Sexual Ornaments Research Articles

The mechanistic origin of amber pigmentation of Perithemis tenera (Say, 1840) wings (Odonata: Libellulidae) and its function in conspecific signalling

Animal coloration serves various signaling and non-signaling functions. In damselflies and dragonflies (Odonata), such colors may not only play photoprotective and/or thermoregulatory roles but also serve as visual signals during courtship and/or agonistic interactions. Here, we analyzed the coloration of Perithemis tenera wings, a potential secondary sexual ornament, applying spectrophotometry and visual modeling to gain a deeper understanding of their color mechanisms and functions. The amber coloration of the P. tenera wings results from the interaction of light with both the melanized chitin matrix and possibly ommochrome pigments. Additionally, by fitting the absorbance curve of P. tenera wings to the extinction coefficient of different melanins, we deduced that pheomelanin is likely the pigment embedded in the wing’s chitinous matrix. The amber coloration of P. tenera wings stands out against their natural habitat, making it detectable by conspecifics. Finding multiple pigments in the P. tenera wings not only enhances our understanding of the functional roles of pigmentation in Odonata but also offer broader insights into how structural and pigment-based colorations evolve as multifunctional traits.

The fossil record of female sexual ornamentation in Empididae (Diptera: Empidoidea), with description of four new species

Females of the subfamily Empidinae (Diptera: Empididae s.s.) often exhibit sexual ornamentation, an adaptation that, within the animal world at large, is most often associated with males, especially in vertebrates. Ornaments of female Empidinae include: 1) legs with rows of relatively large pennate scales, 2) enlarged and/or darkly pigmented wings and 3) inflated abdominal sacs. Ornamentation makes the females appear larger, a characteristic that may make them appear, albeit deceptively, more fecund, and therefore more attractive to potential mates. Given the rarity of female sexual ornamentation, these flies, particularly those of the tribe Empidini, have become a model system for study of this phenomenon. The family’s fossil record consists of 111 known occurrences including several genera from the middle Jurassic. This fossil record however is dominated by two genera, Empis and Rhamphomyia, which account for nearly 60% of all fossil occurrences. Unfortunately, there has been no fossil record of empidine sexual ornamentation other than pennate leg scales. Herein, we review this fossil record and describe the first empidine fossils, all from the Middle Eocene Kishenehn Formation, with enlarged and/or darkly pigmented wings, Rhamphomyia kitadai sp. nov. and R. brunnipennis sp. nov., and two new species of Rhamphomyia which display pennate leg scales, R. decens sp. nov. and R. pennipes sp. nov. Rhamphomyia enena Cockerell, 1921, the oldest known fossil that exhibits pennate leg scales, is redescribed.

The effects of intersexual interactions on survival can drive the evolution of female ornaments in the absence of mate limitation.

The evolution of sexual ornaments in animals is typically attributed to reproductive competition. However, sexual ornaments also arise in contexts where the ornamented sex is neither mate nor gamete limited, and explanations for ornamentation in these cases remain incomplete. In many species, particularly those with slow life histories, lifetime reproductive success depends more strongly on adult survival than fecundity, and survival can depend on intersexual interactions. We develop a population genetic model to investigate how the effect of intersexual interactions on survival may contribute to ornament evolution in the absence of competition for mates. Using female ornamentation in polygynous mating systems as a case study, we show that, indeed, ornaments can evolve when the ornament functions to modify interactions with males in ways that enhance a female's own survival. The evolutionary dynamics depend qualitatively on the specific behavioral mechanism by which the ornament modifies social interactions. In all cases, the ornament's long-term persistence is ultimately determined by the coevolution of the male locus that determines how males affect female survival. We outline the scenarios that are most likely to favor the evolution of female ornaments through the effects of intersexual interactions on survival, and we urge empirical researchers to consider the potential for this social selection mechanism to shape traits of interest across taxa.

