Adaptive significance of synchronous chorusing in an acoustically signalling wolf spider

Synchronous sexual signalling is a behavioural phenomenon that has received considerable theoretical interest, but surprisingly few empirical tests have been conducted. Here, we present a set of experiments designed to determine (i) whether the sexual signalling of the drumming wolf spider Hygrolycosa rubrofasciata is synchronous, and (ii) whether the synchrony may have evolved through female preference. Using controlled playback experiments, we found that males actively synchronized their drumming bouts with other males and females significantly preferred closely synchronized drumming clusters compared with loose clusters. In loose clusters, the first drumming signals attracted the most female responses, whereas in close clusters, the last drumming signals were the most heeded. We suggest that this female preference for the last drummer can maintain male synchronous signalling in H. rubrofasciata.     

SEXUAL SELECTION IN A WOLF SPIDER: MALE DRUMMING ACTIVITY,BODY SIZE,AND VIABILITY

Females are often believed to actively choose highly ornamented males (males with extravagant morphological signals or intense sexual display), and ornaments should be honest signals of male viability. However, this belief is relying only on some pieces of empirical evidence from birds. Our study reports active female choice on sexual display that indicates male viability in spiders. We established trials in which we studied female choice in relation to male courtship drumming activity and body size. Females chose the most actively drumming males as mating partners, but the body size of the males did not seem to be selected. Male drumming activity turned out to be a good predictor of male viability, whereas male viability was independent of male body mass. Our results suggest that by actively choosing mates according to male drumming performance, but independently of male body mass, females are preferring viable males as mates. Because Hygrolycosa rubrofasciata males do not provide obvious direct benefits to their offspring, females may gain some indirect benefits; offspring may have higher chance of survival, or the offspring may inherit the attractiveness of their father.     

Mate choice for offspring performance: major benefits or minor costs?

There is considerable disagreement over whether or not gaining viability benefits to offspring could be substantial enough to overcome the costs of female choosiness. A recent review suggested that the ''lek paradox'' might be resolved by large indirect benefits as indicated by highly heritable ornamental traits. We selected males of a wolf spider Hygrolycosa rubrofasciata in relation to their sexual signalling rate (audible drumming). The estimated correlated response in offspring viability was rather small (0.12 s.d.). However, it may be large enough if the costs of being choosy are small. In fact, females mate with better-than-average males just by responding passively to a random drumming signal, and the active choice by females seemed to increase this benefit only slightly. In many mating systems, females obtain better-than-average males as a consequence of intense male–male competition or because of the extraordinary variance in male signalling. The costs of any additional choice may be so minor that female choice for honestly signalling males may evolve even with minute benefits in offspring viability. This may be the general solution to the lek paradox, as most studies report no apparent fitness benefits. Publication bias favouring statistically significant results may have led to an overemphasis on the few studies with large effects.     

Acoustic signalling in a wolf spider: can signal characteristics predict male quality?

While there has been considerable interest in female choice for male sexual signals, there have been few studies of the underlying information that different aspects of the signal calls convey. Such studies, however, are essential to understand the significance of signals as honest handicaps, arbitrary Fisherian traits and/or in species recognition. We studied the somewhat exceptional system of audible drumming in the wolf spider Hygrolycosa rubrofasciata. We estimated the repeatabilities of signal components, the levels of between-male variance, the symmetry of the signal, the correlations between different aspects of drumming and their correlations with body weight. While in other taxa the frequency of audible signals may convey honest information of male size, in this species signal frequency was not related to male size and had a low repeatability. The pulse rate within each drum was highly repeatable but had a relatively small between-male coefficient of variation. In previous studies on this species, these traits were not important for male mating success. Among the traits directionally preferred by females, signal volume had considerable repeatability. Signal length was repeatable with high variability between males. In one population, signal length and volume were positively correlated with the rate at which males produced the drumming signals, a trait essential for male mating success. Thus, while signal length may reliably indicate male quality, other signal characteristics such as peak frequency and symmetry were not repeatable or were static and not related to any other male traits. Copyright 2000 The Association for the Study of Animal Behaviour.     

