Ownership-dependent mating tactics of minor males of the beetle <Emphasis Type="Italic">Librodor japonicus</Emphasis> (Nitidulidae) with intra-sexual dimorphism of mandibles

Intra-sexual dimorphism is found in the weapons of many male beetles. Different behavioral tactics to access females between major and minor males, which adopt fighting and alternative tactics, respectively, are thought to maintain the male dimorphism. In these species major males have enlarged weapons that they use in fights with rival males. Minor males also have small weapons in some of these species, and it is unclear why these males possess weapons. We examined the hypothesis that minor males might adopt a fighting tactic when their status was relatively high in comparison with that of other males (e.g., ownership of a territory). We observed the behavioral tactics of major and minor males of the beetle Librodor japonicus, whose males have a dimorphism of their mandibles. Major males fought for resources, whereas minor males adopted two status-dependent tactics, fighting and sneaking, to access females, depending on their ownership of a sap site. We suggest that ownership status-dependent mating tactics in minor males may maintain the intra-sexual dimorphism in this beetle.     

Horn Length Is the Determining Factor in the Outcomes of Escalated Fights Among Male Japanese Horned Beetles, <Emphasis Type="Italic">Allomyrina dichotoma</Emphasis> L. (Coleoptera: Scarabaeidae)

Male horn length in some horned beetles shows a sigmoidal relationship with body size. This has often been considered as the reflection of alternative reproductive tactics of males based on body size. Large males should possess long horns to acquire females through fights with other males using their horns, whereas small males do not require long horns because they usually avoid intermale fights and adopt alternative tactics such as sneaking. This may lead to a prediction that horn length is a reliable indicator of the fighting ability of the male. We examined the effects of both male horn length and body size of Allomyrina dichotoma on the outcomes of escalated fights. Results indicate that male horn length was more important than body size in predicting the outcomes of fight, and this may support the hypothesis that the evolution of the horn dimorphism in male horned beetles is the result of different reproductive tactics.     

Male horn dimorphism in the scarab beetle, Onthophagus taurus: do alternative reproductive tactics favour alternative phenotypes?

In a variety of organisms morphological variation is discrete rather than continuous. Discrete variation within a sex has attracted particular interest as it is thought to reflect the existence of alternative adaptations to a heterogeneous selection environment. The beetle Onthophagus taurus shows a dimorphism for male horns: males that exceed a critical body size develop a pair of long, curved horns on their heads, while smaller males remain hornless. In this study we report on the alternative reproductive tactics used by males with these two morphologies, and present experimental and behavioural data suggesting that these alternative tactics selectively favour discretely different male phenotypes. Horned males aggressively defended tunnel entrances containing breeding females. Fights involved the use of horns, and males with longer horns were more likely to win fights. In contrast, hornless males employed nonaggressive sneaking behaviours when faced with competitively superior males. Sneaking behaviours appeared to require high degrees of manoeuvrability inside tunnels to access and mate with females despite the presence of a guarding male. Comparisons of running performances of males with identical body sizes but different horn lengths suggest that the possession of horns reduces male agility inside tunnels. Thus, horn possession confers a clear advantage to males using fighting behaviours to access females, whereas hornlessness may be favoured in males that rely primarily on sneaking behaviours. Combined, the two alternative reproductive tactics used by male O. taurus appear to favour opposite horn phenotypes, which may explain the paucity of intermediate morphologies in natural populations of O. taurus. Copyright 2000 The Association for the Study of Animal Behaviour.     

Alternate Reproductive Tactics in an African Dung Beetle, Circellium bacchus (Scarabeidae)

Male-male competition is recognized as a potent force of sexual selection. When intra-sexual competition is strong then selection theory predicts that alternative male phenotypes will evolve. Circellium bacchus is a large, hornless, ball-rolling dung beetle with extensive variation in size and subject to intense male-male contest competition. We proposed that small male C. bacchus, which are unlikely to be successful in male contest competition, would adopt the alternative reproductive tactic of sneaking copulations. This alternative tactic is likely to influence not only behavior, but morphology with an expectation that sneaking males would invest more resources in testes development. Investigation of testis allometry revealed that smaller male C. bacchus beetles had relatively larger testes than their bigger conspecifics. Furthermore, as resources may be limited during larval development, differential investment in testes development should result in adult male beetles either competing for fertilizations or for access to mates. This is seen in C. bacchus as two alternate male phenotypes; smaller beetles with a relatively low body mass invest proportionately more in testes development compared to larger, heavier form in which testes size does not scale with condition. To our knowledge this study provides the first investigation of alternative phenotypes in the reproductive tactics of a ball-rolling dung beetle species.     

