gestible material is excessive, or if it contains toxic 

 substances, regurgitation or vomiting may result. 



Size of food item 



The size of the food in relation to the animal is 

 not of major importance to many herbivores or 

 saprovores as they normally feed in or on the organ- 

 ism or substance. With carnivores, the size of the 

 prey must be within their power of conquest. Ordi- 

 narily the size of prey is less than that of the carni- 

 vore that feeds on it, but a high degree of ferocity and 

 audacity, or pack hunting in the manner of wolves, 

 often enables the carnivore to take prey larger than 

 itself. On the other hand a predator cannot profitably 

 prey on species so small that the energy derived from 

 its consumption does not equal the energy expended 

 in its capture. Some very large aquatic animals, how- 

 ever, have become adapted to feed with a minimum 

 of effort on very small organisms occurring in dense 

 concentrations through the evolution of a filtering 

 apparatus in their mouth parts. A good example is 

 the feeding of baleen whales on plankton. 



Availability 



In order to determine if a species is fed upon 

 in proportion to its abundance (McAtee 1932), it is 

 necessary to find out what animals have been eating 

 of that which is available in a habitat. The relation 

 between the two may be shown graphically (Hamil- 

 ton 1940a) and expressed as forage ratio: per cent 



of species in animals' food divided by per cent of spe- 

 cies in habitat (Hess and Swartz 1941). A value of 

 unity indicates that the food item is taken in propor- 

 tion to its abundance ; a value greater than unity in- 

 dicates that it is taken more frequently ; values of less 

 than unity indicate that the item is either inaccessible, 

 of the wrong size, too difficult to obtain, or is actu- 

 ally avoided. Table 13-1 is an example of such a 

 study conducted upon a carnivorous species. It is 

 apparent that while there is a relationship between 

 the relative abundance of various species and the 

 degree to which they were taken as food, there are 

 also several discrepancies. Weed-inhabiting organ- 

 isms, more accessible to fish than organisms buried 

 in the mud, are accordingly fed upon heavily, in dis- 

 proportion to their relative abundance in the total 

 fauna. The fingernail clam is fed upon heavily in 

 spring, since it lies on the surface of the mud. But 

 there is no explanation of why it is fed upon less 

 heavily during the summer. Water boatmen and 

 water mites, however, are generally not acceptable. 

 .Although the alderfly larva is a mud dweller, it is fed 

 upon in large numbers, suggesting that there may be 

 something in its behavior that makes it especially 

 vulnerable, or that brown trout have evolved special 

 methods for securing it. The caddisfly larva Lep- 

 tocerus also appears to be easily taken, as it is de- 

 voured in numbers greatly disproportionate to its 

 relative abundance. The mayfly naiad Caenis is not 

 much fed upon in spring, at which time it is buried 

 in the mud, but it is fed upon in large numbers in 

 summer, at which time it comes to the surface of the 

 water to emerge. Midge flies also become more vul- 

 nerable during the process of emergence. Zooplank- 



Food and feeding relationships 191 



