Tactic-dependent plasticity in ejaculate traits in the swordtail Xiphophorus nigrensis

In species with alternative reproductive tactics, males that sneak copulations often have larger, higher quality ejaculates relative to males that defend females or nest sites. Ejaculate traits can, however, exhibit substantial phenotypic plasticity depending on a male's mating role in sperm competition, which may depend on the tactic of his competitor. We tested whether exposure to males of different tactics affected sperm number and quality in the swordtail Xipophorus nigrensis, a species with small males that sneak copulations and large males that court females. Sperm swimming speed was higher when the perceived competitor was small than when the competitor was large. Plasticity, however, was only exhibited by small males. Sperm number and viability were invariant between social environments. Our results suggest sperm quality is role-dependent and that plastic responses to the social environment can differ between male reproductive tactics.     

Investigating the Consistency of Mate-Locating Behavior in the Territorial Butterfly Hypolimnas bolina (Lepidoptera: Nymphalidae)

The study of butterfly behavior has afforded valuable insights into the evolution of alternative mating tactics. Two hypotheses derived from this area of research contend that (1) territoriality is only viable under low to moderate conspecific densities (due to the costs of site defence) and (2) perching may be employed only when thermal conditions constrain flight activity. These hypotheses were evaluated by investigating mate locating behavior in Hypolimnas bolina, a territorial species that is naturally subject to variation in population density and weather conditions. Male behavior was charted throughout the day during a period of high population density at an encounter site in tropical Australia. Perching was the primary tactic, although a small proportion of individuals patrolled nonaggressively in the afternoon. Population-level male behavior failed to support predictions drawn from either the territory economics or thermal constraint hypotheses. First, the proportion of perching males and the number of aggressive conspecific interactions (per male) increased with increasing male density at the site. Second, few males patrolled at the hottest, brightest time of day (approximately midday), and the diel distribution of perchers did not emulate the U-shaped distribution shown by the occurrence of dorsal basking behavior. These results show that perching in this species is not a suboptimal tactic employed when temperatures constrain flight activity but may represent the best method of locating receptive females. At this stage the reproductive significance of the observed patrolling behavior remains obscure.     

Predators, reproductive parasites, and the persistence of poor males on leks

Lekking males are thought to face strong directional selectionon secondary sexual traits. How variation in male traits canpersist under these conditions remains problematic (the lekparadox). Here, we present several game-theoretic models thatshow that avoidance of costly and mobile predators, sneakers,or brood parasites (enemies) leads to variation in female choice.This can result in maintenance of variation in male quality."Enemies" will congregate around higher quality males. Femalesmust then trade-off the benefits of mating with high-qualitymales against the increased risk of enemies. At equilibrium,the models predict a positive correlation between the qualityof a male and the proportions of both enemies and females visitinghim. In the first model, we use this framework to predict thelowest quality male on the lek that will receive any matings.In the second model, we examine the influence of this female-enemygame on the maintenance of variation in male quality. Low-qualitymales are likely to persist when enemies are costly to femalesor occur at high density, and when there is some spatial structureon the lek, so that neighboring males are typically of similarquality. If enemies are more costly to males than to females,high-quality males may benefit from receiving fewer female visits.In the third model, we consider the special case when enemiesare male reproductive parasites. These models illustrate theimportance of considering the simultaneous decisions of multipleplayers in mate choice games.     

Do alternative reproductive strategies in the Wellington tree weta represent different behavioural types?

