The evolution of empty nuptial gifts in a dance fly, Empis snoddyi (Diptera: Empididae): bigger isn't always better

Adaptive female choice is thought to have led to the evolution of nutritionally valuable nuptial gifts in many insect species. However, in several dance fly species, males offer and females accept “empty gifts” with no nutritional value. In the species studied here, Empis snoddyi Steyskal, males produce empty balloons comprised of hundreds of silk bubbles and form mating swarms that females approach to investigate males. Males within the swarm engage in agonistic interactions. The empty balloon has been hypothesized to be an indicator of male condition such that males with larger balloons are predicted to have higher mating success and be more successful in male-male interactions than males with smaller balloons. We examined the role of male body size and balloon size in the context of intersexual and intrasexual selection. We found that neither male body size nor balloon size affected the outcome of pairwise male-male interactions. Using multiple-regression techniques, we found significant linear selection for increasing male body size and decreasing balloon size associated with mating success, a surprising result given a positive relationship between male body size and balloon size. A visualization of selection showed the highest peak of male mating success for larger males with intermediate-size balloons. These results can be explained by a trade-off between long-range attraction of females using large balloons and close-range attraction of females via improved flying efficiency associated with smaller balloons. Both male body size and balloon size are important components in determining male mating success; however, the empty balloon does not appear to play a typical role as a sexually selected ornament. Received: 29 December 1997 / Accepted after revision: 7 October 1998     

Allocation in reproduction is not tailored to the probable number of matings in common toad (<Emphasis Type="Italic">Bufo bufo</Emphasis>) males

The theory of life history evolution assumes trade-offs between competing fitness traits such as reproduction, somatic growth, and maintenance. One prediction of this theory is that if large individuals have a higher reproductive success, small/young individuals should invest less in reproduction and allocate more resources in growth than large/old individuals. We tested this prediction using the common toad (Bufo bufo), a species where mating success of males is positively related to their body size. We measured testes mass, soma mass, and sperm stock size in males of varying sizes that were either (1) re-hibernated at the start of the breeding season, (2) kept without females throughout the breeding season, or (3) repeatedly provided with gravid females. In the latter group, we also estimated fertilization success and readiness to re-mate. Contrary to our predictions, the relationship between testes mass and soma mass was isometric, sperm stock size relative to testes mass was unrelated to male size, fertilization success was not higher in matings with larger males, and smaller males were not less likely to engage in repeated matings than larger males. These results consistently suggest that smaller males did not invest less in reproduction to be able to allocate more in growth than larger males. Causes for this unexpected result may include relatively low year-to-year survival, unpredictable between-year variation in the strength of sexual selection and low return rates of lowered reproductive investment.     

Call structure may affect male mating success in the quacking frog<Emphasis Type="Italic"> Crinia georgiana</Emphasis> (Anura: Myobatrachidae)

We investigated the relationship between mating success, male size and variation in the advertisement call in the frog Crinia georgiana under field conditions. Mating success in 91 males was determined by following 32 females as they moved through the chorus. Our analyses indicated that successful males had a higher number of pulses in the first note and/or called at a higher rate. However, we did not detect a significant relationship between mating success and dominant frequency, the property that varied most strongly with body size, suggesting that size is not an important influence on mate choice in this species. Even so, smaller males were more successful if they called at a higher rate whereas larger males were more successful if they had more pulses in their first note. Accordingly, males of different size may use different calling tactics to attract females. These results provide a framework for further experimental studies aimed at investigating the independent impacts of both inter- and intra-sexual selection on the advertisement call and body size of C. georgiana.     

