Nonlinear and correlational sexual selection on 'honest' female ornamentation

Female ornamentation has long been overlooked because of the greater prevalence of elaborate displays in males. However, the circumstances under which females would benefit from honestly signalling their quality are limited. Females are not expected to invest in ornamentation unless the fitness benefits of the ornament exceed those derived from investing the resources directly into offspring. It has been proposed that when females gain direct benefits from mating, females may instead be selected for ornamentation that deceives males about their reproductive state. In the empidid dance flies, males frequently provide nuptial gifts and it is usually only the female that is ornamented. Female traits in empidids, such as abdominal sacs and enlarged pinnate leg scales, have been proposed to 'deceive' males into matings by disguising egg maturity. We quantified sexual selection in the dance fly Rhamphomyia tarsata and found escalating, quadratic selection on pinnate scales and that pinnate scales honestly reflect female fecundity. Mated females had a larger total number and more mature eggs than unmated females, highlighting a potential benefit rather than a cost of male mate choice. We also show correlational selection on female pinnate scales and fecundity. Correlational selection, equivalent investment patterns or increased nutrition from nuptial gifts may all maintain honesty in female ornamentation.     

Contrasting sexual selection on males and females in a role-reversed swarming dance fly, Rhamphomyia longicauda Loew (Diptera: Empididae)

Although there are several hypotheses for sex-specific ornamentation, few studies have measured selection in both sexes. We compare sexual selection in male and female dance flies, Rhamphomyia longicauda (Diptera: Empididae). Swarming females display size-enhancing abdominal sacs, enlarged wings and decorated tibiae, and compete for nuptial gifts provided by males. Males preferentially approach large females, but the nature of selection and whether it is sex-specific are unknown. We found contrasting sexual selection for mating success on structures shared by males and females. In females, long wings and short tibiae were favoured, whereas males with short wings and long tibiae had a mating advantage. There was no assortative mating. Females occupying potentially advantageous swarm positions were large and, in contrast to selection for mating success, tended to have larger tibiae than those of rivals. We discuss our findings in the context of both the mating biology of dance flies, and the evolution of sexual dimorphism in general.     

An invasion of cheats; the evolution of worthless nuptial gifts

Nuptial gifts are food items or inedible tokens that are transferred to females during courtship or copulation . Tokens are of no direct value to females, and it is unknown why females require such worthless gifts as a precondition of mating. One hypothesis is that token giving arose in species that gave nutritious gifts and males exploited female preferences for nutritional gifts by substituting more easily obtainable but worthless items. An invasion of such behavior would require that females accept the substitute gift and copulate for a period of time similar to that with genuine gifts. We show that both these prerequisites are met in the dance fly Rhamphomyia sulcata, in which females normally accept a nutritious gift. We removed the gift from copulating pairs and replaced it with either a large or small prey item or inedible token. We found that although pairs copulated longest with a large genuine gift, the tokens resulted in copula durations equivalent to those with a small genuine gift. We also observed that males that returned to the lek with tokens re-paired successfully. These findings suggest that female behavior in genuine gift-giving species is susceptible to the invasion of male cheating on reproductive investment.     

Swarming Behavior, Sexual Dimorphism, and Female Reproductive Status in the Sex Role-Reversed Dance Fly Species Rhamphomyia marginata

Dance flies are predaceous insects which often form male mating swarms. In many species males prior to swarming catch an insect prey, which is presented to the female at mating. In Rhamphomyia marginata, females in contrast to males gather to swarm, while males carrying a prey visit swarms for mating. Here I describe the swarming and courtship behavior in R. marginata and provide data on sexual dimorphism and swarming female reproductive status. Females swarm in small clearings in the forests. There was no specific swarm-maker. The swarming period lasted for 2–3 h and peaked around sunset. Identical swarm sites were used each evening and for several years. The mean number of females in swarms (swarm sites with at least one female) was 9.9 ± 9.1 (range, 1–40; n = 107) in 1993 and 7.1 ± 7.0 (range, 1–35; n = 68) in 1994. No obvious competition between females in swarms was observed. The operational sex ratio in swarms was extremely female biased (all swarms, 0.04). Less than one-third of male visits to swarms resulted in mating and males were found more often in larger swarms. Nuptial prey consisted of male midges. Females seem to mate more than once. Swarming females had undeveloped eggs, whereas mated females in swarms had further developed eggs than unmated females. Amount of sperm in the spermatheca was correlated with egg size. Amount of sperm and egg size did not correlate with wet weight, wing length, or wing load, except for egg size and weight. The wing coloration pattern and shape in R. marginata females are unique among dance flies, being greatly enlarged (1.6 times larger than that of males) and bicolored (gray part, 60% of wing area). When females, instead of males, possess extravagant secondary sexual characters, it is predicted from sexual selection theory that females should compete for males and that males should be selective in their choice of partner. A sex-role reversal will evolve when assess to males limit female reproductive success. The dance fly species R. marginata, like Empis borealis, another dance fly species studied earlier and discussed here, seems to fit these predictions.     