Large and interactive pipefish females display ornaments for longer with many males around

Why do animals display sexual ornaments – to attract mates, to compete for access to them, or both? In the broad-nosed pipefish (Syngnathus typhle), ornamented females commonly compete for access to males, whereas choosy males provide uniparental care. During courtship, females show a dynamic ornament, consisting of a row of dark B-shaped signs along the sides of their trunk. Here, we focus on the social context of female ornamentation and mating success (egg transfer) under experimentally female and male biased adult sex ratios. We found that the duration of female B-sign displays was not explained by adult sex ratios alone, but rather by sex ratios in combination with female body length and the number of female interactions. In detail, duration of B-sign display increased more steeply with the number of female-female interactions under the male biased adult sex ratio: that is, when more mating opportunities were available. Similarly, display duration increased with female body length, but only under the male biased adult sex ratio. In addition, successful egg transfer occurred more often in the male biased treatment and correlated significantly with female display duration in the same treatment. Taken together, these results suggest that in the context of the broad-nosed pipefish, the ornamental B-signs in females primarily serve as a sexually selected attractive signal. Our results highlight the importance of an interactive mating environment, male availability, and female size.

Sexual color ornamentation, microhabitat choice, and thermal physiology in the common wall lizard (Podarcis muralis).

Common wall lizards (Podarcis muralis) in Italy show a striking variation in body coloration across the landscape, with highly exaggerated black and green colors in hot and dry climates and brown and white colors in cool and wet climates. Males are more intensely colored than females, and previous work has suggested that the maintenance of variation in coloration across the landscape reflects climatic effects on the strength of male-male competition, and through this sexual selection. However climatic effects on the intensity of male-male competition would need to be exceptionally strong to fully explain the geographic patterns of color variation. Thus, additional processes may contribute to the maintenance of color variation. Here we test the hypothesis that selection for green and black ornamentation in the context of male-male competition is opposed by selection against ornamentation because the genes involved in the regulation of coloration have pleiotropic effects on thermal physiology, such that ornamentation is selected against in cool climates. Field observations revealed no association between body coloration and microhabitat use or field active body temperatures. Consistent with these field data, lizards at the extreme ends of the phenotypic distribution for body coloration did not show any differences in critical minimum temperature, preferred body temperature, temperature-dependent metabolic rate, or evaporative water loss when tested in the laboratory. Combined, these results provide no evidence that genes that underlie sexual ornamentation are selected against in cool climate because of pleiotropic effects on thermal biology.

Human‐modified habitats imperil ornamented dragonflies less than their non‐ornamented counterparts at local, regional, and continental scales

AbstractBiologists have long wondered how sexual ornamentation influences a species' risk of extinction. Because the evolution of condition‐dependent ornamentation can reduce intersexual conflict and accelerate the fixation of advantageous alleles, some theory predicts that ornamented taxa can be buffered against extinction in novel and/or stressful environments. Nevertheless, evidence from the wild remains limited. Here, we show that ornamented dragonflies are less vulnerable to extinction across multiple spatial scales. Population‐occupancy models across the Western United States reveal that ornamented species have become more common relative to non‐ornamented species over >100 years. Phylogenetic analyses indicate that ornamented species exhibit lower continent‐wide extinction risk than non‐ornamented species. Finally, spatial analyses of local dragonfly assemblages suggest that ornamented species possess advantages over non‐ornamented taxa at living in habitats that have been converted to farms and cities. Together, these findings suggest that ornamented taxa are buffered against contemporary extinction at local, regional, and continental scales.

Small-bodied males invest in larger testes when highly ornamented.

