Homosexual Mating Displays in Penguins

More than 50 yr ago, field studies recorded the same‐sex pairs (and trios) of penguins displaying to each other during the mating season, using behavior patterns typical of heterosexual mating displays. Such observations led to a hypothesis that due to a lack of sex recognition pairing occurs at random with respect to sex, an idea countered by the argument that sex recognition is highly accurate. No quantification of same‐sex mating displays has tested the frequency of such displays in penguins or tested the hypothesis of random display partners with respect to sex. During their mating season, we studied displaying and paired king penguins, Apenodytes patagonicus, at Kerguelen Island and sexed them using a DNA marker, to quantify any occurrence of this behavior. Indeed, same‐sex courtship displays were common (28.3% of 53 displaying pairs), the great majority of which were between males. Some homosexually displaying males eventually paired with females, but such males were significantly slower in heterosexual pairing than males that did not display homosexually. In two extraordinary cases, same‐sex pairs learned each other’s calls, an essential step in the pairing process. The frequency of such pairs was much lower than among displaying couples, significantly so for males. Finally, the frequency of homosexually displaying pairs was significantly lower than expected from random assortment of displaying birds, for both males and females. We examined possible explanations for same‐sex display and its biological significance. A population sex‐ratio bias in favor of males and high concentration of male sex hormones may help to explain non‐reproductive homosexually displaying pairs.     

Effects of Environmental and Social Conditions on Homosexual Pairing in the Japanese Beetle (Popillia japonica Newman)

Homosexual pairing between males occurs under natural conditions in a wide variety of taxa, including many insect species, but few studies have investigated how environmental and social conditions affect same-sex pairing in insects. We investigated factors affecting homosexual pairing in male Japanese beetles (Popillia japonica Newman) in the field and in the laboratory. Specifically, we investigated how time of day, sex ratio, beetle density, and temperature affected the likelihood of homosexual pairing. In the field, male–male pairs constituted 1–6% of the pairs we collected. Homosexual pairs were more common in the afternoon than in the morning and the evening. Sex ratio, density, and temperature were all related to the likelihood of finding a homosexual pair, but the relationships were not linear. In the laboratory, higher male densities and relatively male-biased sex ratios were associated with an increase in the frequency of homosexual pairs. Homosexual pairs were more frequent at relatively low and relatively high temperatures. Males that mounted other males tended to be smaller than the males that they mounted. In addition, compared to males that were not homosexually paired, there was some indication that the mounting males were smaller, and the mounted males larger, than the unpaired males. Our data suggest that homosexual pairs are a result of males mistaking other males for females, and we hypothesize that the environmental and social factors cause changes in homosexual pairing through their effects on the frequency of pair formation and pair duration.     

Sexual Selection Under Scramble Competition: Mate Location and Mate Choice in the Eucalypt Leaf Beetle Chrysophtharta agricola (Chapuis) in the Field

For males of many species, the number of offspring sired can depend on the number of females mated. While pre- and postcopulatory choice by females can affect the outcome of potential mate encounters, mate location is a necessary prerequisite to any possible courtship and subsequent mating. Mate location of Chrysophtharta agricola in the field was examined using sticky traps baited with sexually receptive conspecific beetles. More beetles were caught on traps baited with conspecific beetles of either sex than on control traps that contained foliage only. Furthermore, 94% of beetles captured on control traps were males, indicating that the mating system of Chrysophtharta agricola can be labeled prolonged searching scramble competition polyandry, in which receptive females are evenly dispersed spatially and temporally, and males search competitively for them. Operational sex ratios were 1:1 throughout the season. By sampling paired and unpaired beetles in the field, we found that beetles generally did not select mates based on body size. Furthermore, neither sex mated preferentially with partners that were uninfected by parasitic mites or with beetles of the same generation. In the absence of postcopulatory female choice, the ability of males to locate females may therefore be the most important trait in determining male mating success.     

