Male mealworm beetles increase resting metabolic rate under terminal investment

Harmful parasite infestation can cause energetically costly behavioural and immunological responses, with the potential to reduce host fitness and survival. It has been hypothesized that the energetic costs of infection cause resting metabolic rate (RMR) to increase. Furthermore, under terminal investment theory, individuals exposed to pathogens should allocate resources to current reproduction when life expectancy is reduced, instead of concentrating resources on an immune defence. In this study, we activated the immune system of Tenebrio molitor males via insertion of nylon monofilament, conducted female preference tests to estimate attractiveness of male odours and assessed RMR and mortality. We found that attractiveness of males coincided with significant down‐regulation of their encapsulation response against a parasite‐like intruder. Activation of the immune system increased RMR only in males with heightened odour attractiveness and that later suffered higher mortality rates. The results suggest a link between high RMR and mortality and support terminal investment theory in T. molitor.     

Do male mealworm beetles, <Emphasis Type="Italic">Tenebrio molitor</Emphasis>, sustain the honesty of pheromone signals under immune challenge?

Immune function is potentially costly and traded-off against the expression of sexual signals, thus, making sexual advertisement a condition-dependent and honest indicator of current immunocompetence. We conducted a series of laboratory experiments using mature mealworm beetles, Tenebrio molitor, to examine whether an immunological challenge (nylon implant(s), microbial lipopolysacharides, microlatex beads) could affect the attractiveness of male pheromones or male’s survival in ad libitum and ‘water-only’ food regimes or whether pheromones signal more long-term qualities of males. We treated another set of newly-emerged males with a single nylon implant and gave first ad libitum food but later only water. We used several immune challengers and feeding regimes to examine if the expected trade-off between immune function and sexual signaling would be expressed differently depending on the resource level and the type of the immune challenge. None of the treatments affected the female preference for male pheromones or the male longevity except for the acute mortality caused by two-implant treatments. However, males lost less body mass when immune challenged and given an opportunity to feed. We conclude that females seem to prefer certain males, and the effect of immune challenge in males cannot systematically override these preferences.     

Enhanced male coloration after immune challenge increases reproductive potential

In many animal species, females select a mate on the basis of the expression of secondary sexual traits. A prevalent theory suggests that male ornaments are reliable indicators of immunocompetence, because the cost of immune function prevents cheating. However, sexual signalling is a component of male reproductive effort, and an immune challenge may also alter his perceived future prospects and hence signalling effort. In this study, blue‐footed booby males (Sula nebouxii) were inoculated with a diphtheria–tetanus vaccine during courtship to investigate the consequences of mounting an immune response on signalling effort. We found that, after this immune challenge, on average, males increased their signalling effort but lost more body mass compared with control males. Importantly, vaccination affected the partner's reproductive decisions: compared with control females, females paired with vaccinated males laid eggs earlier and increased clutch volume in pairs that laid early. Overall, our results suggest that blue‐footed booby males invest more in sexual signals when future breeding opportunities are at risk, eliciting a greater reproductive investment by their partners. Increased signalling effort by infected individuals may contrast the idea of sexual ornaments as signals of infection status.     

Previous encapsulation response enhances within individual protection against fungal parasite in the mealworm beetle Tenebrio molitor

Immune defenses of insects show either broad reactions or specificity and durability of induced protection against attacking parasites and pathogens. In this study, we tested whether encapsulation response against nylon monofilament increases between two attempts of activation of immune system in mealworm beetles Tenebrio molitor, and whether previous exposure to nylon monofilament may also increase protection against an entomopathogenic fungus. We found that survival of beetles subjected to immune activation by nylon implant and subsequent fungal exposure a week later was significantly higher than survival of beetles which had been subjected to fungal infection only. This result suggests that previous immune activation by the nylon implant may be considered as broad spectrum “immune priming” which helps to fight not only the same intruder but also other parasites.     

