Animal Groups
Different species of animals live in different ways varying from solitary to massive groups. Some groups are ordered and structured, and others are aggregations with no order (individuals at the same place at the same time; eg: flocks of seed-eating birds). Some groups are permanent and others are temporary (eg: just for breeding).
Groups vary in size from two individuals to millions (if not more) at the largest (eg: insects in super-colonies). There are many different terms used for groups(table 1).
SMALL GROUPS
Bonded pair.
Family group; eg red fox.
Sibling group.
Harem - one male with females and offspring.
Matriline - dominant female, daughters and young.
LARGER GROUPS
Band.
Covey; eg quail.
Pack; eg: African wild dog.
Party.
Pride.
Pod; eg killer whale.
Troop; eg baboons.
QUITE LARGE GROUPS
Shoal/school - fish.
Flock - birds; eg American white pelican.
Herd; eg African buffalo.
Swarm.
Aggregation - no order or structure.
LARGEST GROUPS
Colony.
Eusocial group eg ants.
Super-colony.
Super-organism - combined colonies.
Table 1 - Examples of terms used to describe different sized groups.
Slater and Halliday (1978) distinguished seven types of group situation (table 2).
1. Solitary (eg sloth).
2. Aggregation for single activity - eg: meet in one place to mate, but no group structure or organisation (eg: seals).
3. Aggregation for most activity - group without order or structure (eg some grazing mammals.
4. Aggregation for some activity/stable group for others - eg: aggregate at feeding sites, but organised group for rearing offspring (eg: some birds).
5. Aggregation for some activity/solitary for some/stable group for others (eg: bats).
6. Stable group - clear hierarchy and organisation to group (eg: pack of wolves).
7. Solitary within stable group - solitary individuals join stable group for a period (eg: dominant male takes over pride of lionesses).
Table 2 - Seven types of group structure according to Slater and Halliday (1978).
How the species comes to live - alone or in a group, and the group size - depends upon the evolutionary advantages and disadvantages of doing so (table 3). Put at its simplest, evolution is about staying alive (feeding and not being eaten) and mating (passing genes into the next generation). Thus group living must be seen in relation to these behaviours.
Advantages
1. Safety from predators.
2. Food; eg: co-operative hunting.
3. Mate access.
4. Communal care.
5. Social transmission of information.
6. Other benefits like thermoregulation.
Disadvantages
1. Increased competition.
2. Increased risk of infection.
3. Exploitation by other animals.
4. More obvious to predators.
5. Risk of inbreeding.
6. Risk to young; eg: misdirected parental care.
Table 3 - Summary of the advantages and disadvantages of group living for animals.
There will always be a trade-off between the costs and benefits of group living. This does mean that some groups are dynamic and changing, known as "fission-fusion" groups, depending upon food availability or period in breeding cycle.
Popa-Lisseanu et al (2008) found both fission-fusion groups and stable societies among giant noctule bats (Nyctalus lasiopterus) studied in a Spanish park. The composition of a single tree roost varied daily, but the overall roosting groups were stable for over fourteen years of the observations. The authors believed that switching trees was a means of maintaining social bonds in the whole of the colony. There are other advantages, and disadvantages of sub-groups and small group switching for animals in larger groups (table 4).
Advantages
1. Flexible response to food availability.
2. Maintain social bonds in large group.
3. Anti-parasite strategy.
Disadvantages
1. Conflict between sub-groups within the larger group.
2. Less defence against predators.
3. Less opportunity for benefits of thermoregulation.
Table 4 - Advantages and disadvantages of forming sub-groups and small group switching within the larger group.
References
Popa-Lisseanu, A.G et al (2008) Highly structured fission-fusion societies in an aerial-hawking, carnivorous bat Animal Behaviour 75, 471-482
Slater, P.J.B & Halliday, T.R (1978) Behaviour and Evolution Cambridge: Cambridge University Press