Genital damage in the orb-web spider Argiope bruennichi (Araneae: Araneidae) increases paternity success

The morphology of male genitalia often suggests functions besidessperm transfer that may have evolved under natural or sexualselection. In several species of sexually cannibalistic spiders,males damage their paired genitalia during mating, limitingthem to one copulation per pedipalp. Using a triple-mating experiment,we tested if genital damage in the orb-web spider Argiope bruennichiincreases male fitness either through facilitating his escapefrom an aggressive female or by obstructing the female's inseminationducts against future copulation attempts from other males. Wefound no survival advantage for males damaging their pedipalps;however, copulations into a previously used insemination ductwere significantly shorter when the previous male had left partsof his genitalia inside the insemination duct. Because copulationduration determines paternity in this species, our result suggeststhat male genital damage in A. bruennichi is sexually selected.By breaking off parts of their intromittent organs inside avirgin female, males can reduce sperm competition and therebyincrease their paternity success.     

Securing paternity: mating plugs in the dwarf spider Oedothorax retusus (Araneae: Erigoninae)

One of the various male strategies to prevent or impede female remating is the production of a mating plug that covers the female genital opening or remains inside of the female genital tract after mating. Such structures have been described for many species in many animal taxa; however, in most cases, we know little or nothing about their specific adaptive value. Our investigations demonstrate that females of the dwarf spider species Oedothorax retusus (Westring, 1851) (Linyphiidae, Erigoninae) exhibit a substance on one or both of her paired genital openings only after copulation. We performed double-mating trials and forced the second male to mate into the previously used or unused spermathecal duct of the female by amputating one of his paired male gonopods (pedipalps). Furthermore, to investigate whether the duration of the first mating has an effect on the size and efficiency of the mating plug, we interrupted first matings after either 1 or 3 min, categorized plug size and recorded mating behaviour of subsequent males. The amount of secretion transferred was larger in long compared to short copulations. A long first copulation successfully prevented subsequent males from mating into the used ducts, whereas mating success after short first matings was similar to matings into unused copulatory ducts of the females. The present study demonstrates that a male O. retusus can prevent a rival from transferring sperm into the same spermatheca by applying a mating plug, but only if he mates for long enough.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 574–583.     

Male Gryllus bimaculatus Guard Females to Delay Them from Mating with Rival Males and to Obtain Repeated Copulations

Three hypotheses for the function of postcopulatory mate guarding were tested in the field cricket Gryllus bimaculatus De Geer. The duration of spermatophore attachment was greater in the absence than in the presence of a guarding male. The ejaculate protection hypothesis was, therefore, rejected. The duration of mate guarding was found to be equal to the interval between copulations, supporting the spermatophore renewal hypothesis. In support of the rival exclusion hypothesis, the presence of a guarding male did increase the duration of spermatophore attachment when a rival male was also present. The presence of a guarding male also delayed the female from mating with the rival male. Female mating status had a significant effect on the duration of spermatophore attachment. Females mating for the first time retained the spermatophore for a significantly longer period of time than females that had mated previously.     

Differential sperm expenditure reveals a possible role for post‐copulatory sexual selection in a lekking moth

Male reproductive success in the lesser wax moth Achroia grisella is strongly determined by pre‐copulatory mate choice, during which females choose among males aggregated in small leks based on the attractiveness of ultrasonic songs. Nothing is known about the potential of post‐copulatory mechanisms to affect male reproductive success. However, there is evidence that females at least occasionally remate with a second male and that males are unable to produce ejaculates quickly after a previous copulation. Here we investigated the effects of mating history on ejaculate size and demonstrate that the number of transferred sperm significantly decreased from first (i.e., virgin) to second (i.e., nonvirgin) copulation within individual males. For males of identical age, the number of sperm transferred was higher in virgin than in nonvirgin copulations, too, demonstrating that mating history, is responsible for the decrease in sperm numbers transferred and not the concomitant age difference. Furthermore, the number of transferred sperm was significantly repeatable within males. The demonstrated variation in ejaculate size both between subsequent copulations as well as among individuals suggests that there is allocation of a possibly limited amount of sperm. Because female fecundity is not limited by sperm availability in this system, post‐copulatory mechanisms, in particular sperm competition, may play a previously underappreciated role in the lesser wax moth mating system.     

