Stinging hairs on the Japanese nettle Urtica thunbergiana have a defensive function against mammalian but not insect herbivores

Abstract

Thorns and hairs of plants can serve as defenses against herbivores, although they may not have evolved under selection by herbivory. Japanese nettles, Urtica thunbergiana, in Nara Park, Nara Prefecture, Japan, where sika deer have been protected for 1200 years, bear many more stinging hairs than those in areas with few or no deer. Previous studies suggested that such hairy nettles evolved under natural selection imposed by intense deer browsing, because stinging hairs deterred deer browsing and because among-population variation in hair density was associated with deer abundance. To confirm this hypothesis, we examined (1) whether stinging hairs affected oviposition and feeding preferences of herbivorous insects and (2) the degree to which they deterred deer via laboratory and field experiments with hairy nettles from Nara Park and with almost-hairless nettles from another area. A specialist butterfly, Indian red admiral, showed no oviposition or larval feeding preferences for either hairy or hairless nettles. Insect damage levels did not significantly differ between the two variants. In contrast, deer browsed hairless nettles more heavily than hairy ones. In hairy nettles, however, the level of deer browsing was not proportional to stinging-hair density, presumably because the hairy nettle population had reached a plateau for resistance as a result of long-term strong directional selection for stinging hairs. These results corroborate the hypothesis that hairy nettles in Nara Park evolved through natural selection under intense deer browsing.