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Predicted rapid increases in urbanization in the face of accelerating biodiversity loss underscores the need for urban development that promotes rather than displaces native plants and animals. One approach for increasing urban biodiversity is through the development of “green infrastructure”. Although research has explored urban-rural gradients and the overall value of urban green infrastructure, few studies have investigated the habitat value for wildlife of different types of urban greenspace. Here, we use a well-established metric in ecology, giving up-densities (GUDs), to compare foraging costs for a common urban wildlife species, the eastern gray squirrel (Sciurus carolinensis), among three green infrastructure categories: municipal parks, college campuses, and residential yards. We found that GUDs for gray squirrels did not differ significantly among location categories after controlling for proximity to roads, but proximity to roads was associated with significantly higher GUDs in all locations. In an explicit test, we also found that both proximity to roads and traffic volume were associated with higher GUDs. We also found that maximum distance from roads was significantly higher for campuses and parks than for residential yards, indicating a greater proportion of the area of campuses and parks is “away from roads” compared to residential yards. Our results indicate that vehicle traffic may contribute significantly to an “urban landscape of fear” for gray squirrels and suggest that campus and park configurations that reduce road effects could improve habitat quality for squirrels and possibly other animals.

Hemispheric brain lateralization can drive the expression of behavioral asymmetry, or laterality, which varies notably both within and among species. To explain these left–right behavioral asymmetries in animals, predator-mediated selection is often invoked. Recent studies have revealed that a relatively high degree of lateralization correlates positively with traits known to confer survival benefits against predators, including escape performance, multitasking abilities, and group coordination. Yet, we still know comparatively little about 1) how consistently predators shape behavioral lateralization, 2) the importance of sex-specific variation, and 3) the degree to which behavioral lateralization is heritable. Here, we take advantage of the model system of the radiation of Bahamas mosquitofish (Gambusia hubbsi) and measure behavioral lateralization in hundreds of wild fish originating from multiple blue holes that differ in natural predation pressure. Moreover, we estimated the heritability of this trait using laboratory-born fish from one focal population. We found that the degree of lateralization but not the particular direction of lateralization (left or right) differed significantly across high and low predation risk environments. Fish originating from high-predation environments were more strongly lateralized, especially females. We further confirmed a genetic basis to behavioral lateralization in this species, with significant additive genetic variation in the population examined. Our results reveal that predation risk represents one key ecological factor that has likely shaped the origin and maintenance of this widespread behavioral phenomenon, even potentially explaining some of the sex-specific patterns of laterality recently described in some animals.

Female response to male sexual signals is context dependent and influenced by a variety of environmental factors. For instance, females often adjust mating preferences in response to predation risk. Social experiences as a juvenile, such as exposure to conspecific sexual signals, have also been shown to influence female mating preferences as adults. Experiments that examine the influence of both environmental factors and early social experiences on female preferences are needed to understand female mating behaviors in the wild. In Pacific field crickets (Teleogryllus oceanicus), males produce a calling song that females evaluate during mate choice. We examined the influence of both risk perception and juvenile exposure to calling song in shaping adult T. oceanicus female phonotaxis to either a high- or low-quality calling song. We predicted that females raised in song-less conditions would be more responsive to calling song to maximize mating opportunities, and that females would show reduced response to calling in a risky, open area. Females raised in a song-deprived environment were much more responsive to calling song compared to those reared with song. In contrast to our predictions, females raised in song-less conditions were more responsive to the high-quality song in the risky, open arena. These results suggest that juvenile acoustic rearing influences female responsiveness to male calling song, but it has less influence on strength of preference for signal quality.

