Chapter 14 Summary

Prey may use predator deterrent signals to dissuade predators from attacking them at any of the typical stages in a predator/prey interaction: detection, classification, interception, attack, and consumption. Predators should attend to such signals only if there are likely mutual benefits to doing so, and the signals include honesty guarantees.
Prey may try to avoid detection by using different forms of camouflage, including crypsis (blending into the background), masquerade (mimicking inedible objects), or decoys (placing preylike structures nearby). Once a prey determines that it has been detected, it may have time to signal to the predator that it is an unsuitable or less suitable target. Predator detection signals indicate to stalking predators that they have lost any element of surprise and may do better to abandon further approach. Fish, small birds, and ungulates often perform predator inspection as a group while emitting detection signals. Prey condition signals are given to coursing predators to demonstrate relative agility, health, or energy reserves. Honesty of predator detection and condition signals is guaranteed by the use of indices or handicaps. Prey that are unpalatable, toxic, or physically armed may declare their unsuitability with conspicuous aposematic signals. These are conventional signals whose coding scheme must be learned by predators through experience. Since those prey who provide that experience pay the costs but may gain no immediate direct benefit, the evolutionary economics of aposematic signals are complicated. Aposematism often leads to mimicry rings in which many unpalatable species (Müllerian mimics) and more palatable cheaters (Batesian mimics) converge on a common set of signals. Once under attack by a predator, some species produce deimatic displays that use unexpected colors, patterns, movements, or sounds to startle and distract the predator. Distress signals may attract other predators who interrupt the attack of the first and increase the chances that the prey can escape in the ensuing tussle.
Alarm signals warn nearby prey of the presence of predators. Possible compensatory benefits to senders to justify the costs of alarm signals include mutualistic coordination of flight, by-product mutualism of predator detection calls, nepotistic benefits to nearby kin, reciprocal surveillance by members of stable groups, subsequent mating advantages, status enhancement, or manipulation of nearby prey to minimize sender risks.
Most species face a trade-off between surveillance for predators and foraging. In some species, any member in a group may interrupt foraging to survey for predators; such species often show synchronous surveys since no member wants to be conspicuously delayed if the group flees. In other species, group members take turns as sentinels while others concentrate on feeding. Sentinels give alarm signals when predators are detected and may give regular “all clear” signals when not. They minimize costs by taking on sentinel duty only after feeding to satiation, selecting safe surveillance stations, and using alarm signal designs whose source is difficult for predators to locate. Terrestrial birds and mammals may have up to five distinct alarm signals (although two is most common), and some continuous variation within a signal type. Alarm signal variants are more often associated with appropriate flight strategies and urgency than with predator class.
Whereas surveillance alarms usually cause nearby prey to flee, mobbing signals attract prey for inspection, harassment, and attack of predators. In several bird species, neighbors contribute to mobbing only if the callers contributed to prior mobbing events, suggesting that reciprocity may play a role in the economics. Victim signals are emitted by prey once they are contacted by a predator. Many aquatic animals respond to schreckstoff, chemicals released from damaged conspecifics and even from heterospecifics. While these chemicals act as cues in many cases, some frogs actively secrete victim signals when attacked. Social insects such as aphids and treehoppers have pheromonal and acoustic victim signals that alert nearby kin. Bees, ants, wasps, and termites all release victim pheromones that attract colony mates, mark the apparent threat, and induce conspecifics to attack it.
A large number of aquatic and terrestrial species attend to the alarm signals of other sympatric species. This is particularly common in mixed-species bird flocks and monkey troops. Taxonomic relatedness is not a necessary condition for this eavedropping, and there are many examples where reptiles, birds, and mammals attend to the alarm signals of another class. False alarms are surprisingly common in both single species and mixed-species assemblages. Models show that they can be increasingly tolerated economically as group size increases, predator attacks become less common, or group responses provide an index of veracity. Templates for the production and recognition of conspecific alarm signals are largely innate; experience is needed to refine and focus these signals on appropriate entities. Recognition and interpretation of heterospecific alarms is nearly always learned.
Like alarm signals, food advertisement signals impose immediate costs on senders and confer immediate benefits on receivers. The list of economic compensations to senders for providing food signals is nearly identical to that for alarms. Nevertheless, food signals are much less common than alarm signals, and food signalers are much more selective than alarm signalers about when and to whom they give their signals.
The emission rate or call structure of food signals usually reflects food preferences and only rarely food type. The location of a food find is often as important as its preferability. Many species remain at the find and broadcast a signal to recruit fellow foragers. This type of advertisement is especially vulnerable to eavesdropping by unintended receivers. Some ants, birds, and bats minimize eavesdropping by returning to a communal roost or nest, recruiting foragers with special signals, and then leading them back to the food find. Ants, termites, and stingless bees lay pheromone trails between food finds and colony nest sites. This can lead to mass recruitment, in which returning workers reinforce the trail if the food find is valuable, and this in turn recruits even more workers to follow it. Trails are vulnerable to both conspecific and specialized heterospecific eavesdroppers. Honeybees do not use pheromone trails, and thus avoid any eavesdropping. Instead, they return to the nest and perform a waggle dance that provides recruits with the approximate azimuth and distance to a food find. The cost is a high cognitive burden on both senders and receivers.
Autocommunication occurs when sender and receiver are the same individual. The relevant economics can ignore honesty guarantees and focus on signaling efficacy. Examples include electroreception in mormyrid and gymnotid fish, and echolocation in bats and toothed cetaceans (some whales and all porpoises). In each of these groups, efficacy is extremely high, close to the limits imposed by the laws of physics.
Both bats and cetaceans echolocate with frequencies (12–200 kHz) whose wavelengths are similar to the sizes of their prey. Both use short-duration pulses to prevent an outgoing pulse from overlapping and masking a returning echo. Pulse durations of bats (0.5–25 msec) are long enough for them to modify pulse fine structure according to contextual needs: the pulses of open-country bats tend to be long in duration, with narrow bandwidths (CF) for the detection of small and distant prey; during interception, these shift to shorter-duration frequency-modulated (FM) pulses with wide bandwidths to monitor the distance, angular location, size, and shape of targets. Because sound in air does not penetrate solid targets, bats cannot determine target compositions. Bats that glean static prey close to vegetation both search for and intercept prey using FM pulses at lower amplitudes and often use additional sound, odor, or visual cues to complement echolocation information. Those hunting moving prey close to substrates exploit Doppler shifts to detect prey movements. Bats foraging near to substrates often emit their pulses through elaborate nose leaf structures to create directional sound beams and recover the echoes using elaborately directional pinnae.
The higher speed of sound in water requires echolocating cetaceans to use even shorter-duration pulses than those used by bats. However, most species use pulses (0.03–0.10 msec) far shorter than necessary to avoid pulse/echo overlap. These short-duration pulses generate very broad bandwidths without the detailed within-pulse regulation required by bats. The wide bandwidths are needed for the determination of target distance, angular location, size, and shape. Sound in water usually penetrates cetacean targets, and the resulting interactions during echo formation allow cetaceans to assess target composition. The reduced attenuation of sounds propagating in water allows cetaceans to detect prey at large distances without the long-duration CF pulses used by bats. All toothed cetaceans create highly directional echolocation beams using the oily melons on their foreheads, and most achieve directional reception using fatty channels in their lower jaws.