A meta-analysis on alternative mating tactics: when the main and the alternative yield similar reproductive success

In many species, individuals of the same sex exhibit different mating behaviours, a phenomenon known as alternative mating tactics (AMTs). These AMTs may occur in species in which morphology prevents individuals from adopting different tactics (fixed expression of AMTs) as well as in species in which individuals can alternate between them (flexible expression of AMTs). Regardless of the way different mating tactics are expressed, a key point to understanding the selective forces driving AMT evolution relies on identifying differences in the reproductive success between tactics. Empirical studies often indicate that individuals adopting the main tactic (generally the more aggressive) have higher mating success than those adopting the secondary tactic. However, while this is expected for species showing flexible expression of AMTs, the same should not hold for species showing fixed expression of AMTs. In species with fixed expression of AMTs, individuals adopting each tactic have morphological specialisations in acquiring mates that may be responsible for eliminating differences in reproductive success between tactics. Despite such expectations, a comprehensive investigation across species regarding differences in the reproductive success of AMTs is lacking. Using a meta-analytical approach, we investigated if there is a difference in the reproductive success between tactics and whether this difference is related to how these tactics are expressed (fixed or flexible). We focused on males since information on reproductive success for females with AMTs is still scarce. We hypothesised that (i) males adopting the main tactic have higher reproductive success than males adopting the secondary tactic, and (ii) the difference in reproductive success between tactics will be higher in species with flexible rather than fixed expression of AMTs. When all species were pooled, we found that males adopting the main tactic had a similar reproductive success to males adopting the secondary mating tactic. However, this trend changed between species with fixed and flexible expression of AMTs. In species with flexible expression of AMTs, males exhibiting the main tactic have higher mating success than those adopting secondary tactics, while in species with fixed expression of AMTs, males adopting distinct tactics have similar reproductive success. Thus, the widespread assumption that males adopting the main tactic have higher reproductive success than males adopting the secondary tactic may be restricted to species in which individuals can alternate between tactics during their lifespan. However, we found that most data on the reproductive success of males adopting AMTs is restricted to short reproductive windows and may not reflect differences in lifetime reproductive success between tactics. Therefore, we highlight the importance of examining the selective pressures associated with fixed and flexible expression of AMTs on lifetime reproductive success to attain a deeper understanding of AMT evolution.

Condition-dependent survival and movement behavior in an endangered endemic damselfly

Movement is essential for the maintenance of populations in their natural habitats, particularly for threatened species living in fluctuating environments. Empirical evidence suggests that the probability and distance of movement in territorial species are context-dependent, often depending on population density and sex. Here, we investigate the movement behavior of the spring cohort of an endangered endemic damselfly Calopteryx exul in a lotic habitat of Northeast Algeria using capture-mark-recapture (CMR) of adults. By sampling 10 gridded river stretches across a 2 km section of the watercourse, we were able to estimate the distance of movement throughout individual lifespans and estimate movement probability for both males and females. We used multistate models to examine whether individual density and sex ratio influence survival and movement probability. We found that males and females had similar movement kernels with most individuals moving short distances (83% performing movements of < 100 m and only 1% > 1000 m). Of the 547 marked individuals, 63% were residents, and 37% were movers (moved at least 50 m from one sampling occasion to another). Survival probability showed higher estimates for females and was slightly density-dependent (i.e., lower survival probabilities were associated with high male densities). Survival probability did not show a marked difference between residents and movers. Movement probability and distances were positively correlated with individual density, but were not or slightly correlated with sex ratio, respectively. These results are not in line with the hypotheses of sex-biased movement and survival costs of movement. Our results suggest that the species performs mostly short-distance movements that are dependent on intraspecific interactions.

Preference for supernormal stimuli tends to override initially learned associations for conspicuous prey traits: implications from a laboratory study

How predators select on conspicuous prey traits is not well understood. We used a laboratory setup to investigate the role of learning in predator choice of conspicuous visual traits. We used a generalist predator, the great tit, and coloured wings of males of two species of damselflies as prey. Wing pigmentation differs between the species in colour (green vs. blue) and size (large vs. small). Wing pigmentation is a sexually selected trait that experiences negative selection by avian predators. Inexperienced great tits showed no preference for the colour or the size of wing pigmentation. Great tits were then repeatedly exposed to rewarded wings with either large or small wing patch size. When these experienced birds were exposed to both wing patch sizes for the first time, they tended to prefer the wings with the large patch, irrespective of their previous experience. Our results suggest that the choice of the predator was based on an initial association of the trait to a reward followed by a preference for a supernormal stimulus, probably due to a larger sensory stimulation. We discuss the implications of our laboratory results in the light of previous estimates of damselfly predation risk under field conditions.

