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PubMed2005

The maintenance of sex as a developmental trap due to sexual selection.

The writings of George Williams challenged biologists to think critically about levels of selection, social behavior, and the paradox of sex, whose maintenance by recombination alone has not been convincingly demonstrated in theory or in fact. A solution is suggested by observations of the dependence of females on interaction with males, as a result of sexual selection. This, along with recombination in changing environments and DNA repair during meiosis, may contribute to a pluralistic explanation for the maintenance of sex.

Animals↗

PubMed1999

Sexual selection for white tail spots in the barn swallow in relation to habitat choice by feather lice.

Many bird species have white spots in their tails or wing feathers, and such characters have been hypothesized to be either reliable signals (handicaps) or amplifiers that facilitate the message of a signal. In barn swallows, Hirundo rustica, the size of the white spots in the tail feathers is sexually dimorphic and positively correlated with feather length. We tested whether such spots act as handicaps or amplifiers. These white spots affect sexual selection in barn swallows, as shown by an experiment in which we randomly subjected males to (1) a considerable reduction of the size of all the spots by the use of a black permanent marker pen, (2) a small reduction of the size of the spots, or (3) no reduction. There was a positive association between spot size and the number of offspring produced per season. The white tail spots were preferred by feather-eating Mallophaga as a feeding site: holes made by Mallophaga were more abundant in the white spots than expected by chance. A habitat choice experiment with Mallophaga on barn swallow tail feathers revealed that they preferred white spots over black parts of the tail feathers. We therefore expected long-tailed male barn swallows to have more Mallophaga than short-tailed males. However, the opposite relationship was observed, indicating that long-tailed males may reliably signal their quality by the presence of large white tail spots without parasite damage. Thus white tail spots in barn swallows appear to be a reliable signal of phenotypic quality. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗

PubMed1981

A general analysis of genetic models with frequency-dependent mating. II. Sexual selection for heterozygotes.

Heterozygotes are assumed to mate with a frequency that is any general function, f(v), of their population frequency, v. Models are analysed in which the selection that determines the function f(v) acts either on one sex alone or on both sexes equally. The central equilibrium point v* = 1/2 always exists; it is stable if f(1/2) greater than 1/2. If the central equilibrium is unstable, other asymmetric equilibria may be stable; the fixation states may also be stable. This general analysis is applied to a number of specific models of sexual selection. The models give qualitatively different results. The outcome of selection in population cage experiments could be used to test the alternative models.

Animals↗

PubMed2001

Sexual selection at the protein level drives the extraordinary divergence of sex-related genes during sympatric speciation.

An increasing number of molecular studies are indicating that, in a wide variety of species, genes directly related to fertilization evolve at extraordinarily high rates. We try to gain insight into the dynamics of this rapid evolution and its underlying mechanisms by means of a simple theoretical model. In the model, sexual selection and sympatric speciation act together in order to drive rapid divergence of gamete recognition proteins. In this process, intraspecific competition for fertilizations enlarges male gamete protein variation by means of evolutionary branching, which initiates sympatric speciation. In addition, avoidance of competition for fertilizations between the incipient species drives the rapid evolution of gamete recognition proteins. This mechanism can account for both strong stabilizing selection on gamete recognition proteins within species and rapid divergence between species. Moreover, it can explain the empirical finding that the rate of divergence of fertilization genes is not constant, but highest between closely related species.

Animals↗

PubMed2006

Field use of maximal sprint speed by collared lizards (Crotaphytus collaris): compensation and sexual selection.

To understand how selection acts on performance capacity, the ecological role of the performance trait being measured must be determined. Knowing if and when an animal uses maximal performance capacity may give insight into what specific selective pressures may be acting on performance, because individuals are expected to use close to maximal capacity only in contexts important to survival or reproductive success. Furthermore, if an ecological context is important, poor performers are expected to compensate behaviorally. To understand the relative roles of natural and sexual selection on maximal sprint speed capacity we measured maximal sprint speed of collared lizards (Crotaphytus collaris) in the laboratory and field-realized sprint speed for the same individuals in three different contexts (foraging, escaping a predator, and responding to a rival intruder). Females used closer to maximal speed while escaping predators than in the other contexts. Adult males, on the other hand, used closer to maximal speed while responding to an unfamiliar male intruder tethered within their territory. Sprint speeds during foraging attempts were far below maximal capacity for all lizards. Yearlings appeared to compensate for having lower absolute maximal capacity by using a greater percentage of their maximal capacity while foraging and escaping predators than did adults of either sex. We also found evidence for compensation within age and sex classes, where slower individuals used a greater percentage of their maximal capacity than faster individuals. However, this was true only while foraging and escaping predators and not while responding to a rival. Collared lizards appeared to choose microhabitats near refugia such that maximal speed was not necessary to escape predators. Although natural selection for predator avoidance cannot be ruled out as a selective force acting on locomotor performance in collared lizards, intrasexual selection for territory maintenance may be more important for territorial males.

