The history of sexual selection research provides insights as to why females are still understudied

Synthesis of sexual selection: a systematic map of meta-analyses with bibliometric analysis

Sexual selection has been a popular subject within evolutionary biology because of its central role in explaining odd and counterintuitive traits observed in nature. Consequently, the literature associated with this field of study became vast. Meta-analytical studies attempting to draw inferences from this literature have now accumulated, varying in scope and quality, thus calling for a synthesis of these syntheses. We conducted a systematic literature search to create a systematic map with a report appraisal of meta-analyses on topics associated with sexual selection, aiming to identify the conceptual and methodological gaps in this secondary literature. We also conducted bibliometric analyses to explore whether these gaps are associated with the gender and origin of the authors of these meta-analyses. We included 152 meta-analytical studies in our systematic map. We found that most meta-analyses focused on males and on certain animal groups (e.g. birds), indicating severe sex and taxonomic biases. The topics in these studies varied greatly, from proximate (e.g. relationship of ornaments with other traits) to ultimate questions (e.g. formal estimates of sexual selection strength), although the former were more common. We also observed several common methodological issues in these studies, such as lack of detailed information regarding searches, screening, and analyses, which ultimately impairs the reliability of many of these meta-analyses. In addition, most of the meta-analyses' authors were men affiliated to institutions from developed countries, pointing to both gender and geographical authorship biases. Most importantly, we found that certain authorship aspects were associated with conceptual and methodological issues in meta-analytical studies. Many of our findings might simply reflect patterns in the current state of the primary literature and academia, suggesting that our study can serve as an indicator of issues within the field of sexual selection at large. Based on our findings, we provide both conceptual and analytical recommendations to improve future studies in the field of sexual selection.

Lie to me to lay with me: Females deceive males via terminal investment

Historically, males have frequently been portrayed as the manipulative and deceptive gender, while females are often seen as adopting a coy and passive role. In this context, it is proposed that males use a terminal investment strategy, misleading females about their true poor condition, while females passively opt to mate with these deceptive males. However, we hypothesize that females in suboptimal condition may also engage in a terminal investment strategy by mimicking or enhancing their attractiveness to match that of females in better conditions. We studied this hypothesis in Tenebrio molitor, by subjecting females to three varying doses of lipopolysaccharides of Escherichia coli (LPS; 0.25, 0.5, or 1 mg ml-1), or three doses of the pro-oxidant Paraquat (PQ; 20, 40 or 80 mM), and subsequently assessing their survival and attractiveness to males. The LPS treatments and 20 mM of PQ had no significant effect on the survival or attractiveness of the females. However, females treated with 40 or 80 mM PQ survived fewer days compared to the control group. Those injected with 40 mM were more attractive than their control counterparts, while those treated with 80 mM were less attractive. Since the identical doses of LPS, which induce terminal investment in males, had no effect on females, we suggest sexual dimorphism in terminal investment. Furthermore, similar to males, if the stressor reaches a sufficiently high level, the signal becomes honest. These findings highlight how the quantity of stressors influences support for the terminal investment strategy in both males and females. Notably, this study challenges prevailing notions regarding gender roles in sexual selection, indicating that females, not just males, conceal their poor condition to attract mating partners.

How Do Students Critically Evaluate Outdated Language That Relates to Gender in Biology?

Cisheteronormative ideologies are infused into every aspect of society, including undergraduate science. We set out to identify the extent to which students can identify cisheteronormative language in biology textbooks by posing several hypothetical textbook questions and asking students to modify them to make the language more accurate (defined as "correct; precise; using language that applies to all people"). First, we confirmed that textbooks commonly use language that conflates or confuses sex and gender. We used this information to design two sample questions that used similar language. We examined what parts of the questions students modified, and the changes they recommended. When asked to modify sample textbook questions, we found the most common terms or words that students identified as inaccurate were related to infant gender identity. The most common modifications that students made were changing gender terms to sex terms. Students' decisions in this exercise differed little across three large biology courses or by exam performance. As the science community strives to promote inclusive classrooms and embrace the complexity of human gender identities, we provide foundational information about students' ability to notice and correct inaccurate language related to sex and gender in biology.

