aggregation in pill bugs, armadillidium vulgare, in response to predation laura dunn tomomi kahabata...
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Aggregation in pill bugs, Armadillidium vulgare, in response to predation
Laura DunnTomomi KahabataCorinne Boudreau
Animal Behavior 3401Mount Allison Univerisity
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Introduction• What is Aggregation? – the crowding of
organisms in close proximity to one another
• Why does it occur? – Environmental
conditions, clumped resources, reproduction and predator defence
http://en.wikipedia.org/wiki/Pheromone#mediaviewer/File:Bug_aggregation.jpg
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Aggregation in Pill Bugs• When does this occur in pill
bugs? – Ancestral condition• water loss
– Lack of oxygen– Sheltering behaviour
• Speculated to be involved in:– Growth– Reproduction– Predator Defence
http://ed101.bu.edu/StudentDoc/Archives/ED101fa10/kzantay/Pillbugs.html
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How can it be a Predator Defence?
• Dilution Effect– More individuals, less of a target
• Selfish Herd Hypothesis– Individuals positions themselves
between other individuals to reduce predation
• Warning System– More individuals, more likely to
spot a threat
http://www.masterfile.com/stock-photography/image/848-02811526/Japanese-Tree-Frog-%28Hyla-japonica%29-eating-a-Common-Pillbug-%28Armadillidium-vulgare%29-Shiga-Japan
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Question and Hypothesis• Will Pill bugs aggregate in response to a
predator? • What aspects of a predator will cause an
aggregation response?
• Hypothesis: • Live predators and dead pill bugs will cause
strongest aggregation response
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How to test this?
• To test this theory we exposed a group of pill bugs to a live predator, a control and several predator models and measure aggregation response.
http://waynesword.palomar.edu/redmit1a.htm
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Methods and Materials
Predator models • Toy spider• Live spider• Live spider in a cup• Empty cup • Homogenized spider• Dead pill bug• Bornyl acetate (Cockroach pheromone)
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Preparation for pill bugs
• Started with a group of 30 pill bugs:
– The Animal Biology laboratory stock • Wild-caught pill bugs • Commercial supplier in the Fall of 2014
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Equipment• 2 empty, round glass dishes • Red light
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Experiments • Placing 10 pill bugs in each dish
15 sec
3 min: Acclimation
0 sec
30 sec
45 sec
1 min
2 min
3 min
4 min5 min
Put predator models
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Pictures• Image J software
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the nearest neighbor statistic (Rn)
Rn=D(Obs)0.5√a/n
• D(Obs): the mean of the observed distance between an individual and its closest neighbors
• a : the total area of the container • n : the total number of pill bugs observed
in the chamber
http://geographyfieldwork.com/NearestNeighbourFormula2_small.gif
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Results
Figure 9 – Average Rn value for trial stimuli. Error bars indicate standard error. Averages calculated based on the Rn values for the nine points of observation in each trial.
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Rn value over time:
0 0.25 0.5 0.75 1 2 3 4 50
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Control
Model Spider
Time (minutes)
Rn
valu
e
• No difference in trend for all trials except dead spider trial and cockroach pheromone
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Cockroach pheromone trial:
– Highest average Rn value and higher trend line• Indicates more uniform distribution
– The p-value = 0.0703 (nearly significant)
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Dead spider trial:
– Lowest average Rn and lower trend line• Indicates greater aggregation
– The p-value = 0.0134 (significant)
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Discussion
• Only significant result attained in dead spider trial– Lowest Rn value
• Spiders are a major predator – – Sensitivity to chemical signalling– Does homogenate release a stronger
signal than a live spider?
• Supports the Selfish Herd Hypothesis
https://www.flickr.com/photos/shanblan/5075539180/
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Discussion
• Possible source of error:– Dry environment during trials –
• Lack response to other stimuli:
– Pill bugs have limited visual capabilities-– Does this limit reaction to such stimuli?
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Discussion
• Cockroach pheromone: Nearly significant
• Unexplained - Effect of strong unidentified scent?
http://www.planetnatural.com/pest-problem-solver/household-pests/cockroach-control/
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Discussion
• Dead pill bugs: – Contrary to hypothesis
• Reasons? – Manner of death– Homogenization– Age of carcass
http://www.flickriver.com/photos/trombamarina/173563682/
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Discussion
• Further source of error:
– Origin/sourcing of pill bugs– Experiences of the pill bugs – Trials conducted on different
days
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Future Directions
• Investigations into the manner of:− pill bug death− age − condition of carcass
–Do certain conditions cause release of chemical signals on the part of dead or dying bugs?
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Future Directions
• Findings should offer insight into strategies employed by isopods for protection and survival
• Contribute to our knowledge of social adaptive behaviors in isopods
http://forums.permaculturenews.org/showthread.php?5737-Rollie-pollies-pill-bugs
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Questions?