Chapter 16 Summary

This chapter extends the principles of animal communication to microbes, plants, and humans. The current tree of life recognizes three primary domains: Bacteria, Archaea, and Eukaryota. Single-celled microbes include bacteria, archaea, the polyphyletic eukaryotes lumped within the Protista, and some fungi. These organisms do not have male and female sexes, but they possess equivalent mate choice mechanisms to achieve genetically diverse offspring. Archaea and bacteria reproduce by binary fission and mitosis (simple chromosomal duplication) into two equivalent daughter cells, but they occasionally undergo the process of conjugation, in which two cells connect and unidirectionally transfer some genetic material. Ciliates (Protista) and fungi produce gametes via meiosis that are all of the same size (isogamy) but comprise two or more cryptic mating types. Only different types can fuse to produce a zygote. Many species have two mating types, and some of these species show an alternation of generations between clonal multicellular diploid and haploid organisms. Other species have several to many mating types, a strategy that primarily serves a self-recognition/self-incompatibility function to reduce inbreeding.
Microbes cooperate in several ways, using signals or cues to mediate the interaction. Bacterial populations must attain a certain density in order to obtain the public goods benefits of releasing prey digestion enzymes, toxins, surfactants, or virulence factors. They have evolved quorum sensing mechanisms to communicate population density. Each cell releases low levels of an autoinducer signal molecule that accumulates in the environment, and has tuned signal receptors on the cell surface to monitor autoinducer levels. When the concentration reaches a certain threshold, production of the public good product is triggered. This altruistic behavior is vulnerable to mutant cheaters that do not signal or produce the product, but high relatedness among nearby cells tends to keep cheating at a low level. Biofilms are dense aggregations of bacteria that form on many kinds of surfaces. Their tight adhesion protects them from antimicrobial toxins and predators. Extracellular chemical signals are responsible for attraction, settlement, and the polymer production that maintains the strong cohesive matrix. Both beneficial and manipulative interactions may occur in multispecific biofilms, and are probably mediated by cues rather than signals. Soil-dwelling myxobacteria and social amoebae have independently evolved kin discrimination mechanisms that enable more closely related cells to converge under poor conditions and form multicellular fruiting bodies. Only a fraction of the cells survive and contribute genes to the next generation. Some bacteria produce bacteriocins that are selectively toxic to less-related conspecific individuals, an example of spiteful behavior.
Plants are sessile multicellular organisms, and their modular branching structure enables them to forage, move, and behave in ways that maximize resource intake and avoid competitors. Sensory systems for detecting environmental information have been co-opted for communication among conspecifics, with other plant species, and with animal mutualists. Plants have three light detection systems: cryptochromes for regulating flowering time and circadian rhythms, phototrophin for sensing the incoming direction of light and light gradients, and phytochromes for detecting the presence of nearby plants. Plants detect several types of mechanical stimulation, including gravity, wind, physical wounding, and touch. Chemical detection does not involve an olfactory organ, as it does in animals, but occurs in a variety of tissues: root hair cells contain membrane-embedded carrier and channel proteins that take up specific ions in the soil; the stomata on the underside of the leaf takes in volatile airborne chemicals; and the waxy cuticle of the leaf surface absorbs lipophilic compounds.
Plants compete with adjacent neighbors for access to light and soil nutrients using several strategies. The phytochrome red/far-red shade-sensing system can detect the horizontal flux of light reflected of adjacent green plants before they cause real shade. Activation of this system leads to an aggressive response syndrome of stem elongation, reduced branching, and increased apical dominance to increase plant height. Alternatively, plants can use this detector system to avoid competition by reducing growth in the direction of the neighbor and increasing branch growth away from the neighbor. Other plants tolerate the shade of neighbors with adjustments in leaf area. Analogous strategies are used to avoid or tolerate root competition. Some species employ aggressive deterrence against root growth of competitors with toxic territorial signals.
Most plants reproduce sexually, but are hermaphroditic in the sense that male and female function reside in the same individual. In monoclinous flowering plants, male and female reproductive organs are contained in the same flower. Pollen grains on the anthers are the male gametes, which, like sperm, are mobile and move via wind or animal vectors among plant individuals. The pistil, comprising stigma, style, and ovary, contains the eggs. Pollen grains land on the stigma, germinate, and grow a pollen tube down the style into the ovary. There is both male–male competition among pollen for access to ovaries, and the potential for female choice of mates, as pollen tube growth requires interaction with the style. Many plants have a self-recognition system, the hypervariable S-locus, that blocks self-pollen from fertilizing the eggs in self-incompatible species. Incongruity is the process by which nonconspecific pollen is rejected. There is limited evidence that females promote the pollination success of genetically high-quality mates by sensing pollen tube growth performance. Sexual conflict is expected to be stronger in dioecious species (in which the sexes are on separate individuals) than in hermaphroditic species.
Plants employ several defensive strategies against herbivorous predators and parasites. Constitutive physical barriers and deterrents are the first line of defense, and include a tough cuticle, hard woody covers, sharp silica crystals, thorns, and trichomes (hairs) to directly impede herbivory. Secondary plant compounds that are toxic to herbivores are another line of defense. Wounding by an herbivore releases the plant hormone jasmonate, which is circulated through the plant’s vascular system to build up toxic defenses elsewhere. A final strategy involves plants crying for help by producing signals that attract predators of the herbivore. These herbivore-induced plant volatiles are specific to the herbivore, and predators learn to associate these volatile chemical mixes with their prey species. In highly sectorial plant species without a continuous vascular system, a similar chemical warning system may be used to prepare defenses in other parts of the same plant. Eavesdropping neighbors may detect these autocommunication signals.