Genetic analysis of parentage within experimental populations of a male dimorphic beetle,<Emphasis Type="Italic"> Onthophagus taurus</Emphasis>, using amplified fragment length polymorphism

We used the multilocus DNA fingerprinting technique, amplified fragment length polymorphism, to examine parentage of 902 offspring from eight experimental populations of the dung beetle, Onthophagus taurus. The males of this species exhibit a suit of morphological and behavioural traits that characterise alternative mating tactics. Hornless minor males sneak copulations with females that are guarded by horned major males. Our aims were to provide a prospective assessment of the potential role of frequency dependence in the maintenance of alternative mating tactics, to assess the levels of polyandry, and to determine the patterns of sperm usage by multiply mated females. The average proportion of offspring sired by major and minor males did not co-vary with the relative frequency of each morph present in experimental populations. However, there was some indication that the effective mating frequency (number of females producing offspring sired by a given male) of major and minor males may exhibit frequency dependence. Mating success of both male types declined with increasing numbers of major males. Paternity was positively associated with effective mating frequency. Females produced offspring sired from between one and eight males and, on average, paternity was distributed equally amongst a females mates, regardless of the number of males mated. Differences in fertilisation success among males were not associated with alternative male phenotypes. Neither did a males fertilisation success depend on his genetic dissimilarity with the female. These results are discussed in the context of the evolution of alternative mating strategies, and mechanisms of postcopulatory sexual selection.Communicated by N. Wedell     

Estimating paternity with spatial behaviour and DNA fingerprinting in the striped plateau lizard, Sceloporus virgatus (Phrynosomatidae)

The association between spatial proximity and paternity was studied in a population of the striped plateau lizard, Sceloporus virgatus. The relationship between estimated mating success and male phenotypic traits was examined for a sample of 55 males. DNA samples were obtained from 13 female-offspring families. The males with the closest spatial proximity to each female were tested as possible sires within each family. Fingerprinting with two multilocus hypervariable minisatellite probes revealed a strong correspondence between male-female spatial proximity and actual paternity. Paternity could be assigned for 72 of the 100 hatchlings. Most hatchlings with identifiable sires were attributed to a male with the highest category of spatial proximity to the mother. However, there was a low to moderate level of multiple paternity within clutches, and for some clutches probable sires could not be identified even though the most likely behavioural candidates were tested. Thus, nonterritorial males or other males lacking strong social and spatial relationships with females may achieve some degree of reproductive success. Analysis of mating success revealed that male success increased with body size, up to a point beyond which larger size conferred no advantage. Received: 7 January 1997 / Accepted after revision: 16 June 1997     

Sexual size dimorphism in fallow deer (Dama dama): do larger, heavier males gain greater mating success?

Sexual size dimorphism may evolve as a result of both natural and sexual selection. In polygynous mammals, the main factor resulting in the evolution of large body size in males is the advantage conferred during competition for mates. In this study, we examined whether sexual selection acts on body size in mature fallow bucks (Dama dama) by examining how the following traits are inter-related: age, body (skeletal) size, body mass, prerut dominance rank, rut dominance rank and mating success. This is the first study to examine how all these factors are together related to the mating success of a large sexually dimorphic and polygynous mammal. We found that male mating success was directly related to body size, but not to body mass. However body mass was related to prerut dominance rank which was in turn strongly related to rut dominance rank, and thus there was an indirect relationship between mating success and body mass. Rut dominance rank was the variable most strongly related to mating success. Mating success among mature males was unrelated to age. We conclude that larger mature fallow bucks have advantages over other males when competing for matings, and sexual selection therefore continues to act on sexual size dimorphism in this species. Heavier fallow bucks also have advantages, but these are mediated through the dominance ranks attained by males before the rut.     

Selective harvesting and habitat loss produce long-term life history changes in a mouflon population.

