The Evolution of Animal Communication: Reliability and Deception in Signaling Systemsby William A. Searcy, Stephen Nowicki
Gull chicks beg for food from their parents. Peacocks spread their tails to attract potential mates. Meerkats alert family members of the approach of predators. But are these--and other animals--sometimes dishonest? That's what William Searcy and Stephen Nowicki ask in The Evolution of Animal Communication. They take on the fascinating yet perplexing/i>
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Gull chicks beg for food from their parents. Peacocks spread their tails to attract potential mates. Meerkats alert family members of the approach of predators. But are these--and other animals--sometimes dishonest? That's what William Searcy and Stephen Nowicki ask in The Evolution of Animal Communication. They take on the fascinating yet perplexing question of the dependability of animal signaling systems.
The book probes such phenomena as the begging of nesting birds, alarm calls in squirrels and primates, carotenoid coloration in fish and birds, the calls of frogs and toads, and weapon displays in crustaceans. Do these signals convey accurate information about the signaler, its future behavior, or its environment? Or do they mislead receivers in a way that benefits the signaler? For example, is the begging chick really hungry as its cries indicate or is it lobbying to get more food than its brothers and sisters?
Searcy and Nowicki take on these and other questions by developing clear definitions of key issues, by reviewing the most relevant empirical data and game theory models available, and by asking how well theory matches data. They find that animal communication is largely reliable--but that this basic reliability also allows the clever deceiver to flourish. Well researched and clearly written, their book provides new insight into animal communication, behavior, and evolution.
"The book is well written and informative. . . . Searcy and Nowicki are well-known experts in the field of animal communication and sexual selection, and they provide a thorough and careful overview of this important, but often under-discussed, topic."R. Andrew Hayes, Austral Ecology
"The book is a fascinating evaluation of the present state of reliability and deception in animal signaling systems. It would make a perfect, albeit somewhat controversial, focus for an honors biology or graduate seminar course on animal communication."H.Jane Brockmann, BioScience
Katherine E. LeVan and Noah Wilson-Rich
R. Andrew Hayes
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The Evolution of Animal CommunicationReliability and Deception in Signaling Systems
By William A. Searcy Stephen Nowicki
Princeton University PressCopyright © 2005 Princeton University Press
All right reserved.
Whether signals are reliable or deceptive has been a central question in the study of animal communication in recent years. The crux of the issue is whether animal signals are honest, in the sense of conveying reliable information from signaler to receiver, or deceitful, in the sense of conveying unreliable information, the falsity of which somehow benefits the signaler. This issue arises in a variety of contexts. When a male courts a female, do his signals honestly convey his quality relative to other males? Or does he exaggerate his quality in order to win over females that would otherwise choose some other male? When one animal signals aggressively in a contest over a resource, does the signaler honestly convey its likelihood of attack? Or does the signaler exaggerate that likelihood in order to intimidate competitors that would otherwise defeat him? The question of reliability versus deceit arises even in interactions that, on the face of things, seem to be predominantly cooperative. When an offspring begs for food from its parents, does it honestly convey its level of need? Or does the offspring exaggerateits need in order to get more food than the parents would otherwise provide?
The issue of reliability and deceit in animal communication resonates with human observers for a variety of reasons. One is that the occurrence of deceit is fraught with moral implications. In the view of many, human communication is permeated with deceit. Do humans stand apart in this regard, or are other animals as bad or worse? The answer might have considerable effect on how we view ourselves, as well as on how we view other animals. A second reason for interest in this issue is that the occurrence of deceit, if deceit is defined appropriately, can have considerable implications for our understanding of animal cognition. Some definitions of deceit are framed so as to require cognitive processes of considerable sophistication, such as the ability to form intentions and beliefs and to attribute beliefs to other individuals. If we employ such a definition, and if we can then determine that nonhuman animals deceive each other according to this definition (a big "if"), then we have provided support for a greater level of cognitive capacity than many earlier views of animal behavior have allowed.
Our own interest in reliability and deceit revolves around neither morality nor cognition, but instead derives from the evolutionary implications of the issue. The way one expects animal communication systems to function in terms of reliability and deceit depends on how one views the operation of natural selection. Early students of animal behavior often assumed implicitly that selection operates at the level of groups, so that behavior evolves toward what is best for the population or species as a whole, leading to the view that animal communication consists primarily of the cooperative exchange of reliable information. The predominant view nowadays, however, is that selection acts largely at the level of the individual, so that behavior evolves toward what is best for the individual performing the behavior, and not toward what is best for the group. If behavior is commonly selfish, in this sense, then it is not always obvious why animals should exchange information cooperatively. Instead, one might expect many instances in which signalers would attempt to profit individually by conveying dishonest information. But because individual selection works on the receiver as well as the signaler, receivers ought to respond to signals only if doing so is to their advantage, on average. Therefore, if dishonesty is common, it also is not obvious why receivers should respond to signals.
