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And truth is that both gender and sex are on a spectrum and fluid.

Oh, second comment: your framing is disengenuine.

These are scientists, I'm a scientist, we're held to standards of peer review and methodological scrutiny.

I don't need to establish how and whether we're doing science with authority-- that's the beauty of the invention of the scientific method. I also don't need to establish whether these are facts or opinions, because the body of research is so large and well-discussed, for decades now, that peer review has had plenty of time to do it's work.

To humor you-- the methods used have been all of the above: surveys, experiments, studies, etc.

To humble you-- it's extremely arrogant of you to ask a scientist, to their face, whether their research is real or just opinion. If you think all the research in this field is wrong, you can fix it the way we fix all our science: by conducting your own research and subjecting it to review by other experts in the field.

It's a little unclear if you are asking for resources about the diversity of sex, sexual orientation, and sexual selection strategies in nature, OR about the ways in which they are misconstrued by society-- either by ignoring the diversity of nature to favor a heternormative and gender essentialist narrative, or by too closely feeling that what is natural is what should be considered good and just (the naturalistic fallacy).

I, myself, am authoring studies on the latter topic, but the field is so small that by sharing specific examples, I seriously risk doxxing myself (and others with whom I work closely on a politically fraught topic).

One also needs to understand the former before meaningfully engaging with the latter anyway, so I highly recommend the book Evolution's Rainbow by Stanford ecologist Joan Roughgarden. The book is written in plain language (intended for a wider audience than just biology researchers) and details the (at the time-- 2009, with an updated edition from 2016) present summary of known ecological examples of organisms behaving in ways that counter the human social norms surrounding sexual orientation and gender identity. She goes on to discuss the molecular basis of sex and gender in humans, including what is know about difference in brain structures and gene regulation, and then she contextualizes these examples in sociological terms. I think the book is a little dated at this point, and there was some conflict amongst biologists about aspects of the book that aren't related to what we're discussing (related to her modification of Darwin's theory of sexual selection), so it isn't perfect, but it's basically the first thing any junior scholar is asked to read regarding this discussion topic. I think it will provide you with what you are looking for, seeing as it cites hundreds of studies in tens of fields of biological sciences relating to sex, gender, and sexual orientation in humans and other organisms.

Some key facts (mostly covered in the book) that you or others might find interesting:

• Homosexual behavior (same sex mating behavior) has been documented in fish, insects, birds, amphibians, reptiles, Invertibrates, and over 100 species of mammals
• Intersex bodies are common in nature, with many organisms either having characteristics of both sexes at one time or changing sex over the course of their lifetime. This is commonly found in "higher order" animals as well, and some populations of large mammals (like deer or bears) have been documented to have MOSTLY intersex individuals.
• Heterosexual mating is not required for reproduction, with documented examples of two sperm, two eggs, or just a single sperm or egg being enough to produce viable offspring in some circumstances. When homosexual mating is observed, it often increases the reproductive fitness of the participants
• The "sex roles" (unfortunate terminology, imo) of organisms are not a fixed norm, and we have many examples where the expectations you might have based on human societies for division of labor and activities are inverted or ignored entirely (such as birds where the males stay home and incubate eggs while the females compete with each other using displays of dominance and promiscuity)
• Although it doesn't make sense to describe animals as having gender identities (due to those terms being intended for self-description among human social groups), it is worth pointing out that we do have many examples among animals where individuals of the same sex may have distinct categorical social dynamics, morphologies, and social roles (and that human gender identity does appear to have some underlying biological basis, although I will emphasize that it is non-deterministic and non-essentialist). There's a discussion happening in the field about whether it is worth the potential downsides to describe these animals as having multiple genders within a single sex, so as to normalize and explain to non-biologists that human gender identities are mirrored by other organisms and are valid according to biological science

Studies and surveys of what? That some animals look different based on sex? Go ask some ducks.

You could have at least Google searched it before attempting a gotcha, there are three methods. One doesn't actually change the color, but the other two do.