586
15 Lipids
long-chain fatty acids (C8 to C12) are found in the
milk of all terrestrial mammals, whereas butyric
acid (C4) is present only in the milk of ruminants.
The fatty acid spectrum in the milk of marine
mammals is similar to that ofthe storage fat [159].
The crop milk of pigeons (Columbiformes) is
in many ways comparable with mammalian milk.
Male and female pigeons feed the young with a
cheese-like white mass from the crop, the secretion of which, like the secretion of mammalian
milk, is regulated by prolactin from the hypophysis. Crop milk consists of fat-laden epithelial cells
which contain little carbohydrate but much protein and lipid. In addition to about 80 % TAG,
the lipids also include phospholipids and cholesterol. The fatty acid spectrum corresponds to that
of the body fat and consists of more than 90 % of
the usual CI6 and CI8 fatty acids [239]. The lipidrich oesophagal fluids of the flamingo Phoenicopteris ruber (total fat content 18 %) and of the penguin Aptenodytes forsteri (total fat content 29 %)
also serve to feed the young. In contrast, the
stomach oils found in the proventricules of the
albatrosses (Diomedeiae), petrels (Procellariidae) and stormy petrels (Hydrobatidae), but not
the diving petrels (Pelecanoididae), which form
the fourth family of the order of the tube-nosed
swimmers (Procelariiformes), have a variety of
functions. These are yellow to brown oils that
occur in volumes of up to 200 ml in the proventriculus in both sexes, even outside of the breeding
season, and also in young animals. They are fed
not only to the young but also to the sexual partner during courtship feeding. Finally, the stomach
oils are used in the defence reaction of this avian
order; this involves so-called oil spitting, which is
also seen in young animals. The composition of
the oil is very variable and differs between individuals of the same species. It can therefore probably be assumed that the oils have their origin in
foodstuffs, and their variability reflects that of the
diet. The most important components, which are
found in very different proportions, are TAG and
then alkyldiacylglycerols, wax esters and hydrocarbons. The TAG includes large amounts of the
fatty acids 20:5 and 22:6, just like in marine fats;
this further suggests that the origin of the oils lies
in nutrition. The components of the wax esters
are predominantly monounsaturated fatty acids
and fatty alcohols of CI4 to C22 • Hydrocarbons are
seldom found in large amounts but include,
above all, squalene, and also pristane (Fig. 15.12)
[109].
Lipid-rich nutritional secretions are also found
in the insects. The cockroach Diploptera punctata, which bears living young, releases a milky
secretion into the breeding sac and this brings
about a 60-fold increase in the weight of the first
larval stage compared with the weight of the egg.
The secretion contains proteins as 45 % of the dry
weight; 25 % is carbohydrate and 16-22 % lipids,
mainly phospholipids and cholesterol. A lipidrich secretion is produced in the Dufour glands of
some hymenopterans; these are associated with
the sting apparatus. This secretion serves as food
for the larvae of the solitary bees of the genus
Centris. It consists of odd-numbered alkanes with
19-29 C atoms, and in C. analis also contains wax
esters with C8 to C14 fatty alcohols and C2 to C14
fatty acids [32]. Anthophora abrupta secretes
from the Dufour gland a complex mixture of fluid
TAGs that always contains a palmitic acid residue
and two short-chain acyl residues (C2, C4 or C6).
The breeding cavities are lined with DAG, mainly
dipalmitoylglycerol. The easily transportable
TAGs produced in the Dufour gland are presumably hydrolysed and re-esterified for production
of the honeycomb, perhaps by enzymes from the
digestive glands. The pheromones produced in
the Dufour glands of the ants will be discussed in
Chapter 19.
15.3 Lipids of the Body Surface
The surfaces of terrestrial plants and animals are
covered with a lipid layer whose general function
is to protect the organism from unfavourable
environmental factors. In some groups of organisms, the main function is protection against
dehydration (plants, terrestrial arthropods),
whilst in others it is protection against water penetration (birds, mammals). In addition, the lipid
layer hinders the entry of bacteria and other foreign organisms, and may playa role in "chemical
communication" between members of a species.
The term "wax" is often used for the surface
lipids, although they include not only wax esters
but usually complex mixtures of very different
neutral lipids. Despite the structural variety, surface lipids share certain common properties that
can be considered as functional adaptations.
Independent of whether they are liquid or solid,
they always have very long hydrocarbon chains
and are thus relatively non-volatile. They contain
a relatively low number of double bonds, which
can be oxidized by atmospheric oxygen, and are
particularly resistant to hydrolytic and other
metabolic processes.
