Implications For Sexual Selection Research Articles

Within-population variation in signal timing preferences and its implications for sexual selection on male displays

Abstract Female mate choice decisions are guided by preferences for male display features, but in chorusing species the displays of different males may temporally overlap. Here, mate choice decisions may be guided by preferences based on signal timing in addition to signal features. Which type of preference dominates has implications for our understanding of the dynamics of sexual selection in group-displaying animals. I presented female treefrogs with a series of playback treatments varying the amounts of calls in leader/follower position to establish the lowest proportion of leading calls resulting in a preference. About half the females expressed leader preferences when fewer than 15% of calls are in leading position (the maximum produced by chorusing males). This suggests that mate choice decisions will be dominated by call timing preferences in some females, and by call features preferences others, overall lowering the strength of selection on either male display trait.

Characterization of the vocal behavior of the miniature and transparent fish model, Danionella cerebruma).

Danionella cerebrum has recently been proposed as a promising model to investigate the structure and function of the adult vertebrate brain, including the development of vocal-auditory neural pathways. This genetically tractable and transparent cypriniform is highly vocal, but limited information is available on its acoustic behavior and underlying biological function. Our main goal was to characterize the acoustic repertoire and diel variation in sound production of D. cerebrum, as well as to investigate the relationship between vocal behavior and reproduction. Sound recordings demonstrated high vocal activity, with sounds varying from short sequences of pulses known as "bursts" (comprising up to 15 pulses) to notably longer sounds, termed "long bursts", which extended up to 349 pulses with over 2.7 s. Vocal activity peaked at midday and it was very low at night with only a few bursts. While the number of pulses was higher during the daytime, the interpulse interval was longer at night. In addition, calling time was positively associated with the number of viable eggs, suggesting that acoustic communication is important for reproduction. These preliminary findings reveal the potential of using D. cerebrum to investigate vocal plasticity and the implications for sexual selection and reproduction in a novel vertebrate model for neuroscience.

How thermal challenges change gene regulation in the songbird brain and gonad: Implications for sexual selection in our changing world.

In a rapidly warming world, exposure to high temperatures may impact fitness, but the gene regulatory mechanisms that link sublethal heat to sexually selected traits are not well understood, particularly in endothermic animals. Our experiment used zebra finches (Taeniopygia guttata), songbirds that experience extreme temperature fluctuations in their native Australia. We exposed captive males to an acute thermal challenge (43°C) compared with thermoneutral (35°C) and lower (27°C) temperatures. We found significantly more heat dissipation behaviours at 43°C, a temperature previously shown to reduce song production and fertility, and more heat retention behaviours at 27°C. Next, we characterized transcriptomic responses in tissues important for mating effort-the posterior telencephalon, for its role in song production, and the testis, for its role in fertility and hormone production. Differential expression of hundreds of genes in the testes, but few in the brain, suggests the brain is less responsive to extreme temperatures. Nevertheless, gene network analyses revealed that expression related to dopaminergic signalling in the brain covaried with heat dissipation behaviours, providing a mechanism by which temporary thermal challenges may alter motivational circuits for song production. In both brain and testis, we observed correlations between thermally sensitive gene networks and individual differences in thermoregulatory behaviour. Although we cannot directly relate these gene regulatory changes to mating success, our results suggest that individual variation in response to thermal challenges could impact sexually selected traits in a warming world.

Inheritance of social dominance is associated with global sperm DNA methylation in inbred male mice.

Dominance relationships between males and their associated traits are usually heritable and have implications for sexual selection in animals. In particular, social dominance and its related male pheromones are heritable in inbred mice; thus, we wondered whether epigenetic changes due to altered levels of DNA methylation determine inheritance. Here, we used C57BL/6 male mice to establish a social dominance-subordination relationship through chronic dyadic encounters, and this relationship and pheromone covariation occurred in their offspring, indicative of heritability. Through transcriptome sequencing and whole-genome DNA methylation profiling of the sperm of both generations, we found that differential methylation of many genes was induced by social dominance-subordination in sires and could be passed on to the offspring. These methylated genes were mainly related to growth and development processes, neurodevelopment, and cellular transportation. The expression of the genes with similar functions in whole-genome methylation/bisulfite sequencing was also differentiated by social dominance-subordination, as revealed by RNA-seq. In particular, the gene Dennd1a, which regulates neural signaling, was differentially methylated and expressed in the sperm and medial prefrontal cortex in paired males before and after dominance-subordination establishment, suggesting the potential epigenetic control and inheritance of social dominance-related aggression. We suggest that social dominance might be passed on to male offspring through sperm DNA methylation and that the differences could potentially affect male competition in offspring by affecting the development of the nervous system.

