not currently breeding were less likely to commence breeding on the next day, but showed no patterns in breeding cessation. Overall, transitions between both breeding states declined with age, suggesting that males that breed tend to continue breeding while those that do not breed continue to be unsuccessful. Female mating rates were consistently high across all ages with no age-related decline apparent. While previous research has demonstrated actuarial senescence in this population, as does this study, we find little evidence of either age-related declines in reproductive behaviour or breeding-related declines in survival, which might indicate functional senescence or costs of mating, respectively. Indeed, the greater survival in mating individuals of both sexes suggests that variations in individual quality may mediate both reproductive success and longevity. Contrary to recent studies, we found no compelling evidence for reproductive senescence or a cost of mating in an important and well-studied model odonate. The possible link between condition and ageing suggests that individual quality needs to be taken into account when studying senescence. We recommend the use of multistrata models for the future investigation of these phenomena.