Is Bigger Better? Size and Pheromone Production in the Mountain Pine Beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Scolytidae)

Reproductive success in the mountain pine beetle, Dendroctonus ponderosae Hopkins, is determined by the production of aggregation and antiaggregation pheromones, as well as body size. In a laboratory experiment with beetles that emerged from naturally attacked hosts, there was no relationship between body size and the production of aggregation pheromones in either sex. In contrast, there were significant relationships between body size and the production of antiaggregation pheromones in males that were paired with females. Pheromone amounts decreased in paired females and antiaggregation pheromones correspondingly increased in paired males, suggesting that after pairing, males take over the role of pheromone production. Although males could potentially select large females by evaluating gallery size, and females could select large males on the basis of their strength in stridulation or physical courtship, we propose that mate choice occurs primarily by olfaction. Small individuals that produce large amounts of pheromone during initial attack could sabotage mate choice based on size-related criteria. This hypothesis is consistent with a lack of evidence for size assortative mating in 92 pairs of beetles. The production of antiaggregants by large male D. ponderosae after pairing with females appears to be an important factor in intraspecific resource partitioning, population regulation, and reproductive success.     

Assortative pairing with respect to parasite load in the beetle Timarcha maritima (Chrysomelidae)

Because of their effects on host reproductive behaviour, parasites are theoretically expected to create sometimes assortative mating among hosts, with heavily parasitized individuals pairing together and lightly parasitized ones pairing among themselves. We investigated the influence of protozoan gut parasites on the pairing pattern of the chrysomelid beetle Timarcha maritima. In the field, fecundity was negatively correlated with the parasite load of females, unpaired males were significantly more heavily infected than paired ones and, among pairs, males and females were matched for parasite load. Mate choice experiments in the laboratory showed that males have some ability to avoid heavily infected partners when given the choice between two females. Male competitiveness, measured as their mobility, was also negatively correlated with parasite load. These results indicate that parasite-related assortative pairing in this beetle could result from parasitized females being less fecund and parasitized males less competitive.     

Monogamy does not last long in Pontonia mexicana, a symbiotic shrimp of the amber pen-shell Pinna carnea from the southeastern Caribbean Sea

Theory suggests that symbiotic species should be monogamous and form long-lasting heterosexual pairs in/on hosts when inhabiting scarce and small hosts in environments where predation risk away from hosts is high (e.g., tropical subtidal). This prediction was tested with Pontonia mexicana which inhabits the relatively small and scarce amber pen-shell Pinna carnea in the tropical Caribbean. In agreement with theory, P. mexicana were found dwelling as heterosexual pairs in the mantle cavity of pen-shell individuals with more frequency than expected by chance alone. However, additional observations suggested that male-female pairing of P. mexicana does not last long. First, males paired with females that were close to molt and become sexually receptive more frequently than expected by chance alone. In monogamous species in which pairing appears to be long lasting, males occur with females in the same host, independent of the reproductive condition of the female. Second, the body size of paired shrimps was poorly correlated and the relationship between host and shrimp body size was weak. If males and females of P. mexicana were staying within host individuals for long periods of time, a tight correlation between host and shrimp size and between males and females in a pair would have been found. Lastly, sexual dimorphism in terms of cheliped size was evident; males invested considerably more resources to this body structure compared to females. In monogamous species in which pairing appears to be long-term, sexual dimorphism is low or absent given that sexual selection is weak in this mating system. Overall, our data suggests that monogamy in P. mexicana does not last long and that males switch among host individuals in search of receptive females. Manipulative experiments are necessary to understand the conditions favoring short- and long-term monogamy in symbiotic crustaceans.     

State-dependent pairing behaviour in male Gammarus pulex (L.) (Crustacea, Amphipoda): effects of time left to moult and prior pairing status

Because mating can be costly in terms of time and energy, an individual's propensity to engage in courtship and mating activities might be modulated by its physiological state. However, so far, state-dependent mate choice has received little attention The present study examined the effect of both prior pairing status and time left to the moult on the ability of male Gammarus pulex (Crustacea, Amphipoda) to enter in precopula with receptive females. In the lab, males that were freshly collected in precopula pairs in the field had a higher probability of re-pairing and were quicker to enter in precopula with receptive females compared to males of similar size that were freshly collected unpaired. In addition, unpaired males found in the field were closer to their moult than paired males. Considered together, our results strongly suggest that time left to the moult and prior mating status directly influence male propensity to pair in G. pulex.     

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.     

