Variance in male reproductive success and sexual size dimorphism in pinnipeds: testing an assumption of sexual selection theory

相似文献   

The influence of male age and simulated pathogenic infection on producing a dishonest sexual signal

In recent years, studies have shown that reproductive effort decelerates in response to pathogenic infection. If infection substantially reduces a host''s residual reproductive value (RRV), however, then an acceleration of effort may instead occur (e.g. terminal investment). Reproductive acceleration would theoretically allow hosts to maintain or exaggerate their sexual signal upon infection. This would create a deceptive message from the perspective of the chooser, who may unwittingly copulate with an infected mate to their detriment. Using the cricket Allonemobius socius, we assessed the potential for reduced RRV to accelerate male reproductive effort and create a dishonest signal. RRV was manipulated through male age and simulated pathogenic insult. Reproductive effort was measured as calling song energetics, mating success, latency to mate and nuptial gift size. We show that males adopted either an accelerated or decelerated reproductive strategy upon infection, and that this decision was probably mediated by RRV. Moreover, males who accelerated their effort produced a dishonest signal by increasing their song energetics while providing fewer paternal resources (i.e. smaller gifts). Our study is one of the few to document the existence of dishonest signals and relate dishonesty to a potential reduction in female fitness, underscoring the conflict inherent in sexual reproduction.     

Maternal inheritance, sexual conflict and the maladapted male

Females differ from males in transmitting not only nuclear genes but also cytoplasmic genetic elements (CGEs), including DNA in mitochondria, chloroplasts and microorganisms that are present in the cell. Until recently, evolutionary research has adopted a nucleocentric approach in which organelles have been viewed as subservient energy suppliers. In this article, we propose that a more equitable view of nuclear genes and organelle genomes will lead to a better understanding of the dynamics of sexual selection and the constraints on male adaptation. Maternal inheritance of CGEs intensifies sexually-antagonistic coevolution and provides a parsimonious explanation for the relatively high frequency in males of such apparently maladaptive traits as infertility, homosexuality and baldness.     

Multiple sexual ornaments in satin bowerbirds: ultraviolet plumage and bowers signal different aspects of male quality

Much attention has been devoted to understanding the evolutionof elaborate male ornaments and how they may signal male quality.However, the evolution of multicomponent sexual signals remainspoorly understood, and past research on this type of signalinghas been largely theoretical. Satin bowerbirds, Ptilonorhynchusviolaceus, are polygynous, are sexually dichromatic, and constructsexually selected display structures (bowers): a model systemfor investigating the evolution and signal function of multiplesexual signals. We studied the interrelationship between bowerfeatures, plumage coloration, and indicators of male qualityin this species. To do this, we located the bowers of male satinbowerbirds in rainforest in Queensland, Australia, and quantifiedbower quality. We captured the male bower owners and used reflectancespectrometry to objectively measure the plumage coloration ofseveral body regions. We measured various indicators of malehealth and condition, including the intensity of infection fromectoparasites and blood parasites. Bower quality and male ultravioletplumage coloration were significantly correlated. By using multipleregression analyses, we show that bower quality predicts ectoparasiteload and body size, whereas ultraviolet plumage coloration predictsthe intensity of infection from blood parasites, feather growthrate, and body size. Our findings support the multiple messageshypothesis of multicomponent signals: Female satin bowerbirdsshould assess both male and bower features to choose the highestquality mates.     

The bed nucleus of the stria terminalis has developmental and adult forms in mice,with the male bias in the developmental form being dependent on testicular AMH

Canonically, the sexual dimorphism in the brain develops perinatally, with adult sexuality emerging due to the activating effects of pubescent sexual hormones. This concept does not readily explain why children have a gender identity and exhibit sex-stereotypic behaviours. These phenomena could be explained if some aspects of the sexual brain networks have childhood forms, which are transformed at puberty to generate adult sexuality. The bed nucleus of stria terminalis (BNST) is a dimorphic nucleus that is sex-reversed in transsexuals but not homosexuals. We report here that the principal nucleus of the BNST (BNSTp) of mice has developmental and adult forms that are differentially regulated. In 20-day-old prepubescent mice, the male bias in the principal nucleus of the BNST (BNSTp) was moderate (360 ± 6 vs 288 ± 12 calbindin^+ ve neurons, p < 0.0001), and absent in mice that lacked a gonadal hormone, AMH. After 20 days, the number of BNSTp neurons increased in the male mice by 25% (p < 0.0001) and decreased in female mice by 15% (p = 0.0012), independent of AMH. Adult male AMH-deficient mice had a normal preference for sniffing female pheromones (soiled bedding), but exhibited a relative disinterest in both male and female pheromones. This suggests that male mice require AMH to undergo normal social development. The reported observations provide a rationale for examining AMH levels in children with gender identity disorders and disorders of socialization that involve a male bias.     