Sperm competition and male mating rate are two non-mutually exclusive key evolutionary pressures selecting for larger testes within and across animal taxa. A few studies have tried to test the role of mating rate in the absence of sperm competition. Under the mating rate hypothesis, particular phenotypes of a given population that are expected to gain more mates (e.g., more ornamented males) are expected to make higher investments in testes size (a proxy for sperm production). We test this prediction in Polistes simillimus, a neotropical paper wasp in which females are single mated (no sperm competition) and males can mate with multiple partners. Testes size was predicted by body size (positive association), sexual ornamentation (negative association), and their interaction (among small males, testes size was positively related to ornamentation, but the opposite pattern was observed among large males). We propose that small-bodied well-ornamented males may face the highest risk of sperm depletion. Small-bodied males make relatively higher investment in testes size when highly ornamented. This strategy might be less profitable to large males, as they have overall larger testes. Our results provide strong evidence for the mating rate hypothesis.

How important is hidden phenotypic plasticity arising from alternative but converging developmental trajectories, and what limits it?

Developmental plasticity -- the capacity for a genotype to develop into different phenotypes, depending on the environment - is typically viewed from the perspective of the resulting phenotype. Thus, if development is viewed as a trajectory towards a target, then developmental plasticity allows environmentally induced alterations to the target. However, there can also be variations in the trajectory. This is seen with compensatory responses, for instance where growth accelerates after an earlier period of food shortage, or where investment in sexual ornaments is maintained even when resources are limiting. If the compensation is complete, the adult phenotype can appear 'normal' (i.e. the different developmental trajectories converge on the same target). However, alternative trajectories to a common target can have multiple long-term consequences, including altered physiological programming and rates of senescence, possibly owing to trade-offs between allocating resources to the prioritized trait versus to body maintenance. This suggests that plasticity in developmental trajectories towards a common target leads to variation in the resilience and robustness of the adult body. This form of developmental plasticity is far more hidden than plasticity in final adult target, but it may be more common. Here, I discuss the causes, consequences and limitations of these different kinds of plasticity, with a special focus on whether they are likely to be adaptive. I emphasize the need to study plasticity in developmental trajectories, and conclude with suggestions for future research to tease apart the different forms of developmental plasticity and the factors that influence their evolution and expression.

No effect of testosterone or sexual ornamentation on telomere dynamics: A case study and meta-analyses.

Life-history theory predicts that reproductive investments are traded-off against self-maintenance. Telomeres, the protective caps on the ends of chromosomes, offer a promising avenue for assessing life-history trade-offs, as they shorten in response to stressors and are predictive of the remaining lifespan. In males, testosterone frequently mediates life-history trade-offs, in part, through its effects on sexual ornamentation, which is an important aspect of reproductive investment. However, studies of within-individual associations between telomere dynamics and sexual ornamentation are limited in number and have produced mixed results. Furthermore, most such studies have been observational, making it difficult to discern the nature of any causal relationship. To address this, we used short-acting testosterone implants in free-living male superb fairy-wrens (Malurus cyaneus) to stimulate the production of a sexual ornament: early moult into a costly blue breeding plumage. We found no evidence that elevated testosterone, and the consequent earlier moult into breeding plumage, accelerated telomere shortening. We therefore followed up with a systematic review and two meta-analyses (28 studies, 54 effect sizes) exploring the associations between telomeres and (1) testosterone and (2) sexual ornamentation. In line with our experimental findings, neither meta-analysis showed an overall correlation of testosterone or sexual ornamentation with telomere length or telomere dynamics. However, meta-regression showed that experimental, compared to observational, studies reported greater evidence of trade-offs. Our meta-analyses highlight the need for further experimental studies to better understand potential responses of telomere length or telomere dynamics to testosterone or sexual ornamentation.

Blind date: female fossorial amphisbaenians prefer scent marks of large and healthy males.

