Pairing Patterns in Japanese Beetles (Popillia japonica Newman): Effects of Sex Ratio and Time of Day

Size-related patterns between unpaired and paired individuals and between males and females of a given pair give clues about both a species' sexual behavior and the environmental factors affecting its sexual behavior. We studied the mating patterns of Japanese beetles (Popillia japonica) in east–central Illinois. The frequency of male–female pairs varied significantly among days and within a day, with pairs being significantly more common in the morning and the evening. The sex ratio on the food plants was significantly male biased, but although the sex ratio fluctuated among days and among time periods, the variation in the frequency of mating pairs was not explained by variation in the sex ratio. We found no assortative pairing with respect to size, but sizes of paired and unpaired individuals did differ. Paired females were larger than unpaired females at all time periods. In contrast, paired males were larger at 0700 and smaller at 1000, and little difference existed at other times of the day. The size of males and females, sex ratio, and pairing frequency also differed among days. Much of this variation in size and pairing frequency was related to a seasonal effect: later in the summer, beetles of both sexes were smaller and pairs were less common. Interestingly, pairs were also less frequent on days with higher average temperatures. This between-day variation in pairing, in combination with the within-day pairing differences, suggests that the temperature may alter the cost, and hence likelihood, of pairing in this species.     

Mating Patterns of Red-Eyed Treefrogs, Agalychnis callidryas and A. moreletii

Deviations from random mating in frogs are often explained by two different size‐based patterns. The large‐male mating advantage predicts that males found in amplexus with females will be larger on average than non‐amplectant males, whereas size‐assortative mating predicts that males and females found in amplexus will maintain an optimal size ratio. Both these pairing patterns are consistent with a female mating preference for larger males, or for males of a given size relative to the choosy female. I examined pairing patterns of two species of Neotropical hylids, Agalychnis callidryas and A. moreletii for three consecutive breeding seasons in Belize, Central America to evaluate whether mating behavior was influenced by either a large‐male mating advantage or size‐assortative mating. For each species, I compared size traits between amplectant and non‐amplectant males, and within amplectant pairs, and I quantified fertilization success for each amplectant pair. For both species I found evidence of deviations from random mating by size, but the nature of the deviations varied between species and among years. The proportion of eggs fertilized was consistently high among years for both species and there was no relationship between fertilization success and the size ratio of amplectant pairs. These data are consistent with female mate preference, but a role for male–male competition cannot be excluded. My findings suggest that mating patterns may be density‐dependent and that the nature and intensity of sexual selection may be increased by extreme environmental conditions.     

Ecological context and the importance of body and gnathopod size for pairing success in two amphipod ecomorphs

Ecological context generates interspecific variation in mating behavior by imposing differential constraints on the action of sexual selection, but operation of these constraints in nature is not well understood. We used field and laboratory studies to examine the importance of body size and size of sexually dimorphic appendages, the gnathopods, for pairing success in two freshwater amphipod species within the Hyalella azteca species complex. We focused on a large-bodied species found in habitats where ecological factors tend to favor large body size, and a small-bodied species in habitats where small body size is favored by size-selective predation. A field study indicated that although male pairing success was greater for larger males in both species, pairing success increased throughout the range of variation in male size in the large species, whereas, in the small species, pairing success was low for smaller individuals, but roughly constant across intermediate to larger sizes. A laboratory mate choice experiment was consistent with the field study, finding that females of the large species exhibited a preference for larger males that was independent of absolute male size, but females of the small species were indifferent when choosing between males of intermediate to larger size. Species also differed in the direction of sexual size dimorphism in the field, with males being the larger sex in the large species but the smaller sex in the small species, a pattern consistent with the species differences in mate preference. Large gnathopod size relative to body size was associated with enhanced pairing success across all body sizes for the large species, but, in the small species, large gnathopod size enhanced pairing success only in smaller males. Species differences in mating behavior appear consistent with change driven by differing forms of the interaction between sexual and natural selection.     

