Social Systems, Altruism, and Cooperative Behavior
Altruism in biology is behavior that reduces the reproductive success of the actor while increasing that of the recipient, and it can evolve through kin selection (helping relatives, who share genes) or reciprocal altruism (helping those who will return the help). In brief: Hamilton’s rule says that an altruistic act is favored by natural selection when r × B > C, where r is the relatedness between the actor and the recipient, B is the benefit to the recipient and C is the cost to the actor. Social systems range from loose groups to eusocial colonies of ants, bees and naked mole-rats, where most individuals do not reproduce.
Helping others seems to conflict with natural selection, because genes for self-sacrifice should disappear. This guide explains the types of animal societies, the benefits and costs of group living, kin selection and reciprocal altruism, eusociality and cooperative breeding, compares the explanations in a table, works through a Hamilton’s rule calculation and corrects common mistakes.
Why do animals live in groups?
Living in a group has benefits and costs, and group size evolves to balance them.
- Benefits: better detection of predators (many eyes), dilution of the risk of being the victim, collective defense (musk oxen form a circle around calves), cooperative hunting (lions, wolves and orcas), sharing of information about food, and warmth (penguins huddle).
- Costs: competition for food and mates, easier spread of parasites and disease, greater visibility to predators, and the risk of being exploited by cheats.
Societies differ in organization. Some are simple aggregations (a school of fish), some have dominance hierarchies (the “pecking order” in hens, in which a dominant bird has priority for food), and some have division of labor and cooperative care of young.
What is altruism?
In evolutionary terms, an act is altruistic if it lowers the fitness of the actor and raises the fitness of the recipient. Fitness here means reproductive success, not the intention. Examples:
- Belding’s ground squirrels give alarm calls that warn others but attract the attention of the predator. Females, who live near their relatives, call more than males.
- Worker honeybees sting intruders to defend the hive, and die because the sting is torn out.
- Vampire bats regurgitate blood to feed roost-mates that have failed to feed.
- Helper birds, such as Florida scrub-jays, help their parents to raise younger siblings instead of breeding.
Because natural selection acts on genes, the puzzle is how genes for helping can spread.
What is kin selection?
W. D. Hamilton showed in 1964 that a gene for altruism can spread if it helps relatives, who may carry copies of the same gene. An individual’s success in passing on genes has a direct part (through its own offspring) and an indirect part (through the offspring of relatives), and the total is called inclusive fitness.
Hamilton’s rule: r × B > C
- r = coefficient of relatedness, the probability that two individuals share a gene by common descent. Parent and offspring: 0.5. Full siblings: 0.5. Half-siblings: 0.25. Grandparent and grandchild: 0.25. First cousins: 0.125.
- B = benefit to the recipient, in extra offspring.
- C = cost to the actor, in lost offspring.
J. B. S. Haldane is said to have joked that he would give his life for two brothers or eight cousins, which is the same calculation: 2 × 0.5 = 1 and 8 × 0.125 = 1.
What is reciprocal altruism?
Robert Trivers proposed in 1971 that unrelated individuals can help each other if the help is returned later. It requires repeated interactions, recognition of individuals, and a way of punishing cheats who accept help but do not return it. The cost of helping must be smaller than the later benefit. Vampire bats that share blood meals with those who have shared with them are an example, as are some primates that groom each other. In the game theory model of the Prisoner’s Dilemma, the strategy “tit for tat” (cooperate first, then copy the partner’s last move) was successful in Robert Axelrod’s tournaments in the 1980s.
Other explanations include group selection (selection among groups), which is controversial and generally considered weaker, and mutualism, where both partners gain at once, so there is no true altruism, such as in cooperative hunting.
What is eusociality and cooperative breeding?
Eusociality is the highest level of social organization. It has three features: cooperative care of young, overlapping generations in the colony, and a reproductive division of labor, with sterile or nearly sterile workers and fertile queens (and sometimes kings).
- Social insects: ants, termites, honeybees and many wasps. A honeybee colony has a queen, thousands of sterile female workers and seasonal males (drones).
- Haplodiploidy: in ants, bees and wasps, males develop from unfertilized eggs (haploid) and females from fertilized eggs (diploid). Full sisters share on average 0.75 of their genes, since they receive identical genes from their father, so they are more related to sisters than to their own offspring (0.5), which may have promoted the evolution of worker sterility. However, termites are not haplodiploid and are also eusocial, so haplodiploidy cannot be the whole explanation, and ecological factors such as defendable nests and food are also important.
- Naked mole-rats (Heterocephalus glaber) and Damaraland mole-rats are eusocial mammals. One queen breeds in a colony of up to a few hundred, and the other animals dig tunnels and defend the colony.
- Cooperative breeding is found in many birds (jays, bee-eaters) and mammals (meerkats, wolves, African wild dogs) where helpers, often older offspring, assist the breeders. Helpers gain through kin selection and also experience and the chance of inheriting a territory.
Explanations of cooperation compared
| Explanation | Who is helped? | Condition | Example |
|---|---|---|---|
| Kin selection | Relatives | r × B > C | Helper scrub-jays |
| Reciprocal altruism | Unrelated individuals | Repeated interaction, return of help | Vampire bats |
| Mutualism | Partners that gain at the same time | Immediate benefit for both | Cooperative hunting |
| Parental care | Own offspring | Direct fitness | Bird feeding chicks |
Worked example: applying Hamilton’s rule
A young bird can either breed alone or stay and help its parents raise siblings. If it helps, the parents raise 3 extra chicks (benefit B = 3 offspring), and the helper gives up 1 chick of its own (cost C = 1 offspring). The extra chicks are the helper’s full siblings (r = 0.5).
- Write the rule: helping is favored if r × B > C.
- Substitute for full siblings: 0.5 × 3 = 1.5, and 1.5 > 1, so helping is favored.
- Check helping cousins instead: the same 3 extra chicks would be cousins (r = 0.125), so 0.125 × 3 = 0.375, and 0.375 < 1, so helping is not favored.
- Benefit needed for cousins: B > C ÷ r = 1 ÷ 0.125 = 8, so cousins would need to receive more than 8 extra offspring.
The calculation shows why helpers are usually found among close relatives. In practice, B and C are hard to measure, and the model is a simplification, but studies of birds such as the Seychelles warbler have found that helpers are mainly related to the breeders, and that helping increases the survival of young.
How was social behavior explained?
Darwin was worried about sterile worker insects in On the Origin of Species, and suggested that selection could act on the family. Haldane’s remarks in the 1950s and Hamilton’s papers in 1964 gave the mathematical theory of kin selection. Trivers published on reciprocal altruism in 1971, and E. O. Wilson summarized the field in Sociobiology (1975). Axelrod and Hamilton published on the evolution of cooperation in 1981. The idea of kin selection and the role of haplodiploidy are still debated, and some biologists, including Wilson later in his career, argued for a larger role for group selection.
Which mistakes are common?
Mistake 1: Saying that animals act “for the good of the species”
Natural selection acts on individuals and genes. Helping behavior is explained by benefits to the genes of the helper through relatives or reciprocation.
Mistake 2: Thinking that the animal calculates relatedness
Rules such as “help individuals that smell like me” evolved by selection. Animals do not do the calculation.
Mistake 3: Confusing altruism in biology with human motives
Biological altruism is defined by the effect on fitness, not by intentions.
Mistake 4: Assuming that queens control workers
In many colonies, workers affect the queen’s behavior and conflict can occur over reproduction and the sex ratio.