Skin as a Toxin Storage Organ in the Endemic New Guinean GenusPitohui

Abstract

Several bird species in the endemic New Guinean genus Pitohui contain potent defensive toxins of the batrachotoxin family of steroidal alkaloid neurotoxins. We surveyed toxin concentrations in various tissues of Hooded Pitohui (Pitohui dichrous) using radioligand binding assays. The highest concentrations were found in the skin and feathers, on the outside of the birds, where predators or parasites are likely to encounter toxins. Significant levels of toxins also were found in skeletal muscle, heart, and liver. Muscle and liver would normally be poisoned by batrachotoxins; thus, Hooded Pitohuis must be insensitive to the toxins. The presence of toxins in internal organs further argues against the hypothesis that Hooded Pitohuis merely apply toxins topically to skin and feathers. Finally, we used scanning and transmission electron microscopy to examine skin and feathers for unusual morphological or histological adaptations for storing and secreting toxins. The skin of Hooded Pitohuis appears to have typical dermal and epidermal morphology, and we speculate on possible ways in which this species may sequester and secrete toxins using typical avian skin structural features, unique among vertebrates.