Body mass and territorial defence strategy affect the territory size of odonate species

The territory is a distinct mating place that a male defends against intruding conspecific males. The size of a territory varies between species and most of the variation between species has been found to scale allometrically with body mass. The variation that could not be explained by body mass has been explained with several variables such as habitat productivity, trophic level, locomotion strategy and thermoregulation. All previous interspecific comparative studies have been done on vertebrate species such as birds, mammals, reptiles and fishes, meaning that studies using invertebrate species are missing. Here, we studied the relationship of a species's territory size with its fresh body mass (FBM) in addition to other ecologically relevant traits using 86 damselfly and dragonfly (Odonata) species. We found that territory size is strongly affected by species FBM, following an allometric relationship similar to vertebrates. We also found that the territory size of a species was affected by its territorial defence strategy, constantly flying species having larger territories than species that mostly perch. Breeding habitat or the presence of sexual characters did not affect territory sizes, but lotic species and species without wing spots had steeper allometric slopes. It seems that an increase in a species's body mass increases its territory size and may force the species to shift its territory defence strategy from a percher to a flier.

Brain size affects responsiveness in mating behaviour to variation in predation pressure and sex ratio

Despite ongoing advances in sexual selection theory, the evolution of mating decisions remains enigmatic. Cognitive processes often require simultaneous processing of multiple sources of information from environmental and social cues. However, little experimental data exist on how cognitive ability affects such fitness-associated aspects of behaviour. Using advanced tracking techniques, we studied mating behaviours of guppies artificially selected for divergence in relative brain size, with known differences in cognitive ability, when predation threat and sex ratio was varied. In females, we found a general increase in copulation behaviour in when the sex ratio was female biased, but only large-brained females responded with greater willingness to copulate under a low predation threat. In males, we found that small-brained individuals courted more intensively and displayed more aggressive behaviours than large-brained individuals. However, there were no differences in female response to males with different brain size. These results provide further evidence of a role for female brain size in optimal decision-making in a mating context. In addition, our results indicate that brain size may affect mating display skill in male guppies. We suggest that it is important to consider the association between brain size, cognitive ability and sexual behaviour when studying how morphological and behavioural traits evolve in wild populations.

Testes mass in the livebearing fish Brachyrhaphis rhabdophora (Poeciliidae) varies hypoallometrically with body size but not between predation environments

In this study, we considered potential causes of variation in testis size in the livebearing fish Brachyrhaphis rhabdophora. We evaluated variation in testes mass among individual males and among populations that occupy different selective environments. First, we predicted that small males should allocate more to testes mass than large males (i.e., hypoallometric pattern) based on a sperm competition argument. Second, based on life history theory and associated differences in mortality rates between populations that coexist with many fish predators and those with few predators, we predicted that males in high-predation environments should allocate more to testes mass than males in habitats with few predators. Our results showed that small males allocated proportionally more to testes mass than larger males (slope of testes mass to body mass was hypoallometric). However, there was no effect of predator environment on testes mass independent of body size differences. In this system, size-specific patterns of reproductive allocation in males (hypoallometry) differ from that seen in females (hyperallometry). Allocation to testes mass may respond to differences in mortality rate through selection on body size.

Predator-mediated natural selection on the wings of the damselfly Calopteryx splendens: differences in selection among trait types

Traits that increase mating success in males may come at a cost, such as an increased risk of predation. However, predator-mediated selection is challenging to document in natural populations, hampering our understanding of the trade-offs between sexual selection and predation. Here we report on a study of predator-mediated natural selection on wing traits in the damselfly Calopteryx splendens, the males of which possess conspicuous wing patches. Wagtails (genus Motacilla) are important avian predators of C. splendens, capturing them in flight and removing the wings prior to consumption. Using geometric morphometric techniques, we quantified the strength and mode of selection on wing traits by comparing wings from depredated individuals with the standing variation present in the population. Our results reveal that predator-mediated selection is stronger on secondary sexual characters than on size and shape, suggesting that traits related to flight performance are closer to their adaptive peaks. This could be a consequence of the long-term evolutionary association with avian predators, whereas stronger selection on conspicuous secondary sexual traits may reflect trade-offs between sexual and natural selection. Finally, even though C. splendens possesses nearly identical fore- and hindwings, we found evidence for divergent selection between them.

