Sexism in animal testing

In accordance with the principle of reduction, one of the 3Rs in animal research, it is essential to ensure that as few animals as possible are used for research purposes. There are many ways to achieve this goal. For example, statistical methods and breeding programs can be used to calculate in advance exactly how many animals are needed to study certain parameters. This also avoids the breeding of surplus animals, i.e. animals that cannot be used in the experiment. The use of animals of both sexes in the experiment also contributes to reduction.

For decades, mainly male laboratory animals were used in science. Similarly, the "standard human" for which various therapies were developed was always assumed to be a 70 kg male, although it is obvious that society is far more variable. However, one of the main reasons why males are increasingly used in experiments is the assumption that female conspecifics are more variable, for example due to hormonal fluctuations to which they are subject during the estrous cycle. In a study by Levy et al., in which the behaviour of female and male C57BL/6J was investigated using an open-field test, it was found that the cycle has no influence on the behavioural pattern of the females. It was also found that the female mice are less variable in their behaviour compared to the male animals. Even in numerous experiments for which females would be better suited due to a knowingly higher prevalence of certain parameters, males were increasingly used exclusively in the past. One example of this would be a number of experiments investigating fear, anxiety or stress behaviour, for which female mice would be better suited due to their nature, but more than two thirds of which were carried out on males.¹

Apart from the assumption that males are more uniform, there is also the misconception that all animals in the experiment must be absolutely identical and that there should be as few variables as possible within the experimental group. However, the actual aim of research is to reflect reality as closely as possible in a model, and in real life not everything is homogeneous. The focus should therefore not only be on characteristics that are the same, but should also include those that are different. From the planning of the experiment to the execution itself, the inclusion of both sexes should be a central element. This applies not only to basic research, but also to preclinical and clinical research. If this is disregarded, the scientific output is reduced and, in the worst case, can even have devastating consequences for the health of the non-studied sex, for example in the development of drugs.²

For some scientific questions, however, it is understandable and justifiable why only one sex can be of interest. For example, if research is being carried out on certain hormonal conditions during pregnancy or on hormone preparations for contraception in the female organism, it is clear that only female animals can be considered for this. However, if nothing stands in the way of using both sexes, then both should be included. Projects in which, for example, only males are used for no reason should be rejected by journals and reviewers, as this should not be supported under any circumstances.²

In view of the facts mentioned above and for animal welfare reasons, it is therefore desirable to use animals of both sexes in experiments.

Sources:

1) Beery A. K., Zucker I. (2011) Sex bias in neuroscience and biomedical research. Neurosci. Biobehav. Rev. 35, 565–572
Yoon D. Y., Mansukhani N. A., Stubbs V. C., Helenowski I. B., Woodruff T. K., Kibbe M. R. (2014) Sex bias exists in basic science and translational surgical research. Surgery 156, 508–516
Levy DR, Hunter N, Lin S, Robinson EM, Gillis W, Conlin EB, Anyoha R, Shansky RM, Datta SR. Mouse spontaneous behavior reflects individual variation rather than estrous state. Curr Biol. 2023 Apr 10;33(7):1358-1364.e4. doi: 10.1016/j.cub.2023.02.035 . Epub 2023 Mar 7. PMID: 36889318 ; PMCID: PMC10090034.
Graham BM. Battle of the sexes: who is more variable, and does it really matter? Lab Anim (NY). 2023 May;52(5):107-108. doi: 10.1038/s41684-023-01164-7. PMID: 37024612; PMCID: PMC10156595.
Collins F. S., Tabak L. A. (2014) Policy: NIH plans to enhance reproducibility. Nature 505, 612–613
U.S. National Institutes of Health. Consideration of Sex as a Biological Variable in NIH-funded Research (NOT-OD-15-102). Retrieved October 2, 2015, from http://grants.nih.gov/grants/guide/notice-files/NOT-OD-15-102.html

2) Graham BM. Battle of the sexes: who is more variable, and does it really matter? Lab Anim (NY). 2023 May;52(5):107-108. doi: 10.1038/s41684-023-01164-7. PMID: 37024612; PMCID: PMC10156595. Clayton JA. Studying both sexes: a guiding principle for biomedicine. FASEB J. 2016 Feb;30(2):519-24. doi: 10.1096/fj.15-279554. Epub 2015 Oct 29. PMID: 26514164; PMCID: PMC4714546.