1. Mount Allison Univerisity
Aggregation in pill bugs, Armadillidium vulgare, in response to predation
Laura Dunn
Tomomi Kahabata
Corinne Boudreau
Animal Behavior 3401
Mount Allison Univerisity

2. Introduction What is Aggregation? Why does it occur?
the crowding of organisms in close proximity to one another
Why does it occur?
Environmental conditions, clumped resources, reproduction and predator defence

3. 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

4. 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

5. 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

6. 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.

7. 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)

8. 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

9. Equipment
2 empty, round glass dishes
Red light

10. Experiments Placing 10 pill bugs in each dish 3 min: Acclimation
Put predator models
3 min: Acclimation
0 sec
15 sec
30 sec
45 sec
1 min
2 min
3 min
4 min
5 min

11. Pictures
Image J software

12. 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

13. 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.

14. Rn value over time:
No difference in trend for all trials except dead spider trial and cockroach pheromone

15. Cockroach pheromone trial:
Highest average Rn value and higher trend line
Indicates more uniform distribution
The p-value = (nearly significant)

16. Dead spider trial: Lowest average Rn and lower trend line
Indicates greater aggregation
The p-value = (significant)

17. 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

18. Discussion Possible source of error: Lack response to other stimuli:
Dry environment during trials –
Lack response to other stimuli:
Pill bugs have limited visual capabilities-
Does this limit reaction to such stimuli?

19. Discussion Cockroach pheromone: Nearly significant
Unexplained - Effect of strong unidentified scent?

20. Discussion Dead pill bugs: Reasons? Contrary to hypothesis
Manner of death
Homogenization
Age of carcass

21. Discussion Further source of error: Origin/sourcing of pill bugs
Experiences of the pill bugs
Trials conducted on different days

22. 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?

23. 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

24. Questions?