Sexual Size Dimorphism,Canine Dimorphism,and Male-Male Competition in Primates

Sexual size dimorphism is generally associated with sexual selection via agonistic male competition in nonhuman primates. These primate models play an important role in understanding the origins and evolution of human behavior. Human size dimorphism is often hypothesized to be associated with high rates of male violence and polygyny. This raises the question of whether human dimorphism and patterns of male violence are inherited from a common ancestor with chimpanzees or are uniquely derived. Here I review patterns of, and causal models for, dimorphism in humans and other primates. While dimorphism in primates is associated with agonistic male mate competition, a variety of factors can affect male and female size, and thereby dimorphism. The causes of human sexual size dimorphism are uncertain, and could involve several non-mutually-exclusive mechanisms, such as mate competition, resource competition, intergroup violence, and female choice. A phylogenetic reconstruction of the evolution of dimorphism, including fossil hominins, indicates that the modern human condition is derived. This suggests that at least some behavioral similarities with Pan associated with dimorphism may have arisen independently, and not directly from a common ancestor.     

The Relationship Between Objective Sperm Competition Risk and Men’s Copulatory Interest Is Moderated by Partner’s Time Spent with Other Men

Men who spend a greater proportion of time apart from their female partner since the couple’s last copulation are at greater “objective” sperm competition risk. We propose a novel cue to sperm competition risk: the time she spends with her male friends. Four hundred and twenty men in a committed, heterosexual, sexual relationship completed a questionnaire. The results indicate that men at greater objective sperm competition risk report less time desired until the couple’s next copulation, greater interest in copulating with their partner, and greater anger, frustration, and upset in response to their partner’s sexual rejection, but only among men whose partner spends more time with her male friends. These results remain after controlling statistically for the participant’s age and their partner’s age. We discuss limitations of the current research, and discuss how research in human sperm competition can inform social issues, including men’s partner-directed sexual coercion.     

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.     

The relative importance of intra- and intersexual selection on human male sexually dimorphic traits

Recent evidence suggests that in sexual selection on human males, intrasexual competition plays a larger role than female choice. In a sample of men (N?=?164), we sought to provide further evidence on the effects of men's physical dominance and sexual attractiveness on mating success and hence in sexual selection. Objective measures and subjective ratings of male sexually dimorphic traits purportedly under sexual selection (height, vocal and facial masculinity, upper body size from 3D scans, physical strength, and baseline testosterone) and observer perceptions of physical dominance and sexual attractiveness based on self-presentation video recordings were assessed and associated with mating success (sociosexual behaviour and number of potential conceptions) in a partly longitudinal design. Results from structural equation models and selection analyses revealed that physical dominance, but not sexual attractiveness, predicted mating success. Physical dominance mediated associations of upper body size, physical strength, as well as vocal and facial physical dominance and attractiveness with mating success. These findings thus suggest a greater importance of intrasexual competition than female choice in human male sexual selection.     

A sexual selection model for hominid evolution

The following paper develops a sexual selection model for the evolution of bipedal locomotion, canine reduction, brain enlargement, language and higher intelligence. The model involves an expansion of Darwin’s ideas about human evolution based on recent elaborations of sexual selection theory. Modern notions about intrasexual competition and female and male choice and their ecological correlates are summarized along with a new model for the role of sexual selection in speciation. Rapid evolution of bipedal locomotion as a male adaptation for nuptial feeding of females is proposed as a model for ape-hominid divergence through sexual selection; canine reduction is attributed to selection for associated epigamic displays. The analogy with male specialization through sexual selection speciation in hamadryas baboons is noted. Subsequent changes in female reproductive physiology are attributed to female competition for increased male parental investment during the time of early Homo andHomo erectus. The origin of higher intellectual and language abilities inHomo sapiens is attributed to male competition through technology and rule production to control resources and females; intellectual abilities involved in social manipulation are attributed to female competition for male parental investment and maintenance of polyandry. The course of hominid evolution is characterized as involving a trend from a promiscuous mating system toward increasing intensity of adaptations for male control of females, and by increasing intensity of female adaptation to maintain male parental investment while circumventing male control.     

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.     

