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
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
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
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
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
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
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)
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
Equipment• 2 empty, round glass dishes • Red light
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
Pictures• Image J software
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
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.
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
Cockroach pheromone trial:
– Highest average Rn value and higher trend line• Indicates more uniform distribution
– The p-value = 0.0703 (nearly significant)
Dead spider trial:
– Lowest average Rn and lower trend line• Indicates greater aggregation
– The p-value = 0.0134 (significant)
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/
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?
Discussion
• Cockroach pheromone: Nearly significant
• Unexplained - Effect of strong unidentified scent?
http://www.planetnatural.com/pest-problem-solver/household-pests/cockroach-control/
Discussion
• Dead pill bugs: – Contrary to hypothesis
• Reasons? – Manner of death– Homogenization– Age of carcass
http://www.flickriver.com/photos/trombamarina/173563682/
Discussion
• Further source of error:
– Origin/sourcing of pill bugs– Experiences of the pill bugs – Trials conducted on different
days
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?
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
Questions?