EJACULATE QUALITY AND THE SUCCESS OF EXTRA-PAIR COPULATIONS IN THE ZEBRA FINCH

IN many passerine birds, sperm competition(1,2) is intense and extrapa ir paternity frequent(3). The outcome of competition is often determin ed by relative sperm numbers(4,5), theory predicts that males should m aximize the number of sperm they ejaculate during extra-pair copulatio ns(6,7). Differences In sperm quality between males also affect the ou tcome of sperm competition(4). Here we report that the swimming veloci ty of sperm of the zebra finch, Taeniopygia guttata, varies predictabl y within males, and is determined, together with sperm numbers, by the time since last ejaculation. By performing extra-pair copulations out side their own-pair copulation period, males maximize both the quality and number of sperm in ejaculates. These effects are a consequence of the way sperm are stored and mature in the male reproductive tract. T he disportionate success of extra-pair copulations(8), also seen in ot her birds(9), may therefore be explained in terms of the independent e ffects of sperm numbers and velocity.