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Sexual size dimorphism (SSD) is caused by differences in selection pressures and life‐history trade‐offs faced by males and females. Proximate causes of SSD may involve sex‐specific mortality, energy acquisition, and energy expenditure for maintenance, reproductive tissues, and reproductive behavior. Using a quantitative, individual‐based, eco‐genetic model parameterized for North Sea plaice, we explore the importance of these mechanisms for female‐biased SSD, under which males are smaller and reach sexual maturity earlier than females (common among fish, but also arising in arthropods and mammals). We consider two mechanisms potentially serving as ultimate causes: (a) Male investments in male reproductive behavior might evolve to detract energy resources that would otherwise be available for somatic growth, and (b) diminishing returns on male reproductive investments might evolve to reduce energy acquisition. In general, both of these can bring about smaller male body sizes. We report the following findings. First, higher investments in male reproductive behavior alone cannot explain the North Sea plaice SSD. This is because such higher reproductive investments require increased energy acquisition, which would cause a delay in maturation, leading to male‐biased SSD contrary to observations. When accounting for the observed differential (lower) male mortality, maturation is postponed even further, leading to even larger males. Second, diminishing returns on male reproductive investments alone can qualitatively account for the North Sea plaice SSD, even though the quantitative match is imperfect. Third, both mechanisms can be reconciled with, and thus provide a mechanistic basis for, the previously advanced Ghiselin–Reiss hypothesis, according to which smaller males will evolve if their reproductive success is dominated by scramble competition for fertilizing females, as males would consequently invest more in reproduction than growth, potentially implying lower survival rates, and thus relaxing male–male competition. Fourth, a good quantitative fit with the North Sea plaice SSD is achieved by combining both mechanisms while accounting for sex‐specific costs males incur during their spawning season. Fifth, evolution caused by fishing is likely to have modified the North Sea plaice SSD. The paper presents a model to evolutionarily explain the sexual size dimorphism with an eco‐genetic model adapted to detailed empirical data of North Sea plaice. Behavioral investments and diminishing fitness returns are considered as alternative explanations with its implication in individual energy allocation, and put into context with the ecology of the species.

Sexual selection on male body size in species with a female-biased sexual size dimorphism is common yet often poorly understood. In particular, in the majority of bee species, the relative contribution of intrasexual competition and female choice to patterns of male body size is unknown. In this field study, we examined two possible components of male mating success with respect to body size in the solitary bee Diadasia rinconis Cockerell (Hymenoptera: Apidae): 1) ability to procure a mate and 2) the duration of copulation. We found that larger males were better able to procure mates and copulated for shorter periods of time. Although consistent with sperm competition theory, differences in copulation duration were slight; possibly, the shorter copulations of larger males instead reflect in copulo female choice. Consistent with this notion, males engaged in complex courtship while mounted, characterized for the first time in any bee in such detail via audio recordings and high-speed, high-definition video. The number of pulses in male courtship behavior was also positively associated with copulation duration and may have stimulated females to continue copulating, thereby potentially allowing smaller males to transfer a full ejaculate. Females were shown to be potentially polyandrous and although we did not observe precopulatory rejection in the field, captive females frequently rejected copulation attempts by captive males. Our work indicates that intrasexual competition selects for increased body size in a solitary bee.

Mating systems and factors affecting reproductive success are much studied especially for mammals that are large bodied, have marked sexual size dimorphisms, and have a female-defense mating system. For species that deviate from these patterns, we need more information on mating systems and reproductive success. Here, we study mating system and factors related to reproductive success in a solitary rodent, the Siberian flying squirrel (Pteromys volans). In contrast to most other mammals, males are not larger than females in flying squirrels. Similarly to most mammalian species, we observed multimale paternity within litters and reproductive success of males being positively related to body mass. Variation in reproductive success was clearly higher for males than for females, although remained lower than observed in species with highly male-biased sexual size dimorphism. Female flying squirrels lived in nonoverlapping home ranges and reproductive success was positively related to body mass, in line with earlier predictions for large female size in mammals.