Attraction of forest cockchafer Melolontha hippocastani to (Z)-3-hexen-1-ol and 1,4-benzoquinone: application aspects

Abstract The application aspects of chemicals which attract the forest cockchafer, Melolontha hippocastani F., were investigated in field and laboratory experiments. Previous studies have shown that males of M. hippocastani are attracted by a synthetic mixture of green leaf volatiles (GLV) and the sex pheromone 1,4‐benzoquinone (BQ), that synergistically enhances the male response to GLV. In the present study, we demonstrated that BQ also synergised the male response to one single component of the GLV mixture, the leaf alcohol (Z)‐3‐hexen‐1‐ol (Z‐3‐ol). BQ enhanced the attractiveness of Z‐3‐ol at doses between 0.05 and 5 mg per trap, reaching a maximum at 5 × 10^?1 mg day^?1. The addition of an insecticide (cyhalothrin) to traps baited with BQ and Z‐3‐ol did not affect the lures’ attractiveness. However, when a conidiospore formulation of the entomopathogenic fungus Beauveria brongniartii (Saccardo) Petch was added, the attractiveness of baited traps was significantly reduced. Furthermore, two types of dispensers baited with a solution of BQ in Z‐3‐ol at 20 mg ml^?1 were tested over the entire 4‐week flight season. Both a membrane dispenser and a dispenser based on a porous polyethylene (PPE) absorbent disk attracted more males than controls over the entire 4 weeks. The membrane dispenser attracted as many males each week as a reference formulation that was renewed daily. Furthermore, the membrane dispenser attracted more males than the PPE dispenser in weeks 2–4, although laboratory experiments showed that the latter released even higher or at least equal amounts of the joined lure over the entire 4 weeks. However, estimation of the BQ/Z‐3‐ol ratios of the released material by solid phase microextraction (SPME) and coupled gas chromatography–mass spectrometry (GC‐MS) revealed that the membrane dispenser released a higher proportion of BQ than the PPE dispenser in weeks 2–4. Therefore, a higher BQ/Z‐3‐ol ratio might be responsible for the advantage of the membrane dispenser in the field.     

Testosterone Manipulation of Male Attractiveness has no Detectable Effect on Female Home-Range Size and Behavior During the Fertile Period

Female dark‐eyed juncos (Junco hyemalis) are socially monogamous, but they engage in extra‐pair copulations (EPCs). We examined spatial activity and behavior of female juncos during their fertile period to determine whether they engaged in tactics likely to facilitate EPCs and whether any such tactics varied with the attractiveness of their social mates. We manipulated the attractiveness of social mates by implanting experimental males with tubes containing testosterone (T‐males) and control males with empty tubes (C‐males). Previous findings in free‐living juncos showed that females mated to C‐males were more likely to produce extra‐pair young than females mated to T‐males. We radio‐tracked 13 females (eight C‐mated, five T‐mated) for an average of 15 h each over 3 d during their fertile periods. We predicted that C‐mated females, to compensate for the induced relative unattractiveness of their social mates, would foray from their territories to seek EPCs and as a result would have larger home ranges than T‐mated females. Females of both treatment groups made extra‐territorial forays, some of considerable distances, but we observed no EPCs during forays. Further, neighboring T‐ and C‐males frequently made incursions into the home ranges of T‐ and C‐mated females but we saw no EPCs during these incursions. Our ability to detect statistical differences was limited by sample size, but given that constraint, we found no detectable difference in female home‐range size in relation to the treatment of their mates, nor did other female behavior differ according to male treatment. Male behavior was significantly affected by testosterone treatment. C‐males guarded their mates more closely than did T‐males. We conclude that female juncos make extra‐territorial movements during their fertile period without regard to male attractiveness (testosterone treatment), but we found no evidence that these function as a special tactic to gain EPCs.     