A mismatch between the perceived fighting signal and fighting ability reveals survival and physiological costs for bearers

Signals of fighting indicate an animal's intention to attack and so they serve to prevent costly aggressive encounters. However, according to theory, a signal that is different in design (i.e. a novel signal) but that fails to inform fighting intentions will result in negative fitness consequences for the bearer. In the present study we used males of the territorial damselfly Hetaerina americana, which have a red wing spot during territory defense that has evolved as a signal of fighting ability. By producing a novel signal (covering the red spot with blue ink) in territory owners, we investigated: a) the behavioral responses by conspecific males; b) survival cost and c) three physiological mediators of impaired survival: muscular fat reserves, muscle mass and immune ability. We predicted that males with the novel signal would be attacked more often by conspecifics as the former would fail to convey fighting ability and intentions adequately. This will result in lower survival and physiological condition for the novel signal bearers. We found that, compared to control males (males whose red spot was not changed), experimental males had reduced survival, were less able to hold a territory, and had a reduced muscle mass. It seems that spot modified males were not able to effectively communicate their territory tenancy, which may explain why they lost their defended sites. Our results provide support for theoretical models that a novel signal that fails to informing fighting ability may lead to a fitness cost for bearers.

Predation selects for increased immune function in male damselflies, Calopteryx splendens

Predation selects for numerous traits in many animal species, with sick or parasitized prey often being at high risk. When challenged by parasites and pathogens, prey with poor immune functions are thus likely to be at a selective disadvantage. We tested the hypothesis that predation by birds selects for increased immune function in a wild population of male damselflies Calopteryx splendens, while controlling for a trait known to be under selection by bird predation, dark wing-spots. We found that selection on both immune function and wing-spot size was significantly positive, and that selection on either trait was independent of selection on the other. We found no evidence of nonlinear quadratic or correlational selection. In contrast to previous studies, we found no phenotypic correlation between immune function and wing-spot size. There was also no difference in immune response between territorial and non-territorial males. Our study suggests that predation may be an important agent of selection on the immune systems of prey, and because the selection we detected was directional, has the potential to cause phenotypic change in populations.

Condition and phenotype-dependent dispersal in a damselfly, Calopteryx splendens

Individual dispersal decisions may be affected by the internal state of the individual and the external information of its current environment. Here we estimated the influence of dispersal on survival and investigated if individual phenotype (sex and wing length) and environmental condition (conspecific density and sex-ratio) affected dispersal decisions in the banded damselfly, Calopteryx splendens. As suspected from the literature, we showed that the proportion of dispersing individuals was higher in females than in males. We also found negative-density dependent dispersal in both sexes and influence of sex-ratio on dispersal. Individuals moved less when sex-ratio was male biased. These results are consistent with a lek mating system where males aggregate in a place and hold mating territories. Contrary to our expectations, neither dispersal nor survival was affected by wing length. Nevertheless, mean adult survival was about 8% lower in dispersing individuals than in residents. This might reflect a mortality cost due to dispersal.

Immune system activation interacts with territory-holding potential and increases predation of the damselfly Calopteryx splendens by birds

Activation of the immune system in insects has been shown to be costly in the laboratory setting, but experimental studies in the field are lacking. The costs of immunity in the wild may be different to those in the laboratory because animals in the wild are simultaneously subjected to a suite of selective agents. We have measured the costs of immune system activation in a wild population of the territorial damselfly Calopteryx splendens. Immune-challenged males were found to be less likely to be territorial and had lower overall survival rates than control or sham-manipulated males. Because territorial males have a higher mating success than nonterritorial males, this result suggests that immune-challenged males are also likely to suffer reduced mating success. However, the activation of the immune system as such did not increase predation risk; this occurred due to a combination of the former with a reduced territory-holding potential. As such, immune-challenged males not holding a territory were most susceptible to predation by birds. The size of the wing spots, a known sexually selected male trait, predicted territorial behaviour in control and sham-manipulated males, but not in immune-challenged males. Our data show that immune system activation can have several costs acting in unison and that ubiquitous ecological interactions, such as predation, may affect trade-offs between immunity and other life history traits.