Living in groups

what is a population’s distribution?

the spatial pattern in which individuals are dispersed within a given area. varies with time, available resources, and intent

what are the three types of spatial distribution?

• clumped

• uniform

• random

clumped distribution

• includes family and social groups and other group types

• pros: many eyes to search for food, confuse predators with sheer numbers, cooperative hunting

uniform distribution

• common among territorial animals defending scarce resources or defending breeding grounds

• pros: helps ensure adequate resources for each individual

random distribution

• rare, exhibited in individuals that do not form social groups

• occurs when resources are not scarce enough to require territorial spacing or cooperative behaviour

• pros: not resource limited, no need to attract or repel

types of groups: protective groups

individuals associating intermittently to reduce environmental stress. minimizes individual exposure

eg huddling for warmth

types of groups: feeding groups

use the same food resource at the same time

• passive or active aggression

types of groups: foraging groups

search and hunt together

types of groups: breeding groups

formation of breeding units

types of groups: group defense

collectively defend / detect from biotic factors

types of groups: social groups

individuals of the same species that remain and interact with one another to a distinctly greater degree than with other conspecific orgnisms

reasons why to form groups

1. protect from physical factors and/or predators

2. assemble the sexes for reproduction

3. efficient searching for food, habitats, breeding sites

4. social (kin association - cooperation and altruism)

why dont all animals form groups?

1. competition for resources

2. Increased risk of parasitism or disease

3. Increased conspicuousness

4. Increased opportunities for reproductive interference or suppression

5. inbreeding

theres lots of examples in the ppt, make sure to check it out

benefits of group living for prey

1. Dilution effect

2. Confusion Effect

3. Many Eyes Hypothesis

4. Cooperative Defense

1) Dilution of individual risk

• within a group, each individual has a reduced probability of being attacked by a predator

• predator is likely to take someone else

• not all individuals within a group experience equal risk

selfish herd theory

• Group members appear to coordinate defense, but each individual behaves selfishly

• Individuals approach others to reduce their domain of danger

• Individuals in the middle enjoy lower risk

• Individuals on periphery have higher risk as they are more likely to encounter predator first

• Tradeoff: Exterior positions may be better for foraging

exceptions to the selfish herd theory

• best position may depend on predator’s hunting strategy

• dominants protect weaker individuals

• dominants choose peripheral locations (muskox)

2) confusion and cocktail party effects

• Predators may hesitate or become confused when confronted with several prey items simultaneously

• Occurs as a result of limited information processing ability of predators - processing spatial information of multiple targets declines when prey aggregate

• Regardless of attack technique, in larger group sizes predators had difficulty focusing on a single target.

3) enhanced detection of predators - the many eyes hypohthesis

• Many individuals, each with their own finite probability of detecting an approaching predator, increase the overall probability of a predator being detected

• increase overall probability of avoidance or escape

• enhances overall vigilance

the many eyes hypothesis assumes:

• Individuals in a group monitor the alert behaviour of other individuals in the group.

• Individuals detecting predator will actively or passively alert other group members

4) cooperative defence

• approach and harass en masse to deter predators

• makes defencive formations to protect individuals

why does cooperative defense deter predators?

1. confuses predators

2. pronouncement of vigilance

3. stress or risk of physical injury

4. alert others to predator’s presence

5. teach offspring to recognize predators

benefits of grouping to predators / foragers

1. Information transfer

2. Cooperative strategies

3. Catch difficult prey

1) information transfer

• Organisms living in groups can benefit from observing successful foragers

• Forager joins group to acquire information about food availability, location, and/or predator risk

• By observing foraging success of others in the group, a naïve forager can estimate the quality of a food patch

2) cooperative strategies

• Animals may cooperate to trap elusive prey, or in taking down larger prey.

• Permits capture of prey larger than possible by singletons.

• Coordinated effort also involves subsequently protecting the kill from scavengers

• Social bonds derived from kinship or mating may enhance cooperation.

3) catching difficult prey

• coalition - a short-term association that forms in competitive situations.

• Two or more animals may cooperate to defeat or steal from another individual or to secure a resource

optimal group size

• Adaptation to maximize energy intake by reducing search & handling times and/or predation risks

• In small groups, an individual’s ‘fitness’ may increase with group size, because the sum of benefits are increasing.

• leads to optimality in group size (no further advantage to expanded group)

when is an intermediate group size not optimal?

• for prey: small groups are not conspicuous and very large groups dilute predation risk

• for predators: when prey are scarce: hunt alone – take small prey & don’t need to share, OR form larger group to cooperatively search & take down large prey

mechanisms to keep group size optimal:

1. Established group members may repel joiners (cost of aggression can’t be too high)

2. When joiners and group members are genetically related, the group size is predicted to be closer to the optimal value (kin are good to group with, but too large leads to inbreeding)

3. Movements occur from one group to another group by single foragers aiming to maximize their expected fitness (intergroup movement leads to breakdown of the larger group)