Interest In Sexual Selection Research Articles

Sexual selection and 'species recognition' revisited: serial processing and order-of-operations in mate choice.

Following the modern synthesis, mating signals were thought of principally as species recognition traits, a view later challenged by a burgeoning interest in sexual selection-specifically mate choice. In the 1990s, these different signal functions were proposed to represent a single process driven by the shape of female preference functions across both intra- and interspecific signal space. However, the properties of reliable 'recognition' signals (stereotyped; low intraspecific variation) and informative 'quality' signals (condition dependent; high intraspecific variation) seem at odds, perhaps favouring different signal components for different functions. Surprisingly, the idea that different components of mating signals are evaluated in series, first to recognize generally compatible mates and then to select for quality, has never been explicitly tested. Here I evaluate patterns of (i) intraspecific signal variation, (ii) female preference function shape and (iii) phylogenetic signal for male cricket call components known to be processed in series. The results show that signal components processed first tend to have low variation, closed preference functions and low phylogenetic signal, whereas signal components processed later show the opposite, suggesting that mating signal evaluation follows an 'order-of-operations'. Applicability of this finding to diverse groups of organisms and sensory modalities is discussed.

Parental care and UV coloration in blue tits: opposite correlations in males and females between provisioning rate and mate’s coloration

Parental investment and sexually‐selected signals can be intimately related, either because the signals indicate the amount of investment that an individual is prepared to make, and hence its value as a mate (the ‘good parent process’), or because individuals are selected to vary their own investment in relation to their mate’s signals (‘differential allocation’ or ‘reproductive compensation’). Correlations between parental investment and the sexually selected signals of both an individual and its mate are therefore of central interest in sexual selection. Blue tits Cyanistes caeruleus are an ideal study species to investigate such correlations because they provide substantial amounts of biparental care and possess sexually‐selected structural UV coloration that seems to signal attractiveness in both sexes. We investigated whether feeding rates of male and female blue tits were correlated with either their own or their mate’s UV coloration, and whether any such correlation was affected by the sex ratio of the brood. We also investigated whether any such correlations were reflected in offspring phenotype. Feeding rates were not correlated with either sex of parent’s own UV coloration. However, they were correlated with the mate’s UV coloration, but in opposite directions in males and females: females had higher feeding rates when mated to bright UV males, implying differential allocation, while males had lower feeding rates when mated to bright UV females, implying reproductive compensation. These relationships were unaffected by the sex ratio of the brood. In addition, fledgling tarsus length, but not mass, was related to male UV coloration, and to female UV coloration in interaction with male age. These results suggest that both male and female attractiveness influence parental investment of the mate, and that this in turn affects offspring phenotype. We found no evidence for differential sex allocation.

Female sexual selection in light of the Darwin–Bateman paradigm

As sexual selection theory and research have until recently focused mainly on male behavior and traits, I fully support the intent of this review to bring forth sexual selection in females. Nonetheless, I have some objections that relate to the definition used and that the review is restricted by some of the Darwin--Bateman paradigm's notions that are clearly gender biased. Researchers working with a perspective have had a longstanding interest in sexual selection in females and have also pointed out flaws in this paradigm, such as overfocus on male behavior and traits, overemphasis on sex differences, neglect of female variation, the notion of passive females, and emphasis on competition for number and not quality of mates. The Darwin--Bateman paradigm still dominates sexual selection theory and research. Going beyond this paradigm opens up for a fuller understanding of selection on females and males and starts out with gender-neutral expectations on female and male behavior and traits. Recent theory emphasizes chance effects on reproductive success for the evolution of female and male behavior and suggests that all individuals, irrespective of sex, are flexible in their mating decisions, adjusting them moment-by-moment to social, ecological, and internal constraints. Copyright 2011, Oxford University Press.

Intrasexual competition in females: evidence for sexual selection?