Males,but not females,perform strategic mate searching movements between host plants in a leaf beetle with scramble competition polygyny

Mate searching is assumed to be performed mostly by males, but when females benefit from multiple mating or are under risk of failing to mate, they may also perform mate searching. This is especially important in scramble competition polygynies, in which mate searching is the main mechanism of mate competition. Typically, more mobile individuals are expected to achieve higher mating success because mobility increases their probability of finding mates. If we assume individual movements are mainly explained by mate searching in scramble competition polygynies, we can investigate searching strategies by asking when individuals should leave their location and where they should go. We hypothesize that individuals will leave their locations when mating opportunities are scarce and will seek spatially close sites with better mating opportunities. We tested these hypotheses for males and females of Leptinotarsa undecimlineata, a leaf beetle with scramble competition polygyny in which both sexes are promiscuous. Individuals mate and feed exclusively on Solanum plants, and thus, individual movements can be described as switches between plants. Females were less likely than males to leave isolated plants, and both males and females moved preferentially to neighboring plants. Males were more likely to leave when the local number of females was low, and the number of males was high. They moved to plants with more females, a behavior consistent with a mate searching strategy. Females were more likely to move to plants with fewer males and many females, a behavior consistent with male harassment avoidance. Strategic movement is widely considered in foraging context, but seldom in a mate searching context. Considering that selection to minimize searching costs, maximize mating success, and minimize harassment may be ubiquitous in nature, we argue that strategic movements by mate searching individuals are likely to occur in many species.     

Sexual cannibalism, competition, and size dimorphism in the orb-weaving spider Nephila plumipes Latreille (Araneae: Araneoidea)

The degree and direction of sexual dimorphism varies widely,but in several taxa of orb-weaving spiders, including Nephila,males may be less than one-tenth the size of females. This differenceis commonly attributed to selection through precopulation sexualcannibalism: females may either fail to detect very small males,or ignore them as potential prey items. However, there is oftenthe potential for male-male competition in these species becauseseveral males can be found on the web of a single female. Weinvestigated experimentally the effects of sexual cannibalismand male-male competition on male body size and hence sexualdimorphism in the Australian golden orb-weaver (Nephila plumipes).Small males were less likely to be detected and cannibalizedthan larger males. However, larger males excluded small malesfrom the central hub of the web, where mating takes place. Theconflicting effects of sexual cannibalism and male-male competitionmay be responsible for the relatively large variation in malebody size in this species.     

Early diversification of sperm size in the evolutionary history of the old world leaf warblers (Phylloscopidae)

Sperm morphological traits are highly variable among species and are commonly thought to evolve by post‐copulatory sexual selection. However, little is known about the evolutionary dynamics of sperm morphology, and whether rates of evolutionary change are variable over time and among taxonomic groups. Here, we examine sperm morphology from 21 species of Old World leaf warblers (Phylloscopidae), a group of generally dull, sexually monochromatic birds, which are known to have high levels of extra‐pair paternity. We found that sperm length differs markedly across species, spanning about 40% of the range observed across a larger selection of passerine birds. Furthermore, we found strong support for an ‘early‐burst’ model of trait evolution, implying that the majority of divergence in sperm length has occurred early in the evolutionary history of this clade with subsequent evolutionary stasis. This large early divergence matches the early divergence reported in ecological traits (i.e. body size and feeding behaviour). Our findings demonstrate that rates of evolution in sperm morphology can change over time in passerine taxa, and that evolutionary stasis in sperm traits can occur even in species exhibiting characteristics consistent with moderate‐to‐high levels of sperm competition. It remains a major challenge to identify the selection mechanisms and possible constraints responsible for these variable rates of sperm evolution.     