Effect of energy availability, seasonality, and geographic range on brown bear life history

Life-history theory allows predictions of how changes in environmental selection pressures along a species' geographic distribution result in discrete shifts in life-history traits. We tested for spatial patterns of 24 populations of brown bears Ursus arctos across North America that grouped according to the following environmental and population parameters: evapotranspiration as a correlate of primary productivity of vegetation, coefficient of variation of monthly evapotranspiration values as a measure of seasonality. population density, and adult female weight. Cluster analysis grouped brown bear populations into two regions: Pacific-coastal populations characterized by high population density and large females that lived in areas of high primary productivity and low seasonality. and inland and barren-ground populations characterized by relatively low density and small bears that lived in areas of low productivity and high seasonality. For each region, we tested whether life-history traits (age at maturity and interbirth interval) related to primary productivity or seasonality. High altitude (interior: > 1000 m) and high latitude (barren-ground; >65°N) populations respond to extremes in seasonality with risk-spreading adaptations. For example, age at maturity and interbirth interval increased with greater seasonality. In contrast, Pacific-coastal populations living on the western edge of brown bear geographic range respond to intraspecific competition at high densities by maximizing offspring competitive ability. For example, age at maturity increased with greater primary productivity and high population density. In each region, the female parent decided on the life-history trade-offs required to reduce the risks of offspring mortality depending on the environmental pattern.     

The effects of sexual selection on life‐history traits: an experimental study on guppies

Sexual selection is often prevented during captive breeding in order to maximize effective population size and retain genetic diversity. However, enforcing monogamy and thereby preventing sexual selection may affect population fitness either negatively by preventing the purging of deleterious mutations or positively by reducing sexual conflicts. To better understand the effect of sexual selection on the fitness of small populations, we compared components of female fitness and the expression of male secondary sexual characters in 19 experimental populations of guppies (Poecilia reticulata) maintained under polygamous or monogamous mating regimes over nine generations. In order to generate treatments that solely differed by their level of sexual selection, the middle‐class neighbourhood breeding design was enforced in the monogamous populations, while in the polygamous populations, all females contributed similarly to the next generation with one male and one female offspring. This experimental design allowed potential sexual conflicts to increase in the polygamous populations because selection could not operate on adult‐female traits. Clutch size and offspring survival showed a weak decline from generation to generation but did not differ among treatments. Offspring size, however, declined across generations, but more in monogamous than polygamous populations. By generation eight, orange‐ and black‐spot areas were larger in males from the polygamous treatment, but these differences were not statistically significant. Overall, these results suggest that neither sexual conflict nor the purging of deleterious mutation had important effects on the fitness of our experimental populations. However, only few generations of enforced monogamy in a benign environment were sufficient to negatively affect offspring size, a trait potentially crucial for survival in the wild. Sexual selection may therefore, under certain circumstances, be beneficial over enforced monogamy during captive breeding.     

Populations with elevated mutation load do not benefit from the operation of sexual selection

Theory predicts that if most mutations are deleterious to both overall fitness and condition-dependent traits affecting mating success, sexual selection will purge mutation load and increase nonsexual fitness. We explored this possibility with populations of mutagenized Drosophila melanogaster exhibiting elevated levels of deleterious variation and evolving in the presence or absence of male-male competition and female choice. After 60 generations of experimental evolution, monogamous populations exhibited higher total reproductive output than polygamous populations. Parental environment also affected fitness measures - flies that evolved in the presence of sexual conflict showed reduced nonsexual fitness when their parents experienced a polygamous environment, indicating trans-generational effects of male harassment and highlighting the importance of a common garden design. This cost of parental promiscuity was nearly absent in monogamous lines, providing evidence for the evolution of reduced sexual antagonism. There was no overall difference in egg-to-adult viability between selection regimes. If mutation load was reduced by the action of sexual selection in this experiment, the resultant gain in fitness was not sufficient to overcome the costs of sexual antagonism.     

