Section Summary

In this section, we first explored the concept of sexual reproduction, examining its underlying mechanisms and the evolutionary advantages, including genetic diversity and masking of harmful mutations, that have made it so widespread across nature. We also learned that many times, animals engage in sexual behavior without the intent of reproduction, and that same-sex sexual behavior is widespread in nature. We then discussed how the initial reproductive investment of each sex, such as the larger, more energetically costly eggs produced by females compared to the smaller, less costly sperm produced by males, can lead to different strategies between males and females to maximize reproductive success and evolutionary fitness. We also explored the dynamics of intrasexual competition, where individuals of the same sex compete for access to mates, often resulting in the development of traits like larger body size or aggressive behaviors. Additionally, we examined intersexual competition, where mate choice drives the evolution of traits that are attractive to the opposite sex, such as elaborate plumage or intricate courtship behaviors. Finally, we explored how sex differences affect many physiological and behavioral processes beyond reproductive traits, including sensory perception and stress responses, highlighting the critical importance of incorporating biological sex as a variable in biomedical research.

In this section, we embarked on a fascinating journey to understand the complex mechanisms that determine whether an individual develops as male or female. We began by exploring how, in many species, the presence of specific genes located on sex chromosomes (X and Y in the case of humans) initiates a series of events that lead to the development of male or female gonads. However, we also learned that in many other species, sex determination is not driven by genetics but by environmental factors. For example, we learned that temperature can dictate sex in certain reptiles, and that some species of fish can even change their sex based on social dynamics. Lastly, we delved into the world of steroid hormones and discovered how these powerful chemicals, secreted by male and female gonads, differentially shape the early development of tissues throughout the body, including the brain, and how these hormones further continue to influence the physiology of tissues throughout an individual's lifespan.

In this section, we explored how genes located on sex chromosomes, steroid hormones, and environmental factors contribute to sex differences in the brain. We learned that proteins encoded by sex chromosome-linked genes can directly affect the physiology of neurons and brain function. Additionally, we examined mouse models with variations in the number of X or Y chromosomes, where the function of the gonads remains intact. These models have shown that changes in the dosage of sex chromosomes can significantly contribute to sex differences in brain anatomy and function.

Further, we explored how steroid hormones can act through both genomic and non-genomic mechanisms to influence brain function. Genomic actions involve hormones like estradiol binding to intracellular receptors, such as estrogen receptor alpha (ERα), which then act as transcription factors to regulate gene expression. This process is typically slower and results in long-lasting changes in cell function. Non-genomic actions, on the other hand, involve rapid responses initiated by hormone-receptor interactions at the cell membrane, leading to quick changes in cell signaling pathways.

Lastly, we learned that environmental factors, including the physical and social environment, can play a significant role in contributing to brain sex differences. Importantly, we also discovered that exposure to various man-made chemicals can disrupt hormonal balance and brain development, underscoring the urgent need for comprehensive studies and stringent regulations to better understand and mitigate the impact of these environmental factors on brain development and overall health.

In this section, we delved into the neurobiological underpinnings of sex differences in brain circuits underlying behavioral responses related to stress, motivation, and social interactions, which has implications for understanding sex differences in the prevalence and manifestation of psychological and psychiatric disorders. We learned about specific brain circuits that use neurotransmitters like CRH, serotonin, and oxytocin, and how these are different between sexes largely stemming from differences in circulating steroid hormones secreted by male and female gonads throughout the lifespan. Lastly, we introduced the possible role that immune cells like microglia and mast cells, who also reside in the brain along neurons, have in contributing to these sex differences.