Deadly hairs, lethal feathers--convergent evolution of poisonous integument in mammals and birds

Hairs and feathers are textbook examples of the convergent evolution of the follicular appendage structure between mammals and birds. While broadly recognized for their convergent thermoregulatory, camouflage and sexual display functions, hairs and feathers are rarely thought of as deadly defence tools. Several recent studies, however, show that in some species of mammals and birds, the integument can, in fact, be a de facto lethal weapon. One mammalian example is provided by African crested rats, which seek for and chew on the bark of plants containing the highly potent toxin, ouabain. These rats then coat their fur with ouabain-containing saliva. For efficient toxin retention, the rodents have evolved highly specialized fenestrated and mostly hollow hair shafts that soak up liquids, which essentially function as wicks. On the avian side of the vertebrate integumental variety spectrum, several species of birds of New Guinea have evolved resistance to highly potent batrachotoxins, which they acquire from their insect diet. While the mechanism of bird toxicity remains obscure, in a recently published issue of the journal, Dumbacher and Menon explore the intriguing idea that to achieve efficient storage of batrachotoxins in their skin, some birds exploit the basic permeability barrier function of their epidermis. Batrachotoxins become preferentially sequestered in their epidermis and are then transferred to feathers, likely through the exploitation of specialized avian lipid-storing multigranular body organelles. Here, we discuss wider implications of this intriguing concept.