Living in Groups I

Question 1

What are the big two reasons for forming groups? What are the sub-reasons in each (4 for one, 2 for the other)? What are the three types of dilution? What are the four parts of vigilance?

Answer 1

1. Anti-predator
   Dilute risk of attack, predator confusion, communal defense, and improved vigilance
2. Foraging
   Better food finding (information centers), better food capture (group hunting)
3. Pure, selfish herd, synchrony
4. Early detection, public information, cheaters, sentinels

Question 2

What is dilution? When does it occur? What is an example? Why does vigilance not help here?

Answer 2

1. Increase group size = decreased probability of being picked for food (1/N)
2. Occurs if: # prey attacked does not increased with group size faster than the increase in group size itself
3. Water skaters - sardines eat them from below. attack rate is scaled to 1/N (increase size by x10, decrease attack rate by x10)
4. The insects cannot see the fish below them

Question 3

What is a second example of the dilution effect? What does the table look like displaying how many bites horses receive? What are the two main points this graph showed?

Answer 3

1. Wild horses against biting flies
2. Small vs large group of horses VS # biting flies per group vs per horse. results:

Small group - 30/group, 10/horse

Large group - 108/group, 3/horse

3. 1) Large group attract more flies BUT 2) # flies/horse decreases

Question 4

What is the selfish herd effect (dilution)? What happens to the domain of danger, why? What is an example? What was found with Styrofoam seals?

Answer 4

1. middle of the group is less danger than the periphery
2. Decreases because predator is unlikely to strike at you if other peeps are in the way
3. Cape fur seals vs great white sharks
4. Farther away = more likely to be attacked (increased domain of danger)

Question 5

What is synchrony (dilution)? What is an example? What is the benefit of doing this?

Answer 5

1. Dilution over time
2. Mayflies and their emergence (two weeks in late May) to breed and die
3. They get eaten a lot, so randomly popping out in large numbers overwhelms the ability of preds to eat them

Question 6

What is predator confusion? What were the details of the experiment that tested this? What were the results? What is the implication of these results?

Answer 6

1. Predators cannot lock on moving prey
2. 4 groups (3 ambush: squid, cuttlefish, pike and 1 chaser: perch) were given food.

Squid –> silversides
Cuttlefish –> young mullet
Pike –> minnows
Perch –> female guppies

3. All four were less efficient at catching larger groups of prey
4. Hard to pick out a single in a group

Question 7

What is communal defense? What are two example?

Answer 7

1. Prey actively defending themselves by attacking/mobbing a predator
2. Guillemonts against predator gulls (mobbing), white-breasted nuthatches/downy woodpeckers/carolina chickadees (mobbing calls)

Question 8

What does it mean to have improved vigilance? What are two examples? How is overall flock vigilance impacted by the change in vigilance of the individual due to being in a group, why?

Answer 8

1. early detection

2.
1) Wood pigeons vs Goshawks (increase flock = decrease hunting success)
2) Ostriches raising their heads

3. Vigilance of the individual decreases, but this has 0 impact on vigilance for group because raising YOUR head is independent of other individuals raising theirs

Question 9

What is public information? What is an example of an experiment done to test this? What was the result? What was the control of this experiment?

Answer 9

1. Information that you gather from individuals around you of your group (not that you gain yourself)
2. Minnow experiment, two types: transmitter (sees pike directly) and receiver (can only see another minnow through a one-way mirror).
3. Pike comes near = transmitter stops foraging for food and receiver reduces foraging after seeing trans.
4. no pike

Question 10

Are cheats stable when talking about vigilance in a group and predation risk? What are the two reasons why or why not? What was an experiment to demonstrate?

Answer 10

1. No
2. ESS - cheating not stable b/c of reciprocity, policing, and kin effects
   Asymmetry - individual that sees the predator first may gain a personal advantage
3. Cheetahs - target the least vigilant gazelle (14/16 times) AND more vigilant indvs. escaped more

Rolling a ball at tree sparrow/junco target bird (responds first) and more vigilant birds followed first (early detection is important)

Question 11

What are sentinels? What are two examples? Why do sentinels do what they do and what is the value of “quiet calls”?

Answer 11

1. Primary vigilant indvs. for the group
2. Meerkats/pied babblers
3. Being a sentinel is selfish because once you have eaten, the best thing to do for your own safety is to watch.

b) “Quiet calls” are good because you don’t want to alert the predator of your presence but also let other friends know you’re watching (watchman’s song) so they can eat.