Sexual signal evolution may present fitness consequences for the non-signaling sex due to shared genes and altered social conditions, but this is rarely studied in natural populations. On the Hawaiian Island of Kauai, most male Teleogryllus oceanicus (Pacific field crickets) lack the ability to sing because of a novel wing mutation (flatwing) that arose and spread in

Animals in captivity experience drastically different selective pressures than their wild counterparts. This can cause evolutionary divergence in behavior between captive and wild populations. While most research on evolution under captivity has focused on vertebrates, we expect similar behavioral changes in insects that live and breed in commercial facilities. Using the common house cricket, Acheta domesticus, we tested how crickets reared in captivity for many generations differed from wild-caught counterparts in two aspects of social behavior: male aggression and female responsiveness to male calling song. Acheta domesticus is an important model organism for behavioral research and are often reared in dense, commercial facilities with ad-libitum access to food and no risk of mortality from predators— very different conditions from the wild. We predicted that commercially-derived males would exhibit less intrasexual aggression due to selection from living in dense conditions. We predicted that commercially-derived females would be less responsive to male calling song because they are more likely to encounter many males at random. Instead, we found that commercially-derived males were more aggressive than wild ones, and that commercially-derived and wild females did not differ in responsiveness to calling song. Insects serve as model systems for a great deal of research in evolutionary and behavioral biology. If these animals are evolving in captivity, they may not provide an accurate representation of the natural phenomena we aim to understand.

Associations between heritable polymorphisms and life-history traits, such as development time or reproductive investment, may play an underappreciated role in maintaining polymorphic systems. This is because selection acting on a particular morph could be bolstered or disrupted by correlated changes in life history or vice versa. In a Hawaiian population of the Pacific field cricket (Teleogryllus oceanicus), a novel mutation (flatwing) on the X-chromosome is responsible for a heritable polymorphism in male wing structure. We used laboratory cricket colonies fixed for male wing morph to investigate whether males and females bearing the flatwing or normal-wing (wild-type) allele differed in their life-history traits. We found that flatwing males developed faster and had heavier testes than normal-wings, whereas flatwing homozygous females developed slower and had lighter reproductive tissues than normal-wing homozygous females. Our results advance our understanding of the evolution of polymorphisms by demonstrating that the genetic change responsible for a reproductive polymorphism can also have consequences for fundamental life-history traits in both males and females.

How populations adapt, or not, to rapid evolution of sexual signals has important implications for population viability, but is difficult to assess due to the paucity of examples of sexual signals evolving in real time. In Hawaiian populations of the Pacific field cricket (Teleogryllus oceanicus), selection from a deadly parasitoid fly has driven the rapid loss of a male acoustic signal, calling song, that females use to locate and evaluate potential mates. In this newly quiet environment where many males are obligately silent, how do phonotactic females find mates? Previous work has shown that the acoustic rearing environment (presence or absence of male calling song) during late juvenile stages and early adulthood exposes adaptive flexibility in locomotor behaviors of males, as well as mating behaviors in both sexes that helps facilitate the spread of silent (flatwing) males. Here, we tested whether females also show acoustically induced plasticity in walking behaviors using laboratory‐reared populations of T. oceanicus from Kauai (HI; >90% flatwings), Oahu (HI; ~50% flatwings), and Mangaia (Cook Islands; no flatwings or parasitoid fly). Though we predicted that females reared without song exposure would increase walking behaviors to facilitate mate localization when song is rare, we discovered that, unlike males, female T. oceanicus showed relatively little plasticity in exploratory behaviors in response to an acoustic rearing environment. Across all three populations, exposure to male calling song during development did not affect latency to begin walking, distance walked, or general activity of female crickets. However, females reared in the absence of song walked slower and showed a marginally non‐significant tendency to walk for longer durations of time in a novel environment than those reared in the presence of song. Overall, plasticity in female walking behaviors appears unlikely to have facilitated sexual signal loss in this species.