In spite of recent interest in sexual selection in females, debate exists over whether traits that influence female-female competition are sexually selected. This review uses female-female aggressive behavior as a model behavioral trait for understanding the evolutionary mechanisms promoting intrasexual competition, focusing especially on sexual selection. I employ a broad definition of sexual selection, whereby traits that influence competition for mates are sexually selected, whereas those that directly influence fecundity or offspring survival are naturally selected. Drawing examples from across animal taxa, including humans, I examine 4 predictions about female intrasexual competition based on the abundance of resources, the availability of males, and the direct or indirect benefits those males provide. These patterns reveal a key sex difference in sexual selection: Although females may compete for the number of mates, they appear to compete more so for access to high-quality mates that provide direct and indirect (genetic) benefits. As is the case in males, intrasexual selection in females also includes competition for essential resources required for access to mates. If mate quality affects the magnitude of mating success, then restricting sexual selection to competition for quantity of mates may ignore important components of fitness in females and underestimate the role of sexual selection in shaping female phenotype. In the future, understanding sex differences in sexual selection will require further exploration of the extent of mutual intrasexual competition and the incorporation of quality of mating success into the study of sexual selection in both sexes.

Maintenance of variation in sexually selected traits in females: a case study using intrasexual aggression in tree swallows Tachycineta bicolor

A fundamental question in evolutionary biology is how phenotypic variation is maintained in the face of selection that ought to deplete that variation. Much research has investigated this question in traits favored via sexual selection in males, with a common solution implicating the condition dependence of sexually selected phenotypes. Despite growing interest in sexual selection on females, it is not clear if the same mechanisms maintain variation in female ornaments, weaponry or other female behaviors targeted by sexual selection. An important step in testing condition dependence in females is thus to identify whether sexually selected female phenotypes are associated with condition and also with potential costs. Here, I examine these two components of condition dependence for a sexually selected behavior, intrasexual aggression, in female tree swallows Tachycineta bicolor. I asked whether high levels of intrasexual aggression map onto natural variation in female condition and whether aggression is associated with one potential behavioral cost: performance in a vertically challenging test of flight. More aggressive females were heavier for their body size, heavier for their wing size and showed decreased flight ability, relative to less aggressive females. These findings are consistent with condition dependence, where only females in better condition are able to be highly aggressive. The association between high aggression and reduced flight ability may result from the additional lift required to power these relatively heavier birds. These associations between natural variation in aggressive behavior, morphology and flight ability are consistent with condition dependence because they confirm two basic assumptions of condition dependence: a link between aggression and condition, and a link between aggression and a behavioral cost, the speed of escape flight. As the first study to examine these assumptions for a conspicuous behavior favored by intrasexual selection in females, this study suggests broad relevance of condition dependence.

Sexual selection: Another Darwinian process

Why was sexual selection so important to Darwin? And why was it de-emphasized by almost all of Darwin's followers until the second half of the 20th century? These two questions shed light on the complexity of the scientific tradition named “Darwinism”. Darwin's interest in sexual selection was almost as old as his discovery of the principle of natural selection. From the beginning, sexual selection was just another “natural means of selection”, although different from standard “natural selection” in its mechanism. But it took Darwin 30 years to fully develop his theory, from the early notebooks to the 1871 book The Descent of Man, and Selection in Relation to Sex. Although there is a remarkable continuity in his basic ideas about sexual selection, he emphasized increasingly the idea that sexual selection could oppose the action of natural selection and be non adaptive. In time, he also gave more weight to mate choice (especially female choice), giving explicit arguments in favor of psychological notions such as “choice” and “aesthetic sense”. But he also argued that there was no strict demarcation line between natural and sexual selection, a major difficulty of the theory from the beginning. Female choice was the main reason why Alfred Russel Wallace, the co-discoverer of the principle of natural selection, engaged in a major controversy with Darwin about sexual selection. Wallace was suspicious about sexual selection in general, trying to minimize it by all sorts of arguments. And he denied entirely the existence of female choice, because he thought that it was both unnecessary and an anthropomorphic notion. This had something to do with his spiritualist convictions, but also with his conception of natural selection as a sufficient principle for the evolutionary explanation of all biological phenomena (except for the origin of mind). This is why Wallace proposed to redefine Darwinism in a way that excluded Darwin's principle of sexual selection. The main result of the Darwin–Wallace controversy was that most Darwinian biologists avoided the subject of sexual selection until at least the 1950 s, Ronald Fisher being a major exception. This controversy still deserves attention from modern evolutionary biologists, because the modern approach inherits from both Darwin and Wallace. The modern approach tends to present sexual selection as a special aspect of the theory of natural selection, although it also recognizes the big difficulties resulting from the inevitable interaction between these two natural processes of selection. And contra Wallace, it considers mate choice as a major process that deserves a proper evolutionary treatment. The paper's conclusion explains why sexual selection can be taken as a test case for a proper assessment of “Darwinism” as a scientific tradition. Darwin's and Wallace's attitudes towards sexual selection reveal two different interpretations of the principle of natural selection: Wallace's had an environmentalist conception of natural selection, whereas Darwin was primarily sensitive to the element of competition involved in the intimate mechanism of any natural process of selection. Sexual selection, which can lack adaptive significance, reveals this exemplarily.