Is size assortative mating in Paracalliope fluviatilis (Crustacea: Amphipoda) explained by male–male competition or female choice?

Field and laboratory studies were used to assess: (1) whether size assortative mating occurred in the New Zealand amphipod Paracalliope fluviatilis and (2) hypotheses developed to explain size assortative mating. We found that assortative mating occurred and that larger females carried more eggs, suggesting they may be more valuable as mates. Laboratory experiments were then used to determine whether: (1) male size influenced the size of the female selected (mechanical constraints hypothesis); (2) male size influenced pairing success in the presence of competition (intrasexual selection hypothesis); (3) take‐overs of females occurred and whether large males were more successful (intrasexual selection hypothesis); (4) guard duration varied relative to male and female size (guard duration hypothesis); and (5) females had control over pairing success and guard duration (intersexual selection hypothesis). Although there was evidence to suggest the existence of intrasexual competition for mates (i.e. both small and large males preferred large females), there was no evidence of overt competition (i.e. takeovers of paired females). There was also no difference with respect to how long small and large males guarded females, but large females were guarded longer by both male size classes. Females handicapped by having their mobility reduced were guarded for the same duration as control females but males were more likely to pair with handicapped females, suggesting that they were easier to amplex. Given the lack of evidence for direct male–male competition or female choice, we suggest that assortative mating may be the result of: (1) indirect competition (e.g. in situ large males may be better able to access and amplex the largest females) or (2) female resistance to small males in combination with higher costs that small males may incur in securing large females. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 92 , 173–181.     

Adult nutrition,but not inbreeding,affects male primary sexual traits in the leaf‐footed cactus bug Narnia femorata (Hemiptera: Coreidae)

Adverse conditions may be the norm rather than the exception in natural populations. Many populations experience poor nutrition on a seasonal basis. Further, brief interludes of inbreeding can be common as population density fluctuates and because of habitat fragmentation. Here, we investigated the effects of poor nutrition and inbreeding on traits that can be very important to reproductive success and fitness in males: testes mass, sperm concentration, and sperm viability. Our study species was Narnia femorata, a species introduced to north‐central Florida in the 1950s. This species encounters regular, seasonal changes in diet that can have profound phenotypic effects on morphology and behavior. We generated inbred and outbred individuals through a single generation of full‐sibling mating or outcrossing, respectively. All juveniles were provided a natural, high‐quality diet of Opuntia humifusa cactus cladode with fruit until they reached adulthood. New adult males were put on a high‐ or low‐quality diet for at least 21 days before measurements were taken. As expected, the low‐quality diet led to significantly decreased testes mass in both inbred and outbred males, although there were surprisingly no detectable effects on sperm traits. We did not find evidence that inbreeding affected testes mass, sperm concentration, and sperm viability. Our results highlight the immediate and overwhelming effects of nutrition on testes mass, while suggesting that a single generation of inbreeding might not be detrimental for primary sexual traits in this particular population.     

Female choice for male cuticular hydrocarbon profile in decorated crickets is not based on similarity to their own profile

Indirect genetic benefits derived from female mate choice comprise additive (good genes) and nonadditive genetic benefits (genetic compatibility). Although good genes can be revealed by condition‐dependent display traits, the mechanism by which compatibility alleles are detected is unclear because evaluation of the genetic similarity of a prospective mate requires the female to assess the genotype of the male and compare it to her own. Cuticular hydrocarbons (CHCs), lipids coating the exoskeleton of most insects, influence female mate choice in a number of species and offer a way for females to assess genetic similarity of prospective mates. Here, we determine whether female mate choice in decorated crickets is based on male CHCs and whether it is influenced by females' own CHC profiles. We used multivariate selection analysis to estimate the strength and form of selection acting on male CHCs through female mate choice, and employed different measures of multivariate dissimilarity to determine whether a female's preference for male CHCs is based on similarity to her own CHC profile. Female mating preferences were significantly influenced by CHC profiles of males. Male CHC attractiveness was not, however, contingent on the CHC profile of the choosing female, as certain male CHC phenotypes were equally attractive to most females, evidenced by significant linear and stabilizing selection gradients. These results suggest that additive genetic benefits, rather than nonadditive genetic benefits, accrue to female mate choice, in support of earlier work showing that CHC expression of males, but not females, is condition dependent.     

