Adaptation:

Also know as function, adaptive value refers to the increase in survival fitness and reproductive success of the animal due to its performance of the behavior.

 

The migratory behavior exhibited by the Pacific Salmon (Oncorhynchus) has been a focal organism behavior for many years. Migration of Pacific Salmon provides many benefits to those that do exhibit anadromous behavior. When Pacific Salmon reach the Ocean they avoid many pitfalls of river life. Migrating Salmon find abundant food sources in the ocean creating better growing conditions, are able to avoid higher summer river temperatures that correlate to low oxygen levels, and are able to easily locate mature mating partners. Pacific Salmon vary their migration to avoid high temperatures in rivers [13]. Pacific Salmon only feed in the ocean indicating that their primary objective in the ocean is feed and their primary objective in freshwater is mating.

            An alternate to the migrating behavior of Pacific Salmon exists as “sneakers,” which are males that do not migrate and use their small size, attributed to the lack of feed in the river system, to sneak in as the female drops her eggs. In order to understand the adaptive value of Migration we must observe the interaction of the two.

Innate competitive dominance:
           
            The growth of Salmon due to anadromity is considered the main adaptive function of migration, since it establishes the competitive hierarchy used to determine mating. In jack Salmon or “sneakers” (see top of page or Ontogeny) the dominance created by the larger size in migrating salmon is circumvented through a maintained small size, which allows jacks to remain hidden, and subsequently allows them to mate.

However, when jacks are paired with other jacks the males will fight in order to establish dominance[14]. As the number of sneakers grows the benefit of sneaking decreases, sneaking doesn’t work if there is no dominate male to guard the female from other males. This competitive behavior of the jacks implies that competition to establish dominance is an ancestral trait, which in turn aids in the understanding of migration’s adaptive value. Migration allows individuals to gain a substantial size advantage over sneakers and in a population full of sneakers this size advantage would easily create a dominant male which will be able to reproduce passing on the migration behavior. [14] has shown that differing populations of Pacific Salmon demonstrate different values of fitness for migration vs. sneaking. This indicates that the adaptive value is variable based on the cost-benefit of anadromous behavior.

Migration as a female coercion behavior:

            Coho Salmon (Oncorhynchus kisutch) is a species in which sneaking is the primary reproduction tactic[14], yet a significant number of Coho migrate. Their migration provides an interesting insight into the adaptive value of migration, since in this situation migrating is not to establish dominance in other males to gain female choice it is in order to coerce female choice by creating a cost for female avoidance of male coercion. Females of Coho Salmon prefer to mate with sneaker males, but regularly mate with Coho migratory individuals in order to avoid the energetic cost of her avoiding large males [15]. This indicates that the value of migration is to get large in order to have the strength to force a female to mate. There has even been an observed instance where a female was dragged back to the spawning nest in the jaws of a migratory Coho after leaving. The potential for injury by large males is enough to influence mate choice.
The differing adaptive values for the growth benefit of migration show how even the same behavior can have a radically different purpose between species.