Some writers in animal research used gender and sex interchangeably, especially in the 1980s and 1990s, though there is recent movement to set the record straight.
In biology, the distinction between gender and sex is often defined by culturally-influenced behavior versus anatomy. In normative definitions, gender is assigned to humans, sex to most animals including humans. Suk Kyeoung Lee (2018) explains:
Sex and gender are occasionally used in an interchangeable manner. Both sex and gender affect research results, but they have different meanings. Thus, it is important to know the correct meanings of them and to avoid interchangeable use. According to the US Institute of Medicine (IOM) (5), sex is “the classification of living things, generally as male or female, according to their reproductive organs and functions assigned by chromosomal complement”, while gender is “a person’s self-representation as male or female or how that person is responded to by social institutions on the basis of the individual’s gender presentation. Gender is shaped by environment and experience.” Thus, sex is related to reproductive organs, sex hormone, gene expression, anatomy, and physiology. Gender refers to socio-culturally constructed roles, norms, identities, and power relations (9) that, together, shape ‘feminine’ and ‘masculine’ behaviors (10). Sex can be used for both human and animals as whole organisms or materials derived from them such as cells and tissues, while gender is in general used only for humans. Importantly, sex and gender affect each other, as gender is rooted in biology and can influence biological outcomes.
The explanation offers a few dicta for how to understand the usages of sex and gender:
Some scholars use the two terms interchangeably (a result that should be avoided).
Others distinguish between "classification ... according to their reproductive organs" (sex) and "how that person is responded to," perceives themselves, and how behaviors are thereby shaped (gender).
Gender should be reserved for humans, while sex may pertain to either humans or animals.
The author's insistence on knowing the correct usage comes from a history of authors in biology interchanging the two. Indeed, the citations this question gives of articles in the 1990s and 2000s using gender confirm that, for some time before 2018, it was common for biologists to either perceive gender and sex as interchangeable or to apply gender to the behavior of animals.
A website article by Moriah L. Jacobson (2021) agrees that the interchangeable use of sex and gender in animal research is confusing and attempts to answer why that happened. The short answer is that something happened starting in the 1960s to break down the prior distinction between the terms:
We didn't always refer to animal research subjects as having genders. But how did we get here? The term gender first appeared in the research literature in the 1950-60s. Since then, the use of both terms has grown over time. A PubMed search for either sex or gender appearing within just the title or abstract of all manuscripts is illustrated in Figure 2. The use of both words has exponentially increased over time. The word sex continues to be predominant, but the use of the word gender has immensely grown, recently nearing almost 80% of the same number of publications observed with sex in the title or abstract within the past two decades, shown in Figure 3.
The terms sex and gender were, for the most part, accurately distinguished in the literature until the 1980-90s where the two words began to be used interchangeably and as synonyms. While it is possible that many of the clinical publications use the words accurately, the vast majority of these papers use the word gender when they mean sex, and many of them use the mixed term sex/gender when it may be difficult to distinguish between them.
Here is Figure 2 from the article, which shows the gradual growth in the use of both terms in rodent research:
It's not as if every author in the period went that direction. Walker and Cook (1998) write a brief article arguing that the distinction "prevalent in the social sciences" should be preserved:
The distinction prevalent in the social sciences between the terms sex and gender is a useful one and ought to be preserved. Sex refers to the anatomical or chromosomal categories of male and female. Gender refers to socially constructed roles that are related to sex distinctions. Use of these terms as synonyms is becoming increasingly frequent in physical anthropology, especially among bioarchaeologists and primatologists. A failure to make the distinction between gender and sex is analytically incapacitating in a field such as physical anthropology, whose strength lies in the integration of biological and cultural information.
So this is a point where technical, normative explanations of use are conflicting with actual use by writers getting published. It's too early to say whether a side is winning.