Sexual selection in the gift-giving dance fly, Rhamphomyia sulcata, favors small males carrying small gifts

In some species of insects males transfer a gift to females during courtship or copulation. In the dance flies these nuptial gifts vary from nutritious prey items to inedible tokens such as a leaf, stone, or silk balloon. Nuptial gifts in dance flies are presumed to increase male mating success. We examined the strength and form of sexual selection on male Rhamphomyia sulcata, an empidid in which males provide females with a nutritious prey item as a nuptial gift. We found that whereas large males carried large gifts, neither large males nor gifts were targets of sexual selection. Indeed, correlational selection analysis and nonparametric examination of the fitness surfaces revealed that small males carrying small gifts were the most successful. Males may be more maneuverable or flight efficient with small gifts, or small males with large gifts may be unable to carry both a large gift and a female in the paired descent flight. These results suggest carrying constraints may be an important factor in determining selection on nuptial gift size. The largest target of sexual selection was old males. Old males were also paired with the largest and most fecund females, highlighting the role mate quality can further contribute to selection on males. Correlational selection analysis also revealed selection for an increase in covariance between male wing length and body size, and for an increase in slope between these traits. Males who deviate away from the optimal phenotypic relationship for two tightly related morphological traits, such as tibia and wing length, may have overall reduced performance. These findings highlight the role correlational sexual selection can play in optimizing nonsexual male morphology and scaling relationships. This study questions the role of the nuptial gift in dance flies as a resource for females.     

Comparative evidence supports a role for reproductive allocation in the evolution of female ornament diversity

1. Sexually selected ornaments are highly variable, even among closely related species, and the ultimate causes of variation in ornament evolution are unclear, including in rare cases of female ornament expression. One hypothesis is that differences across species in female reproductive allocation may help to explain patterns of female ornament expression among insects with nuptial gifts. 2. Dance flies (Diptera: Empididae: Empidinae) vary considerably among species in the presence and extravagance of female ornaments, which probably evolved through female contests for mates. In most dance flies, adult females appear to acquire all their dietary protein from nuptial gifts provided by males during mating. The importance of nuptial feeding on egg development is not yet known. 3. To test the prediction that the presence of female ornaments reflects differences in the degree to which females rely on nuptial feeding for egg development, egg development was examined in wild females of two species, one ornamented and the other unornamented. An ageing technique based on cuticular bands was validated, which permitted a regression of egg size on adult age. 4. We found that egg development depended on mating status in the ornamented species alone, meaning the eggs of unmated females of the ornamented species did not develop. This contrast across species is consistent with expectations that females of different species vary in their dependence on nuptial gifts for egg development. 5. These findings provide preliminary support for the hypothesis that differences in reproductive allocation mediate the intensity of female contests for nuptial gifts.     

Absence of direct sexual selection for parasitoid encapsulation in Drosophila melanogaster

Resistance against parasites may play a role in female mate choice, especially if males that have actually survived parasitism can be discriminated from males that have not been parasitised. Larvae of several Drosophila species are subject to attack by hymenopteran parasitoids, but have the ability to kill the parasitoid egg through the process of encapsulation. Because an encapsulated egg remains visible in the abdomen of the adult fly throughout its life, its presence in a male signals to a female that the male has the genes to survive parasitism. The hypothesis that females preferentially mate with males bearing an encapsulated egg in their abdomen was tested using D. melanogaster. No indication was found for this female preference. The absence of preference for males with “good genes” could result from sensory constraints in the female or a negative correlation between encapsulation ability and some other fitness component. Alternatively, it is hypothesised that the black abdominal ends of the males of many species in the melanogaster-group evolved to mimic encapsulated eggs, leading to the breakdown of capsule recognition by the female.     

