Recent evidence indicates that variation in intensity of aggressiveness and competitive ability among nestlings of a clutch may be attributed, at least partly, to differences in maternally deposited yolk testosterone that affects embryonic development (Schwabl 1993). We developed a radioimmunoassay (RIA) for measuring yolk testosterone levels in house wren (Troglodytes aedon) eggs. This procedure was used in initial studies to determine whether yolk testosterone varies within a clutch of house wren eggs. The analysis revealed the presence of higher levels of testosterone in yolks of eggs laid later in the clutch sequence. Because later laid eggs tend to hatch later (Winkler 1993), variation in the amount of testosterone deposited by the female may ensure a greater chance of survival for younger, less developed offspring.



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