Selection For Large Male Size Research Articles

The origins and drivers of sexual size dimorphism in sharks.

While sexual size dimorphism (SSD) is abundant in nature, there is huge variation in both the intensity and direction of SSD. SSD results from a combination of sexual selection for large male size, fecundity selection for large female size and ecological selection for either. In most vertebrates, it is variation in the intensity of male-male competition that primarily underlies variation in SSD. In this study, we test four hypotheses regarding the adaptive value of SSD in sharks-considering the potential for each of fecundity, sexual, ecological selection and reproductive mode as the primary driver of variation in SSD between species. We also estimate past macroevolutionary shifts in SSD direction/intensity through shark phylogeny. We were unable to find evidence of significant SSD in early sharks and hypothesise that SSD is a derived state in this clade, that has evolved independently of SSD observed in other vertebrates. Moreover, there is no significant relationship between SSD and fecundity, testes mass or oceanic depth in sharks. However, there is evidence to support previous speculation that reproductive mode is an important determinant of interspecific variation in SSD in sharks. This is significant as in most vertebrates sexual selection is thought to be the primary driver of SSD trends, with evidence for the role of fecundity selection in other clades being inconsistent at best. While the phylogenetic distribution of SSD among sharks is superficially similar to that observed in other vertebrate clades, the relative importance of selective pressures underlying its evolution appears to differ.

Comparative sexual selection in field and laboratory in a guild of sepsid dung flies

Phenomenological and behavioural studies have greatly advanced the study of natural selection. Field studies of selection well appraise the natural situation, but is this also true for laboratory studies, which are typically more mechanistic? We compared precopulatory sexual selection (mating differential based on pairing success) in field and laboratory of several closely related, ecologically similar black scavenger dung flies (Diptera: Sepsidae). Selection on fore femur (sexual trait) and wing size (nonsexual trait) and shape varied considerably among seven species and continental populations in agreement with variation in their mating system and sexual size dimorphism. Selection on trait size was mostly positive or nil, but never significantly negative, implying mating advantages of large males in most species. Strongest selection was found in species/populations with male-biased size dimorphism, associating evolutionary shifts from female- to male-biased dimorphism with intensified sexual selection for large male size by adding male –male competition to a mating system previously driven primarily by female choice. Although sexual selection on shape was closely aligned with allometric shape variation, selection on fore femur shape was more consistent than selection on wing shape, which was absent in most species. Sexual selection intensities, but not necessarily the underlying behavioural mechanisms, were overall similar in field and laboratory, suggesting that laboratory assessments well represent the natural situation. If this conclusion can be generalized, it would lend credence to the strategy of using controlled laboratory mating studies to better understand natural selection, behaviour and ecology, at least for smaller animals that can be held in captivity.

Andrew meets Rensch: sexual size dimorphism and the inverse of Rensch's rule in Andrew's toad (Bufo andrewsi).

Variation in sexual size dimorphism (SSD) is a widespread phenomenon and is commonly attributed to variation in sex-specific patterns of selection. According to Rensch's rule, SSD increases with increasing body size when males are the larger sex, and decreases when females are the larger sex. Using data from 17 populations of Andrew's toad (Bufo andrewsi), we tested whether the patterns of SSD conform to Rensch's rule. Using field experiments, we also evaluated the hypothesis that sexual selection favours large male body size and that fecundity selection favours large female body size. The results revealed that the degree of SSD increased with increasing mean size in females, consistent with the inverse of Rensch's rule. Although experiments revealed evidence for a large-male mating advantage, selection for large male size was weak at best, and hence unlikely to be an important source of variation in SSD. However, fecundity selection favouring large females was evident, and likely to explain the observed inverse of Rensch's rule. After correcting male and female body size for age differences, the patterns of SSD remained the same, suggesting that the intra- and interpopulational variation in SSD is not driven by sex differences in age structure. Hence, these findings suggest that the strong fecundity selection favouring large females drives the evolution of female-biased SSD in B. andrewsi, providing an explanation for the inverse of Rensch's rule. As such, the study provides an important addition to the small body of literature that uses an intraspecific approach to demonstrate the inverse of Rensch's rule.

