Stomach oil
Encyclopedia
Stomach oil is the light oil
composed of neutral dietary lipid
s found in the proventriculus
(fore-gut) of birds in the order
Procellariiformes
. All albatross
es, procellarids
(gadfly petrels and shearwaters) and storm-petrel
s use the oil. The only Procellariiformes that do not are the diving-petrels.
The chemical make up of stomach oil varies from species
to species and between individuals, but almost always contains both wax
ester
s and triglyceride
s. Other compounds found in stomach oil include glycerol ethers, pristane
and squalene
. Stomach oil has low viscosity and will solidify into a hard wax if allowed to cool.
It was once thought that stomach oil was a secretion of the proventriculus, but it is now known to be a residue of the diet created by digestion
of the prey items such as krill
, squid
, copepod
s and fish
. It is thought to serve several functions for Procellariiformes
, primarily as an energy store; its calorific
value is around 9.6 kcal
per gram, which is only slightly lower than the value for diesel oil. For this reason a great deal more energy can be stored in oil form as opposed to undigested prey. This can be a real advantage for species that range over huge distances to provide food for hungry chicks, or as a store for lean times when ranging across the sea looking for patchy areas of prey.
Surface nesting petrel
s and albatross
can eject this oil out of their mouths (not nostrils, as has sometimes been suggested) towards attacking predators or conspecific rivals. This oil can be deadly to birds, as it can cause matting of the feathers leading to the loss of flight or water repellency. Against threatening mammal
s (including humans) it is not outright dangerous, but due to its extremely offensive smell it is usually highly repulsive and liable to spoil a predator's hunting success for quite some time. The smell of the hydrophobic oil cannot be removed with water, and can persist (e.g. on clothing) for months or even years.
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
composed of neutral dietary lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s found in the proventriculus
Proventriculus
The proventriculus is part of the digestive system of birds, invertebrates and insects.-Birds:The proventriculus is a standard part of avian anatomy...
(fore-gut) of birds in the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Procellariiformes
Procellariiformes
Procellariiformes is an order of seabirds that comprises four families: the albatrosses, petrels and shearwaters, storm petrels, and diving petrels...
. All albatross
Albatross
Albatrosses, of the biological family Diomedeidae, are large seabirds allied to the procellariids, storm-petrels and diving-petrels in the order Procellariiformes . They range widely in the Southern Ocean and the North Pacific...
es, procellarids
Procellariidae
The family Procellariidae is a group of seabirds that comprises the fulmarine petrels, the gadfly petrels, the prions, and the shearwaters. This family is part of the bird order Procellariiformes , which also includes the albatrosses, the storm-petrels, and the diving petrels.The procellariids are...
(gadfly petrels and shearwaters) and storm-petrel
Storm-petrel
Storm petrels are seabirds in the family Hydrobatidae, part of the order Procellariiformes. These smallest of seabirds feed on planktonic crustaceans and small fish picked from the surface, typically while hovering. The flight is fluttering and sometimes bat-like.Storm petrels have a cosmopolitan...
s use the oil. The only Procellariiformes that do not are the diving-petrels.
The chemical make up of stomach oil varies from species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
to species and between individuals, but almost always contains both wax
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s and triglyceride
Triglyceride
A triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....
s. Other compounds found in stomach oil include glycerol ethers, pristane
Pristane
Pristane is a natural saturated terpenoid alkane obtained primarily from shark liver oil, from which its name is derived . It is also found in mineral oil and some foods...
and squalene
Squalene
Squalene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...
. Stomach oil has low viscosity and will solidify into a hard wax if allowed to cool.
It was once thought that stomach oil was a secretion of the proventriculus, but it is now known to be a residue of the diet created by digestion
Digestion
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
of the prey items such as krill
Krill
Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...
, squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
, copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s and fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
. It is thought to serve several functions for Procellariiformes
Procellariiformes
Procellariiformes is an order of seabirds that comprises four families: the albatrosses, petrels and shearwaters, storm petrels, and diving petrels...
, primarily as an energy store; its calorific
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...
value is around 9.6 kcal
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...
per gram, which is only slightly lower than the value for diesel oil. For this reason a great deal more energy can be stored in oil form as opposed to undigested prey. This can be a real advantage for species that range over huge distances to provide food for hungry chicks, or as a store for lean times when ranging across the sea looking for patchy areas of prey.
Surface nesting petrel
Petrel
Petrels are tube-nosed seabirds in the bird order Procellariiformes. The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group...
s and albatross
Albatross
Albatrosses, of the biological family Diomedeidae, are large seabirds allied to the procellariids, storm-petrels and diving-petrels in the order Procellariiformes . They range widely in the Southern Ocean and the North Pacific...
can eject this oil out of their mouths (not nostrils, as has sometimes been suggested) towards attacking predators or conspecific rivals. This oil can be deadly to birds, as it can cause matting of the feathers leading to the loss of flight or water repellency. Against threatening mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s (including humans) it is not outright dangerous, but due to its extremely offensive smell it is usually highly repulsive and liable to spoil a predator's hunting success for quite some time. The smell of the hydrophobic oil cannot be removed with water, and can persist (e.g. on clothing) for months or even years.