Swarm Site Fidelity in the Sex Role-Reversed Dance Fly Empis borealis

In the dance fly species Empis borealis (L.), females (1–40) gather to swarm at landmarks (swarm markers, like trees and bushes), and males carrying an insect prey visit these swarms for mating. We noticed earlier that some swarm sites were used for several years and that they appeared to be frequented by a similar number of swarming females in each year, although the numbers of females varied greatly among swarm sites and certain sites attracted more swarming individuals than others. To explore swarm site fidelity in this mating system, in 1993 we monitored the same swarm sites that we studied in 1989, addressing the questions, Would the same swarm sites still attract the same number of females and males after 4 years? and Why do some swarm sites attract more displaying females than others? The number of females swarming at the different markers in 1993 was approximately the same as 4 years earlier. Some of these swarm sites are known to have been used for 18 years. The swarm sites with the largest number of flies had a high sun exposure during the day and were found at coniferous swarm marker trees and in a mixed forest habitat. A swarm site with few females attending and with a low amount of insolation during the day can be predicted to be abandoned as a swarming site soon. Empis borealis swarm sites thus persist over many years and are attended by a similar number of individuals each year. To our knowledge, such site fidelity has not been demonstrated for any swarming insect species earlier.     

Acoustic orientation of male Aedes diantaeus during mating

The reaction of males of Aedes diantaeus to different auditory stimuli was studied in natural environment during swarming and under laboratory conditions. The males were attracted by 250-420 Hz that corresponds to the main frequencies of flight tones of sympatric females and the range of sensitivity of male Johnston's organs. Behavioural data show that in natural environment swarming males are attracted by flying conspecific and heterospecific females rather than by dead or immobilized females. Contact with heterospecific females always ended in parting of mosquitoes while contact with cospecific females resulted in pairing. Thus, swarming males of Aedes diantaeus localize females by their flight tones but recognize the females of their own species in contact.     

Genetic continuity of brood-parasitic indigobird species

Speciation in brood-parasitic indigobirds (genus Vidua) is a consequence of behavioural imprinting in both males and females. Mimicry of host song by males and host fidelity in female egg laying result in reproductive isolation of indigobirds associated with a given host species. Colonization of new hosts and subsequent speciation require that females occasionally lay eggs in the nests of novel hosts but the same behaviour may lead to hybridization when females parasitize hosts already associated with other indigobird species. Thus, retained ancestral polymorphism and ongoing hybridization are two alternative explanations for the limited genetic differentiation among indigobird species. We tested for genetic continuity of indigobird species using mitochondrial sequences and nuclear microsatellite data. Within West Africa and southern Africa, allopatric populations of the same species are generally more similar to each other than to sympatric populations of different species. Likewise, a larger proportion of genetic variation is explained by differences between species than by differences between locations in alternative hierarchical AMOVAS, suggesting that the rate of hybridization is not high enough to homogenize sympatric populations of different species or prevent genetic differentiation between species. Broad sharing of genetic polymorphisms among species, however, suggests that some indigobird species trace to multiple host colonization events in space and time, each contributing to the formation of a single interbreeding population bound together by songs acquired from the host species.     

Nestling sex ratios in a population of Bluethroats Luscinia svecica inferred from AFLP™ analysis

We studied the sex ratio of Bluethroat Luscinia svecica broods using AFLPs. Our aim was to test whether there is a bias towards males that could be explained by sexual selection theories, or conversely, a bias towards females that could help explain the female-biased sex ratio among juveniles observed at a wintering site. The AFLP technique was reliable in sexing the nestlings from even small initial DNA quantities. Given the large number of polymorphic markers that can be obtained for each primer combination, the probability of detecting a W-chromosome-linked fragment is reasonably high. As a consequence, this method could be used in other species for sex-ratio studies and for other genetic purposes. Among 246 nestlings, we found an overall proportion of males of 50.8% at hatching and the sex-ratio variation using broods as independent units was not significantly different from expectation under a binomial distribution. None of the parental and environmental variables tested changed significantly the deviance to the model. Thus, sex determination in the Bluethroat seems to match the classical Mendelian model of a 1:1 sex ratio and cannot explain the biased sex ratio towards juvenile females found at the wintering site.     