Shifts in sexual dimorphism across a mass extinction in ostracods: implications for sexual selection as a factor in extinction risk.

Sexual selection often favours investment in expensive sexual traits that help individuals compete for mates. In a rapidly changing environment, however, allocation of resources to traits related to reproduction at the expense of those related to survival may elevate extinction risk. Empirical testing of this hypothesis in the fossil record, where extinction can be directly documented, is largely lacking. The rich fossil record of cytheroid ostracods offers a unique study system in this context: the male shell is systematically more elongate than that of females, and thus the sexes can be distinguished, even in fossils. Using mixture models to identify sex clusters from size and shape variables derived from the digitized valve outlines of adult ostracods, we estimated sexual dimorphism in ostracod species before and after the Cretaceous/Palaeogene mass extinction in the United States Coastal Plain. Across this boundary, we document a substantial shift in sexual dimorphism, driven largely by a pronounced decline in the taxa with dimorphism indicating both very high and very low male investment. The shift away from high male investment, which arises largely from evolutionary changes within genera that persist through the extinction, parallels extinction selectivity previously documented during the Late Cretaceous under a background extinction regime. Our results suggest that sexual selection and the allocation of resources towards survival versus reproduction may be an important factor for species extinction during both background and mass extinctions.

Individual variation in two types of advertisement calls of Pacific tree frogs, Hyliola (=Pseudacris) regilla, and the implications for sexual selection and species recognition

ABSTRACT Anuran advertisement calls play a fundamental role in social interactions related to reproduction, like territory defence and mate attraction. As such, advertisement calls are often shaped by natural and sexual selection. Therefore, to understand the evolution, function, and mechanisms of signal production and perception, it is important to characterise sources and patterns of variation in call properties. We recorded and analysed the two types of advertisement calls produced by Pacific tree frogs, Hyliola regilla: diphasic and monophasic calls. Specifically, we examined (i) the relationship between call properties and both body size and physical condition, (ii) patterns of within- and among-individual variation in call properties, and (iii) acoustic differences between diphasic and monophasic calls. While only one property of diphasic calls correlates with physical condition, spectral properties of both types of calls correlate with body size. Patterns of within- and among-individual variation were similar between call types and, overall, consistent with patterns reported for closely related species. Monophasic calls have lower frequencies, are delivered faster, and have higher call effort and duty cycle than diphasic calls. We discuss our results in relation to sexual selection and formulate hypotheses about the evolution, function, and mechanisms of acoustic communication in Pacific tree frogs.

Adult sex-ratio in ostracods and its implications for sexual selection

ABSTRACTThe Ostracoda – ubiquitous aquatic micro-crustaceans – show an exceptionally high incidence of female-biased adult sex ratio. Intraspecific sex ratio is known to vary in extant species and yet in the fossil record a species’ adult sex ratio can be highly stable across time. Sex ratio conditions the intensity of sexual selection and influences which sex undergoes stronger selective pressure. However, the impact of variation in spatial and temporal intraspecific sex ratio on the evolution of sexual selection remains an open question, calling for further investigations on the factors controlling adaptive sex ratio. This mini-review aims to introduce the system, and explores some of the key literature addressing factors influencing intraspecific variation in adult sex ratio (ASR) and its implication in the intensity of sexual selection and evolution of mating systems.

Sexual dimorphism in human arm power and force: implications for sexual selection on fighting ability.

Sexual dimorphism often arises from selection on specific musculoskeletal traits that improve male fighting performance. In humans, one common form of fighting includes using the fists as weapons. Here, we tested the hypothesis that selection on male fighting performance has led to the evolution of sexual dimorphism in the musculoskeletal system that powers striking with a fist. We compared male and female arm cranking power output, using it as a proxy for the power production component of striking with a fist. Using backward arm cranking as an unselected control, our results indicate the presence of pronounced male-biased sexual dimorphism in muscle performance for protracting the arm to propel the fist forward. We also compared overhead pulling force between males and females, to test the alternative hypothesis that sexual dimorphism in the upper body of humans is a result of selection on male overhead throwing ability. We found weaker support for this hypothesis, with less pronounced sexual dimorphism in overhead arm pulling force. The results of this study add to a set of recently identified characters indicating that sexual selection on male aggressive performance has played a role in the evolution of the human musculoskeletal system and the evolution of sexual dimorphism in hominins.

Age-specific patterns of infection with haemosporidians and trypanosomes in a warbler: implications for sexual selection.