Male Choice and Male-male Competition in Idotea baitica (Crustacea,Isopoda)

Sexual selection in mate-guarding Crustacea may involve several processes: male choice, male-male competition, and female choice. To evaluate the relative importance of the different processes in mate choice of the aquatic isopod I. baitica we studied 1) the mate-choice criteria of males, 2) effects of sex ratio on the outcome of the mating contest, and 3) the role of size in male-male interactions. When given a choice between a small and a large female, males most often chose the one that matured earlier for parturial ecdysis. Maturity was a more important choice criterion than female size, but these also correlated positively. Large males had a mating advantage in both male- and female-biased sex ratios; pairing was size-assortative only in the male-biased ratio where guarding was also longer. If an extra male was placed with a precopulatory pair, 30 % take-overs occurred, large males surpassing. Present and earlier work suggests that male size is an asset in both intra- and intersexual interactions. There is little or no direct phenotypic sexual selection on female size: sexual selection for large males presumably contributes to the evolution of sexual size dimorphism in I. baitica.     

FEMALE-FEMALE COMPETITION IN KATYDIDS: SEXUAL SELECTION FOR INCREASED SENSITIVITY TO A MALE SIGNAL?

In contrast to studies of sex-specific weaponry and other sexually selected traits, there has been no examination of Darwin's (1871, p. 418) suggestion that elaborations or enlargements of “the organs of sense” function to enhance mating success. In certain katydids the size of thoracic spiracles, which are a main input into the hearing system, determines auditory sensitivity of females. Here we present evidence that sexual dimorphism in the spiracle size of a pollen katydid, Kawanaphila nartee, is a result of sexual selection on females competing to locate nuptial-gift giving males. In field experiments in which female K. nartee were attracted to a calling male, we show a pairing advantage to females with larger auditory spiracles. The spiracle-size advantage was not a correlated result of a larger body size or mass of winners. Finally, there was no spiracle-size advantage or body-mass advantage for mating females in a later stage of competition when experimental females struggled for access to a silent male. We suggest that research on the detection of displays has lagged behind work on the displays themselves; the focus has been on the species specificity of signal perception rather than on the fitness consequences of variation in the ability to detect cues from mates or predators.     

RAPID LOSS OF BEHAVIORAL PLASTICITY AND IMMUNOCOMPETENCE UNDER INTENSE SEXUAL SELECTION

Phenotypic plasticity allows animals to maximize fitness by conditionally expressing the phenotype best adapted to their environment. Although evidence for such adjustment in reproductive tactics is common, little is known about how phenotypic plasticity evolves in response to sexual selection. We examined the effect of sexual selection intensity on phenotypic plasticity in mating behavior using the beetle Callosobruchus maculatus. Male genital spines harm females during mating and females exhibit copulatory kicking, an apparent resistance trait aimed to dislodge mating males. After exposing individuals from male‐ and female‐biased experimental evolution lines to male‐ and female‐biased sociosexual environments, we examined behavioral plasticity in matings with standard partners. While females from female‐biased lines kicked sooner after exposure to male‐biased sociosexual contexts, in male‐biased lines this plasticity was lost. Ejaculate size did not diverge in response to selection history, but males from both treatments exhibited plasticity consistent with sperm competition intensity models, reducing size as the number of competitors increased. Analysis of immunocompetence revealed reduced immunity in both sexes in male‐biased lines, pointing to increased reproductive costs under high sexual selection. These results highlight how male and female reproductive strategies are shaped by interactions between phenotypically plastic and genetic mechanisms of sexual trait expression.     

Intraspecific mating system evolution and its effect on complex male secondary sexual traits: Does male–male competition increase selection on size or shape?

Sexual selection is generally held responsible for the exceptional diversity in secondary sexual traits in animals. Mating system evolution is therefore expected to profoundly affect the covariation between secondary sexual traits and mating success. Whereas there is such evidence at the interspecific level, data within species remain scarce. We here investigate sexual selection acting on the exaggerated male fore femur and the male wing in the common and widespread dung flies Sepsis punctum and S. neocynipsea (Diptera: Sepsidae). Both species exhibit intraspecific differences in mating systems and variation in sexual size dimorphism (SSD) across continents that correlates with the extent of male–male competition. We predicted that populations subject to increased male–male competition will experience stronger directional selection on the sexually dimorphic male foreleg. Our results suggest that fore femur size, width and shape were indeed positively associated with mating success in populations with male‐biased SSD in both species, which was not evident in conspecific populations with female‐biased SSD. However, this was also the case for wing size and shape, a trait often assumed to be primarily under natural selection. After correcting for selection on overall body size by accounting for allometric scaling, we found little evidence for independent selection on any of these size or shape traits in legs or wings, irrespective of the mating system. Sexual dimorphism and (foreleg) trait exaggeration is therefore unlikely to be driven by direct precopulatory sexual selection, but more so by selection on overall size or possibly selection on allometric scaling.     

