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Results for “Sexual selection”

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PubMed2006

Environmental stress increases skeletal fluctuating asymmetry in the moor frog Rana arvalis.

Whether fluctuating asymmetry (FA) provides a useful metric indicator of the degree of environmental stress experienced by populations is still a contentious issue. We investigated whether the degree of FA in skeletal elements is useful in elucidating the degree of environmental stress experienced by frog populations, and further, tested the proposition that a trait's sensitivity to stress--as reflected in the degree of FA--is related to the degree of directional selection experienced by the given trait. We compared the degree of FA in four bilateral skeletal elements of male and female moor frogs (Rana arvalis) originating from low (acidified) and neutral pH populations. While the degree of uncorrected FA was unrelated to the degree of acidity, the growth rate and age of the individuals, the size-corrected FA was significantly higher in low than in neutral pH populations and decreased with individual ages and growth rates. In addition, both measures of FA were significantly higher in males and in particular in traits presumably under high sexual selection as indicated by the degree of sexual size dimorphism. All in all, the results indicate that individuals from acidified localities are smaller, younger and exhibit a significantly higher degree of FA than individuals from neutral pH populations. These results constitute the first assessment of FA in amphibians and suggest that the degree of FA in skeletal traits can be a useful indicator of the degree of environmental stress experienced by amphibian populations.

Age Factors↗

PubMed2005

No discrimination against previous mates in a sexually cannibalistic spider.

In several animal species, females discriminate against previous mates in subsequent mating decisions, increasing the potential for multiple paternity. In spiders, female choice may take the form of selective sexual cannibalism, which has been shown to bias paternity in favor of particular males. If cannibalistic attacks function to restrict a male's paternity, females may have little interest to remate with males having survived such an attack. We therefore studied the possibility of female discrimination against previous mates in sexually cannibalistic Argiope bruennichi, where females almost always attack their mate at the onset of copulation. We compared mating latency and copulation duration of males having experienced a previous copulation either with the same or with a different female, but found no evidence for discrimination against previous mates. However, males copulated significantly shorter when inserting into a used, compared to a previously unused, genital pore of the female.

Animals↗

PubMed1998

Sex-biased mortality in woodrats occurs in the absence of parental intervention

Male-biased mortality in young animals is often viewed as adaptive discrimination against male offspring by parents unable to raise reproductively competitive sons. Unequivocal evidence of the presence or absence of parental discrimination against males is lacking, however, and the adaptive interpretation of male-biased mortality is confounded by an alternative explanation that it reflects differential energetic requirements between the sexes (due to sexual selection for large size in mature males) independent of parental manipulation. To determine whether maternal discrimination against offspring explains postnatal mortality in a sexually dimorphic rodent, we examined patterns of growth and mortality in offspring of food-restricted and food-enriched lactating bushy-tailed woodrats, Neotoma cinerea. We also monitored mothers and their litters daily throughout lactation for evidence of maternal discrimination against offspring. Offspring of food-restricted mothers showed depressed growth, and mortality of offspring born to both food-restricted and food-enriched mothers was male-biased, but in the absence of maternal discrimination. Offspring that died were no less likely to be attached to their mother's teats in the 10 days prior to death than were offspring that successfully weaned. Similarly, offspring of food-restricted mothers were attached as often as were offspring of food-enriched mothers. In a series of behavioural arena trials in the first 10 days after birth, restricted mothers were no less attentive toward their sons than they were to their daughters, nor did mothers treat their offspring that did not survive to weaning differently from those that survived. Our findings provide empirical evidence that postnatal, sex-biased mortality in offspring is not necessarily due to parental intervention, and they call into question the adaptive interpretations of previous examples of sex-biased offspring mortality. Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.

Journal Article↗

PubMed2001

Positive genetic correlation between parasite resistance and body size in a free-living ungulate population.