Mating Patterns of Red-Eyed Treefrogs, Agalychnis callidryas and A. moreletii

Deviations from random mating in frogs are often explained by two different size‐based patterns. The large‐male mating advantage predicts that males found in amplexus with females will be larger on average than non‐amplectant males, whereas size‐assortative mating predicts that males and females found in amplexus will maintain an optimal size ratio. Both these pairing patterns are consistent with a female mating preference for larger males, or for males of a given size relative to the choosy female. I examined pairing patterns of two species of Neotropical hylids, Agalychnis callidryas and A. moreletii for three consecutive breeding seasons in Belize, Central America to evaluate whether mating behavior was influenced by either a large‐male mating advantage or size‐assortative mating. For each species, I compared size traits between amplectant and non‐amplectant males, and within amplectant pairs, and I quantified fertilization success for each amplectant pair. For both species I found evidence of deviations from random mating by size, but the nature of the deviations varied between species and among years. The proportion of eggs fertilized was consistently high among years for both species and there was no relationship between fertilization success and the size ratio of amplectant pairs. These data are consistent with female mate preference, but a role for male–male competition cannot be excluded. My findings suggest that mating patterns may be density‐dependent and that the nature and intensity of sexual selection may be increased by extreme environmental conditions.     

Age-associated male mating success in three swarming caddis fly species (Trichoptera: Leptoceridae)

Abstract. 1. In the three caddis fly species, Athripsodes albifrons (L.), A. cinereus (Curtis) and Mystacides azurea (L.) (Leptoceridae; Trichoptera), males swarm above the water surface of lakes or rivers. Receptive females fly to swarms and are chased and/or courted by males. After one of the swarming males has grasped an approaching female, the pair flies in tandem to the shore where they copulate.
2. In males, wing wear indices were negatively correlated with the ratio of fat/dry weight. In the only species in which comparisons were possible between newly emerged and swarming males (M. azurea), the former had significantly lower indices. Unmated females on average had lower wing wear indices than spent females. These facts suggest that wing wear reflects relative age.
3. The tandem males had significantly less wing wear than those in swarms, and are probably therefore younger. Age is therefore likely to be significant in relation to mating success.
4. Among males of the same relative age, tandem males had higher fat ratio than swarming ones, indicating that male mating success was also influenced by traits other than age. It is suggested that the shortest possible duration of the period of adult prematurity is adaptive, especially in insects with marginal adult food intake.     

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.     

Genetic structure in a swarming brown long-eared bat (<Emphasis Type="Italic">Plecotus auritus</Emphasis>) population: evidence for mating at swarming sites

Plecotus auritus, a small, gleaning bat species, lives in small, isolated summer colonies in which both males and females show a high degree of natal philopatry. Despite this, colonies have high gene diversities and low inbreeding coefficients. It has been suggested that inbreeding is avoided because mating occurs during autumnal and spring swarming at hibernation sites. We tested this hypothesis by comparing microsatellite profiles, based on eight loci, of bats from six summer colonies and two swarming sites they were known to visit from radiotelemetry studies. We found high gene diversities (H _s = 0.77) at both swarming sites and summer colonies which were not statistically different. There was no detectable isolation by distance and F_ST was low (0.001). Together, these results suggest high gene flow between sites. Despite this, there was small but significant genetic differentiation amongst summer colonies and between summer colonies and the primary swarming site. We suggest that swarming is important for gene flow and for maintaining genetic diversity in this highly philopatric species and discuss possible reasons for the genetic differentiation observed. The identification and protection of swarming sites should be a major conservation priority for this and other temperate bat species.     