Reproductive characteristics of a captive colony of big-eared climbing rats (Ototylomys phyllotis)

The authors analyzed the breeding characteristics of a colony of Ototylomys phyllotis (big-eared climbing rat) from Campeche, México, that was bred in captivity for 6 y. The big-eared climbing rat is a reservoir of Leishmania (Leishmania) mexicana, a causal agent of localized cutaneous leishmaniasis on the Yucatán Peninsula. The colony had been established to facilitate studies analyzing the effectiveness of O. phyllotis as an experimental model for L. (L.) mexicana. The authors describe the housing and husbandry of the colony, the procedures used for mating the animals and the behavior of the animals during mating. They report that the animals showed social behavior and could be bred successfully. Most breeding pairs successfully produced litters; some pairs produced more than one litter. The authors also report data for other parameters, such as the interval between pairing and birth or between births of consecutive litters, litter size, survival to weaning, the timing of sexual maturity and the effects of breeder age on breeding success.     

Reproductive Success and Sexual Selection in Wild Eastern Tiger Salamanders (<Emphasis Type="Italic">Ambystoma t. tigrinum</Emphasis>)

Variation in reproductive success is most pronounced in species with strongly biased operational sex ratios, prominent sexual dimorphisms, and where mate competition and choice are likely. We studied sexual selection in eastern tiger salamanders (Ambystoma t. tigrinum) and examined the role of body size on reproductive success. We genotyped 155 adults and 1,341 larvae from 90 egg masses at six microsatellite loci. Parentage analyses revealed both sexes engaged in multiple matings, but was more common among females (64%) than males (27%). However, the standardized variance in mating and reproductive success was higher in males. Bateman gradients were significant and nearly identical in both sexes, suggesting that sexual selection was roughly equal between sexes. Body size was not correlated with mating or reproductive success in either sex. The apparent lack of sexual selection on body size may be a result of sperm storage, sperm competition, alternative mating tactics, and/or random induction of spermatophores.     

Opportunistic and restrictive matings among wild chimpanzees in the Mahale Mountains,Tanzania

The mating patterns of free-ranging chimpanzees (Pan troglodytes) of the Mahale Mountains, Tanzania, were studied. Opportunistic mating (non-competitive and temporary mating) was frequently observed in a large-sized unit-group, among young, low-ranking males, and among young, newcomer, non-ovulating females. Restrictive mating (a continuous sexual relationship between a particular pair which includes possessiveness and consortship) was frequently observed in a small-sized unit-group, among middle- and old-aged, high-ranking males, and among old, resident, ovulating females. Relations between those characteristics, such as group size and composition, ages of the individuals of both sexes, female estrous stages, and life history, and the 2 mating patterns are discussed.     

Influence of Nutrition on Courtship and Mating in the Scorpionfly Panorpa cognata (Mecoptera, Insecta)