Aging↗

PubMed2000

Human sexual selection, good genes, and special design.

Evolutionary psychology seeks to understand the functional design underlying psychological processes and behavior. Theories of selection pressures developed within evolutionary biology are key components of this understanding. Because past selection pressures responsible for current design cannot be directly observed, theoretical understandings of the psychological processes and behavior must be inferred. The most important epistemological concept within evolutionary psychology is that of special design-evidence that a feature exhibits specificity, efficiency, and economy for producing a particular beneficial effect. A variety of sexual-selection processes have been proposed to account for aspects of human mating. These processes are not mutually exclusive. More than one may account for aspects of human mating. A core task of evolutionary psychology within this domain is to identify which processes account for which phenomena. I have attempted to illustrate how the search for special design is central to this endeavor.

Adaptation, Physiological↗

PubMed2006

Post-mating sexual selection increases lifetime fitness of polyandrous females in the wild.

Females often mate with several males before producing offspring. Field studies of vertebrates suggest, and laboratory experiments on invertebrates confirm, that even when males provide no material benefits, polyandry can enhance offspring survival. This enhancement is widely attributed to genetic benefits that arise whenever paternity is biased towards males that sire more viable offspring. Field studies suggest that post-mating sexual selection biases fertilization towards genetically more compatible males and one controlled experiment has shown that, when females mate with close kin, polyandry reduces the relative number of inbred offspring. Another potential genetic benefit of polyandry is that it increases offspring survival because males with more competitive ejaculates sire more viable offspring. Surprisingly, however, there is no unequivocal evidence for this process. Here, by experimentally assigning mates to females, we show that polyandry greatly increases offspring survival in the Australian marsupial Antechinus stuartii. DNA profiling shows that males that gain high paternity under sperm competition sire offspring that are more viable. This beneficial effect occurs in both the laboratory and the wild. Crucially, there are no confounding non-genetic maternal effects that could arise if polyandry increases female investment in a particular reproductive event because A. stuartii is effectively semelparous. Our results therefore show that polyandry improves female lifetime fitness in nature. The threefold increase in offspring survival is not negated by a decline in maternal lifespan and is too large to be offset by an equivalent decline in the reproductive performance of surviving offspring.

Animals↗

PubMed2004

Disposable-soma senescence mediated by sexual selection in an ungulate.

Senescence may result from an optimal balance between current reproductive investment and bodily repair processes required for future reproduction, a theoretical prediction difficult to prove especially in large, long-lived animals. Here we propose that teeth that have fixed dimensions early in life, but that wear during chewing, can be taken as a measure of total lifetime 'repair', and their wear rate as a measure of current expenditure in performance. Our approach also considers the sexual selection process to investigate the advance of senescence in males compared with females, when selection favouring competition over mates reduces the reproductive lifespan of males. We studied carcasses of 2,141 male and 739 female red deer (Cervus elaphus) of different ages, finding that male molariform teeth emerged at a far smaller size than expected from body size dimorphism. This led to higher workload, steeper wear rate and earlier depletion of male teeth than in females, in concordance with sex-specific patterns of lifetime performance and reproduction. These findings provide the empirical support for the disposable-soma hypothesis of senescence, which predicts that investment in bodily repair will decrease when the return from this investment may not be realized as a result of other causes that limit survival or reproduction.

Aging↗

PubMedDate not supplied

Sexual selection, seminal coagulation and copulatory plug formation in primates.

This study examines the question of whether multipartner matings by female primates, with resulting sperm competition among males, may have favored the evolution of biochemical mechanisms to enhance seminal coagulationand copulatory plug formation. Comparative ratings of seminal coagulation (using a four-point scale where 1 = no coagulation and 4 = copulatory plug formation) were obtained for 40 species representing 26 primate genera. Coagulation ratings were highest (mean = 3.64) in those genera where females commonly mate with multiple partners, and lowest (mean = 2.09) in genera where females are primarily monogamous or belong to polygynous (one male) units(p < 0.0001). This result remained significant (p < 0.001) after the use of comparative analysis of independent contrasts (CAIC) to control for possible phylogenetic biases in the data set. Results indicate that sexual selection has played an important role in the evolution of seminal coagulation, and copulatory plug function, in primates.

Adaptation, Biological↗

PubMed1991

The influence of parasites on host sexual selection.