Parasite resistance and body size are subject to directional natural selection in a population of feral Soay sheep (Ovis aries) on the island of St. Kilda, Scotland. Classical evolutionary theory predicts that directional selection should erode additive genetic variation and favor the maintenance of alleles that have negative pleiotropic effects on other traits associated with fitness. Contrary to these predictions, in this study we show that there is considerable additive genetic variation for both parasite resistance, measured as fecal egg count (FEC), and body size, measured as weight and hindleg length, and that there are positive genetic correlations between parasite resistance and body size in both sexes. Body size traits had higher heritabilities than parasite resistance. This was not due to low levels of additive genetic variation for parasite resistance, but was a consequence of high levels of residual variance in FEC. Measured as coefficients of variation, levels of additive genetic variation for FEC were actually higher than for weight or hindleg length. High levels of additive genetic variation for parasite resistance may be maintained by a number of mechanisms including high mutational input, balancing selection, antagonistic pleiotropy, and host-parasite coevolution. The positive genetic correlation between parasite resistance and body size, a trait also subject to sexual selection in males, suggests that parasite resistance and growth are not traded off in Soay sheep, but rather that genetically resistant individuals also experience superior growth.

Alleles↗

PubMed2005

The effect of Wolbachia versus genetic incompatibilities on reinforcement and speciation.

Wolbachia is a widespread group of intracellular bacteria commonly found in arthropods. In many insect species, Wolbachia induce a cytoplasmic mating incompatibility (CI). If different Wolbachia infections occur in the same host species, bidirectional CI is often induced. Bidirectional CI acts as a postzygotic isolation mechanism if parapatric host populations are infected with different Wolbachia strains. Therefore, it has been suggested that Wolbachia could promote speciation in their hosts. In this article we investigate theoretically whether Wolbachia-induced bidirectional CI selects for premating isolation and therefore reinforces genetic divergence between parapatric host populations. To achieve this we combined models for Wolbachia dynamics with a well-studied reinforcement model. This new model allows us to compare the effect of bidirectional CI on the evolution of female mating preferences with a situation in which postzygotic isolation is caused by nuclear genetic incompatibilities (NI). We distinguish between nuclear incompatibilities caused by two loci with epistatic interactions, and a single locus with incompatibility among heterozygotes in the diploid phase. Our main findings are: (1) bidirectional CI and single locus NI select for premating isolation with a higher speed and for a wider parameter range than epistatic NI; (2) under certain parameter values, runaway sexual selection leads to the increase of an introduced female preference allele and fixation of its preferred male trait allele in both populations, whereas under others it leads to divergence in the two populations in preference and trait alleles; and (3) bidirectional CI and single locus NI can stably persist up to migration rates that are two times higher than seen for epistatic NI. The latter finding is important because the speed with which mutants at the preference locus spread increases exponentially with the migration rate. In summary, our results show that bidirectional CI selects for rapid premating isolation and so generally support the view that Wolbachia can promote speciation in their hosts.

Animals↗

PubMed2006

Male and female effects on sperm precedence in the giant sperm species Drosophila bifurca.

Sperm competition is expected to be a driving force in sexual selection. In internally fertilized organisms, it occurs when ejaculates from more than one male are present simultaneously within the female's reproductive tract. It has been suggested that greater sperm size may improve the competitive ability of sperm, but studies provide contradictory results depending on the species. More recently, the role of females in the evolution of sperm morphology has been pointed out. We investigate here the male and female effects that influence sperm precedence in the giant sperm species, Drosophila bifurca Patterson & Wheeler. Females were mated with two successive males, and the paternity outcomes for both males were analyzed after determining sperm transfer and storage. We found very high values of last male sperm precedence, suggesting a strong interaction between rival sperm. However, the data also indicate high frequencies of removal of the sperm of the first male from the female reproductive tract prior to any interaction with the second male. This implies that successful paternity depends mainly on successful sperm storage. Knowing what happens to the sperm within females appears to be a prerequisite for disentangling post-copulatory sexual interactions between males and females.

Animals↗

PubMed1990

Group selection among alternative evolutionarily stable strategies.

Many important models of the evolution of social behavior have more than one evolutionarily stable strategy (ESS). Examples include co-ordination games, contests, mutualism, reciprocity, and sexual selection. Here we show that when there are multiple evolutionarily stable strategies, selection among groups can cause the spread of the strategy that has the lowest extinction rate or highest probability of contributing to the colonization of empty habitats, and that this may occur even when groups are usually very large, migration rates are substantial, and "extinction" entails only the disruption of the group and the dispersal of its members. The main requirements are: (1) individuals drawn from a single surviving group make up a sufficiently large fraction newly formed groups, and (2) the processes increasing the frequency of successful strategies within groups are strong compared to rate of migration among groups. The latter condition suggests that this form of group selection will be particularly important when behavioral variation is culturally acquired.