We examined the long-term effects (28 years) of habitat loss and phenotype-based selective harvest on body mass, horn size, and horn shape of mouflon (Ovis gmelini musimon) in southern France. This population has experienced habitat deterioration (loss of 50.8% of open area) since its introduction in 1956 and unrestricted selective hunting of the largest horned males since 1973. Both processes are predicted to lead to a decrease in phenotype quality by decreasing habitat quality and by reducing the reproductive contribution of individuals carrying traits that are targeted by hunters. Body mass and body size of both sexes and horn measurements of males markedly decreased (by 3.4-38.3%) in all age classes from the 1970s. Lamb body mass varied in relation to the spatiotemporal variation of habitat closure within the hunting-free reserve, suggesting that habitat closure explains part of these changes. However, the fact that there was no significant spatial variation in body mass in the early part of the study, when a decline in phenotypic quality already had occurred, provided support for the influence of selective harvesting. We also found that the allometric relationship between horn breadth and horn length changed over the study period. For a given horn length, horn breadth was lower during the second part of the study. This result, as well as changes in horn curve diameter, supports the interpretation that selective harvesting of males based on their horn configuration had evolutionary consequences for horn shape, since this phenotypic trait is less likely to be affected by changes in habitat characteristics. Moreover, males required more time (approximately four years) to develop a desirable trophy, suggesting that trophy hunting favors the reproductive contribution of animals with slow-growing horns. Managers should exploit hunters' desire for trophy males to finance management strategies which ensure a balance between the population and its environment. However, for long-term sustainable exploitation, harvest strategy should also ensure that selectively targeted males are allowed to contribute genetically to the next generations.     

Body size preferences in the pot-bellied seahorse <Emphasis Type="Italic">Hippocampus abdominalis:</Emphasis> choosy males and indiscriminate females

Male seahorses (genus Hippocampus) provide all post-fertilization parental care, yet despite high levels of paternal investment, these species have long been thought to have conventional sex roles, with female mate choice and male–male competition. Recent studies of the pot-bellied seahorse (Hippocampus abdominalis) have shown that sex-role reversal occurs in high-density female-biased populations, indicating that male mating preferences may lead to sexual selection on females in this species. Egg size, egg number, and offspring size all correlate positively with female body size in Hippocampus, and by choosing large mating partners, male seahorses may increase their reproductive success. While male brood size is also positively correlated with body size, small H. abdominalis males can carry exceptionally large broods, suggesting that the fecundity benefits of female preference for large partners may be limited. We investigated the importance of body size in reproductive decisions of H. abdominalis, presenting focal individuals of both sexes with potential mating partners of different sizes. Mating preferences were quantified in terms of time spent courting each potential partner. Male seahorses were highly active throughout the mate-choice trials and showed a clear behavioral preference for large partners, while females showed significantly lower levels of activity and equivocal mating preferences. The strong male preferences for large females demonstrated here suggest that sexual selection may act strongly on female body size in wild populations of H. abdominalis, consistent with predictions on the importance of female body size for reproductive output in this species. An erratum to this article can be found at     

Temporary alteration of local social structure in a threatened population of Cuban iguanas (Cyclura nubila)

In small, insular populations, behavioral patterns that lead to increased variance in individual reproductive success can accelerate loss of genetic variation. Over a 1-year period, we documented behavior and hormone levels in a breeding group of adult Cuban iguanas (Cyclura nubila) at Guantánamo Bay. Male dominance was associated with body and head size, display behavior, testosterone levels, home-range size, and proximity to females. Based on their success in agonistic encounters, we ranked males in a linear dominance hierarchy. During the subsequent breeding season, we conducted a removal experiment in which the five highest-ranking males were temporarily relocated from the study site. Although we were unable to assess reproductive success directly, previously lower-ranking males assumed control of vacated territories, won more fights, and increased their proximity to females in the absence of the dominant males. When it results in greater mating opportunities for otherwise socially suppressed individuals, temporary alteration of local social structure may help limit erosion of genetic variation in small, insular populations. Electronic Publication     

Swarming and mating behavior of a mayfly Baetis bicaudatus suggest stabilizing selection for male body size

Large size often confers a fitness advantage to female insects because fecundity increases with body size. However, the fitness benefits of large size for male insects are less clear. We investigated the mating behavior of the mayfly Baetis bicaudatus to determine whether the probability of male mating success increased with body size. Males formed mating aggregations (swarms) ranging from a few to hundreds of individuals, 1-4 m above the ground for about 1.5-2 h in the early morning. Females that flew near swarms were grabbed by males, pairs dropped to the vegetation where they mated and then flew off individually. Some marked males returned to swarms 1, 2 or 3 days after marking. Larger males swarmed near spruce trees at the edges of meadows, but the probability of copulating was not a function of male body size (no large male advantage). Furthermore, the potential fitness advantage of mating with larger, more fecund females was not greater for large males (no size-assortative mating). However, the sizes of copulating males were significantly less variable than those of non-mating males collected at random in swarms. Intermediate male size may be optimal during mating because of trade-offs between flight agility and longevity or competitive ability. Results of this study are consistent with the hypotheses that there is stabilizing selection on adult male body size during mating, and that male body size in this species may be influenced more by selection pressures acting on larvae than on adults.     

