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​Sex Ratio

Updated: Feb 11, 2023

What is it? Sex ratio or gender ratio is an index to indicate the ratio of male to female individuals in a population. In most species of animals and plants, the ratio of males to females in a population is roughly equal (50:50). However, the exact ratio can vary based on several factors, including genetics, environmental conditions (e.g., temperature), and migration. Examples in Some Groups

  • Insects The gender ratio of insects varies widely among different species, and can be influenced by a variety of biological and environmental factors. In many social insects, such as ants, bees, and wasps, there is a high proportion of females, as the reproductive females (queens) are responsible for the majority of reproduction in the colony, while males play a relatively minor role. In these species, the gender ratio can be influenced by environmental factors, such as the availability of food and the size of the colony. In other insect species, such as some species of moths and butterflies, the gender ratio can be influenced by the availability of mates, as females may lay eggs that hatch into either males or females depending on the availability of potential mates. It is also worth noting that some species of insects are parthenogenic, meaning that they are capable of reproducing without fertilization by a male, resulting in a population of only female individuals.

  • Fishes Similar to insects, the gender ratio of fish can be influenced by a wide range of environmental factors, such as temperature, water chemistry, and food availability. For example, some species of fish have temperature-dependent sex determination, where the temperature during incubation can determine the sex of the offspring. In these species, changes in water temperature can cause shifts in the gender ratio of the population over time. Some fish species (e.g., clownfish) can also change sex over the course of their lives, and the gender ratio in these species can thus be influenced by additional factors such as food availability, social status, and the presence of other more dominant males/females.

  • Reptiles In some species of reptiles, sex ratio is influenced by temperature. This is termed "temperature-dependent sex determination", whereby the temperature during incubation can influence the proportion of male and female offspring. Temperature-dependent sex determination is species specific, meaning some species might produce more females at lower temperature, some might produce more females at higher temperature, while some might only produce more females at both higher and lower temperatures, with intermediate temperatures causing mostly male development.

  • Birds Some species of birds exhibit sex-role reversal, where females are the dominant sex and compete for mates, while males provide care for the offspring. In these species, the sex ratio can be influenced by the quality of the male's territory and the resources he can provide for the female and her offspring.

  • Mammals In mammals, the ratio of male to female offspring can be influenced by the presence of certain hormones, such as testosterone, in the mother's bloodstream. Higher testosterone is associated with the reproduction of more sons. Some species of mammals, such as elephants and certain species of primates, have a matriarchal social structure, where females are the dominant sex and compete for mates, while males provide care for the offspring. In these species, the sex ratio can be influenced by the quality of the male's territory and the resources he can provide for the female and her offspring.


Why is it important that we study it? The gender ratio of a population can have important ecological and evolutionary implications, as a skewed ratio can impact the mating dynamics and genetic diversity of a population.

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