ESTIMATING THE MATING-BEHAVIOR OF A PAIR OF HYBRIDIZING HELICONIUS SPECIES IN THE WILD

Premating isolation between incipient species is rarely studied in nat ure, even though mating tests in captivity may give an inaccurate pict ure of natural hybridization. We studied premating barriers between th e warningly colored butterflies Heliconius erato and H. himera (Lepido ptera) in a narrow contact zone in Ecuador, where hybrids are found at low frequency. Eggs obtained from wild-mated females, supplemented wi th eggs and young larvae collected from the wild, were reared to adult hood. Adult color patterns of these progeny were then used to infer ho w their parents must have mated. Likelihood was used to estimate both the frequencies of potential parental genotypes from adult phenotypes collected in the wild, and the degree of assortative mating from the i nferred parents. The frequencies of parental genotypes varied across t he hybrid zone, but our statistical method allowed estimates of hybrid deficit and assortative mating to be integrated across all sites samp led. The best estimate of the frequency of F-1 and backcross hybrid ad ults in the center of the hybrid zone was 10%, with support limits (7. 1%, 13.0%; support limits are asymptotically equivalent to 95% confide nce limits). Mating was highly assortative: in the center of the hybri d zone the cross-mating probability between H. erato and H. himera was only 5% (0.3%, 21.4%). Wild hybrids themselves mated with both pure f orms, and the probabilities that they mated in any direction were not significantly lower than those among conspecifics. These results are c onsistent with earlier laboratory studies on mate choice, and suggest that selection against hybrids must be strong to prevent formation of a hybrid swarm. Unfortunately, the wide support limits on mating behav ior precluded a measure of the strength of selection from these data a lone. Our statistical approach provides a useful general method for es timating mate choice in the wild.