Reproductive success and symmetry in zebra finches

Fluctuating asymmetries are small, random developmental accidents in morphological traits that would otherwise be described as being bilaterally symmetric. Experimental evidence indicates that in mate choice situations individuals possessing symmetric arbitrary traits or symmetric secondary sexual characters are preferred over those with larger asymmetries. This experiment investigated the effect of symmetric and asymmetric arbitrary traits (arrangements of coloured leg-bands) on reproductive success in captive zebra finches, Taeniopygia guttata. Six groups of six males and six females were separated into six free-flight breeding aviaries. Nestboxes and nesting material were freely available throughout the experiment. Males wore symmetric and asymmetric arrangements of the same coloured leg-bands. Each male wore two orange and two light green bands, two on each leg. There were six possible arrangements of these leg-bands. One male from each of these six leg-band treatments was present in each aviary, along with six females wearing numbered orange bands. Symmetrically banded males produced more offspring that survived past the period of parental care than males in either of the asymmetric treatments; this appeared to be the effect of female choice processes and female-based parental investment and not male intra-sexual dominance. This is the first experiment to indicate that symmetrically manipulated males gain reproductive advantages in controlled laboratory conditions and further supports recent theories indicating the evolutionary importance of symmetry in signalling-trait design. (C) 1996 The Association for the Study of Animal Behaviour