Abstract
The wide-ranging adaptations of birds has astounded scientists for many years and the crested auklet (Aethia cristatella) is no exception. Aside from its physical ornamentation and vocalisations, the auklet’s citric plumage-odour is one of its most well-known characteristics. The tangerine-like secretion is used in courtship displays as well as acting as an ectoparasite repellant, thus, determining the survival of an individual. The workings of natural selection allows the selection of the fittest birds for survival. The noticeable fragrance is due to the presence of aldehydes, especially octanal, which have been identified through mass spectrometry.
Introduction
“What wild creature is more accessible to our eyes and ears, as close to us and everyone in the world, as universal as a bird?” That pensive comment was made by the national treasure who is none other than Sir David Attenborough. Be it the iridescence of the endangered Ribbon-tailed astrapia (Astrapia mayeri), or the entrancing song of the Common nightingale (Luscinia megarhynchos), birds have always fascinated humans and continue to attract our aural and visual attention. However, did you know that some birds also appeal to our sense of smell?
First proposed by Darwin and Wallace, sexual selection is one of the driving factors of survival and is therefore a mode of natural selection.[1] Avian intersexual selection has resulted in both behavioural and physiological adaptations. Expression of visual, acoustic and olfactory ornamentation has led to the evolution of some of the most incredible birds. One such bird is the crested auklet (Aethia cristatella) of the Bering Sea. With colonies of over one million individuals, this monogamous seabird has evolved various iconic characteristics, through the pressures of sexual selection, to ensure its survival.
The black, forward-curving bristle feathers found on their foreheads, befitting of their name, are used to attract mates and to assert dominance. The existence of these showy monomorphic traits is indicative of intense sexual selection for both sexes.[2] Their notable trumpet-like call is used for both individual recognition and sexual advertising. The loud volume and harsh timbre of the call may not be ‘music to our ears’, but it still does serve its evolutionary purpose: to attract a mate.
However, one of the most distinctive and unique features of this bird is its citrus-like plumage odour. The ‘tangerinesque’ fragrance is an integral part of the courtship process as it secures the long-term pair formation of mates. During the ruff sniff display, pairs all anointed one another by rubbing the perfume on each other’s napes, back and breast. These strong scent molecules attract individuals to form mating pairs through a courtship ritual whilst signalling the quality of the potential mate; this function gives the secretion a pheromone-like purpose.[3][4] Another function of this odorant is its defence role as a repellent for ectoparasites.
But what causes this ‘orangey’ aroma?
Through mass spectra and organic analysis, it appears that this perfume-like secretion contains even-numbered carbon aldehydes such as hexanal, octanal (Figure 2) and decanal. In addition to the aldehydes, the mixture also contains unsaturated aldehydes: cis-4-decanal, (Z)-4-dodecenal and (Z)-6-dodecenal.[5] Our ability to taste and smell is regulated by chiral molecules in our mouths and noses that act as receptors to \”sense\” and recognise foreign substances. We can anticipate, then, that enantiomers may interact differently with the receptor molecules and induce different sensations. This is why there is high stereospecificity in the production of the chemicals found within this avian organism to ensure and maintain its communicatory purpose. 
As shown by the bar graph (Figure 3), around 40% of the secreted mixture contains Caprylic aldehyde or octanal. This short chain aldehyde gives the bird its notorious ‘waxy, citrus orange with a green peely nuance’ smell.[6] This odour is known to repel parasitic organisms and recent evidence shows that crested auklets with a low chemical emission rate tend to be heavily parasitized by ticks.[7] This puts these individuals at a biological disadvantage as it reduces the likelihood of their survival and therefore lowers their chances of mating. This shows the evolutionary significance of this physiological adaptation. 
In fact, research has been done to investigate the human uses of this auklet odour. By synthetically creating the potion, researchers studied the effectiveness of the fragrance in repelling mosquitoes. Even at very low concentrations, the tangerine-like secretion deterred mosquitoes almost entirely.[7] Although we should not be expecting ‘birdie bug spray’ stocked up in pharmacys anytime soon, scientists can use this research to safely apply these chemical components to daily life.
The seemingly ornamental feature is underlined with intricate workings of natural selection. Without its chemically created citric smell, this peculiar and wonderful bird may not even be alive today. Life’s ability to thrive in the face of adversity shows the beautiful yet ruthless randomness of nature.
So, the next time you think you can smell a tangerine being peeled near you, remember this; you could actually be standing right next to an avian Eau De Parfum producer!
References
1. Brennan, P. (2010) Sexual Selection. Nature Education Knowledge 3(10):79,
URL:https://www.nature.com/scitable/knowledge/library/sexual-selection-13255240/, Last Accessed: [Monday 30th September 19:53]
2. Jones I. L. (1993), Crested Auklet- The Birds of North America (Aethia cristatella), 2nd ed, Cornell Lab of Ornithology, Ithaca, https://doi.org/10.2173/bna.70, Last Accessed: [Monday 30th September 19:34]
3. Hagelin J, Jones I, Rasmussen L (2013), A tangerine-scented social odour in a monogamous seabird, 10.1098/rspb.2003.2379, Last Accessed: [Monday 30th September 20:22]
4.Hwee Ng, Shruti Shankar, Yasumasa Shikichi, Kazuaki Akasaka, KenjiMori, Joanne Y. Yew (2014), Pheromone evolution and sexual behaviour in Drosophila are shaped by male sensory exploitation of other males, Proceedings of the National Academy of Sciences,
https://www.pnas.org/content/111/8/3056, Last Accessed: [Monday 30th September 19:45]
5. Jones, Ian L., et al., An Experimental Field Study of the Function of Crested Auklet Feather Odour. The Condor, vol. 106, no. 1, 2004, pp. 71–78. JSTOR, www.jstor.org/stable/1370517., Last Accessed: [Monday 30th September 20:39]
6. Sorensen, P. W., & Hoye, T. R. (2010). Pheromones in vertebrates. In L. Mander, & H-W. Lui (Eds.), Comprehensive Natural Products II: Chemistry and Biology: Chemistry and Biology (Vol. 4, pp. 225-262). Elsevier Ltd.
7. Douglas III, Hector. (2006), Measurement of Chemical Emissions in Crested Auklets (Aethia cristatella). Journal of Chemical Ecology.
https://www.researchgate.net/publication/6735245_Measurement_of_Chemical_Emissions_in_Crested_Auklets_Aethia_cristatella, Last Accessed: [Monday 30th September 19:42]
Figure References
1. ‘The Crested Auklet (Aethia cristatella)’, Living on Earth, http://www.loe.org/shows/segments.html?programID=15-P13-00003&segmentID=5 , Last Accessed: [Monday 30th September 20:50]
2. ‘Octanal also known as Caprylic aldehyde’, Wikidata, https://www.wikidata.org/wiki/Q416673 , Last Accessed: [Wednesday 1st April 20:05]
3. ‘Chemical composition of the crested auklet (Aethia cristatella) odorant’, Douglas III, Hector. (2006), Measurement of Chemical Emissions in Crested Auklets (Aethia cristatella). Journal of Chemical Ecology.
https://www.researchgate.net/publication/6735245_Measurement_of_Chemical_Emissions_in_Crested_Auklets_Aethia_cristatella, Last Accessed: [Monday 30th September 19:42]
About The Author
Sathvika Krishnan is a Year 13 student at Bablake School, Coventry, where she is also the School Captain. She is passionate about the welfare of the environment and is an aspiring zoologist. Aside from her academic interests, Sathvika is an avid South-Indian classical vocalist and Western diploma-level pianist. In addition to this, she enjoys acting and public speaking.