Geographic Variation in Reliance on Trial-and-Error Signal Derivation by Portia labiata, an Araneophagic Jumping Spider from the Philippines

Signal-generation behavior of Portia labiata, a web-invading araneophagic jumping spider (Salticidae), was investigated in the laboratory. Individuals derived from two habitats in the Philippines were compared: Los Baños, a low-elevation tropical rainforest site where prey (spider) diversity is especially high, and Sagada, a high-elevation pine-forest site where prey (spider) diversity is less. Maternal effects and variation in experience were minimized because all individuals tested were from laboratory rearing to second and third generation under standardized conditions. Individuals from both populations used a trial-and-error (generate-and-test) algorithm to derive appropriate aggressive-mimicry signals. However, in laboratory experiments, the Los Baños P. labiata relied on trial and error significantly more often than did the Sagada P. labiata. Selection pressures that may have been responsible for evolution of different levels of flexibility are considered, including the different arrays of prey to which the Los Baños and the Sagada P. labiata are exposed.     

Behavioural evidence of UV sensitivity in jumping spiders (Araneae: Salticidae)

Jumping spiders are known to possess ultraviolet (UV) receptors in the retinas of their large-principal eyes. The existence of UV visual cells, however, does not prove that jumping spiders can see into the UV part of spectrum (300–400 nm) or whether such an ability plays any role in salticid intra-specific interactions. In the study reported herein, we performed behavioural experiments to test whether a UV−reflecting jumping spider, Cosmophasis umbratica, is sensitive to UV wavelengths and whether UV cues are important in intra-specific communication. The absence of UV cues not only affected intra-specific behaviour by significantly reducing the frequency of agonistic displays, but also elicited unprecedented courtship displays in males towards their own mirror images and conspecific opponents. Furthermore, C. umbratica males were able to respond rapidly to changes in UV cues of conspecific mirror images by switching between agonistic and courtship displays. These findings clearly demonstrate that C. umbratica males are capable of seeing UV wavelengths and that UV cues are necessary and sufficient for this species to enable the agonistic displays. Hence, UV light may have an important role to play in intra-specific communication in jumping spiders.     

The Consequences of Predation Risk on the Male Territorial Behavior in a Solitary Bee

Territorial males may adopt a mating tactic that yields greater reproductive success but that at the same time increases the risk of predation. Plasticity in reproductive behavior can reflect a balance between sexual selection and natural selection. In this study, we sought to verify the effect of predation risk on territorial behavior of males of the solitary bee Ptilothrix fructifera (Apidae). Males of the species employ alternative mating tactics and can be territorial in defense of larval food sources. By manipulating predator models in the field, we tested whether (1) males avoid perch flowers containing predator models; (2) males alternate between mating tactics when their territory is associated with a predation risk; and (3) female foraging on flowers in a territory is altered in the presence of a predator model. We measured the responses of males and females in flowers containing and not containing a model of a spider or a stuffed bird. The results show that territorial males of P. fructifera alter their territorial behavior when faced with a high risk of predation. They do not abandon their territory or change to a non‐territorial mating tactic, but instead change the use of their territory, avoiding flowers containing predator models or perching on other flowers when the previous flower presented a potential predation risk. Female P. fructifera decreases the frequency of their visits to flowers and the length of time spent there in the presence of a spider model. In short, in the face of predation risk, females and males alter their behaviors at the cost of less efficient foraging and searching for partners, respectively.     

