In the dynamic between predator and prey, survival often depends on communication. A prey communication device (PCD) is any structure or behavior that transmits a signal to a potential hunter, influencing the predator’s decision to attack. This presents a biological paradox: why would a vulnerable animal deliberately draw attention to itself? Advertising one’s presence can sometimes be less costly than being silently pursued and captured. By sending a clear message, prey animals attempt to manage the interaction and deter the attack before a chase begins.
Defining Communication Devices Used by Prey
A Prey Communication Device (PCD) is an evolved signal used to transmit information specifically to a predator to alter its hunting behavior. Signals operate under signaling theory, meaning the message is either honest or deceptive. Honest signaling accurately reflects a trait of the prey, such as its speed or toxicity. Since the display is often costly to produce, weaker individuals are prevented from faking it. Deceptive signaling attempts to mislead the predator, such as when a harmless species mimics a dangerous one. The PCD’s effectiveness depends on the predator’s ability to learn and interpret the signal.
Diverse Mechanisms of Signal Transmission
Prey animals employ various physical and chemical methods to transmit anti-predator messages.
Visual Signals
Visual signals are often the most conspicuous, frequently involving bright, contrasting colors known as aposematism. Aposematism advertises a chemical defense or unpalatability. Visual communication also includes specific body postures or rapid movement patterns, such as the sudden flashing of a hidden color patch. These displays are designed to be immediately noticeable, maximizing recognition speed by the predator.
Chemical Signals
Chemical signals are widely used, especially in aquatic environments and among insects, utilizing specialized molecules called pheromones. Certain fish, like minnows, release alarm pheromones, often called Schreckstoff, when their skin is damaged during an attack. This distress signal is detected by nearby conspecifics, causing them to scatter or aggregate into a defensive school. These messages can linger in the environment as a warning after the initial threat has passed.
Auditory and Vibrational Signals
Auditory and vibrational signals serve to startle or warn an approaching threat. The rapid, high-frequency clicks produced by some tiger moths can jam the echolocation systems of hunting bats. Other animals use mechanical sounds, such as the distinct rattling of a rattlesnake’s tail. This provides an audible warning before a physical confrontation is necessary.
Adaptive Functions and Messages to Predators
Prey communication devices serve several distinct adaptive functions, all aimed at increasing the probability of survival.
Pursuit Deterrence
Pursuit deterrence occurs when the prey signals its alertness and readiness to escape, communicating, “I see you, and a chase will be fruitless.” This often involves a momentary pause or a specific display that demonstrates the prey’s physical fitness. This signal benefits both species by preventing a high-energy, low-probability chase.
Signaling Unprofitability
Unprofitability signaling advertises that the prey is toxic, venomous, or dangerous to consume. Aposematic coloration is the classic example. This message is an honest signal because the cost of developing and maintaining the defense ensures that only genuinely unprofitable prey can sustain the display.
Startle Signals
Startle signals are designed to momentarily interrupt a predator’s attack sequence, buying the prey a crucial moment for escape. These displays rely on sudden, unexpected visual or acoustic stimuli, such as the flash of large, false eye-spots on a moth’s wings. The momentary hesitation caused by the surprise allows the prey to flee before the predator can re-initiate its strike.
Kin Warning and Alarm Signaling
Kin warning or alarm signaling is directed toward conspecifics but indirectly influences the predator. When a group of prey is alerted by an alarm call or pheromone, the resulting chaotic scattering or tight grouping makes it harder for the predator to select and focus on a single target. The sudden movement and collective defense reduce the predator’s chance of a successful kill.
Specific Examples in the Animal Kingdom
The behavior known as stotting, exhibited by Thomson’s gazelles, serves as a textbook example of pursuit deterrence signaling. When a gazelle detects a predator, it leaps into the air with a stiff-legged, arched-back posture. This behavior is understood as an honest signal of the gazelle’s physical condition, advertising that it is fit enough to perform the costly jump and still escape, thereby convincing the predator to target a less vigorous individual.
In the realm of unprofitability signaling, the brilliant colors of the monarch butterfly and the poison dart frog represent true aposematism. Monarch caterpillars sequester toxins from milkweed, making the adult butterflies unpalatable to birds. The bright orange and black wing patterns act as a visual reminder to predators that have learned to associate the colors with sickness.
Moth eye-spots illustrate the effectiveness of the startle function. The sudden display of large, circular patterns on the wings mimics the eyes of a much larger animal. The sudden appearance of these markings can cause a predator to hesitate its attack for the fraction of a second needed for the moth to fly away. Alarm pheromones released by minnows cause others in the school to rapidly change their behavior to minimize their risk.