AbstractThe present investigation is concerned with chemoreceptivity in the male three-spined stickleback (Gasterosteus aculeatus L.) during reproductive behaviour. The functions of cranial nerves were studied with regard to nest building (N), the increase in the zigzag dance (a measure of the sexual tendency (S)) between N and the day of fertilization (F) of the eggs, the zigzag dance between F and the day of hatching (H), suppression of the zigzag dance (if still present) at H, and fanning activity (P) during these days of a reproductive cycle (F-H). The methods of quantifying S and P, as well as the methods of sectioning the olfactory nerves and the branches of cranial nerves possibly involved in the conduction of chemical stimuli are described. The behavioural changes observed after sectioning the olfactory nerves, branches of cranial nerves or combinations of these nerves are compared with behavioural data of unoperated fish. Special attention is given to behavioural changes following regeneration of transected nerves. Nest building is still possible after sectioning of the olfactory nerves, but occurs only in a few fish. The functioning of the olfactory nerves might influence the development of nest building behaviour by inducing hormonal changes, necessary to start such reproductive activities. During the period between N and F (the latter determined by the observer), the number of zigzags (S) increases from zero up to 100 or more per 5 minutes. The olfactory nerves are indispensable for the promotion of sexual behaviour during this period. It seems likely that these nerves are necessary to induce hormonal changes in such a way that the reproductive cycle can proceed from nest building to courtship stage. In the exceptional case that a nest was built by a fish in which the olfactory nerves had been sectioned, the zigzag score remaining low, all other reproductive activities still occurred, including fertilization. Fanning activity in such fish appeared to be quantitatively normal. When the olfactory nerves were sectioned between N and F, at a stage when the zigzag scores had reached an essential level ( 100 per 5 min), both sexual and fanning activity between F and H were normal. This is explained by the supplementary functioning of both the ramus posttrematicus IX and the ramus pretrematicus X1. Both eggs and embryos stimulate sexual behaviour between F and H. The olfactory nerves exert an exciting function with regard to sexual behaviour during the first days of the cycle (F-H). The area in the roof of the pharynx, situated between the first and second gill arches appeared to perceive stimuli from eggs and embryos, exciting sexual behaviour (referred to as sex-excitation area). The sensory fibres from this area run in a dorsal direction, some joining the rami pharyngei X1, others one of the rami pharyngei IX (referred to as the S-anastomosis), their excitation increasing during the course of the cycle. The S-anastomosis which joins the ramus posttrematicus IX near or just dorsal of the second arteria branchialis dorsalis, exites sexual behaviour during most days of the cycle (F-H). Similarly, the rami pharyngei X1 which join the ramus pretrematicus X1 at a point dorsal of the second arteria branchialis dorsalis, excite sexual behaviour during the last days of the cycle. The presence of young following hatching suppresses sexual behaviour, if still present, at H. Such stimuli are perceived by the rami pharyngei IX, which join the ramus posttrematicus IX near or dorsal to the first arteria branchialis dorsalis. Perceiving such stimuli results in an immediate suppression of sexual behaviour. Both eggs and embryos also stimulate fanning behaviour between F and H. Our experiments have shown that the following three nerves are involved in the conduction of such stimuli: nervus olfactorius, ramus posstrematicus IX (including its anastomosis with the ramus pretrematicus X1, referred to as the F-anastomosis), and the ramus pretrematicus X1. The involvement of the cranial nerves I, IX and X in the reproductive activities of the male three-spined stickleback is illustrated in a schematic way in Fig. 27.
