The functional significance of the marked directional asymmetry in testes size observed in many bird
species is obscure. Møller suggested that (i) the smaller of the two testes serves a compensatory role and
increases in size (and hence reduces asymmetry) when the larger one is defective in some way, and (ii) as a
consequence, the degree of directional asymmetry in testes size reflects male quality and covaries positively
with the expression of secondary sexual traits.We conducted an experimental test of these two hypotheses
in the zebra finch,Taeniopygia guttata. Neither hypothesis was supported. First, there was no significant relationship between the size of the left testis and relative testes asymmetry. Second, we obtained no support
for the hypothesis that the degree of directional asymmetry in testes mass covaried with condition. On the
contrary, directional asymmetry in testes mass was signifcantly greater in birds whose condition was
experimentally reduced, compared with control birds. Moreover, we found no significant relationships
between testes asymmetry and secondary sexual traits. We conclude that directional asymmetry in testes
size does not reflect male condition in the zebra finch
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