Predator–Prey Interactions Between Aggressive-Mimic Jumping Spiders (Salticidae) and Araneophagic Spitting Spiders (Scytodidae) from the Philippines

In Los Baños in the Philippines (Laguna, Luzon), Scytodes sp. indet. is a web-building spitting spider (Scytodidae) that preys primarily on jumping spiders (Salticidae) and Portia labiata is an aggressive-mimic jumping spider that preys especially frequently on Scytodes. Tactics by which three species of Portia (P. africana, P. fimbriata, and P. labiata) and, for Portia labiata, three disjunct populations (Sri Lanka and, in the Philippines, Sagada and Los Baños) capture this especially dangerous prey are compared. Local adaptation to Scytodes by the Los Baños P. labiata is discussed. The Los Baños P. labiata uniquely made consistent use of tactics (soft plucking with palps and signal–detour–leap sequences) that were apparently responsible for greater prey-capture success and more effective avoidance of being spat on. Inter- and intraspecific differences were evident despite having used Portia that were reared in the laboratory with no prioir experience with scytodids.     

Female response to predation risk alters conspecific male behaviour during pre‐copulatory mate guarding

Mating behaviour often increases predation risk, but the vulnerability within mating pairs differs between the sexes. Such a sex difference is expected to lead to differences in responses to predation risk between the sexes. In the two‐spotted spider mite Tetranychus urticae, males engage in pre‐copulatory mate guarding because only the first mating results in fertilisation. We investigated (i) whether pre‐copulatory pairs are more conspicuous to the predatory mite Phytoseiulus persimilis than solitary females, (ii) whether the vulnerability to the predator differs between sexes within the pre‐copulatory pair, (iii) whether each sex of T. urticae responds to predation risk during pre‐copulatory mate guarding and (iv) whether T. urticae's response to predation risk affects predator behaviour. Because T. urticae females are immobile during pre‐copulatory mate guarding, we observed male behaviour to evaluate effects of predation risk. We found that the predators detect more pre‐copulatory pairs than solitary females and that more females than males of the pre‐copulatory pairs are preyed upon by the predators. The preference of spider mite males for pre‐copulatory pairs versus solitary females was affected by whether or not the female had been exposed to predators during development. Male T. urticae exposed to predation risk did not alter their behaviour. These results suggest that only the most vulnerable sex, that is the female, responds to predation risk, which modifies male behaviour. Regardless of T. urticae females’ experience, however, P. persimilis detected more T. urticae pre‐copulatory pairs than solitary females, suggesting that pre‐copulatory mate guarding itself is dangerous for T. urticae females when these predators are present. We discuss our results in the context of sex‐dependent differences in predation risk.     

Wind turbine noise limits propagation of greater prairie‐chicken boom chorus,but does it matter?

Acoustic signals are often critical elements of mating displays, and lekking male greater prairie‐chickens (Tympanuchus cupido pinnatus) use their boom vocalization for this purpose. We quantified the acoustic characteristics of the boom chorus created by multiple male greater prairie‐chickens vocalizing simultaneously at leks in Brown County, NE, USA, in 2013 and 2014. We used these data to evaluate (a) the role of the boom chorus in prairie‐chicken breeding dynamics and (b) the impact of a wind energy facility on the acoustic signal of the boom chorus. We sampled the chorus using audio recorders placed in transects extending from leks; the chorus exhibited an average peak frequency of 297 ± 13 Hz. The mean chorus signal‐to‐noise ratio declined from 15.7 dB (50 m) to 2.6 dB (800 m), and wind speed and direction, topography, and relative humidity caused variation in signal‐to‐noise ratio at a given distance and location. Chorus recordings from leks within 1,000 m of a wind turbine had lower signal‐to‐noise ratio (β_turbine = ?5.659, SE = 1.289) than leks farther from turbines. The chorus signal‐to‐noise ratio increased slightly with more males present on the lek (~0.1 dB for each additional male; β_males = 0.177; SE = 0.037) and considerably more as more females visited the lek (~1.4 dB for each additional female; β_females = 2.498, SE = 0.235; β_females² = ?0.309, SE = 0.039). Our results provide support for the signal enhancement hypothesis that proposes the boom chorus is influenced notably by male–male competition for females on the lek, rather than functioning solely to advertise the presence of the lek to recruit females. Our results also suggest the choruses emanating from small leks have the greatest potential to be masked by anthropogenic (wind turbine) noise, which may affect the breeding success of male and female prairie‐chickens.     

