12 FLEAS, FLUKES AND CUCKOOS 



fish such as sword-fish and sun-fish and are carried about with them, 

 Hterally taking the food from their mouths. 



In most cases of commensaHsm it is easy enough to see why the 

 active beneficiary cHngs to the association, but it is far more difficult to 

 understand why the passive partner tolerates the other's presence. One 

 is sometimes tempted to beheve there must be a psychological explana- 

 tion which cannot be observed from the outside. 



For instance, a man from Mars would find it a simple matter to 

 understand the relationship between a shepherd and a sheep dog, or a 

 hunter and his hounds. On the other hand, he might well be puzzled 

 at the seemingly one-sided benefits enjoyed by pekinese and pug-dogs. 

 At present no one is in a position to say whether or not the starfish 

 receives agreeable sensations from the worm wriggling up its ambulacral 

 grooves. Is it possible that a remora, twitching in their buccal cavities, 

 assuages feelings akin to loneliness and boredom with which the sun- 

 fish and sword-fish are otherwise afflicted ? We do not know. 



On the whole commensal relationship among birds is unusual and 

 when it does occur it is chiefly limited to the breeding season. It is, 

 however, common knowledge that many species of birds associate in 

 flocks, especially for feeding and during migration. The significance o 

 most of these associations — if indeed they have any special signific- 

 ance — is completely unknown. In the case, however, of wigeon and 

 brent geese, which are often seen together at their feeding grounds, 

 the duck can be described as commensals, for on such occasions they 

 feed on the debris of eel grass [Zoster a) which the geese have pulled 

 up and broken into convenient lengths. Nevertheless, the wigeon 

 are quite capable of breaking up the ^ostera themselves if there are 

 no geese about. In Lapland, waxwings have been seen feeding on 

 the berries of mountain ash, and scattering a quantity of pulped fruit 

 on the ground below. There, flocks of redpolls were waiting to 

 eat up the debris. They appeared unwilling to feed on the whole 

 berries and followed the waxwings from tree to tree. In south east 

 Africa, Swynnerton has watched swallows and bee-eaters accompany- 

 ing a party of bulbuls (Pycnonotidae) which were feeding on ripe 

 guavas. They were catching the insects accidentally disturbed by 

 the bulbuls. 



In more familiar surroundings a casual commensal relationship may 

 be observed between the robin and the mole. As the mole tunnels just 

 below the turf and throws up fresh soil in the form of the familiar mole 



