Animal collectives I: Pros, cons and leaders of groups

Question 1

Why live collective: anti predator defence

Answer 1

Predator / parasite dilution
- safety in numbers = less likely consumed in larger group
- Decreased per capita risk of predation and impact of parasites like mosquitoes
- e.g. Wildabeasts while migrating

Swamping
- predators quickly satiated by prey in large groups
- so synchronise mass emergence / mating events
- eg. oak masting to overwhel seed predators or decreased predation w/ increased no. mayfly emerging

Predator confusion
- many prey means hard to focus on one target
- Single prey > coordinated movement > random movement (shown in fish tank study)
- e.g. Leopard ghecko must fix on single prey item when striking -> increased time to strike with larger number

Many eyes + predator vigilance
- More time spent foraging
- eg. Starlings while foraging
- e.g. Mixed herds – zebra + giraffe -> zebra spend more time looking at giraffe as spot preator with their long neck (vigilance down 60% o spend more time foraging)

Protection -> Selfish herd (Hamilton)
- Often safer in central position in group as decreased domain of danger (zone around animal where predator may attack) -> compete for safest position

Mobbing behaviour
- fend of predators together eg. meerkats + birds
- e.g. Chickadees, noisily and aggressively mob hawks and owls

Question 2

Why live collective: Improved hunting success

Answer 2

Collective groups allow group hunting and group size alters the probability of success for different prey

Example: yellow stone wolves
- Probaility of successful capture depends on group size
- 2-6 wolves: Els
- 9-13 wolves: Bison

Question 3

Why live collective: thermoregulation

Answer 3

save energy -> requires lower basal metabolic rate

Example: more bats in roost increases temp
- Birds roosting together save 53% of energy budget per day

Example: huddling in voles
- Resting metabolic rate (RMR) of huddling groups was 37.2% lower than in separated groups
- reduced food intake by 29% and saved digestible energy 156.99 kJ/day

Question 4

Why live collective: enhanced locomotion

Answer 4

• V-formation of birds saves energy by utilising upwash made by birds in front

Example: pelicans = 11-15% lower heart rate in formation than when fly alone

Example: Northern bald ibis ..
- GPS tracker monitors speed, locomotion and wing beat
- <1/2 wavelength behind bird in front flap in sync (in phase) w/ bird in front
- >1 wavelength flap out of sync (anti-phase)
- can fly further in groups

Question 5

Why live collective: Social support

Answer 5

Cooperative breeders provide alloparental care to non-kin (protect from predators + provide food)

But also in non-kin groups as social living can have non-kin related benefits -> better quality of life

Example:
- in yellow baboons more socially integrated have higher fitness eg. infant survival (independent of rank or env)
- groom more + groomed more lowering heart rate + promotes endorphin expression

Example:
- in NZ horses – foaling success increases and harassement decreases by males if females more more social
- better success as social contact reduces stress

Question 6

Costs of collective: Disease and increase energy demands

Answer 6

Disease = transmission depends on density and size of susceptible population
- Group living therefore increases R0
- Behavioural adaptations to decrease disease in groups

Example: Lockdowns reduced covid transmission between households but if one individual within a house hold got infected, likely to infect others.

Question 7

Costs of collective: higher stress levels

Answer 7

Example: Both for alpha and for subordinates are constantly repressed in baboon troups

• Alpha-> constant challenging from other males
• Sub-> harrasment, social exclusion and reduced access to preferred foraging areas, water sources, and sleeping sites within the troop.

Question 8

Costs of collective: Increased competition

Answer 8

Larger groups means increased competition for resources (food, space, mates)

Question 9

Optimal groups sizes: ecological constraint model

Answer 9

Optimal group size varied between species and environment

Ecological constrants model shows the possible group sizes along an environmental continuum (e.g. rainfall)

1) minimum beneficial size
- minimum size where achieve group benefit (e.g. predator avoidance)
- Decreases with rainfall as individuals can hide in grass

2) ecological maximum
- Maximum size that environment can support
- Increases with rainfall as less competition then decreases as rainfall make foraging challenging leading to competition

3) cognative max (constant)
- Related to social dynamics, communication abilities, tracking abilities
- e.g. must be able to track heiarchies -> cannot do with too many individuals

Group size is not linear:
Intermediate group size = optimal

Small -> between-group competition (small groups displaced by large groups from food patch)
Large -> within-group competition (large groups must move more often as deplete patches rapid + so travel further to meet energetic requirements, greater competition)

Question 10

Who leads the group?

Answer 10

1) Better informed

example: Ants:
- Show distributed leadership
- Ants carrying food cannot see or follow scent trails
- Leadership is temporary for 10 seconds when know direction + get constant turnover of leaders w/ up-to-date info
- Too big object -> no one indivs can impact direction enough so no specific directional movement

2) fastest

Example: Pigeons
- GPS trackers on pigeons as they flew back to Ox
- No correlation between navigation ability and heiarchy -> fastest pigeon leads -> over time they become better navigators
- Hierchical leadership -> follow the one in front

2) Age/ experinence

• Older individuals have critical environmental knowledge gained through experience

Example: Orcha pods
- Older / post-reproductive or menopausal females are leaders (matriarchs)
- Older = have ecological knowledge so buffers against env change eg. if year has low salmon pop’ns then older know where to go to find salmon
- When mother dies daughter mortality increases 2.7 fold and son increases 8.7 fold

3) Dominant -> heirarchy

Example: Africa Lion pride

4) More motivated

Example: Group of young children looking for ice cream van
- More motivated individuals will work harder to find it and lead the other

Question 11

Conclusion

Answer 11

Benefits of group living
- anti- predator defense
- Information sharing
- Hunting success
- locomotion
- social support
- Thermoregulation

Negatives
- competition
- disease
- Stress levels

Who is the leader?
- Older/ more experienced
- fastest
- Most dominant
- Most motivated
- Most informed