The adaptive significance of mate guarding in the soapberry bug,Jadera Haematoloma (Hemiptera: Rhopalidae)

Male soapberry bugs (Jadera haematoloma)face severe mating competition at the northern edge of their range due to male-biased adult sex ratios. Copulations lasting up to 11 days may serve a mate guarding function (encompassing four or more ovipositions), but copulation duration is highly variable, with some pairings lasting as little as 10 min. Data were gathered to describe factors that influence the reproductive costs and benefits of prolonged copulation. Estimated copulation durations (mean ± SD) were 20 ± 23 h in the lab and 50 ± 8 h in the field and were only weakly affected by sex ratio. Females mated for 5 min produced as many fertile eggs as those mated for 600 min laid; they became depleted of fertile sperm after about 25 days. In twicemated females, the first male's paternity was reduced by about 60%, and all females (N = 13) whose mates were removed experimentally mated again within an average of 6 min. The outcome of sperm competition on a perclutch basis was not highly predictable. The possibility of increased sperm displacement in longer copulations was not tested. Males often guarded females during oviposition and successfully defended them from intruding single males by recopulating. Such intrusions occurred in the majority of oviposition attempts observed in nature. Even though most females mated promiscuously, in a focal aggregation with a mean sex ratio of 2.2 ± 0.4 males/female, the interval between matings by males was commonly several days. Males appeared to respond facultatively to several aspects of the distribution and availability of females. The intensities of mating competition and sperm competition indicate that monogamous mate guarding should be favored over nonguarding in nature. Unpredicted brief. pairings may result from assessment by males of female reproductive value or of their own physical condition, or from female resistance.     

Male phenotypes in a female framework: Evidence for degeneration in sperm produced by male snails from asexual lineages

How changes in selective regimes affect trait evolution is an important open biological question. We take advantage of naturally occurring and repeated transitions from sexual to asexual reproduction in a New Zealand freshwater snail species, Potamopyrgus antipodarum, to address how evolution in an asexual context—including the potential for relaxed selection on male‐specific traits—influences sperm morphology. The occasional production of male offspring by the otherwise all‐female asexual P. antipodarum lineages affords a unique and powerful opportunity to assess the fate of sperm traits in a context where males are exceedingly rare. These comparisons revealed that the sperm produced by ‘asexual’ males are markedly distinct from sexual counterparts. We also found that the asexual male sperm harboured markedly higher phenotypic variation and was much more likely to be morphologically abnormal. Together, these data suggest that transitions to asexual reproduction might be irreversible, at least in part because male function is likely to be compromised. These results are also consistent with a scenario where relaxed selection and/or mutation accumulation in the absence of sex translates into rapid trait degeneration.     

May the (selective) force be with you: Spatial sorting and natural selection exert opposing forces on limb length in an invasive amphibian

Spatial sorting on invasion fronts drives the evolution of dispersive phenotypes, and in doing so can push phenotypes in the opposite direction to natural selection. The invasion of cane toads (Rhinella marina) through tropical Australia has accelerated over recent decades because of the accumulation of dispersal‐enhancing traits at the invasion front, driven by spatial sorting. One such trait is the length of the forelimbs: invasion‐front toads have longer arms (relative to body length) in comparison with populations 10–20 years after invasion. Such a shift likely has fitness consequences: an increase of forearm length would decrease the strength with which a male could cling to a female during amplexus and so render such a male less competitive in competition for mates, compared to short‐armed conspecifics. Our laboratory trials of attachment strength confirmed that males with relatively longer arms were easier to displace, and competition trials show higher duration of amplexus for males with shorter arms. Together with the sharp cline in limb length observed behind the invasion front, these results imply an opposition of selective forces: spatial sorting optimizes dispersal, but as this force wanes behind the invasion front, we see the primacy of natural selection reassert itself.     