Male-Male Competition in a Mixed-Mating Fish

Mating systems that comprise a mixture of pure males and self-fertilising hermaphrodites remain an evolutionary enigma. In particular, our understanding of the sexual selection pressures associated with such mating systems is nascent. Males can only reproduce by fertilising hermaphrodites’ eggs, but hermaphrodites can also fertilise their own eggs and gain a genetic advantage by doing so. Consequently, there should be intense competition among males to access hermaphrodites. Here, we test the importance of male size, colour and heterozygosity in predicting the outcome of male–male competition using the mangrove rivulus, which has a male-hermaphrodite mixed-mating system. We pitted males against one another in dyadic laboratory trials to develop a dominance score for each male. We then correlated these scores with male length, several components of male colour, and heterozygosity. Male size was the only significant correlate of dominance: larger males dominated smaller males, implying selection for large male size. However, male mangrove rivulus are similar in size to hermaphrodites, indicating that directional selection for large body size in males is no greater than it is in hermaphrodites. Across all trials, colour was unrelated to dominance, but contests between similarly sized males were usually won by more colourful individuals. As mangrove rivulus are dichromatic, we suspect that male colour may prove to be more important in mate choice than we found it to be in intrasexual competition. Heterozygosity did not explain dominance directly, but correlated strongly with male size, implying an indirect role in intrasexual competition.

Growth and survivorship as proximate causes of sexual size dimorphism in peninsula dragon lizards Ctenophorus fionni

AbstractMales and females differ in body size in many animals, but the direction and extent of this sexual size dimorphism (SSD) varies widely. Males are larger than females in most lizards of the iguanian clade, which includes dragon lizards (Agamidae). I tested whether the male larger pattern of SSD in the peninsula dragon lizard, Ctenophorus fionni, is a result of sexual selection for large male size or relatively higher mortality among females. Data on growth and survivorship were collected from wild lizards during 1991–1994. The likelihood of differential predation between males and females was assessed by exposing pairs of male and female lizards to a predator in captivity, and by comparing the frequency of tail damage in wild‐caught males and females. Male and female C. fionni grew at the same rate, but males grew for longer than females and reached a larger asymptotic size (87 mm vs. 78 mm). Large males were under‐represented in the population because they suffered higher mortality than females. Predation may account for some of this male‐biased mortality. The male‐biased SSD in C. fionni resulted from differences in growth pattern between the sexes. The male‐biased SSD was not the result of proximate factors reducing female body size. Indeed SSD in this species remained male‐biased despite high mortality among large males. SSD in C. fionni is consistent with the ultimate explanation of sexual selection for large body size in males.

The function of female behaviours adopted during premating struggles in the seaweed fly, Coelopa frigida

The mating system of sciomyzoid flies is typified by vigorous premating struggles and a large male mating advantage. Females commonly exhibit three behaviours (shaking, kicking and curling) during struggles. We tested the predictions of three hypotheses proposed to explain the evolution of female resistance in Coelopa frigida . Both shaking and curling were associated with reduced female mating rate and thus appear to be resistance traits. Mounts that ended while the female was curling appeared to be terminated by the male dismounting from the female rather than as a result of resistance. The communication hypothesis proposes that female resistance functions by signalling to the male that the female is unwilling to mate to encourage males to dismount them. Curling therefore seems to fit the predictions of the communication hypothesis. Shaking was associated with sexual selection for large male size and also males that could withstand resistance the longest. These observations fit with the predictions of both the female reluctance and screening hypotheses. Furthermore, shaking was associated with a pattern of mating that increased offspring fitness. This did not occur in females that did not shake. This corresponds with the predictions of the screening hypothesis that resistance generates indirect sexual selection. However, our results are also largely consistent with the female reluctance hypothesis. It appears that different resistance behaviours may have different functions, and furthermore that individual resistance behaviours might generate different mating rates and patterns of sexual selection between female karyotypes.

