Szabolcs Számadó

Cost and conflict in animal signals and human language

The “costly signaling” hypothesis proposes that animal signals are kept honest by appropriate signal costs. We show that to the contrary, signal cost is unnecessary for honest signaling even when interests conflict. We illustrate this principle by constructing examples of cost-free signaling equilibria for the two paradigmatic signaling games of Grafen (1990) and Godfray (1991). Our findings may explain why some animal signals use cost to ensure honesty whereas others do not and suggest that empirical tests of the signaling hypothesis should focus not on equilibrium cost but, rather, on the cost of deviation from equilibrium. We use these results to apply costly signaling theory to the low-cost signals that make up human language. Recent game theoretic models have shown that several key features of language could plausibly arise and be maintained by natural selection when individuals have coincident interests. In real societies, however, individuals do not have fully coincident interests. We show that coincident interests are not a prerequisite for linguistic communication, and find that many of the results derived previously can be expected also under more realistic models of society.

Cheating as a mixed strategy in a simple model of aggressive communication

The possibility that frequency-dependent cheating can persist in an evolutionarily stable communication system has frequently been proposed. Although there is empirical evidence for this idea, however, it has not been investigated in terms of game theory. In the present paper I show for a simple symmetric game that cheating can be part of a mixed evolutionarily stable strategy (ESS). Furthermore, despite the widespread assumption that cheaters must be rare, I show that most of the population can be cheaters, while the signalling system remains evolutionarily stable. Consequences for signalling theory and experiments to detect such mixed ESS are discussed. Copyright 2000 The Association for the Study of Animal Behaviour.

How threat displays work: species-specific fighting techniques, weaponry and proximity risk

Whether threat displays can reveal information about the strength or condition of the contestants is a long-debated issue. Enquist (1985, Animal Behaviour, 33, 1152–1161) showed that communication of such information is possible by means of choice of action in aggressive encounters. The key assumption of Enquists model is that weak individuals signalling that they are strong (i.e. cheaters) cannot get away without fighting even if they want to, if they meet an honest strong individual. However, this assumption was not elaborated further and Enquists model is often cited in support of Zahavis handicap principle. Here I elaborate this assumption and show in terms of Enquists model, by introducing spatial distance between opponents as a continuous variable, that it is the proximity of the opponent, what I call ‘proximity risk’, that maintains the honesty of threat displays. I show that the honest use of threat displays, sensu Enquist, is evolutionarily stable only within a certain distance threshold. Outside this threshold there may or may not be a zone where a mixture of honest and cheating displays can be evolutionarily stable. Outside this second zone threat displays are unreliable and thus expected not to be used and attended to. The model gives specific predictions about weaponry, species-specific fighting techniques and the value of these thresholds. Finally, I show that key predictions of the model, namely the relation between signal intensity, riskiness and proximity has strong support in the literature.

Beneficial laggards: multilevel selection, cooperative polymorphism and division of labour in threshold public good games

BackgroundThe origin and stability of cooperation is a hot topic in social and behavioural sciences. A complicated conundrum exists as defectors have an advantage over cooperators, whenever cooperation is costly so consequently, not cooperating pays off. In addition, the discovery that humans and some animal populations, such as lions, are polymorphic, where cooperators and defectors stably live together -- while defectors are not being punished--, is even more puzzling. Here we offer a novel explanation based on a Threshold Public Good Game (PGG) that includes the interaction of individual and group level selection, where individuals can contribute to multiple collective actions, in our model group hunting and group defense.ResultsOur results show that there are polymorphic equilibria in Threshold PGGs; that multi-level selection does not select for the most cooperators per group but selects those close to the optimum number of cooperators (in terms of the Threshold PGG). In particular for medium cost values division of labour evolves within the group with regard to the two types of cooperative actions (hunting vs. defense). Moreover we show evidence that spatial population structure promotes cooperation in multiple PGGs. We also demonstrate that these results apply for a wide range of non-linear benefit function types.ConclusionsWe demonstrate that cooperation can be stable in Threshold PGG, even when the proportion of so called free riders is high in the population. A fundamentally new mechanism is proposed how laggards, individuals that have a high tendency to defect during one specific group action can actually contribute to the fitness of the group, by playing part in an optimal resource allocation in Threshold Public Good Games. In general, our results show that acknowledging a multilevel selection process will open up novel explanations for collective actions.

Long-term commitment promotes honest status signalling

Although there are many examples of status signalling in nature, the mechanisms that maintain the stability of these systems are still poorly understood. Here I show in terms of a simple game of aggressive communication that commitment to the need to defend a given resource repeatedly in the long term greatly increases the conditions under which signalling of strength by means of conventional badges can be honest and evolutionarily stable. Such commitment can efficiently prevent the invasion of potentially weak cheaters pretending to be strong at a very low cost of harassment suffered by such weak individuals. Moreover, as long as such commitment is present, this cost is not a function of the contested resource; thus the value of the resource can be orders of magnitude higher than the potential cost imposed on cheaters, yet the system remains honest and evolutionarily stable. While liar–strong strategies (i.e. Trojans) can invade under some conditions, in most cases there is a broad region of honesty and this honest region is always open ended towards large resource values. In this region of honesty, no other costs, either production costs or handicaps, are necessary to maintain the evolutionary stability of the system.