Divergent sexual selection via male competition: ecology is key

Sexual selection and ecological differences are important drivers of speciation. Much research has focused on female choice, yet the role of male competition in ecological speciation has been understudied. Here, we test how mating habitats impact sexual selection and speciation through male competition. Using limnetic and benthic species of threespine stickleback fish, we find that different mating habitats select differently on male traits through male competition. In mixed habitat with both vegetated and open areas, selection favours two trait combinations of male body size and nuptial colour: large with little colour and small with lots of colour. This matches what we see in reproductively isolated stickleback species, suggesting male competition could promote trait divergence and reproductive isolation. In contrast, when only open habitat exists, selection favours one trait combination, large with lots of colour, which would hinder trait divergence and reproductive isolation. Other behavioural mechanisms in male competition that might promote divergence, such as avoiding aggression with heterospecifics, are insufficient to maintain separate species. This work highlights the importance of mating habitats in male competition for both sexual selection and speciation.     

Female remating, sperm competition and sexual selection in Drosophila

Female remating is fundamental to evolutionary biology as it determines the pattern of sexual selection and sexual conflict. Remating in females is an important component of Drosophila mating systems because it affects sperm usage patterns and sexual selection. Remating is common in females of many species of Drosophila in both natural and laboratory populations. It has been reported in many insect species and also in vertebrates. Female remating is a prerequisite for sperm competition between males, and the consequences of this competition, such as sperm precedence or sperm displacement, have been reported for many species of Drosophila. Female remating is dependent on the amount of sperm stored, the male seminal fluid components, nutrition, the quantity of eggs laid, experimental design and density of flies in laboratory. Remating by a female is an insurance against male sterility and sub-fertility and increases genetic heterogeneity of female offspring. Remating gives greater female productivity in many species of Drosophila. We examined female remating with respect to sperm competition and sexual selection in Drosophila and addressed the possible benefits for females. We also reviewed the role of accessory gland fluid in remating, costs associated with remating, the genetic basis of female remating and some possible mechanisms of sperm competition in the light of last male sperm priority and paternity assurance in Drosophila and other insects. We also suggest future areas of research.     

Sexual conflict resulting from adaptations to sperm competition

Recent research on diverse animal taxa has revealed that male adaptations to sperm competition often lead to a conflict with female interests. That is, male attempts to increase their own fertilization success can result in a reduction of female fitness. This sexual conflict has led to selection for a variety of female adaptations that apparently reduce male-imposed costs. Understanding the causes and consequences of sexual conflict arising from adaptations to sperm competition offers much potential for new insight into the coevolution of male and female sexual strategies.     

Sexual selection in complex communities: Integrating interspecific reproductive interference in structured populations

The social structure of populations plays a key role in shaping variation in sexual selection. In nature, sexual selection occurs in communities of interacting species; however, heterospecifics are rarely included in characterizations of social structure. Heterospecifics can influence the reproductive outcomes of intrasexual competition by interfering with intraspecific sexual interactions (interspecific reproductive interference [IRI]). We outline the need for studies of sexual selection to incorporate heterospecifics as part of the social environment. We use simulations to show that classic predictions for the effect of social structure on sexual selection are altered by an interaction between social structure and IRI. This interaction has wide‐ranging implications for patterns of sexual conflict and kin‐selected reproductive strategies in socially structured populations. Our work bridges the gap between sexual selection research on social structure and IRI, and highlights future directions to study sexual selection in interacting communities.     

Female competition and its evolutionary consequences in mammals

Following Darwin's original insights regarding sexual selection, studies of intrasexual competition have mainly focused on male competition for mates; by contrast, female reproductive competition has received less attention. Here, we review evidence that female mammals compete for both resources and mates in order to secure reproductive benefits. We describe how females compete for resources such as food, nest sites, and protection by means of dominance relationships, territoriality and inter‐group aggression, and by inhibiting the reproduction of other females. We also describe evidence that female mammals compete for mates and consider the ultimate causes of such behaviour, including competition for access to resources provided by mates, sperm limitation and prevention of future resource competition. Our review reveals female competition to be a potentially widespread and significant evolutionary selection pressure among mammals, particularly competition for resources among social species for which most evidence is currently available. We report that female competition is associated with many diverse adaptations, from overtly aggressive behaviour, weaponry, and conspicuous sexual signals to subtle and often complex social behaviour involving olfactory signalling, alliance formation, altruism and spite, and even cases where individuals appear to inhibit their own reproduction. Overall, despite some obvious parallels with male phenotypic traits favoured under sexual selection, it appears that fundamental differences in the reproductive strategies of the sexes (ultimately related to parental investment) commonly lead to contrasting competitive goals and adaptations. Because female adaptations for intrasexual competition are often less conspicuous than those of males, they are generally more challenging to study. In particular, since females often employ competitive strategies that directly influence not only the number but also the quality (survival and reproductive success) of their own offspring, as well as the relative reproductive success of others, a multigenerational view ideally is required to quantify the full extent of variation in female fitness resulting from intrasexual competition. Nonetheless, current evidence indicates that the reproductive success of female mammals can also be highly variable over shorter time scales, with significant reproductive skew related to competitive ability. Whether we choose to describe the outcome of female reproductive competition (competition for mates, for mates controlling resources, or for resources per se) as sexual selection depends on how sexual selection is defined. Considering sexual selection strictly as resulting from differential mating or fertilisation success, the role of female competition for the sperm of preferred (or competitively successful) males appears particularly worthy of more detailed investigation. Broader definitions of sexual selection have recently been proposed to encompass the impact on reproduction of competition for resources other than mates. Although the merits of such definitions are a matter of ongoing debate, our review highlights that understanding the evolutionary causes and consequences of female reproductive competition indeed requires a broader perspective than has traditionally been assumed. We conclude that future research in this field offers much exciting potential to address new and fundamentally important questions relating to social and mating‐system evolution.     

