Numbers and Distribution of Sperm in the Uterovaginal Sperm Storage Tubules of the Zebra Finch

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Abstract
The number and distribution of sperm in the sperm storage tubules (SSTs) in the uterovaginal junction (UVJ) of the Zebra Finch (Poephila guttata) oviduct was examined under three copulation patterns: single insemination, multiple insemination by the same male, and multiple insemination by two males in sequence. The proportion of SSTs containing sperm varied markedly between birds (range = 22-99%), and was positively correlated with the mean number of sperm per SST. All SSTs were apparently ''active'' and capable of storing sperm. Multiply mated females had higher sperm loads than those mated once, with one exception. The maximum number of sperm in the UVJ of a single bird was 62,000; the minimum, 540. Within birds, sperm were distributed nonrandomly between SSTs. In all birds most sperm were located at the distal end of SSTs in a single clump. There was no evidence that sperm from separate ejaculates from either the same or different males remained stratified within SSTs. The degree of stratification was low and was determined by the number of sperm in the UVJ. It seems unlikely that stratification of sperm within SSTs accounts for last male sperm precedence in the Zebra Finch.