Although the selective loss of individuals susceptible to disease can favor the evolution of female preference for older males, the interrelationship between age, infection, longevity, and mating success remains poorly characterized in natural populations. In a longitudinal study of 61 male common yellowthroats (Geothlypis trichas), we found that the probability of infection with hematozoa increased as males aged from 1 to 5years. Despite a significant, negative association between infection and longevity that partially masked age-effects, the odds that a male was infected with Trypanosoma, Plasmodium, or Leucocytozoon increased 71-212% per year. Nearly 75% of males in their first breeding season were either uninfected or infected with only a single parasite, while 50% of older males were infected with at least two parasites and 16% were infected with all three. No males escaped infection after their second breeding season. Older males were also more likely to sire extra-pair young (EPY) and, as a consequence, infection with multiple parasites was associated with a fourfold increase in the odds of producing EPY. Unlike younger males, 80% of the oldest males had a history of either surviving chronic infection or recovering. Combined with previous work showing higher diversity at the major histocompatibility complex among older males, our results suggest that the song and plumage traits that signal male age in common yellowthroats also, perforce, signal resistance to parasites. By preferring older males, females may obtain good genes for disease resistance even in the absence of any effect of infection on male ornamentation.

Mating behaviour and spermatophore morphology of four Cuban whip spiders (Arachnida, Amblypygi, Phrynidae) – Taxonomic relevance and evolutionary trends

Mating behaviour and the species-specific spermatophore morphology of four Cuban whip spider species are described. Whereas the courtship and spermatophore transfer is more or less typical for phrynid whip spider species, the entire spermatophore morphology is species-specific suggesting new undiscovered evolutionary trends and further, in two Paraphrynus species the male modify the spermatophore head with its chelicera and this differs remarkably between related species. The purpose of this unexpected behaviour is not clear and further research should resolve its function and the implications for sexual selection. Thus, we emphasizes the importance of inclusion of the species-specific spermatophore descriptions into taxonomic works.

Adult sex ratio, sexual dimorphism and sexual selection in a Mesozoic reptile.

The evolutionary history of sexual selection in the geologic past is poorly documented based on quantification, largely because of difficulty in sexing fossil specimens. Even such essential ecological parameters as adult sex ratio (ASR) and sexual size dimorphism (SSD) are rarely quantified, despite their implications for sexual selection. To enable their estimation, we propose a method for unbiased sex identification based on sexual shape dimorphism, using size-independent principal components of phenotypic data. We applied the method to test sexual selection in Keichousaurus hui, a Middle Triassic (about 237 Ma) sauropterygian with an unusually large sample size for a fossil reptile. Keichousaurus hui exhibited SSD biased towards males, as in the majority of extant reptiles, to a minor degree (sexual dimorphism index -0.087). The ASR is about 60% females, suggesting higher mortality of males over females. Both values support sexual selection of males in this species. The method may be applied to other fossil species. We also used the Gompertz allometric equation to study the sexual shape dimorphism of K. hui and found that two sexes had largely homogeneous phenotypes at birth except in the humeral width, contrary to previous suggestions derived from the standard allometric equation.

Condition-dependent mate assessment and choice by peahens: implications for sexual selection

Condition-dependent mate choice is thought to affect the strength and direction of sexual selection. Although there is ample evidence of this from studies that experimentally manipulate female condition, few studies to date have examined condition dependence of natural mate choice wherein females interact with a pool of available males. We examined mate assessment by free-living Pavo cristatus peafowl, focusing on 3 measures of the choosing female’s condition: mass condition (scaled mass index), ectoparasite load, and white blood cell (WBC) count. Females with greater mass condition were more active on peacock leks, approaching and visiting males more often, and spending more time near the males they visited. Condition did not affect the total number of males visited or the probability that a female would visit a male that she had already approached. Peacock mating success is strongly correlated with the iridescent colors of the eyespots on their train, suggesting that color signals are a major focus of female choice. Here, we show that females in better condition (higher mass condition and lower WBC count) allocate a greater proportion of their visit time to the most iridescent males displaying on leks. Our results provide evidence of strong condition-dependent effects on female mate assessment and choice in the absence of experimentally induced stress. We suggest that condition-dependent mate choice may help maintain variation in sexually selected male color traits.

