On the importance of including both sexes in animal studies - insights from home-cage monitoring.

A review of behavioural studies using home-cage monitoring (HCM) systems revealed that over 61% of studies used only male subjects, with only 24% including both sexes, despite evidence of substantial behavioural differences between male and female animals. This bias could influence the outcomes of biomedical research. This preference for male subjects has introduced a significant bias in preclinical animal studies, compromising the generalisability of research findings and potentially overlooking important sex differences in disease manifestation and response to therapies. This review highlights such sex differences in laboratory rodent behaviour, as observed through HCM systems. HCM provides an automated method for observing the behaviour of undisturbed animals in a familiar environment, offering valuable insights into sex-specific differences in behaviour and disease progression. In particular, we emphasise how this technology revealed sex differences in spontaneous activity and behaviour in models of neurodegenerative diseases, while demonstrating that, contrary to common belief, female rodents are not more variable than males in their behaviour. We further discuss the reasons underlying the prevalent sex bias in research and advocate for the inclusion of both sexes in neuroscience and behavioural studies, to enhance the generalisability and informative value of preclinical research, avoid unethical waste of animals and resources, and thereby support the Three Rs principles (Replacement, Reduction, and Refinement of animal use).