Nutritional implications for sexual cannibalism in a sexually dimorphic orb web spider

Models predicting mechanisms driving sexual cannibalism in spiders with sexual size dimorphism (SSD) often assume that spiders use post‐copulatory food to channel nutrients into eggs and fecundity is altered through changes in clutch size or egg mass. I tested these assumptions using an orb web spider with extreme SSD, Argiope keyserlingi. I fed mated female spiders prey of either high protein‐low energy or low protein‐high energy composition. I measured egg energy density; a measure of the relative volumes of yolk and albumen. I predicted that if A. keyserlingi increase their egg energy density upon feeding on prey of a specific nutrient composition, they could enhance their fecundity by investing in more energy dense eggs. However, if the egg energy densities are dissimilar to their post‐copulatory prey they must be drawing energy from their somatic reserves to invest in eggs. In a further experiment I allowed female spiders to mate with and cannibalize males to determine if cannibalism induces similar effects on egg energy density. Male spider protein energy ratio was measured and found to resemble the high protein‐low energy prey. I found disagreement between the composition of post‐copulatory food and eggs in both experiments. Additionally, spiders fed high protein‐low energy prey lost weight indicating that they draw on their energy reserves to invest in eggs. I thus concluded that spiders that feed on high protein‐low energy prey or on males increase their egg energy density and, possibly, fecundity. However, the nutrient content of the prey or males cannot provide for investment in eggs. The energy invested in eggs is drawn from somatic reserves, probably induced by an as yet undescribed physiological trigger.     

Nuptial gifts of male spiders function as sensory traps

While nuptial food gifts come in various forms in arthropods, their evolutionary origins are unclear. A previous study on insects has shown that such gifts may arise as a sensory trap that exploits a female's underlying motivation to feed. Here I present independent evidence of a sensory trap in spiders. In certain visually oriented spiders, I suggest that males initially exploit the maternal care instinct by producing a nuptial gift that closely resembles the female egg sac. Males of the spider Pisaura mirabilis cover their prey gift with a silk layer, transforming it into a white round object. In a laboratory experiment I tested whether the colour of the gift affected the rate that females accepted males displaying their gifts. I found that the brighter and the more alike the nuptial gift to a female's egg sac, the faster the female responded by grabbing the gift. My results support the hypothesis that the nuptial gift in P. mirabilis works as a sensory trap.     

Neue Mecopteren aus dem oberen Jura von Karatau (Kasachstan) (Insecta, Mecoptera: ‘Orthophlebiidae’)

Structures of the wings, legs and abdomen of three new species of Jurassic ‘Orthophlebiidae’ from Karatau in Kazachstan (Orthophlebia elenae, O. heidemariae, O. longicauda) are described in detail. On their hind legs, the males of two of the species have a so-called metatarsal organ of unknown function, which certainly is a synapomorphy.Orthophlebia longicauda is very similar to the Tertiary speciesHolcorpa maculosa, as they both possess a fork consisting of two long processes arising from the hind margin of abdominal segment 6 as well as an elongate abdomen. A female specimen ofO. elenae shows, in its 9th abdominal segment, the medigynium, a genital structure of major importance for reconstructing the phylogenetic position of Mecoptera. As in the Panorpodidae andHolcorpa, the medigynium ofO. elenae does not bear two tongue-shaped processes which are characteristic of the Panorpidae. This is the first time that female genitalia are described from a Mesozoic mecopteran species. The males of the three new species show the tergites and sternites of abdominal segments 6 to 8 to be unfused or incompletely fused, as the sclerites’ margins are still visible. This implies that contrary to earlier assumptions, completely fused sclerites in segment 8 leaving no trace of their delimitations are not a synapomorphy of the Panorpomorpha (= Eomeropidae,Apteropanorpa, Choristidae,Choristopanorpa and ‘Orthophlebiidae’ plus descendants).     