Body size,morphological constraints,and mated pair formation in four populations of horseshoe crabs (Limulus polyphemus) along a geographic cline

The relationship of body size to mating success was studied in four populations of horseshoe crabs, Limulus polyphemus L., along the east coast of the U.S.A. in the spring and summer from 1986 to 1989. Crabs of both sexes from Great Bay, New Hampshire, were significantly smaller than crabs from three middle Atlantic coast populations: Sandy Hook Bay and Delaware Bay, New Jersey, and Chincoteague Bay, Virginia. The formation of mated pairs was independent of body size in each population; there were no significant size differences between mated and single individuals, and size assortative mating did not occur. A comparision of male clasper dimensions with the corresponding point of attachment on the female indicates that there are no morphological constraints limiting amplexus between any male and any female within a population. The ratios of male to female prosoma width within amplexed mated pairs averaged from 0.78 to 0.80 in each population, despite the large difference in absolute size between southern and nothern populations. This may suggest a role for natural selection in regulating the relative sizes of each sex.     

Sexual dimorphism and male mating success in the tentacled blenny, <Emphasis Type="Italic">Parablennius tentacularis</Emphasis> (Teleostei: Blenniidae)

Although external sexually dimorphic traits are commonly found in males of combtooth blenny species, little is known about the benefit they can convey to male mating success. Indeed, while female preferences for large males have been demonstrated in some species, the possible role played by dimorphic ornaments has been neglected. We now report on the tentacled blenny, Parablennius tentacularis, a species where males are characterized by bulb glands on the anal fin and both sexes exhibit a dark spot on the dorsal fin and orbital tentacles. Males are territorial, make nests in empty bivalve shells, and provide solitary parental care for the eggs. Using morphometric analysis and field collected data on male and female external features, nest characteristics and number of eggs in the nests, we have assessed the development of dimorphic traits in both sexes and male mating success. The results reveal that orbital tentacles of males are more developed and more variable in size than those of females. Larger males exhibit longer orbital tentacles and larger anal glands but do not necessarily occupy larger nests. Male mating success is significantly correlated with the inner nest surface area and with orbital tentacle size but not with body size. These results provide support for a primary role of male ornaments in enhancing blenny male mating success and are discussed in the context of mate choice for direct and indirect benefits.     

Sexual selection and the function of a melanin-based plumage ornament in polygamous penduline tits <Emphasis Type="Italic">Remiz pendulinus</Emphasis>

Melanin-based ornaments are often involved in signaling aggression and dominance, and their role in sexual selection is increasingly recognized. We investigated the functions of a melanin-based plumage ornament (facial ‘mask’) in male Eurasian penduline tits Remiz pendulinus in the contexts of male–male aggression, mating success, and parental care. The penduline tit is a passerine bird with a unique mating system in which both sexes may mate with several mates in a breeding season, and one (or both) parent deserts the clutch. Our study revealed that mask size of males is more likely an honest signal used by females in their mate choice decisions than a trait involved in male–male competition. First, mask size increased with both age and body condition, indicating that the mask may signal male quality. Second, males with larger masks paired more quickly and had more mates over the breeding season than males with smaller masks. Third, we found no evidence that male mask size signals male–male aggression or dominance during competitive encounters. The increased mating success of large-masked males, however, did not translate into higher reproductive success, as nestling survival decreased with mask size. Therefore, we conclude that there is either no directional selection on male mask size or males with larger masks receive indirect, long-term benefits.     

