And that's if they included female subjects at all. Historically, they generally didn't. This is actually one of those places where researchers trying to make science more replicable introduces failures of generalizability: that is, by controlling sex by simply not including female subjects, you can artificially derive strong, clear, reproducible results... that don't fucking generalize to females. It is maddening.
Now, since 2016, the NIH has been requiring the use of female subjects in grant proposals for rodent and cell work. In 2018, Annalise Beery published this excellent paper finding that estrous cycle and sex do not introduce any more general variability into rodent work than using males only. And.... still I see papers that use perhaps a dozen females in an experimental cohort of 60 animals, find no difference in the first round, and happily continue using males only without even noting the sex of animals in any included experiment. MADDENING.
Of course, now on the rodent work end my lab/I also have to start raising awareness that sex differences in rodents are not necessarily identical to sex differences in humans but that does not mean we should stop looking at female rodents, for fuck's sake. Yes, even if it introduces a new axis of variation for you to think about.
See, behavioral differences across animal sexes are often extremely variable between species. For example, I was complaining yesterday about work that purports to be focusing on social motivation in mice that are supposed to be models of autism, using as their study paradigm the mouse's motivation to interact with a strange male mouse. (They typically use same sex stimulus individuals to rule out the possibility that focal animals are motivated by sex rather than social interaction.)
But social interactions between unfamiliar adult male lab mice inevitably decompose into territorial fights. We have a number of behavioral paradigms that explicitly take advantage of that reality. On the other hand, interactions among unfamiliar adult female mice usually don't become immediately antagonistic, and there's even some compelling evidence that female house mice in the wild often find substantial fitness advantages from co-raising litters with a familiar female that has pups of a similar age--if they're allowed to choose that partner, anyway.
Obviously, these sex differences are not the same in humans. Male humans are quite capable of making friends with strange males. This is not difficult. But if you're doing poorly thought out social behavior work using mouse models because we have more fine tuned genetic control of mice, maybe you want to be using female mice as your analog to humans rather than male mice.
On the other hand, if you are trying to use a paradigm of social stress where you expose your focal animal to an interaction with a stranger, and you pick a stranger that is going to fight with the focal animal and definitely going to win... maybe you DO use males, because trying to apply the paradigm to two female mice doesn't fucking work unless you either use drugs to make your aggressive female REALLY absurdly aggressive to literally anything that moves, or you use a male aggressive mouse that has been trained to be the same way. Which is one of the reasons I hate that paradigm; it claims to be ethologically relevant but if you think about the actual social landscape of mice it's incredibly extreme. Especially given that the aggressor mouse is generally a totally different strain, full of hatred, and twice the size of the focal animal, and also it's albino so it's got relatively little vision and therefore is not going to catch some of the social signals that the focal mouse is sending.
On the other hand, male lab rats are quite social with other males and can often form affiliative interactions with strange males. Even within rodent work, species matters! But there is a distinct tendency in neuroscience workers to think of mice as basically miniature rats in mouse suits, and of course to think of rats as miniature humans in rat suits. It is extremely frustrating and I suspect I am going to spend my career screaming at people about this. I can already feel myself becoming a crank about it.
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