Abstract
MALES of most animal species will enhance their reproductive success if they mate often and with many different partners, whereas promiscuous mating is unlikely to increase a female's reproductive success. Why then is multiple copulation by females so common1–6? Many theoreticians have suggested that multiple copulations might enhance the viability of a female's offspring, either because of inadequate quantities of sperm from the first mating1,7, additional nutrients derived from the seminal fluid7,8 or some genetic advantage9–14. Our field studies on Swedish adders provide the first empirical evidence that multiple copulations, with different partners each time, increase offspring viability. This advantage apparently results from more intense sperm competition in the female's reproductive tract, resulting in a higher proportion of her ova being fertilized by genetically superior males.
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