Provisional rejection of three alternative hypotheses on the maintenance of a size dichotomy in males of Dawson’s burrowing bee,Amegilla dawsoni (Apidae,Apinae, Anthophorini)

I examine three alternative hypotheses on the male size dimorphism of Dawson’s burrowing bees (Amegilla dawsoni) in which there are large (major) and small (minor) males. One possibility is that minor males are simply the incidental byproduct of environmental conditions that prevent females from provisioning brood cells optimally. This hypothesis is not supported by the finding that males of intermediate size are consistently rare in populations sampled across years and in different regions, nor can it easily account for the absence of a size dichotomy in females. A second possibility is that minors represent a “best of a bad job” response of those females that are small or otherwise disadvantaged. However, presumptive male siblings sometimes include both majors and minors, a result not predicted from this hypothesis. A third explanation is that female brood provisioning strategy results in the production of minors and majors with equal fitness benefit to fitness cost ratios. However, although it is true that minor males weigh on average about half what a major weighs, and so represent approximately half the provisioning expense of a major, minor males on average appear to secure far fewer than half the number of matings of majors. If the estimate of mating success of minors is accurate, the net gain to females from producing a minor son is unlikely to equal that derived from a major son. Therefore the third hypothesis must also be tentatively rejected, although with caution given the uncertainties in estimating the relative costs and benefits of producing major and minor sons. Received: 12 January 1996/Accepted after revision: 27 April 1996     

Size-assortative mating and non-reciprocal copulation in a hermaphroditic intertidal limpet: test of the mate availability hypothesis

Mating behaviour and mating patterns are affected to a large extent by body size in both hermaphrodites and gonochorists. Detailed research on mating patterns, mate choice, pre- and post-copulatory sexual selection in marine pulmonates is wanting, thus warranting more attention as a study system in the future. The simultaneous hermaphroditic limpet Siphonaria capensis (Pulmonata: Basommatophora) shows size-dependent fecundity, and acts as a suitable organism to test the effects of body size on mating patterns, mating success, gender expression and reciprocity in hermaphroditic marine gastropods. We mainly used bootstrap resampling techniques to estimate the effects of different factors on mating patterns. In the populations studied, a strong size-assortative mating pattern was observed, where small-scale spatial distribution of potential mating partners (the mate availability hypothesis) could explain 65% of this pattern, while mate choice and mating constraints explaining the rest. No significant difference in mating success between limpets with different body size was found, even though in one population, the sperm recipients were larger than the non-copulating limpets. Interestingly, we found that intromission was non-reciprocal during copulations. This may mean that this species does not conform to the common rule of reciprocity predicted for hermaphrodites, unless there is sex-role alternation between individuals in a mating pair. The mating partners consisted of similar sized, acting males and females, thus without any indication of the body size determining the gender expression. The matings took place in early mornings only at spring tides and the animals were observed laying egg masses only during neap tides. Since siphonariid limpets possess both a spermatheca and a seminal vesicle, which may have either a sperm storing and/or digestive function, post-copulatory sexual selection (e.g., sperm competition) cannot be dismissed.     

The role of body size in early mating behavior in a simultaneous hermaphrodite, <Emphasis Type="Italic">Chelidonura sandrana</Emphasis>

Intraspecific variation in mating behavior is widespread among simultaneous hermaphrodites but its underlying sources remain largely unexplored. In the sea slug Chelidonura sandrana, most matings are reciprocal. However, despite non-conditional sperm exchange and potential polygamy-mediated benefits, 30% of matings end after unilateral insemination. To resolve this apparent inconsistency, we here investigated the effect of body size on the frequency of reciprocal matings by testing the following two hypotheses. First, sex-allocation theory predicts that the likelihood of reciprocity depends on the size difference between mating partners. Second, if both sex functions temporally differ in reaching maturity, reciprocal matings should be more frequent with increasing absolute body size of the smaller partner. The likelihood of reciprocity increased with body size of the smaller partner. Moreover, smaller individuals acted more often as males among unilateral matings. These findings suggest that the ability to donate sperm develops prior to female functionality in C. sandrana.     