Body‐colour variation in an orb‐web spider and its effect on predation success

Animal body coloration serves several functions such as thermoregulation, camouflage, aposematism, and intraspecific communication. In some orb‐web spiders, bright and conspicuous body colours are used to attract prey. On the other hand, there are other species whose body colour does not attract prey. Using a spider species showing individual body‐colour variation, the present study aimed to determine whether or not the variation in body colour shows a correlation with predation rates. We studied the orb‐web spider (Cyclosa argenteoalba) using both field observations and T‐maze experiments, in which the prey were exposed to differently coloured spiders. Cyclosa argenteoalba has silver‐ and black‐coloured areas on its dorsal abdomen, with the ratio of these two colours varying continuously among individuals. The bright and conspicuous silver area reflects ultraviolet light. Results of both field observations and colour choice experiments using Drosophila flies as prey showed that darker spiders have a greater chance of capturing prey than silver spiders. This indicates that body‐colour variation affects predation success among individuals and that the bright silver colour does not function to attract prey in C. argenteoalba.     

Laboratory evaluation of predation rates of two native natural enemies on the exotic pest Bagrada hilaris

Laboratory studies were conducted to determine the consumption rates of two native predators found attacking the exotic invasive stink bug Bagrada hilaris (Burmeister) (Hempitera: Pentatomidae) in field plots in New Mexico, USA. Individual field‐collected adults of the spined soldier bug, Podisus maculiventris (Say) (Hempitera: Pentatomidae) and the soft‐winged flower beetle, Collops vittatus (Say) (Coleoptera: Melyridae), were provided daily with fixed numbers of different life stages of B. hilaris under controlled conditions. Consumption rates were recorded daily for ten consecutive days for a total of 20 adult P. maculiventris and 20 adult C. vittatus per prey life stage. For P. maculiventris, predation rates were obtained in relation to adult, third and fifth instar prey, and for C. vittatus for first, second and third instar prey. On average, predation on third and fifth instar B. hilaris nymphs by P. maculiventris was 0.6 ± 0.1 and 0.9 ± 0.1 per day respectively. Predation rates on adults were slightly higher (1.3 ± 0.1 per day), with female prey being consumed at a significantly higher rate than male prey when three mating pairs of B. hilaris were provided per day (0.8 ± 0.1 females per day vs. 0.5 ± 0.1 males per day). Collops vittatus adults provisioned daily with 20 first instar B. hilaris nymphs killed a mean total of 4.7 ± 0.4 and 9.3 ± 0.6 prey each day (for male and female beetles respectively), with only approximately half that number of prey being fully consumed. Partial consumption of prey by this species was also observed with second and third instar nymphs, but to a lesser degree. Female beetles consumed significantly more prey than did male beetles when fed first and third instar B. hilaris, but not when given second instar prey.     

Warning Signal Efficacy: Assessing the Effects of Color,Iridescence, and Time of Day in the Field

Warning coloration deters predators from attacking distasteful or toxic prey. Signal features that influence warning color effectiveness are not well understood, and in particular, we know very little about how effective short‐wavelength and iridescent colors are as warning color elements in nature and how warning signal effectiveness changes throughout the day. We tested the effect of these factors on predation risk in nature using specimens of the distasteful pipevine swallowtail butterfly, Battus philenor. B. philenor adults display both iridescent blue and diffusely reflecting orange components in their warning signal. We painted B. philenor wings to create five different model types: all‐black, only‐iridescent‐blue, only‐orange, iridescent‐blue‐and‐orange (intact signal), and matte‐blue‐and‐orange. We placed 25 models in each of 14 replicate field sites for 72 h and checked for attacks three times each day. Model type affected the likelihood of attack; only‐orange models were, the only model attacked significantly less than the all‐black model. Iridescence did not enhance or decrease warning signal effectiveness in our experiment because matte‐blue‐and‐orange models were attacked at the same rate as iridescent‐blue‐and‐orange models. Time of day did not differentially affect model type. Video recordings of attacks revealed that insectivorous birds were responsible. The results of this experiment, when taken with previous work, indicate that the response to blue warning coloration is likely dependent on predator experience and context, but that iridescence per se does not affect warning signals in a natural context.     

