Males do not prolong copulation in response to competitor males in the polyandrous fly Drosophila bifasciata

Males in many taxa exhibit behavioural plasticity in response to the perceived threat of sperm competition. Drosophila males prolong mating in response to the presence of competitor males before copulation. The benefits of this behaviour to males are evident in Drosophila melanogaster. However, the adaptive nature of the trait is challenged by the observation that it is present in four other Drosophila species, two of which are largely monandrous, raising the possibility that this plasticity is not evolutionarily labile. In the present study, behavioural plasticity and the mating system in Drosophila bifasciata Pominini (Diptera, Drosophilidae) are analyzed. By contrast to other Drosophila species, male D. bifasciata do not exhibit plasticity in copulation duration when competitor males are present before mating. Thus, plasticity in mating duration is not fixed in the genus Drosophila. The mating system of D. bifasciata is also examined. The species is polyandrous but, uncommonly for the genus Drosophila, males transfer a mating plug composed of sperm to females, which forms very shortly after copulation and fills the female uterus. The absence of plasticity observed in D. bifasciata may arise from the sperm plug.     

Delineating the roles of males and females in sperm competition

Disentangling the relative roles of males, females and their interactive effects on competitive fertilization success remains a challenge in sperm competition. In this study, we apply a novel experimental framework to an ideally suited externally fertilizing model system in order to delineate these roles. We focus on the chinook salmon, Oncorhynchus tshawytscha, a species in which ovarian fluid (OF) has been implicated as a potential arbiter of cryptic female choice for genetically compatible mates. We evaluated this predicted sexually selected function of OF using a series of factorial competitive fertilization trials. Our design involved a series of 10 factorial crosses, each involving two ‘focal’ rival males whose sperm competed against those from a single ‘standardized’ (non-focal) rival for a genetically uniform set of eggs in the presence of OF from two focal females. This design enabled us to attribute variation in competitive fertilization success among focal males, females (OF) and their interacting effects, while controlling for variation attributable to differences in the sperm competitive ability of rival males, and male-by-female genotypic interactions. Using this experimental framework, we found that variation in sperm competitiveness could be attributed exclusively to differences in the sperm competitive ability of focal males, a conclusion supported by subsequent analyses revealing that variation in sperm swimming velocity predicts paternity success. Together, these findings provide evidence that variation in paternity success can be attributed to intrinsic differences in the sperm competitive ability of rival males, and reveal that sperm swimming velocity is a key target of sexual selection.     

The timing of mating influences reproductive success in Drosophila melanogaster: implications for sexual conflict

Despite its potential importance, the role of the timing of mating(s) as a source of variation in female lifetime reproductive success has been largely overlooked. Here, using a laboratory‐adapted population of the model species Drosophila melanogaster, we explore how temporal variation in the patterns of single and multiple matings influences female fecundity. We find that the boost to fecundity known to occur after a virgin female’s initial mating also extends to subsequent matings as nonvirgins, but only for a short duration. This fecundity boost at least partially offsets the direct costs of multiple matings to females in this population of D. melanogaster. The implications of these results for our understanding of the evolution and maintenance of polyandry in this species are discussed in the context of sexual conflict.     

Role of male-male competition and female choice in the development of breeding coloration in pupfish (Cyprinodon pecosensis)

The function of male breeding coloration and the conditionsunder which it is expressed were investigated using two experimentalprotocols that differed in the degree of control of social factors.In tank experiments, focal males were separated by opaque orclear partitions or allowed to interact freely without partitions.In lake experiments, males were introduced into opaque or clearenclosures in the lake from which they were collected. Thisdesign tested the relative importance of breeding substrate,social interactions with other males, and presence of femalesand juveniles on the expression of breeding coloration and agonisticand mating behavior in pupfish. Breeding coloration was notexpressed in the absence of conspecifics, even in the presenceof suitable breeding substrate, or in the presence of juveniles.Breeding coloration was poorly developed when only males werepresent, was more intense in the presence of females, and darkenedsignificantly after a spawning bout Males in clear enclosuresin their natural lake habitat were in visual contact with conspecificsof both sexes and developed more intense blue breeding colorationthan males in the tank experiments that were in visual contactonly with other males. Uncon-fined males with territories inthe lake developed the most intense breeding coloration. Breedingcoloration of male pupfish is a facultative trait that is welldeveloped when breeding females are present and plays a majorrole in the attraction and courtship of mates. However, maximumdevelopment of breeding coloration by territorial males suggeststhat intrasexual and intersexual selection act concordantlyon this trait.[Behav Ecol 7: 431-437 (1996)]     

Genetic composition of social groups influences male aggressive behaviour and fitness in natural genotypes of Drosophila melanogaster

Indirect genetic effects (IGEs) describe how an individual''s behaviour—which is influenced by his or her genotype—can affect the behaviours of interacting individuals. IGE research has focused on dyads. However, insights from social networks research, and other studies of group behaviour, suggest that dyadic interactions are affected by the behaviour of other individuals in the group. To extend IGE inferences to groups of three or more, IGEs must be considered from a group perspective. Here, I introduce the ‘focal interaction’ approach to study IGEs in groups. I illustrate the utility of this approach by studying aggression among natural genotypes of Drosophila melanogaster. I chose two natural genotypes as ‘focal interactants’: the behavioural interaction between them was the ‘focal interaction’. One male from each focal interactant genotype was present in every group, and I varied the genotype of the third male—the ‘treatment male’. Genetic variation in the treatment male''s aggressive behaviour influenced the focal interaction, demonstrating that IGEs in groups are not a straightforward extension of IGEs measured in dyads. Further, the focal interaction influenced male mating success, illustrating the role of IGEs in behavioural evolution. These results represent the first manipulative evidence for IGEs at the group level.