Energetic costs of size and sexual signalling in a wolf spider

A prerequisite for honest handicaps is that there are significant condition-dependent costs in the expression of sexual traits. In the wolf spider Hygrolycosa rubrofasciata (Ohlert), sexual signalling (drumming) is costly in terms of increased mortality. Here we investigated whether this mortality may be caused by increased energy expenditure. During sexual signalling, metabolic rate was 22 times higher than at rest and four times higher than when males were actively moving. Metabolic rate per unit mass was positively related to absolute body mass during sexual signalling but not during other activities. This positive relationship is novel to any studies of metabolic rates. Indeed, it seems that the largest males can drum only 12 times per minute before reaching the maximum sustainable metabolic rate, whereas the smallest males may drum up to 39 times per minute. However, there is no relationship between body mass and drumming rate, indicating that larger males are able to compensate for the higher cost of drumming. There was a quadratic relationship between relative abdomen mass and overall body mass, which may provide a partial explanation for the increased energy expenditure of largest males while drumming. Altogether, our results indicate that sexual signalling is highly energetically demanding, which may be the main reason for the honesty of signalling in this species. In addition, the energetic costs are surprisingly strongly size dependent, which may compensate any disadvantage of small male size.     

Spatial and temporal variation in a suite of life-history traits in two species of wolf spider

Abstract.  1. Life-history traits and density were assayed in seven populations of two sympatric species of wolf spider for three consecutive years. The goal of the study was to quantify population dynamics and its relation to spatial and temporal life-history variation.
2. Adult female body size and fecundity varied significantly, among field sites and among years, in both species. Female spiders of both species differed in mean relative reproductive effort among sites, but not among years. The size of offspring was invariable, with no significant differences due to site or year.
3. All populations of both species tended to either decrease or increase in density during a given year and this was tightly correlated with changes in prey consumption rates.
4. Since life-history patterns are determined primarily by selection, it is concluded that size at sexual maturity for females is phenotypically plastic and responds to changes in prey availability. Offspring size however is not plastic and it is likely that other selection forces have determined offspring size. Temporal fluctuations in population size are correlated over a large area relative to dispersal capabilities for these species and conservation efforts for invertebrates must take this into consideration.     

Anisogamy,sexual selection,and the evolution and maintenance of sex

Summary In the present paper we distinguish between two aspects of sexual reproduction. Genetic recombination is a universal features of the sexual process. It is a primitive condition found in simple, single-celled organisms, as well as in higher plants and animals. Its function is primarily to repair genetic damage and eliminate deleterious mutations. Recombination also produces new variation, however, and this can provide the basis for adaptive evolutionary change in spatially and temporally variable environments.The other feature usually associated with sexual reproduction, differentiated male and female roles, is a derived condition, largely restricted to complex, diploid, multicellular organisms. The evolution of anisogamous gametes (small, mobile male gametes containing only genetic material, and large, relatively immobile female gametes containing both genetic material and resources for the developing offspring) not only established the fundamental basis for maleness and femaleness, it also led to an asymmetry between the sexes in the allocation of resources to mating and offspring. Whereas females allocate their resources primarily to offspring, the existence of many male gametes for each female one results in sexual selection on males to allocate their resources to traits that enhance success in competition for fertilizations. A consequence of this reproductive competition, higher variance in male than female reproductive success, results in more intense selection on males.The greater response of males to both stabilizing and directional selection constitutes an evolutionary advantage of males that partially compensates for the cost of producing them. The increased fitness contributed by sexual selection on males will complement the advantages of genetic recombination for DNA repair and elimination of deleterious mutations in any outcrossing breeding system in which males contribute only genetic material to their offspring. Higher plants and animals tend to maintain sexual reproduction in part because of the enhanced fitness of offspring resulting from sexual selection at the level of individual organisms, and in part because of the superiority of sexual populations in competition with asexual clones.     

Male mating success and risk of predation in a wolf spider: a balance between sexual and natural selection?

1. Traits that benefit males through sexual selection are simultaneously expected to impair males by provoking costs through natural selection. If we consider the two male fitness components, mating success and viability, then we may expect that the increase in male mating success resulting from a larger trait size will be counterbalanced by an increase in viability costs.
2. We studied the benefits and costs of male mate searching and sexual signalling activity in the wolf spider Hygrolycosa rubrofasciata . In the field, males search females actively and court them by drumming dry leaves with their abdomen. Females have been shown to prefer males with high drumming rate. Male moving and especially drumming is energetically highly demanding and drumming results in significant mortality costs.
3. Our objective in this study was to determine whether male mate-searching activity or drumming activity affect male mating success and the risk of males being predated.
4. It was evident that both higher mate-searching activity and higher drumming activity benefited males by increasing their mating success. Higher mate-searching activity clearly impaired males by causing direct increase in predation risk. There was also a slight tendency that more actively drumming males had higher risk of predation and from all of the predated males 13.3% were caught directly after they had drummed. Furthermore, male drumming activity decreased drastically in the presence of the predator.
5. We conclude that in H. rubrofasciata both increased mate-searching activity and drumming activity benefit males through sexual selection, but at the same time natural selection provokes direct balancing costs on the same traits.     