Taking the argument one step further, if receivers fail to respond to signals, it is not obvious how signaling systems can exist at all. Thus if one accepts the view that selection acts predominantly at the level of the individual, as we do, and if one at the same time accepts the idea that animals do communicate with each other, as seems obvious, then one is left with a series of evolutionary puzzles. Are animal signals in reality reliable or unreliable? If animal signals are reliable, what mechanisms maintain reliability despite the tempting advantages of dishonesty? If animal signals are deceitful, do receivers respond to them anyway, and, if so, why? Our principal purpose in this book is to work through possible answers to evolutionary puzzles such as these.
Definitions Before we get to these puzzles, we need to define some terms. First, we need to define what we mean by "signal," in order to delimit the set of traits whose honesty and dishonesty we will examine. In one of the first rigorous evolutionary analyses of communication, Otte (1974, p. 385) defined "signals" as "behavioral, physiological, or morphological characteristics fashioned or maintained by natural selection because they convey information to other organisms." Otte explicitly rejected group-selectionist explanations for the evolution of traits, so in his view the transmission of information had to confer some reasonable advantage on the signaler itself in order to satisfy the definition. Thus Otte excluded as signals those traits that convey information to predators or parasites without any benefit to their possessors; he cited the chemicals in human sweat that attract disease-carrying mosquitoes as a possible example. Otte also rejected as signals those traits, such as body size, that may be used by other individuals of the species to assess their possessors but did not evolve for that function. Clearly included under Otte's definition would be vocalizations, color patterns, and body movements that have evolved because they transmit information in a way that benefits the individual that exhibits those traits. More ambiguous are traits, such as the form of a bird's tail, that originally evolved for some other function but have been modified by selection for information transmittal. We will regard such traits, or more precisely their modified properties, as signals; thus the bird's tail itself is not a signal but the tail's length is, if that length has been exaggerated beyond its aerodynamic optimum in order to influence receivers.
This brings us to our definitions of reliability and deceit. In everyday English, "reliable" means that "in which reliance or confidence may be put; trustworthy, safe, sure" (Little et al. 1964). An animal signal, then, would be reliable if one could have confidence in its veracity, or truthfulness-if, that is, one could trust the signal to convey whatever it is supposed to convey. The difficulty with this formulation is in ascertaining what the signal is "supposed to" convey. "Supposed to" in this context must be interpreted from the viewpoint of the receiver rather than the signaler; what matters is whether the signal conveys something that the receiver would benefit from knowing. If we are certain what it is that the receiver benefits from knowing, such as some attribute of the signaler or its environment, then we can ascertain the reliability of the signal by measuring the correlation between the signal and the attribute of interest.
Suppose, for example, that we think that female frogs are interested in the size of conspecific males, and we find that calls communicate information on male size by a negative correlation between call frequency and caller size (males with deeper croaks are larger). We can then determine the reliability of this information by measuring the correlation between call frequency and caller size. The trouble is that we can never really be certain that caller size is what the females "want" or "need" to know. Even if we can show that call frequency is well correlated with caller size, and that the females show a behavioral preference for calls of lower frequency, we cannot be sure that their true interests are not in some other characteristic-perhaps, in this example, male age. The best we can do is to measure as carefully as we can the benefits that the receivers obtain from different types of information. If we can show that female frogs benefit from mating with larger males but not from mating with older ones, we at least can have some confidence that size is what matters to the receivers, and then evaluate reliability of call frequency in terms of its correlation with signaler size.
To formalize this definition, we suggest that an animal signal is reliable if:
1. Some characteristic of the signal (including, perhaps, its presence/absence) is consistently correlated with some attribute of the signaler or its environment; and
2. Receivers benefit from having information about this attribute.
A remaining problem is how to specify what we mean by "consistently correlated." We can never expect a perfect correlation between signal characteristic and the attribute being signaled. Even if signalers are striving for perfect honesty, errors must be expected in the production of the signal and in our measurements of it, either of which would prevent our observing perfect reliability. How good, then, does the correlation have to be for us to conclude that the signal is on the whole reliable? One answer is provided by the concept of "honest on average" (Johnstone and Grafen 1993, Kokko 1997). A signal can be considered honest on average if it contains enough information, sufficiently often, that the receiver on average is better off assessing the signal than ignoring it. Consider again the example of male frogs communicating their size to females via the frequency of their call. The correlation between male size and call frequency can never be expected to be perfect, and in reality is often rather low (see chapter 4). The male's call can be considered honest on average if the correlation between male size and call frequency is good enough that the female benefits on average from using the call to assess male size, instead of ignoring this signal feature. In practice, it will be difficult to determine whether this criterion is being met, but at least it provides a theoretical standard against which reliability can be judged.