Carotenoid maintenance handicap and the physiology of carotenoid-based signalisation of health

Despite a reasonable scientific interest in sexual selection, the general principles of health signalisation via ornamental traits remain still unresolved in many aspects. This is also true for the mechanism preserving honesty of carotenoid-based signals. Although it is widely accepted that this type of ornamentation reflects an allocation trade-off between the physiological utilisation of carotenoids (mainly in antioxidative processes) and their deposition in ornaments, some recent evidence suggests more complex interactions. Here, we further develop the models currently proposed to explain the honesty of carotenoid-based signalisation of heath status by adding the handicap principle concept regulated by testosterone. We propose that under certain circumstances carotenoids may be dangerous for the organism because they easily transform into toxic cleavage products. When reserves of other protective antioxidants are insufficient, physiological trade-offs may exist between maintenance of carotenoids for ornament expression and their removal from the body. Furthermore, we suggest that testosterone which enhances ornamentation by increasing carotenoid bioavailability may also promote oxidative stress and hence lower antioxidant reserves. The presence of high levels of carotenoids required for high-quality ornament expression may therefore represent a handicap and only individuals in prime health could afford to produce elaborate colourful ornaments. Although further testing is needed, this 'carotenoid maintenance handicap' hypothesis may offer a new insight into the physiological aspects of the relationship between carotenoid function, immunity and ornamentation.

Sexual selection research on spiders: progress and biases

The renaissance of interest in sexual selection during the last decades has fuelled an extraordinary increase of scientific papers on the subject in spiders. Research has focused both on the process of sexual selection itself, for example on the signals and various modalities involved, and on the patterns, that is the outcome of mate choice and competition depending on certain parameters. Sexual selection has most clearly been demonstrated in cases involving visual and acoustical signals but most spiders are myopic and mute, relying rather on vibrations, chemical and tactile stimuli. This review argues that research has been biased towards modalities that are relatively easily accessible to the human observer. Circumstantial and comparative evidence indicates that sexual selection working via substrate-borne vibrations and tactile as well as chemical stimuli may be common and widespread in spiders. Pattern-oriented research has focused on several phenomena for which spiders offer excellent model objects, like sexual size dimorphism, nuptial feeding, sexual cannibalism, and sperm competition. The accumulating evidence argues for a highly complex set of explanations for seemingly uniform patterns like size dimorphism and sexual cannibalism. Sexual selection appears involved as well as natural selection and mechanisms that are adaptive in other contexts only. Sperm competition has resulted in a plethora of morphological and behavioural adaptations, and simplistic models like those linking reproductive morphology with behaviour and sperm priority patterns in a straightforward way are being replaced by complex models involving an array of parameters. Male mating costs are increasingly being documented in spiders, and sexual selection by male mate choice is discussed as a potential result. Research on sexual selection in spiders has come a long way since Darwin, whose spider examples are reanalysed in the context of contemporary knowledge, but the same biases and methodological constraints have persisted almost unchanged through the current boom of research.