Scorpionflies have been used as model organisms for the study of alternative male mating tactics as well as sexual conflict and coercive mating. Here we describe the courtship and mating behaviour of the scorpionfly Panorpa cognata at different levels of nutrition. Alternative mating tactics in scorpionflies involve nuptial food gifts, and we expected an effect of nutrient availability and male individual condition on the relative frequency of these mating tactics. Subsequent to female attraction by means of male pheromonal emission (calling) and a conspicuous pairing prelude, the majority of matings were initiated by male secretion of one relatively large salivary mass on which females feed during copulation. Usually, males produced only a single salivary mass per mating, and the copulation was terminated after the female had consumed the salivary mass. Alternatively, in 40% of the copulations, males offered females a dead arthropod as nuptial gift. However, these matings were neither preceded by male calling nor by the pairing prelude. Copulations with no gifts were extremely rare, and forced copulations were absent. The manipulation of the clamp‐like notal organ used by male scorpionflies in coercive matings had no effect on the duration of copulation, suggesting that P. cognata males are not able to enforce longer matings. Copulations involving salivary mass gifts were significantly longer than copulations with prey provided as gifts. Although contrary to our expectations, nutrition had no effect on the relative frequency of the different male mating tactics, it had several effects on courtship and mating. First, well‐fed individuals copulated significantly more often, both with prey and salivary secretions, than individuals with limited nutrient resources available. This was true for both sexes, although the effect was stronger for males. Higher availability of nutrients decreased the time until male and female sexual maturity and increased male calling duration per day. Furthermore, high nutrient availability decreased the duration of the pairing prelude, and consequently pairs started copulating earlier at night in the high nutrient treatment.     

Is allometry of sexual traits adaptive? A field test with territorial damselflies

Recent studies have linked static allometry of sexual traits to selective advantages, in terms of sexual selection. An underlying, yet untested, assumption is that the allometry of sexual traits confers higher mating success and/or survival. Here, we investigated whether the allometry of two sexual traits is related to male mating success and survival in two species of damselflies: wing size in Paraphlebia zoe and the red‐pigmented wing spot in Hetaerina americana. We used large field‐based data sets of marked‐recaptured animals, in which we recorded male mating success and survival. Both sexual traits exhibited hyperallometric patterns; however, allometry was not linked to either mating success or survival. These results indicate that, at least during the period of sexual competition, allometry does not seem to be adaptive. Although our results may only apply to our damselfly study subjects (which nevertheless would require further tests in different seasons and/or study sites), our findings should encourage researchers to evaluate at least whether the assumed adaptiveness of sexual trait allometry holds for their study animals. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 327–334.     

Is bigger better? Male body size affects wing‐borne courtship signals and mating success in the olive fruit fly,Bactrocera oleae (Diptera: Tephritidae)

Variations in male body size are known to affect inter‐ and intrasexual selection outcomes in a wide range of animals. In mating systems involving sexual signaling before mating, body size often acts as a key factor affecting signal strength and mate choice. We evaluated the effect of male size on courtship displays and mating success of the olive fruit fly, Bactrocera oleae (Diptera: Tephritidae). Wing vibrations performed during successful and unsuccessful courtships by large and small males were recorded by high‐speed videos and analyzed through frame‐by‐frame analysis. Mating success of large and small males was investigated. The effect of male–male competition on mating success was evaluated. Male body size affected both male courtship signals and mating outcomes. Successful males showed wing‐borne signals with high frequencies and short interpulse intervals. Wing vibrations displayed by successful large males during copulation attempt had higher frequencies over smaller males and unsuccessful large males. In no‐competition conditions, large males achieved higher mating success with respect to smaller ones. Allowing large and small males to compete for a female, large males achieve more mating success over smaller ones. Mate choice by females may be based on selection of the larger males, able to produce high‐frequency wing vibrations. Such traits may be indicative of “good genes,” which under sexual selection could means good social‐interaction genes, or a good competitive manipulator of conspecifics.     

EVOLUTION OF DIVERGENT FEMALE MATING PREFERENCE IN RESPONSE TO EXPERIMENTAL SEXUAL SELECTION

Sexual selection is predicted to drive the coevolution of mating signals and preferences (mating traits) within populations, and could play a role in speciation if sexual isolation arises due to mating trait divergence between populations. However, few studies have demonstrated that differences in mating traits between populations result from sexual selection alone. Experimental evolution is a promising approach to directly examine the action of sexual selection on mating trait divergence among populations. We manipulated the opportunity for sexual selection (low vs. high) in populations of Drosophila pseudoobscura. Previous studies on these experimental populations have shown that sexual selection manipulation resulted in the divergence between sexual selection treatments of several courtship song parameters, including interpulse interval (IPI) which markedly influences male mating success. Here, we measure female preference for IPI using a playback design to test for preference divergence between the sexual selection treatments after 130 generations of experimental sexual selection. The results suggest that female preference has coevolved with male signal, in opposite directions between the sexual selection treatments, providing direct evidence of the ability of sexual selection to drive the divergent coevolution of mating traits between populations. We discuss the implications in the context sexual selection and speciation.     

