716 



ECOLOGY AND EVOLUTION 



Let us now consider some of the re- 

 ported examples of mutualism found be- 

 tween species of animals. 



Mc Anally and Phillipson (1944) state 

 that the diet of ruminants consists of large 

 quantities of cellulose and other carbohy- 

 drates that are indigestible except by the 

 aid of bacteria (p. 712), and that pro- 

 tozoans probably do not play a vital part 

 in the digestion of these materials. Species 

 of protozoans of the genus Diplodinium 

 inhabit the rumen of cattle, are capable of 

 digesting cellulose (Hungate, 1943), and 

 may be considered mutuaUstic. Other pro- 

 tozoan genera in the rumen of cattle do 

 not digest cellulose. 



In many ways the most remarkable ex- 

 ample of evolutionary mutualism known is 

 that between the intestinal flagellates of 

 wood-eating termites and roaches and their 

 hosts. Largely through the work of Cleve- 

 land (1924, 1925, 1925a, 1926, 1928), 

 Cleveland, Hall, Sanders, and Collier 

 (1934), Hungate (1938, 1939, 1941, 

 1944a), and Kirby (1937, 1944), we now 

 have a fairly clear understanding of the 

 mutualism that has evolved between these 

 insects and their intestinal flagellates. Ex- 

 perimental evidence is conclusive that an 

 interdependence between these associated 

 animals exists. Some of the protozoans seem 

 to be commensals or parasites in the hind- 

 gut of the insects, but a large number of 

 species ingest wood particles, and the cellu- 

 lose is broken down by the specific enzymes 

 cellulase and cellobiase in the body of the 

 protozoan. The resulting products, includ- 

 ing sugar (dextrose), are then squeezed 

 back into the midgut, where they are ab- 

 sorbed by the termite or roach. Neither 

 the roaches nor the termites that normally 

 harbor these intestinal flagellates can sur- 

 vive long without the protozoans unless 

 they are fed a diet other than cellulose. It 

 is now certain that the protozoans are the 

 agents that enable these insects to feed 

 upon hard wood. 



Cleveland, Hall, Sanders, and Collier 

 (1934) give many details of the morpho- 

 logical adjustments of the wood-roach 

 {Cryptocercus punctulatus) to the proto- 

 zoan faunule (Fig. 254). Because of the 

 remarkable functional adaptations of the 

 hindgut of Cryptocercus to the sym- 

 biotic protozoans, there would seem to be 

 no question that the evolution of man\ 



of these structures was the result of selec- 

 tion of the efiicient interspecies system as 

 a functional unit. The hindgut of the ter- 

 mites does not show such striking mor- 

 phological adaptations for harboring and 

 controlling the protozoa, but the physiolo- 

 gical adjustments are doubtless the result 



k 



Fig. 254. Alimentary canal of the protozoan- 

 harboring roach, Cryptocercus punctulatus, 

 showing position and structure of valves. Cen- 

 tral figure: flattened inner surface; A, B, C, 

 longitudinal sections; D, E, F, G, surface 

 views of portions of valves showing one or 

 more of the chitinous bars of which the valves 

 are composed. (From Cleveland, Hall, Sanders, 

 and Collier.) 



of a long evolution of the mutuaUstic re- 

 lationship. 



Hungate (1939) postulates the mutualis- 

 tic relations of termites and protozoans as 

 follows: 



" ( 1 ) The termite comminutes the wood and 

 transports it to the hind- gut where (2) the 

 protozoa digest it and (3) metabolize it 

 anaerobically, thereby deriving their necessary 

 energy and forming metabolic products which 

 (4) are absorbed by the termite and oxidized 

 to satisfy its energy requirements, an action 

 which not only serves the termite but also 

 aids the protozoa by (5) removing harmful 