Adaptive inter-population differences in blue tit life-history traits on Corsica

Few studies of natural populations have investigated how phenotypic variation across populations relates to key factors in the environment and landscape structure. In the blue tits of southern France, inter-population differences in reproductive life-history traits (e.g. laying date and clutch size) are small, whatever the timing of maximum caterpillar availability, a key factor for offspring survival in tits. These small differences are attributed to gene flow between local populations occupying different habitat types. In contrast, in blue tits on the island of Corsica, we noted large differences in reproductive life-history traits between two populations, where each population is synchronized with the peak-date of caterpillar abundance. These occur over a short geographical distance (25km). Considering our study within a framework of long-term population studies in tits, our results support the hypothesis that different blue tit populations on Corsica show adaptive differences in life-history traits, and suggest that landscape structure at a small spatial scale can have profound effects on adaptive between-population differentiation in life-history traits that are closely linked with fitness.     

Important population viability analysis parameters for giant pandas (Aliuropoda melanoleuca)

Population viability analysis(PVA) is a tool to evaluate the risk of extinction for endangered species and aid conservation decision-making.The quality of PVA output is dependent on parameters related to population dynamics and life-history;however,it has been difficult to collect this information for the giant panda(Aliuropoda melanoleuca),a rare and endangered mammal native to China,confined to some 30 fragmented habitat patches.Since giant pandas are long-lived,mature late,have lower reproductive rates,and show little sexual dimorphism,obtaining data to perform adequate PVA has been difficult.Here,we develop a parameter sensitivity index by modeling the dynamics of six giant panda populations in the Minshan Mountains,in order to determine the parameters most influential to giant panda populations.Our data shows that the giant panda populations are most sensitive to changes in four female parameters:initial breeding age,reproductive rate,mortality rate between age 0 and 1,and mortality rate of adults.The parameter sensitivity index strongly correlated with initial population size,as smaller populations were more sensitive to changes in these four variables.This model suggests that demographic parameters of females have more influence on the results of PVA,indicating that females may play a more important role in giant panda population dynamics than males.Consequently,reintroduction of female individuals to a small giant panda population should be a high priority for conservation efforts.Our findings form a technical basis for the coming program of giant panda reintroduction,and inform which parameters are crucial to successfully and feasibly monitoring wild giant panda populations.     

The song of the old mother: Reproductive senescence in female drosophila

Among animals with multiple reproductive episodes, changes in adult condition over time can have profound effects on lifetime reproductive fitness and offspring performance. The changes in condition associated with senescence can be particularly acute for females who support reproductive processes from oogenesis through fertilization. The pomace fly Drosophila melanogaster is a well-established model system for exploring the physiology of reproduction and senescence. In this review, we describe how increasing maternal age in Drosophila affects reproductive fitness and offspring performance as well as the genetic foundation of these effects. Describing the processes underlying female reproductive senescence helps us understand diverse phenomena including population demographics, condition-dependent selection, sexual conflict, and transgenerational effects of maternal condition on offspring fitness. Understanding the genetic basis of reproductive senescence clarifies the nature of life-history trade-offs as well as potential ways to augment and/or limit female fertility in a variety of organisms.     

The song of the old mother: Reproductive senescence in female drosophila

Among animals with multiple reproductive episodes, changes in adult condition over time can have profound effects on lifetime reproductive fitness and offspring performance. The changes in condition associated with senescence can be particularly acute for females who support reproductive processes from oogenesis through fertilization. The pomace fly Drosophila melanogaster is a well-established model system for exploring the physiology of reproduction and senescence. In this review, we describe how increasing maternal age in Drosophila affects reproductive fitness and offspring performance as well as the genetic foundation of these effects. Describing the processes underlying female reproductive senescence helps us understand diverse phenomena including population demographics, condition-dependent selection, sexual conflict, and transgenerational effects of maternal condition on offspring fitness. Understanding the genetic basis of reproductive senescence clarifies the nature of life-history trade-offs as well as potential ways to augment and/or limit female fertility in a variety of organisms.     