The role of functional constraints in nonrandom mating patterns for a dance fly with female ornaments

Most hypotheses to explain nonrandom mating patterns invoke mate choice, particularly in species that display elaborate ornaments. However, conflicting selection pressures on traits can result in functional constraints that can also cause nonrandom mating patterns. We tested for functional load‐lifting constraints during aerial copulation in Rhamphomyia longicauda, a species of dance fly that displays multiple extravagant female‐specific ornaments that are unusual among sexual traits because they are under stabilizing selection. R. longicauda males provide females with a nuptial gift before engaging in aerial mating, and the male bears the entire weight of the female and nuptial gift for the duration of copulation. In theory, a male's ability to carry females and nuptial gifts could constrain pairing opportunities for the heaviest females, as reported for nonornamented dance flies. In concert with directional preferences for large females with mature eggs, such a load‐lifting constraint could produce the stabilizing selection on female size previously observed in this species. We therefore tested whether wild‐caught male R. longicauda collected during copulation were experiencing load‐lift limitations by comparing the mass carried by males during copulation with the male's wing loading traits. We also performed permutation tests to determine whether the loads carried by males during copulation were lighter than expected. We found that heavier males are more often found mating with heavier females suggesting that whereas R. longicauda males do not experience a load‐lift constraint, there is a strong relationship of assortative mating by mass. We suggest that active male mate choice for intermediately adorned females is more likely to be causing the nonrandom mating patterns observed in R. longicauda.     

Macronutrient intake affects reproduction of a predatory insect

Prey quality has previously been shown to affect the growth and reproduction of predatory arthropods, however relatively little is known about the specific nutrients responsible for these effects. We tested if the macronutrient content (i.e. lipid and protein) of live prey affected mate attraction, reproductive behavior, egg production and nutrient reserves of adult female praying mantids, Pseudomantis albofimbriata. Females on a high‐protein diet produced more than twice as many eggs as females on a high‐lipid diet despite being fed the same overall biomass of prey. Furthermore, the lipid and protein composition of eggs and the female body was directly related to the diet that females were fed (i.e. high lipid content on the high‐lipid diet). Even more striking was the effect of diet treatment on the number of males attracted to females – only one male was attracted to females on the high‐lipid treatment and 56 males were attracted to females on the high‐protein treatment. Although it is not unexpected that females with more eggs would attract more males, the extreme nature of this difference is certainly surprising because previous studies have shown that females with only a couple of eggs can attract multiple males. Hence, our results suggest that female pheromone production may be affected by the quality/nutritional composition of eggs rather than simply the number of eggs. We found no significant difference in any of the other behaviours measured during mating trials, including the frequency of sexual cannibalism. The positive effects of prey protein content on mate attraction and egg production suggest that praying mantids might be expected to choose more protein‐biased prey in nature or, if prey choice is limited, to have higher reproductive output or population growth in communities dominated by protein‐rich prey.     

宁波滑蜥两性异形和雌性繁殖

蜥蜴的雌性繁殖特征对理解两性异形的进化原因起着重要作用。于2011年4月在安徽滁州采集宁波滑蜥(Scincella modesta),定量研究该种形态特征的两性异形和雌性繁殖特征,检验成体形态特征两性异形与雌性繁殖的相关性。研究共采集43条(17♀♀,26♂♂)宁波滑蜥,雄性和雌性个体的最大体长分别为47.4 mm和46.6 mm。雌雄两性在体长和头宽上没有差异,而在腹长和头长上差异显著,雄性有较大的头长,雌性有较大的腹长。宁波滑蜥年产单窝卵。窝卵数和窝卵重与雌体体长及腹长呈正相关,卵重与雌体体长无相关性。窝卵数及卵重的变异系数分别为0.20和0.12。卵长径与窝卵数呈负相关,而卵短径与窝卵数无关。雌体主要通过增加窝卵数来增加繁殖输出。这些结果表明,宁波滑蜥是雌雄个体大小同形的两性异形模式,性选择使得雄性有着较大的头长,以具有较高的交配成功率,生育力选择使得雌性有着较大的腹长,以具有较大的生育力和繁殖输出。     

Egg Load Decreases Mobility and Increases Predation Risk in Female Black-Horned Tree Crickets (Oecanthus nigricornis)

