It Takes Two to Tango: Including a Female Perspective in Reproductive Biology

A Practical Guide to Avoiding Biased Communication in Reproductive Biology

When cultural biases pervade communication, whether visual or text-based, objectivity is impaired. Anthropocentrism (human-centered bias) and androcentrism (male-centered bias) in particular distort perspectives in mammalian reproductive biology. This paper provides a resource for professionals who understand how cultural biases can be reinforced with language, visuals, and conceptual framing. After brief explanations, we present neutral alternatives to biased terminology as well as ways to avoid bias in illustrations. Since this paper is animal-centric, we hope to inspire the creation of similar resources across a more diverse biota and, thus, move towards a more neutral perspective across reproductive biology.

Evolution of Litter Size: Proximate and Ultimate Mechanisms

Relative reproductive success and failure are the ultimate determinants of Darwinian fitness. As such, reproductive traits and variations therein have an immediate and considerable impact on the evolutionary trajectory of lineages. Historically, significant attention has been paid to the ecological and evolutionary processes (ultimate factors) that shape the diversity and canalization of reproductive traits within groups to better our understanding of organismal diversity and population or species resilience. In contrast, the physiological systems that mediate variation within and among species (i.e., the proximate factors) in reproductive traits remain a significant black box. To date, there is comparatively little information about how proximate mechanisms constrain or promote evolutionary potential in reproductive traits. In this mini-review, we focus on litter size in Eutherian mammals as a trait with relatively well-defined diversity (litter sizes are well-described both within and across species) and for which some genetic determinants have been identified. We discuss both the ultimate and potential proximate determinants of litter size with special attention to the breadth of physiological traits that may act as "toggle" switches for evolution of litter size. We close with a brief discussion of the role that physiological plasticity may play in the evolution of litter size and lay out several forward-looking areas for future research.

Balancing Act: An Interdisciplinary Exploration of Trade-offs in Reproducing Females

Trade-offs resulting from the high demand of offspring production are a central focus of many subdisciplines within the field of biology. Yet, despite the historical and current interest on this topic, large gaps in our understanding of whole-organism trade-offs that occur in reproducing individuals remain, particularly as it relates to the nuances associated with female reproduction. This volume of Integrative and Comparative Biology (ICB) contains a series of papers that focus on reviewing trade-offs from the female-centered perspective of biology (i.e., a perspective that places female reproductive biology at the center of the topic being investigated or discussed). These papers represent some of the work showcased during our symposium held at the 2024 meeting of the Society for Integrative and Comparative Biology (SICB) in Seattle, Washington. In this roundtable discussion, we use a question-and-answer format to capture the diverse perspectives and voices involved in our symposium. We hope that the dialogue featured in this discussion will be used to motivate researchers interested in understanding trade-offs in reproducing females and provide guidance on future research endeavors.

Introduction to the Symposium: An Integrative Look at Whole-organism Trade-offs from the Female-centered Perspective of Biology

Trade-offs during reproduction have long been a central focus within biology and much of the foundational work within life history evolution has focused on females, as the fitness of females is more easily quantified for use in theoretical models. However, in many regards, the field of organismal biology has deviated from this early focus on females, particularly as it relates to the nuances and dynamic nature of female reproduction. Regardless, at the organismal level, reproduction is thought to trade-off with other simultaneously occurring processes. Recent papers have sought to outline the issues with our current understanding of whole-organism trade-offs, though the field as a whole has not come to a consensus on what trade-offs mean to a reproducing female. To rectify this important gap in how trade-offs are discussed in organismal biology as well as confusion about what constitutes a trade-off, our overarching goal of this symposium was to discuss trade-offs from an integrative perspective that places female reproduction at the center. By answering what trade-offs are and what they mean to reproducing females, what has been neglected in the context of whole-organism physiology, and how maternal effects fit within this framework, our group of speakers and their associated papers will crystalize nuances of measuring and determining presence (if any) of trade-offs in reproducing females in a range of taxa and subfields.