How sexual traits are gained and lost in the wild remains an important question in evolutionary biology. Pacific field crickets (Teleogryllus oceanicus) in Hawaii provide an unprecedented opportunity to investigate the factors facilitating evolutionary loss of a sexual signal in real time. Natural selection from an acoustically orienting parasitoid fly drove rapid evolution of a novel, silent male morph. While silent (flatwing) males enjoy protection from the fly, they face difficulty attracting mates. We tested how offspring production varies in association with three male attributes affected by the spread of flatwing: wing morph (flatwing or normal-wing), age (flatwings should survive longer than singers) and exposure to calling song during rearing (wild populations with many flatwings lack ambient calling song). Per mating event, flatwings sired more offspring than singers and older males were mounted more quickly by females when presented with standard courtship song. Despite prior work showing that male age and acoustic experience influence sperm characteristics associated with fertilization, age and song exposure had no influence on male offspring production per mating. This represents the first evidence that the silent male morph possesses a reproductive advantage that may help compensate for precopulatory barriers to mate attraction.

Pleiotropy between male signals and female preferences can facilitate evolution of sexual communication by maintaining coordination between the sexes. Alternatively, it can favor variation in the mating system, such as a reproductive polymorphism. It is unknown how common either of these scenarios is in nature. In Pacific field crickets (Teleogryllus oceanicus) on Kauai, Hawaii, a mutation (flatwing) that segregates as a single locus is responsible for the rapid loss of song production in males. We used outbred cricket colonies fixed for male wing morph to investigate whether homozygous flatwing and normal-wing (wild-type) females differ in responsiveness to male calling song and propensity to mate when paired with either a flatwing or normal-wing male in the presence or absence of courtship song. Flatwing females were less likely to mount a male than normal-wing females. Females of both genotypes showed a preference for normal-wing males and were more likely to mate in the presence of courtship song; normal-wing females were particularly likely to mate with song. Our results show that negative pleiotropy between obligate male silence and female mating behavior can constrain the evolution of sexual signal loss and contribute to the maintenance of a male reproductive polymorphism in the wild.

Many animals show complex behaviours that can have important ecological and evolutionary consequences. Environmental variation can lead to divergent selection that consistently favours particular behaviours in different environments; but how predictably multiple aspects of animal behaviour diverge in response to different environmental conditions remains unclear. We tested whether populations evolving under different levels of predation risk show predictable and repeatable population-level behavioural differences in all five primary components of animal personality: aggression, sociability, boldness, activity and exploration. We formulated and tested a priori predictions of divergence for each behaviour using the adaptive radiation of Bahamas mosquitofish, Gambusia hubbsi (family Poeciliidae), inhabiting vertical water-filled caves (blue holes) where they have evolved for thousands of years in either the presence or absence of predatory fish. Mosquitofish behaviours differed consistently, and largely predictably, between predation regimes: low-predation mosquitofish showed reduced sociability and greater exploration of a novel environment compared to high-predation counterparts. However, some differences were sex dependent: only females showed greater boldness and only males displayed reduced aggressiveness in low-predation populations. Activity levels did not differ between predation regimes. All populations showed a behavioural syndrome characteristic of either proactive or reactive stress-coping styles with regard to exploration. Exploration behavioural syndromes were more similar among populations that evolved in similar predation regimes, regardless of genetic relatedness. Using laboratory-born, high-predation mosquitofish, we confirmed that exploratory behaviours have a genetic basis and show significant within-individual repeatability. Our results suggest that environmental variation, such as chronic predation risk, can lead to repeatable, and often predictable, changes in multifarious animal behaviours, and that various aspects of behaviour can diversify more or less independently of one another. Considering the ecological importance of these behaviours, the ability to forecast behavioural shifts in a rapidly changing world could serve as a valuable conservation tool.

