Common Orientation 



313 



ber of scavengers will be attracted. Other sources of food or shelter 

 similarly serve as a focal point at which animals from the surrounding 

 areas tend to congregate (Fig. 9.1). In these instances each individ- 

 ual reacts independently, and the group is formed as a secondary 

 consequence. 



Sometimes the aggregation is the result of more complex and long- 

 continuing reactions. Several kinds of marine worms form aggrega- 

 tions by reacting individually to some aspect of the lunar cycle and 

 swimming to the surface of the sea, as described in Chapter 6. When 

 the animals have formed a huge swarm in this way, they initiate their 

 breeding activity. Since the eggs and sperm are thus discharged 

 into the water at the same time and at close quarters, the chances of 

 successful fertilization are much improved. Consequently, the re- 

 action of the animals to the physical stimulus that leads to the forma- 

 tion of the aggregation has survival value in this instance. 



The dense schools of salmon preparing to breed in the headwaters 

 of streams have come into being following the orientation of each in- 

 dividual separately during the previous weeks while the fish was find- 

 ing its way for hundreds of miles up from the ocean (Fig. 9.2). 



Photo Inlernalional Pacific Salmon Fisheries Commission 



Fig. 9.2. Sockeye salmon in Adams River, British Columbia, schooling near bank 

 before moving onto the spawning beds. Note dead, spawned-out salmon washed 



up on the bar. 