The cost of producing a sexual signal: testosterone increases the susceptibility of male lizards to ectoparasitic infestation

According to current evolutionary theory, advertising traitsthat honestly indicate an organism's genetic quality might becostly to produce or maintain, though the kind of costs involvedin this process are controversial. Recently the immunocompetencehypothesis has proposed that testosterone (T) stimulates theexpression of male sexually selected traits while decreasingimmunocompetence. Even though some recent studies have shownan effect of T on ectoparasite load, the dual effect of thehormone has not been addressed in free-living populations. Herewe report results of an experiment in a free-living populationof the lizard Psammodromus algirus during the mating season.Males implanted with T had larger patches of breeding colorand behaved more aggressively than control males. In T-implantedmales, the increase in number of ticks during the mating seasonwas significantly higher than in control males and this negativelyaffected several hematological parameters. T-males sufferedsignificantly higher mortality than control males during theexperiment The results from the manipulation of T are consistentwith the dual effect of this hormone.     

Aromatization and the induction of male sexual behavior in male, female, and androgenized female hamsters

Eight weeks after gonadectomy male, female, and androgenized [10 μg testosterone propionate (TP), 24 hr after birth] female hamsters were given daily treatment with: 150 μg dihydrotestosterone (DHT), 5 μg estradiol benzoate (EB), 150 μg DHT + 5 μg EB, 150 μg DHT + 1 μg EB, 30 μg DHT + 5 μg EB, 30 μg DHT + 1 μg EB, or the oil vehicle. Treatment of castrated male hamsters with 5 μg EB fully restored mounting but relatively few of these animals intromitted and none ejaculated. Treatment with 150 μg DHT restored all components of male sexual behavior but only in a small proportion of the males. Combined treatment with EB and DHT restored mounts, intromissions, and ejaculations in the majority of the males. Although as little as 30 μg DHT + 1 μg EB restored the full complement of male behavior, the males which received 150 μg DHT + 5 μg EB or 150 μg DHT + 1 μg EB required fewer intromissions to achieve ejaculation than the males which received 30 μg DHT + either dose of EB. The response of the androgenized females was similar to that of the males except that the androgenized females had lower intromission rates and none ejaculated. Relatively few of the nonandrogenized females responded to EB and DHT treatment and those that did mounted only a few times each test. These results demonstrate that both EB and DHT can stimulate male sexual behavior in the hamster and that the sensitivity to EB and DHT for copulatory behavior is determined by early postnatal androgen exposure.     

Strong, silent types: the rapid, adaptive disappearance of a sexual signal

To see evolution in action, it helps to be in the right place at the right time. In a recent study, Zuk et al. document the rapid evolution of a sexual signal in crickets on a Hawaiian island and suggest that it is a response to parasitism. A new male morph has appeared that cannot sing, and so does not attract an acoustically orientating parasite. The disadvantage of this, however, is that silent males might not be able to attract mates. The authors suggest that plasticity in male aggregating behaviour can provide an escape from the costs of this potentially maladaptive trait.     

An investigation of the IQM signal variation and error detection sensitivity for patient specific pre-treatment QA

PurposeTo evaluate the Integral Quality Monitor (IQM) as a clinical dosimetry device for detecting photon beam delivery errors in clinically relevant conditions.Materials and methodsThe IQM’s ability to detect delivery errors introduced into clinical VMAT plans for two different treatment sites was assessed. This included measuring 103 nasopharynx VMAT plans and 78 lung SBRT VMAT plans with introduced errors in gantry angle (1–5°) and in MLC-defined field size and field shift (1–5 mm). The IQM sensitivity was compared to ArcCheck detector performance. Signal dependence on field position for on-axis and asymmetrically offset square field sizes from 1 × 1 cm² to 30 × 30 cm² was also investigated.ResultsThe IQM detected almost all introduced clinically-significant MLC field size errors, but not some small gantry angle errors or most MLC field shift errors. The IQM sensitivity was comparable to the ArcCheck for lung SBRT, but worse for the nasopharynx plans. Differences between IQM calculated/predicted and measured signals were within ± 2% for all on-axis square fields, but up to 60% for the smallest asymmetrically offset fields at large offsets.Conclusion The IQM performance was consistent and reproducible. It showed highest sensitivity to the field size errors for these plans, but did not detect some clinically-significant introduced gantry angle errors or most MLC field shift errors. The IQM calculation model is still being developed, which should improve small offset-field performance. Care is required in IQM use for plan verification or online monitoring, especially for small fields that are off-axis in the detector gradient direction.     