A simple way to define "deceptive" would be as the opposite of reliable, but for many the concept of deception carries more baggage, and consequently requires a more complex definition. A relatively simple definition of deception is provided by Mitchell (1986, p. 20), who suggested that deception occurs when:
1. A receiver registers something Y from a signaler;
2. The receiver responds in a way that is appropriate if Y means X; and
3. It is not true here that X is the case.
Note that the definition requires specifying what the signal (Y) means to the receiver. The meaning of Y to the receiver is judged by the response of the receiver to Y together with an observed correlation between Y and X, across many such signals. In other words, we infer that Y meansX to the receiver because signalers usually produce Y in association with X, and because the receiver responds to Y in a way that is appropriate if X is true. To make this more concrete, let Y be an alarm call given by the signaler. The alarm call is usually produced when a predator (X) is present, and the receiver typically responds to the alarm call by fleeing, an appropriate (i.e., beneficial) response if a predator is indeed nearby. Deception occurs if the signaler produces the alarm and the receiver reacts by fleeing when in fact no predator is present.
A difficulty with Mitchell's (1986) definition, which he himself points out, is that deception so defined cannot be distinguished from error on the part of the signaler. If the signaler has produced an alarm in error, would we want to call such an action deceptive? This problem can be solved if the definition of deception further stipulates that the signaler benefits from the receiver 's response to the signal. Mitchell (1986) himself is uncomfortable with the notion of benefit, remarking that the "idea of benefit is taken from human affairs" and when applied to nonhuman animals typically refers to what a human observer "believes is good for them." For an evolutionary biologist, however, "benefit" has a straightforward meaning-an individual benefits from an action if that action increases the individual's fitness, in the sense of the representation of the individual's genes in subsequent generations. Benefit in this sense is not an anthropocentric idea, but one that applies equally well to all organisms. With the added stipulation about a benefit to the signaler, we will define deception as occurring when:
1. A receiver registers something Y from a signaler;
2. The receiver responds in a way that
a. benefits the signaler and
b. is appropriate if Y means X; and
3. It is not true here that X is the case.
Deception defined in this way has sometimes been termed "functional deception" (Hauser 1996), meaning that the behavior has the effects of deception without necessarily having the cognitive underpinnings that we would require of deception in humans.
Other definitions specify that deception must have more complex cognitive underpinnings, that is, that the signaler has an "intention" to cause the receiver to form a false "belief" about the true situation (Russow 1986, Miller and Stiff 1993). Deception defined in this way has been termed "intentional deception" (Hauser 1996). "Intentions" and "beliefs" are mental states, and as such are difficult to measure in nonhuman animals, to say the least. Whether animals possess such mental states, and whether they can ascribe them to others, is of great interest to philosophers (Dennett 1988) and cognitive ethologists (Cheney and Seyfarth 1990, Seyfarth and Cheney 2003, Byrne and Whiten 1992), as well as to the general public. A major goal of some researchers studying deception in nonhuman animals is to use this type of interaction as a window onto the mental states of those animals, in an effort to determine whether they do indeed form intentions, beliefs, and so forth. Although we applaud such efforts, we repeat that our own interests lie elsewhere, in the analysis of reliability and deceit from a functional, evolutionary viewpoint. Another way of saying this is that we are interested in how natural selection shapes animal communication to be either honest or dishonest. From this viewpoint, the question of mental states is largely irrelevant; the costs and benefits to the signaler of giving a false alarm, and to the receiver of responding, ought to be the same whether or not the signaler is able to form an intention and the receiver to form a belief.
Another issue in defining deception is whether to include the withholding of signals. Some authors have argued in favor of this inclusion, suggesting that under certain circumstances, a failure to signal can be considered just as deceptive as producing a dishonest signal (Cheney and Seyfarth 1990, Hauser and Marler 1993a, Hauser 1996). Hauser (1996), for example, states that if an animal fails to produce a signal in a certain context in which that signal is typically produced, and if the animal benefits from failing to signal, that failure constitutes functional deception. This idea seems to us to have little application to a large majority of signaling contexts, such as those involving aggression or mate choice, in which cooperation is not expected from the interactants. In practice, the idea that withholding information is deceptive has most often been applied to cooperative interactions, most notably to interactions in which an animal signals the discovery of a food source to others of the same species (Hauser and Marler 1993a,b). Even here, the concept seems to us to be problematic. Say, for example, that a signaler follows the convention of calling when it finds a large amount of food, more than it can eat itself, and not calling when it finds a smaller amount. The signal then is consistently correlated with an aspect of the environment that receivers benefit from knowing, and so meets our criteria for reliability. Of course the receivers would be even better served by knowing more (i.e., from hearing about the small amount of food as well), but the signaler has not broken its convention in denying them this information.
Excerpted from The Evolution of Animal Communication by William A. Searcy Stephen Nowicki Copyright © 2005 by Princeton University Press. Excerpted by permission.
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Peter McGregor, Cornwall College
Robert Seyfarth, University of Pennsylvania, author of "How Monkeys See the World: Inside the Mind of Another Species"
Meet the Author
William A. Searcy is the Robert E. Maytag Professor of Ornithology at the University of Miami. He is the author, with Ken Yasukawa, of "Polygyny and Sexual Selection in Red-Winged Blackbirds" (Princeton). Stephen Nowicki is Bass Fellow and Professor of Biology, Psychological and Brain Sciences, and Neurobiology at Duke University, where he currently serves as Dean of the Natural Sciences. He has published more than 65 scientific papers on animal communication and behavior, including work on birds, insects, spiders, and mammals.
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