The stinging hairs of Urtica dioica (stinging nettle) have often been described as defences against grazing; however, no experimental studies have tested this proposition. Variability in the number of stinging hairs borne by plants has been described, and is known to have a genetic basis. This variability extends to plants which are virtually stingless. Comparison of grazing preferences of two species of herbivores (rabbits and sheep) on plants of known stinging hair density revealed that: (1) Both herbivores preferentially grazed plants with lower stinging hair densities under controlled conditions; (2) In a field experiment in an area with intense rabbit grazing, higher grazing damage was observed on plants with lower stinging hair densities; (3) Patterns of grazing damage, and observations of herbivore behaviour, suggested that stinging hairs act to deter consumption of significant amounts of plant matter, even by herbivores which had not developed learned avoidance.
It is likely that grazing by large mammals could act as a strong selective force for higher stinging hair densities in nettles. Theoretical treatments of plant-herbivore interactions based on invertebrate grazing patterns may be too simplistic to apply to mammalian herbivores that are large, mobile, behaviorally complex, and yet dietarily generalist.