Sexual size dimorphism and sexual selection in primates

1. In most animals, females are larger than males. Paradoxically, sexual size dimorphism is biased towards males in most mammalian species. An accepted explanation is that sexual dimorphism in mammals evolved by intramale sexual selection. I tested this hypothesis in primates, by relating sexual size dimorphism to seven proxies of sexual selection intensity: operational sex ratio, mating system, intermale competition, group sex ratio, group size, maximum mating percentage (percentage of observed copulations involving the most successful male), and total paternity (a genetic estimate of the percentage of young sired by the most successful male).
2. I fitted phylogenetic generalised least squares models using sexual size dimorphism as the dependent variable and each of the seven measures of intensity of sexual selection as independent variables. I conducted this comparative analysis with data from 50 extant species of primates, including Homo sapiens, Pan troglodytes, and Gorilla spp.
3. Sexual dimorphism was positively related to the four measures of female monopolisation (operational sex ratio, mating system, intermale competition, and group sex ratio) and in some cases to group size, but was not associated with maximum mating percentage or total paternity. Additional regression analyses indicated that maximum mating percentage and total paternity were negatively associated with group size.
4. These results are predicted by reproductive skew theory: in large groups, males can lose control of the sexual behaviour of the other members of the group or can concede reproductive opportunities to others. The results are also consistent with the evolution of sexual size dimorphism before polygyny, due to the effects of natural, rather than sexual, selection. In birds, the study of molecular paternity showed that variance in male reproductive success is much higher than expected by behaviour. In mammals, recent studies have begun to show the opposite trend, i.e. that intensity of sexual selection is lower than expected by polygyny.
5. Results of this comparative analysis of sexual size dimorphism and sexual selection intensity in primates suggest that the use of intramale sexual selection theory to explain the evolution of polygyny and sexual dimorphism in mammals should be reviewed, and that natural selection should be considered alongside sexual selection as an evolutionary driver of sexual size dimorphism and polygyny in mammals.

     

EVOLUTION,PHYLOGENY, SEXUAL DIMORPHISM AND MATING SYSTEM IN THE GRACKLES (QUISCALUS SPP.: ICTERINAE)

According to theory, two consequences of sexual selection are sexual dimorphism in size and secondary sexual characteristics, due to either intra- or intersexual selection. In this paper I suggest three criteria for the test of an evolutionary hypothesis involving quantitative morphological characters. First, the postulated change must be shown to have occurred in evolutionary time. Second, this change must be positively correlated with a change in the proposed selective agent. Third, given two taxa with different degrees of sexual size dimorphism and different mating system, the possible influence of drift must be rejected. If the hypothesis is not rejected by these three criteria, then we still have no proof of causality, but we can at least be more confident about its plausibility. This is applied to the particular hypothesis that sexual dimorphism in the Boat-tailed and Great-tailed grackles (Quiscalus spp; Icterinae; Aves) is caused by the highly polygynous mating system in these species. In relation to an outgroup, both species have increased disproportionately in male tarsus and tail size, creating an increased sexual dimorphism. This has cooccurred with the evolution of their particular mating system. However, the variance among species in male tarsus size can be accounted for by drift, and need not be a result of selection for increased size. In contrast, the variance among species in male tail size was much larger than expected under a null model of drift, indicating directional selection for long tails. The variance in female tail size was not larger than expected by drift, whereas the variance in female tarsus size was in fact lower than expected by drift, indicating stabilizing selection. The data are consistent with the hypothesis with regard to tail size, but not with regard to body size.     

