Large males fight and court more across a range of social environments: an experiment on the two spotted goby Gobiusculus flavescens

The present study explored how male size relates to mating competition across a natural range of male and female densities in the two-spotted goby Gobiusculus flavescens. Across this range of social environments, large males were more than twice as likely as small ones to chase other males, to become nest-holders, and to court females, but large males were not significantly more likely to engage in agonistic fin displays. Overall, the study showed that large males court and fight more than small ones across a wide, yet natural, span of social environments. Having a large body size appears to confer competitive advantage for males in any social environment of the study species. Further studies are needed to disentangle whether the benefit of large size is mainly in competition over resources, over matings as such, or both.     

Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus

In crustacean species with precopulatory mate-guarding, sexual size dimorphism has most often been regarded as the consequence of a large male advantage in contest competition for access to females. However, large body size in males may also be favoured indirectly through scramble competition. This might partly be the case if the actual target of selection is a morphological character, closely correlated with body size, involved in the detection of receptive females. We studied sexual selection on body size and antennae length in natural populations of Asellus aquaticus, an isopod species with precopulatory mate guarding. In this species, males are larger than females and male pairing success is positively related to body size. However, males also have longer antennae, relative to body size, than females, suggesting that this character may also be favoured by sexual selection. We used multivariate analysis of selection to assess the relative influences of body size and antennae length in five different populations in the field. Selection gradients indicated that, overall, body size was a better predictor of male pairing success than antennae length, although some variation was observed between sites. We then manipulated male antennae length in a series of experiments conducted in the lab, and compared the pairing ability of males with short or long antennae. Males with short antennae were less likely to detect, orient to, and to pair with a receptive female compared with males with long antennae. We discuss the implications of our results for studies of male body size and sexual dimorphism in relation to sexual selection in crustaceans.     

Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus

In crustacean species with precopulatory mate-guarding, sexual size dimorphism has most often been regarded as the consequence of a large male advantage in contest competition for access to females. However, large body size in males may also be favoured indirectly through scramble competition. This might partly be the case if the actual target of selection is a morphological character, closely correlated with body size, involved in the detection of receptive females. We studied sexual selection on body size and antennae length in natural populations of Asellus aquaticus, an isopod species with precopulatory mate guarding. In this species, males are larger than females and male pairing success is positively related to body size. However, males also have longer antennae, relative to body size, than females, suggesting that this character may also be favoured by sexual selection. We used multivariate analysis of selection to assess the relative influences of body size and antennae length in five different populations in the field. Selection gradients indicated that overall body size was a better predictor of male pairing success than antennae length, although some variation was observed between sites. We then manipulated male antennae length in a series of experiments conducted in the laboratory, and compared the pairing ability of males with short or long antennae. Males with short antennae were less likely to detect, orient to and to pair with a receptive female compared to males with long antennae. We discuss the implications of our results for studies of male body size and sexual dimorphism in relation to sexual selection in crustaceans.     

Factors driving extreme sexual size dimorphism of a sit-and-wait predator under low density

Sexual size dimorphism is often a likely outcome of the interplay between natural selection and sexual selection, with female size dictated primarily by natural selection that maximizes fecundity and male size by sexual selection that maximizes reproductive opportunities. Attention to male fitness has focused heavily on direct male-male conflict selecting for superior male size and/or fighting ability, although male reproductive traits vary immensely among animals. An alternative, advanced by Michael Ghiselin, posits highly mobile dwarf males as a strategy for finding relatively immobile females in low-density populations. Adult male crab spiders Misumena vatia , sit-and-wait predators, are strikingly smaller, much more active, and relatively longer-legged than their females. This size difference results largely from males having two fewer instars than females, which simultaneously results in marked protandry. Populations of M. vatia often were small and of low density, with a female-biased sex ratio and an operational sex ratio that changed strikingly over the season. Sexual selection through scramble competition (locating the female first) should favour this suite of characters in males of low-density populations. Although direct male-male contests favoured large males, the low densities of adult males and the dispersed, relatively immobile females led to low levels of direct intrasexual contest. Females exaggerated the problem of males in finding them by providing few cues to their presence, a pattern consistent with indirect mate choice. In addition to favouring high mobility, scramble competition favoured males that selected flowers attracting many prey, the sites most often occupied by females.     

