The importance of selection and reporting of the sex of experimental animals

Abstract

Biased use of males and females in animal studies or omitting specific details of the sex of animals used in publications limits reproducibility, hampers the pace and likelihood of new discoveries and invites adverse events in ensuing clinical research. Hence unbiased use of males and females in animal studies and specific reporting of animal details are increasingly required by funding bodies and scientific journals worldwide. An analysis to determine how males and females are used in animal studies involving the Commonwealth Scientific and Industrial Research Organisation (CSIRO) was undertaken as part of a process to review and further support best practice. In the study 178 publications that contain animal studies and include CSIRO researchers published between January 2014 and December 2016 were analysed for the sex of animals used. The overall sex distribution was males only 26.4% (47/178), females only 15.7% (28/178), males and females 18.0% (32/178) and sex of animals unspecified 39.9% (71/178). Reasons for this distribution include species biology, farming practices and commercial relevance. Although including sex as an experimental variable provides the most information, using both sexes in an animal study requires careful consideration and planning. Furthermore, there are valid biological and experimental reasons why sex distribution in a study may not be balanced. Biological reasons include cases where the severity of disease in a given model differs between males and females, superior husbandry or production traits in one sex and hermaphroditic species that change sex with age. Examples where experiments can only be undertaken in one sex of animal include animal models of female breast cancer, female reproductive traits, male fertility studies and post-castration welfare outcomes. Where there is no biological or experimental reason for using a single sex of animal, future studies should obtain an estimate of sex effect either from the literature or with a pilot study, and experiments should be planned and reported accordingly.