Sexual dimorphism in relation to swarming and pair formation patterns in leptocerid caddisflies (Trichoptera: Leptoceridae)

North European Leptoceridae (Trichoptera) perform three types of swarming flight patterns: (1) swarming males of Athripsodesand Ceracleafly in horizontal zigzag patterns over the water surface, (2) the Mystacidesspp. perform vertical zigzag movements, and (3) the flight of males of Triaenodes unanimisMcLach. is a mixture of the horizontal and vertical zigzagging. Also three groups of pair formation behavior can be distinguished. In the first group, of Athripsodesand Ceraclea,the females fly into the male swarms, where they are grasped and carried to the riparian vegetation by the flying males with the females hanging upside-down in genitalia coupling. In the second group, a Mystacidesfemale is caught by a male, when approaching a swarm and both use their wings to fly in tandem to the shore where they copulate. In the third group, of Triaenodes bicolor(Curt.) and Oecetis lacustris(Curt.), the males fly searching for females sitting on aquatic plants and when a female is found the male lands and they copulate immediately while clinging to the plant. The different swarming and mating behaviors might have favored selection for three types of sexual dimorphism: (1) longer forewings in males than females in species which fly in copula, (2) larger eyes in males of the vertically zigzagging species, and (3) much smaller males in the group where males search for females sitting on aquatic plants. In the second group approaching females are detected by males before reaching the swarm and in the third group the female almost always mates with the male which is the first to find her. In conclusion, we suggest that females of Athripsodesand Ceracleahave a greater choice among swarming males than do females of Mystacides, T. bicolor,and O. Lacustris.     

Structure and function of the Johnston’s organ in <Emphasis Type="Italic">Fleuria lacustris</Emphasis> Kieff. males (Diptera,Chironomidae)

The majority of chironomid midges mate in swarms, where males find females by their flight sounds using specialized Johnston’s organs for acoustic perception. Males of Fleuria lacustris do not swarm and copulate on the ground. Both the flagellum and antennal fibrils are shortened and the pedicel is reduced in comparison with that typical of swarming midge species. Our results demonstrate that, in swarming midge species, the number of antennal fibrils and of A-type chordotonal sensilla in the male Johnston’s organ is by 1.33 and by 21 times higher, respectively, than in F. lacustris. Though Johnston’s organ of the latter species responds to female flight tones, it is significantly (about 70 times) less sensitive to these stimuli in comparison with the Johnston’s organ of swarming species. The decrease in the sensitivity of the Johnston’s organ in F. lacustris can be explained by the decrease in the size and the number of antennal structures, whereas their ability to perceive acoustic signals is determined by the presence of a relatively high number of A-type sensilla. Our results demonstrate that F. lacustris males are not able to search for conspecific females by perception of acoustic signals produced during the flight; however, it cannot be ruled out that females produce sounds during copulation on the substrate and thus affect male behavior.     

Variation in ectoparasite load reflects life history traits in the lesser mouse-eared bat Myotis blythii (Chiroptera: Vespertilionidae) in western Iran

We studied seasonal variation of ectoparasite load (number of parasites per individual bat) in free-ranging populations of the lesser mouse-eared bat Myotis blythii in western Iran. Data for 1 species each of batfly (Nycteribidae), tick (Ixodidae), and mite (Spinturnicidae) are reported for a 1 yr period. Patterns of parasite load during this time differed considerably among species. However, the parasite load increased markedly in pregnant females in spring and early summer. During the same time frame, parasite load decreased in solitary males when they roosted apart from maternity clusters. However, in late summer, when bats began swarming, males showed a significant (P < 0.05) increase in parasite load. Using the ratio of body mass to length of forearm as an index of body condition, no significant correlation was found.     

Parentage Analysis with Genetic Markers in Natural Populations. I. the Expected Proportion of Offspring with Unambiguous Paternity

