predator-prey interactions P2
Aposematism: Warning Colouration and Signals
Definition:
Aposematic colouration is a warning signal used by defended prey to deter predators.
It is an honest signal, advertising toxicity, unpalatability, or physical defences (e.g., spines, venom).
A. Features of Aposematic Signals
High contrast colouration (e.g., black and yellow, red and black).
Repetition and redundancy (e.g., stripes, spots) to enhance detection.
Tailored to predator sensory systems (e.g., bright colours for birds, high contrast for mammals).
B. Why Are Certain Colours Common?
Shared across different taxa (e.g., insects, amphibians, reptiles, mammals).
Differences in terrestrial vs. aquatic species:
Terrestrial species often use yellow, red, and black (effective in daylight).
Marine species (e.g., nudibranchs, blue-ringed octopus) use blues and purples due to underwater light absorption filtering out red wavelengths.
3. The Effectiveness of Aposematism: Learning and Avoidance
A. The Evolution of Warning Colouration
Wallace (Letter to Darwin):
"There must be an outward sign of distastefulness to indicate to a would-be predator that the prey is disgusting."
No point being toxic if a predator has to eat you to find out!
B. Predator Learning and Conspicuousness
Experiment: Chicks and Dyed Prey
Chicks foraged on green (cryptic) and blue (conspicuous) dyed food.
Early in trials, chicks preferred conspicuous blue food.
Later, they avoided the blue food more than the cryptic green food, despite both being distasteful.
Conclusion:
Conspicuousness enhances learning and memory.
Predators avoid bright colours faster than cryptic prey.
4. Mimicry Systems in Predator-Prey Interactions
Mimicry is when one species imitates another’s warning signals to gain protection from predators.
A. Müllerian Mimicry (Mutualistic)
Two or more defended species evolve similar warning colouration.
Both species reinforce the predator’s avoidance response.
Examples:
Heliconius butterflies share similar colour patterns despite not being closely related.
Poison dart frogs in different regions develop similar colouration.
Millipedes with bright bands indicating toxicity.
B. Batesian Mimicry (Parasitic)
A harmless species evolves to mimic a toxic/dangerous species.
Predators avoid the mimic, but the model species bears the cost.
For Batesian mimicry to work:
The mimic must be less common than the model.
If mimics become too abundant, predators may stop avoiding them.
Examples:
Scarlet king snake (non-venomous) mimicking the coral snake (venomous).
Flatworms mimicking toxic nudibranchs.
C. Field Experiment on Snake Mimicry
Researchers placed clay models of coral snakes and mimics in the wild.
Models with the correct colour order were avoided more than poorly mimicked versions.
Results: Mimicry works best when proportions and colours closely match the model.
5. Startle Responses and Dramatic Displays
Definition:
A sudden, conspicuous display used by prey when detected, to startle predators and deter attack.
Examples:
Eye spots on butterflies, caterpillars, and moths.
Flash colouration (e.g., frogs revealing bright inner legs when jumping).
Cuttlefish expanding and intensifying skin patterns.
A. Cuttlefish Startle Displays
Cuttlefish change defensive strategies depending on the predator:
Against crabs: No dramatic display—just direct escape.
Against dogfish (poor vision): Darkening body before escaping.
Against sea bass (good vision): Full dramatic display (expanding body, bright patterns, eye spots).
Conclusion:
Displays are tailored to predator perception abilities.
6. Predator-Prey Arms Races Beyond Visual Signals
Most predator-prey research focuses on visual interactions, but other senses play a role.
Example: Bat-insect acoustic arms race
Bats use echolocation to hunt.
Many insects have evolved ultrasonic hearing to detect approaching bats.
Six different insect orders have evolved the ability to hear echolocation independently.
7. Costs of Defensive Behaviours
Secondary defences come with trade-offs.
Example: Snails retracting into their shells
Energetically costly (muscle contraction).
Reduces feeding time and slows shell growth.
Increases risk of parasite infection when rehydrating after re-emerging.
Trade-off between predator avoidance and other survival costs.
8. Summary & Key Takeaways
A. Aposematism and Warning Signals
Bright colours and high contrast patterns act as honest signals of toxicity or danger.
Predators learn to avoid these signals faster than cryptic prey.
B. Mimicry Systems
Müllerian Mimicry: Mutualistic—multiple defended species share warning signals.
Batesian Mimicry: Parasitic—harmless species mimic toxic species for protection.
C. Startle Displays and Defensive Strategies
Some prey use sudden dramatic displays (e.g., cuttlefish, eye spots) to deter predators.
Displays are often tailored to predator sensory capabilities.
D. Costs of Defence
Secondary defences have metabolic and survival trade-offs (e.g., reduced feeding time, higher parasite risk).