Let's Talk About Sex: Instructor Views and Hesitancies Related to Sex and Gender in the Biology Classroom

There is much current debate in the United States regarding how sex and gender are approached in science and medical classrooms. There does not seem to be sufficient consensus around why it must be taught and how it should be implemented. State-enacted restrictions to both education and healthcare in recent years demonstrate the relevance and importance of sex and gender in the college classroom, not only including but especially in the biology classroom. Given the areas comprising the Society for Integrative and Comparative Biology (SICB), these topics of sex and gender in biology instruction are incredibly salient to our members. Thus, this survey aimed to determine instructors' views of and experiences with sex-diverse gender-inclusive biology. College-level biology instructors who are members of SICB were surveyed about their views of science, views of sex and gender, teaching philosophy, and their experiences with inclusive teaching and with sex-diverse gender-inclusive teaching. The resulting data lead us to implore academic biology to provide more sex-diverse and gender-inclusive teaching tools and resources to educators, while minimizing potential fear of retaliation and backlash to instructors who utilize these teaching methods.

Sex and Biology: Broader Impacts Beyond the Binary

What are the implications of misunderstanding sex as a binary, and why is it essential for scientists to incorporate a more expansive view of biological sex in our teaching and research? This roundtable will include many of our symposium speakers, including biologists and intersex advocates, to discuss these topics and visibilize the link between ongoing reification of dyadic sex within scientific communities and the social, political, and medical oppression faced by queer, transgender, and especially intersex communities. As with the symposium as a whole, this conversation is designed to bring together empirical research and implementation of equity, inclusion, and justice principles, which are often siloed into separate rooms and conversations at academic conferences. Given the local and national attacks on the rights of intersex individuals and access to medical care and bodily autonomy, this interdisciplinary discussion is both timely and urgent.

How Female-Female Competition Affects Male-Male Competition: Insights into Postcopulatory Sexual Selection from Socially Polyandrous Species

AbstractSexual selection is a major driver of trait variation, and the intensity of male competition for mating opportunities has been linked with sperm size across diverse taxa. Mating competition among females may also shape the evolution of sperm traits, but the effect of the interplay between female-female competition and male-male competition on sperm morphology is not well understood. We evaluated variation in sperm morphology in two species with socially polyandrous mating systems, in which females compete to mate with multiple males. Northern jacanas (Jacana spinosa) and wattled jacanas (J. jacana) vary in their degree of social polyandry and sexual dimorphism, suggesting species differences in the intensity of sexual selection. We compared mean and variance in sperm head, midpiece, and tail length between species and breeding stages because these measures have been associated with the intensity of sperm competition. We found that the species with greater polyandry, northern jacana, has sperm with longer midpieces and tails as well as marginally lower intraejaculate variation in tail length. Intraejaculate variation was also significantly lower in copulating males than in incubating males, suggesting flexibility in sperm production as males cycle between breeding stages. Our results indicate that stronger female-female competition for mating opportunities may also shape more intense male-male competition by selecting for longer and less variable sperm traits. These findings extend frameworks developed in socially monogamous species to reveal that sperm competition may be an important evolutionary force layered atop female-female competition for mates.

Sexual selection drives the coevolution of male and female reproductive traits in Peromyscus mice

When females mate with multiple partners within a single reproductive cycle, sperm from rival males may compete for fertilization of a limited number of ova, and females may bias the fertilization of their ova by particular sperm. Over evolutionary timescales, these two forms of selection shape both male and female reproductive physiology when females mate multiply, yet in monogamous systems, post-copulatory sexual selection is weak or absent. Here, we examine how divergent mating strategies within a genus of closely related mice, Peromyscus, have shaped the evolution of reproductive traits. We show that in promiscuous species, males exhibit traits associated with increased sperm production and sperm swimming performance, and females exhibit traits that are predicted to limit sperm access to their ova including increased oviduct length and a larger cumulus cell mass surrounding the ova, compared to monogamous species. Importantly, we found that across species, oviduct length and cumulus cell density are significantly correlated with sperm velocity, but not sperm count or relative testes size, suggesting that these female traits may have coevolved with increased sperm quality rather than quantity. Taken together, our results highlight how male and female traits evolve in concert and respond to changes in the level of post-copulatory sexual selection.

