The Origin of Petrel Stomach Oils-a Review

Among stomach contents of procellariiform birds, one of the commonest components is oil which varies in color from pale yellow to red. This oil occurs in greatest quantities in stomachs of young birds on the nest and may be regurgitated by them when disturbed. Regurgitation is apparently more frequent, and may be more voluntarily controlled, in chicks of surface nesters than in those of burrow nesters, thereby possibly serving as a defense mechanism for the former. Because the oil floats on top of the stomach contents it is ejected most frequently but solid and partly-digested food may follow it. Early analyses of stomach oil from Sooty Shearwaters (Puffinus griseus; Smith 1911, Carter 1921, Carter and Malcolm 1927) and from a Fulmar (Fulmarus glacialis; Rosenheim and Webster 1927) revealed high concentrations of wax esters. In these lipids the fatty acids are esterified with long-chain alcohols, whereas in typical fats (triglycerides) they are esterified with glycerol. Because wax esters were then virtually unknown in marine animals, though known to occur in large quantities in Sperm Whales (Physeter macrocephalus; Smith 1911, Carter 1921), hypotheses were put forward to explain their origin, Carter and Malcohn (1927) considered that they might be indigestible residues from food but failed to find them in an analysis of fish and shell-fish fats (littoral and epipelagic species). They and Rosenheim and Webster (1927) independently made similar hypotheses, based on similar reasoning: the stomach oils might be secretions of the birds themselves, similar to preen gland wax but possibly originating near the nasal cavity (Rosenheim and Webster 1927), or actual preen gland wax accidentally ingested (Carter and Malcolm 1927). Matthews (1949) found evidence suggesting that the proventriculus could be the site of secretion but he emphasized that proof of a secretory origin was still required. No such proof appears to have been published but, until very recently, some ornithologists have given unwarranted credence to this hypothesis ( see Ashmole 1971) . be the predominant lipids, and proportions have varied widely even among samples taken from the same species at the same time. It was mainly this variability that cast doubt on the secretion hypothesis (Lewis 1969), for it seemed unlikely that a secretory mechanism would vary its product so much. The hypothesis, first advanced by Hagerup (1926), of a dietary origin for the oils seemed more likely. Kritzler’ s (1948) experiment with a captive fulmar, which regurgitated oil resembling pork fat after being fed fatty pork (but no biochemical tests were made) tended to support this hypothesis. On the basis of their studies of petrel stomach oils, Lewis (1969) and Cheah and Hansen (1970 a,b) independently concluded that the oils were not secretions but derivatives of the diet. Earlier studies of lipids in the marine environment had revealed mainly triglycerides. However, Nevenzel et al. (1965, 1966) discovered in some deeper-living pelagic fish large amounts of wax esters that were characterized by a predominance of oleic acid. Then Lewis (1967) found that the concentration of oleic acid in marine animals increased with increasing depth of habitat. He suggested, after considering the findings of Nevenzel et al., that this indicated increasing amounts of wax esters in deeper-living animals. Among the animals whose lipids he analyzed were crustaceans of the genera Gnathophausia and Acanthephyra, and of the family Pasiphaeidae, and fishes of the family Myctophidae, all of which contained moderate to high proportions of oleic acid among their fatty acids. All of these groups of animals were subsequently found to be important prey of Grey-faced Petrels (Pterodroma macropteru gouldi; Imber 1973) and Cook’ s Petrels (P. cooki cooki; Imber unpubl. data). Further studies by Lee et al. (1971) and Lee and Hirota (1973) of the lipids of marine animals taken from varying depths have confirmed Lewis’ s hypothesis. Many of the animals whose lipids they analyzed belonged to groups which have recently been found to be important prey of Procellariiformes (table 2).