Niche variation and the maintenance of variation in body size in a burying beetle

1. In burying beetles (Nicrophorinae), body size is known to provide both a fecundity advantage (in females) and successful resource defence (in males and females). Despite this, considerable variation in body sizes is observed in natural populations. 2. A possible explanation for the maintenance of this variation, even with intra‐ and inter‐specific resource competition, is that individuals might assort according to body size on different‐sized breeding resources. 3. We tested prediction that ‘bigger is always better’, in the wild and in the laboratory, by experimentally manipulating combinations of available breeding‐resource size (mouse carcasses) and competitor's body size in Nicrophorus vespilloides (Herbst 1783). 4. In the field, large female beetles deserted small carcasses, without breeding, more often than they did larger carcasses, but small females used carcasses indiscriminately with respect to size. In the laboratory, large beetles reared larger broods (with more offspring) on larger carcasses than small beetles, but on small carcasses small beetles had a reproductive advantage over large ones. Offspring size covaried with carcass size independently of parental body size. 5. The present combined results suggest breeding resource value depends on an individual's body size, and variation in body size is environmentally induced: maintained by differences in available carcass sizes. This produces a mechanism by which individual specialisation leads to an increase in niche variation via body size in these beetles.     

Time and energy constraints and the evolution of sexual size dimorphism — to eat or to mate?

Summary We present an empirical test of the Ghiselin—Reiss small-male hypothesis for the evolution of sexual size dimorphism (SSD). In mating systems dominated by scramble competition, where male reproductive success is a function of encounter rate with females, small males may be favoured when food is limiting because they require lower absolute amounts of food. Given a trade-off between time and energy devoted to foraging and to mate acquisition, small males should be able to devote more time to the latter. If at the same time larger females are favoured, this mechanism will contribute to the evolution of SSD and may be the major determinant of the female-biased SSDs that characterize most animal taxa. We tested this hypothesis using the water strider,Aquarius remigis (Heteroptera: Gerridae), a scramble competitor which mates many times over a prolonged mating season and which shows female-biased SSD. Laboratory experiments demonstrated that foraging success and giving up times (GUTs) are lower for males than for females during the reproductive season and that male water striders flexibly alter their time budgets under conditions of energy limitation. Controlled feeding experiments showed that male and female longevity, female fecundity and male mating success are positively related to food availability. As predicted, male body size is negatively correlated with several indices of male fitness (longevity, number of mating attempts and mating success), while female body size is positively correlated with longevity. These results are consistent with the hypothesis that scramble competition for mates favours small males in this species and provides empirical support for the Ghiselin—Reiss small-male hypothesis.     

Is size assortative mating in Paracalliope fluviatilis (Crustacea: Amphipoda) explained by male–male competition or female choice?

Field and laboratory studies were used to assess: (1) whether size assortative mating occurred in the New Zealand amphipod Paracalliope fluviatilis and (2) hypotheses developed to explain size assortative mating. We found that assortative mating occurred and that larger females carried more eggs, suggesting they may be more valuable as mates. Laboratory experiments were then used to determine whether: (1) male size influenced the size of the female selected (mechanical constraints hypothesis); (2) male size influenced pairing success in the presence of competition (intrasexual selection hypothesis); (3) take‐overs of females occurred and whether large males were more successful (intrasexual selection hypothesis); (4) guard duration varied relative to male and female size (guard duration hypothesis); and (5) females had control over pairing success and guard duration (intersexual selection hypothesis). Although there was evidence to suggest the existence of intrasexual competition for mates (i.e. both small and large males preferred large females), there was no evidence of overt competition (i.e. takeovers of paired females). There was also no difference with respect to how long small and large males guarded females, but large females were guarded longer by both male size classes. Females handicapped by having their mobility reduced were guarded for the same duration as control females but males were more likely to pair with handicapped females, suggesting that they were easier to amplex. Given the lack of evidence for direct male–male competition or female choice, we suggest that assortative mating may be the result of: (1) indirect competition (e.g. in situ large males may be better able to access and amplex the largest females) or (2) female resistance to small males in combination with higher costs that small males may incur in securing large females. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 92 , 173–181.     

DOES LARGE BODY SIZE IN MALES EVOLVE TO FACILITATE FORCIBLE INSEMINATION? A STUDY ON GARTER SNAKES

A trend for larger males to obtain a disproportionately high number of matings, as occurs in many animal populations, typically is attributed either to female choice or success in male-male rivalry; an alternative mechanism, that larger males are better able to coercively inseminate females, has received much less attention. For example, previous studies on garter snakes (Thamnophis sirtalis parietalis) at communal dens in Manitoba have shown that the mating benefit to larger body size in males is due to size-dependent advantages in male-male rivalry. However, this previous work ignored the possibility that larger males may obtain more matings because of male-female interactions. In staged trials within outdoor arenas, larger body size enhanced male mating success regardless of whether a rival male was present. The mechanism involved was coercion rather than female choice, because mating occurred most often (and soonest) in females that were least able to resist courtship-induced hypoxic stress. Males do physically displace rivals from optimal positions in the mating ball, and larger males are better able to resist such displacement. Nonetheless, larger body size enhances male mating success even in the absence of such male-male interactions. Thus, even in mating systems where males compete physically and where larger body size confers a significant advantage in male-male competition, the actual selective force for larger body size in males may relate to forcible insemination of unreceptive females. Experimental studies are needed to determine whether the same situation occurs in other organisms in which body-size advantages have been attributed to male-male rather than male-female interactions.     