Effect of UV-reflecting markings on female mate-choice decisions in Cosmophasis umbratica, a jumping spider from Singapore

Earlier studies have shown that Cosmophasis umbratica, a jumpingspider from Singapore, is sexually dimorphic in the reflectanceof ultraviolet (UV) (males, but not females, have UV-reflectingmarkings). Here we present the first experimental evidence thatthe male's UV markings influence mate choice of C. umbraticafemales. When presented with males whose appearance was manipulatedby the use of a UV-blocking filter, females spent more timewatching UV+ males (i.e., males with UV present) and less timewatching UV– males (UV absent). We also manipulated thelevels of male brightness by using 2 UV-transmitting neutraldensity filters and showed that UV reflectance was used specificallyfor hue discrimination instead of being used for detecting differencesin brightness alone. This is not only the first strong evidenceof UV influence on female mate-choice decisions for a spiderbut also the best experimental demonstration of color vision,whatever the wavelength.     

Seed‐swallowing Toucans are Less Effective Dispersers of Guettarda viburnoides (Rubiaceae) than Pulp‐feeding Jays

For many tropical plants, birds are the most important seed dispersers. Not all birds, however, will provide equally effective dispersal services. Behavioral differences, during and after feeding, can result in different establishment probabilities of new individuals. During 3 yr, we examined species‐specific quantitative and qualitative aspects of Guettarda viburnoides seed dispersal by avian frugivores, focusing on how these aspects modify seed dispersal effectiveness. Fruits of G. viburnoides were consumed by ten species of birds, two of which, Cyanocorax cyanomelas and Pteroglossus castanotis, removed 80 percent of the fruits. These two species differ in qualitative aspects of seed dispersal. First, they select for fruits of different sizes; C. cyanomelas feeds on larger fruits than P. castanotis, which results in the former dispersing larger endocarps than the latter. Second, they differ in their fruit handling treatment; C. cyanomelas are pulp consumers, whereas P. castanotis swallow the fruit whole, and are thus traditionally considered ‘legitimate’ dispersers. The probability of seedling emergence, the temporal pattern of emergence, the number of emerged seedlings per endocarp, and the probability of post‐dispersal seed predation differs between endocarps dispersed by C. cyanomelas and P. castanotis; endocarps dispersed by the former have higher emergence probabilities, higher number of seedlings, faster emergence times, and lower predation probabilities than those dispersed by the latter. Finally, these birds differ in their landscape patterns of endocarp deposition; C. cyanomelas disperses endocarps to habitats with higher recruitment probabilities. Ultimately, the pulp consumer C. cyanomelas is a more effective disperser of G. viburnoides than P. castanotis.     

Risky Courtship: Background Contrast,Ornamentation, and Display Behavior of Wolf Spiders Affect Visual Detection by Toad Predators

Males that search widely for females and perform conspicuous courtship displays run a high risk of being detected by their predators. Therefore, gains in reproductive success might be offset by increased mortality due to predation. Male brush‐legged wolf spiders (Schizocosa ocreata) with larger decorative traits (foreleg tufts) are preferred by females as mates, but are more readily detected by predators. However, predation risk may also be influenced by the interaction between components of signals and the environment in which signaling occurs. Courting male spiders were readily accepted as prey by a sympatric predator, the American toad (Anaxyrus americanus). We used video playback to tease apart the interactive effect between visual signals and the signaling environment on the ability of toads to detect courting spiders as a function of distance, background contrast, the presence or absence of male foreleg tufts, and behavioral activity. The response of toads to video sequences of male spiders was similar to their response to live male spiders. Toad response varied over distance toward spiders displayed against high contrast (sunny) vs. low contrast (shaded) backgrounds. Beyond 30 cm, more toads detected courting male spiders against light, ‘sunny’ backgrounds and detected them faster when compared to the same spider stimulus against darker, ‘shady’ backgrounds. In choice tests, toads oriented more often toward courting males with leg tufts than those without. Toad responses also varied with male spider behavior in that only videos of moving males were attacked. Latency to orient and detection by toads was significantly greater for walking males than courting males, and this effect was most evident at distances between 30 cm and 50 cm. Results supported that courting wolf spiders are at significant risk of predation by visually acute predators. Distance, background contrast, and the presence of foreleg decorations influence detection probability. Thus, the same complex visual signals that make males conspicuous and are preferred by females can make males more vulnerable as prey to toads.     