Many plants rely on mobile animal vectors to disperse their pollen and seeds. In addition to offering rewards such as shelter, breeding sites, and roost sites, plants typically display multimodal visual and olfactory signals and offer food rewards to obtain these dispersal services. Flowers and fruits use pigments such as carotenoids, anthocyanins, flavonols, and betacyanins to produce colorful displays. Olfactory signaling components are more diverse and have the potential to target specific pollinators. Pollination requires that visitors show flower constancy by visiting multiple individuals of the same species to pick up and then unload pollen, so flower appearance varies significantly among simultaneously flowering species. Flowers show some evidence of pollinator syndromes—convergent appearance, color, and structure among species designed to attract the same type of pollinator (e.g., bee, beetle, butterfly, or bird). Fruit dispersal requires that visitors move the seeds far from the parent plant, so fruit consumer constancy is not favored. Plants have thus converged on a smaller number of fruit syndromes. Because fruits are unicolored and their signal and reward are derived from the same structure, there is greater potential for signal variants to honestly reflect reward quality. Flowers, on the other hand, have separately derived signaling (petals) and reward (nectar and pollen) components, so there is no constraint on honesty. Plants may cheat by failing to fill some portion of their flowers with nectar. Orchid species are the ultimate plant cheaters, employing a variety of nonrewarding and deceptive strategies to lure pollinators.
Human speech, characterized by voluntary utterances of arbitrary and learned symbolic sounds that are syntactically ordered and recombined with grammatical rules, has no animal precedent. It enables people to make strategic plans, imagine the future, negotiate exchanges of goods, and cooperate to achieve common goals. It likely evolved gradually in our Homo ancestors as social organization underwent the troop-to-tribe transition, and seasonal gatherings and trading networks increased among-group interactions and the exchange of technical information. Nonverbal communication signals, on the other hand, are more obviously derived from ancestral primate signals. The human nonverbal repertoire has expanded to meet the needs of more complex social interactions and is strongly integrated with speech, by some measures encoding 65% of the meaning of communication exchanges.
Emotions are episodic, short-term, biologically based patterns of perception, experience, physiology, communication, and action that occur in response to specific external events. Universal emotions such as fear, anger, happiness, sadness, surprise, disgust, and contempt are signaled with characteristic affect displays, including facial expressions and associated vocalizations. Such expressions are both bidirectional (event-triggered feelings produce the expression, called feedback, and enacting the expression induces some physiological responses) and regulated by mirror neurons (observing expression in another individual activates the same emotion in receiver). These combined processes synchronize dyadic sharing of emotions and produce emotional contagion effects in group situations, which mobilizes adaptive cooperative and defensive responses that benefit all parties. Other emotions in humans such as embarrassment, shame, remorse, and guilt promote reciprocity and honesty and reduce cheating and dishonesty.
Social power in human interactions refers to the ability to do what one wants without interference from others, to exert influence on others, and to avoid being influenced. Five bases of power are traditionally recognized: coercive power, legitimate power, reward power, expert power, and referent power. Dominance refers to the communication behaviors that are effective in exerting power and influence. Physical fighting ability rarely determines dominance in human societies, but taller height and deeper voice pitch do contribute to the perception of dominance. For most power bases, dominance is communicated by gestures of confidence, relaxed and open body postures, and conversational control. Turn-taking in conversations is managed by a set of regulators, including patterns of intonation, gazing, and body language that two people employ unconsciously to avoid speaking at the same time.
Deception is widespread in human interactions. Although people tend to believe the nonverbal cues and signals over verbal messages when information from the two sources is contradictory, nonverbal signals can also be used deceptively. Some facial expressions are difficult to fake, and true emotions may leak out in the form of fleeting microfacial expressions. There are two forms of smiling, the truly felt Duchenne smile and the social smile, distinguished by the eye and cheek components of the display. Duchenne smiling appears to be honestly related to altruistic intentions. Liars give deceitful or misleading verbal information to improve their reputation, gain resources, or avoid punishment, and simultaneously attempt to manage their nonverbal signals to promote the deception. The consequences of high arousal, intense cognitive processing, and attempted control efforts from telling a concocted story often yield leaked cues of lying to discerning receivers.<.li>
Nonverbal signals play an important role during mate attraction and courtship in humans. Humans are more sexually dimorphic than their closest primate relatives: males have greater height, more lean muscle mass and strength, more facial and body hair, and deeper voices than women. Women have fat deposits on breasts and hips, higher voices, and neotenous (youthful) facial features. Human females have lost specific signals of estrus (concealed estrus), but cues of estrus nevertheless leak out. Women show stronger preferences for slightly more masculine facial and body features when they are in the fertile phase of their cycle, but in general prefer average male features. Male facial and body characteristics most likely have been selected via intrasexual selection (male–male competition and dominance interactions) rather than via intersexual female choice. Female traits, on the other hand, have been selected primarily via intersexual male mate choice for fertile mates. The hourglass figure, a low waist-to-hip ratio in particular, is correlated with greater endocrine cycling and ovulation regularity, overall health, and ability to support fetal brain development, growth, and lactation. The initial stages of courtship in humans rely on assessments of beauty in women and resources and confidence in men, regulated by nonverbal signals such as darting glances, body postures, and touching. During the interaction phase of courtship, couples converse intimately. Men employ creative displays and seduction strategies to persuasively court the woman, while women are ambivalently interested and coy and attempt to learn about the resourcefulness, status, and paternal commitment of the man. Mate choice is thus usually mutual, except in societies where parents select mates for their offspring. Both sexes value intelligence, kindness, and a sense of humor in a long-term mate.