Female-biased predation is an uncommon phenomenon in nature since males of many species take on riskier behaviours to gain more mates. Several species of sphecid wasps have been observed taking more female than male prey, and it is not fully understood why. The solitary sphecid Isodontia mexicana catches more adult female tree cricket (Oecanthus nigricornis) prey. Previous work has shown that, although female tree crickets are larger and thus likely to be more valuable as prey than males, body size alone cannot fully explain why wasps take more females. We tested the hypothesis that wasps catch adult female tree crickets more often because bearing eggs impedes a female’s ability to escape predation. We compared female survivors to prey of I. mexicana, and found that females carrying more eggs were significantly more likely to be caught by wasps, regardless of their body size and jumping leg mass. We also conducted laboratory experiments where females’ jumping responses to a simulated attack were measured and compared to her egg load and morphology. We found a significant negative relationship between egg load and jumping ability, and a positive relationship between body size and jumping ability. These findings support the hypothesis that ovarian eggs are a physical handicap that contributes to female-biased predation in this system. Predation on the most fecund females may have ecological-evolutionary consequences such as collapse of prey populations or selection for alternate life history strategies and behaviours.     

Fatty acid signatures of female Brünnich's guillemots Uria lomvia suggests reliance on local prey for replacement egg production

The laying of smaller replacement eggs has been described as a time-saving adaptation because chicks generally grow faster once hatched than inside the egg. However, chicks hatched from smaller eggs have, potentially, lower survival. Consequently, the smaller replacement egg represents a benefit to the female in terms of preserving its own condition at a cost to their offspring. We test these ideas by measuring adult mass changes and plasma lipid concentration changes in male and female Brünnich's guillemots Uria lomvia breeding on Coats Island, Nunavut. Though males lost more mass than females, these differences were not significant. Between laying the first and replacement egg, plasma fatty acid concentrations declined in females and increased in males, suggesting that females mobilise less lipid to preserve their condition after laying the replacement egg. In females, plasma lipid concentrations of the dominant fatty acids found in the eggs (16:0 and 18:1) declined between the laying of first and replacement eggs while plasma concentrations of 20:1 increased in both males and females. We compared the fatty acid signatures of first and replacement egg to look for evidence of differences between the lipid sources for their production. Principal component and discriminant function analyses showed that the fatty acid signatures of replacement eggs were closer to the signatures of the local prey than those of first eggs. We suggest that females rely on local sources of energy to a greater degree for the production of the replacement egg than the first egg, but that endogenous reserves of certain nutrients are important for the production of both eggs.     

Genetic variation in male effects on female reproduction and the genetic covariance between the sexes

Abstract.— Males of many insect species increase the fecundity and/or egg size of their mates through the amount or composition of their nuptial gifts or ejaculate. The genetic bases of such male effects on fecundity or egg size are generally unknown, and thus their ability to evolve remains speculative. Likewise, the genetic relationship between male and female investment into reproduction in dioecious species, which is expected to be positive if effects on fecundity are controlled by at least some of the same genes in males and females, is also unknown. Males of the seed beetle Stator limbatus contribute large ejaculates to females during mating, and the amount of donated ejaculate is positively correlated with male body mass. Females mated to large males lay more eggs in their lifetime than females mated to small males. We describe an experiment in which we quantify genetic variation in the number of eggs sired by males (mated to a single female) and found that a significant proportion of the phenotypic variance in the number of eggs sired by males was explained by their genotype. Additionally, the number of eggs sired by a male was highly positively genetically correlated with his body mass. The between-sex genetic correlation, that is, the genetic correlation between the number of eggs sired by males and the number of eggs laid by females, was highly positive when eggs were laid on Acacia greggii seeds. This indicates that males that sire many eggs have sisters that lay many eggs. Thus, some of the genes that control male ejaculate size (or some other fecundity-enhancing factor) when expressed in males appear to control fecundity when expressed in females. We found no significant interaction between male and female genotype on fecundity.     