Learning provides mating opportunities for males of a parasitoid wasp

The ability of insects to learn locations of future resources has rarely been studied. Here, we show that males of the solitary parasitoid wasp Pimpla disparis Viereck (Hymenoptera: Ichneumonidae) learn locations of future mates. Male P. disparis reportedly arrest on parasitized pupae of wax moth, Galleria mellonella L. (Lepidoptera: Pyralidae), and gypsy moth, Lymantria dispar L. (Lepidoptera: Erebidae), when mate emergence is imminent. We tested the hypothesis that male P. disparis identify, memorize, and revisit the location(s) of parasitized host pupae as a strategy to attain mates. We colour‐coded P. disparis males in the field and noticed that they revisit parasitized moth pupae on consecutive days, and arrest on those pupae with a near‐emergence P. disparis parasitoid. In a laboratory experiment with two large corrugated cardboard cylinders (CCCs) as surrogate trees, each CCC bearing two parasitized moth pupae with a near‐emergence P. disparis parasitoid or two pupae not parasitized, males on day 1 of the experiment visited parasitized pupae more often than pupae not parasitized. On day 2, when each CCC had been replaced and now carried pupae that were not parasitized, males returned to the same CCC, or the same micro‐location on that CCC, which on day 1 had carried parasitized pupae. Field and laboratory data combined indicate that male P. disparis learn the location of future mates. With female P. disparis being haplodiploid and capable of reproducing without mating experience, the onus to find a mate is on males. They accomplish this by detecting parasitized pupae, learning their location, revisiting them frequently, and then arresting on them when the prospective mate nears emergence, taking a 50% chance that it is indeed a female.     

Laboratory adaptation reduces female mating resistance in the sweet potato weevil

Selection for genetic adaptation might occur whenever an animal colony is maintained in the laboratory. The laboratory adaptation of behavior such as foraging, dispersal ability, and mating competitiveness often causes difficulties in the maintenance of biological control agents and other beneficial organisms used in procedures such as the sterile insect technique (SIT). Sweet potato weevil, Cylas formicarius (Summers) (Coleoptera: Brentidae), is an important pest in sub‐tropical and tropical regions. An eradication program targeting C. formicarius using SIT was initiated in Japan with weevils being mass‐reared for 95 generations to obtain sufficient sterile males. The mass‐reared strain of C. formicarius exhibits weaker female resistance to male mating attempts compared with the wild strain. This could affect the success of SIT programs because mating persistence of mass‐reared males might be expected to decrease in response to weak female resistance. We show that high success of sperm transfer to mass‐reared females was due to weak female resistance to male mating attempts. However, the mating behavior of mass‐reared males did not change. In C. formicarius, the trait of male persistence to mate was not correlated with the female resistance traits. Our results suggest that mass‐rearing conditions do not have negative effects on the mating ability of the sterile males of this species, and thus that the current mass‐rearing procedures are suitable for production of sterile males for the weevil eradication program.     

