A hard nut to crack: rapid evolution in the Kona Grosbeak of Hawaii for a locally abundant food source (Drepanidini:Chloridops kona)

Abstract

Abstract The Kona Grosbeak (Chloridops kona), last seen in 1892, was restricted to a small mid-elevation area on the leeward (Kona) side of the island of Hawaii. It was reported to feed almost exclusively on the fruits of the naio tree (Myoporum sandwicense), which are extremely hard, requiring a force of perhaps 400 newtons or more to open. The morphology of the skull and mandible of the Kona Grosbeak was accordingly much more modified for the hypertrophied adductor musculature needed to crack naio fruits than its closest relative, the Wahi Grosbeak (Chloridops wahi), which has been found as fossils on most of the other main Hawaiian islands. The Wahi Grosbeak is here hypothesized to have fed on less resistant seeds, such as those of the Hawaiian prickly-ash (Zanthoxylum spp.). Naio is widely distributed in diverse habitats in the Hawaiian Islands but its fruits were apparently an energetically viable food source only on relatively young (ca. 1200 year old) aa lava flows on the dry side of Hawaii, where ...