15 Lipids
long-chain fatty acids (C8 to C12) are found in the
milk of all terrestrial mammals, whereas butyric
acid (C4) is present only in the milk of ruminants.
The fatty acid spectrum in the milk of marine
mammals is similar to that ofthe storage fat [159].
The crop milk of pigeons (Columbiformes) is
in many ways comparable with mammalian milk.
Male and female pigeons feed the young with a
cheese-like white mass from the crop, the secretion of which, like the secretion of mammalian
milk, is regulated by prolactin from the hypophysis. Crop milk consists of fat-laden epithelial cells
which contain little carbohydrate but much protein and lipid. In addition to about 80 % TAG,
the lipids also include phospholipids and cholesterol. The fatty acid spectrum corresponds to that
of the body fat and consists of more than 90 % of
the usual CI6 and CI8 fatty acids [239]. The lipidrich oesophagal fluids of the flamingo Phoenicopteris ruber (total fat content 18 %) and of the penguin Aptenodytes forsteri (total fat content 29 %)
also serve to feed the young. In contrast, the
stomach oils found in the proventricules of the
albatrosses (Diomedeiae), petrels (Procellariidae) and stormy petrels (Hydrobatidae), but not
the diving petrels (Pelecanoididae), which form
the fourth family of the order of the tube-nosed
swimmers (Procelariiformes), have a variety of
functions. These are yellow to brown oils that
occur in volumes of up to 200 ml in the proventriculus in both sexes, even outside of the breeding
season, and also in young animals. They are fed
not only to the young but also to the sexual partner during courtship feeding. Finally, the stomach
oils are used in the defence reaction of this avian
order; this involves so-called oil spitting, which is
also seen in young animals. The composition of
the oil is very variable and differs between individuals of the same species. It can therefore probably be assumed that the oils have their origin in
foodstuffs, and their variability reflects that of the
diet. The most important components, which are
found in very different proportions, are TAG and
then alkyldiacylglycerols, wax esters and hydrocarbons. The TAG includes large amounts of the
fatty acids 20:5 and 22:6, just like in marine fats;
this further suggests that the origin of the oils lies
in nutrition. The components of the wax esters
are predominantly monounsaturated fatty acids
and fatty alcohols of CI4 to C22 • Hydrocarbons are
seldom found in large amounts but include,
above all, squalene, and also pristane (Fig. 15.12)
[109].
Lipid-rich nutritional secretions are also found
in the insects. The cockroach Diploptera punctata, which bears living young, releases a milky
secretion into the breeding sac and this brings
about a 60-fold increase in the weight of the first
larval stage compared with the weight of the egg.
The secretion contains proteins as 45 % of the dry
weight; 25 % is carbohydrate and 16-22 % lipids,
mainly phospholipids and cholesterol. A lipidrich secretion is produced in the Dufour glands of
some hymenopterans; these are associated with
the sting apparatus. This secretion serves as food
for the larvae of the solitary bees of the genus
Centris. It consists of odd-numbered alkanes with
19-29 C atoms, and in C. analis also contains wax
esters with C8 to C14 fatty alcohols and C2 to C14
fatty acids [32]. Anthophora abrupta secretes
from the Dufour gland a complex mixture of fluid
TAGs that always contains a palmitic acid residue
and two short-chain acyl residues (C2, C4 or C6).
The breeding cavities are lined with DAG, mainly
dipalmitoylglycerol. The easily transportable
TAGs produced in the Dufour gland are presumably hydrolysed and re-esterified for production
of the honeycomb, perhaps by enzymes from the
digestive glands. The pheromones produced in
the Dufour glands of the ants will be discussed in
Chapter 19.
15.3 Lipids of the Body Surface
The surfaces of terrestrial plants and animals are
covered with a lipid layer whose general function
is to protect the organism from unfavourable
environmental factors. In some groups of organisms, the main function is protection against
dehydration (plants, terrestrial arthropods),
whilst in others it is protection against water penetration (birds, mammals). In addition, the lipid
layer hinders the entry of bacteria and other foreign organisms, and may playa role in "chemical
communication" between members of a species.
The term "wax" is often used for the surface
lipids, although they include not only wax esters
but usually complex mixtures of very different
neutral lipids. Despite the structural variety, surface lipids share certain common properties that
can be considered as functional adaptations.
Independent of whether they are liquid or solid,
they always have very long hydrocarbon chains
and are thus relatively non-volatile. They contain
a relatively low number of double bonds, which
can be oxidized by atmospheric oxygen, and are
particularly resistant to hydrolytic and other
metabolic processes.