TESTES ASYMMETRY, CONDITION AND SEXUAL SELECTION IN BIRDS - AN EXPERIMENTAL TEST

The functional significance of the marked directional asymmetry in tes tes size observed in many bird species is obscure. Moller suggested th at (i) the smaller of the two testes sera es a compensatory role and i ncreases in size (and hence reduces asymmetry) when the larger one is defective in some way, and (ii) as a consequence, the degree of direct ional asymmetry in testes size reflects male quality and covaries posi tively with the expression of secondary sexual traits. We conducted an experimental test of these two hypotheses in the zebra finch, Taeniop ygia guttata. Neither hypothesis was supported. First, there was no si gnificant relationship between the size of the left testis and relativ e testes asymmetry Second, we obtained no support for the hypothesis t hat the degree of directional asymmetry in testes mass covaried with c ondition. On the contrary directional asymmetry in testes mass was sig nificantly greater in birds whose condition was experimentally reduced , compared with control birds. Moreover, we found no significant relat ionships between testes asymmetry and secondary sexual traits. We conc lude that directional asymmetry in testes size does not reflect male c ondition in the zebra finch.