DEPLETION DETERMINES SPERM NUMBERS IN MALE ZEBRA FINCHES

This study tests the idea that male zebra finches, Taeniopygia guttata, can control the numbers of spermatozoa in their ejaculates. The first experiment compared the numbers of spermatozoa transferred by males in extra-pair copulations during and after their own pair copulation period, but taking the time since the last copulation into account. If males can control ejaculate size then, after controlling for sperm depletion effects, extra-pair ejaculates occurring after the male's own pair copulation period should be larger, but this was not the case. In the second experiment males were presented simultaneously with two model females: if males could adjust their ejaculate size the number of spermatozoa transferred to each female in turn should differ from that when the two females were presented separately. However, the numbers of spermatozoa were similar to those occurring during depletion with model females presented sequentially These experiments provide no evidence that male zebra finches can adjust the size of their ejaculate, and support the hypothesis that males operate under a physiological constraint and that spermatozoa numbers are determined largely by sperm depletion.