Mating system in the tarantula spider Eupalaestrus weijenberghi (Thorell, 1894): evidences of monandry and polygyny

The road tarantula Eupalaestrus weijenberghi shows a strongly female-biased sex ratio since adult females live several years while adult males live only for 2 months. In this scenario selective males could be expected. However, several factors such as the rates of reproduction of each sex, degree of sexual selectivity and synchronicity of female receptiveness determine the operational sexual ratio and mating system of the species. Our objective was to determine the mating rates and mating tactics for females and males of E. weijenberghi and their variation throughout the reproductive period. Four hundred sexual encounters among 20 females and 20 males in all possible pair-wise combinations were carried out during 29 days, a brief but intense experimental period, as it also occurs in the field. Mating success differed strongly between sexes. Females mated once: five females mated at the first attempt, eight initially rejected males and copulated in subsequent attempts. Half of the males did not copulate and the others copulated 1-3 times. Mated females actively rejected males. Results indicate a mating system with monogamous females and polygamous males. Not all the females were receptive in every reproductive season. We suggest that female monogamy drastically affects the operational sex ratio, since several females were unreceptive after a single copulation, directly diminishing the male potential reproductive rate. This is the first experimental approach to estimate tarantula mating systems, their determinants and the consequences of the strategies shown by each sex.     

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.     

Conflict over multiple-partner mating between males and females of the polygynandrous common lizards

The optimal number of mate partners for females rarely coincides with that for males, leading to a potential sexual conflict over multiple-partner mating. This suggests that the population sex ratio may affect multiple-partner mating and thus multiple paternity. We investigate the relationship between multiple paternity and the population sex ratio in the polygynandrous common lizard (Lacerta vivipara). In six populations the adult sex ratio was biased toward males, and in another six populations the adult sex ratio was biased toward females, the latter corresponding to the average adult sex ratio encountered in natural populations. In males the frequency and the degree of polygyny were lower in male-biased populations, as expected if competition among males determines polygyny. In females the frequency of polyandry was not different between treatments, and polyandrous females produced larger clutches, suggesting that polyandry might be adaptive. However, in male-biased populations females suffered from reduced reproductive success compared to female-biased populations, and the number of mate partners increased with female body size in polyandrous females. Polyandrous females of male-biased populations showed disproportionately more mating scars, indicating that polyandrous females of male-biased populations had more interactions with males and suggesting that the degree of multiple paternity is controlled by male sexual harassment. Our results thus imply that polyandry may be hierarchically controlled, with females controlling when to mate with multiple partners and male sexual harassment being a proximate determinant of the degree of multiple paternity. The results are also consistent with a sexual conflict in which male behaviors are harmful to females.     

Role of Contests in the Scramble Competition Mating System of a Leaf Beetle

Male competition for mates can occur through contests or a scramble to locate females. We examined the significance of contests for mates in the leaf beetle Chrysomela aeneicollis, which experiences a short breeding season. During peak mating season, 18–52% of beetles are found in male-female pairs, and nearly half of these are copulating. Sex ratios do not differ from parity, females are larger than males, and positive size-assortative mating occurs. Males fight (2–4% of beetles) over access to females, and disruption of mating usually follows these contests. In the laboratory, we compared mating and fighting frequencies for males found in mating pairs (field-paired) and single males placed into an arena with a field-paired female. Mating pairs were switched in half of arenas (new male-female pairs) and maintained in the other half. For 2 days, each male was free to move about and fight; thereafter males were tethered to prevent contests. Mating frequencies were significantly greater for field-paired than single males in both situations. Male size was not related to mating frequency; however, large females received more matings than small ones. These data suggest that males fight for high quality females, but otherwise search for as many matings as possible.     

Male Distribution and Mate Searching in the Yellow Dung Fly Scathophaga stercoraria: Comparison between Paired and Unpaired Males

Movements by individual males were examined in the yellow dung fly, Scathophaga stercoraria. Males were observed from their arrival until they found a female (paired males) or departed (unpaired males). The focal variables were the male mating status, body size and the number of males and females at the site. Paired males, independent of size, spent more time in the best mate-searching area (pat and the first up-wind zone) than unpaired males. Paired males in all size classes moved around and attacked other males more often than unpaired males. Among paired males, males that caught a single female and those that took over a female from another male were very similar in their mate-searching behaviour. The total time spent searching at the pat was positively related to its resource value as indicated by the number of pairs. Time spent in the best mate-searching area was negatively related with male numbers. The causes of differences in movements and aggression between paired and unpaired males are discussed. The male distribution around cow pats can be understood only if the differences in movement patterns by paired and unpaired males are taken into account.     