Effects of methoxyfenozide-treated surfaces on the attractiveness and responsiveness of adult leafrollers

Abstract The attractiveness and responsiveness of the adult redbanded leafroller, Argyrotaenia velutinana (Walker), and obliquebanded leafroller, Choristoneura rosaceana (Harris), exposed to surfaces treated with the ecdysteroid agonist methoxyfenozide were investigated in wind tunnel and orientation tube assays. For both species, when males were exposed to water‐treated surfaces for 48 h, and regardless of what treatment surfaces the females had been exposed to, the mean percentage recaptures of such treated males in the wind tunnel assays were always significantly greater than the recaptures of males that had been exposed to methoxyfenozide. However, exposure to surfaces treated with surfactant (Latron B‐1956) also negatively affected the responsiveness (or attractiveness) of both moth species. In the orientation tube assays, males exposed to methoxyfenozide‐treated surfaces always had significantly lower mean levels of individuals exhibiting sexual excitability, and the mean distances traveled upwind, regardless of females’ exposure. In addition, exposure to surfactant‐treated surfaces did not affect mean levels of sexual excitibility and distances traveled upwind. The two bioassays demonstrated that male A. velutinana and C. rosaceana exposed to methoxyfenozide‐treated surfaces were not as responsive to calling females (treated and non‐treated) as were the non‐treated males. Conversely, females exposed to methoxyfenozide‐treated surfaces were just as attractive to non‐treated males as water‐treated females. It appears that a male's ability to respond to a calling female is more negatively affected by the ecdysone agonist than a female's ability to call and attract males.     

Inbreeding affects sexual signalling in males but not females of Tenebrio molitor

In many species of animals, individuals advertise their quality with sexual signals to obtain mates. Chemical signals such as volatile pheromones are species specific, and their primary purpose is to influence mate choice by carrying information about the phenotypic and genetic quality of the sender. The deleterious effects of consanguineous mating on individual quality are generally known, whereas the effect of inbreeding on sexual signalling is poorly understood. Here, we tested whether inbreeding reduces the attractiveness of sexual signalling in the mealworm beetle, Tenebrio molitor, by testing the preferences for odours of inbred and outbred (control) individuals of the opposite sex. Females were more attracted to the odours produced by outbred males than the odours produced by inbred males, suggesting that inbreeding reduces the attractiveness of male sexual signalling. However, we did not find any difference between the attractiveness of inbred and outbred female odours, which may indicate that the quality of females is either irrelevant for T. molitor males or quality is not revealed through female odours.     

Attractiveness and toxicity of an attracticide formulation on adult males of ash leaf cone roller, Caloptilia fraxinella

A biodegradable attracticide formulation containing the pyrethroid insecticide permethrin and the sex pheromone of the ash leaf cone roller, Caloptilia fraxinella (Ely) (Lepidoptera: Gracillariidae), was tested for attractiveness and toxicity to males of this invasive pest of horticultural ash, Fraxinus spp. Marsh. (Oleaceae). Trap capture in attracticide‐baited traps was equal to capture in traps baited with rubber septa lures releasing the known attractive pheromone. Pheromone response by male C. fraxinella was not affected by the presence of permethrin as equal numbers of males were captured in traps baited with the attracticide formulation with and without permethrin. Attraction to the attracticide containing both pheromone and insecticide active ingredients did not vary with dose as 10, 50, and 100 mg droplets of the formulation attracted similar numbers of male moths in a field experiment. Equal numbers of male moths were captured in traps baited with droplets that had aged for 0–5 weeks in a laboratory fume hood before deployment in the field. The permethrin constituent of the attracticide formulation was toxic to male moths treated through tarsal contact and resulted in 73 and 100% mortality at 24 and 72 h post‐treatment, respectively. Leg autotomy occurred in over 80% of males 24 h post‐treatment to formulations containing permethrin. Mortality of moths was greater 24 h after treatment with a fresh attracticide source as compared to a 5‐week‐old source. However, mortality as a result of treatment with fresh and aged attracticide droplets was equal at 48, 72, and 96 h post‐treatment. This formulation has the potential to become a useful tool in an integrated pest‐management system for C. fraxinella on horticultural ash.     

