J ^4 Comparative Animal Physiology 



emergence and normal appearance of the scales, and lack of linoleic acid 

 causes an appearance similar to that of certain mutants; possibly a given gene 

 controls linoleic acid metabolism, and hence scale production (Fig. 28). 

 Linoleic acid synthesis occurs in Tenehrio but not in Ephestia, as shown by 

 the following figures: 



LINOLEIC ACID CONTENT OF FAT" 



Tenehrio Ephestia 



with linoleic acid in diet 20 per cent 15 per cent 



without Hnoleic acid in diet 10 per cent 1 per cent 



Rats on a low fat diet develop hematuria, loss in weight, and scaly feet and 

 tail, all of which symptoms are then cured by feeding linoleic acid as sodium 

 linoleate, about 30 mg. per day. Linolenic acid cannot be substituted, and 

 vitamin E or some other antioxidant must be present for proper utilization of 

 the linoleate. ■"^' ^^' ^'' 



Men have lived well for six months on a low fat diet which would have 

 been insufficient for rats; however, their blood concentration of linoleic acid 

 declined; hence man needs linoleic acid in metabolism, but can synthesize 

 relatively more of it than the rat can. ^■' Chickens also grow well on a diet 

 containing less than 0.1 per cent of fat, and the depot fat is more saturated 

 under this condition than on a normal fat ration. "^ Apparently, then, the 

 ability to synthesize fatty acids is universal among animals, but Hnoleic acid 

 is not synthesized in sufficient amounts by some species and hence is a dietary 

 essential for them. 



Cholesterol. Sterols are commonly associated with fatty acids in a variety 

 of lipoidal compounds, and by far the most common animal sterol is cholesterol. 

 As with the fatty acids, there is much variation in ability to synthesize the 

 needed cholesterol, and some species require it in their diet. For example, 

 Lwoff ^^' ^-^ states that cholesterol is not needed by free-living Protozoa or by 

 primitive trypanosomes (Strigoynonas, Leptomonas), but it is needed by 

 specialized trypanosomes (Trichonwnas and Eiitrichomastix). ^^ Probably all 

 insects, beetles (Lucilia, Triholium), flies (Drosophila), and roaches (Blatella) 

 require dietary cholesterol (Table 24). Dermestes, normally a wool feeder, 

 uses only animal sterols, not ergosterol or sitosterol, whereas flour beetles do 

 well on the plant sterols; none require calciferol (vitamin D). ^^ Drosophila 

 can use ergosterol or cholesterol but not calciferol. ''■' Larvae of Aedes aegypti 

 fail to metamorphose without some lipid in the diet, a need which is satisfied 

 by cholesterol or lecithin but not by fatty acids. ^^' Vertebrates can synthesize 

 the cholesterol which they need. 



It was formerly believed that the sterol components of invertebrate animals 

 depended largely on the diet, herbivorous animals having predominantly 

 higher sterols (C2;)) and carnivorous animals having lower sterols like chole- 

 sterol CC07). This dietary correlation now appears not to hold, but the sterols 

 of different animals are often complex and may be useful in taxonomy of some 

 groups. '^^ '■' For example, gastropod and cephalopod molluscs contain mainly 

 cholesterol, whereas pelecypods have mixtures of higher melting point sterols, 

 mostly with 28 carbons. Sponges contain a mixture including Ci>7, Cjh, and 

 Cj<, sterols, some of which are unique and may be useful in tracing taxonomic 