Increased fecundity, as a function of multiple mating, in an arctiid moth, Utetheisa ornatrix

1. Female Utetheisa ornatrix mate multiply and may receive up to thirteen spermatophores. Spermatophores provide the female not only with sperm but also with a nuptial gift of pyrrolizidine alkaloid that she transmits to the eggs, protecting them against predation. Thus, through multiple mating the female accrues nuptial gifts that add to the defence of her offspring.
2. Multiple mating in female U. ornatrix brings about increases in fecundity, but not in longevity or egg mass. These increases occur through the third mating; the fourth and fifth matings, however, have no additional effect on fecundity.
3. These results, taken with the fact that spermatophores are sizeable and are digested within the female, suggest that they bear nutritive gifts that the female uses in egg construction. Multiple mating thus allows females to accrue the resources to build additional eggs.
4. These results are discussed in relation to the potential benefits accruing to donating such gifts and to male and female mating strategies.     

Stabilizing sexual selection for female ornaments in a dance fly

Ornamental traits function by improving attractiveness and are generally presumed to experience directional selection for mating success. However, given the greater investment of females in offspring than males, female-specific ornaments can in theory signal fecundity yet be constrained by fecundity costs. Theoretical work predicts that such constraints can lead to stabilizing selection via male choice for intermediately ornamented females. Female dance flies Rhamphomyia longicauda (Diptera: Empididae) display two female-specific ornaments in mating swarms - inflatable abdominal sacs and pinnate tibial scales. We investigated the intensity and form of sexual selection on female traits including ornaments and found no evidence for directional sexual selection. Instead, we found marginally nonsignificant quadratic selection for all three measures of ornament expression. Canonical analysis confirmed that the strongest vectors of nonlinear selection were associated with ornamental traits, although the significance of the quadratic coefficients associated with these vectors depended on the statistical approach. Direct Mitchell-Olds and Shaw tests for the location of the maximum fitted fitness value for both raw morphological traits and canonical axes revealed only one marginally nonsignificant result for the multivariate axis loading most heavily on pinnate leg scales. Together, these results provide the first tentative support for stabilizing selection on female-specific ornaments.     

Why Do Males of the Spider Pisaura mirabilis Wrap Their Nuptial Gifts in Silk: Female Preference or Male Control?

Males of the spider Pisaura mirabilis present a nuptial prey gift to the female during courtship as a mating effort. The gift is usually round and wrapped in white silk. It was suggested that the wrapped gift functions as a sensory trap by mimicking the female's eggsac implying that males exploit the female maternal care instinct and not her foraging motivation in a sexual context. The shape of the gift (round) and appearance (white) should then increase female acceptance of males. We tested these predictions experimentally and found that neither gift shape (round or oblong) nor silk wrapping (wrapped or unwrapped) facilitated female acceptance, in contrast unwrapped gifts were accepted faster than wrapped ones. Instead, we found that silk wrapping benefited the males because it significantly decreased the risk of females stealing the gift without copulation and consequently directly increased male mating success. Large oblong gifts were difficult for males to handle during copulation, resulting in shorter copulations for oblong vs. round gifts. Thus, round gifts were not preferred by the females but were beneficial to the males. Our results indicate two adaptive benefits to males of wrapping the nuptial gift: to reduce the risk of losing the gift to females without copulation, and make it possible to reshape an oblong prey into a round gift that facilitates the male's access to the female's genitalia. Our results suggest that the male gift wrapping trait may be selected though sexual conflict over remating rate.     

Abdominal stretch in the regulation of protein ingestion by the black blowfly, Phormia regina

ABSTRACT. Negative feedback to protein feeding by Phormia is transmitted to the brain via the abdominal nerve. The inhibition is apparently generated by stretch receptors in the abdominal wall. This feedback makes a protein meal self-limiting, supplying progressively more inhibition as the fly's swelling crop enlarges the abdomen during a meal. Abdominal stretch resulting from a mature egg mass also inhibits protein feeding by gravid females. This stretch seems to terminate protein feeding 'prematurely', leaving the female 'primed' to consume protein as soon as abdominal stretch is relaxed. The adaptive value of such a mechanism can be envisioned when one notes that the abdominal stretch normally will be relaxed only at a source of proteinaceous material, where the female lays her eggs. This would permit the fly to take advantage of the protein source before she flies away. The amount of protein consumed at such a time would normally be greater than the total amount stored in the fat body, thereby providing an important contribution toward the development of the next batch of eggs.