Size-biased Mating in Both Sexes of the Black-horned Tree Cricket, <Emphasis Type="Italic">Oecanthus nigricornis</Emphasis> Walker (Orthoptera: Gryllidae: Oecanthinae)

Previous studies on tree crickets have demonstrated female choice of males based on size and courtship feeding but less is known about sexual selection under conditions of direct mating competition. I studied courtship, aggression and mating of the black-horned tree cricket Oecanthus nigricornis (Walker) to test size-related sexual selection under conditions of direct sexual competition. Results show that larger individuals of both sexes mated more frequently than their smaller counterparts, and this was due to the ability of large individuals to out compete rivals. Large males achieved the advantage by aggressively reducing courtship by small males, whereas large females responded to male courtship more quickly but with little aggression. Although there was no evidence here for mate choice, there were advantages for having larger mates; fecundity increased with female size and spermatophores (which females consume after mating) increased with male size. Size of the specialized metanotal courtship gift, however, was not related to male size.     

Competing dwarf males: sexual selection in an orb-weaving spider

Hypotheses for the adaptive significance of extreme female-biased sexual size dimorphism (SSD) generally assume that in dimorphic species males rarely interfere with each other. Here we provide the first multivariate examination of sexual selection because of male-male competition over access to females in a species with 'dwarf' males, the orb-weaving spider Argiope aurantia. Male A. aurantia typically try to mate opportunistically during the female's final moult when she is defenceless. We show that, contrary to previous hypotheses, the local operational sex ratio (males per female on the web) is male-biased most of the season. Both interference and scramble competition occur during opportunistic mating, the former leading to significant selection for large male body size. Male condition and leg length had no effect on mating success independent of size. We discuss these findings in the context of the evolution of extreme female-biased SSD in this clade.     

Sperm competition and small size advantage for males of the golden orb‐web spider Nephila edulis

Sexual selection, through female choice and/or male–male competition, has influenced the nature and direction of sexual size dimorphism in numerous species. However, few studies have examined the influence of sperm competition on size dimorphism. The orb‐web spider Nephila edulis has a polygamous mating system and extreme size dimorphism. Additionally, the frequency distribution of male body size is extremely skewed with most males being small and few large. The duration of copulation, male size and sexual cannibalism have been identified as the significant factors determining patterns of sperm precedence in spiders. In double mating trials, females were assigned to three treatments: either they mated once with both males or the first or the second male was allowed to mate twice. Paternity was strongly associated with the duration of copulation, independent of mating order. Males that were allowed to mate twice not only doubled the duration of copulation but also their paternity. Small males had a clear mating advantage, they copulated longer than large males and fertilized more eggs. Males of different sizes used different tactics to mate. Large males were more likely to mate through a hole they cut into the web, whereas small males approached the female directly. Furthermore, small males usually mated at their first attempt but large males required several attempts before mating took place. There was no obvious female reaction towards males of different sizes.     

THE EVOLUTION OF FEMALE-BIASED SEXUAL SIZE DIMORPHISM: A POPULATION-LEVEL COMPARATIVE STUDY IN HORNED LIZARDS (PHRYNOSOMA)

Female-biased sexual size dimorphism is uncommon among vertebrates and traditionally has been attributed to asymmetric selective pressures favoring large fecund females (the fecundity-advantage hypothesis) and/or small mobile males (the small-male advantage hypothesis). I use a phylogenetically based comparative method to address these hypotheses for the evolution and maintenance of sexual size dimorphism among populations of three closely related lizard species (Phrynosoma douglasi, P. ditmarsi, and P. hernandezi). With independent contrasts I estimate evolutionary correlations among female body size, male body size, and sexual size dimorphism (SSD) to determine whether males have become small, females have become large, or both sexes have diverged concurrently in body size during the evolutionary Xhistory of this group. Population differences in degree of SSD are inversely correlated with average male body size, but are not correlated with average female body size. Thus, variation in SSD among populations has occurred predominantly through changes in male size, suggesting that selective pressures on small males may affect degree of SSD in this group. I explore three possible evolutionary mechanisms by which the mean male body size in a population could evolve: changes in size at maturity, changes in the variance of male body sizes, and changes in skewness of male body size distributions. Comparative analyses indicate that population differentiation in male body size is achieved by changes in male size at maturity, without changes in the variance or skewness of male and female size distributions. This study demonstrates the potential of comparative methods at lower taxonomic levels (among populations and closely related species) for studying microevolutionary processes that underlie population differentiation.     