Age‐dependent male mating tactics in a spider mite—A life‐history perspective

Males often fight with rival males for access to females. However, some males display nonfighting tactics such as sneaking, satellite behavior, or female mimicking. When these mating tactics comprise a conditional strategy, they are often thought to be explained by resource holding potential (RHP), that is, nonfighting tactics are displayed by less competitive males who are more likely to lose a fight. The alternative mating tactics, however, can also be explained by life‐history theory, which predicts that young males avoid fighting, regardless of their RHP, if it pays off to wait for future reproduction. Here, we test whether the sneaking tactic displayed by young males of the two‐spotted spider mite can be explained by life‐history theory. We tested whether young sneaker males survive longer than young fighter males after a bout of mild or strong competition with old fighter males. We also investigated whether old males have a more protective outer skin—a possible proxy for RHP—by measuring cuticle hardness and elasticity using nanoindentation. We found that young sneaker males survived longer than young fighter males after mild male competition. This difference was not found after strong male competition, which suggests that induction of sneaking tactic is affected by male density. Hardness and elasticity of the skin did not vary with male age. Given that earlier work could also not detect morphometric differences between fighter and sneaker males, we conclude that there is no apparent increase in RHP with age in the mite and age‐dependent male mating tactics in the mite can be explained only by life‐history theory. Because it is likely that fighting incurs a survival cost, age‐dependent alternative mating tactics may be explained by life‐history theory in many species when reproduction of old males is a significant factor in fitness.     

Seasonal variation in male alternative reproductive tactics

Genetic parentage analyses reveal considerable diversity in alternative reproductive behaviours (e.g. sneaking) in many taxa. However, little is known about whether these behaviours vary seasonally and between populations. Here, we investigate seasonal variation in male reproductive behaviours in a population of two‐spotted gobies (Gobiusculus flavescens) in Norway. Male two‐spotted gobies guard nests, attract females and care for fertilized eggs. We collected clutches and nest‐guarding males early and late in the breeding season in artificial nests and used microsatellite markers to reconstruct parentage from a subset of offspring from each nest. We hypothesized that mating, reproductive success and sneaking should be more prevalent early in the breeding season when competition for mates among males is predicted to be higher. However, parentage analyses revealed similar values of mating, reproductive success and high frequencies of successful sneaking early (30% of nests) and late (27% of nests) in the season. We also found that multiple females with eggs in the same nest were fertilized by one or more sneaker males, indicating that some males in this population engage in a satellite strategy. We contrast our results to previous work that demonstrates low levels of cuckoldry in a population in Sweden. Our results demonstrate marked stability in both the genetic mating system and male alternative reproductive tactics over the breeding season. However, sneaking rates may vary geographically within a species, likely due to local selection influencing ecological factors encountered at different locations.     

Ejaculatory strategies associated with experience of losing

Modulation of behaviours as a result of fighting experience has been observed in many animals and can influence pre-copulatory sexual selection. This study investigated how fighting experience affects ejaculatory strategies. In male flour beetles, Gnatocerus cornutus, experience of losing a fight decreases a male''s aggressiveness for up to 4 days. We found that males losing a fight show increased ejaculatory investment, but there was no ejaculatory modulation owing to winning. However, the increase in ejaculate investment following a loss was no longer observed after 5 days. These results indicate that males adjust their investment in sperm competition according to their experience, and that fighting experience can significantly influence pre- and post-copulatory reproductive tactics.     

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

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

Influences of establishment and maintenance of territory on reproductive activity in the male dwarf gourami Colisa lalia

We examined the importance of establishment and maintenance of territory on reproductive activity in the male dwarf gourami, Colisa lalia. After three males had been forced to fight for territory (five sets) for three weeks, social status was divided into three classes: the territorially dominant male, which guarded the territory under the floating nest; the second male which remained near the nest and occasionally attacked the dominant male; and the third male which was non-aggressive and remained at a distance from the other two males. Comparing testicular size by gonadosomatic indices (GSI) after three weeks of aggression, GSI of the dominant male (1.19 ± 0.07) was significantly larger than that of the second (0.81 ± 0.15) and the third (0.62 ± 0.08) males, as well as the initial control (not involved in any experiments: 0.85 ± 0.10, n = 5), indicating that the testes of the dominant males enlarge during territory defense. Histological observations of testes revealed that sperm production in the dominant males was more active compared to males of other classes, although spermatogenesis was confirmed in all males examined, suggesting that dominance accelerates sperm production. Social-status dependent development of testes suggests an absence of sperm competition due to the lack of sneaking by subordinate males. Since non-territorial males do not engage in alternative tactics (e.g., sneaking) leading to emission of semen, male C. lalia must obtain and defend territory if they are to increase their reproductive success.     

Females solicit sneakers to improve fertilization success in the bitterling fish (Rhodeus sericeus)

In most species there is greater variance in reproductive fitness among males than females. A consequence is the evolution of alternative male mating tactics, including sneaking behaviour whereby males attempt to mate with females courted by other males, which could undermine female choice. In bitterling (Rhodeus sericeus), fish that lay their eggs on the gills of living freshwater mussels, sneaking is common and successful. Here we show that female bitterling can significantly improve their fertility with an increase in the number of spawning partners. Females perform conspicuous behaviours associated with spawning more frequently close to sneakers, and spawn more eggs close to high-quality sneakers. This is the first time that females have been shown to engage in behaviour that increases the probability of sneaking, and raises the possibility of a sexual conflict over the number and source of ejaculates during spawning in this species.