Honest and cheating strategies in a simple model of aggressive communication

The honesty of communication in competitive situations has long been debated. We investigated the coexistence of a diverse set of strategies in a simple model of aggressive communication by means of individual-based computer simulations. The game is an extended Hawk–Dove game in which there are two types of individual, weak and strong, and in which individual can communicate by means of cost-free signals before deciding whether to attack. The available strategies can be classified into three categories: honest, cheaters and those that ignore the signalling system. We found a diverse set of equilibria, most of them consisting of a mixture of honest and cheating individuals. We found that when starting populations consist of all strategies (1) the honest equilibrium can evolve, (2) communication is almost always present when signals are informative, and (3) strategies that ignore signalling are generally rare. Honest individuals need not be the majority in these populations yet communication will be present and stable in the long run. In contrast, the pure honest equilibrium is unlikely to evolve when the starting populations consist of strategies that ignore signals. Strategies that ignore signals are more frequent in these types of run however, signalling strategies are still present in the most frequently evolved equilibria. Even in this simple system two different kinds of use of signals can evolve: the first when signals refer to resource-holding potential and a second where signals are used to create a payoff-irrelevant asymmetry. In general, regardless of the starting conditions, a low resource value favours weak individuals, both honest and cheaters, and cowards, medium values favour strong individuals that use and listen to signals, and a high resource value favours strong individuals that ignore the signalling system and attack under all conditions. Although it is possible to find parameter combinations with a negative value of information, the value of information is positive in the overwhelming majority of equilibria. Thus one can conclude that for the majority of parameter combinations an equilibrium evolved that might not be honest, not even on average, but communication is present and signals are worth listening to.

The effect of dispersal and neighbourhood in games of cooperation

The prisoners dilemma (PD) and the snowdrift (SD) games are paradigmatic tools to investigate the origin of cooperation. Whereas spatial structure (e.g. nonrandom spatial distribution of strategies) present in the spatially explicit models facilitates the emergence of cooperation in the PD game, recent investigations have suggested that spatial structure can be unfavourable for cooperation in the SD game. The frequency of cooperators in a spatially explicit SD game can be lower than it would be in an infinitely large well-mixed population. However, the source of this effect cannot be identified with certainty as spatially explicit games differ from well-mixed games in two aspects: (i) they introduce spatial correlations, (ii) and limited neighbourhood. Here we extend earlier investigations to identify the source of this effect, and thus accordingly we study a spatially explicit version of the PD and SD games with varying degrees of dispersal and neighbourhood size. It was found that dispersal favours selfish individuals in both games. We calculated the frequency of cooperators at strong dispersal limit, which in concordance with the numerical results shows that it is the short range of interactions (i.e. limited neighbourhood) and not spatial correlations that decreases the frequency of cooperators in spatially explicit models of populations. Our results demonstrate that spatial correlations are always beneficial to cooperators in both the PD and SD games. We explain the opposite effect of dispersal and neighbourhood structure, and discuss the relevance of distinguishing the two effects in general.

Being human: Language: A social history of words

Language evolved as part of a uniquely human group of traits, the interdependence of which calls for an integrated approach to the study of brain function, argue Eors Szathmary and Szabolcs Szamado.Language evolved as part of a uniquely human group of traits, the interdependence of which calls for an integrated approach to the study of brain function, argue Eors Szathmary and Szabolcs Szamado.

Why does costly signalling evolve? Challenges with testing the handicap hypothesis

Zahavi’s handicap hypothesis (Grafen, 1990; Zahavi, 1975; Zahavi & Zahavi, 1997) is a popular explanation for the evolution of honest and costly signalling. The general idea is that individuals honestly signal their quality because signalling is costly and therefore low-quality individuals cannot afford to produce dishonest signals. However, this hypothesis is controversial for several reasons. (1) Zahavi suggested that selection favours the evolution of honest signalling because (and not despite) of their costs, and he made the radical suggestion that when it comes to the evolution of signalling, natural selection favours waste rather than efficiency. (2) Zahavi argued that this idea is a general principle, not merely a hypothesis, which explains honest signalling in most or all contexts. (3) There are several versions of the handicap hypothesis, but attempts to provide theoretical support have largely failed. The main exception is a model proposed by Grafen (1990), which has become widely accepted among behavioural ecologists; however, his conclusions have been challenged (Bergstrom, Szamado, & Lachmann, 2002; Getty, 1998, 2006; Hurd, 1995; Lachmann, Szamado, & Bergstrom, 2001; Szamado, 1999, 2000, 2011). (4) There have been many attempts to empirically test the handicap hypothesis, but there is no consensus regarding how it might be tested (Kotiaho, 2001). Despite these difficulties, Polnaszek and Stephens (2014) recently conducted a study with trained blue jays, Cyanocitta cristata, to experimentally test the handicap hypothesis. They concluded that their findings provide the first experimental evidence that signal costs enforce honesty, and they interpreted their results to support the handicap principle. This experiment is unusually clever and insightful, and the findings provide important implications for honest signalling and receiver psychology (Guilford & Dawkins, 1991). However, we raise several caveats about the theoretical background, interpretations and conclusions of the study, and we explain why this study and other attempts to test the handicap hypothesis will be problematic as long as there is not a clear theoretical model to test.

In silico Evolutionary Developmental Neurobiology and the Origin of Natural Language

It is justified to assume that part of our genetic endowment contributes to our language skills, yet it is impossible to tell at this moment exactly how genes affect the language faculty. We complement experimental biological studies by an in silico approach in that we simulate the evolution of neuronal networks under selection for language-related skills. At the heart of this project is the Evolutionary Neurogenetic Algorithm (ENGA) that is deliberately biomimetic. The design of the system was inspired by important biological phenomena such as brain ontogenesis, neuron morphologies, and indirect genetic encoding. Neuronal networks were selected and were allowed to reproduce as a function of their performance in the given task. The selected neuronal networks in all scenarios were able to solve the communication problem they had to face. The most striking feature of the model is that it works with highly indirect genetic encoding–-just as brains do.