Social behaviour: costs and benefits

Question 1

Define social behaviour and sociality

Answer 1

Social behaviour
–Broad definition: Behavioural interactions between 2 or more animals, usually of the same species
–Narrow definition: Cooperative interactions extending beyond reproductive and parental behaviour

Sociality
–The tendency to form social groups

Question 2

Why form social groups?

Answer 2

*For the benefit of the species or group?
*For the benefit of the individual?

Group selection original concepts (~1960’s)
it was thought that individual animals “behave in the interests of their species or social group” but this concept of ‘selflessness’ is questionable – it leaves a society open to exploitation by ‘selfish’ individuals (see games theory next lecture).

Question 3

Group selection

Answer 3

*Wynne-Edwards (1962) Animal Dispersion in Relation to Social Behavior:

*Concept that natural selection ‘chooses’ between groups, populations or species on the basis of how well their individuals serve the collective unit’s interests.

*Animal populations regulate their size to track fluctuations in limited resources:

*Plenty → populations ‘allowed’ to expand,

*Scarcity → reproduction reduced so that populations do not exceed resources.

Question 4

Social conventions

Answer 4

*territories, dominance, etc
*regulate access to resources & thus breeding opportunity.
*Individuals selflessly obey conventions,
*to breed or not according to social status.

*Social systems that regulate population better are then selected for………….supposedly………

Question 5

Group selection: the argument against

Answer 5

*Populations generally track resource levels - OK.
BUT:
*animals adopting social systems to bring this about – controversial.
*Social systems based on interests of population are vulnerable to invasion by selfish individuals who ignore rules.
*Selfish individuals are likely to occur (through immigration, mutation or recombination & outcrossing) .
*Selfish traits have reproductive advantage & spread rapidly.
*A group-selected social system would thus collapse.

(See games theory: next lecture)

Question 6

Group selection: current thinking

Answer 6

*Group selection may work BUT it is unlikely to be a major evolutionary force under natural conditions.

Question 7

One main problem in group selection theory

Answer 7

*group selection requires unrealistically low levels of gene flow between groups, & low mutation rates within groups.

*Groups would have to be genetically isolated with little chance of a selfish individuals appearing (via immigration or mutation).

*Generally unlikely to be significant form of selection in natural conditions

*Selection acts at the level of the individual

*Genes that avoid replication for the good of a larger entity (group, society, species) – not favoured (selfish genes, tragedy of the commons; Hardin 1968)

Question 8

group living, why do individuals ‘join’ groups?

Answer 8

*Relatedness – kin selection – to allow the continuation of your genes
*Heterogeneity and repeated interactions (this & next lecture)
*Costs and benefits to individual (this lecture)

Question 9

Benefits of group living

Answer 9

Benefits:

*Foraging success
-Groups indicate rich food sources
-Cooperative hunting
-Cooperative defence of resources
-Information sharing

*Reduced predation risk
-Predator dilution
-Synchrony and satiation
-Predator confusion
-Cooperative vigilance
-Cooperative defence

*Energetic advantages
-Social thermoregulation
-Slipstreaming in flight/swimming

*Other
-Cooperative mate displays
-Easier to find mates
-Division of labour
-Enhanced learning of young

Question 10

Costs of group living

Answer 10

Costs
Dominants in the group have first claim on food and mates resulting in some cases in subordinate starvation, lack of food to provide their offspring or no chance to reproduce

*Foraging success
- Local depletion
- Exploitative competition (food displacement)
- Interference competition (intraspecific aggression)
- Time investment in social behaviours (e.g shows of dominance/ submission)

*Mating success
- Risk of accidental / deceptive investment in others’ offspring
- Risk of inbreeding
- Risk of infanticide/cannibalisation (e.g. When a new lion takes over a pack it may kill the young from the previous alpha, Some male seals cannibalise pups on the breeding colony allowing them to hold territory longer)