Frequency of interspecific mating in salamanders of the plethodontid genus Desmognathus: different experimental designs may yield different results

Heterosexual courtship trials were staged between dusky salamanders of two species ( Desmognathus fuscus and D. santeetlah ) from two allopatric populations in the southern Appalachian Mountains of eastern North America. Two experiments were conducted in order to test for effects of experimental design on estimates of the strength of sexual isolation between these species. In Experiment 1, frequencies of interspecific mating were determined when: (1) a male was presented with a conspecific and a heterotypic female successively; and (2) both types of females were presented simultaneously to a male. The frequency of interspecific mating by male D. fuscus was greatest when females were presented successively. The frequency of interspecific mating by male D. santeetlah was not influenced by method of female presentation. An index of sexual isolation of +0.29 was obtained between D. fuscus and D. santeetlah when females were presented successively, and of +0.63 when females were presented simultaneously. In Experiment 2, interspecific mating frequencies were determined in courtship arenas of two different sizes. No significant effect of arena size on frequency of interspecific mating was found for D. fuscus. In D. santeetlah , a trend towards greater mating frequency in small arenas was almost significant. These results suggest that interspecific mating frequencies (and thus estimates of the strength of sexual isolation between populations or species) may be sensitive to the experimental design employed. The validity and generality of this conclusion for other animals is an urgent, empirical issue.     

Size-mediated dominance and aggressive behavior of male Japanese fluvial sculpin Cottus pollux (Pisces: Cottidae) reduce nest-site abundance and mating success of conspecific rivals

Despite growing evidence for plasticity in the mating patterns of nest-holding animals in relation to the changes in nest abundance, the effects of aggressive interaction by dominant males on nest availability for conspecific rivals remains unclear. To quantify the effects of male–male competition on nest-site choice and mating success of the male Japanese fluvial sculpin Cottus pollux, we conducted experiments on 5 males from different 5 size classes under both sufficient and shortage nest-abundance conditions. Nest-choice experiments showed that both male size class and nest-abundance condition had significant effects on the nesting rates of males. Following the nest-choice experiments, 10 gravid females were added in the experimental tanks. Mating experiments revealed that male size, nesting rate before addition of females, and the number of courtship attempts on females were valid variables of male mating success, regardless of nest-abundance conditions. After achieving initial mating success, the largest nesting male exhibited more frequent aggressive interaction with other conspecific males than he did before obtaining eggs in his nest. Our results suggest that size-mediated dominance and aggressive behavior of males may disrupt nest acquisition of other conspecific males, and may consequently result in extreme variation in mating success among males even under sufficient nest-abundance conditions.     

Male mating success,sexual size dimorphism,and site fidelity in two species of Malacoctenus (Labrisomidae)

Synopsis In both Malacoctenus hubbsi and Malacoctenus macropus, males defended preferred oviposition sites from both other males and potential egg predators. In M. hubbsi, adult females were larger than adult males. Larger M. hubbsi males were not associated with territory parameters that were correlated with higher mating success, and male size was not correlated with mating success. Male size did affect mating success when territory parameters were statistically controlled for, but the failure of large males to associate with better territories eliminated any mating advantage for larger males. In M. macropus, males are larger than females. Larger males defended preferred oviposition sites, and had higher mating success than did smaller males. Male M. macropus also had much higher site fidelity than male M. hubbsi. These results suggest that the evolution of the differences in site fidelity and sexual size dimorphism between these two species may be due to sexual selection acting differentially in these two species.     

Pairing and insemination patterns in a giant weta (Deinacrida rugosa: Orthoptera; Anostostomatidae)

Positive size assortative mating can arise if either one or both sexes prefer bigger mates or if the success of larger males in contests for larger females leaves smaller males to mate with smaller females. Moreover, body size could not only influence pairing patterns before copulation but also the covariance between female size and size of ejaculate (number of spermatophores) transferred to a mate. In this field study, we examine the pre-copulatory mate choice, as well as insemination, patterns in the Cook Strait giant weta (Deinacrida rugosa). D. rugosa is a large orthopteran insect that exhibits strong female-biased sexual dimorphism, with females being nearly twice as heavy as males. Contrary to the general expectation of male preference for large females in insects with female-biased size dimorphism, we found only weak support for positive size assortative mating based on size (tibia length). Interestingly, although there was no correlation between male body size and the number of spermatophores transferred, we did find that males pass more spermatophores to lighter females. This pattern of sperm transfer does not appear to be a consequence of those males that mate heavier females being sperm depleted. Instead, males may provide lighter females with more spermatophores perhaps because these females pose less of a sperm competition risk to mates.