Context-Dependent Mating Success in Murgantia histrionica (Hemiptera: Pentatomidae)

Abstract Animals must find and select mates in environments that naturally vary in both space and time. Of particular importance for many species are the availability and phenology of host plants for feeding and oviposition sites. Here, we used Murgantia histrionica (Hahn) (Hemiptera: Pentatomidae), a herbivorous insect, to determine mating preferences and success using mating assays and preferences based on odor cues.Wemanipulated the natal and encounter contexts using two different host plants, mustard and broccoli. We found that both the natal host plant and the host plant where insects encountered potential mates significantly affected mating success in this insect. Broccolireared individuals were more likely to mate in general; however, all individuals were more likely to mate when placed in an encounter context of mustard. Using a separate assay, we also found that the natal host plant affected female preference based on odor cues; females preferred the odors of males from the same natal environment that the females experienced as nymphs. In addition, we examined color and body size for host plant effects as both size and color are important factors for many species during mate choice. We found that individuals reared on mustard were larger than those reared on broccoli. However, the proportion of orange color on the dorsum of insects was not significantly affected by the natal host plant. In summary, these results suggest that changes in host plants, as commonly occurs seasonally, may have implications for sexual selection via mate choice.

Size-dependent temperature and desiccation constraints on performance capacity: Implications for sexual selection in a fiddler crab

Reproductively active males of the sand fiddler crab Uca pugilator are routinely exposed to high body temperatures and desiccation while defending breeding-burrows from other males or displaying to attract females. Such males are generally larger than individuals in other parts of the marsh, in part, because large males win more contests and have greater energy stores. We propose an additional explanation: that higher thermal inertia and lower mass-specific water loss rates may make large males less susceptible to thermal and hydration stresses than small males. We experimentally investigated the effects of variation in body temperature (Tb) and desiccation on physiological performance and behavior of male sand fiddler crabs as a function of body size. Treadmill endurance and sprint speed increased to a maximum before decreasing rapidly with further increases in Tb. Moderate dehydration (7–10% body water loss) resulted in significantly reduced locomotor performance, with small crabs exhibiting the greatest reductions in both body water and performance capacity. Fiddler crabs appear to be sensitive to the potentially conflicting demands of body temperature and hydration state. In a laboratory thermal gradient, crabs on wet sand chose Tb's associated with high locomotor performance capacity. In contrast, crabs on dry sand selected lower Tb's that presumably minimized water loss, with small crabs choosing the lowest Tb's. In addition to providing empirical support for a general model of ectotherm locomotion, our results are consistent with the idea that larger individuals are better able to tolerate harsh environmental conditions above-ground, increasing the proportion of time they can spend engaging in fitness-enhancing reproductive activities rather than sheltering in a burrow.

Repeatability of antler characteristics in mature white-tailed deer in South Texas: consequences of environmental effects

Antler traits are both genetically determined and environmentally influenced. However, the degree to which environmental factors affect antler expression has rarely been quantified. We captured 30 to 150 male white-tailed deer (Odocoileus virginianus) annually at 7 South Texas sites during 1985 to 2009 to determine repeatability of antler traits from a semiarid environment with variable rainfall. Repeatability is defined as the intraclass correlation between repeated measures of the same trait over time. Repeatability was moderate to high (0.42–0.82) for all antler traits. Overall, number of antler points had the lowest repeatability, whereas inside spread of main beams and length of main beams had the highest repeatability. Repeatability of total antler score and number of antler points from sites with variable rainfall was 16% and 24% lower than sites with consistent rainfall, respectively. Sites with variable rainfall had 13–18% higher repeatability when enhanced nutrition was available. Studies of cervids reveal a tendency for lower repeatability of antler traits as the environmental conditions become more variable. The association between repeatability and variable environmental conditions illustrates the magnitude of environmental effects and supports the role of antlers as an honest advertisement of individual condition or quality. Our results help to understand potential of microevolution in antlers and have implications for sexual selection and harvest management.

Visible skin condition and perception of human facial appearance

Evolutionary psychology suggests that certain human beauty standards have evolved to provide reliable cues of fertility and health. Hence, preferences for some physical characteristics of the face and body are thought to reflect adaptations for the promotion of mate choice. Studies that have investigated facial attractiveness have concentrated mainly on features such as symmetry, averageness and sex-typical traits, which are developed under the influence of sex steroids. Few studies, however, have addressed the effect of human skin condition on perception of facial appearance in this context, and possible implications for sexual selection. There is now accumulating evidence that skin pigmentation and skin surface topography cues, particularly in women, have a significant influence on attractiveness judgements, as they seem primarily to signal aspects of age and health. This article (i) reviews briefly some of the main determinants of visible skin condition, (ii) presents recent evidence on its signalling value in face perception and (iii) suggests areas for future research with reference to an evolutionary psychology framework.