*Other
- Stress from competition
- Increased risk of disease and parasite transmission
- Free-riders

Question 11

Benefits of sociality #1: Exploitation of resources: Food

Answer 11

*feeding efficiency – experienced coordinators lead the hunt
*information sharing (see Barta & Giraldeau 2001, Doligez et al 2002)
* increased competitive ability/capture rates
* cooperative hunting → capture larger prey & defense of prey once captured
*e.g. wolves, hyenas, lions (Packer et al. 1990)

Question 12

Benefits of sociality #2: Exploitation of resources: Mates

Answer 12

*access to potential mates,
*comparison of mates – mate choice- making informed choices from longer observation
*helpers & coalitions, deferred reproduction
*experience & potential territory inheritance in future

Question 13

Benefits of sociality #3:Anti-predator effects

Answer 13

e.g. increased vigilance (e.g. Redshanks; Cresswell 1994)

*dilution effects & confusion effects
*selfish-herd effects
*mobbing e.g. crows mobbing eagles
*group defense e.g. musk-oxen
*alarm calling e.g Belding’s Ground Squirrel
(many examples in Alcock/Dugatkin/Nordell & Valone)

Question 14

Benefits of sociality #4: Energetic advantages

Answer 14

Social thermoregulation e.g. in emperor penguins
slipstreaming e.g. wolves travelling through snow follow the leader who creates a path thus saving their energy

This is also why large birds fly in a V-formation as it provides energetic advantages

e.g. A study by Weimerskirch et al. 2001 on White pelicans (Pelecanus onocrotalus) Placed heart rate monitors on trained birds and recorded video flight formations + wingbeat frequency, along with the location of each bird in formation. They found that birds in formation behind the leader had slower wingbeat frequency and lower heart rate.

Question 15

Benefits of sociality #5: Enhanced learning

Answer 15

*Young individuals in social groups can learn from adults
*e.g. antipredator information

How do young prairie dogs learn their predators & how to avoid them? A study by Shier & Owings 2007 on Black-tailed prairie dogs (Cynomys ludovicianus)

method
They captured 36 juveniles and carried out a pretraining assessment
Then, conducted a five-week training period for juveniles in 3 conditions:
1.With experienced adult
2.With inexperienced sibling
3.Alone

Post-training tests:
Measured activity, fleeing, vigilance, and calls when released back into wild

Results:
Individuals trained with an experienced adult; showed higher levels of antipredator behaviour and higher survivorship

Question 16

Costs of living in groups #1: Increased competition for resources

Answer 16

Resulting in local depletion, exploitative competition (displacement) and interference competition (intraspecific aggression)

Food:
- increased chance of starvation as dominant individuals monopolise food.

Reproduction:
- Increased time to first reproduction
(e.g. Blumstein & Armitage 1998)
- reductions in offspring viability
(e.g. Tella et al. 2001)
- chance of not breeding at all - reproductive skew theory

Question 17

Costs of living in groups #2: Mating success:

Answer 17

*Risk of inbreeding → inbreeding depression
*Risk of misdirected parental care
* brood parasitism, e.g. cliff swallows
*extra-pair copulation in colonial birds
*cross-fostering e.g. grey seals
*Risk to offspring from conspecifics.

Question 18

Costs of living in groups #3: Increased exposure to parasites & disease

Answer 18

exposure to disease and parasites
e.g. cliff swallows (Brown et al 2001).

*Increased conspicuousness to predators
e.g. Whirligig beetles (Watt & Chapman 1998)

*large groups more likely to be attacked but risk per individual is decreased

*Cost in time spent in social behaviours,
-e.g. submissive behaviours
-and stress from competition

Question 19

The extent of sociality

Answer 19

*Extent & form of social organisation of a population is regulated by balance of costs & benefits of social interactions for individuals.

*Most animal spp. with 2 sexes are to some extent social, with the minimal social behaviour necessary between sexually reproducing animals being that associated with mating.

