Fitness implications of sex-specific catch-up growth in Nephila senegalensis, a spider with extreme reversed SSD

Honey, What's for Dinner? Dietary Overlap and Size Dimorphism Between Female and Male Joro Spiders (Trichonephila clavata)

Selection on body size tends to favor larger males that outcompete smaller males to mate with females, and larger, more fecund females. For many web-building spiders in the Nephilidae family, reproductive success increases with body size, which in turn, is related to diet. The diet of female spiders may overlap with males who share her web, but diet patterns could depend on size if certain males have better access to prey ensnared in the web. Therefore, we tested the hypothesis that Joro spider (Trichonephila clavata) dietary patterns are sex-specific and related to body size and condition. We combined morphometric size analysis with molecular gut content analysis via DNA metabarcoding and high-throughput sequencing to characterize male and female Joro spider diets from 60 webs at 25 locations. Female Joro spiders were larger than males and their diets were more diverse. While male Joro spiders consumed the same prey taxa as females, more than 50 additional taxa were detected in female diets, which led to significant dissimilarity in prey composition between sexes. As male body size increased, diet diversity and body condition decreased, suggesting a potential trade-off between time spent foraging versus active defense of their position on the web. Female and male spiders captured from the same web shared prey taxa. However, the proportion of prey overlap was not related to male size. Combined, Joro spiders show sex-specific dietary patterns, and males appear to have less access to the diverse food captured in female webs.

Sex-specific developmental trajectories in an extremely sexually size dimorphic spider

Adult body size, development time, and growth rates are components of organismal life histories, which crucially influence fitness and are subject to trade-offs. If selection is sex-specific, male and female developments can eventually lead to different optimal sizes. This can be achieved through developmental plasticity and sex-specific developmental trajectories. Spiders present suitable animals to study differences in developmental plasticity and life history trade-offs between the sexes, because of their pronounced sexual dimorphism. Here, we examine variation in life histories in the extremely sexually size dimorphic African hermit spider (Nephilingis cruentata) reared under standardized laboratory conditions. Females average 70 times greater body mass (and greater body size) at maturity than males, which they achieve by developing longer and growing faster. We find a small to moderate amount of variability in life history traits to be caused by family effects, comprising genetic, maternal, and early common environmental effects, suggesting considerable plasticity in life histories. Remarkably, family effects explain a higher variance in male compared to female life histories, implying that female developmental trajectories may be more responsive to environment. We also find sex differences in life history trade-offs and show that males with longer development times grow larger but exhibit shorter adult longevity. Female developmental time also correlates positively with adult body mass, but the trade-offs between female adult mass, reproduction, and longevity are less clear. We discuss the implications of these findings in the light of evolutionary trade-offs between life history traits.

Two sexes respond equally to food restriction in a sexually dimorphic but not body mass dimorphic jumping spider

Natural selection favors animals that evolve developmental and behavioral responses that buffer the negative effects of food restrictions. These buffering responses vary both between species and within species. Many studies have shown sex-specific responses to environmental changes, usually in species with sexual size dimorphism (SSD), less found in species with weak or no SSD, which suggests that sizes of different sexes are experiencing different selections. However, previous studies usually investigated development and behavior separately, and the balanced situation where males and females of sexually dimorphic species respond in the same way to food restriction remains little known. Here, we investigated this in Phintelloides versicolor (Salticidae) that presents sexual dimorphism in color and shape but weak SSD. We examined whether food restriction induced the same responses in males and females in development duration, adult body size and weight, daily time allocated to foraging, and hunting. We found food restriction induced similar responses in both sexes: both exhibited longer development duration, smaller adult body size and weight, higher probability of staying outside nests and noticing prey immediately, and higher hunting success. However, there were sexual differences regardless of food condition: females showed faster development, smaller adult body size, higher probability of staying outside of nests, and higher hunting success. These indicated the differential selection on male and female sizes of P. versicolor could be under a balanced situation, where males and females show equal developmental and behavioral plasticity to environmental constraints.

Giant and dwarf females: how to explain the fourfold variation in body size and fecundity in Trichonephila senegalensis (Aranea: Nephilidae)

Variation in life-history traits within a population is caused by genetic, maternal and environmental factors. We explored the high variability in development time, adult body weight and fecundity in females of the sexually size dimorphic spider Trichonephila senegalensis. Their mothers originated from two habitats—strongly seasonal Namibia and mildly seasonal South Africa—and we reared F1 females under standardized laboratory conditions. We found that a considerable part of the variability in recorded life-history traits is caused by family-specific effects, comprising genetic, maternal and early environmental influences. Furthermore, we show population differences in development time, where females originating from Namibia matured within shorter periods than females from South Africa. Also, the relationship between development time and adult weight differs between the two populations, as a significant correlation is only found in females with Namibian origin. Against common wisdom, there was a weak overall correlation between adult weight and clutch mass. We also found that females make different life-history decisions under increasing rather than under decreasing daylength. Although a considerable part of variability in life-history traits is family-specific, we discuss how the between-population differences in life histories and their trade-offs reflect adaptation to diverse habitats.

Sex differences in spiders: from phenotype to genomics

Sexual reproduction is pervasive in animals and has led to the evolution of sexual dimorphism. In most animals, males and females show marked differences in primary and secondary sexual traits. The formation of sex-specific organs and eventually sex-specific behaviors is defined during the development of an organism. Sex determination processes have been extensively studied in a few well-established model organisms. While some key molecular regulators are conserved across animals, the initiation of sex determination is highly diverse. To reveal the mechanisms underlying the development of sexual dimorphism and to identify the evolutionary forces driving the evolution of different sexes, sex determination mechanisms must thus be studied in detail in many different animal species beyond the typical model systems. In this perspective article, we argue that spiders represent an excellent group of animals in which to study sex determination mechanisms. We show that spiders are sexually dimorphic in various morphological, behavioral, and life history traits. The availability of an increasing number of genomic and transcriptomic resources and functional tools provides a great starting point to scrutinize the extensive sexual dimorphism present in spiders on a mechanistic level. We provide an overview of the current knowledge of sex determination in spiders and propose approaches to reveal the molecular and genetic underpinnings of sexual dimorphism in these exciting animals.