Predation Risk Reduces a Female Preference for Heterospecific Males in the Green Swordtail

The presence of a predator can result in the alteration, loss or reversal of a mating preference. Under predation risk, females often change their initial preference for conspicuous males, favouring less flashy males to reduce the risk of being detected by predators. Previous studies on predator‐induced plasticity in mate preferences have given females a choice between more and less conspicuous conspecific males. However, in species that naturally hybridize, it is also possible that females might choose an inconspicuous heterospecific male over a conspicuous conspecific male under predation risk. Our study addresses this question using the green swordtail (Xiphophorus helleri) and the southern platyfish (Xiphophorus maculatus), which are sympatric in the wild. We hypothesized that X. helleri females would prefer the sworded conspecific males in the absence of a predator but favour the less conspicuous, swordless, heterospecific males in the presence of a predator. Contrary to our expectation, females associated more with the heterospecific male than the conspecific male in the control (no predator) treatment, and they were non‐choosy in the predator treatment. This might reflect that females were attracted to the novel male phenotype when there was no risk of predation but became more neophobic after predator exposure. Regardless of the underlying mechanism, our results suggest that predation pressure may affect female preferences for conspecific versus heterospecific males. We also found striking within‐population, between‐individual variation in behavioural plasticity: females differed in the strength and direction of their preferences, as well as in the extent to which they altered their preferences in response to changes in perceived predation risk. Such variation in female preferences for heterospecific males could potentially lead to temporal and spatial variation in hybridization rates in the wild.     

Tritrophic interactions between a fungal pathogen,a spider predator,and the blacklegged tick

The blacklegged tick Ixodes scapularis is the primary vector for the bacterium causing Lyme disease in eastern North America and for other medically important pathogens. This species is vulnerable to attack by fungal pathogens and arthropod predators, but the impacts of interactions between biocontrol agents have not been examined. The biocontrol agent Met52^®, containing the entomopathogenic fungus Metarhizium brunneum (=M. anisopliae), controls blacklegged ticks with efficacy comparable to chemical acaricides. The brush‐legged wolf spider Schizocosa ocreata is a predator of I. scapularis that reduces their survival under field conditions. We conducted a field microcosm experiment to assess the compatibility of Met52 and S. ocreata as tick biocontrol agents. We compared the fits of alternative models in predicting survival of unfed (flat) and blood‐fed (engorged) nymphs. We found the strongest support for a model that included negative effects of Met52 and S. ocreata on flat nymph survival. We found evidence for interference between biocontrol agents, with Met52 reducing spider survival, but we did not find a significant interaction effect between the two agents on nymph survival. For engorged nymphs, low recovery rates resulted in low statistical power to detect possible effects of biocontrol agents. We found that nymph questing activity was lower when the spider was active above the leaf litter than when the spider was unobserved. This provides the first evidence that predation cues might affect behavior important for tick fitness and pathogen transmission. This study presents field microcosm evidence that the biopesticide Met52 and spider Schizocosa ocreata each reduced survival of blacklegged ticks Ixodes scapularis. Met52 reduced spider survival. Potential interference between Met52 and the spider should be examined at larger scales, where overlap patterns may differ. Ticks were more likely to quest when the spider was inactive, suggesting the ticks changed their behavior to reduce danger.     

A further cost for the sicker sex? Evidence for male‐biased parasite‐induced vulnerability to predation

Males are typically the sicker sex. Data from multiple taxa indicate that they are more likely to be infected with parasites, and are less “tolerant,” or less able to mitigate the fitness costs of a given infection, than females. One cost of infection for many animals is an increased probability of being captured by a predator. A clear, hitherto untested, prediction is therefore that this parasite‐induced vulnerability to predation is more pronounced among males than females. We tested this prediction in the sexually size dimorphic guppy, Poecilia reticulata, in which females are typically larger than males. We either sham or experimentally infected guppies with Gyrodactylus turnbulli, elicited their escape response using an established protocol and measured the distance they covered during 60 ms. To discriminate between the effects of body size and those of other inherent sex differences, we size‐matched fish across treatment groups. Infection with G. turnbulli reduced the distance covered during the escape response of small adults by 20.1%, whereas that of large fish was unaffected. This result implies that parasite‐induced vulnerability to predation is male‐biased in the wild: although there was no difference in escape response between our experimentally size‐matched groups of males and females, males are significantly smaller across natural guppy populations. These results are consistent with Bateman's principle for immunity: Natural selection for larger body sizes and longevity in females seems to have resulted in the evolution of increased infection tolerance. We discuss the potential implications of sex‐ and size‐biased parasite‐induced vulnerability to predation for the evolutionary ecology of this host–parasite interaction in natural communities.     