Animals↗

PubMed2001

The genetic mating system and tests for cuckoldry in a pipefish species in which males fertilize eggs and brood offspring externally.

Highly variable microsatellite loci were used to study the mating system of Nerophis ophidion, a species of pipefish in which pregnant males carry embryos on the outside of their body rather than in an enclosed brood pouch. Despite this mode of external fertilization and brooding, otherwise rare in the family Syngnathidae, the genotypes of all embryos proved to be consistent with paternity by the tending male, thus indicating that cuckoldry by sneaker males is rare or nonexistent in this species. N. ophidion is a phylogenetic outlier within the Syngnathidae and its reproductive morphology is thought to be close to the presumed ancestral condition for pipefishes and seahorses. Thus, our genetic results suggest that the evolutionary elaboration of the enclosed brood pouch elsewhere in the family was probably not in response to selection pressures on pregnant males to avoid fertilization thievery. With regard to maternity assignments, our genotypic data are consistent with behavioural observations indicating that females sometimes mate with more than one male during a breeding episode, and that each male carries eggs from a single female. Thus, the polyandrous genetic mating system in this species parallels the social mating system, and both are consistent with a more intense sexual selection operating on females, and the elaboration of secondary sexual characters in that gender.

Animals↗

PubMed1998

Correlated response, competition, and female canine size in primates.

Recently, comparative analyses of female canine tooth size in primates have yielded two hypotheses to explain interspecific variation in female relative canine size. Greenfield ([1992] Int. J. Primatol. 13:631-657; [1992] Yrbk. Phys. Anthropol. 35:153-184; [1996] J. Hum. Evol. 31:1-19) suggested that covariation in male and female canine size across species indicates that female canine size reflects correlated response (in which the expression of a trait in one sex causes the expression of the same trait in the other sex). Plavcan et al. ([1995] J. Hum. Evol. 28:245-276) noted that female canine size in primates is associated with variation in categorical estimates of the intensity of female-female agonistic competition, suggesting that selection favors large female canine size in many species. While it may seem that the two models are in conflict, they are not. To simultaneously evaluate these two models, this analysis examines the joint relations between male canine size, female canine size, and estimates of female-female competition in a sample of 108 primate species. Overall, female canine size is correlated with variation in male canine size. Controlling for variation in male canine size, female canine size is also correlated with estimates of the intensity of female-female agonistic competition. The relation between these variables differs strongly between anthropoid and strepsirhine primates. In anthropoids, the data suggest that selection for the development of large canines in females is not constrained by any affect of correlated response. In strepsirhines, the evidence suggests that sexual selection may affect male canine size but that correlated response affects female canine size, resulting in monomorphism for most species. These observations help reconcile the observations of Greenfield ([1992] Int. J. Primatol. 13:631-657; [1996] J. Hum. Evol. 31:1-19) and Plavcan et al. ([1995] J. Hum. Evol. 28:245-276) and provide a more precise model for understanding interspecific variation in female canine size and hence canine dimorphism.

Animals↗

PubMed2024

Rapid vertebrate speciation via isolation, bottlenecks, and drift.