Understanding reversals in the relative strength of sexual selection on males and females: a role for sperm competition?

Sperm competition affects sexual selection intensity on males, but models suggest it cannot affect the relative intensity of sexual selection on males compared to females. However, if sperm competition depresses the payoff for male multiple mating, it could affect the relative intensity of sexual selection and even cause sexual selection to be more intense on females than males (reversal of typical pattern). To evaluate how sperm competition, energy availability, and parental investment affect the intensity of sexual selection on each sex, I constructed a simulation model using the relationship between fecundity and number of mates to estimate sexual selection gradients. Unlike earlier models, I include a trade-off between paternal investment and sperm competition ability. The amount of energy available for reproduction affects the sexual selection gradient for each sex. Reversals in the sex experiencing stronger sexual selection do occur when additional paternal investment reduces a male's ability to compete for fertilizations within females. The shape of the distribution of mates for each sex (determined by mate competition) is also important. Output from the model is qualitatively similar to empirical data from insects with paternal investment. This model challenges previous thinking about the role of sperm competition in sex-role reversal.     

Sexual selection and animal personality

Consistent individual behavioural tendencies, termed “personalities”, have been identified in a wide range of animals. Functional explanations for personality have been proposed, but as yet, very little consideration has been given to a possible role for sexual selection in maintaining differences in personality and its stability within individuals. We provide an overview of the available literature on the role of personality traits in intrasexual competition and mate choice in both human and non‐human animals and integrate this into a framework for considering how sexual selection can generate and maintain personality. For this, we consider the evolution and maintenance of both main aspects of animal personality: inter‐individual variation and intra‐individual consistency.     

Sexual competition and N supply interactively affect the dimorphism and competiveness of opposite sexes in Populus cathayana

Several important dioecious species show sexual spatial segregation (SSS) along environmental gradients that have significant ecological effect on terrestrial ecosystem. However, little attention has been paid to understanding of how males and females respond to environmental gradients and sexual competition. We compared eco‐physiological parameters of males and females of Populus cathayana under different sexual competition patterns and nitrogen (N) supply levels. We found that males and females interacting with the same or opposite sex showed significant differences in biomass partition, photosynthetic capacity, carbon (C) and N metabolism, and leaf ultrastructure, and that the sexual differences to competition were importantly driven by N supply. The intersexual competition was enhanced under high N, while the intrasexual competition among females was amplified under low N. Under high N, the intersexual competition stimulated the growth of the females and negatively affected the males. In contrast, under low N, the males exposed to intrasexual competition had the highest tolerance, whereas females exposed to intrasexual competition showed the lowest adaptation among all competition patterns. Sexual competition patterns and N supply levels significantly affected the sexual dimorphism and competitiveness, which may play an important role in spatial segregation of P. cathayana populations.     

Polyandry as a mediator of sexual selection before and after mating

The Darwin–Bateman paradigm recognizes competition among males for access to multiple mates as the main driver of sexual selection. Increasingly, however, females are also being found to benefit from multiple mating so that polyandry can generate competition among females for access to multiple males, and impose sexual selection on female traits that influence their mating success. Polyandry can reduce a male''s ability to monopolize females, and thus weaken male focused sexual selection. Perhaps the most important effect of polyandry on males arises because of sperm competition and cryptic female choice. Polyandry favours increased male ejaculate expenditure that can affect sexual selection on males by reducing their potential reproductive rate. Moreover, sexual selection after mating can ameliorate or exaggerate sexual selection before mating. Currently, estimates of sexual selection intensity rely heavily on measures of male mating success, but polyandry now raises serious questions over the validity of such approaches. Future work must take into account both pre- and post-copulatory episodes of selection. A change in focus from the products of sexual selection expected in males, to less obvious traits in females, such as sensory perception, is likely to reveal a greater role of sexual selection in female evolution.     