The study of genital diversity has experienced rapidly burgeoning attention over the past few decades. This research has shown that male genitalia in internally fertilizing animals exhibit remarkably rapid and complex evolution. In recent years, a consensus has emerged that sexual selection is responsible for much of the observed genital diversity, with natural selection largely playing a subsidiary role. Despite enhanced understanding of the key proximate forms of selection responsible for genital evolution, we still have a poor grasp of the broader, ultimate causes and consequences of the striking diversity of genitalia. Here, we highlight three topics that have so far received comparatively little attention and yet could prove critically important. First, we encourage investigation of ecology's direct and indirect roles in genital diversification, as ecological variation can influence selection on genitalia in several ways, perhaps especially by influencing the context of sexual selection. Second, we need more research into the effects of genital divergence on speciation, as genital differences could enhance reproductive isolation through either a lock-and-key process (where selection directly favors reproductive isolation) or as an incidental by-product of divergence. Third, we echo recent calls for increased research on female genitalia, as non-trivial female genital diversity exists, and multiple mechanisms can lead to rapid diversification of female genitalia. For all three topics, we review theory and empirical data, and describe specific research approaches for tackling these questions. We hope this work provides a roadmap toward increased understanding of the causes and consequences of the conspicuous diversity of primary sexual traits, and thus toward new insights into the evolution of complex traits and the phenotypic causes of speciation.

Sex and reproduction lie at the heart of studies of insect behavior. We begin by providing a brief overview of insect anatomy and physiology, followed by an introduction to the overarching themes of parental investment, sexual selection, and mating systems. We then take a sequential approach to illustrate the diversity of phenomena and concepts behind insect reproductive behavior from pre-copulatory mate signalling through copulatory sperm transfer, mating positions, and sexual conflict, to post-copulatory sperm competition, and cryptic female choice. We provide an overview of the evolutionary mechanisms driving reproductive behavior. These events are linked by the economic defendability of mates or resources, and how these are allocated in each sex. Under the framework of economic defendability, the reader can better understand how sexual antagonistic behaviors arise as the result of competing optimal fitness strategies between males and females.

A longstanding hypothesis in evolutionary biology is that trade-offs between natural and sexual selection often underlie the diversification of sexual signals in the wild. A classic example of this "selection trade-off hypothesis" proposes that males evolve elaborate and conspicuous ornamentation in low-risk environments where female preferences dominate selection on sexual traits, but they evolve muted and relatively cryptic sexual traits in high-risk environments where selection from predators acts against conspicuous sexual traits and female preferences potentially weaken or reverse. However, little direct empirical evidence supports this notion. Using the model system of Bahamas mosquitofish (Gambusia hubbsi)—where males have recently evolved greater orange coloration in their dorsal fins in blue holes lacking predatory fish relative to populations with fish predators—we tested this hypothesis using fish replicas differing only in dorsal-fin color. Specifically, we employed plastic fish models in a combination of field and lab experiments to directly examine conspicuity to predators and female preferences for dorsal-fin color. We found that orange-shifted dorsal fins resembling the color exhibited in predator-free populations appeared more conspicuous to predatory bigmouth sleepers (Gobiomorus dormitor) that are evolutionarily naive to mosquitofish. Wild-caught female mosquitofish preferred the orange-shifted dorsal-fin model during dichotomous choice tests; evolutionary history with predators did not affect female preferences. Similar mate-choice trials with lab-born virgin females also found preferences for the orange-shifted dorsal-fin model and revealed significant genetic variation for female preferences. Our study provides direct empirical evidence documenting a trade-off between natural and sexual selection in a colorful sexual signal.

The aim of this study rests on three premises: (i) humans are altering ecosystems worldwide, (ii) environmental variation often influences the strength and nature of sexual selection, and (iii) sexual selection is largely responsible for rapid and divergent evolution of male genitalia. While each of these assertions has strong empirical support, no study has yet investigated their logical conclusion that human impacts on the environment might commonly drive rapid diversification of male genital morphology. We tested whether anthropogenic habitat fragmentation has resulted in rapid changes in the size, allometry, shape, and meristics of male genitalia in three native species of livebearing fishes (genus: Gambusia) inhabiting tidal creeks across six Bahamian islands. We found that genital shape and allometry consistently and repeatedly diverged in fragmented systems across all species and islands. Using a model selection framework, we identified three ecological consequences of fragmentation that apparently underlie observed morphological patterns: decreased predatory fish density, increased conspecific density, and reduced salinity. Our results demonstrate that human modifications to the environment can drive rapid and predictable divergence in male genitalia. Given the ubiquity of anthropogenic impacts on the environment, future research should evaluate the generality of our findings and potential consequences for reproductive isolation.

