Animal Collectives 2: Collective behavior Flashcards
Challenges to overcome with group living
1) Coming to a consensus to ensure the group stays together as a cohesive unit and maintains group benefits (self organisation, decision making, behavior regulation, locomotion, navigation)
2) Getting over the increased risk of infection
Self organisation
simple repeated interactions between indivs producing complex adaptive patterns at group level
eg. starling murmuration / Mexican wave (has common properties worldwide)
Self orgainsation requires:
- simple rules
- local interactions
- no sophisticated decision making
- no singlalling
- no individual recongition
Results in decentralised control
- Robust to perturbation (e.g. important for our immune system so doesn’t have single point of failure)
- Single leader may make erroneous decision
- hard to choose/ locate leader or leader may die
o pattern at the global level of a system emerges solely from numerous interactions among the lower-level components of a system
The importance of collective decisions
Need unanimous decision to prevent group dissolution when opinions vary
Benefit of collective decisions -> wisdom of the crowds
- Median value often accurate (eg. actual ox weight less than 1% difference) but large range
- Central limits theorum: larger pool of estimates -> smaller standard error -> more likely accurate mean
Reaching a decision
a) Leader -> Autocracy
- leader makes the decision and the rest of the group follow
- Many different ways in which leaders are chosen (previous lecture)
Example: Matriarch leads African elephatns
b) Voting -> democracy
Quorum = minimum number of individuals needed to “agree” for entire group to reach decision
Example: WHEN to move (African hunting dogs)
- pack = despotic w/ dominant breeding pair
- When deciding to move, votes using rallies by sneazing
- Rallies initiated by dominants are more likely successful rally + leave
- Dominants have 77% success and sub have 27%
Example: WHERE to move (House hunting ants)
- Live in small cracks in stones (frequently relocate as outgrow crack or it is dislodged)
- Good location: small entrve, large and dark
- Individuals go of to find new house -> Determine cavity size in dark using pheromone trails + how often recross trail when re-enter site
- Once found nest return and tandem run back with another individuals (follower does orientation loops)
- If new scout approves -> makes own tandem run
- +ve feedback of tandem running – more ants approve -> more recruited to run
o Quorum = 10 ants – sensed by no. times ants bump into nestmate at new site
o Quorum achieved -> transport runs (ants at old nest carried to new one eg. queen + brood)
Regulating collective behavior
Behaviors must be regulated to ensure optimum number of individuals are carrying them out -> benefits> costs
Example: Honey Bees
- indivs have different thresholds of temperature increase tolerance before they start fanning
- Fanning increases air circulation + cools colony
- -ve feedback loop to maintain stable hive temp
Collective locomotion
Locomotion can be explained by self organisation with no cooperation
Simple model:
- Maintain nearest neigbor distance (repulsion to avoid crashing, attraction to reduce predation risk) -> 2 body lengths
- Fly at same speed with same head orientation (alignment)
Example: locust
* Marching = self-organised phenomenon not emerging via cooperation
* Marching migratory locust in plague years
* high locust densities -> phenotypic change of juveniles from solitary to gregarious marching
* high locust densities -> spontaneously switch from disordered to ordered state
* Marching triggered by contact on rear of abdomen (if cut ventral nerve cord then move less as don’t feel contact from behind)
* They are cannibals and eat stationary non-marching indivs
Example: Wildabeast
- excess of 100,000 individuals in one herd as they cross serengeti
- Wavelike herd front with no peturbations -> keep constant distance between wildaebeast -> there is a range within which they can slow down and speed up, afterwhich gap is filled
Collective navigation
Collective navigation = collective locomotion + decision making (not always determined by leader)
Example:
- Olive baboons search for forage (despotic collective w/ alpha indiv)
- baboons more likely follow indiv if many initiators of movement are moving in same direction
- If initiators disagreed on direction less likely followed
- If angle <90 -> group compromises
- if angle >90 -> Groups chooses one of the directions (majority) - follow larger group rather than dominant individuals
Collective disease defence
**Group size offsets impact of infection **
- Benefits of group living can offset the risk of mortality
-
Example: Mortality risk of Yellowstone wolves due to mange caused by mites
- Risk decreases with increasing pack size
- Larger pack -> more food as better hunting success and social benefits offset risk of mortality
- The impact of 1 Infected indiv can be almost completely offset extra mortality by having 5 pack mates
Avoiding infection
- Ill indivs + busy places appear more disgusting so are avoided reducing transmission
- Sick animals emit chemical odours
Example: Caribbean spiny lobster
- Normal = several in same den under sponge
- Healthy indivs avoid dens w/ infected indivs in (infecteds have no preference over which den)
- Chemical odour in urine of infected – allows healthy to avoid indivs before risk of contracting infection is too high
Example: mandrills
- Smell each other poos to detect intestinal parasites and avoid
Example: ant pupa
- Workers detect infected puppae due to emitted chemical signals (4 cuticular hydrocarbons increase upon immune stimulation just after infection so ants can act before infectious)
- Rip them out their silk cocoon then spray with antimicrobial acid, make holes and spray acid inside.
**Organisational immunity **
- Colonies organise themselves so that different groups interact predominantly within themselves
- Multiple groups: nurses, cleaners, foragers
- Oldest workers do out of nest tasks + have limited interactions w/ vulnerable + valuable eg. young workers, brood + queen
- Post exposure colony changes to isolate foragers + other exposed indivs from colony = social distancing (caregivers look after isolating indivs)
Overview
collective groups must overcome challenges to ensure they get benefits of living in a group
- Group decision making (via voting or leader)
- Locomotion (often via self organisation)
- Navigation (locomotion + decision making)
- Regulating collective behavior
- Limiting risk of disease