Mate discrimination by females in the burying beetle Nicrophorus orbicollis: the influence of male size on attractiveness to females

1. Female burying beetles behave differently towards males of different sizes, avoiding mating with large males that are not defending resources but mating with small males regardless of the presence of resources. Females of the burying beetle Nicrophorus orbicollis were therefore examined to determine whether they discriminate among males using only pheromonal signals. The influence of female size on its own mate choice was also examined. 2. Females do use male pheromonal signals to discriminate among males and these signals do appear to convey information about male body size to females. Overall, females were more likely to be attracted to larger males than to smaller males. 3. Female choice of a male was influenced by both the female's own body size and the size of the female relative to the size of the two males available to it. 4. While there is an overall mating advantage for larger males, resulting from female preferences based on odour cues, smaller males are also attractive to some females under some circumstances. 5. It is argued that there are different costs and benefits of mating with different sized males, leading to the evolution of context‐dependent mate choice for females and the need to be able to discriminate males of different sizes from a distance.     

Mutual mate choice in a female-dominant and sexually monomorphic primate

Sexual dimorphism is common in polygynous species, where intrasexual competition is often thought to drive the evolution of large male body size, and in turn, male behavioral dominance over females. In Madagascar, the entire lemur radiation, which embraces diverse mating systems, lacks sexual dimorphism and exhibits frequent female dominance over males. The evolution of such morphological and behavioral peculiarities, often referred to as "the lemur syndrome," has proven difficult to understand. Among other hypotheses, a potential role of intersexual selection has been repeatedly proposed but hardly ever tested. Here, we investigate whether female choice favors small and compliant males, and whether male choice favors large females in captive gray mouse lemurs (Microcebus murinus). Detailed analysis of a combination of behavioral observations and hormonal data available for both sexes shows that (1) females accept more matings from males with higher fighting abilities, (2) males adjust their investment in intrasexual competition to female fertility, and (3) both male and female strategies are weakly influenced by the body mass of potential partners, in directions contradicting our predictions. These results do not suggest a prominent role of intersexual selection in the evolution and maintenance of the lemur syndrome but rather point to alternative mechanisms relating to male-male competition, specifically highlighting an absence of relationship between male body mass and fighting ability. Finally, our findings add to the growing body of evidence suggesting flexible sex roles, by showing the expression of mutual mate choice in a female-dominant, sexually monomorphic and promiscuous primate.     

Male size does not affect the strength of male mate choice for high-quality females in Drosophila melanogaster

Theory predicts that the strength of male mate choice should vary depending on male quality when higher-quality males receive greater fitness benefits from being choosy. This pattern extends to differences in male body size, with larger males often having stronger pre- and post-copulatory preferences than smaller males. We sought to determine whether large males and small males differ in the strength (or direction) of their preference for large, high-fecundity females using the fruit fly, Drosophila melanogaster. We measured male courtship preferences and mating duration to show that male body size had no impact on the strength of male mate choice; all males, regardless of their size, had equally strong preferences for large females. To understand the selective pressures shaping male mate choice in males of different sizes, we also measured the fitness benefits associated with preferring large females for both large and small males. Male body size did not affect the benefits that males received: large and small males were equally successful at mating with large females, received the same direct fitness benefits from mating with large females, and showed similar competitive fertilization success with large females. These findings provide insight into why the strength of male mate choice was not affected by male body size in this system. Our study highlights the importance of evaluating the benefits and costs of male mate choice across multiple males to predict when differences in male mate choice should occur.     

The mating chances of small males of the cerambycid beetleTrachyderes mandibularis differ in different environments (Coleoptera: Cerambycidae)

Some males of the cerambycid beetle Trachyderes (Dendrobias) mandibularisgained access to mates by defending a patchily distributed food resource, the fruits of saguaro cactus (Cereus giganteus).Male beetles differed greatly in fighting ability because of extreme variation in body size and a striking dimorphism in mandibular weaponry. As is typical in resource defense mating systems, larger males had an advantage in combat. Major males with their large pincer jaws invariably defeated minor males with small cutting jaws, and larger majors usually defeated smaller majors. However, although minor males were at a competitive disadvantage on saguaro fruits, they did not suffer a great penalty in terms of mating probability. In contrast, minor males have a considerably lower probability of mating at desert broom (Baccharis sarothroides)where sap ooze sites are few in number and effectively monopolized by major males (Goldsmith, S. K., Behav. Ecol. Sociobiol.20,111–115, 1987). On saguaros, minor males successfully obtained mates through scramble competition while avoiding direct physical competition with larger, territorial major males. Smaller males of either morph may have succeeded in acquiring mates in part because there were many more ripe saguaro fruits than beetles, which made it impossible for larger major males to monopolize females effectively under these conditions.     

