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?