  Why do some females menstruate at all? Answering this question has
implications for understanding the tight links between reproductive function
and organismal immunity. Here we build on the growing evidence that
menstruation is the by-product of a "choosy" uterus to (i) make the theoretical
case for the idea that female immunity is cyclical in menstruating species,
(ii) evaluate the evidence for the menstrual modulation of immunity and health
in humans and (iii) speculate on the implications of cyclical female health for
female behaviour, male immunity and host-pathogen interactions. We argue that
an understanding of females' evolved reproductive system is foundational for
both tackling the future challenges of the global women's health agenda and
predicting eco-evolutionary dynamics in cyclically reproducing species.
