1. Associational resistance occurs when one plant species gains protection from its consumers by association with a defended species. In semi-arid ecosystems of Chile the perennial herb Convolvulus chilensis (Convolvulaceae) suffers heavy herbivory by small mammals when growing prostrate but plants seem to be protected when they are climbing onto cacti or thorny shrubs (nurse species). 2. Field observations suggest that damaged main stems have a greater number of small lateral stems which greatly enhance climbing success. In this field study we addressed whether C. chilensis shows associational resistance by climbing onto nurse species and evaluated whether damage and also shade (a cue for neighbour presence) elicit architectural responses that enhance climbing probability. 3. We documented associational resistance for C chilensis: climbing individuals growing associated with cacti and thorny shrubs suffered less herbivory (percentage of damaged stems: 35 vs. 98) and had greater reproductive output (number of reproductive structures at the end of the season: 150 vs. 0) than prostrate individuals growing in isolation. Experimental clipping of the main stem caused plants to produce a greater number of lateral stems both in sun (70% increase) and in shade (66% increase) treatments. Moreover stem damage caused a 50% increase in the number of stems actually climbing on a support but only in shade. 4. Synthesis. We show for the first time that phenotypic responses to herbivory of the potentially protected species may enhance association with the defended nurse species thereby promoting associational resistance. This finding adds a new perspective for mechanistic studies of positive interactions among plants suggesting that behavioural responses of the protv©gv©e may mediate the magnitude of nurse effects.