I received my PhD for studies into the how rat caging affects the rats' welfare. Every single rat used in my four and a half year course of study was female. The main reason for this was that one animal facility supplied both the biology and psychology departments. When I visited this facility in the basement of a neighboring building I was told there were almost always 'excess' female rats. The reason being that most of the researchers in the biology department wanted only male rats. I immediately decided to use female rats to avoid further exacerbating the unnecessary breeding of litters from which only the males were wanted. Over the following months and year I heard the same question over and over:
Why did you use only female rats?
Initially out of naive sincerity, and later out of perverse dogmatism, I gave the same answer. I used them because they were there, and because they had a longer happier life being in my experiment rather then being culled as excess animals. Besides, almost all of the respected studies on the effects of environmental enrichment on rats used single-sex groups. All of them used males. If it is not a flaw in a study to use only male rats it cannot be a flaw to use female rats, using female rats replicates these findings with the other sex, and using female rats serves the goal of the 3Rs--that the use of animal in the laboratory should, whenever possible, be refined, reduced and replaced.
This explanation was usually met with mild consternation, silence and bemused acceptance. But each new reviewer would repeat it again. I must have delivered this explanation about thirty times. In fact I delivered long after I realised that I was just perpetuating an error, and they were just letting me. Research that is not sex-specific (for example on testicular cancer, or pregnancy) should not be carried out on single sex groups. The old justification that using single sex-groups reduces 'error variation' is bunk.
Sex is not random and it is not error, it is in fact a fundamental quality of most species. Any study that uses only one sex should not, no matter how obvious it seems, extrapolated to the species as a whole. Not because males and females are radically different in every way, but because it creates a subtle but dangerously skewed science in which the male is normal, and the female is aberrant. It tailors our ideas of biological truth, our understanding of sickness and our approach to healing, primarily to the service of the male. And if a few more animals are needed to account for sex-based variation... well, I bet many fewer animals will ultimately be needed due to the improved validity of the outcome of the study.
I was reminded of all this by Chelsea Wald and Corinna Wu who wrote an article in Science magazine called Of Mice and Women: the Bias in Animal Models . They point out that, to this day, the vast majority of research us conducted on male animals, ultimately undermining the effectiveness of medical treatment on women--and also I suspect leading to the unnecessary death of female animals not needed for the breeding population of the animal colony. In retrospect I still think I made the right choice in taking those 'excess' female rats--but I should have had a word with those biologists.