CLIMATIC AND TEMPORAL EFFECTS ON THE EXPRESSION OF SECONDARY SEXUAL CHARACTERS: GENETIC AND ENVIRONMENTAL COMPONENTS

Despite great interest in sexual selection, relatively little is known in detail about the genetic and environmental determinants of secondary sexual characters in natural populations. Such information is important for determining the way in which populations may respond to sexual selection. We report analyses of genetic and large-scale environmental components of phenotypic variation of two secondary sexual plumage characters (forehead and wing patch size) in the collared flycatcher Ficedula albicollis over a 22-year period. We found significant heritability for both characters but little genetic covariance between the two. We found a positive association between forehead patch size and a large-scale climatic index, the North Atlantic Oscillation (NAO) index, but not for wing patch. This pattern was observed in both cross-sectional and longitudinal data suggesting that the population response to NAO index can be explained as the result of phenotypic plasticity. Heritability of forehead patch size for old males, calculated under favorable conditions (NAO index > or = median), was greater than that under unfavorable conditions (NAO index < median). These changes occurred because there were opposing changes in additive genetic variance (VA) and residual variance (VR) under favorable and unfavorable conditions, with VA increasing and VR decreasing in good environments. However, no such effect was detected for young birds, or for wing patch size in either age class. In addition to these environmental effects on both phenotypic and genetic variances, we found evidence for a significant decrease of forehead patch size over time in older birds. This change appears to be caused by a change in the sign of viability selection on forehead patch size, which is associated with a decline in the breeding value of multiple breeders. Our data thus reveal complex patterns of environmental influence on the expression of secondary sexual characters, which may have important implications for understanding selection and evolution of these characters.

CLIMATIC AND TEMPORAL EFFECTS ON THE EXPRESSION OF SECONDARY SEXUAL CHARACTERS: GENETIC AND ENVIRONMENTAL COMPONENTS

Despite great interest in sexual selection, relatively little is known in detail about the genetic and environmental determinants of secondary sexual characters in natural populations. Such information is important for determining the way in which populations may respond to sexual selection. We report analyses of genetic and large- scale environmental components of phenotypic variation of two secondary sexual plumage characters (forehead and wing patch size) in the collared flycatcher Ficedula albicollis over a 22-year period. We found significant heritability for both characters but little genetic covariance between the two. We found a positive association between forehead patch size and a large-scale climatic index, the North Atlantic Oscillation (NAO) index, but not for wing patch. This pattern was observed in both cross-sectional and longitudinal data suggesting that the population response to NAO index can be explained as the result of phenotypic plasticity. Heritability of forehead patch size for old males, calculated under favorable conditions (NAO index ≥ median), was greater than that under unfavorable conditions (NAO index < median). These changes occurred because there were opposing changes in additive genetic variance (VA) and residual variance (VR) under favorable and unfavorable conditions, with VA increasing and VR decreasing in good environments. However, no such effect was detected for young birds, or for wing patch size in either age class. In addition to these environmental effects on both phenotypic and genetic variances, we found evidence for a significant decrease of forehead patch size over time in older birds. This change appears to be caused by a change in the sign of viability selection on forehead patch size, which is associated with a decline in the breeding value of multiple breeders. Our data thus reveal complex patterns of environmental influence on the expression of secondary sexual characters, which may have important implications for understanding selection and evolution of these characters.

Pollen Removal, Paternity, and the Male Function of Flowers

Much recent research suggests that floral characteristics of hermaphroditic plants are often selected primarily because of their effects on male reproduction. Two recent challenges to this view have been based on the fate of pollen after it is removed by pollinators, something that is rarely measured in field studies. One objection is theoretical, holding that the large variance in the success of pollen after it is removed weakens selection through the male side (Wilson et al. 1994). The other objection is empirical. In a species in which pollen removal studies had suggested that flowers are primarily male, new paternity data have been used to argue that they are not (Broyles and Wyatt 1990a, 1990b, 1995). Here I argue that both objections are invalid, the theoretical argument because it uses the wrong measure of selection, and the empirical study because a confounding variable was not controlled. Bateman (1948) provided a foundation for understanding why males often act as if they take a greater interest in having many mates than females do. This difference arises because of an asymmetry in reproductive costs. Males commonly invest little or nothing in offspring, whereas female investment may be large. Therefore, reproductive success of females is limited by resources, and mating frequently has comparatively little effect on their fitness, whereas male success is limited by access to females and may increase linearly with the number of mates. Bateman's principle helped fuel an explosion of interest in sexual selection in animals, recently summarized by Andersson (1994). Bateman believed his principle to be nearly universal and explicitly included plants among the relevant organisms. However, sexual selection research on plants lagged behind that on animals, partly for historical reasons and perhaps partly because it was unclear how to apply the ideas to hermaphroditic organisms that were both male and female (reviewed in Willson 1994). It was eventually recognized how Bateman's principle might apply to these organisms (e.g., Charnov 1979; Wilson 1979; Willson and Burley 1983). For example, consider an animal-pollinated hermaphroditic plant. It needs to attract pollinators for two reasons: to receive pollen for its own seed (female function) and to have its pollen