Quantifying the strength and form of sexual selection on men's traits

Although recent research has increasingly focused on human sexual selection, fundamental questions remain concerning the relative influence of individual traits on success in competition for mates and the mechanisms, form, and direction of these sexual selective pressures. Here, we explore sexual selection on men's traits by ascertaining men's dominance and attractiveness from male and female acquaintances. On a large American university campus, 63 men from two social fraternities provided anthropometric measurements, facial photographs, voice recordings, and reported mating success (number of sexual partners). These men also assessed each other's dominance, and 72 women from two socially affiliated sororities assessed the men's attractiveness. We measured facial masculinity from inter-landmark distances and vocal masculinity from acoustic parameters. We additionally obtained facial and vocal attractiveness and dominance ratings from unfamiliar observers. Results indicate that dominance and the traits associated with it predict men's mating success, but attractiveness and the traits associated with it do not. These findings point to the salience of contest competition on men's mating success in this population.     

Disruptive natural selection predicts divergence between the sexes during adaptive radiation

Evolution of sexual dimorphism in ecologically relevant traits, for example, via resource competition between the sexes, is traditionally envisioned to stall the progress of adaptive radiation. An alternative view is that evolution of ecological sexual dimorphism could in fact play an important positive role by facilitating sex‐specific adaptation. How competition‐driven disruptive selection, ecological sexual dimorphism, and speciation interact during real adaptive radiations is thus a critical and open empirical question. Here, we examine the relationships between these three processes in a clade of salamanders that has recently radiated into divergent niches associated with an aquatic life cycle. We find that morphological divergence between the sexes has occurred in a combination of head shape traits that are under disruptive natural selection within breeding ponds, while divergence among species means has occurred independently of this disruptive selection. Further, we find that adaptation to aquatic life is associated with increased sexual dimorphism across taxa, consistent with the hypothesis of clade‐wide character displacement between the sexes. Our results suggest the evolution of ecological sexual dimorphism may play a key role in niche divergence among nascent species and demonstrate that ecological sexual dimorphism and ecological speciation can and do evolve concurrently in the early stages of adaptive radiation.     

Sexual selection and its evolutionary consequences in female animals

For sexual selection to act on a given sex, there must exist variation in the reproductive success of that sex as a result of differential access to mates or fertilisations. The mechanisms and consequences of sexual selection acting on male animals are well documented, but research on sexual selection acting on females has only recently received attention. Controversy still exists over whether sexual selection acts on females in the traditional sense, and over whether to modify the existing definition of sexual selection (to include resource competition) or to invoke alternative mechanisms (usually social selection) to explain selection acting on females in connection with reproduction. However, substantial evidence exists of females bearing characters or exhibiting behaviours that result in differential reproductive success that are analogous to those attributed to sexual selection in males. Here we summarise the literature and provide substantial evidence of female intrasexual competition for access to mates, female intersexual signalling to potential mates, and postcopulatory mechanisms such as competition between eggs for access to sperm and cryptic male allocation. Our review makes clear that sexual selection acts on females and males in similar ways but sometimes to differing extents: the ceiling for the elaboration of costly traits may be lower in females than in males. We predict that current and future research on female sexual selection will provide increasing support for the parsimony and utility of the existing definition of sexual selection.     

Sexual dimorphism in postcranial skeletal shape suggests male-biased specialization for physical competition in anthropoid primates

Sexual dimorphism often arises as a response to selection on traits that improve a male's ability to physically compete for access to mates. In primates, sexual dimorphism in body mass and canine size is more common in species with intense male–male competition. However, in addition to these traits, other musculoskeletal adaptations may improve male fighting performance. Postcranial traits that increase strength, agility, and maneuverability may also be under selection. To test the hypothesis that males, as compared to females, are more specialized for physical competition in their postcranial anatomy, we compared sex-specific skeletal shape using a set of functional indices predicted to improve fighting performance. Across species, we found significant sexual dimorphism in a subset of these indices, indicating the presence of skeletal shape sexual dimorphism in our sample of anthropoid primates. Mean skeletal shape sexual dimorphism was positively correlated with sexual dimorphism in body size, an indicator of the intensity of male–male competition, even when controlling for both body mass and phylogenetic relatedness. These results suggest that selection on male fighting ability has played a role in the evolution of postcranial sexual dimorphism in primates.