Speciation is often driven by selective processes like those associated with viability, mate choice, or local adaptation, and "speciation genes" have been identified in many eukaryotic lineages. In contrast, neutral processes are rarely considered as the primary drivers of speciation, especially over short evolutionary timeframes. Here, we describe a rapid vertebrate speciation event driven primarily by genetic drift. The White Sands pupfish (Cyprinodon tularosa) is endemic to New Mexico's Tularosa Basin where the species is currently managed as two Evolutionarily significant units (ESUs) and is of international conservation concern (Endangered). Whole-genome resequencing data from each ESU showed remarkably high and uniform levels of differentiation across the entire genome (global FST ≈ 0.40). Despite inhabiting ecologically dissimilar springs and streams, our whole-genome analysis revealed no discrete islands of divergence indicative of strong selection, even when we focused on an array of candidate genes. Demographic modeling of the joint allele frequency spectrum indicates the two ESUs split only ~4 to 5 kya and that both ESUs have undergone major bottlenecks within the last 2.5 millennia. Our results indicate the genome-wide disparities between the two ESUs are not driven by divergent selection but by neutral drift due to small population sizes, geographic isolation, and repeated bottlenecks. While rapid speciation is often driven by natural or sexual selection, here we show that isolation and drift have led to speciation within a few thousand generations. We discuss these evolutionary insights in light of the conservation management challenges they pose.

Animals↗

PubMed2005

The influence of territoriality and mating system on the evolution of male care: a phylogenetic study on fish.

Evolution of male care is still poorly understood. Using phylogenetically matched-pairs comparisons we tested for effects of territoriality and mating system on male care evolution in fish. All origins of male care were found in pair-spawning species (with or without additional males such as sneakers) and none were found in group-spawning species. However, excluding group spawners, male care originated equally often in pair-spawning species with additional males as in strict pair-spawning species. Evolution of male care was also significantly related to territoriality. Yet, most pair-spawning taxa with male care are also territorial, making their relative influence difficult to separate. Furthermore, territoriality also occurs in group-spawning species. Hence, territoriality is not sufficient for male care to evolve. Rather, we argue that it is the combination of territoriality and pair spawning with sequential polygyny that favours the evolution of male care, and we discuss our results in relation to paternity assurance and sexual selection.

Animals↗

PubMedDate not supplied

Epidemiologic study of prostatic cancer: preliminary report.

In a preliminary report on a small fraction of the projected subject population, some evidence in support of both sexual activity and venereal transmission hypotheses is accumulating. Prostatic cancer cases, in contrast to hospitalized and/or neighborhood controls, are beginning to show greater proportions of selected sexual activities compatible with venereal transmission of an infectious agent, such as number of sexual partners, use of prostitutes, prior venereal disease, and genital infections in the spouse. Patients with prostatic cancer also appear to have had higher fertility and more prostatic cancer in blood relatives than controls. Age at first intercourse and at first marriage are lower among the cancer patients than among the controls. Antibody titrations for herpesvirus and cytomegalic virus, although currently not revealing striking disparities in positivity, tend to show higher titers among the cancer cases.

Adult↗

PubMed2002

Homosexual tandem running as selfish herd in Reticulitermes speratus: novel antipredatory behavior in termites.

We investigated the predator avoidance mechanism of post-swarming alates of the lower subterranean termite, Reticulitermes speratus Kolbe. In some lower termites, homosexual tandem running is observed in addition to ordinary heterosexual tandem running. An experiment designed to compare the risk of predation by a termite-hunting ant, Brachyponera chinensis Emery, showed that homosexual tandem running reduced the predation risk until termites encounter the opposite sex. Since an individual ant cannot capture two dealates at once, one of the two dealates forming a tandem can escape while the ant captures its partner. Therefore, the "post-encounter risk" of individuals running in tandem was lower than that of single individuals. The "encounter risk" with predatory ants was also examined using a mathematical model considering the increased detectability of the predator due to enhanced size of the prey unit. It was suggested that tandem running reduces the predation risk of both participants, even when the enhanced encounter risk was taken into account. In males, competition for the back position was often observed, and consequently, the male at the back was always larger than the male in front. When a male-male tandem encountered a female, the back male won the female more often than the front male. This result suggested that male-male tandem running should result in selection pressure in favor of vigorous males. In conclusion, tandem running decreases the individual predation risk through the dilution effect, and it also plays a role as a mechanism of indirect sexual selection.

Animals↗

PubMed2002

Possible roles for products of polymorphic MHC and linked olfactory receptor genes during selection processes in reproduction.