Quantitative Genetic Interpretations of Postpollination Reproductive Traits in Plants

Plant reproduction includes a series of critial "postpollination" stages that begin with deposition of pollen and culminate in formation of mature seeds. Interest in sexual selection, along with advances in empirical methods, has fostered efforts to characterize genetic variation of reproductive traits expressed during these stages, for example, traits of ovules, of pollen growing through pistillar tissue, and of mature seeds. However, because such phenotypes are attributable in many cases to triploid endosperms or haploid gametophytes alone or in combination with diploid tissue of the maternal plant, the standard quantitative genetic theory does not apply. The complex and nonstandard determination of postpoilination reproductive traits makes it necessary to develop models that are appropriate to the genetic participants in question. We present four models for postpollination traits and discuss them in the context of diallel and equivalent reciprocal crossing designs. These models can be used to examine hypotheses of evolutionary interest. We also propose multigeneration designs to resolve components of particular models that are confounded in data from diallels.

On the modelling of sexual selection.

THE FIRST chapter of this detailed treatment of the conceptually difficult topic of sexual selection is an admirable account of the history of thought on the subject from Darwin to the present. The second is a thoughtful review of the natural history of sexual competition and mate choice. We can heartily recommend these chapters to those with even a casual interest in sexual selection. The rest of the book will be understood only by those willing to pay really close attention to both verbal and mathematical representations. Our efforts were not as rewarding as we had hoped they would be.

RELATIONSHIP BETWEEN REPRODUCTIVE SECURITY AND INTERSEXUAL SELECTION IN A JUMPING SPIDER PHIDIPPUS JOHNSONI (ARANEAE: SALTICIDAE).

The jumping spiders (Salticidae) are noted foir their highly developed vision (Land, 1972), sexually dimorphic and often extravagant coloration, and elaborate courtship displays in which the males dance, gesture, and posture in front of the watching females (Crane, 1949). Early interest in these spiders was very much associated with debate over the importance of sexual selection (e.g., Poulton, 1890; Wallace, 1890). Peckham and Peckham (1889, 1890, 1909) described the displays of many North American species and used their observations to support the Darwinian concept that choice by females of males with particular characteristics (intersexual selection) has been an important evolutionary process. During subsequent years, interest in sexual selection apparently subsided; but after the clarification of certain concepts, such as proximate and ultimate causation (Mayr, 1961), that were blurred during earlier discussions, interest in sexual selection has again become intense (Campbell, 1972; Blum and Blum, 1979). However, current studies of salticid sexual behavior have been little affected by this renaissance. The early objections to the Peckhams' assertions concerning female choice were multiple and complex, and many of these do not presently seem very germane. However, two stand out as particularly damaging. (1) The Peckhams made many of their observations under laboratory conditions with spiders in abnormally high densities. Under these conditions, females sometimes rejected courting males, but the Peckhams provided few data concerning the characteristics of the rejected males. (2) Later workers often indicated that salticid females mate only once, that females probably encounter courting males in nature one-at-a-time, and that females tend to copulate with the first conspecific male that courts. During the course of a recent study of the mating strategy of a North American salticid, Phidippus johnsoni Peckham and Peckham (Jackson, 1980), I made some observations that are relevant to these two objections. It was found that females of this species sometimes copulate with more than one male, and two specific hypotheses were suggested by the data: (1) females of P. johnsoni discriminate between males and copulate preferentially with ones having particular characteristics, and (2) non-virgin females discriminate more strongly than virgin females.