RESUMO

The Darwin-Bateman paradigm predicts that females enhance their fitness by being choosy and mating with high-quality males, while males should compete to mate with as many females as possible. In many species, males enhance their fitness by defending females and/or resources used by females. That is, males directly defend access to mating opportunities. However, paternity analyses have repeatedly shown that females in most species mate polyandrously, which contradicts traditional expectations that male defensive behaviours lead to monandry. Here, in an extensive meta-analysis, encompassing 109 species and 1026 effect sizes from across the animal kingdom, we tested if the occurrence of defensive behaviours modulates sexual selection on females and males. If so, we can illuminate the extent to which males really succeed in defending access to mating and fertilisation opportunities. We used four different indices of the opportunity for sexual selection that comprise pre-mating and/or post-mating episodes of selection. We found, for both sexes, that the occurrence of defensive behaviours does not modulate the potential strength of sexual selection. This implies that male defensive behaviours do not predict the true intensity of sexual selection. While the most extreme levels of sexual selection on males are in species with male defensive behaviours, which indicates that males do sometimes succeed in restricting females' re-mating ability (e.g. elephant seals, Mirounga leonina), estimates of the opportunity for sexual selection vary greatly across species, regardless of whether or not defensive behaviours occur. Indeed, widespread polyandry shows that females are usually not restricted by male defensive behaviours. In addition, our results indicate that post-mating episodes of selection, such as cryptic female choice and sperm competition, might be important factors modulating the opportunity for sexual selection. We discuss: (i) why male defensive behaviours fail to lower the opportunity for sexual selection among females or fail to elevate it for males; (ii) how post-mating events might influence sexual selection; and (iii) the role of females as active participants in sexual selection. We also highlight that inadequate data reporting in the literature prevented us from extracting effect sizes from many studies that had presumably collected the relevant data.

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RESUMO

Males are expected to mate with as many females as possible, but can maximize their reproductive success through strategic mating decisions. For instance, males can increase their own fitness by mating with high quality females that produce more offspring. Additionally, males can adjust mating effort based on the relative distribution of females and male competitors. To test factors that influence male mate choice, we assessed male mating decisions in the golden silk orb-weaver spider, Trichonephila clavipes (Nephilidae), a species in which females are polyandrous, males guard females before and after copulation occurs and large males are the most successful at guarding mates. We tested the hypothesis that males spend more time guarding high quality females that are spatially isolated, and when the risk of sperm competition is higher. We also hypothesized that this effect increases with male body size. We assessed solitary and aggregated female webs in the field and quantified female quality (i.e., female body condition), male size (i.e., male body size), the risk of sperm competition (i.e., number of males in each female web), and mate-guarding duration (i.e., number of days each male spent in each web). We found that mate-guarding behaviour is largely influenced by the presence of male competitors. In addition, male body size seems to moderately influence male guarding decisions, with larger males guarding for a longer time. Finally, female body condition and type of web (i.e., solitary or aggregated) seem to play small roles in mate-guarding behaviour. As mate-guarding duration increased by 0.718 day per each additional male competitor in the web, and guarding behaviour prevents males from seeking additional mates, it seems that guarding females can be considerably costly. We conclude that failing to guard a sexual partner promotes high costs derived from sperm competition, and a male cannot recover his relative loss in fertilization success by seeking and fertilizing more females. In addition, the search for more sexual partners can be constrained by possible high costs imposed by weight loss and fights against other males, which may explain why the type of web only moderately influenced male mate choice. Following the same rationale, if high-quality females are not easy to find and/or mating with a high-quality female demands much effort, males may search females and guard them regardless of female quality. In conclusion, the factor that most influences male mate-guarding behaviour among T. clavipes in the field is the risk of sperm competition.

