209 APS Animal Behaviour
Lecture 8 The Evolution of Antipredator Behaviour
1. Antipredator behaviour as an example of adaptive behaviour 2. Camouflage 3. Attack Avoidance Behaviour 4. Selfish Herding
Aims & Objectives
Aims 1. To present antipredator behaviour as an example of adaptive behaviour, and for testing adaptationist hypotheses. 2. To present examples of antipredator behaviour. 3. To present examples and the logic behind selfish herding. Objectives 1. Learn specific examples 2. Understand testing of adaptationist hypotheses by means of comparative tests and current value tests. 3. Understand how selfish herding, a behaviour that is bad for the group, can be selected for and how this is different to group vigilance.
What is an Adaptation?
A heritable trait which enhances the fitness of its bearers Current benefits Past benefits and evolutionary history
Note: not every factor that reduces predation is an adaptation against predation
Current Benefits of an Adaptive Trait: Mobbing
Mobbing
Nesting gulls mob an intruder Risky behaviour for the gulls Could it protect their chicks? Does it distract predators? Is mobbing a behavioural adaptation against predators?
Does Mobbing Protect Eggs?
Prediction If mobbing is behavioural adaptation against egg predators, then mobbing should reduce egg predation.
Test of Prediction Hans Kruuk tested this experimentally. He placed a hen egg every 10m along a line running from inside to outside a gull colony. He measured mobbing and egg predation. The results supported this prediction.
Mobbing
Using Comparative Study to Investigate Changes in the Benefits of an Adaptive Trait
Comparative Method
A method for testing evolutionary hypotheses, including adaptationist hypotheses, by comparing different taxa to see who does what. Correlate occurrence of traits with benefit of trait. Example: If ground predator mobbing is not needed or not beneficial it will not occur Where unneeded? cliff dwelling gull (no predators?) Prediction no mobbing in cliff dwelling gulls Data support the prediction lost in gulls which evolved cliff nesting To use the comparative method you ideally need a phylogeny to show that evolutionary changes have occurred.
Gull Phylogeny: Cliff Nesting is Derived
Todays ground-nesting gulls + mobbing (e.g., black-headed gull) Todays cliff-nesting gulls - mobbing (e.g., kittiwake) Recent cliff-nesting ancestor
Ground-nesting behaviour Cliff-nesting behaviour
The first gull = ground-nesting
Alternative Phylogeny
The alternative phylogeny does not alter the interpretation that species that switched from ground nesting to cliff nesting have lost mobbing.
Comparative Method
The comparative method is a way of testing evolutionary hypotheses by comparing taxa. The basic idea is to determine if one factor causes another by determining if the presence or absence of two factors are correlated. Thus, if mobbing is an adaptation we expect it only in species in which it is necessary or effective in reducing predation. In both swallows and gulls some species have evolutionarily lost mobbing, as shown by being derived within their phylogenies. This loss is correlated with cliff dwelling in kittiwakes and solitary nesting in rough-winged swallows. Their relatives, the black-headed gull and the bank swallow, both have mobbing because both are vulnerable to predation and can do something about it by mobbing. Kittiwake nests are not predated, and rough-winged swallows are solitary and so cannot mob effectively.
Camouflage
Diversity of Antipredator Behaviours
Antipredator adaptations are diverse and many involve a behavioural component. Alcock informally refers to behaviours which reduce the chance of being detected, attacked, captured, or consumed. Many antipredator adaptations fit into one of these categories, while some straddle several. It is easy to see the adaptive significance of anti-predator adaptations such as that of cryptic insects or the puffer fish, which inflates itself with water becoming a spine-covered sphere. Others, such as selfish herds and stotting in Thompsons gazelles reflect the sometimes strange logic of natural selection and require careful theorizing and ingenious hypothesis testing. Some antipredator adaptations (not all are from Alcock) Anti-detection crypsis, camouflage Anti-attack stotting in Thompsons gazelles, selfish herding warning colouration and mimicry Anti-capture run, swim or fly fast, vigilance body part autotomy (e.g., tail loss in lizards) Anti-consumption fighting back, feigning death releasing noxious chemicals hard to swallow (e.g., inflation by puffer fish)
Camouflage
Make yourself less detectable to predators Some things to remember: Camouflage might not just be for vision. Predators have different senses than we do. What might appear camouflaged/ visible to us may visible/camouflaged to a predator. Behavioural part may be in the prey, the predator, or both.
Camouflage: Peppered Moth
Camouflage: Peppered Moth Larva
Camouflage: White Underwing
In the White Underwing moth, Catocola relicta, resting position and orientation are important. Moths rest head up on the pale bark of birch trees.
Testing Whether Camouflage Works
Pietrewicz and Kamil trained captive blue-jays to respond to moths Showed the jays images for short periods: if a moth is detected then the jays pecked at a key: reward and new slide If they got it wrong, they got no food and had to wait a long time for the next slide: Operant conditioning technique Jays detected less moths on pale bark Head up moths on pale bark seen least
Conclusions: behaviour of moths in where to settle affects behaviour of birds in detecting.
