It sometimes a "technical" descission. You need more female than male rats for breeding.

This question has been asked and answered before:

https://tinyurl.com/y7svrfd8

Why more MALE than FEMALE rats in studies involving hormones in general may relate to the fact that the estrous cycle is 4 days, during which the female hormonal milieu is a bit different on each day. If one is testing acute effects of some treatment, there is the possibility that the female response may interact with the stage of estrous and therefore increase the variability. A controlled study would use 4 times as many female rats, with a group in each of the 4 stages of estrous. I am not defending this rationale, just stating that it is what I heard for years. Effects of chronic treatments, like studies of obesity, would not require such a control, so there is little justification for it at all. Michael Tordoff's answer explains why females might actually be PREFERRED for such studies.

Thank you very much for the answers!

Because sex-dependent factors including the X-chromosome and female sex hormone levels.

Please check

Becker JB, Prendergast BJ, Liang JW. Female rats are not more variable than male rats: a meta-analysis of neuroscience studies. Biol Sex Differ. 2016;7:34. Published 2016 Jul 26. doi:10.1186/s13293-016-0087-5

simply to avoid the interaction of pregnancy, menustral, and hormonal fluctuations that may occur during experiment, and then interferes with our findings.

Dear Sedanur Macit

i am not sure for this human-made gender bias but still above reason may be valid.

What I am more interested in are the reasons why the poor male rats suffer more?

Is there a scientific reason?

Thank you all for the valuable answers! It seems like it would be better to choose male rats due to eliminate hormanal effects. Studying with the both genders is also an option. However, this make the group numbers double which is harder to control.

I hope more studies would be conducted with both genders to remove bias if there is any.

Individual experiments involve hard choices. Studying both genders might reveal important interactions, but not all research budgets allow it. Meta-analyses across many studies could turn up such interactions when some studies use each gender, but there is much to be said for a controlled study in the same laboratory with the same strain, source, methods, drugs, etc. Many studies suffer from having too small a sample size to get a precise estimate of effect size

, and they turn out not to be replicable. If one cuts down on group sizes to include both genders, one could wind up wasting all the animals in an uninterpretable study. Welcome to real-world science.