Eviction-driven infanticide and sexually selected adoption and infanticide in a neotropical parrot

Infanticide and adoption have been attributed to sexual selection, where an individual later reproduces with the parent whose offspring it killed or adopted. While sexually selected infanticide is well known, evidence for sexually selected adoption is anecdotal. We report on both behaviors at 346 nests over 27 y in green-rumped parrotlets (Forpus passerinus) in Venezuela. Parrotlets are monogamous with long-term pair bonds, exhibit a strongly male-biased adult sex ratio, and nest in cavities that are in short supply, creating intense competition for nest sites and mates. Infanticide attacks occurred at 256 nests in two distinct contexts: 1) Attacks were primarily committed by nonbreeding pairs (69%) attempting to evict parents from the cavity. Infanticide attacks per nest were positively correlated with population size and evicting pairs never adopted abandoned offspring. Competition for limited nest sites was a primary cause of eviction-driven infanticide, and 2) attacks occurred less frequently at nests where one mate died (31%), was perpetrated primarily by stepparents of both sexes, and was independent of population size. Thus, within a single species and mating system, infanticide occurred in multiple contexts due to multiple drivers. Nevertheless, 48% of stepparents of both sexes adopted offspring, and another 23% of stepfathers exhibited both infanticide and long-term care. Stepfathers were often young males who subsequently nested with widows, reaching earlier ages of first breeding than competitors and demonstrating sexually selected adoption. Adoption and infanticide conferred similar fitness benefits to stepfathers and appeared to be equivalent strategies driven by limited breeding opportunities, male-biased sex ratios, and long-term monogamy.

New estimates indicate that males are not larger than females in most mammal species

Sexual size dimorphism has motivated a large body of research on mammalian mating strategies and sexual selection. Despite some contrary evidence, the narrative that larger males are the norm in mammals-upheld since Darwin's Descent of Man-still dominates today, supported by meta-analyses that use coarse measures of dimorphism and taxonomically-biased sampling. With newly-available datasets and primary sources reporting sex-segregated means and variances in adult body mass, we estimate statistically-determined rates of sexual size dimorphism in mammals, sampling taxa by their species richness at the family level. Our analyses of wild, non-provisioned populations representing >400 species indicate that although males tend to be larger than females when dimorphism occurs, males are not larger in most mammal species, suggesting a need to revisit other assumptions in sexual selection research.

Long-term effects of neonatal pain and sucrose treatment

Purpose

In neonatal intensive care units, applying sucrose solution for analgesia is now a routine treatment for mild procedural pain. Studies of animal and human infants provide clear evidence of benefits in the short term, but few studies have investigated the long term benefits. Thus, we determined whether sucrose could ameliorate painful stimulation during infancy in Sprague-Dawley rats and also explored the long-term effects of repeated sucrose administration during infancy. Female and male rats were included to investigate sex-related differences.

Methods

Rat pups were stimulated either with painful or tactile stimuli for the first 14 days of their lives. Pups were pretreated either with sucrose or not treated before stimulation. Behavioral tests were conducted during adolescence and adulthood. Hotplate, rotarod, open field, elevated plus maze, and radial arm water maze tests were employed to assess the behavioral consequences of early life manipulations and treatments.

Results

Painful stimulation during infancy increased the sensitivity to pain later in life, and sucrose did not remedy this effect. Motility, coordination, anxiety, and cognition tests in adulthood obtained mixed results. Pain during infancy appeared to increase anxiety during adulthood. Learning and memory in adulthood were affected by pain during infancy, and sucrose had a negative effect even in the absence of pain. No sex-related differences were observed in any of the behavioral tests by employing this model of neonatal pain.

Conclusion

Painful stimulation during infancy resulted in deficiencies in some behavioral tests later in life. Sucrose pretreatment did not mitigate these shortcomings and it actually resulted in negative outcomes.

Neural mechanisms involved in female mate choice in invertebrates

Mate choice is a critical decision with direct implications for fitness. Although it has been recognized for over 150 years, our understanding of its underlying mechanisms is still limited. Most studies on mate choice focus on the evolutionary causes of behavior, with less attention given to the physiological and molecular mechanisms involved. This is especially true for invertebrates, where research on mate choice has largely focused on male behavior. This review summarizes the current state of knowledge on the neural, molecular and neurohormonal mechanisms of female choice in invertebrates, including behaviors before, during, and after copulation. We identify areas of research that have not been extensively explored in invertebrates, suggesting potential directions for future investigation. We hope that this review will stimulate further research in this area.

Ancestral neural circuits potentiate the origin of a female sexual behavior

Courtship interactions are remarkably diverse in form and complexity among species. How neural circuits evolve to encode new behaviors that are functionally integrated into these dynamic social interactions is unknown. Here we report a recently originated female sexual behavior in the island endemic Drosophila species D. santomea, where females signal receptivity to male courtship songs by spreading their wings, which in turn promotes prolonged songs in courting males. Copulation success depends on this female signal and correlates with males' ability to adjust his singing in such a social feedback loop. Functional comparison of sexual circuitry across species suggests that a pair of descending neurons, which integrates male song stimuli and female internal state to control a conserved female abdominal behavior, drives wing spreading in D. santomea. This co-option occurred through the refinement of a pre-existing, plastic circuit that can be optogenetically activated in an outgroup species. Combined, our results show that the ancestral potential of a socially-tuned key circuit node to engage the wing motor program facilitates the expression of a new female behavior in appropriate sensory and motivational contexts. More broadly, our work provides insights into the evolution of social behaviors, particularly female behaviors, and the underlying neural mechanisms.