Selecting a good mate is a decision with important fitness consequences. For this reason, mate choice has promoted the evolution of sexual ornaments signaling the quality of an individual. In fossorial animals, inhabiting visually restricted underground environments, chemical senses should be very important for mate choice. We examined whether sexual chemical signals (substrate scent marks) produced by males of the Iberian worm lizard, Blanus cinereus, a strictly fossorial blind amphisbaenian, provide information to females on morphological traits and health state. We administered corticosterone (CORT) to males simulating a continuous stressor affecting their health. Females preferred settling at sites scent-marked by males in comparison with similar sites with female scent or unmarked sites, but the attractiveness of males' scent differed between individuals. Females preferred scent marks of larger/older males and with a higher immune response, while their body condition and CORT treatment were unrelated to female preferences. Chemical analyses showed that proportions of some compounds in precloacal secretions of males (used to produce scent marks) were correlated with the morphological (body size) and health state (immune response and body condition, but not CORT treatment) of these males. These results suggest that females may make site-selection decisions based on assessing the chemical characteristics of males' scent marks, which were reliably related to some of the traits of the male that produced the scent. Therefore, females might use chemical senses to increase the opportunities to find and mate with males of high quality, coping with the restrictions of the subterranean environment.

Spectral variation and pigmentary basis of ornamental and mimetic wing colour patches of swallowtail butterflies

Abstract Colours and colour patterns are diverse traits that are often used as visual signals. To test ecological and evolutionary drivers of these visual signals, a clear understanding of their nature and variation is necessary. Here we characterize variation in wing coloration of mormon swallowtail butterflies (Papilio, subgenus Menelaides). These species exhibit two kinds of colour patches on black wings: creamy white/yellow/green patches that are presumably used as sexual ornaments, and pure white patches that are presumably used as mimetic signals. Using reflectance spectrophotometry we quantified spectral properties of black wing background and colour patches between sexes, wing surfaces, and mimicry status. We found that brightness and saturation of the black background were less variable across sexes, wing surfaces, and mimetic/nonmimetic status. However, colour contrast and saturation were higher on dorsal surfaces than on ventral surfaces, and colour contrast between the black background and colour patches was higher in males than in females. Colour patches in nonmimetic butterflies were brighter and more saturated across the whole colour spectrum compared to mimetic butterflies. Thus, our study characterizes colour variation in an extraordinary signal radiation on the wings of swallowtail butterflies, a model clade in ecology, evolution, and genetics. Our results generate numerous hypotheses about the ecological and sexual functions of these colour patches, which may guide further behavioural experiments. Finally, using liquid chromatography-mass spectrometry we identified the pigment papiliochrome-II to be the chemical basis of the presumed sexual ornaments in mormon swallowtails.

The great tit abdominal stripe contains a sexually dichromatic colour patch hidden from the human eye

Information on the exact nature of sexual dichromatism might be incomplete, often leading to the treatment of dichromatic species as monochromatic. This error is evident when the two sexes of a species look identical to the human eye, as in the great tit (Parus major). We measured reflectance in three sections (throat, breast, belly) of the abdominal black stripe of great tits during the pairing and nestling feeding periods and assessed differences between patch sections, sexes, and seasons using visual models. We found that the stripe, which had previously been considered a single, contiguous patch, consists of multiple patches. In males, the breast section differed markedly from the throat and belly sections in having higher total brightness and ultraviolet chroma, while the female’s breast seemed to be less bright than the two other regions, resulting in strong sexual dichromatism hidden from the human eye. Colouration was more pronounced in winter, but dichromatism was present in both periods. The hidden breast ultraviolet patch we discovered in males may act as a sexual ornament or a signal amplifier.

Does capacity to produce androgens underlie variation in female ornamentation and territoriality in White-shouldered Fairywren (Malurus alboscapulatus)?