PROBLEM: Polymorphic genes of the human major histocompatibility complex [MHC; human leukocyte antigen (HLA)] are probably important in determining resistance to parasites and avoidance of inbreeding. We investigated whether HLA-associated sexual selection could also involve HLA-linked olfactory receptor (OR) genes, which might not only participate in olfaction-guided mate choice, but also in selection processes within the testis. METHOD OF STUDY: The testicular expression status of HLA class I molecules (by immunohistology) and HLA-linked OR genes (by transcriptional analysis) was determined. RESULTS: Various HLA class I heavy chains, but not beta2-microglobulin (beta2m), were expressed, mainly at the spermatocyte I stage. Of 17 HLA-linked OR genes analyzed, eight were found to be transcribed in the testis. They exhibited varying numbers of 5'- or 3'-non-coding exons as well as differential splicing. CONCLUSIONS: We suggest that testis-expressed polymorphic HLA and OR proteins are functionally connected and serve the selection of spermatozoa, enabling them to distinguish 'self from 'non-self [the sperm-receptor-selection (SRS) hypothesis].

Epithelium↗

PubMed2000

Male calling song provides a reliable signal of immune function in a cricket.

A critical prediction of the immunocompetence handicap hypothesis is that the expression of secondary sexual traits should be positively correlated with pathogen resistance ability This correlation is necessary if females are to be able to use a particular sexual trait as an indicator of a male's resistance ability. In this study we document a positive correlation between a sexually selected component of the calling song of male house crickets (the number of syllables per chirp) and haemocyte load, an important determinant of the ability to encapsulate pathogens in insects. The results indicate that, by favouring males which produce more syllables per chirp, females may also select males with higher pathogen resistance ability, potentially generating either direct or indirect selection on female mating preferences.

Animals↗

PubMed2002

Degree of male ornamentation affects female preference for conspecific versus heterospecific males.

Several studies have shown female preference for conspecific males with the attached artificial ornaments of more elaborate heterospecifics. However, preference for heterospecifics under natural conditions is relatively rare. We tested what factors affect behavioural mechanisms of species isolation using three species of estrildid finch (genus Uraeginthus) that occur in both sympatry and allopatry. These finches differ in degree of sexual dimorphism; male ornamentation; behavioural and morphological similarity; and phylogenetic distance. Paired mate-choice trials were used in which females were presented with a conspecific and heterospecific male to test which of the above between-species differences best predicted the degree of premating isolation. The three species differed in the degree of species-specific mate preference shown. Females from the brighter two species discriminated against dull males, independently of sympatry-allopatry, similarity and phylogenetic distance. Females from the dull species reacted to conspecific males and brighter heterospecific males equally strongly, independently of similarity and phylogenetic distance. In contrast to previous studies, an equal preference for heterospecific and conspecific males was found under natural conditions. It is suggested that differences between closely related species in male ornamentation affect the likelihood that premating isolation will occur due to the fact that sexual selection tends to drive preferences for exaggerated ornamentation.

Animals↗

PubMed2006

Linking color polymorphism maintenance and speciation.

Here, we review the recently burgeoning literature on color polymorphisms, seeking to integrate studies of the maintenance of genetic variation and the evolution of reproductive isolation. Our survey reveals that several mechanisms, some operating between populations and others within them, can contribute to both color polymorphism persistence and speciation. As expected, divergent selection clearly can couple with gene flow to maintain color polymorphism and mediate speciation. More surprisingly, recent evidence suggests that diverse forms of within-population sexual selection can generate negative frequency dependence and initiate reproductive isolation. These findings deserve additional study, particularly concerning the roles of heterogeneous visual environments and correlational selection. Finally, comparative studies and more comprehensive approaches are required to elucidate when color polymorphism evolves, persists, or leads to speciation.

Animals↗

PubMed2000

A gender-specific intervention for at-risk women in the USA.

Women are the fastest growing group in the USA to become infected with HIV. Also, the mode of transmission is changing with heterosexual behaviour being the predominant source. As these changes occur, HIV infection becomes more common in women who have not typically been considered at high risk. This paper describes an intervention designed to decrease unsafe sexual encounters and to focus on a highly meaningful concern in the lives of these women: relationships with men. The sessions encouraged making decisions, choice, partner selection, sexual rights, refusal of any unwanted sex, female controlled methods and other elements of empowerment. An eight-session and a four-session curriculum were created to assess dose factors as well.

Adolescent↗