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Chapter 1 Summary: Signals and Communication

  1. While most people acknowledge that animals communicate, they often lack the background needed to recognize common patterns and principles amidst the enormous diversity of animal signals.
  2. The field of animal communication research, like other sciences, seeks to identify the basic principles that are required to explain and organize animal signal diversity. As is customary in modern science, these principles must be concordant with principles derived in other fields such as physics, chemistry, physiology, and economics, and they are proving general enough to provide insights for other disciplines such as psychology, medicine, conservation and wildlife management, and linguistics.
  3. All animals monitor the ambient physical, ecological, and social conditions around them. Because many of the relevant conditions are not directly measurable, animals use their sense organs to monitor cues that are correlated with the conditions of interest. Because cues are only correlated with conditions, they typically provide imperfect information.
  4. Signals are stimuli produced by one animal (the sender) and propagated to another animal (the receiver). Like cues, they are correlated with conditions of interest to the receiver, but unlike cues, which are generated inadvertently or as a by-product of some function other than communication, signals are generated expressly to provide additional information to receivers.
  5. Because all animals have evolved the sense organs and sampling behaviors required to monitor ambient cues, the evolution of signals is often a relatively simple process. A mutant sender displays a slightly exaggerated anatomical feature or performs a slightly enhanced version of some action that another animal is already monitoring as a cue. If both parties benefit by this exaggerated or enhanced display, subsequent coevolution of the sender trait and receiver perception leads to further exaggeration and refinement and a new signal is created.
  6. The relevant principles needed to explain animal signal diversity can be grouped into general categories: (A) those due to physical constraints imposed by the habitat in which each species lives and reflected in the physiological mechanisms that serve as preadaptations for signal generation and reception; (B) those due to different evolutionary histories for each taxon, that limit the kinds of preadaptations available for signal evolution; (C) those imposed by the economics of communication, which require that both senders and receivers benefit on average when exchanging signals; (D) those imposed by conflicts of interest between sender and receiver and the need for honesty guarantees in the design of signals; and (E) those imposed by the hordes of eavesdroppers that form the ambient communications network and may exploit the exchanges between any pair of animals.
  7. As with all other fields of biology, evolutionary theory provides the backbone for studying all aspects of animal communication. Basic evolutionary principles such as the genetic heritability of traits, sexual and natural selection for some variants over others, phylogenetic affinity, and the perpetual introduction of new variation through mutation are all critical to understanding the origins and maintenance of animal signal diversity.
  8. Animal communication systems can be classified in several ways. One axis focuses on the modality (light, sound, chemicals, touch, hydrodynamics, or electricity) used to create signal stimuli, the medium (air, water, or solids) through which the signal propagates, and the preadaptations for that modality and medium that are present in each taxon.
  9. A second axis for signal classification focuses on the alternative kinds of information that signals might provide. Since the sender knows more about its own status than any other animal, many signals provide information about sender conditions. Such information may include sender identity, sex, age, dominance status, and location. In some cases, the sender can provide information about the receiver that the latter cannot know on its own. Finally, the sender may provide receivers with information about other parties, such as predators or approaching conspecifics, or about objects, such as food.
  10. A third method of classifying signals focuses on the mechanisms by which signals are kept honest when sender and receiver have a conflict of interest. Index signals are constrained in ways that make it impossible for a sender to be deceitful. Handicap signals impose higher costs or lower benefits on deceitful senders. Receivers can punish deceitful senders using conventional signals, and in aggressive contests, confident senders can confirm their self-assessment by making themselves vulnerable with proximity signals. Where there is minimal conflict of interest between sender and receiver, no honesty guarantees are needed.
  11. A fourth axis classifies signals into one of four contextual categories that differ in the relevant payoffs that each party receives following communication exchanges. Usually, a species’ signal repertoire can be divided into its aggressive signals, mating signals, social integration signals, and environmental signals. Different species may have more or fewer signals in each category, but most species have at least a few signals of each type.
  12. Animals may use signals that mix categories both within and across the above classifications. Multimodal signals utilize two or more modalities at once, and each modality may invoke a different honesty guarantee. Birds often combine acoustic and visual displays, whereas insects combine visual and chemical components. Animals may also provide information about multiple conditions in the same signal: a frog’s call can indicate the caller’s body size and its species in the same signal. Finally, the same signal may be used in multiple contexts: the songs of many birds are designed both to repel male competitors and to attract potential female mates.
  13. All communication requires the same seven steps. For the sender, this consists of (1) generating an initial stimulus, (2) modifying it to ensure proper pattern, and (3) coupling it to the propagation medium. These three sender steps are followed by (4) propagation in the medium, which usually results in some distortion of the released signal, depending on its design, the modality, the medium, and the distance between sender and receiver. The final three steps occur at the receiver and are the reverse of those undertaken by the sender: (5) coupling the propagated signal from the medium into the receiver’s sense organs, (6) modifying it as necessary to improve detectability and resolution, and finally (7) identifying and classifying the perceived signal. Variations among species at each step contribute to the overall patterns of signal diversity seen in animals.