Causal and Functional Organization of the Mating Behaviour Sequence in Helix Pomatia (Pulmonata, Gastropoda)

Abstract

Copulation in Helix pomatia is reciprocal. The partners behave identically, but not necessarily at the same time. Eleven different behaviour patterns are recognized and described. The mating behaviour is of long duration, about 8 hours. It can be divided into four phases which occur in a fixed sequence and each of which is characterized by a certain type of behaviour. Also the behaviour within the phases I-III is sequential, and the sequences are usually repeated many times. During phase I the snails adopt a conspicuous upright posture, which also occurs in phases II and III alternating with other behaviour. At the end of phase II a calcareous dart is shot into the body of the partner; the place of dart penetration is rather variable. A number of unsuccessful copulatory attempts occur during phase III before copulation takes place. A nearly immobile posture is maintained throughout the post-copulatory phase IV. Mating behaviour occurs in a cyclic manner. A complete (primary) mating sequence may during the next few days be followed by one or more secondary matings which do not include the dart shooting phase, then the next few days no mating takes place until a new primary mating is performed. The number of secondary matings are increased after deprivation of mating. No stimulating effect of dart-shooting on the mating behaviour of the receiver could be found, only decremental effects were observed. There are, however, short and long termed effects of dart-shooting on the actor: it causes the transition phase IT-III and has at least a main influence on the cyclic nature of mating activity. The mating behaviour is a highly complex behaviour as defined by duration, occurrence of phases of behaviour, number of behaviour patterns involving action of the whole body and interaction of individuals. In spite of this the sequential organization of the behaviour is basically determined by endogenous consequences of the behaviour of each individual. Changes of behaviour occur spontaneously or may be triggered by stimuli from the partner, but in the latter case the exogenous stimuli are never decisive for the behaviour to follow. Stimuli from the partner are significant for the functional organization by facilitating, orienting and synchronizing the behaviour of the partners. The sequential behaviour during phases II and III can be explained by increasing shooting and copulatory tendencies, respectively, and threshold differences between successive behaviour patterns. The increase in these tendencies is probably due to a self-stimulatory effect of the mating activities, and these activities again are facilitated by unspecific touch stimuli from the partner. Localized touch stimuli release dart-shooting and sometimes copulatory attempts. Both of these acts are stereotyped, but they are preceded by orientation movements. The orienting and releasing stimuli increase the chance that the dart hits the partner and that the copulatory attempts are synchronized. Several factors are found to he the cause of the high numbers of unsuccessful copulatory attempts. The separation of the first three phases is explained by an inhibitory influence of the shooting tendency above a certain threshold on copulatory behaviour and a disinhibitory effect of dart-shooting. Receptivity is obtained during phase III independently of the copulatory tendency, i.e., male and female tendencies of each individual can be separated. A complete post-copulatory behaviour and probably a temporary reduction of the mating tendency depend on endogenous factors correlated with spermatophore formation and is independent of delivery as well as receipt of the spermatophore. During phase IV all behaviour except the post-copulatory posture is strongly inhibited for about 3 hours, and when this inhibitory influence diminishes, the mating behaviour sequence is terminated and may be followed by phase I of a new mating sequence or other activities.