AP Syllabus focus:
‘Signaling behaviors can alter the behavior of other organisms and result in differential reproductive success.’
Signaling links an organism’s traits to the behavior of others, especially during mating. Because mates and rivals respond to signals, signaling can increase or decrease reproductive success, shaping which phenotypes leave more offspring.
Signaling and reproductive success
Signaling behavior influences who mates, how often mating occurs, and which individuals gain access to resources needed for reproduction (territories, nests, or protection). When receivers change their behavior in response, the signaler may gain a fitness advantage.
Reproductive success: the relative number of viable offspring an individual contributes to the next generation.
Reproductive success is “differential” when some individuals consistently leave more offspring than others because their signals attract mates, deter rivals, or coordinate reproductive timing.
The polygyny threshold model graph shows how a chooser’s expected fitness can differ depending on mate/territory quality, even when it involves sharing a mate. It provides a quantitative example of how receiver choice rules can produce differential reproductive success by favoring individuals associated with higher-quality resources or traits. Source
Key outcomes of signaling
Mate attraction: increasing the probability of being chosen as a mate.
Mate assessment: enabling receivers to evaluate potential mates (quality, compatibility, or readiness to reproduce).
Rival deterrence: reducing costly fights by advertising ownership, size, or willingness to escalate.
Coordination: synchronising reproductive behaviours (courtship sequences, spawning events, parental care roles).
How signals alter receiver behaviour
For signaling to affect reproduction, it must produce a measurable change in the receiver’s actions.
This diagram breaks communication into sender, encoding, message, medium, decoding, and receiver, with feedback and noise affecting the process. In biological signaling, the same logic applies: signalers encode information into behavioral/visual/chemical signals, the environment modifies transmission (noise), and receiver decoding drives the behavioral response that ultimately changes mating outcomes. Source
Receivers often show choice (approach, acceptance, rejection) or avoidance (retreat, submission), which then shifts mating opportunities.
Receiver responses that change mating patterns
Female choice (or mate choice by either sex): preferential mating with certain signalers increases those signalers’ mating frequency.
Male–male competition: signals that reduce fighting can still increase mating access by securing territories or social rank.
Species recognition: correct signals reduce wasted reproductive effort on the wrong species.
Reproductive state detection: signals indicating fertility or receptivity focus mating attempts when fertilisation is most likely.
Sexual selection: natural selection arising from differences in mating success, often driven by mate choice or competition for mates.
Sexual selection explains why traits used in signaling can evolve even when they are costly for survival, as long as they increase reproductive output.
Why “honest” signals can evolve
Signals may be reliable indicators of quality when they are difficult to fake or when dishonesty is penalised by receiver behaviour.
This graph illustrates a core idea behind costly (handicap-style) signaling: as signal intensity increases, low-quality individuals incur steeper costs than high-quality individuals, even if both gain benefits from signaling. Because the marginal cost of exaggeration is higher for low-quality signalers, the stable outcome is that high-quality individuals can afford higher-intensity signals, making the signal informative to receivers. Source
Sources of signal reliability
Energetic or resource costs: only high-condition individuals can sustain intense displays, frequent calling, or elaborate ornaments.
Performance constraints: physical limits (speed, endurance, neural control) link display quality to underlying phenotype.
Social enforcement: rivals challenge exaggerated signals; repeated defeat reduces the benefit of bluffing.
Receiver resistance: if receivers ignore unreliable signals, dishonest strategies yield fewer matings.
Trade-offs and fitness consequences
Because signaling can be conspicuous, it can impose survival costs that offset reproductive benefits. The balance of these costs and benefits determines whether a signaling trait increases overall fitness.
Common trade-offs
Predation risk: bright coloration or loud calls can attract predators as well as mates.
Time and energy: courtship reduces time for feeding, parental care, or immune function.
Injury risk: escalation from signaling to combat can cause harm, reducing future reproduction.
If the reproductive gain exceeds these costs, signaling increases fitness; if not, selection may favor reduced signaling or alternative strategies.
Variation in signals and differential reproductive success
Populations often show variation in signaling traits (intensity, timing, pattern). Differential reproductive success arises when:
receivers consistently prefer certain variants,
preferred signalers gain more mates or higher-quality mates,
and offspring viability or number increases as a result.
This connects signaling to evolutionary change: signaling traits that boost reproductive success tend to become more common over generations.
FAQ
They use manipulative experiments to isolate the signal.
Alter the signal while holding other traits constant (e.g., playback, model presentations, cosmetic changes).
Measure changes in approach, acceptance, or mating rate.
Include controls to rule out disturbance or novelty effects.
Sensory exploitation occurs when a signal evolves to tap into pre-existing biases in the receiver’s nervous system.
A novel trait can spread if it triggers strong attention or preference, even before it reliably indicates “quality,” after which selection may refine or constrain it.
Cheaters can persist when detection is difficult or when cheating is rare.
If most signals are honest, receivers may still respond because ignoring signals is costly. Frequency-dependent dynamics can limit cheating: as cheating rises, receivers evolve discrimination or reduced responsiveness.
Researchers often use proxies that predict offspring contribution.
Genetic parentage (DNA-based paternity/maternity)
Number of mates or copulations
Number of fledglings/young surviving to a defined stage
Each metric targets a different component of reproductive output.
Pollution can mask or distort signals, reducing effective communication.
Noise can reduce call transmission or force shifts in pitch/timing.
Artificial light can disrupt signalling schedules and receiver responsiveness.
These changes can lower encounter rates, increase mate rejection, or favour alternative signalling strategies.
Practice Questions
Explain how a courtship signal can lead to differential reproductive success in a population. (2 marks)
States that the signal changes receiver behaviour (e.g., mate choice/acceptance) (1)
Links this to some individuals obtaining more matings and/or producing more offspring than others (1)
In a species of bird, males vary in song rate. Females preferentially mate with males that sing at higher rates, but high song rate also increases predation risk. Explain how selection could maintain variation in song rate in the population. (5 marks)
Female preference increases mating success of high song-rate males (1)
High song rate incurs a survival cost via increased predation (1)
Net fitness depends on trade-off between increased reproduction and decreased survival (1)
Some contexts/individuals may benefit more from high song rate (e.g., better condition, safer territories), while others benefit from lower song rate (1)
Therefore both high and low song-rate phenotypes can persist because neither has universally higher fitness across conditions (balancing selection) (1)
