Behavioral hierarchy in the medicinal leech, Hirudo medicinalis: feeding as a dominant behavior

The effect of feeding behavior on other behaviors (swimming, crawling and shortening) was investigated in the leech, Hirudo medicinalis. The stimulus locations and intensities required to produce mechanically elicited behaviors were first determined in the non-feeding leech. Stimuli were delivered while the leech was in various body positions to determine whether stimulus location affected behavioral response. Response thresholds were determined for the mechanically elicited behaviors. The same stimuli were then applied to feeding leeches to determine if response thresholds had changed. A solution with NaCl and arginine was used to elicit feeding. The same sets of stimuli were applied at intervals for an hour after feeding, to determine the duration of feeding-induced changes in behavior. Depending on the body position and stimulus location, stimuli produced different combinations of behaviors that included shortening, swimming and crawling. Anterior stimuli generally elicited shortening, whereas posterior stimuli generally elicited crawling and swimming, with swimming more likely to ventral stimulation than to dorsal stimulation. Having the front sucker attached changed these behavioral patterns. During feeding, the response thresholds changed dramatically, from 3-5 V to greater than 9 V. This increase in threshold began with the start of feeding, even before ingestion commenced. Suppression of the behaviors lasted up to 1 h after the end of feeding, with the effect on swimming being the most pronounced and longest lasting.