Widow spiders alter web architecture and attractiveness in response to same-sex competition for prey and mates, and predation risk

Female-female competition in animals has rarely been studied. Responses of females that compete context-dependently for mates and prey, and seek safety from predators, are ideally studied with web-building spiders. Cobwebs possess unique sections for prey capture and safety, which can be quantified. We worked with Steaoda grossa females because their pheromone is known, and adjustments in response to mate competition could be measured. Females exposed to synthetic sex pheromone adjusted their webs, indicating a perception of intra-sexual competition via their sex pheromone. When females sequentially built their webs in settings of low and high intra-sexual competition, they adjusted their webs to increase prey capture and lower predation risk. In settings with strong mate competition, females deposited more contact pheromone components on their webs and accelerated their breakdown to mate-attractant pheromone components, essentially increasing their webs' attractiveness. We show that females respond to sexual, social and natural selection pressures originating from intra-sexual competition.

Sex-specific assumptions and their importance in models of sexual selection

Sexual selection is a field coloured by tension and contrasting views. One contested claim is the causal link from the definition of the sexes (anisogamy) to divergent selection on the sexes. Does theory truly engage with this claim? We survey the extent to which theory makes sex-specific assumptions and engages with anisogamy, and discuss these issues in a broader context. The majority of theory in sexual selection makes sex-specific assumptions and does not engage with the definition of the sexes. While this does not invalidate existing results, debates and criticisms regarding sexual selection force us to think deeper about its logical foundations. We discuss ways to strengthen the foundations of sexual selection theory by relaxing central assumptions.

The coevolutionary dynamics of cryptic female choice

In contrast to sexual selection on traits that affect interactions between the sexes before mating, little theoretical research has focused on the coevolution of postmating traits via cryptic female choice (when females bias fertilization toward specific males). We used simulation models to ask (a) whether and, if so, how nondirectional cryptic female choice (female-by-male interactions in fertilization success) causes deviations from models that focus exclusively on male-mediated postmating processes, and (b) how the risk of sperm competition, the strength of cryptic female choice, and tradeoffs between sperm number and sperm traits interact to influence the coevolutionary dynamics between cryptic female choice and sperm traits. We found that incorporating cryptic female choice can result in males investing much less in their ejaculates than predicted by models with sperm competition only. We also found that cryptic female choice resulted in the evolution of genetic correlations between cryptic female choice and sperm traits, even when the strength of cryptic female choice was weak, and the risk of sperm competition was low. This suggests that cryptic female choice may be important even in systems with low multiple mating. These genetic correlations increased with the risk of sperm competition and as the strength of cryptic female choice increased. When the strength of cryptic female choice and risk of sperm competition was high, extreme codivergence of sperm traits and cryptic female choice preference occurred even when the sperm trait traded off with sperm number. We also found that male traits lagged behind the evolution of female traits; this lag decreased with increasing strength of cryptic female choice and risk of sperm competition. Overall, our results suggest that cryptic female choice deserves more attention theoretically and may be driving trait evolution in ways just beginning to be explored.

Gendered conflict in the human family

Sexual conflict is a thriving area of animal behaviour research. Yet parallel research in the evolutionary human sciences remains underdeveloped and has become mired by controversy. In this special collection, we aim to invigorate the study of fitness-relevant conflicts between women and men, advocating for three synergistic research priorities. First, we argue that a commitment to diversity is required to innovate the field, achieve ethical research practice, and foster fruitful dialogue with neighbouring social sciences. Accordingly, we have prioritised issues of diversity as editors, aiming to stimulate new connections and perspectives. Second, we call for greater recognition that human sex/gender roles and accompanying conflict behaviours are both subject to natural selection and culturally determined. This motivates our shift in terminology from sexual to gendered conflict when addressing human behaviour, countering stubborn tendencies to essentialise differences between women and men and directing attention to the role of cultural practices, normative sanctions and social learning in structuring conflict battlegrounds. Finally, we draw attention to contemporary policy concerns, including the wellbeing consequences of marriage practices and the gendered implications of market integration. Focus on these themes, combined with attendance to the dangers of ethnocentrism, promises to inform culturally sensitive interventions promoting gender equality worldwide.