Recent studies indicate that polymorphic genetic markers are potentially helpful in resolving genealogical relationships among individuals in a natural population. Genetic data provide opportunities for paternity exclusion when genotypic incompatibilities are observed among individuals, and the present investigation examines the resolving power of genetic markers in unambiguous positive determination of paternity. Under the assumption that the mother for each offspring in a population is unambiguously known, an analytical expression for the fraction of males excluded from paternity is derived for the case where males and females may be derived from two different gene pools. This theoretical formulation can also be used to predict the fraction of births for each of which all but one male can be excluded from paternity. We show that even when the average probability of exclusion approaches unity, a substantial fraction of births yield equivocal mother-father-offspring determinations. The number of loci needed to increase the frequency of unambiguous determinations to a high level is beyond the scope of current electrophoretic studies in most species. Applications of this theory to electrophoretic data on Chamaelirium luteum (L.) shows that in 2255 offspring derived from 273 males and 70 females, only 57 triplets could be unequivocally determined with eight polymorphic protein loci, even though the average combined exclusionary power of these loci was 73%. The distribution of potentially compatible male parents, based on multilocus genotypes, was reasonably well predicted from the allele frequency data available for these loci. We demonstrate that genetic paternity analysis in natural populations cannot be reliably based on exclusionary principles alone. In order to measure the reproductive contributions of individuals in natural populations, more elaborate likelihood principles must be deployed.     

Widespread 'hilltopping' in Acraea butterflies and the origin of sex-role-reversed swarming in Acraea encedon and A. encedana

In some populations of the butterflies Acraea encedon and A. encedana, most females are infected with a bacterium that kills their sons. The resulting shortage of males is associated with females adopting a sex‐role‐reversed mating system, in which females swarm at landmarks such as hilltops and compete for males. We have observed the mating behaviour of Acraea species that are not known to be infected with the male‐killer. In over half of these species, males were found to aggregate on hilltops. It is likely that this behaviour was ancestral to the sex‐role‐reversed swarms of Acraea encedon and A. encedana, and we discuss how the spread of the male‐killing infection may have converted this mating system into sex‐role‐reversed swarming.     

Mating system ofTokunagayusurika akamusi (Diptera: Chironomidae): I. Copulation in the air by swarming and on the ground by searching

Observations on the mating system of the midge,Tokunagayusurika akamusi, revealed mating to occur both in the air by swarming and on the ground by searching. At the shores of Lake Biwa, midges appeared from November to early December. Newly emerged adults arrived at the resting place, lakeside vegetation, in the morning, during which time a number of males also walked about in search of mates. Many copulating pairs were observed at the resting place. Huge swarms occurred chiefly before sunset but the frequency of copulation observed in the swarm was extremely low. It is likely that, in the Lake Biwa population, the proportion of females inseminated by searching males at the resting place was much larger than that by swarming males in the air. Furthermore, by searching, males copulated with younger females than by swarming. The differences between the searching and swarming tactics are discussed.     

Sex Differences in Discrimination of Shoal Size in the Guppy (Poecilia reticulata)

For a diversity of species, differences in sexual and parental roles, along with differences in body morphology, often result in males and females having different diets, distinct predators and even different patterns of habitat use. As a consequence, the two sexes often face different environmental challenges and selection may favour the evolution of sex differences in cognition. We tested this prediction in the guppy (Poecilia reticulata). Under perceived hazard, individual guppies join the larger available social group, a behaviour that is thought to minimise predation risk. In this species, females are more frequently exposed to predation and more averse to predation risk; we therefore expected greater accuracy in shoal size discrimination in females. We compared the accuracy of male and female guppies in discriminating shoals of 4 and 6 conspecifics, which represents the upper limit of discrimination for this species. Overall, we found no sex differences in the accuracy of discriminating the two shoals. However, while females showed this ability at the beginning of the test, males began to select the larger group only after several minutes. In three control experiments, we found indications that this sex difference cannot be accounted for by differences in motivation or antipredator strategies between the two sexes, suggesting female guppies are more efficient at rapidly estimating shoal size.     

Burrow ornamentation in the fiddler crab (Uca leptodactyla): female mate choice and male–male competition

Non-biological ornamentation is found in the nests and burrows of different kinds of animals. We evaluated here whether sand hoods constructed by male fiddler crabs (Uca leptodactyla) are one of the signals used by males to attract females during courtship. We observed females when they were walking among the males, and we quantified the proportion of females that visited male burrows with and without ornamentation and the choice to stay in a male’s burrow. Females visited more burrows with hoods than burrows without hoods, and they chose significantly more builder males. Male investment in ornamentation nevertheless decreased when the proportion of females increased in the area. Male investment was not correlated with the proportion of non-builder males nearby, but was positively correlated with overall density. The density sex ratio, however, was more male-biased in high-density than in low-density areas suggesting that even if building attracts females, the function could be related to male competition for mates.     