Historic bias toward study of sex hormones and sexual ornamentation in males currently constrains our perspective of hormone—behavior—phenotype relationships. Resolving how ornamented female phenotypes evolve is particularly important for understanding the diversity of social signals across taxa. Studies of both males and females in taxa with variable female phenotypes are needed to establish whether sexes share mechanisms underlying expression of signaling phenotypes and behavior. White-shouldered Fairywren (Malurus alboscapulatus) subspecies vary in female ornamentation, baseline circulating androgens, and response to territorial intrusion. The moretoni ornamented female subspecies is characterized by higher female, but lower male baseline androgens, and a stronger pair territorial response relative to pairs from the lorentzi unornamented female subspecies. Here we address whether subspecific differences in female ornamentation, baseline androgens, and pair territoriality are associated with ability to elevate androgens following gonadotropin releasing hormone (GnRH) challenge and in response to simulated territorial intrusion. We find that subspecies do not differ in their capacity to produce androgens in either sex following GnRH or simulated territorial intrusion (STI) challenges. STI-induced androgens were predictive of degree of response to territorial intrusions in females only, but the direction of the effect was mixed. GnRH-induced androgens did not correlate with response to simulated intruders, nor did females sampled during intrusion elevate androgens relative to flushed controls, suggesting that increased androgens are not necessary for the expression of territorial defense behaviors. Collectively, our results suggest that capacity to produce androgens does not underlie subspecific patterns of female ornamentation, territoriality, and baseline plasma androgens.

Size rather than complexity of sexual ornaments prolongs male metamorphosis and explains sexual size dimorphism in sepsid flies.

Male sexual ornaments often evolve rapidly and are thought to be costly, thus contributing to sexual size dimorphism. However, little is known about their developmental costs, and even less about costs associated with structural complexity. Here, we quantified the size and complexity of three morphologically elaborate sexually dimorphic male ornaments that starkly differ across sepsid fly species (Diptera: Sepsidae): (i) male forelegs range from being unmodified, like in most females, to being adorned with spines and large cuticular protrusions; (ii) the fourth abdominal sternites are either unmodified or are converted into complex de novo appendages; and (iii) male genital claspers range from small and simple to large and complex (e.g. bifurcated). We tracked the development of 18 sepsid species from egg to adult to determine larval feeding and pupal metamorphosis times of both sexes. We then statistically explored whether pupal and adult body size, ornament size and/or ornament complexity are correlated with sex-specific development times. Larval growth and foraging periods of male and female larvae did not differ, but the time spent in the pupal stage was ca 5% longer for sepsid males despite emerging 9% smaller than females on average. Surprisingly, we found no evidence that sexual trait complexity prolongs pupal development beyond some effects of trait size. Evolving more complex traits thus does not incur developmental costs at least in this system.

A learning experience elicits sex-dependent neurogenomic responses in Bicyclus anynana butterflies.

Sexually dimorphic behaviour is pervasive across animals, with males and females exhibiting different mate selection, parental care, foraging, dispersal, and territorial strategies. However, the genetic underpinnings of sexually dimorphic behaviours are poorly understood. Here we investigate gene networks and expression patterns associated with sexually dimorphic imprinting-like learning in the butterfly Bicyclus anynana. In this species, both males and females learn visual preferences, but learn preferences for different traits and use different signals as salient, unconditioned cues. To identify genes and gene networks associated with this behaviour, we examined gene expression profiles of the brains and eyes of male and female butterflies immediately post training and compared them to the same tissues of naïve individuals. We found more differentially expressed genes and a greater number of associated gene networks in the eyes, indicating a role of the peripheral nervous system in visual imprinting-like learning. Females had higher chemoreceptor expression levels than males, supporting the hypothesized sexual dimorphic use of chemical cues during the learning process. In addition, genes that influence B. anynana wing patterns (sexual ornaments), such as invected, spalt, and apterous, were also differentially expressed in the brain and eye, suggesting that these genes may influence both sexual ornaments and the preferences for these ornaments. Our results indicate dynamic and sex-specific responses to social scenario in both the peripheral and central nervous systems and highlight the potential role of wing patterning genes in mate preference and learning across the Lepidoptera.