A trade-off between reproduction and a condition-dependent sexually selected ornament in the house sparrow Passer domesticus

Reproductive effort was manipulated in a free-living population of house sparrows (Passer domesticus) to investigate the trade-off between reproductive investment and the expression of a condition-dependent sexually selected ornament. Phenotypic plasticity in the expression of this trait was related to the experimentally manipulated size of the brood reared by a male. Males that invested more in current reproduction subsequently became more attractive to females in this population as they showed a preference for males with smaller badges. This supports the argument that direct benefits are a primary focus for mate choice by females. Trade-offs between reproductive effort and the expression of sexual ornaments are a potentially important source of phenotypic variation in both sexual ornaments and life-history traits.     

Female choice and annual reproductive success favour less-ornamented male house sparrows

Theories of sexual selection usually assume that female preferences for male ornamental traits are fixed and always likely to favour the largest or most extravagant sexual ornaments. This is not, however, always necessary or likely, particularly in resource-based monogamous systems. Here we show that in a closed population of house sparrows, small-badged males were preferred by females as both social and genetic mates and produced a higher number of viable offspring. Previous studies of other house sparrow populations have shown females to prefer large-badged males. Given the likely trade-offs operating between different male behavioural and morphological traits, we propose that female choice is a flexible adaptive strategy through which females can select those males likely to supply the male-acquired benefits that are locally most important.     

Offspring viability benefits but no apparent costs of mating with high quality males

Traditional models of sexual selection posit that male courtship signals evolve as indicators of underlying male genetic quality. An alternative hypothesis is that sexual conflict over mating generates antagonistic coevolution between male courtship persistence and female resistance. In the scarabaeine dung beetle Onthophagus taurus, females are more likely to mate with males that have high courtship rates. Here, we examine the effects of exposing females to males with either high or low courtship rates on female lifetime productivity and offspring viability. Females exposed to males with high courtship rates mated more often and produced offspring with greater egg-adult viability. Female productivity and lifespan were unaffected by exposure to males with high courtship rates. The data are consistent with models of sexual selection based on indirect genetic benefits, and provide little evidence for sexual conflict in this system.     

Temporal change in inbreeding depression in life‐history traits in captive populations of guppy (Poecilia reticulata): evidence for purging?

Inbreeding depression, which generally affects the fitness of small populations, may be diminished by purging recessive deleterious alleles when inbreeding persists over several generations. Evidence of purging remains rare, especially because of the difficulties of separating the effects of various factors affecting fitness in small populations. We compared the expression of life-history traits in inbred populations of guppy (Poecilia reticulata) with contemporary control populations over 10 generations in captivity. We estimated inbreeding depression as the difference between the two types of populations at each generation. After 10 generations, the inbreeding coefficient reached a maximum value of 0.56 and 0.16 in the inbred and control populations, respectively. Analysing changes in the life-history traits across generations showed that inbreeding depression in clutch size and offspring survival increased during the first four to six generations in the populations from the inbred treatment and subsequently decreased as expected if purging occurred. Inbreeding depression in two other traits was weaker but showed similar changes across generations. The loss of six populations in the inbred treatment indicates that removal of deleterious alleles also occurred by extinction of populations that presumably harboured high genetic load.     

The immunocompetence handicap hypothesis: testing the genetic predictions

The immunocompetence handicap hypothesis suggests that the immune system competes for resources with sexually selected ornaments; variation in ornaments might reflect genetic variation for immunocompetence. We tested this genetic prediction by mating scorpionfly females to males differing in the expression of a condition-dependent ornament trait, saliva secretion, and then comparing offspring immunocompetence. We found several indications of an immunocompetence handicap in our study: females had superior immunocompetence compared with males, the different immune traits were positively correlated, and there were indications of genetic variation in immune traits. However, we found no significant difference in the immunocompetence of offspring derived from males differing in ornament expression, only a tendency for sons of ornamented males to possess slightly better immunocompetence. The estimated effect of fathers on offspring immunocompetence was rather small, but it might be a sufficient benefit of female choice, provided that the costs of choice are small. We conclude that the genetic benefit of female choice is small concerning offspring immunocompetence, but the immunocompetence handicap principle might nevertheless work in scorpionflies.