The Human Anger Face Likely Carries a Dual-Signaling Function

摘要

Introduction Anger is an integral part of interpersonal aggression (Baumeister et al., 1990 ; Sell et al., 2009b ) and has a cross-culturally recognizable facial expression (Ekman, 1973 ). This expression typically entails simultaneously lowering one's browridge, raising the cheeckbones and mouth, widening the nose, and pressing the lips (Ekman and Friesen, 1978 ; Sell et al., 2014 ). Given these species-typical features, recent studies sought to reveal their signaling function. That is, what does the human anger face communicate? Sell et al. ( 2014 ) argued that the anger face mainly enhances facial cues of physical strength, thereby increasing the angry person's perceived fighting ability. In a paper published in the same year, Reed et al. ( 2014 ) argued that the anger face communicates the angry person's commitment to carry out threats. We believe that these two hypotheses complement each other to provide a more complete analysis of the signaling function of the anger face. In our discussion, we focus on men because interpersonal aggression is primarily a male activity (Puts, 2010 ). At the same time, though this opinion piece concerns the signaling function of face, we use research on vocal signals to build up our arguments. This is because (to our knowledge) a major component of this opinion piece, namely aggressive-intent signaling, has been mostly demonstrated with vocal signals. Aggressive signals Aggressive signals are naturally-selected structures or acts that communicate signalers' threat potential, including their resource-holding potential (RHP; e.g., physical strength) and aggressive intent (i.e., the willingness to escalate in a fight; Hurd and Enquist, 2005 ). Both types of aggressive signals are prevalent in animals (see below), and the use of those signals helps reduce the cost of combats. For example, adult red deer stags weigh ~330 pounds on average and carry large, piercing antlers, and both features are capable of causing serious physical damages. However, the annual rate of permanent injuries is ~6% among stags that engage in rutting fights (Clutton-Brock et al., 1979 ). This is partly because roaring contests, where two stags stand apart from and take turn to roar at each other, resolve ~50% of the fights on average (Clutton-Brock and Albon, 1979 ). The roaring contests can resolve conflicts of interest because stags' roars convey information predictive of their chance of winning a pending fight against each other, and stags use such information to make fight-or-flight decisions. Signals of RHP Stags' roars are an RHP signal because the roaring rate correlates with stag's physical condition (e.g., deterioration caused by aging; Clutton-Brock and Albon, 1979 ) and the minimum formant frequency of the roars correlates with stags' body weight (Reby and McComb, 2003 ). With all else being equal, stags that are in better conditions and/or heavier are more likely to win physical fights. Importantly, only stags in better physical conditions can roar faster because roaring fast is energetically demanding. Stags in worse conditions may be able to roar faster than its condition allows, but this cannot last long and will quickly exhausts the stags, impairing their ability to make the next move. As such, an energetic cost proportional to signalers' condition prevents weaker stags from faking greater RHP, and the honesty of roaring rates as an RHP signal is maintained (i.e., the handicap principle; Zahavi, 1975 ). At the same time, because body weight is almost impossible to fake, stags' roars also constitute an “unfakeable” index signal of RHP (Maynard Smith and Harper, 2003 ). Signals of aggressive intent Aggressive-intent signals broadcast one's willingness to escalate in combats, and, as RHP signaling, animals that signal stronger aggressive intent are more likely to win the contested resources without fighting (Searcy and Nowicki, 2005 ). Much research showed that aggressive-intent signals exist (e.g., Vehrencamp, 2001 ; Searcy et al., 2006 ; Ak?ay et al., 2011 ), contrary to earlier arguments (e.g., Maynard Smith, 1982 ) that aggressive-intent signals could not have evolved. Those arguments are based on the observation that the association between the form of most aggressive-intent signals (e.g., song singing) and their content (e.g., aggressive intent) is often arbitrary. This would render the signals prone to bluff and thus useless in the long run in resolving conflicts of interest. However, the retaliation-cost model (Enquist, 1985 ) suggests that aggressive-intent signals can be honest if they elicit aggression from signal receivers. Specifically, Enquist considered how an animal can use one of two signals, S (strong), and W (weak), to signal different levels of aggressive intent. S is more intense and more effective in repelling opponents than W is, and both strong and weak animals can use S and W equally well (i.e., the signals do not entail production or maintenance costs). In the