Summary
The role of courtship behaviour in ethological isolation was studied by analysing courtship sequences between different crosses of pumpkinseed (Lepomis gibbosus), bluegill (L. macrochirus), and their hybrid sunfish. Crosses were established in large laboratory tanks, under an artificially advanced photoperiod and temperature regime, and in outside pools. Male-female interactions were filmed and videotaped. In the field, underwater films of naturally occurring courtship and spawning were taken. A behavioural repertoire was described and the data converted to pairs of male and female acts (dyads). Two contexts were distinguished (spawn and nonspawn) depending on whether courtship led to spawning or not. Several methods of analysis were used. Multidimensional contingency table analysis with factors of preceding act, following act, and cross, enabled examination of the dependence of the response of any individual (sex or species) upon the species of partner. Multidimensional Scaling of the crosses, based on a chi-square measure of overall dissimilarity for each context, revealed the relative proximities among the different interspecific and hybrid crosses and the parentals. The results generally agreed well with those produced by a clustering technique. The conclusions are: (1) The differences between the courtship and spawning behaviour of the parentals are evidence for a behavioural isolating mechanism. (2) Interspecific and hybrid crosses tend to be intermediate to parentals. (3) Courting and spawning behaviour is different in the laboratory and field environments. (4) Hybrids likely originate from crosses of male bluegills and female pumpkinseeds.
Methodology
Data collection.
Data were collected during the spring and early summer of 1976 and 1977. These included : 1) observations of courtship (and spawning) in Lake Opinicon, and 2) observations of behaviour in artificial (laboratory and pool) habitats where specific pairings were isolated. Observations of courtship in the lake were made at three locations. Behavioural sequences were filmed underwater at a distance of about 1 m from the male's nest. This had no noticeable effect on the behaviour of fish. No heterospecific pairings were observed during these lake observations but female bluegills were observed spawning with hybrid males. Additional data were collected from fish kept in artificial habitats. During May, June and July of 1976 these observations were made on fish kept in wading pools (3 m diameter) erected on the shore of Lake Opinicon. In early spring 1977 similar observations were made on fish kept in large tanks in the laboratory.