Behaviour & Camouflage: Decorator Crab
Behaviour & Camouflage: Decorator Crab
Behaviour & Camouflage: Decorator Crab
Algae species consumed by crab
Behaviour and Camouflage
Algae species used for decoration by crab
Behaviour & Camouflage: Decorator Crab
Prediction: Juvenile crabs decorated with Dictyota menstrualis will be less likely to be killed by predators (fish) than crabs unable to use this alga Test: Tether crabs in areas with and without the alga Result: Crabs in areas without the favoured alga disappear at five times the rate as crabs kept with the favoured alga. Problem of interpretation: But is this because of better camouflage or because the alga is repellent/distasteful to fish?
Behaviour & Camouflage: Decorator Crab
The alga contains a chemical that repels omnivorous fish
Is the crab protected because the fish cant see it through the weed, or because the fish can see the weed and doesnt to eat it? Or both? Shows the importance of clear thinking in behavioural work
Behavioural Mimicry
You are probably familiar with Batesian mimicry Batesian mimics resemble a harmful or distasteful model in order to gain protection Frequency dependent models common, mimics rare Mimicry can involve also behaviour
Dronefly
(honeybee mimic)
Honeybee
Bumblebee
Volucella
Behavioural Mimicry
Behavioural Mimicry
Behavioural Mimicry
For all plants, droneflies match honeybee flower visit duration, but not bumble-bee visit duration. Non-mimic flies do not match honeybee duration times
Strange Logic: Stotting in Thomsons Gazelles
Stotting by Thomsons Gazelles
When Thomsons Gazelles and some other gazelles spot a cheetah or other predator they may stot. Stotting: jumping 0.5-1.0m in the air with legs held straight and white rump patch full visible. Why advertise yourself to predator? Stotting may signal to predators that Ive seen you and I am very fit and ready to flee. So predators dont bother to chase animals that stot. Stotting is an honest signal.
Stotting by Thomsons Gazelles
The Ive seen you and am ready to flee explanation is called the Unprofitability hypothesis. Other hypotheses include Anti-ambush hypothesis: Stotting lets gazelles see what is ahead and reduces the chance that they will be ambushed lions often ambush their prey. Prediction: Since stotting is costly, it should only occur in long grass, mixed grass or shrub habitats Observation: Stotting occurs in all habitats including short grass. This indicates that the anti-ambush hypothesis is incorrect.
Stotting by Thomsons Gazelles
Alarm signal hypothesis: Stotting warns conspecifics, especially offspring, that a predator is near (indirect fitness explanation). But even solitary animals will stot. Also, the stotting gazelle shows its rump to the predator. Social cohesion hypothesis: Stotting enables gazelles to form groups and flee in a coordinated fashion. But even solitary animals will stot. Also, the stotting gazelle shows its rump to the predator. Confusion effect hypothesis: Stotting confuses and distracts a predator preventing it focusing on one animal. But even solitary animals will stot.
Stotting by Thomsons Gazelles
Only the signal of unprofitability hypotheses does not have a problem. But how can we test whether this is actually the correct explanation for stotting, rather than some other as yet unknown explanation?
Stotting by Thomsons Gazelles
Group Formation: Selfish Herding & Dilution Effect
The Selfish Herd
A grass snake predator is about to prey on one of several frogs sitting at the edge of a pond. Where is the best place for a frog to sit to minimise its risk? Assuming that the snake will move with equal probability in all directions, then the best place is next to another frog. Then, if the snake attacks in your direction, you are not the only target. This is a selfish strategy. It benefits individuals but not the group. By bunching prey, selfish herding may even increase the total predation risk by making a more tempting target. If you can understand the above logic then congratulationsyou have understood an application of game theory to animal behaviour.
Hamilton WD 1971 J Theor Biol 31: 295311
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Predation selects for bunching upselfish herding
The Selfish Herd
Is selfish herding good for the group? If the bunched prey are a more tempting target it will definitely not be good for the group. That is, it will increase predation overall. However, individuals who group will have lower predation that those that do not. Everyone is worse off than if they did not group. What would be the likely fate of a rare mutant frog behaviour which tried to reduce total predation risk on all frogs by leaving the group and sitting on its own? Would this spread out behaviour be selected for or against?
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The Dilution Effect
Grouping may reduce an individuals chances of being eaten. 5 predators, each kills one prey a day 40 prey
In a group of 40, chance of being eaten 5/40 = 0.125 In a group of 400, chance of being eaten is 5/400 = 0.0125
Selfish Herding in Whirligig Beetles
Selfish Herding in Whirligig Beetles
Larger groups are more attractive to predators.
Selfish Herding in Whirligig Beetles
But in larger groups the predation rate per individual is less.
Selfish Herding in Whirligig Beetles
Trade off in obtaining food versus predation risk. Food is more abundant on edge o...