Male parasitism and intrasexual competition in a burrowing barnacle

Summary In sexually dimorphic animals, large male body size is often associated with direct interference competition among males for access to females or resources used in reproduction. In constrast, small male body size may be associated with indirect scramble competition among males for temporal or spatial access to females. Minute, “parasitic” males of the acrothoracican barnacleTrypetesa lampas (Hancock) appear to compete with one another for permanent attachment sites on the external body of the female. Several spatial patterns suggest indirect male-male competition: 1) males were consistently aggregated on the anterior surface of the female ovarian disc; 2) the average distance from attached males to the site of insemination correlated positively with local male density; 3) average male body size on a female decreased as a function of male density; 4) the distribution of males on the left and right hand sides of the female ovarian disc was more even than expected, suggesting that males avoided crowded settlement sites. The number of males attached to a female increased with female body size and matched a null model in which males colonized female “targets” of differing areas. These results suggest that competition between males primarily affected settlement sites and male body sizes within, rather than among, females. Male parasitism may have evolved through both sexual selection for efficient access to females (Ghiselin 1974) and natural selection for reduced burrow density in a space-limited habitat (Turner and Yakovlev 1983).     

Sexual behaviour and evolution of sexual dimorphism in body size in Jaera (Isopoda Asellota)

Individuals of the genus Jaera do not mate at random. In the species from the Mediterranean group, J. italica and. J. nordmanni, large males and medium sized females are at an advantage and their sizes are positively assorted. These effects are attributable to sexual competition between males. In the Ponlo-caspian species J. istri, no advantage of large males exists, but sexual selection could be the cause for a long passive phase prior to copulation and for normalizing selection upon female size at pairing. In the Atlantic species, J. albifrons, no selection can be ascertained.
Differential mating success in males appears as one of the causes of the evolution of sexual dimorphism in body size, which makes males larger, of equal size, or smaller than females according to the species. The reason for this reversal in dimorphism seems to differ in the two sexes. Sexual selection provides an explanation for the evolution of male size, while the interspecific changes in female length are more likely due to ecological factors.     

A biogeographic reversal in sexual size dimorphism along a continental temperature gradient

The magnitude and direction of sexual size dimorphism (SSD) varies greatly across the animal kingdom, reflecting differential selection pressures on the reproductive and/or ecological roles of males and females. If the selection pressures and constraints imposed on body size change along environmental gradients, then SSD will vary geographically in a predictable way. Here, we uncover a biogeographical reversal in SSD of lizards from Central and North America: in warm, low latitude environments, males are larger than females, but at colder, high latitudes, females are larger than males. Comparisons to expectations under a Brownian motion model of SSD evolution indicate that this pattern reflects differences in the evolutionary rates and/or trajectories of sex‐specific body sizes. The SSD gradient we found is strongly related to mean annual temperature, but is independent of species richness and body size differences among species within grid cells, suggesting that the biogeography of SSD reflects gradients in sexual and/or fecundity selection, rather than intersexual niche divergence to minimize intraspecific competition. We demonstrate that the SSD gradient is driven by stronger variation in male size than in female size and is independent of clutch mass. This suggests that gradients in sexual selection and male–male competition, rather than fecundity selection to maximize reproductive output by females in seasonal environments, are predominantly responsible for the gradient.     

INTRASEXUAL COMPETITION ALONE FAVORS A SEXUALLY DIMORPHIC ORNAMENT IN THE RUBYSPOT DAMSELFLY HETAERINA AMERICANA

I studied the sex-limited red spots on the wings of male rubyspot damselflies (Hetaerina americana) in relation to territoriality and fitness in the wild. Both observational and experimental (wing spot manipulation) studies indicated that wing spots were selected through competition among males for mating territories, not through female choice or direct competition for females. Males with naturally or artificially large wing spots were more successful at holding territories and consequently mated at higher rates than males with relatively small wing spots. In contrast, sexual selection on male body size appeared to operate among nonterritorial males at the clasping stage of the mating sequence, perhaps because larger males were better at clasping females forcibly. Of four models proposed to explain the evolution of ornaments through territory competition, only the agonistic handicap model makes predictions consistent with the results of this study.