Experimental evidence for evolved tolerance to avian malaria in a wild population of low elevation Hawai'i 'Amakihi (Hemignathus virens).

Introduced vector-borne diseases, particularly avian malaria (Plasmodium relictum) and avian pox virus (Avipoxvirus spp.), continue to play significant roles in the decline and extinction of native forest birds in the Hawaiian Islands. Hawaiian honeycreepers are particularly susceptible to avian malaria and have survived into this century largely because of persistence of high elevation refugia on Kaua'i, Maui, and Hawai'i Islands, where transmission is limited by cool temperatures. The long term stability of these refugia is increasingly threatened by warming trends associated with global climate change. Since cost effective and practical methods of vector control in many of these remote, rugged areas are lacking, adaptation through processes of natural selection may be the best long-term hope for recovery of many of these species. We document emergence of tolerance rather than resistance to avian malaria in a recent, rapidly expanding low elevation population of Hawai'i 'Amakihi (Hemignathus virens) on the island of Hawai'i. Experimentally infected low elevation birds had lower mortality, lower reticulocyte counts during recovery from acute infection, lower weight loss, and no declines in food consumption relative to experimentally infected high elevation Hawai'i 'Amakihi in spite of similar intensities of infection. Emergence of this population provides an exceptional opportunity for determining physiological mechanisms and genetic markers associated with malaria tolerance that can be used to evaluate whether other, more threatened species have the capacity to adapt to this disease.