Ethologists have long been aware that animals are not uniformly sensitive to experience throughout their lives. 'Sensitive periods' are periods during development when experience has a particularly strong effect on later behavior. For example, it has been shown in birds from several taxonomic families that mate preference is based largely on the phenotypes individuals experience as fledglings, and that those preferences are only slightly altered later in life. Sensitive periods have been described in the ontogeny of behavior related to habitat, host, food, and mate preferences as well as communication (e.g., bird song) and homing.
The occurrence and timing of sensitive periods are shaped by selection such that an individual's behavior is influenced most by those experiences that most improve reproductive success. For example, young salmon (Onchorhyncus sp.) become particularly sensitive to experience with stream odors as their morphological transition from stream-dwelling forms (termed parr) to ocean-dwelling forms (termed smolt) is initiated. In this manner, salmon learn olfactory landmarks as they migrate toward the ocean that they can use later when they move back up those same streams to spawn. Similarly, some bird species have a sensitive period during the fledgling stage in which mate preferences are learned. The phenotype most frequently experienced during this period is that of the parent. Fledglings thus develop a preference for a phenotype that, by evidence of their own existence, represents a reproductively successful conspecific.
Sensitive periods can have important population and evolutionary consequences. Animals whose behavior is shaped early in development may not be able to adjust to environmental changes that occur within their lifetimes, even when adjustments would be beneficial. In a world in which environmental change has been accelerated greatly by human activity, the occurrence of a sensitive period may result in a potentially tragic kind of behavioral obsolescence. For example, in homing species such as sea turtles and salmon, individuals choose their spawning grounds based entirely on early experience, and typically do not alter those choices. Animals may consequently be 'trapped' into reproducing in suboptimal habitats if those habitats are degraded within their lifetime by human activity.
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