Interaction between signal timing and signal feature preferences: causes and implications for sexual selection

Females often choose their mates based on features of the male’s advertisement signal, often preferring louder, deeper, longer, or faster signals. Females of chorusing animals also frequently have preferences for signal timing positions, generally preferring the leading signal. Under natural chorus conditions, females must choose among signals that vary in features as well as relative timing. I conducted playback experiments with female Hyla cinerea treefrogs to evaluate whether one type of preference (for attractive signal feature or attractive timing) dominates the overall attractiveness of a signal, or whether different types of preferences can interact. I found that the overall attractiveness of a signal was determined by a combination of timing and feature preferences. The specific signal feature (amplitude, frequency, duration, or rate) was important for the outcome of this interaction effect; signal frequency preferences were very strong and dominated overall signal attractiveness, while signal amplitude, duration or rate preferences were weaker and overall signal attractiveness was largely determined by signal timing preferences. I suggest that preference interactions can result in changes in the dynamics of sexual selection and in the maintenance of variation in signal traits, because males with unattractive signals could gain higher mating success by producing these signals in a leading position, thus weakening or even disrupting feature-based signal selection.

Effects of an Immune Challenge on Multiple Components of Song Display in Barn SwallowsHirundo rustica: Implications for Sexual Selection

AbstractBird song is believed to honestly reveal male quality including the ability of singers to face parasitism. In a natural population of barn swallowsHirundo rustica, we experimentally imposed a cost on song production by an immune challenge. We therefore vaccinated a group of reproducing males with an antigen (Newcastle disease virus), and injected phosphate‐buffered saline to a control group. Immune challenge significantly reduced one song feature, rattle duration. This decrease was related to male quality, as measured by tail length, because males with short tails reduced the duration of their rattle significantly more than males with long tails. In addition, another song feature, strophe duration, decreased in the control group, while it remained constant in the challenged group. Duration of the rattle has previously been found to be positively related to testosterone level, and it may hence reflect male competitive ability. Thus, male barn swallows may not have the potential to produce long rattles when their immune system is challenged. By maintaining their strophe duration after an immune challenge, males may compensate for the decrease in rattle duration. Our results suggest that different song features may convey different types of information, and that barn swallow song, in particular their rattle, may reliably reveal information about activation of the immune system.

The Intersexual Genetic Correlation for Lifetime Fitness in the Wild and Its Implications for Sexual Selection

BackgroundThe genetic benefits of mate choice are limited by the degree to which male and female fitness are genetically correlated. If the intersexual correlation for fitness is small or negative, choosing a highly fit mate does not necessarily result in high fitness offspring.Methodology/Principal FindingUsing an animal-model approach on data from a pedigreed population of over 7,000 collared flycatchers (Ficedula albicollis), we estimate the intersexual genetic correlation in Lifetime Reproductive Success (LRS) in a natural population to be negative in sign (−0.85±0.6). Simulations show this estimate to be robust in sign to the effects of extra-pair parentage. The genetic benefits in this population are further limited by a low level of genetic variation for fitness in males.Conclusions/SignificanceThe potential for indirect sexual selection is nullified by sexual antagonistic fitness effects in this natural population. Our findings and the scarce evidence from other studies suggest that the intersexual genetic correlation for lifetime fitness may be very low in nature. We argue that this form of conflict can, in general, both constrain and maintain sexual selection, depending on the sex-specific additive genetic variances in lifetime fitness.

Male-biased predation of western green lizards by Eurasian kestrels

Selective predation can be an important force driving the evolution of organisms. In particular, sex-biased predation is expected to have implications for sexual selection, sex allocation and population dynamics. In this study, we analysed sex differences in the predation of the western green lizard (Lacerta bilineata) by the Eurasian kestrel (Falco tinnunculus) during the reproductive season. In addition, we investigated whether the rate of predation differed during the 8-year study period and among the three habitats studied. We collected lizard remains from nest boxes of kestrels. Freshly killed lizards were sexed by visual inspection, whilst the sex of head remains was assigned by analysing the cephalic scale morphology using geometric morphometrics. Our results show that the risk of being predated by a kestrel in our population was overall about 3.55 times higher for males than for females. To our knowledge this is the first study showing a male-biased predation in a lizard species. The selective predation of males was consistent between years over the 8-year study period (1999-2006) and also consistent between the three types of kestrel hunting habitat. Overall predation rates on lizards differed between habitats, depending on the year. We propose that the observed sex-biased predation is mainly due to sex differences in lizard behaviour.