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Sperm competition is an evolutionary force that has shaped the reproductive biology of many species from insects to mammals. It is a consequence of promiscuity in the animal kingdom. It is manifest in the jealous guarding of mates and the size and shape of testes, penises and sperm. But has sperm competition been a significant factor in human evolution? And has it led to men developing a suicidal warrior caste of sperm cells with the sole function of destroying other men’s sperm in the race to fertilise the egg cell? This hypothesis was proposed by evolutionary biologist Robin Baker in the 1990s but Tim Birkhead of Sheffield University says the evidence for it is not good.