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

While it is widely acknowledged that Darwin's descriptions of females were gender-biased, gender bias in current sexual selection research is less recognized. An examination of the history of sexual selection research shows prevalent male precedence-that research starts with male-centered investigations or explanations and thereafter includes female-centered equivalents. In comparison, the incidence of female precedence is low. Furthermore, a comparison between the volume of publications focusing on sexual selection in males versus in females shows that the former far outnumber the latter. This bias is not only a historical pattern; sexual selection theory and research are still male-centered-due to conspicuous traits, practical obstacles, and continued gender bias. Even the way sexual selection is commonly defined contributes to this bias. This history provides an illustrative example by which we can learn to recognize biases and identify gaps in knowledge. I conclude with a call for the scientific community to interrogate its own biases and suggest strategies for alleviating biases in this field and beyond.

Beyond a biased binary: A perspective on the misconceptions, challenges, and implications of studying females in avian behavioral endocrinology

For decades, avian endocrinology has been informed by male perspectives and male-focused research, leaving significant gaps in our understanding of female birds. Male birds have been favored as research subjects because their reproductive behaviors are considered more conspicuous and their reproductive physiology is presumably less complex than female birds. However, female birds should not be ignored, as female reproductive behavior and physiology are essential for the propagation of all avian species. Endocrine research in female birds has made much progress in the last 20 years, but a substantial disparity in knowledge between male and female endocrinology persists. In this perspective piece, we provide examples of why ornithology has neglected female endocrinology, and we propose considerations for field and laboratory techniques to facilitate future studies. We highlight recent advances that showcase the importance of female avian endocrinology, and we challenge historic applications of an oversimplified, male-biased lens. We further provide examples of species for which avian behavior differs from the stereotypically described behaviors of male and female birds, warning investigators of the pitfalls in approaching endocrinology with a binary bias. We hope this piece will inspire investigators to engage in more comprehensive studies with female birds, to close the knowledge gap between the sexes, and to look beyond the binary when drawing conclusions about what is ‘male’ versus ‘female’ biology.

Pearl lecture: Biological normalcy: A new framework for biocultural analysis of human population variation

Biological normalcy is a new analytical framework for understanding the bi-directional relationships between the biology of populations and cultural norms. Populations are characterized by statistical distributions-that is, measures of central tendency and variance-for biological traits, and these co-exist in societies with ideas about what constitutes "normal" human bodies, that is, normative views about what bodies "should" be like. While statistical norms may carry no explicit evaluative weight, the question is how they are related to judgments about what is "normal" or "abnormal." In a 1947 paper, Margaret Mead recognized their potential relationship: "normal…may refer to the statistically usual in the culture-usually without any recognition that this is culturally relative-so that the statistically usual is identified with the basically human…." Despite her observations over 70 years ago, little has been done on this topic, yet such work promises new insights into the relationship between culture and biology, here described at the population level, rather than as individual genetic characteristics. Using examples of sex/gender, race/ethnicity, age, and my work on human variation in the ability to drink milk, I outline the ways in which statistical norms may: influence individuals' perceptions of what is "normal" (Mead's "basically human"); lead to normative judgments about what human biology "should" be ("ethno-biocentrism") that are reinforced by biases in discourse about human variation; and potentially feedback to mold the biological characteristics of a population.

