First things first, sex and gender are not interchangeable! Gender is social construct, that can differ across cultures. Humans can identify as a particular gender, but rodents, tissues, and cells DO NOT HAVE A GENDER.
Sex is also somewhat socially constructed, in the sense that what we consider someone’s sex can change based on what we use to define sex. Are we defining sex based on chromosomes? Internal gonads? External genitalia? Hormonal profile?
In plenty of people, these categories don’t all neatly line up into “male” or “female.” An estimated 1.7% of human births might be intersex. (It’s tough to estimate, because people may not discover this until later in life)
The rats I use in my research have been bred to try to make them as genetically similar as possible, so the likelihood of any being intersex is very low. So we define them as male or female based on external genitalia and body habitus.
A big advantage of using a rodent model for pain is that I can look at sex differences without the influence of gender effects—things like who is “allowed” to express pain and how, who seeks treatment more, etc