Condition-dependent female remating resistance generates sexual selection on male size in a ladybird beetle

Behavioural resistance to remating by females is common, but the causes and consequences of resistance are rarely explained. Prominent hypotheses include resistance as a means of avoiding costly and superfluous mating, or as a means of biasing mating towards high-quality males. In species in which males produce nutritious nuptial gifts, females may further modulate resistance according to their need for nutrition. We investigated these hypotheses in the ladybeetle Adalia bipunctata , in which females frequently display vigorous resistance before copulation and ingest a spermatophore after copulation. In two experiments, we manipulated female nutritional state, depriving or satiating females for a short (16 h) or long (96 h) interval before a remating trial. We found that food-deprived females resisted mating more frequently and for longer periods than satiated females and consequently remated less frequently. This condition dependence of resistance supports the hypothesis that resistance functions to reduce superfluous and costly mating. Our finding that food-deprived females were more resistant suggests that mating imposes energetic costs, and that nuptial feeding does not offset these costs. In a third experiment, we investigated whether the extent of resistance depended on male size or whether resistance itself biased mating towards large males. The extent of female resistance was independent of male size, but resistance itself resulted in a mating bias towards large males. In summary, our results support the hypotheses that females resist mating simply because it is costly and superfluous, and that a side effect of resistance is sexual selection for large male size.

Male‐biased size dimorphism in ichneumonine wasps (Hymenoptera: Ichneumonidae) – the role of sexual selection for large male size

Abstract. 1. Sexual differences in body size are expected to evolve when selection on female and male sizes favours different optima.2. Insects have typically female‐biased size dimorphism that is usually explained by the strong fecundity advantage of larger size in females. However, numerous exceptions to this general pattern have led to the search for selective pressures favouring larger size in males.3. In this study, the benefits of large size were investigated in males of four species of ichneumonine wasps, a species‐rich group of parasitoids, many representatives of which exhibit male‐biased size dimorphism.4. Mating behaviour of all ichneumonine wasps are characterised by pre‐copulatory struggles, in the course of which males attempt to override female reluctance to mate. A series of laboratory trials was conducted to study the determinants of male mating success.5. A tendency was found for larger males as well as those in better condition to be more successful in achieving copulations. Size dimorphism of the species studied, mostly male‐biased in hind tibia length but female‐biased in body weight, indicates that sexual selection in males favours longer bodies and appendages rather than larger weight.6. The qualitative similarity of the mating patterns suggests that sexual selection cannot completely explain the considerable among‐species differences in sexual size dimorphism.7. The present study cautions against using various size indices as equivalents for calculating sexual size dimorphism.8. It is suggested that female reluctance in ichneumonine wasps functions as a mechanism of female mate assessment.

Sexual selection on morphological and physiological traits and fluctuating asymmetry in the yellow dung fly.

Previous univariate studies of the yellow dung fly (Scathophaga stercoraria) have demonstrated strong sexual selection, in terms of mating success, on male size (estimated as hind tibia length). To identify specific target(s) of selection on body size and possible conflicting selection pressures on particular body parts, two multivariate field studies of sexual selection were conducted. In one study using point samples from three populations, we assessed several morphological traits, including genital traits and measures of fluctuating asymmetry (FA) of all paired traits. There was sexual selection for large male size in general, confirming previous, univariate studies. With the possible exception of thorax width, which was selected in the opposite direction, no main target of selection was identified, as most morphological traits were highly correlated. There was no detectable sexual selection on the male external genital structures assessed. In a second study using multiple samples from one population, we included physiological measures of energy reserves (lipids, glucose and glycogen) known to affect mating success, in addition to trait size and FA of wings and legs. Inclusion of physiological traits is rare in phenomenological studies of selection. This study again confirmed the mating advantage of large males, and additionally showed independent positive influences of lipid and glucose but not glycogen levels. FA in paired traits generally did not affect male mating success, but was negatively correlated with energy reserves. Our study suggests that inclusion of physiological measures and genital traits in phenomenological studies of selection would be fruitful in other species.

