Journal of Chemical Ecology, Vol. 30, No. 10, October 2004 (  C 2004) INTERSPECIFIC DIFFERENCES IN AETHIA SPP. AUKLET ODORANTS AND EVIDENCE FOR CHEMICAL DEFENSE AGAINST ECTOPARASITES HECTOR D. DOUGLAS III, 1,∗ JASON E. CO, 2 TAPPEY H. JONES, 2 and WILLIAM E. CONNER 3 1 Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA 2 Department of Chemistry, Virginia Military Institute, Lexington, Virginia 24450, USA 3 Department of Biology, Wake Forest University, Winston-Salem, North Carolina 27109, USA (Received October 23, 2003; accepted June 30, 2004) Abstract—The true auklets (Genus Aethia) are small planktivorous seabirds of the Bering Sea and North Pacific. Two species, the crested and whiskered auklets produce volatile citrus-like odorants. We here show that the whiskered auklet odorant is composed predominantly of two odd-numbered aldehydes (heptanal and nonanal) with no detectable unsaturated aldehydes. By comparison the crested auklet odorant is dominated by even-numbered aldehydes, both saturated and monounsaturated, ranging in size from 6 to 12 carbons. This is evidence of species-specific acquisition or biosynthetic pathways. We clarify the chemistry of the crested auklet odorant. We cite evidence that the C-12:1 aldehyde in crested auklets is actually two isomers, (Z)-4-dodecenal and (Z)-6-dodecenal. We also report on experimental evidence that aldehyde constituents kill and repel ectoparasites. Efficacy of the aldehydes may increase when they are combined in a mixture. The repellency of the mixture increases with chemical concentration. This suggests that individuals with higher chemical production are likely to repel ectoparasites more effectively. Key Words—Crested auklet, whiskered auklet, aldehydes, (Z)-4-dodecenal, (Z)-6-dodecenal, synthesis, chemical defense, ectoparasites, pheromones, seabirds. INTRODUCTION Aethia auklets are some of the most locally abundant avifauna in the Bering Sea, with colony sizes ranging up to 10 7 individuals (Springer et al., 1993). They dive ∗ To whom correspondence should be addressed. E-mail: douglas@ims.uaf.edu 1921 0098-0331/04/1000-1921/0 C  2004 Springer Science+Business Media, Inc.