In 1982 Hamilton and Zuk(1) proposed a provocative solution for the unexplained fact that the males of many species exhibit 'showy' traits such as brightly coloured plumage or vigorous courtship displays. They suggested that showy traits are fully expressed only by males who are resistant to parasites and that females examine such traits in order to choose resistant males as mates. Hamilton and Zuk's proposal has been the topic of extensive research and vigorous debate for nearly a decade. This article reviews the research, relevant criticisms and unanswered questions pertaining to the influence of parasites on sexual selection.

Journal Article↗

PubMedDate not supplied

Sexual selection in mice. VI. Effects of fostering.

Individually, 10 male mice of both the albino (ST/bJ) and black-agouti (CBA/J) strains were taken at birth, fostered to females of the other strain, and reared with their litters. At weaning each male was paired with one of its foster brothers and placed in a separate cage. When the males were 90 days of age, a single albino (ST/bJ) female was placed in each pen. Ten litters were obtained from each competition cage, yielding a total of 100 litters for each type of fostering. In those cases where CBA males had been reared with ST litters, both males were equally successful in reproduction. However, in the reverse situation, where ST males had been reared with CBA litters, the ST males enjoyed a distinct reproductive advantage. These results are compared to those obtained in a previous sexual selection experiment, using the same strains of mice but without fostering. In addition, the results of fighting experiments between males from these strains and the implications of those results for the present study are discussed.

Animals↗

PubMed2000

Sexual selection and the evolution of costly female preferences: spatial effects.

Models of Fisher's runaway process show that if there is a cost to female preference, no preference or male trait exaggeration will evolve. Surprisingly, this is true no matter how small the cost, which reveals that these models of Fisher's process are structurally unstable (Bulmer 1989). Here a model of Fisher's runaway process is presented to demonstrate that costly female preference evolves very easily when space is explicitly included in the model. The only requirement is that the optimal male phenotype changes across the species' range. The model shows that the spatial average of the female preference and male trait reach an evolutionary equilibrium that is identical to those of nonspatial models, but that the preference and male trait can deviate greatly from these averages at any point in space. For example, if random mating results in the lowest cost to females, then at equilibrium the spatial average preference will be zero. Nevertheless, there will be some locations at which females prefer males with larger ornaments and others where they prefer males with smaller ornaments. Results also show that the structural instability of nonspatial models of Fisher's process is less of a problem in spatial models. In particular, many of the main qualitative features of cost-free spatial models of Fisher's process remain valid even when there are small costs of female preference. Finally, the model shows that abrupt changes in the optimal male phenotype across space can result in an amplification of this pattern when preference has a small cost, but it can also result in a pattern similar to reproductive character displacement. Which of these occurs depends on the magnitude of the cost of female preference. This suggests that some patterns of reproductive character displacement in nature might be explained simply by sexual selection rather than by hybrid dysgenesis and reinforcement.

Animals↗

PubMed2004

Quantitative genetic models of sexual selection.

Modeling of R.A. Fisher's ideas about the evolution of male ornamentation using quantitative genetics began in the 1980s. Following an initial period of enthusiasm, interest in these models began to wane when theoretical studies seemed to show that the rapid evolution of ornaments would not occur if there were costs associated with female mate choice. Recent theoretical work has shown, however, that runaway evolution and other kinds of extensive diversification of ornaments and preferences can occur, even when female choice is costly. These new models highlight crucial parameters that profoundly influence evolutionary trajectories, but these parameters have been neglected in empirical studies. Here, we review quantitative genetic models of sexual selection with the aim of fostering communication and synergism between theoretical and empirical enterprises. We also point out several areas in which additional empirical work could distinguish between alternative models of evolution.

Journal Article↗

PubMed2004

Constraints on evolution and postcopulatory sexual selection: trade-offs among ejaculate characteristics.

Ejaculates function as an integrated unit to ensure male fertility and paternity, can have a complex structure, and can experience multiple episodes of selection. Current studies on the evolution of ejaculates typically focus on phenotypic variation in sperm number, size, or related traits such as testes size as adaptations to postcopulatory male-male competition. However, the evolution of the integrated nature of ejaculate structure and function depends on genetic variation in and covariation between the component parts. Here we report a quantitative genetic study of the components of the ejaculate of the cockroach Nauphoeta cinerea, including those we know to experience postcopulatory sexual selection, in the context of functional integration of ejaculate characters. We use the patterns of genetic variation and covariation to infer how the integration of the functions of the ejaculate constrain and shape its evolution. Ejaculate components were highly variable, showed significant additive genetic variance, and moderate to high evolvability. The level of genetic variation in these characters, despite strong directional or truncating selection, may reflect the integration of multiple episodes of selection that occur in N. cinerea. There were few significant phenotypic correlations, but all the genetic correlations among ejaculate characters were significantly different from zero. The patterns of genetic variation and covariation suggest that there are important trade-offs among individual traits of the ejaculate and that evolution of ejaculate characteristics will not proceed unconstrained. Fully describing the genetic relationships among traits that perform as an integrated unit helps us understand how functional relationships constrain or facilitate the evolution of the complex structure that is the ejaculate.