*There are some examples where individuals do NOT meet - even to mate e.g. broadcast spawning in lugworms, sea urchins

Question 20

Economics of sociality

Answer 20

See graph in notes
As with all animal behaviour, must consider economics of benefits and costs

Question 21

Extreme forms of sociality: the classic studies - related individuals

Answer 21

The evolution of such cooperative social behavior is often a result of kin selection:

*helping related individuals → indirect fitness benefits (‘Hamilton’s rule’)
*see lectures on genetics of behaviour
*Often this occurs where ecological constraints limit which or how many individuals can breed

Eusociality: societies that contain specialised non-reproducing castes that assist the reproductive members of the society.
^ e.g. ants, bees (Haplodiploid: ♂ = haploid ♀ = diploid)

Altruism: cooperative behaviour in which the actor’s fitness is reduced by the behaviour but raises the recipients fitness.

e.g: Alarm calling in Belding’s ground squirrels. Alarm calling individuals face risk of immediate costs ( predation) but may also gain recognition and status for their actions

e.g. Cooperative breeders (such as hyenas)

Question 22

Extreme forms of sociality: not limited to kin the classic studies - UNrelated individuals

Answer 22

Reciprocal altruism: one individual performs a cooperative act which is ‘repaid’ at a later date by the recipient of the assistance. A means by which cooperative behaviour can evolve even between non-relatives.

This requires a means by which individuals can ‘distinguish’ reciprocators from those who refuse to cooperate by:

*individual recognition
*probabalistic (frequency dependence)
^ e.g. Allo-grooming in many primate species.

Question 23

Extreme forms of sociality – the classic studies

Answer 23

Attempts at deriving a unified social theory of evolution have focused almost exclusively upon eusocial insects, or cooperatively breeding vertebrates (see Emlen 1997 & Ch 10 in Krebs & Davies) and highly social species.

BUT:

cooperative breeding occurs in only 3% of bird and mammal species.

Highly social species are rare

To understand evolution of sociality need to look at less extreme manifestations of social interactions e.g. conflict reduction.

Question 24

“Simpler” sociality

Answer 24

*Living in groups leads to conflicts of interest between individuals (see list of costs of group living).

Resulting in aggression e.g.:
- chasing off related individuals - inbreeding avoidance
- aggression between neighbouring female seals to protect pups.

Low sociality - e.g. conflict reduction behaviour as seen in seals. This reduction is not cooperation but can lead to reciprocal altruism

Question 25

Non-random association definition

Answer 25

Certain individuals interact with each other repeatedly - more than by chance. Which can lead to simple forms of social behaviour through familiarity and ultimately reciprocation.

Population social structure is the product of local interactions between conspecifics that may vary spatially. Such spatial variation arises from limits imposed upon mobility by physical landscape. Resuting in non-random associations between individuals.

Ferriere & Michod (1996)

Question 26

Why reduce conflict?

Answer 26

Conflict is potentially harmful & energetically ‘expensive’, to all involved.

Non-random associations can lead to mechanisms of conflict reduction - in each individual’s interest to minimise conflict. Most organisms live in heterogeneous landscapes - so non-random associations likely to exist in most animal populations.

Question 27

Mechanisms of conflict reduction likely to be common?
Models for evolution of conflict reduction

Answer 27

Theoretical models based on Games Theory (see next lecture):
- e.g. Iterated Prisoners’ Dilemma (IPD): (Axelrod & Hamilton, 1981)
- tit for tat’ strategy.
- Requires repeated encounters (but unknown number of)
- Does not necessarily require individual recognition.

Cooperation occurs even in species that do not have the ability for individual recognition
AND
does NOT require that individuals be related.

Common social structures and behaviours that promote conflict reduction
*Dominance hierarchies / social status & status signals – games theory – next lecture
*Territories.
*Ritiualisation of conflict - including ornaments (see level 1).

