Most species tend to achieve a balanced sex ratio of approximately 1:1, largely due to a negative feedback mechanism that regulates this balance. For example, in populations where females are more numerous, a male born at the same time has a significantly higher potential for reproductive success throughout his life compared to a female. Conversely, if there’s a male surplus, females typically have access to more high-quality mates than males do. This balancing act results in the prevalent 1:1 sex ratio observed in many dioecious species. However, various factors, such as differing life expectancies or deviations from sexual binary norms, can disrupt this equilibrium.

A notable instance of skewed sex ratios occurs in species where females consume males post-mating. The praying mantis (Mantis religiosa) is a prime example, where a male-biased birth ratio allows females to engage in post-coital cannibalism, leading to a population heavily weighted towards females as the mating season progresses and males are consumed.

Similarly, nursery web spiders from the family Pisauridae, which primarily hunt on land but use webs for rearing their young, exhibit this behavior. Males often present nuptial gifts to delay their inevitable fate, and research indicates that the size of these gifts correlates with longer copulation durations. Some males even attempt to provide fake gifts to buy time for their escape. This behavioral adaptation contributes to a progressively female-biased sex ratio during the mating season, despite the absence of a mechanism to skew birth ratios towards males.

Other species demonstrate extreme forms of sexual dimorphism, such as anglerfish in the teleost order Lophiiformes, where males fuse to females for sperm transfer. In this scenario, males essentially become parasitic, losing their ability to feed and eventually being absorbed by the female. This results in a significant female bias in the population as the season progresses.

Post-coital cannibalism typically involves females consuming males due to anisogamy, where females invest more energy in larger gametes compared to males. This discrepancy in parental investment often leads to males sacrificing themselves, benefiting the survival of their offspring. Such behaviors illustrate kin selection, where males enhance the fitness of their genetic relatives at their own expense, adhering to Hamilton's rule.

In addition to cannibalism, various embryonic sex determination methods can lead to imbalanced sex ratios. For instance, turtles and crocodilians exhibit temperature-dependent sex determination (TSD), where the incubation temperature during a specific embryonic period determines the sex of the offspring. This phenomenon is also found in some lizards and teleost fish, raising concerns that climate change could disrupt these systems, potentially leading to population bottlenecks or extinctions.

Other species employ genetic sex determination systems, such as ants, where fertilization status of an egg dictates its sex. Unfertilized eggs develop into haploid males, while fertilized ones become diploid females. This system, known as arrhenotoky, fosters a strong female bias in ant colonies, as the majority of offspring are female workers responsible for the colony's tasks.

A noteworthy aspect of arrhenotoky is the potential conflict of interest within ant colonies. While queens tend to prefer a balanced sex ratio, worker ants, being more closely related to their sisters, may preferentially raise female offspring and neglect or even eliminate males. This dynamic exacerbates the pre-existing sex ratio bias.

Furthermore, in simultaneous or sequential hermaphrodites, defining sex ratios can be complex. In species like banana slugs, individuals possess both male and female reproductive organs and may engage in aggressive behaviors during mating, often resulting in one slug effectively becoming female by losing its penis. This self-inflicted change leads to greater investment in offspring, complicating discussions about sex ratios.

Sequential hermaphroditism is also seen in clownfish and other species where social structures dictate sex changes. In clownfish, when the dominant female dies, the dominant male transitions into the female role, complicating assessments of sex ratios due to the fluidity of their social hierarchy.

In general, while a 1:1 sex ratio is prevalent in many higher vertebrates due to significant parental investment per offspring, various mechanisms—such as post-coital cannibalism, diverse sex determination strategies, and hermaphroditism—can lead to deviations from this norm or render the concept of sex ratio itself ambiguous, challenging the binary notion of sex we often take for granted.

(This essay has been adapted from one I authored for my Natural Sciences course at the University of Cambridge, where I study evolutionary biology. I find these topics particularly fascinating and plan to share more adapted essays in the future! I hope you enjoyed reading.)