Analysis of Variance↗

PubMed2006

An eye for detail: selective sexual imprinting in zebra finches.

To investigate the idea that sexual imprinting creates incipient reproductive isolation between phenotypically diverging populations, I performed experiments to determine whether colony-reared zebra finches would imprint on details of artificial white crests. In the first experiment, adults in one breeding colony wore white crests with a vertical black stripe, while in another colony adults wore crests having a horizontal black stripe; except for their crests, breeders possessed wild-type plumage and conformation. Offspring of both sexes reared in these colonies developed mate preferences for opposite-sexed birds wearing the crest type with which they were reared; neither sex developed a social preference for crested individuals of the same sex. In a second experiment, females reared by crested parents preferred crested males versus males with red leg bands, while control females (reared in a colony of wild-type, uncrested birds) preferred red-banded males in the same test. Results of a third experiment that used sexually dimorphic crest phenotypes indicate that both sexes of offspring imprinted on maternal crest patterns. Results support the hypothesis that sexual imprinting can facilitate isolation both by engendering a preference for population-typical traits and by prioritizing such an imprinting-based preference over species-typical preferences for other traits used in mate choice. Comparison with results of other recent studies indicates that imprinting tendencies of both sexes vary with the characteristics of traits presented as an imprinting stimuli. Tendency to imprint may vary with the perceived information content (e.g., kin, sex, or population indicator) of parental traits, a process dubbed selective sexual imprinting.

Animals↗

PubMed2000

Song, sexual selection, and a song control nucleus (HVc) in the brains of European sedge warblers.

Female sedge warblers select males that have more complex songs as mates. This study tests two predictions concerning HVc, a telencephalic nucleus that is essential for song learning and production: first, that males with more complex songs will have a larger HVc, and second that males who pair successfully will have a larger HVc than unpaired males. Data on song composition and pairing status were collected from wild sedge warblers breeding in Hungary. We found significant positive correlations between three song attributes (repertoire size, song complexity, and song length) and the size of HVc. Males that paired successfully also had more complex songs (repertoire size and song complexity, though not song length) than males that did not. However, we find no direct evidence that males who paired successfully had a larger HVc than unpaired males. These findings are discussed in relation to the possible functions of HVc and also to current views on sexual selection and the evolution of the song control pathway.

Animal Communication↗

PubMed1997

Sexually selected vigilance behaviour of the grey partridge is affected by plasma androgen levels

In the grey partridge, Perdix perdixvigilance and calling activity are two sex-dimorphic behaviours that are critical for mate choice. To ascertain whether circulating levels of testosterone directly affect vigilance (i.e. the occurrence of upright alert posture), we compared vigilance scores of testosterone-implanted versus control males both during the normal activity of the flock and after the passage of a raptor silhouette; in the latter case, the calling activity was also recorded. Hormone-treated males were more vigilant than controls in both experimental situations. Vigilance was correlated with calling rate. Testosterone seems to act as a link, relating conspicuous behaviours involved in sexual selection to male quality and physical condition, because of the costs of having high levels of both signalling and androgens.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

Journal Article↗

PubMed1998

Male parental care, differential parental investment by females and sexual selection.

Males play a variable parental role in reproduction, ranging from no male parental care to extensive male care. Females may acquire either direct or indirect fitness benefits from their mate choice, and direct fitness benefits include male parental care. Theoreticians have traditionally emphasized direct fitness benefits to females in species with extensive male parental care. We review the literature and show extensive variation in the patterns of male care, related to the attractiveness of males to females. At one extreme of this continuum, females invest differentially in parental care, investing more when paired with attractive males. The costs of female parental care and other aspects of parental investment may be balanced by benefits in terms of more attractive sons and/or more viable offspring. At the other extreme, in species with extensive direct fitness benefits, males with preferred sexual phenotypes provide the largest relative share of parental care. A comparative study of birds revealed that the extent of the differential female parental investment was directly related to the frequency of extra-pair paternity. Since extra-pair paternity may arise mainly as a consequence of female choice for indirect fitness benefits, this result supports our prediction that differential parental investment is prevalent in species where females benefit indirectly from their mate choice. The consequences for sexual selection theory of these patterns of male care in relation to male attractiveness are emphasized. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

Journal Article↗