Getting to the core: Internal body temperatures help reveal the ecological function and thermal implications of the lions’ mane

It has been proposed that there is a thermal cost of the mane to male lions, potentially leading to increased body surface temperatures (T_s), increased sperm abnormalities, and to lower food intake during hot summer months. To test whether a mane imposes thermal costs on males, we measured core body temperature (T_b) continuously for approximately 1 year in 18 free‐living lions. There was no difference in the 24‐hr maximum T_b of males (n = 12) and females (n = 6), and males had a 24‐hr mean T_b that was 0.2 ± 0.1°C lower than females after correcting for seasonal effects. Although feeding on a particular day increased 24‐hr mean and 24‐hr maximum T_b, this phenomenon was true of both male and female lions, and females had higher 24‐hr mean and 24‐hr maximum T_b than males, on both days when lions did not feed, and on days when lions did feed. Twenty‐four‐hour T_b was not influenced by mane length or color, and 24‐hr mean T_b was negatively correlated with mane length. These data contradict the suggestion that there exists a thermal cost to male lions in possessing a long dark mane, but do not preclude the possibility that males compensate for a mane with increased heat loss. The increased insulation caused by a mane does not necessarily have to impair heat loss by males, which in hot environments is primarily through respiratory evaporative cooling, nor does in necessarily lead to increased heat gain, as lions are nocturnal and seek shade during the day. The mane may even act as a heat shield by increasing insulation. However, dominant male lions frequent water points more than twice as often as females, raising the possibility that male lions are increasing water uptake to facilitate increased evaporative cooling. The question of whether male lions with manes compensate for a thermal cost to the mane remains unresolved, but male lions with access to water do not have higher T_b than females or males with smaller manes.     

Transition from monogyny to polygyny in Nephila senegalensis (Araneae: Nephilidae) is not accompanied by increased investment in sperm

A common male adaptation to prevent sperm competition is the placement of a mating plug. Such plugs are considered as an extreme investment if they comprise parts of the genital systems and render the male sterile. Genital mutilation occurs in monogynous spiders of several families and may co‐occur with permanent sperm depletion, meaning that sperm production is terminated once males become mature. Within the orb‐web spider genus Nephila, monogynous mating strategies are considered ancestral, although some species have reverted to a polygynous mating strategy. Although genital mutilation does not occur in these species, permanent sperm depletion (PSD) remained. We compared investment in sperm between an effectively plugging (Nephila fenestrata Thorell, 1859) and a closely‐related nonplugging species [Nephila senegalensis (Walckenaer, 1841)]. Sperm investment should be higher in N. senegalensis because males are able to mate with several females, whereas N. fenestrata males can only achieve a maximum of two copulations, generally performed with the same female. The absence of a plugging mechanism in N. senegalensis and the inability to monopolize females by means of mating plugs results in a higher risk of sperm competition. Thus, we predicted higher investment in sperm producing tissue and larger sperm storage organs in males of N. senegalensis compared to N. fenestrata. We examined the testes and deferent ducts of both species for size and cell‐quality differences using light and transmission electron microscopy and analyzed the volume of the sperm reservoir in the male copulatory organ (i.e. spermophor) using X‐ray microcomputed tomography. In contrast to our prediction, the lumen of testes, deferent ducts, and spermophor of N. senegalensis males were significantly smaller than in N. fenestrata.