RESUMO

John Endler's sensory drive hypothesis posits that physical properties of the environment may shape signalling traits by determining how effective they are in communicating. Evidence abounds of signalling environment driving selection on colours and vocalizations, yet little is known about its influence on visual signals produced via body movements (i.e., gestural displays). Here, we aimed to perform the most taxonomically comprehensive assessment of sensory drive by testing the hypothesis that habitat structure drives the evolution of aerial sexual displays in passerine birds (order Passeriformes). We base this hypothesis on physical properties of habitats that should allow aerial displays to transmit more broadly in open habitats. To test the hypothesis, we performed Bayesian phylogenetic comparative analyses using species-level phylogenetic trees and a dataset of 469 species from across the Americas. We found that evolutionary gain of aerial displays is on average six times more frequent in open-habitat passerines than in forest ones. The influence of habitat structure on the evolution of aerial sexual displays attests to the importance of sensory drive as a deterministic evolutionary force within sexual selection, as opposed to models that assume a purely arbitrary direction of sexual trait evolution.

Resumo A hipótese da condução sensorial de John Endler postula que propriedades físicas do ambiente podem influenciar traços de sinalização ao determinar quão efetivos eles são em comunicar. Enquanto há bastante evidência do ambiente influenciando cores e vocalizações, pouco se sabe de sua influência sobre sinais visuais produzidos por movimentos corporais (i.e., gestos). Nesse estudo, realizamos a avaliação mais taxonomicamente abrangente da condução sensorial ao testar a hipótese de que a estrutura do hábitat influencia a evolução de exibições sexuais aéreas em aves da ordem Passeriformes. Baseamos essa hipótese em propriedades físicas dos hábitats que deveriam permitir uma transmissão mais ampla de exibições aéreas em áreas abertas. Para testar a hipótese, realizamos análises comparativas filogenéticas Bayesianas utilizando árvores filogenéticas ao nível de espécie e um banco de dados de 469 espécies que ocorrem ao longo do continente americano. Encontramos que o ganho evolutivo de exibições aéreas é, em média, seis vezes mais frequente em Passeriformes de áreas abertas do que em florestais. A influência da estrutura do hábitat sobre a evolução de exibições sexuais aéreas evidencia a importância da condução sensorial como uma força evolutiva determinística dentro da seleção sexual, contrastando com modelos que predizem uma direção puramente arbitrária na evolução de traços sexuais.

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RESUMO

The intensity of biotic interactions varies around the world, in such a way that mortality risk imposed by natural enemies is usually higher in the tropics. A major role of offspring attendance is protection against natural enemies, so the benefits of this behaviour should be higher in tropical regions. We tested this macroecological prediction with a meta-regression of field experiments in which the mortality of guarded and unguarded broods was compared in arthropods. Mortality of unguarded broods was higher, and parental care was more beneficial, in warmer, less seasonal environments. Moreover, in these same environments, additional lines of defence further reduced offspring mortality, implying that offspring attendance alone is not enough to deter natural enemies in tropical regions. These results help to explain the high frequency of parental care among tropical species and how biotic interactions influence the occurrence of parental care over large geographic scales. Finally, our findings reveal that additional lines of defences - an oftentimes neglected component of parental care - have an important effect on the covariation between the benefits of parental care and the climate-mediated mortality risk imposed by natural enemies.

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RESUMO

The handicap principle proposes that sexual signals must be costly to be honest. Honesty may be maintained by the costs paid by honest signallers or by the potential costs of cheating. In the latter, handicaps should emerge as a consequence of specific biological constraints, such as life-history trade-offs. Nuptial prey-giving arthropods are good systems to investigate the honesty of sexual signals taking into account trade-offs between self-maintenance and mating effort. We experimentally evaluated if prolonged food shortage during early adulthood imposes long-term negative effects on gift construction by males of the spider Paratrechalea ornata. We also evaluated whether a burst of food availability improved body condition of poorly fed males, increasing their frequency of gift construction. Poorly fed males hardly constructed gifts, even after a marked increase in feeding rate, which clearly improved their body condition. Moreover, initially poorly fed males that latter received high food intake constructed lighter gifts than continuously well fed males. The long-term effects of prolonged dietary restriction on male propensity to construct a gift and on the size of this gift may increase the honesty of this sexually selected signal. From the female's perspective the offer of a gift may bring information on male quality.

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