A signal detection analysis of morphine effects on the response bias of rats in a two-shock discrimination task

Signal detection theory techniques were used to assess the extent to which morphine affects response bias of rats in a two-choice shock discrimination task. The ratio of low shock (“noise”) to high shock (“signal”) trials was varied to produce different response biases prior to drug administration. Morphine significantly reduced signal sensitivity (d′) under all three conditions without systematically affecting any of the measures of response bias assessed in this study. While consistent with most reports with respect to d′, these results conflict with a few previous reports of morphine disrupting performance more when the discriminative stimulus is the presence of shock than when it is the absence of shock.     

A new sexual signal in rutting male red deer: Age related chemical scent constituents in the belly black spot

Rutting behaviour of red deer stags (Cervus elaphus) includes an extensive repertoire of visual and acoustic signals directed either to rival males or to females. As in other mammals, olfactory communication is expected to play a central role in these rutting interactions too, but this has rarely been investigated. Only during the rutting season, red deer males show a conspicuous black spot area throughout most of their underbelly produced by the impregnation of substances with a strong scent. Here, we examined the origin of these compounds and their potential role as chemical signals. By using gas chromatography–mass spectrometry (GC–MS), we identified 67 compounds in the hair from the belly black spot of red deer stags, mainly heterocyclic aromatic organic compounds, such as m-cresol, benzoic acid, cyclohexanecarboxylic acid and ethylphenol, but we also found steroids, such as cholesterol and androstane-3,17-dione, carboxylic acids and their esters between n-C₆ and n-C₂₂, alcohols, squalene and other minor compounds. Many of these compounds are found in the belly black spot but not in other hair areas, and may have originated from several sources, such as the urine or the sebaceous glands of the skin, which impregnated the belly. Moreover, we found differences in chemical profiles depending on age, with older males having higher proportions of benzoic acid and androstane-3,17-dione, but lower proportions of m-cresol. Because most of these compounds are strongly odoriferous, and appear related to male characteristics, our data indicate that scent from the hairs forming the black spot of the belly may be regarded as an overlooked new sexual chemical signal in red deer in the context of competition for mates during the rutting season.     

Mosquitocidal toxins, genes and bacteria: the hit squad

Certain entomopathogenic species of bacilli and Clostridium produce one or more toxins that kill mosquito larvae even at concentrations in the picomolar range. Altogether, 19 distinct genes are known that encode mosquitocidal toxins, which vary in their potency, species specificity and mode of action. Unlike chemical insecticides, mosquitocidal bacilli used as larvicides are safe for animals and the environment, and do not affect non-pest insects. Mosquitocidal bacteria are effective to varying degrees against Culex, Anopheles and Aedes mosquito larvae, but their rapid sedimentation from the larval feeding zone, UV-light sensitivity and narrow host range have hampered their development. New genetic engineering approaches are being investigated that could overcome these limitations and allow stable expression of broad host range combinations of toxins in UV-resistant, buoyant recombinant bacteria, as discussed here by Alan Porter.     

Selfish genetic elements and sexual selection: their impact on male fertility

Females of many species mate with more than one male (polyandry), yet the adaptive significance of polyandry is poorly understood. One hypothesis to explain the widespread occurrence of multiple mating is that it may allow females to utilize post-copulatory mechanisms to reduce the risk of fertilizing their eggs with sperm from incompatible males. Selfish genetic elements (SGEs) are ubiquitous in eukaryotes, frequent sources of reproductive incompatibilities, and associated with fitness costs. However, their impact on sexual selection is largely unexplored. In this review we examine the link between SGEs, male fertility and sperm competitive ability. We show there is widespread evidence that SGEs are associated with reduced fertility in both animals and plants, and present some recent data showing that males carrying SGEs have reduced paternity in sperm competition. We also discuss possible reasons why male gametes are particularly vulnerable to the selfish actions of SGEs. The widespread reduction in male fertility caused by SGEs implies polyandry may be a successful female strategy to bias paternity against SGE-carrying males.