In many animal species, variation in reproductive success among individuals has led to the evolution of alternative mating strategies, which in the case of insects can often be correlated with developmental trajectories. In the Wellington tree weta, Hemideina crassidens, males can mature at the 8th, 9th or 10th instar, while females mature at the 10th instar only. A number of morphological attributes including male head and mandible size correlate with final instar number, and as these attributes represent a form of weaponry, they are often used in mate/site guarding and male–male competition. Tenth instar males have larger head/mandible/body sizes and show a conventional (guarder) reproductive strategy, whereas smaller 8th instar males typically show an unconventional (sneaker) strategy. In contrast, 9th instar males are predicted to adopt a “jack‐of‐all‐trades” strategy whereby they can fight or sneak depending context. Here, we tested whether alternative reproductive morphs exhibit strategy‐specific differences in risk‐taking associated with refuge emergence, activity and antipredator behaviour and further, whether these traits correlate to form a behavioural syndrome. We found that tree weta show consistent and repeatable differences in activity and refuge use at the individual level; however, behavioural covariances suggest that only 8th instar males exhibit a behavioural syndrome. That 9th instar males show high plasticity and variance in their gallery‐related behaviours supports the hypothesis that these males are a “jack‐of‐all‐trades.” Contrary to our predictions, antipredator behaviour was not correlated with other traits, and differences in behaviour overall were consistently more pronounced between individuals rather than between male morphs or sexes.     

Spatial variation in the temporal change of male and female melanic ornamentation in the barn owl

Because the magnitude of selection can vary between sexes and in space and time, sexually antagonistic selection is difficult to demonstrate. In a Swiss population of barn owls (Tyto alba), a heritable eumelanic colour trait (size of black spots on ventral feathers) was positively selected with respect to yearling survival only in females. It remains unclear whether the absence of negative selection in males is typical in this species. To tackle this issue indirectly, we measured the size of black spots in 1733 skin specimens collected by museums from 1816 to 2001 in seven European countries and in the Middle-East. The temporal change in spot size was sex- and country-specific. In males, spots became smaller particularly in three countries (Middle-East, Italy and Switzerland). In females, the size of spots increased significantly in two countries (UK and Spain) and decreased in two others (Germany and Switzerland). Because migration and phenotypic plasticity cannot explain these results, selection is the most likely cause. The weaker temporal change in spot size in females than males may be because of the combined effect of strong genetic correlation between the sexes and stronger negative selection in males than positive selection in females. We thus suggest that in the barn owl, spot size (or genetically correlated traits) is sexually antagonistically selected and that its pattern of selection may account for the maintenance of its variation and sexual dimorphism.     

Male morph determination in Rhizoglyphus echinopus (Acaridae)

In Rhizoglyphus echinopus (Fumouze and Robin) two male morphs occur: heteromorphs, with a thickened and sharply terminated third pair of legs that serve as a weapon in intrasexual conflicts, and homeomorphs, with unmodified legs. This study investigated the system of male morph determination. No significant heritability of male morph was found, but cues emanating from a dense colony were found to suppress the production of heteromorphs. Developmental plasticity was retained throughout the protonymphal stage. Diet did not influence morph expression, but lowered temperature decreased the proportion of heteromorphs emerging.     

Investigating the genetic architecture of conditional strategies using the environmental threshold model

The threshold expression of dichotomous phenotypes that are environmentally cued or induced comprise the vast majority of phenotypic dimorphisms in colour, morphology, behaviour and life history. Modelled as conditional strategies under the framework of evolutionary game theory, the quantitative genetic basis of these traits is a challenge to estimate. The challenge exists firstly because the phenotypic expression of the trait is dichotomous and secondly because the apparent environmental cue is separate from the biological signal pathway that induces the switch between phenotypes. It is the cryptic variation underlying the translation of cue to phenotype that we address here. With a ‘half-sib common environment’ and a ‘family-level split environment’ experiment, we examine the environmental and genetic influences that underlie male dimorphism in the earwig Forficula auricularia. From the conceptual framework of the latent environmental threshold (LET) model, we use pedigree information to dissect the genetic architecture of the threshold expression of forceps length. We investigate for the first time the strength of the correlation between observable and cryptic ‘proximate’ cues. Furthermore, in support of the environmental threshold model, we found no evidence for a genetic correlation between cue and the threshold between phenotypes. Our results show strong correlations between observable and proximate cues and less genetic variation for thresholds than previous studies have suggested. We discuss the importance of generating better estimates of the genetic variation for thresholds when investigating the genetic architecture and heritability of threshold traits. By investigating genetic architecture by means of the LET model, our study supports several key evolutionary ideas related to conditional strategies and improves our understanding of environmentally cued decisions.