Evidence for a recent horizontal transmission and spatial spread of Wolbachia from endemic Rhagoletis cerasi (Diptera: Tephritidae) to invasive Rhagoletis cingulata in Europe

The widespread occurrence of Wolbachia in arthropods and nematodes suggests that this intracellular, maternally inherited endosymbiont has the ability to cross species boundaries. However, direct evidence for such a horizontal transmission of Wolbachia in nature is scarce. Here, we compare the well‐characterized Wolbachia infection of the European cherry fruit fly, Rhagoletis cerasi, with that of the North American eastern cherry fruit fly, Rhagoletis cingulata, recently introduced to Europe. Molecular genetic analysis of Wolbachia based on multilocus sequence typing and the Wolbachia surface protein wsp showed that all R. cingulata individuals are infected with wCin2 identical to wCer2 in R. cerasi. In contrast, wCin1, a strain identical to wCer1 in R. cerasi, was present in several European populations of R. cingulata, but not in any individual from the United States. Surveys of R. cingulata from Germany and Hungary indicated that in some populations, the frequency of wCin1 increased significantly in just a few years with at least two independent horizontal transmission events. This is corroborated by the analysis of the mitochondrial cytochrome oxidase II gene that showed association of wCin1 with two distinct haplotypes in Germany, one of which is also infected with wCin1 in Hungary. In summary, our study provides strong evidence for a very recent inter‐specific Wolbachia transmission with a subsequent spatial spread in field populations.     

Soil respiration is stimulated by elevated CO2 and reduced by summer drought: three years of measurements in a multifactor ecosystem manipulation experiment in a temperate heathland (CLIMAITE)

This study investigated the impact of predicted future climatic and atmospheric conditions on soil respiration (R_S) in a Danish Calluna‐Deschampsia‐heathland. A fully factorial in situ experiment with treatments of elevated atmospheric CO₂ (+130 ppm), raised soil temperature (+0.4 °C) and extended summer drought (5–8% precipitation exclusion) was established in 2005. The average R_S, observed in the control over 3 years of measurements (1.7 μmol CO₂ m^?2 sec^?1), increased 38% under elevated CO₂, irrespective of combination with the drought or temperature treatments. In contrast, extended summer drought decreased R_S by 14%, while elevated soil temperature did not affect R_S overall. A significant interaction between elevated temperature and drought resulted in further reduction of R_S when these treatments were combined. A detailed analysis of short‐term R_S dynamics associated with drought periods showed that R_S was reduced by ~50% and was strongly correlated with soil moisture during these events. Recovery of R_S to pre‐drought levels occurred within 2 weeks of rewetting; however, unexpected drought effects were observed several months after summer drought treatment in 2 of the 3 years, possibly due to reduced plant growth or changes in soil water holding capacity. An empirical model that predicts R_S from soil temperature, soil moisture and plant biomass was developed and accounted for 55% of the observed variability in R_S. The model predicted annual sums of R_S in 2006 and 2007, in the control, were 672 and 719 g C m^?2 y^?1, respectively. For the full treatment combination, i.e. the future climate scenario, the model predicted that soil respiratory C losses would increase by ~21% (140–150 g C m^?2 y^?1). Therefore, in the future climate, stimulation of C storage in plant biomass and litter must be in excess of 21% for this ecosystem to not suffer a reduction in net ecosystem exchange.