Question 28

Further development of sociality

Answer 28

*Conflict reduction as a first step towards sociality?
*This is NOT ‘cooperation’ - but can lead to reciprocal altruism.
*Once the costs of conflict have been reduced
-> secondary benefits of sociality - see benefits of social living. -> further selection for social behaviours.

*The evolution of sociality can have profound effects upon individual reproductive success & therefore fitness

Question 29

Summary

Answer 29

*Non-random associations & therefore social behaviour is likely to be very common.

*Not just related individuals.

Need to study ‘simple sociality’ to understand evolution of social behaviour (& higher levels of organisation within biological systems, ranging from genes to populations - Michod 1999).

Difficulty is doing this in the field:
*how to show individual recognition in wild animals?
*How to demonstrate conflict reduction - how do you show that something is not there!?

Question 30

Examples summary

Answer 30

Examples 1-5 show reduced conflict to familiar individuals leaving more time for other activities

Example 6 shows socialisation in microorganisms

Example 7 ? Greater movement of seal females results in more aggression

Example 8 is an example of pack hunting in worms

Question 31

Example 1: Sticklebacks (Gasterosteus aculeatus): Utne-Palm & Hart, 2000

Answer 31

*Familiar and unfamiliar pairs compared.
*Familiar pairs showed less aggression.
*Aggression decreased after 2 weeks together.

Question 32

Example 2: Sea Trout (Salmo trutta): Hojesjo et al. 1998

Answer 32

*Groups of familiar individuals and groups of strangers.
*Familiar fish → more stable dominance hierarchies, reduced distance between individuals → higher food intake.
*Familiarity stabilises social structure & allows behaviours that promote feeding & growth & therefore fitness

Question 33

Example 3: Red-backed salamanders (Plethodon cinereus): Jaeger & Peterson 2002.

Answer 33

*Females that were familiar (5 days together) cf. Strangers.
*Familiar females spent Less time in threat displays.
*Therefore more time for foraging etc.

Question 34

Example 4: Turnstones (Arenaria interpres): Whitfield 1986.

Answer 34

*Territorial & have variable plumage.
*Removed male territory owner & replaced with models that;
(1) resembled the owner
or
(2) resembled other males
*Neighbouring males showed less aggression towards models of original territory owner compared to models of strangers OR if model was a copy of the neighbour (i.e. self = stranger).
*Plumage variation in this spp. aids individual recognition, & Individual recognition facilitates aggression reduction.

Question 35

Example 5: Lesser white-toothed shrew (Crocidura suaveolens): Zuri & Rado 2000.

Answer 35

*Observed encounters between familiar and unfamiliar pairs - male & female pairs.

*Familiarity decreased aggression & increased tendency to spend time with opponent regardless of its sex

Question 36

Example 6: Micro-organisms: Crespi 2001.

Answer 36

*Suggested social behaviours include;
*cooperation in foraging. e.g. myxobacteria - mass attacks on microbial prey.
*cooperation in building - microbial biofilms.
*cooperation in reproduction - Heterocysts in some cyanobacteria (lose ability to reproduce and specialise in fixing N).
*cooperation in dispersal - fruiting bodies.

Question 37

Example 7: Grey seals (Halichoerus grypus).

Answer 37

*originally pagophillic ancestry (“ice breeding”) - low density.
*Now breed on islands - higher densities - but females protect pups → aggression.
*Site fidelity - c. 30m - many neighbours same year after year.
* comparison of Isle of May cf. North Rona
*Isle of May had greater movement of females -> more aggression and greater intensity of aggression.

Rates of female/female aggression per female per hour:

        North Rona  Isle of May     

1998 0.17 0.44

1999 0.15 0.46

2000 0.11 0.36

(Data from Paula Redman)

Question 38

Example 8: Velvet worms

Answer 38

Velvet worm - social groups pack hunt slugs with a dom matriarch
Reinhard & Rowell (2005)