Reproducing in hot water: Experimental heatwaves deteriorate multiple reproductive traits in a freshwater ectotherm

Abstract Heatwaves are occurring at an increasing frequency and intensity under ongoing climate change. As many reproductive traits—including mating behaviour and gamete traits—are sensitive even to small temperature changes, the impact of heatwaves on reproduction and sexual selection processes is likely to be vast. Also, evaluating whether the sexes respond differently to these extreme events is crucial to understand the impact on fecundity and the consequences at the population level. Nonetheless, our knowledge of the effects of heatwaves on these key aspects of animal life is still limited. Here, we expose recently mated male and female guppies Poecilia reticulata to an experimental heatwave (32°C, 6°C above the control, for 5 days) to determine its effects on several traits, including sexual behaviour, condition, ornamentation and fertility. Using this design, in contrast to alternative experimental setups, we had the possibility to attribute the effects of the heatwave to males' and females' reproductive traits independently. Overall, our results indicate that heatwaves can drastically affect key reproductive traits and unravel sex‐specific responses. In males, there was no effect of the heatwave on survival, but both pre‐ and postcopulatory reproductive traits were affected. After the heatwave, we detected a decrease in orange colouration (the most important ornament on which female choice is based) and the overall level of sexual activity, and a shift in the preferred mating tactic towards forced copulation attempts. The latter suggests implications in sexual conflict dynamics, as forced copulations override female mate choice. Also, after the heatwave, males had more sperm but of lower quality, and, in addition, an increased variance in sperm number. Overall, heatwaves may thus result in a compromised ability to secure matings and fertilizations. In females, the heatwave significantly affected survival, with increased mortality in the short term, and impaired fecundity, with many females from the heatwave treatment not reproducing at all. The negative effects of heatwaves on key reproductive traits unravelled by our study could have major implications for population dynamics and persistence. It highlights the need for further studies on how these extreme events affect reproduction, to improve our understanding of the impacts of climate change. Read the free Plain Language Summary for this article on the Journal blog.

Barn Swallows in East Asia show strong tolerance against nest mites

Abstract Hosts defend against parasites through two main strategies: resistance and tolerance. These two strategies have different influences on the parasite load of the host and so have variable effects on the fitness of parasites. In this study, we investigated the strategy used by the Barn Swallow (Hirundo rustica) to defend against a hematophagous mite (Dermanyssus hirundinis) in East Asia. For the first time, we found Barn Swallows were parasitized by D. hirundinis in Asia and, in northeastern China, over 60% of broods were parasitized. We examined whether swallows with better body condition or the males with more extravagant sexually selected traits (longer tail streamers or redder ventral plumage) showed stronger resistance or tolerance to these parasites. The results showed that males with more extravagant sexual ornaments had weaker resistance (heavier brood mite load) than males with less extravagant traits; however, the heavier mite load was likely to be a byproduct of larger brood size. Moreover, the Barn Swallow population showed strong tolerance against the parasite (i.e. the brood mite load did not affect brood size or nestling body mass). It is worth noting that male swallows with redder breast plumage showed weaker tolerance (lower seasonal reproductive success) than other males. Further, we conducted experimental ectoparasite manipulations in 46 Barn Swallow nests to verify the potential effect of these parasites on the body condition of nestlings. Neither the body mass nor the hemoglobin concentration differed between the designed groups; however, the hemoglobin concentration of nestlings decreased with mite load. We concluded that the Barn Swallow population in East Asia used the tolerance strategy, rather than resistance to defend against nest mites, and the ectoparasitic mite did not appear to be a strong selective factor for reproductive success owing to the strong tolerance of the studied Barn Swallow population.

