Biological Sex and Sexual Behavior

We cannot discuss sexual behavior in animals without acknowledging that biological sex and sexual behavior are interconnected. Many species have more than two sexes, can change sex, reproduce asexually, are hermaphroditic, or develop secondary sex characteristics that are not the expected phenotype of their “biological sex.”
For example, many fish species are sequential hermaphrodites and change from male-to-female, female-to-male, or alternate between male and female \cite{Gemmell_2019}. There are many advantages to this lifestyle. \citet{z} writes that larger females are more fertile than smaller females, so the largest male will change to a female. This happens in clownfish and is also known as protandry. Larger males can also be more dominant, which is advantageous to fish that change from female to male. This is called protogyny, a strategy several parrotfish species use  \citep{z}. Fish that can change between male and female have the most advantage, as they just have to find a mate and can change to the sex needed for reproduction.
There are also numerous examples of species with more than one type of male and female. The bluegill sunfish displays three types of males and one type of female. \citet{1991} describes that each male sex has a distinct size, coloration, and behavior. One of these male sexes is seen as the “feminine male” as it schools with other females and displays courtship behaviors with the other types of males \citep{1991}. \citet{roughgarden2004} theorizes that the “feminine male” appears less threatening than the larger males and can protect other males trying to mate. Therefore, the female often chooses this type of male to mate with.
Another interesting example of a species with multiple types of males and females is the White-throated sparrow. \citet{k2017} describes the more aggressive and territorial males and females as having a white stripe on their heads, while the less aggressive males and females have a tan stripe. These four phenotypes even have distinct genetic differences - meaning this species has four different sexes.
These are just a few examples of animals that differ from our perceived norm of two distinct sexes - male and female. In reality, animals with two distinct sexes that form monogamous male-female relationships are the minority. Only 3-9% of mammals have been observed as socially monogamous species, meaning they find life-long partners and typically raise offspring together \cite{Lambert_2018}. Social monogamy differs from genetic monogamy. Researchers have observed species once considered socially monogamous to rear offspring with different genetics than the original pair. In other words, a male and female bonded together will still reproduce with other males and females outside their pair. Currently, no documentation or evidence suggests that any species is genetically monogamous \cite{Ophir_2008}.
This information is an important find as the heteronormative society we live in today expects one male and one female to pair and mate for life. Any sexual activity outside this pair is considered infidelity, yet it is common amongst socially monogamous species. Based on this information, heteronormative monogamous relationships should not be the baseline for discussing sex and gender, as most animal relationships observed do not fit the criteria for a heterosexual monogamous pair.