How sexual traits are gained and lost in the wild remains an important question in evolutionary biology. Pacific field crickets ( Teleogryllus oceanicus ) in Hawaii provide an unprecedented opportunity to investigate the factors facilitating evolutionary loss of a sexual signal in real time. Natural selection from an acoustically orienting parasitoid fly drove rapid evolution of a novel, silent male morph. While silent (flatwing) males enjoy protection from the fly, they face difficulty attracting mates. We tested how offspring production varies in association with three male attributes affected by the spread of flatwing: wing morph (flatwing or normal-wing), age (flatwings should survive longer than singers) and exposure to calling song during rearing (wild populations with many flatwings lack ambient calling song). Per mating event, flatwings sired more offspring than singers and older males were mounted more quickly by females when presented with standard courtship song. Despite prior work showing that male age and acoustic experience influence sperm characteristics associated with fertilization, age and song exposure had no influence on male offspring production per mating. This represents the first evidence that the silent male morph possesses a reproductive advantage that may help compensate for precopulatory barriers to mate attraction.

Male genital morphology is remarkably diverse across internally fertilizing animals, a phenomenon largely attributed to sexual selection. Ecological differences across environments can alter the context of sexual selection, yet little research has addressed how this may influence the rapid, divergent evolution of male genitalia. Using the model system of Bahamas mosquitofish (Gambusia hubbsi) undergoing ecological speciation across blue holes, we used geometric morphometric methods to test (i) whether male genital shape (the small, approximately 1 mm long, distal tip of the sperm-transfer organ, the gonopodium) has diverged between populations with and without predatory fish and (ii) whether any observed divergence has a genetic basis. We additionally examined the effects of genetic relatedness and employed model selection to investigate other environmental factors (i.e. interspecific competition, adult sex ratio and resource availability) that could potentially influence genital shape via changes in sexual selection. Predation regime comprised the most important factor associated with male genital divergence in this system, although sex ratio and some aspects of resource availability had suggestive effects. We found consistent, heritable differences in male genital morphology between predation regimes: Bahamas mosquitofish coexisting with predatory fish possessed more elongate genital tips with reduced soft tissue compared with counterparts inhabiting blue holes without predatory fish. We suggest this may reflect selection for greater efficiency of sperm transfer and fertilization during rapid and often forced copulations in high-predation populations or differences in sexual conflict between predation regimes. Our study highlights the potential importance of ecological variation, particularly predation risk, in indirectly generating genital diversity.

Peppered with personal stories, historical context, and examples from across the animal kingdom, Nature's Nether Regions provides an accessible and entertaining account of one of the most fundamental, but long overlooked, aspects of biodiversity – animal genitalia. Belying their seemingly simple function, genitalia often exhibit bizarre shapes and sizes. Taxonomists have known this for centuries, employing genital morphology to identify species. But research aimed at unraveling why genitalia evolve such elaborate forms did not take root until recent decades.

Sexual signal evolution can be complex because multiple factors influence the production, transmission, and reception of sexual signals, as well as receivers' responses to them. To grasp the relative importance of these factors in generating signal diversity, we must simultaneously investigate multiple selective agents and signaling traits within a natural system. We use the model system of the radiation of Bahamas mosquitofish (Gambusia hubbsi) inhabiting blue holes to test the effects of resource availability, male body size and other life-history traits, key aspects of the transmission environment, sex ratio, and predation risk on variation in multiple male color traits. Consistent with previous work examining other traits in this system, several color traits have repeatedly diverged between predation regimes, exhibiting greater elaboration in the absence of predators. However, other factors proved influential as well, with variation in resource levels, body size, relative testes size, and background water color being especially important for several color traits. For one prominent signaling trait, orange dorsal fins, we further confirmed a genetic basis underlying population differences using a laboratory common-garden experiment. We illustrate a promising approach for gaining a detailed understanding of the many contributing factors in the evolution of multivariate sexual signals.