Sexually dimorphic head sizes and reproductive success in the sleepy lizard Tiliqua rugosa

In 1993, 458 males and 346 females of the largve Australian skink, Tiliqua rugosa , were captured in a study area near Mt. Mary, South Australia. Females were significantly longer than males, although there was broad overlap in snout-vent length measures. Males had significantly longer and broader heads than females of equivalent snout-vent length. In the spring some, but not all, lizards formed monogamous pairs. Pairing was used as an indirect indicator of reproductive success. When all adult males were considered there was no significant difference in head size between those found paired or unpaired. However, among small adults, paired males had signficantly broader heads than unpaired males. This supports the hypothesis that head size is under sexual selection. Individuals with wider heads could be more successful in male-male combat where jaws are the major offensive weapon. Younger, smaller males with a wise head could gain mates at an earlier age. Females showed a different pattern. In all females, and most strongly amongst larger size classes, paired females had significantly larger heads than unpaired females. An explanation is that larger heads somehow reduce the chance that a female will skip a year of reproduction, although the mechanism is not clear.     

Pairs during hibernation in a temperate frog: an unusual male mating strategy among anurans

Amplexus in which a male grasps a female from behind, characterizes most anurans as their strategy to ensure mating success. The behavior usually takes place some minutes, hours or days before gamete release. In a few species, however, it forms very early and lasts up to months, assumed to be an extreme pattern of mate guarding. Rana chensinensis is a temperate frog endemic to northern China. They hibernate in water in groups and breed explosively. At 15 sites across the species’ range, we found that on average 34% (17–67%) of hibernating adult females were paired, and these pairs were initiated shortly before hibernation and remained until spawning, lasting up to 5 months for the northern populations. Data from a focal study site showed that paired males and females were both larger in body size than their non-paired counterparts, and that there was a positive relationship between the body sizes of the paired frogs. The proportion of females that were paired was positively related with hibernating group size and independent of the sex ratio of the adults in a refuge. We interpret the extended pairing behavior as a male strategy that facilitates the avoidance of intense intra-sexual competition in mating aggregations during the breeding period and the starting of spawning earlier to acquire additional females.     

Lifetime patterns of pairing success in male Ortolan Buntings Emberiza hortulana

Analyses of lifetime fitness in birds are typically based on estimates of breeding success, in particular the number of offspring fledged. Small and isolated bird populations often have a male‐skewed adult sex ratio, so that male lifetime productivity depends to a large degree on pairing success, but few studies have focused on patterns of lifetime pairing success. The Norwegian population of Ortolan Buntings Emberiza hortulana is strongly male‐skewed, such that in any year about half of all males are unpaired. Pairing success of first‐year males (16–44%) was significantly lower than for older males (52–89%). Lifetime pairing success was correlated with lifespan and was strongly skewed, with a majority of males being paired only once or never, and only 11% paired three or more times despite a stable lifetime annual survival rate of 63%. Males that were paired in one year were more likely to be paired the next year than males that were unpaired in the previous year. The shortage of females caused even the older males to have a substantial probability of becoming unpaired, and 49% of long‐lived males (known as adults for at least 4 years) were unpaired after years in which they were paired. Pairing success in the Ortolan Bunting therefore follows similar age‐related and lifetime patterns in breeding success documented in other species. However, even the older males ran a high risk of not being paired, contrasting with earlier distinctions between pre‐breeding and breeding lifespans. I discuss the importance of knowledge of pairing success for the management of endangered and declining populations.     

Female‐biased size dimorphism in a diapausing caddisfly,Mesophylax aspersus: effect of fecundity and natural and sexual selection

1. The effect of mating success, female fecundity and survival probability associated with intra‐sex variation in body size was studied in Mesophylax aspersus, a caddisfly species with female‐biased sexual size dimorphism, which inhabits temporary streams and aestivates in caves. Adults of this species do not feed and females have to mature eggs during aestivation. 2. Thus, females of larger size should have a fitness advantage because they can harbour more energy reserves that could influence fecundity and probability of survival until reproduction. In contrast, males of smaller size might have competitive advantages over others in mating success. 3. These hypotheses were tested by comparing the sex ratio and body size of individuals captured before and after the aestivation period. The associations between body size and female fecundity, and between mating success and body size of males, were explored under laboratory conditions. 4. During the aestivation period, the sex ratio changed from 1 : 1 to male biased (4 : 1), and a directional selection on body size was detected for females but not for males. Moreover, larger clutches were laid by females of larger size. Finally, differences in mating success between small and large males were not detected. These results suggest that natural selection (i.e. the differential mortality of females associated with body size) together with possible fecundity advantages, are important factors responsible of the sexual size dimorphism of M. aspersus. 5. These results highlight the importance of taking into account mechanisms other than those traditionally used to explain sexual dimorphism. Natural selection acting on sources of variation, such as survival, may be as important as fecundity and sexual selection in driving the evolution of sexual size dimorphism.     