Behavioural and Genetic Data Suggest that Bechstein's Bats Predominantly Mate Outside the Breeding Habitat

Investigating mating systems of species with a cryptic lifestyle often requires a combination of behavioural and genetic data. We used such a combined approach to investigate the mating system of the communal breeding Bechstein's bat (Myotis bechsteinii). Although females of this species are philopatric, gene flow among colonies is high. Gene flow occurs if dispersing males mate within the colony to which they moved. Males could gain local matings by defending resources or females in their breeding habitat. Alternatively, mating may take place at swarming sites, apart from the breeding habitat of the females, where males and females of several colonies meet. Whether or not males defend resources or females in the breeding habitat is of importance for understanding the mating system. Detailed observations of individual foraging and roosting behaviour over 4 yr suggest that males do not defend resources or females to gain matings. Moreover, paternity assignment based on microsatellite data of four complete juvenile cohorts showed that local males fathered less than 25% of the juveniles born in the colony where they settled. Even more striking, none of the males that had immigrated into our study area reproduced with the local females.     

Sexual dimorphism in intra- and interspecific competitive ability of the dioecious herb Mercurialis annua

Males and females of dioecious plant species often show different responses to competition with individuals of the same or opposite gender, but almost no data are available on the outcome of competition with members of other species. Here, we show that male and female individuals of the wind-pollinated herb Mercurialis annua are sexually dimorphic in both their intraspecific and interspecific competitive abilities. In a controlled experiment, we found that both sexes of M. annua were negatively affected by interspecific competition, but the sensitivity of males and females depended on the identity of their competitor species, with females tending to suppress the aboveground growth of competitor species more than males. Further, we found that intrasexual and intersexual competition affected the aboveground growth of males but not that of females: only males showed a significant reduction in growth when growing with conspecific competitors (male or female). We discuss our results with reference to related studies that suggest that males and females of M. annua have different resource requirements for reproduction, which in turn affect their competitive abilities.     

Feeding and sexual dimorphism in adult midges (Diptera: Chironomidae)

Adult chironomids feed readily on materials containing sucrose and glucose, and the addition of dyes is an easy way of demonstrating that food passes through the gut. Male and female flies are shown to make very different use of the food they take in. Males show no change in longevity but extend their flight time for an average of 160% over unfed males. Females, by contrast, show no detectable increase in flight time, but increase longevity by about 40%. Sexual dimorphism in the use of food seems appropriate to the roles of the sexes. We infer that males improve their swarming performance while females may benefit from increased longevity both in gaining time to find suitable mates and in the distance dispersed after mating. Males, and to a less extent females, are found on aphid infested trees near fresh water, and the suggested biological value of feeding is in sustaining the swarming flight especially for the males. Trichoptera adults are found feeding on aphid infested trees and the earlier findings that adult Trichoptera feed are confirmed.     

Males Prefer Small Females in a Dichotomous Choice Test in the Poeciliid Fish Heterandria formosa

Male choice is expected to evolve when females differ in quality, even if male investment in each mating is low. The family Poeciliidae is an example of fishes in which males show little parental investment as they only provide sperm. Up until now, a preference for large females has been found in all species studied. Here we show that unexpectedly, males of the least killifish (Heterandria formosa) prefer to interact with small instead of large females in a dichotomous male choice test, even though large females are more fecund. During a free‐swimming choice experiment, males did not discriminate between females based on their size. We suggest that this unique preference for small females, or the lack of preference for large females, results from strong first male sperm precedence in this species. Smaller females are younger and therefore more likely to be virgin, which probably makes them more profitable mates for males. When presented with a virgin and a mated female of similar size, males showed no preference for either type. This suggests that males do not use pheromone cues to assess female mating status but that they are likely to use female size as a proxy for it.