A COMPARATIVE TEST OF ADAPTIVE HYPOTHESES FOR SEXUAL SIZE DIMORPHISM IN LIZARDS

It is commonly argued that sexual size dimorphism (SSD) in lizards has evolved in response to two primary, nonexclusive processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition (intrasexual selection hypothesis), and (2) natural selection for large female size, which confers a fecundity advantage (fecundity advantage hypothesis). However, outside of several well-studied lizard genera, the empirical support for these hypotheses has not been examined with appropriate phylogenetic control. We conducted a comparative phylogenetic analysis to test these hypotheses using literature data from 497 lizard populations representing 302 species and 18 families. As predicted by the intrasexual selection hypothesis, male aggression and territoriality are correlated with SSD, but evolutionary shifts in these categorical variables each explain less than 2% of the inferred evolutionary change in SSD. We found stronger correlations between SSD and continuous estimates of intrasexual selection such as male to female home range ratio and female home range size. These results are consistent with the criticism that categorical variables may obscure much of the actual variation in intrasexual selection intensity needed to explain patterns in SSD. In accordance with the fecundity advantage hypothesis, SSD is correlated with clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, reduced major axis estimator of fecundity slope, length of reproductive season, or latitude). However, evolutionary shifts in clutch size explain less than 8% of the associated change in SSD, which also varies significantly in the absence of evolutionary shifts in reproductive frequency and mode. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD is explained using these variables. Intrasexual selection for large male size and fecundity selection for large female size have undoubtedly helped to shape patterns of SSD across lizards, but the comparative data at present provide only weak support for these hypotheses as general explanations for SSD in this group. Future work would benefit from the consideration of alternatives to these traditional evolutionary hypotheses, and the elucidation of proximate mechanisms influencing growth and SSD within populations.

The effect of hunger on mating behaviour and sexual selection for male body size in Gerris buenoi

The set of mating behaviours expressed by an individual may depend upon the state of that individual and local environmental conditions. Understanding how these factors affect mating behaviours may elucidate how a mating system operates, and its consequences for the form and strength of sexual selection. We conducted two experiments on the water striderGerris buenoi to (1) determine the effect of hunger on the mating behaviour of both sexes and (2) examine female choice for large males. In our first experiment, we manipulated hunger (20h starvation) in both sexes and recorded mating, male harassment, copulation duration and guarding duration. We predicted that hunger would increase female reluctance to mate because mating conflicts with foraging. Female hunger (20h starvation) decreased mating rate by two-thirds but had no significant effect on male mating behaviour. In a second experiment, we examined the effect of female hunger, and resulting reluctance, on sexual selection for large male size. Hungry females (5h starvation) were placed with two fed males (one large, one small) and we recorded male premating and mating behaviours. We observed significant large-male mating advantage when females were hungry, but not when satiated. Mating efforts (harassment, premating struggles) were similar for both male phenotypes in both female hunger treatments, suggesting that the mating advantage of large males resulted from increased reluctance of hungry females to mate. Neither male body size nor female hunger explained a significant amount of variation in copulation duration or guarding duration. We discuss our results in light of two competing hypotheses for female choice (active and passive) on male body size and suggest that passive choice for large males acts in this system. Copyright 2002 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

POPULATION VARIATION IN SEXUAL SELECTION AND ITS EFFECT ON SIZE ALLOMETRY IN TWO DUNG FLY SPECIES WITH CONTRASTING SEXUAL SIZE DIMORPHISM

Body size is one of the most important quantitative traits under evolutionary scrutiny. Sexual size dimorphism (SSD) in a given species is expected to result if opposing selection forces equilibrate differently in both sexes. We document variation in the intensity of sexual and fecundity selection, male and female body size, and thus SSD among 31 and 27 populations of the two dung fly species, Scathophaga stercoraria and Sepsis cynipsea, across Switzerland. Whereas in S. cynipsea females are larger, the SSD is reversed in S. stercoraria. We comprehensively evaluated Fairbairn and Preziosi's (1994) general, three-tiered scenario, hypothesizing that sexual selection for large male size is the major driving force of SSD allometry within these two species. Sexual selection intensity on male size in the yellow dung fly, S. stercoraria, was overall positive, greater, and more variable among populations than fecundity selection on females. Also, sexual selection intensity in a given population correlated positively with mean male body size of that population for both the field-caught fathers and their laboratory-reared sons, indicating a response to selection. In S. cvnipsea, sexual selection intensity on males was lower overall and significantly positive, about equal in magnitude, but more variable than fecundity selection on females. However, there was no correlation between the intensity of sexual selection and mean male body size among populations. In both species, the laboratory-reared offspring indicate genetic differentiation among populations in body size. Despite fulfillment of all key prerequisites, at least in S. stercoraria, we did not find hypoallometry for SSD (Rensch's rule, i.e., greater evolutionary divergence in male size than female size) for the field-caught parents or the laboratory-reared offspring: Female size was isometric to male size in both species. We conclude that S. cynipsea does not fit some major requirements of Fairbairn and Preziosi's (1994) scenario, whereas for S. stercoraria we found partial support for it. Failure to support Rensch's rule within the latter species may be due to phylogenetic or other constraints, power limitations, erroneous estimates of sexual selection, insufficient genetic isolation of populations, or sex differences in viability selection against large size.