Male Defense of the Breeding Cavity and Factors Affecting the Persistence of Breeding Pairs in the Stomatopod,Gonodactylus bredini (Manning) (Crustacea: Hoplocarida)

In Caribbean Panama, nonreproductive male and female stomatopods are solitary and defend their own coral-rubble cavities. When breeding pairs form, however, males assume all responsibility for cavity defense. To compare success in cavity defense and defensive tactics among paired and unpaired males, and to examine the tendency for paired stomatopods to exchange their present mates for larger (higher quality) individuals, we introduced same-sized and 15% larger male, and same-sized and 15% larger reproductive female intruders to paired and unpaired male residents in a balanced design. Paired males were more successful at cavity defense than unpaired males, evidently because paired males strike intruders more than unpaired males, and because intruders fight less intensely against paired males than against unpaired males. Paired males occasionally attempted extrapair copulations, but showed little tendency to abandon their mates in favor of larger females. Paired females, however, mated readily with intruder males that evicted resident males. Populationwide female breeding synchrony and prolonged female receptivity before oviposition reduce variance in male mating success and may force males to guard the breeding cavity to assure their paternity. Uncertainty about the reproductive condition of intruder females may prevent males from exchanging mates.     

Selection becomes visible: enforced sexual dimorphism caused by sexual selection in the weevil Rhopalapion longirostre (Olivier 1807) (Coleoptera: Curculionoidea: Brentidae)

Extremely divergent traits between males and females are often the result of different requirements and behaviours of the sexes and will evolve relatively rapidly under selection forces. Sexual dimorphism in Rhopalapion longirostre is predominately manifested in the length and structure of the rostrum. To estimate how sexual selection shapes mating success in this weevil we compared paired and unpaired individuals collected from three populations in Austria. The mating process in this species is complex and lengthy. Statistical analyses based on detailed observations of their mating behaviour revealed that matched pairs show functional affinities in body size. Females and males with larger elytra, as well as males with large overall body size, are favoured mating partners, while males that are too small have no mating success. This arrangement ensures copulation and consequently successful egg deposition. For efficient egg channel boring into the flower buds of the host plant, Alcea rosea, the extremely long female rostrum is a crucial tool. Natural selection promotes longer rostra in females whereas sexual selection favours the shorter rostra in males. The major evolutionary forces, natural and sexual selection, enhance the sexual dimorphism in this species. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 38–47.     

PROXIMATE MECHANISMS OF SEXUAL SELECTION IN WOOD FROGS

Observations and several types of field experiments on the mating behavior of wood frogs have revealed the proximate mechanisms for a size-related reproductive advantage in both males and females. For females, larger individuals produce larger clutches; for males, larger individuals can better remain clasped to females when contested by rival males and can better depose males clasped to other females. No results obtained support of the existence of mate choice in either males or females. Males were estimated to be 4.74 times as variable as females in the number of zygotes produced per individual per season; however, much of the variation in male RS resulted from a male-biased sex ratio at the breeding site rather than from sexual selection. After taking sex ratio effects into consideration, males were estimated to be only 1.63 times as variable as females. Patterns of variation in RS in males and females are associated with numerous sex-specific differences in life history and morphology. Life history differences include differential growth rates, ages at sexual maturity, and rates of mortality. Interpretation of how the body size dimorphism (females larger than males) in this species relates to sexual selection is consistent with information on how similar variations in body size influence RS for each sex, and how males and females differ in the functional relationship between body size and RS. Average RS increases more with body size in females than in males. Although body size directly influences RS for females, the possibility exists that, for males, other anatomical features correlated with body size more directly affect RS. Preliminary evidence suggests that sexual selection influences male arm length and that the male body size : RS relationship results as an incidental correlation.