Misconceptions about Conception and Other Fallacies: Historical Bias in Reproductive Biology

Natural selection (differential reproduction) is a major tenet of evolutionary theory. In mammals the success of reproduction is primarily controlled by females who provide the majority of offspring care via gestation and lactation. In some species, maternal care also extends post-weaning. This primacy of female reproduction in evolution has not quite crept into our understanding of organismal adaptations in anatomy, physiology, and behavior. This cultural legacy has left its mark and led to misconceptions in our understanding of reproductive biology that are especially prominent in the understanding of reproduction in the general public. Here, I give examples of such misconceptions. I focus on aspects of physiology (the "sperm race," the "estrous cycle," the "28-day" menstrual cycle, "sex" hormones, and meiosis) as well as aspects of terminology in morphology and behavior. The issues I raise are not new, but all remain embedded in the teaching of reproductive biology especially at the introductory level. For each issue, I examine the historical bias, the consequences of that bias, and, more importantly, ways to ameliorate that bias going forward.

An inconvenient tooth: Evaluating female choice in multiple paternity using an evolutionarily and ecologically important vertebrate clade

Understanding mating systems is a pillar of behavioural ecology, placing the complex interactions between females and males into a reproductive context. The field of multiple paternity, the phenomenon whereby many sires contribute to an individual litter, has traditionally viewed females as passive players in a male–male competitive framework. With the emergence of feminist perspectives in ecological fields, novel alternative mechanisms and evolutionary theories across invertebrate and vertebrate taxa recognize females are active stakeholders in the reproductive process. Despite their evolutionary significance, ecological diversity and myriad reproductive modes, elasmobranch (sharks, skates and rays) research lags behind other fields regarding complex biological processes, such as multiple paternity which is often ascribed to convenience polyandry. Here, we layout hypotheses and resynthesize multiple paternity literature from a female and life history perspective to highlight how alternative mechanisms influence the predominance of multiple paternity across elasmobranchs. We draw upon parallels in other invertebrate and vertebrate taxa to demonstrate how female elasmobranchs can influence multiple paternity outcomes that benefit their reproductive success. Our article challenges dogma that has resulted from years of dismissing the female perspective as important and provides a framework for future advancement using more holistic approaches to studying mating systems.

Introduction to “Reproduction: The Female Perspective from an Integrative and Comparative Framework”

This compendium is from a symposium about reproductive biology from the female perspective, but what do we mean by the female perspective? Most obviously, since all of our speakers, and most of our contributors, are female, one meaning is that the female perspective is the view of female scientists. Our diverse contributors are from a spectrum of academic ranks (post-docs to chaired professors) and study a range of animal taxa from insects to mammals. More importantly, we want to examine reproductive biology from the perspective of female organisms themselves. What happens when we examine social behavior, physiology, or ecology strictly from the viewpoint of females? In many cases, the female-centric perspective will alter our prior interpretations. For example, with deoxyribonucleic acid fingerprinting, differences between genetic and behavioral mating-systems became obvious. The scientific community came to realize that assessing parentage is the definitive way to categorize mating systems since using male-mating strategies resulted in flawed conclusions; in fact, the female selection of which sperm is involved in conception is more important in determining parentage than mating events per se. Perhaps parentage systems rather than mating systems would be more appropriate. This difference in interpretation relative to methodology exemplifies how behavioral ecology might change if we examine systems from the female perspective; similar changes may occur for other fields. Another example comes from studies of whole-organism performance. Here, jumping, running, and swimming have been measured in males, usually with the deliberate removal of females and the major facet of female physiology, that is, reproduction. However, female biology may actually set the limits of performance given the need to carry extra weight and the extensive changes in body shape required for reproduction. Female performance is a valuable area for research. In fact, novel insights into metabolic ceilings arose from examining energetics, including metabolic rates, during lactation. In the symposium and the associated papers, our contributors explored the various ways in which a female-focused framework shifts our research conclusions and programs. As a way forward, we also include a table of sex-neutral terminology to replace terms that are currently androcentric or value-laden.