Early Developmental Conditions and Male Attractiveness in Zebra Finches

Traits used in mate choice are often costly to produce or maintain, and thus can reflect an individual’s current condition. Mate choice, however, might not only be influenced by the current condition of a potential partner, but also by the condition it had experienced during its early development which can have strong and long‐lasting effects on various traits. Here we studied the effects of different early developmental conditions, imposed by brood size manipulations (small, medium and large broods), on male attractiveness as measured by female choice experiments in zebra finches (Taeniopygia guttata). In three different experiments, we allowed females to choose between males that had been raised in different experimental brood sizes. In none of the experiments, females showed a significant preference for males which had experienced better developmental conditions, i.e. were raised in the relatively smaller experimental broods. Song rate was higher in males coming from small than large broods, but females did not prefer males that sang more. These results suggest that sexual attractiveness either was not affected by our experimental treatment or that males subsequently had compensated in their overall attractiveness for negative effects of early developmental conditions.     

Differential Sperm Priming by Male Sailfin Mollies (Poecilia latipinna): Effects of Female and Male Size

Recent interest in sperm competition has led to a re‐evaluation of the ‘cheap sperm’ assumption inherent in many studies of sexual selection. In particular, mounting evidence suggests that male sperm availability can be increased by the presence of females. However, there is little information on how this interacts with male traits presumably affected by female mate choice, such as larger size. This study examines the effects on male sperm availability of female presence, male body size, and female body size in the sailfin molly, Poecilia latipinna. Individual males of variable body sizes were isolated in divided tanks for 3 d, after which time either a female or no female was added to the other side of the tank. Prior to the treatments, larger males had more stripped sperm than smaller males. Female presence significantly increased the amount of sperm males primed, but this effect was strongest in small males. Furthermore, males showed a greater priming response in the presence of larger females than in the presence of smaller females. These results demonstrate that the presence of sexually mature females increases the amount of sperm males have for insemination. Furthermore, traits that indicate female fecundity may be used by males as cues in male mate choice.     

Sexually dimorphic immune response in the harem polygynous Wellington tree weta Hemideina crassidens

Abstract Adult males are often less immunocompetent than females. One explanation for this is that intense sexual selection causes males to trade‐off investment in immunity with traits that increase mating success. This hypothesis is tested in the Wellington tree weta (Hemideina crassidens), a large, sexually dimorphic orthopteran insect in which males possess enormous mandibular weaponry used during fights for access to female mates. Field‐collected males have a significantly greater immune response (greater melanotic encapsulation) than females, suggesting that body condition, longevity or an allied trait is important to male fitness, or that females require materials for egg production that would otherwise be used to boost immunity. Although immunity is expected to trade‐off against reproductive traits in both sexes, there is no significant relationship between immune response and weapon or testes size in males, nor fecundity in females.     

Effects of Egg and Circulating Testosterone on Ring‐Necked Pheasant (Phasianus colchicus) Male Traits and Combat Outcome

Studies of avian species have shown that maternal effects mediated by the transfer of egg hormones can profoundly affect offspring phenotype and fitness. We previously demonstrated that the injection of a physiological amount of testosterone (T) in the eggs of ring‐necked pheasants (Phasianus colchicus) disrupted the covariation among male morphological traits at sexual maturity and positively affected male mating success. Here, we investigate whether egg T exposure affected adult male circulating T levels at the onset of the breeding season (reflecting gonadal maturation), and the relationship between circulating T and male traits. Egg T exposure did not affect pre‐mating plasma T. T levels were not associated with the expression of secondary sexual and non‐sexual traits or socio‐sexual behaviour (social rank, overall fighting ability and mating success). However, wattle brightness decreased with increasing circulating T in males hatched from T‐eggs (T‐males) but not among control males. In dyadic encounters during the peak mating period, control males with higher pre‐mating T levels had higher chances of being dominant over other control males. However, higher pre‐mating T levels did not predict success in male‐male competition in encounters involving T‐males. We suggest that the long‐term effects of egg T on male phenotype do not originate from differential gonadal maturation according to egg T treatment. Rather, prenatal androgens may have priming effects on functioning of target tissues, translating into differential phenotypic effects according to androgen exposure during embryonic development.     

