Biology · Behaviour, Communication & Social Life
Social organisation, kinship and the evolution of cooperation
An animal gives up a resource and another gains it. That immediate transfer does not yet tell us whether the giver loses reproductive success, gains a later benefit, helps relatives or participates in an exchange. Social evolution begins by making those possibilities explicit.
Nervous Systems, Senses & Movement supplies the sensory and motor context. Evolutionary Processes & Adaptation introduces heritable variation, selection and fitness. Begin with the first lesson and the ordinary numerical examples; return to those chapters when a mechanism or evolutionary comparison needs more detail. Animal Reproduction, Development & Life Histories is an optional return path for reproductive timing and the distinction between resource expenditure and lifetime outcomes.
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Use explicit fitness effects, relatedness and evidence for contingent exchange to compare possible explanations of social behaviour.
A return can matter without proving a contingent exchange
Cooperation can also yield direct benefits, such as a shared outcome improving both participants' reproductive prospects. It therefore need not require close kinship. Relatedness, direct benefits and repeated interactions can operate together rather than being mutually exclusive explanations.
Reciprocity concerns a benefit returned in relation to a previous interaction. Seeing A help B and B help A establishes that both directions occur; it does not by itself establish that the later help depended on the earlier help. Familiarity, need, opportunity and partner quality can influence both.
The original record has nine returns in twelve familiar-partner opportunities and three in twelve unfamiliar-partner opportunities. A separate matched-partner record has nine returns in twelve opportunities following receipt of help and six in twelve following no receipt. These are conditional associations, without random assignment or control of all relevant states.
A selected vampire-bat research discussion illustrates why food-sharing correlations, direct benefits, kinship and contingent reciprocation need careful separation. Its historical analysis is a guide to evidence, not a claim that all later research remains unresolved or that every social exchange follows one strategy.
Returned help can have several explanations
Read the diagram in words
Familiarity fractions are 9/12 and 3/12, difference 0.5. In the separate matched-pair record, fractions after help and after no help are 9/12 and 6/12, difference 0.25. One candidate graph has shared context leading to both directions of help; another has earlier help leading to later return.
| Specified opportunity class | Opportunities in this row | Opportunities with return | Returns divided by opportunities |
|---|---|---|---|
| Familiar-partner opportunities | 12 | 9 | 9/12 = 0.75 |
| Unfamiliar-partner opportunities | 12 | 3 | 3/12 = 0.25 |
| Familiar-partner opportunities + Unfamiliar-partner opportunities | 12 + 12 = 24 |
|---|---|
| First-minus-second row fraction | 0.5 |
| Specified opportunity class | Opportunities in this row | Opportunities with return | Returns divided by opportunities |
|---|---|---|---|
| Prior help received | 12 | 9 | 9/12 = 0.75 |
| Prior help not received | 12 | 6 | 6/12 = 0.5 |
| Prior help received + Prior help not received | 12 + 12 = 24 |
|---|---|
| First-minus-second row fraction | 0.25 |
| Partner context or opportunity → Earlier help from A to B | Candidate explanation, not demonstrated cause |
|---|---|
| Partner context or opportunity → Later help from B to A | Candidate explanation, not demonstrated cause |
| Earlier help from A to B → Later help from B to A | Candidate explanation, not demonstrated cause |
|---|
Candidate explanation, not demonstrated cause
The two records track separate opportunities
Separate original opportunity records, one matched partner identity for the prior-help comparison, complete stipulated counts and two conceptual candidate graphs. No actual animals, random assignment, independence, causal proof or historical conclusion.
Worked example
Compare a fitness condition and a behavioural association
For the first additive case,0.5 × 3 − 1 = 0.5. For the second,0.25 × 3 − 1 = −0.25. The cost remains one while the relatedness weighting changes. These margins describe the supplied selection condition, not measured probabilities that an animal will help.
The familiar-partner return fraction is 9/12 = 0.75; the unfamiliar-partner fraction is 3/12 = 0.25. Their difference is 0.5. That comparison could reflect a relationship effect without any return specifically contingent on earlier help.
Within the separately supplied matched-partner record, the corresponding fractions are 9/12 = 0.75 after help and 6/12 = 0.5 after no help, a difference of 0.25. This is a more focused association but still does not eliminate need, opportunity or other changing states. The two records cannot be pooled as if they tracked the same opportunities.
Check your understanding
Does an immediate energy cost establish biological altruism?
Show reasoning
No. The classification concerns direct fitness effects over the relevant comparison. Later direct benefits could change the account; resource expenditure alone does not supply lifetime consequences.
What does the additive account predict for r=0.25, benefit three and cost one?
Show reasoning
The weighted benefit is 0.25 × 3 = 0.75, less than cost one; the margin is −0.25. The specified condition is not met. This is a model result, not a measured helping frequency or universal rule.
Why do mutual help and a higher return fraction after help not by themselves prove reciprocity?
Show reasoning
Bidirectional help and conditional associations can reflect familiarity, need, opportunity or partner quality. Evidence for contingent return must distinguish dependence on earlier help from those alternatives.
What to carry forward
Social explanations need a stated time horizon, compatible fitness effects and evidence that separates kinship, direct benefits and contingent exchange.
References
Resources supporting this lesson’s explanations and examples.
- Social semantics: altruism, cooperation, mutualism, strong reciprocity and group selection (Journal of Evolutionary Biology 20(2):415–432, 2007; doi: 10.1111/j.1420-9101.2006.01258.x)S. A. West, A. S. Griffin and A. GardnerScientific review
- Hamilton's rule and the causes of social evolution (Philosophical Transactions B 369:20130362, 2014; doi: 10.1098/rstb.2013.0362)Andrew F. G. BourkeScientific review
- Does food sharing in vampire bats demonstrate reciprocity? (Communicative & Integrative Biology 6(6):e25783, 2013; doi: 10.4161/cib.25783)Gerald G. Carter and Gerald S. WilkinsonResearch discussion
Describe organisation before assigning benefits
Social organisation includes who associates, who interacts and how roles or access are distributed. A group can contain relatives, unrelated individuals or both. Sharing a location does not prove cooperation, stable hierarchy or common interests in every outcome.
Evolutionary classifications of social behaviour concern fitness consequences for an actor and a recipient over a stated comparison. Both gaining is mutual benefit; an actor losing direct fitness while a recipient gains is biological altruism. An actor gaining while the recipient loses is selfishness; both losing is spite in this technical classification.
These labels do not describe moral intentions or emotional kindness. An immediate resource cost can be followed by a direct benefit, so its classification depends on the full relevant account. The original outcome plane stipulates complete comparable fitness changes; it does not infer them from observed food transfers.
Social classifications use two fitness effects
Read the diagram in words
The actor/recipient effects are −3/+4 for biological altruism, +2/+4 for mutual benefit, +2/−1 for selfishness and −1/−2 for spite. The sign combinations classify the stipulated complete effects rather than the immediate resource transfer.
Comparable assigned fitness units
No change relative to the comparison
Effects use the same specified outcome horizon
Technical classification does not identify motive
Original complete comparable fitness-effect stipulations over a defined common outcome horizon. Not resource transfers, intentions, actual lifetime measurements or predictions for a real species.