Territorial males have larger wing spots than non-territorial males in the damselfly Calopteryx splendens (Zygoptera: Calopterygidae)

Males of Calopteryx splendens use two alternative mating tactics, territoriality, and non-territoriality. These different mating tactics are shown to vary between males within the same population and previous studies have shown that territorial males have considerably higher fitness than non-territorial males. In this paper, we tested whether the wing spot size as sexual ornament, wing length, relative wing spot size, asymmetry in wing length, asymmetry in wing spot size, or asymmetry in relative wing spot size differed between the territorial and non-territorial males. We sampled C. splendens males, representing both mating tactics, from a river system in south-west Finland. According to our results, territorial males have larger wing spot than non-territorial males. In contrast, there were no differences in the other tested traits between the territorial and non-territorial males. In conclusion, our data show that the size of pigmented wing spots may predict the alternative mating tactic of C. splendens males.

Is Color Related to Parasite Load in a Sexually Dichromatic Neotropical Toad?

Research on anuran parasites often focuses on the description and identification of parasites and demonstrating that parasitic infection can have deleterious effects on the health of amphibians. There is comparatively little research on how parasitism influences evolutionary dynamics such as sexual selection. Here, we explore anuran blood parasites with a focus on parasite-mediated sexual selection. The Hamilton–Zuk hypothesis posits that parasite–host coevolution drives sexual ornamentation of hosts, and that the degree of ornament expression honestly signals heritable parasite resistance. Although the Hamilton–Zuk hypothesis has received support in a number of animal taxa, it has received little attention in amphibians. Neotropical Yellow Toads (Incilius luetkenii) are sexually dichromatic toads with an unusual sexual ornament: males turn from cryptic brown to bright lemon yellow for a very short period during the brief breeding season. In this study, we identify blood parasites in male Neotropical Yellow Toads, and we investigate one component of the Hamilton–Zuk hypothesis by comparing skin color and body condition to blood parasite levels. We found four types of parasites. Apicomplexa and frog erythrocytic virus were the most common, infecting 73% and 46% of toads, respectively, whereas Nematoda and bacteria were found in 27% and 36% of toads, respectively. We found negative relationships between parasites and male color: males infected with fewer Apicomplexa and Nematoda had higher yellow saturation, and males with fewer Nematoda had longer-wavelength hue. In contrast, we found one positive relationship between parasites and male color: males infected with higher levels of frog erythrocytic virus had higher brightness. Our results suggest that parasites may influence sexual traits in this species, but that the relationship is complex. This research contributes to our understanding of amphibian host–parasite coevolution by demonstrating a relationship between parasites and host coloration.

Mating and Sexual Selection in Empidine Dance Flies (Empididae).

Simple SummaryIn many species of Empis, Rhamphomyia and Hilara dance flies, females rather than males display ornaments prior to mating. These ornaments appear to have evolved by Darwinian sexual selection in which females compete for access to choosy males that supply courtship (nuptial) gifts, which in many species appear to be the main or only source of dietary protein for adult females. We review the diversity of mating in this group of flies, including the different types of ornaments and aspects of behaviour and morphology thought to influence sexual selection on each sex, including nuptial gifts, and the sex ratios of aerial swarms where pairing takes place.Species whose behaviour or morphology diverges from typical patterns can provide unique insights on the evolutionary forces that promote diversity. Darwin recognised that while elaborate sexually selected traits mostly occurred among males, in a few species females possess such traits. Some species from the subfamily Empidinae (Diptera: Empididae) are among the animals that are often invoked to illustrate female ornaments. Empidines include taxa that exhibit varying levels of female ornament expression; some species possess multiple, elaborate female-specific ornaments while others have fewer and more modest adornments, and many species are altogether lacking discernible sexual ornamentation. This continuous variation in display traits in the Empidinae provides unique opportunities to explore the causes and consequences of sexually selected ornament expression. Here, we review the literature on sexual selection and mating systems in these flies and synthesise the evidence for various evolutionary forces that could conceivably create this impressive morphological and behavioural diversity, despite evolutionary constraints on female ornament exaggeration that help to explain its general rarity among animals. We also suggest some aspects of diversity that remain relatively unexplored or poorly understood, and close by offering suggestions for future research progress in the evolutionary ecology of mating behaviour among empidine flies.