Selection for early emergence, longevity and large body size in wingless, sib-mating ant males

Sexual selection has led to male morphologies and behaviours that either increase male attractiveness or their success in male–male competition. We investigated male traits under selection in the ant Hypoponera opacior, in which wingless males mate with pupal queens inside their natal colony and guard their partners for hours. The lack of female choice and fights among adult males makes this species an ideal study system to investigate sexual selection in the absence of these selective forces. We hypothesised that males, which emerge first and live longer, should have a higher mating success because of more mating opportunities, reduced competition and the ability to kill pupal competitors. We recorded the number and length of matings and tested whether these measures of male-mating success were associated with emergence order, lifespan and body size. Indeed, early emerged males mated more often and longer than their later-emerging rivals. Furthermore, longer-lived and larger males obtained more matings. Body size might be important because larger males either produce more sperm or perform better in mounting females. We found no evidence for a trade-off between body size and emergence time. Moreover, male removal manipulations revealed that males quickly adapt their guarding behaviour to changes in the competitive environment. Under reduced competition, males guarded their partners for shorter periods. In conclusion, these sib-mating ant males are under selection to develop fast, to live long, to be large and to be able to respond to the competitive situation in the nest.     

Experimental assessment of ecological and phenotypic factors affecting male mating success and polyandry in northern watersnakes, Nerodia sipedon

To resolve conflicting field observations regarding the action of sexual selection, we used breeding experiments and paternity analysis of the 927 resulting offspring to assess how male size, condition, tail length, genetic similarity to the female, and variation in operational sex ratio (OSR) affected male reproductive success and the incidence of polyandry in northern watersnakes (Nerodia sipedon). Only size affected male mating success. Large males were more successful, but only when male size varied substantially and competition among males was intense (i.e., male-biased OSR). The conditional nature of the size advantage may explain why studies of free-living watersnakes have produced inconsistent results regarding the relationship between male size and mating success. Size differences between males did not affect the proportion of offspring each male sired within multiply sired litters. We found positive size-assortative mating, but only when the OSR was female biased, suggesting that smaller males had improved access to females when competition among males was reduced, but that competition with larger males still restricted mating opportunities of small males to less preferred, smaller females. Most litters (58%) were multiply sired and larger females were more likely to produce multiply sired litters, similar to free-living watersnakes. There was no association between the incidence of multiple paternity and OSR, however, suggesting that polyandry is not simply a function of opportunity, with females passively waiting for males to court them.     

Male mating competitiveness and age-dependent relationship between testosterone and social rank in bighorn sheep

In males, the acquisition and development of behavioral and morphological secondary sexual traits typically depends on testosterone and correlates with mating success. Testosterone level could affect competition for mates and thus be a target of sexual selection. We sought to relate testosterone levels to male mating competitiveness, by teasing apart the relationships between testosterone, behavior, and growth before the mating period. We monitored 24 adult bighorn rams (Ovis canadensis) at Ram Mountain, Alberta, from 2008 to 2011. Using linear mixed models, we tested the relationships between testosterone metabolites in feces, social rank, and both growth and size of two sexually selected traits: horns and body mass. The correlation between testosterone and social rank varied with age. Testosterone and rank were weakly and negatively correlated for young rams, positively correlated for prime-aged rams, and negatively correlated for older rams. Although testosterone had an increasingly positive effect on total horn length until 8 years of age, we could not detect any effects on annual growth rate of horns or body mass. Testosterone may be related to male’s ability to compete for mates through its relationship with behaviors determining social rank, rather than by influencing the development of morphological traits. Differences in testosterone levels among competitors may be a proximate cause of variance in fitness.     

Lifetime mating success in a natural population of the damselfly,Enallagma hageni (Walsh) (Odonata: Coenagrionidae)

Summary Variance in lifetime mating success was measured for individuals of a population of Enallagma hageni, a non-territorial damselfly in northern Michigan. E. hageni is an explosive breeder with scramble competition for mates. Highly skewed operational sex ratios resulted in intense male-male competition which took the form of interference with tandem pairs. 41% of the males failed to mate in their lifetime as opposed to only 3.6% mating failure in females. The effect on mating success of size, age, longevity, and time spent at the breeding site were investigated. Intermediate sized males obtained the most matings, and male lifetime mating success was highly correlated with longevity.