In some genera of small insect-eating marsupials but in no other mammals, escalating stress hormones during the breeding season cause immune system collapse, haemorrhaging, and death after mating in all males (suicidal reproduction). Male death before offspring are born has often been attributed to paternal suicide to avoid food depletion. Comparative analysis of Australian and South American taxa shows that rather than altruism, individual sexual selection leads to apparent self-sacrifice in these mammals, because males gain a competitive advantage under sperm competition. Our continental scale study reveals the ultimate role of climate predictability in shaping reproductive behaviour and sexual selection in insectivorous marsupials.