A trade-off between sexual signalling and immune function in a natural population of the drumming wolf spider Hygrolycosa rubrofasciata

The field of ecological immunology is ultimately seeking to address the question ‘Why is there variation in immune function?’ Here, we provide experimental evidence that costs of ubiquitous sexual signals are a significant source of variation in immune function. In the mating season, males of the wolf spider Hygrolycosa rubrofasciata drum against dry leaves while wandering around the habitat searching for receptive females. According to a previous study, the male metabolic rate during the drumming increases 22‐fold compared to the resting metabolic rate. In the present study, we examined whether investment in costly courtship drumming decreases male immune function in a wild population of H. rubrofasciata. We induced males to increase their drumming rate by introducing females in proximity. As estimates of male immune function, we used lytic activity and encapsulation rate. Lytic activity estimates the concentration of antimicrobial peptides in haemolymph, which have been shown to play an important role in defence against bacteria, viruses and fungi. Encapsulation is an important defence mechanism against nematodes and insect parasitoids, but it also plays a role in defence against viruses. Our results show that males with nonarbitrarily increased investment in drumming rate had considerably lower lytic activities than control males. Also, there was a tendency for males with nonarbitrarily increased investment in drumming rate to have lower encapsulation rates than control males. This study provides experimental evidence for the first time, to our knowledge, that there are direct immunological costs of sexual signalling in natural populations. Therefore, immunological costs of sexual signals may provide significant phenotypic variation to parasite‐mediated sexual selection.     

Precopulatory Sexual Cannibalism Causes Increase Egg Case Production,Hatching Success,and Female Attractiveness to Males

Precopulatory sexual cannibalism is an extreme form of sexual conflict that can entail significant costs to the cannibalized individual and a variety of costs and benefits to the cannibal itself. Characterizing these costs and benefits is fundamental to our understanding of how this behavior evolves. Using the spider Agelenopsis pennsylvanica, we tested the reproductive consequences of precopulatory sexual cannibalism by staging cannibalization events and comparing the performance of experimental cannibals against natural cannibals (i.e., those that cannibalized on their own) and non‐cannibals. We found two performance benefits associated with precopulatory sexual cannibalism: first, experimental cannibals were more likely to produce egg cases than non‐cannibals, and second, egg cases from experimental cannibals and natural cannibals were significantly more likely to hatch than those produced by non‐cannibals. We then tested whether males were more likely to approach the webs of experimental cannibals vs. non‐cannibalistic control females. Our data demonstrate that sexual cannibalism increases female attractiveness to males. Although this result seems counterintuitive, in fact, rates of precopulatory sexual cannibalism were much lower in females that had already cannibalized their first male: 38% of sexually naïve females engaged in precopulatory sexual cannibalism, whereas only 5% of females engaged in cannibalism a second time. Thus, males that approach cannibals receive two benefits: they are less likely to be cannibalized precopula, and they have the possibility of mating with females that have a higher probability of producing viable egg cases. Taken together, our data suggest that precopulatory sexual cannibalism affords females numerous benefits and may have a hand in shaping male mate choice decisions.     

Toxoplasma gondii infection enhances testicular steroidogenesis in rats

The protozoan parasite Toxoplasma gondii enhances the sexual attractiveness of infected male rats and attenuates the innate fear of cat odour in infected individuals. These behavioural changes plausibly lead to greater transmission of parasites through sexual and trophic routes, respectively. Testosterone, a testicular steroid, is known to reduce fear and enhance sexual attractiveness in males. Here, we show that Toxoplasma gondii infection enhances expression of genes involved in facilitating synthesis of testosterone, resulting in greater testicular testosterone production in male rats. In several species, testosterone mediates trade‐offs between sexually selected traits and life history decisions. Augmentation of testosterone synthesis by Toxoplasma gondii suggests that parasites may manipulate these trade‐offs in rats.     