Sexual selection for large male size in a polyandrous butterfly: the effect of body size on male versus female reproductive success in Pieris napi

Sexual selection for large male size in a polyandrous butterfly: the effect of body size on male versus female reproductive success in <i>Pieris napi</i>

Sexual Size Dimorphism in Relation to Female Polygamy and Protandry in Butterflies: A Comparative Study of Swedish Pieridae and Satyridae

In a number of insects male size has been shown to be positively correlated with mating success. Since variance in male mating success should increase with the degree of female polygamy, sexual selection for large male size may be stronger under polyandry than under monandry. To test this hypothesis we correlated male/female sexual size dimorphism, defined as male/female forewing length, with the degree of female polygamy, assessed by counting the number of spermatophores in wild-caught females. Both sets of data were assessed from 23 species of butterflies, 11 in the Pieridae and 12 in the Satyridae. There was a positive correlation between male/ female sexual size dimorphism and female polygamy in both families. Although size is affected by many factors, we contend that this result is best explained by sperm competition; in butterflies, the mass of the ejaculate delivered by a male is positively correlated with male size, and large ejaculates induce a longer period of female unreceptivity, resulting in sexual selection favouring large male size in species where females mate repeatedly. Although there is an obvious conflict between having a short development time and acquiring large size, the expected negative relationship between sexual size dimorphism and protandry was not statistically significant under direct development in the 8 species tested. This suggests that males under direct development achieve both large size and short development time simultaneously by increasing their growth rate. In diapause generations of 14 species (including overwintering generations of the 8 bivoltine species above) there was, however, such a negative relationship. Since protandry in diapause generations is mainly the result of morphogenesis of the adult during postdiapause pupal development, this is consistent with the tendency for small individuals, males, to require shorter morphogenetic development times than larger individuals, females. We found a weak negative correlation between polyandry and protandry, statistically significant only under diapause development. This suggests that mating with virgin females is more beneficial than mating with non-virgins also under female polygamy, and indicates that selection for early male emergence is only weakly relaxed in species where females mate repeatedly.

Sexual size dimorphism and reproductive ecology in relation to mating system in waders

The relationship between sexual size dimorphism, body-weight and different reproductive traits (e.g. clutch size, egg weight and incubation period) in relation to mating system and forms of parental care was studied in waders. Two hypotheses were examined. (1) Sexual size dimorphism is correlated with the intensity of sexual selection. (2) The degree of sexual size dimorphism is the result of an interrelationship between the reproductive strategy of the female and her body size. In the polygynous species the male was significantly larger than the female. This is consistent with the sexual selection hypothesis. However, among waders, a positive correlation exists between egg weight, clutch mass and body-weight. Selection for small eggs or a short incubation period may therefore have an influence on female body-weight. If the lack of paternal care reduces the female's possibility for producing large eggs or incubating a large clutch mass, we would expect a selection pressure for small female size among polygynous species. Thus, large sexual size dimorphism among polygynous waders may be a result of selection for small female size to lack of paternal care, or selection for large male size due to intramale competition or a female preference for large-sized males. In multiple-clutch species (viz. species in which the female regularly lays more than one clutch during the season) egg weight was low both for a given female and male body-weight. The low egg weight of multiple-clutch species is assumed to be a result of the constraints placed on the female from producing several clutches during a single breeding season.