Ejaculate quality and the success of extra-pair copulations in the zebra finch

IN many passerine birds, sperm competition1,2 is intense and extra-pair paternity frequent3. The outcome of sperm competition is often determined by relative sperm numbers4,5, and theory predicts that males should maximize the number of sperm they ejaculate during extra-pair copulations6,7. Differences in sperm quality between males also affect the outcome of sperm competition4. Here we report that the swimming velocity of sperm of the zebra finch, Taeniopygia guttata, varies predictably within males, and is determined, together with sperm numbers, by the time since last ejaculation. By performing extra-pair copulations outside 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. The disportionate success of extra-pair copulations8, also seen in other birds9, may therefore be explained in terms of the independent effects of sperm numbers and velocity.

[1]  A. Wolfson.  Sperm storage at lower-than-body temperature outside the body cavity in some passerine birds. , 1954, Science.

[2]  G. Parker,et al.  SPERM COMPETITION AND ITS EVOLUTIONARY CONSEQUENCES IN THE INSECTS , 1970 .

[3]  P. A. Martin,et al.  The effect of ratios and numbers of spermatozoa mixed from two males on proportions of offspring. , 1974, Journal of reproduction and fertility.

[4]  P. A. Martin,et al.  Assessment of relative fertility of males (cockerels and boars) by competitive mating. , 1977, Journal of reproduction and fertility.

[5]  Why are there so many tiny sperm? Sperm competition and the maintenance of two sexes. , 1982 .

[6]  G. Wishart,et al.  Correlation of the fertilising ability of semen from individual male fowls with sperm motility and ATP content. , 1986, British poultry science.

[7]  R. Zann,et al.  Extra-pair courtship, copulation and mate guarding in wild zebra finches taeniopygia guttata , 1988, Animal Behaviour.

[8]  T. Birkhead,et al.  Extra-pair copulation and sperm competition in the zebra finch , 1988, Nature.

[9]  G. Parker.  Sperm competition games: sneaks and extra-pair copulations , 1990, Proceedings of the Royal Society of London. Series B: Biological Sciences.

[10]  A. Møller,et al.  Sperm competition in birds : evolutionary causes and consequences , 1992 .

[11]  R. J. Robertson,et al.  Extra-pair paternity in monogamous tree swallows , 1993, Animal Behaviour.

[12]  T. Birkhead,et al.  Selection and utilization of spermatozoa in the reproductive tract of the female zebra finch Taeniopygia guttata. , 1993, Journal of reproduction and fertility.

[13]  D. Shapiro,et al.  Sperm Economy in a Coral Reef Fish, Thalassoma Bifasciatum , 1994 .

[14]  T. Burke,et al.  Paternal investment inversely related to degree of extra-pair paternity in the reed bunting , 1994, Nature.

[15]  C. M. Lessells,et al.  Sperm precedence in zebra finches does not require special mechanisms of sperm competition , 1995, Proceedings of the Royal Society of London. Series B: Biological Sciences.