It is reasonable to hypothesize that the degradation of a signal, like a sound changing in intensity across distance, may be transformed into different types of signs, accordingly. For instance, acoustic cues like a song strophe or an alarm call may be effective when organisms are close to the source. However, in this signal degradation, an organism must apply a metric in order to evaluate the threshold of the sign efficacy. We could call such a type of sign a "dimension-dependent sign" that can be distinguished from a "nondimension dependent sign," like symbols or icons. Dimension-dependent signs may be acoustic, visual, or functional, like the amount of food in a place. Such signs are under the control of a process that utilizes a cognitive template for an immediate comparison, like the search image mechanism. For instance, when an organism is hungry, a sign of food presence is quickly identified, even when a low level of food availability (the signal) is offered.
There is evidence of sophisticated mechanisms in sign processing in terms of pattern recognition and learning capacity. For instance, the exposition to different live predators produces, in Black-capped Chickadees (Poecile atricapilla), different alarm calls (Templeton et al. 2005) and this demonstrates the capacity in this species to discriminate signals.
In many insects foraging for nectar or pollen, a learned fidelity to a plant species has been interpreted as a strategy for optimizing energy in food searching. For instance, Goulson et al. (1997) observed 85% flower fidelity in the butterfly Thymelicus flavus, which ignored the presence of other sources of food during the searching behavior. This process is a clear indication of the capacity not only to learn but also to modify the meaning of the sign.
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