The Bruce effect should be defined by function, not mechanism: comments on ‘How to escape male infanticide: mechanisms for avoiding or terminating pregnancy in mammals’

Bartoš et al. (2021; Mammal Review 51: 143–153; https://doi.org/10.1111/mam.12219) reviewed the mechanisms involved in the ‘Bruce effect’ – a phenomenon originally documented in inseminated female house mice Mus musculus, who block pregnancy following exposure to a novel (non-sire) male. They argue that the term ‘Bruce effect’ should be applied in cases that are mechanistically equivalent to this original observation in mice. We argue that the Bruce effect should be defined instead by its function: a phenomenon by which inseminated or pregnant females benefit by blocking or terminating pregnancy following exposure to a non-sire male. Only functional definitions of phenomena allow for the articulation and testing of evolutionary hypotheses.