Colonial seabird's paralytic perfume slows lice down: an opportunity for parasite-mediated selection?

Selection for chemical signals in birds could be influenced by parasitism as has been previously suggested for visual and acoustic displays. Crested auklets (Aethia cristatella), colonial seabirds from Alaska and Siberia, offer an example of how this might occur. Crested auklets secrete lipids in plumage, possibly as an indicator of status and attractiveness. Prominent among these secretions are aldehydes, which are noticeable as a pungent citrus-like odour. Octanal and hexanal, the most abundant aldehydes in the plumage of crested auklets, are potent invertebrate repellents, reported from the chemical defenses of heteropteran insects. These aldehydes occur at high concentrations within specialized secretory structures. Experiments presented here show that these compounds can paralyse lice. Lice obtained from auklets were paralysed or killed within seconds after exposure to volatiles from nicks in the integument of a crested auklet. Chemical analysis demonstrated the presence of aldehydes in the area of integument used for this experiment. Lice exposed to control tissues in the same manner were not affected. A synthetic blend of crested auklet odourant constituents caused a sequence of impaired behaviours in auklet lice comparable to the effects of neuroactive insecticides. The time until onset of effects was dependent on dose, suggesting that the rate of molecular diffusion into louse spiracles was the explanatory factor. Impairment was evident even at very low concentrations that can occur in crested auklet plumage during winter. The same aqueous emulsions were present in both experimental and control treatments but lice in controls experiments were not affected. Crested auklets inhabit crowded social neighbourhoods with larger social groups, closer interindividual spacing and higher rates of contact than sympatric least auklets (Aethia pusilla). This could help to explain why crested auklets can have higher louse abundances. Lice are spread through direct contact between hosts and louse-infected mates could transmit lice to offspring. Large differences in the louse loads on crested auklet fledglings suggest differences in the parental transmission of lice to offspring.