ABSTRACT Aquatic bugs (Nepomorpha) are a species‐rich taxon of insects inhabiting all types of freshwaters, often at high densities, and some are among the dominant predators in aquatic food webs. In contrast to the vertebrates and some other arthropod taxa, sexual dimorphism in aquatic bugs is a poorly investigated issue; the present work is the first comprehensive analysis in this field. Morphological differences between the sexes are generally minor, except that female body size is usually longer than that of males. There is no information on the extent of sexual dimorphism in Nepomorpha, except in Belostomatidae and Corixidae. We examined the patterns of sexual size dimorphism (SSD) of 1195 species and subspecific taxa (about 45% of the known species) and whether they conform to Rensch's rule (RR) at the infraorder, superfamily, family and genus levels. We attempted to identify the potential causes of the observed patterns. Our assumption that females are longer‐bodied in most aquatic bug species is falsified. Species living in habitats near the edges of water bodies had higher SSD than in all other habitats. The most likely reason for the large intersexual size difference here is that, unlike the species living in other habitats, the species here are sit‐and‐wait predators. Our results indicate that sexual selection (including fertility selection) plays an important role in the development of SSD. Moreover, it is probable that water‐edge habitats and the associated sit‐and‐wait lifestyle increase sexual selection pressure and the degree of SSD in Nepomorpha. Converse RR, typical for many insects, characterises Nepomorpha as a whole, and two of its superfamilies. At the family and genus levels, however, roughly half of the taxa obey RR and half of them obey converse RR. We examined the degree of sexual size dimorphism (SSD), based on body length, in approximately 45% of known water bug (Nepomorpha) species. Contrary to our initial hypothesis, we found that females are not significantly longer than males in most species. Our research revealed that habitat plays a significant role in SSD. Species found in nearshore habitats exhibit a higher degree of SSD compared to those in other water bodies. This greater SSD may be associated with a sit‐and‐wait lifestyle.

We developed a Hidden Markov mark–recapture model (R package marked) to examine sex-specific demography in Magellanic Penguins (Spheniscus magellanicus). Our model was based on 33 yr of resightings at Punta Tombo, Argentina, where we banded ~44,000 chicks from 1983 to 2010. Because we sexed only 57% of individuals over their lifetime, we treated sex as an uncertain state in our model. Our goals were to provide insight into the population dynamics of this declining colony, to inform conservation of this species, and to highlight the importance of considering sex-specific vital rates in demographic seabird studies. Like many other seabirds, Magellanic Penguins are long-lived, serially monogamous, and exhibit obligate biparental care. We found that the non-breeding-season survival of females was lower than that of males and that the magnitude of this bias was highest for juveniles. Biases in survival accumulated as cohorts aged, leading to increasingly skewed sex ratios. The survival bias was greatest in years when overall survival was low, that is, females fared disproportionality worse when conditions were unfavorable. Our model-estimated survival patterns are consistent with independent data on carcasses from the species’ non-breeding grounds, showing that mortality is higher for juveniles than for adults and higher for females than for males. Juveniles may be less efficient foragers than adults are and, because of their smaller size, females may show less resilience to food scarcity than males. We used perturbation analysis of a population matrix model to determine the impact of sex-biased survival on adult sex ratio and population growth rate at Punta Tombo. We found that adult sex ratio and population growth rate have the greatest proportional response, that is, elasticity, to female pre-breeder and adult survival. Sex bias in juvenile survival (i.e., lower survival of females) made the greatest contribution to population declines from 1990 to 2009. Because starvation is a leading cause of morality in juveniles and adults, precautionary fisheries and spatial management in the region could help to slow population decline. Our data add to growing evidence that knowledge of sex-specific demography and sex ratios are necessary for accurate assessment of seabird population trends.