Immune Challenge and Pre- and Post-copulatory Female Choice in the Cricket Teleogryllus commodus

Life history theory predicts a trade off between the expression of male sexual traits and the immune system. To test for this trade off, male crickets Teleogryllus commodus were injected with bacterial lipopolysaccharides (LPS) to induce an immune response and their subsequent pre- and post-copulatory sexual attractiveness to virgin and non-virgin females was assessed. Pre-copulatory attractiveness was quantified based on the time taken for males to court and mate with a female. Post-copulatory attractiveness was measured as the time that elapsed between mating and a female interrupting sperm transfer by removing the externally attached spermatophore. We found no difference in pre- or post-copulatory attractiveness between LPS and control males. In contrast, virgin females retained spermatophores for almost twice as long as non-virgins, presumably to enhance fertilization and begin egg-laying. Finally, we note that although LPS is a widely used immune elicitor in insects, its effect might be transitory. After 24 h there was no detectable elevation in haemolymph antibacterial activity of LPS injected crickets compared to that of controls.     

Male Post‐Ejaculatory Mounting in the Ring‐Tailed Lemur (Lemur catta): A Behavior Solicited by Females?

Male mate guarding can take many forms but often involves aggression toward male conspecifics and continued proximity with a female. This study describes a previously undocumented behavior in a promiscuous primate, the ring‐tailed lemur: post‐ejaculatory (PE) mounting. PE mounting was documented across eight mating seasons in a ring‐tailed lemur colony on St. Catherines Island (SCI), USA. During PE mounting, a male remounts a female following ejaculation and clasps her midsection as if to mate again, but copulation does not occur; males showing this behavior typically lack erections, and their mounts show an absence of penile intromission and rhythmic thrusting. Male PE mounting was more common among males mating earlier in the queue, and when PE mounting occurred, it accompanied mate guarding. Four non‐mutually exclusive hypotheses to explain PE mounting were evaluated as follows: (1) gaining additional copulations, (2) prevention of re‐mating, (3) lengthening sperm residence time, and (4) re‐mounting as a function of female proceptivity. Male PE mounting did not aid males in gaining additional copulations nor did PE mounting prevent females from mating with new males. Equivocal support was found for Hypothesis 3: although there was much overlap in copulatory plug residence times for males who did and did not show PE mounting, one‐third of males who practiced PE mounting had plug residence times of 2 h or more, much longer than that of males who did not show PE mounting. PE mounting may therefore be related to increased plug residence time, which may provide an advantage to males in sperm competition. Strong evidence was found in support of Hypothesis 4: males overwhelmingly performed PE mounts in response to continued female sexual presentations, suggesting that females can solicit this male behavior. Females consequently exercise an even greater degree of control over males than was previously realized in this ‘female dominant’ primate.     

Reduced naïve CD8+ T‐cell priming efficacy in elderly adults

Aging is associated with impaired vaccine efficacy and increased susceptibility to infectious and malignant diseases. CD8⁺ T‐cells are key players in the immune response against pathogens and tumors. In aged mice, the dwindling naïve CD8⁺ T‐cell compartment is thought to compromise the induction of de novo immune responses, but no experimental evidence is yet available in humans. Here, we used an original in vitro assay based on an accelerated dendritic cell coculture system in unfractioned peripheral blood mononuclear cells to examine CD8⁺ T‐cell priming efficacy in human volunteers. Using this approach, we report that old individuals consistently mount quantitatively and qualitatively impaired de novo CD8⁺ T‐cell responses specific for a model antigen. Reduced CD8⁺ T‐cell priming capacity in vitro was further associated with poor primary immune responsiveness in vivo. This immune deficit likely arises as a consequence of intrinsic cellular defects and a reduction in the size of the naïve CD8⁺ T‐cell pool. Collectively, these findings provide new insights into the cellular immune insufficiencies that accompany human aging.