SPERM PRECEDENCE IN ZEBRA FINCHES DOES NOT REQUIRE SPECIAL MECHANISMSOF SPERM COMPETITION

Competition between the spermatozoa of different males to fertilize th e eggs of a single female acts as a selection pressure on the behaviou r of males and females. However, quantitative predictions about behavi our fan only be made if the paternity consequences of different patter ns of copulation are known. Because exhaustive empirical measurement o f these consequences may be impractical, interest has centred on deter mining the mechanisms by which sperm competition occurs, knowledge of which may allow consequences to be calculated. One method of elucidati ng mechanisms of sperm competition is to use mathematical models to de termine which mechanisms are necessary or sufficient to account for em pirical observations. We use this approach for zebra finches Taeniopyg ia guttata and show that empirically measured rates of disappearance o f sperm from the reproductive tract, and differences in the number of sperm in the first and subsequent ejaculates of each male, are suffici ent to account for observed levels of sperm precedence. Special mechan isms of sperm competition, such as displacement or stratification of s perm, are therefore unnecessary to explain sperm precedence in this sp ecies.