23. Social Behavior and Evolution Notes

Measuring Social Behaviour

• Dominance Hierarchies and their influence on animal societies are discussed in Martin & Bateson Chapter 10.

Dominance and Offspring Sex Ratio

• Breeding success of hinds depends on their dominance rank within their matrilineal group, termed maternal dominance.
• High maternal dominance hinds are more likely to produce male offspring, which have greater variance in reproductive success.

Dominance Behaviour and Breeding Success

• Jane Goodall studied chimpanzees at Gombe National Park for over 35 years.
• She collected long-term data on the life-time reproductive success of many individuals.

Chimpanzee Dominance

• High-ranking female chimpanzees had offspring that lived longer compared to low-ranking females.
• High-ranking females had daughters who reached sexual maturity sooner.
• High-ranking females had more surviving offspring per year.

Chimpanzee Dominance Conclusion

• High-ranking female chimpanzees had significantly greater breeding success than low-ranking females.
• Expression of high dominance is advantageous to female chimpanzees.

Benefits of Hierarchies

• The position of dominance by an individual is often insignificant in terms of its society.
• Establishing a hierarchy develops and maintains harmony within the society.
• Social hierarchies provide a means by which animals can live in groups and exploit resources in an orderly manner.
• All members of a society are important to the society’s survival and reproductivity.

Darwinian Puzzles

• The evolution of complex social animal societies involves altruism.
• Evolution is the survival of the fittest, where the individual is the unit of evolution.
• This raises the question of whether the group is the unit of selection in these societies.

Group Selection and Altruism

• George Williams (1966) showed that the persistence of a hereditary trait and its underlying genes is more likely determined by differential reproductive success of genetically distinctive individuals than by differential success among distinctive groups.

Parental Care and Altruism

• Parents are selected to invest in their young because the offspring carry many of the parent’s genes.
• Natural selection favors traits that benefit an individual’s genes.
• This leads to the question of what happens when altruism is directed toward non-offspring.

Relatedness and Reproductive Opportunities

• For paper wasps, the greater the relatedness between colony foundresses, the fewer reproductive concessions are made by the dominant reproductive.

Eusocial Behaviour

• Eusocial societies contain specialized non-reproducing castes that work on behalf of reproductive members.
• The evolution of sterile castes presents a puzzle as these workers have no direct opportunity to reproduce.

Inclusive Fitness

• Genetic analyses and the concept of inclusive fitness provide a basis for studying the evolution of behaviour.
• Natural selection favors behaviour that maximizes an individual’s survival and reproduction.
• Many social behaviours are selfish, yet altruistic behaviour is consistent with maximizing inclusive fitness.
• Inclusive fitness is the sum of genes passed on by all your relatives plus what you pass on.

Relatedness Table

• Parent-offspring: r = 1(0.5)^1 = 0.5
• Full-siblings: r = 2(0.5)^2 = 0.5
• Half-siblings: r = 1(0.5)^2 = 0.25
• Grandparent and grandchild: r = 1(0.5)^2 = 0.25
• Aunt/uncle-niece/nephew (both parents shared): r = 2(0.5)^3 = 0.25
• Aunt/uncle-niece/nephew (one parent shared): r = 1(0.5)^3 = 0.125
• Cousins (parents are full-sibs): r = 2(0.5)^4 = 0.125
• Cousins (parents are half-sibs): r = 1(0.5)^4 = 0.0625
• General formula: r= \sum(0.5)^L where L is the number of generation links and \sum is the number of paths.
• r between unrelated individuals is 0.
• r between clones is 1.

Hamilton’s Insight

• Fitness:
  • c = cost (offspring not produced)
  • b = benefits (extra relatives existing because of altruist)
  • r_c = coefficient of relatedness between parent and offspring (fitness lost)
  • r_b = mean coefficient of relatedness between altruist and recipients (fitness gained)
• Many eusocial animals are haplodiploid.
• Hamilton's condition for altruism: c \times rc < b \times rb

Hamilton’s Rule

• Natural selection favours altruism when the benefit to the recipient multiplied by the coefficient of relatedness exceeds the cost to the altruist.
• Hamilton’s rule: rB > C

Haldane's Quote

• "I would gladly give up my life for two brothers or eight cousins"
• This illustrates the concept of relatedness and altruism.

Insect Helpers

• In the paper wasp, queens lay over 95% of all eggs.
• The question arises as to why other females in the nest tolerate this.

Haplodiploidy-1

• Mother-offspring genetic relatedness; daughters share 50% of chromosomes with mother and 100% with father.
• Consideration of sister-sister relatedness.

Haplodiploidy-2

• Sister-sister genetic relatedness averages 75% of genes shared.

Testing Haplodiploid Hypothesis

• Important haplodiploid coefficients of relatedness:
  • Sister to Sister r = 0.75
  • Sister to Brother r = 0.25
  • Mother to son r = 0.50
  • Mother to daughter r = 0.50
• Workers should prefer a 3:1 investment ratio in reproductive sisters to brothers.
• Queens should prefer a 1:1 investment ratio in reproductive daughters to sons.
• The question of who wins in the colonies is raised.

Who Wins?

• In general, it’s the workers. Many ant species have a 3:1 investment sex ratio (Female:Male).
• Amongst slave-making ants, however, a 1:1 investment sex ratio has been reported.
• There is a conflict of interest between the queen and the workers.

Conflict Between Queen and Her Daughters: Queen Control of Sex Ratio in Fire Ants

• Fire ant colonies can be male-biased or female-biased.
• When queens are exchanged between colonies with different biases, the colony takes on the bias of the new queen.
• Queens from male-producing colonies lay more than 50% haploid eggs, while queens from female-producing colonies produce almost no haploid eggs.
• The Queen wins.

Queen vs. Daughter Conflict

• When the queen of a colony mates with more than one male, the relatedness amongst daughters decreases.
• In that case, workers stop favouring the production of females, investing equally in rearing males and females.

Workers and Multiple Queens

• If there are several queens in a colony, the workers selectively favour a brood of closer kin.
• The greater the difference in relatedness between the workers and the two queens, the greater the increase in the reproductive success of the more closely related queen during brood rearing.

Insect Helpers

• In the paper wasp, queens lay over 95% of all eggs; about half the workers are queen's sisters.
• About half of the workers are sisters of the queen, so they may gain indirect fitness through the reproduction of the queen. What about the third of workers that are not sisters or even cousins.

Worker Reproduction

• In some wasps and other social insects, the workers do reproduce.
• The reasons why dominant females don’t suppress that reproduction include the high cost of policing the colony, fitness gain from greater genetic diversity in the colony, and concession to induce workers to remain.

Parasitism & Genetic Variation in the Colony

• For bumblebees, the percentage of colonies with parasites, the total number of parasites per colony, and the number of parasite species per colony are lower in genetically diverse colonies.

Concession Theory

• Dominant females may allow subordinates to breed in order to retain them in the colony.
• By allowing them to breed a little, their fitness will be greater than if they were to try to breed on their own.
• Concession theory predicts:
  • Dominants will tolerate more breeding by subordinates when unrelated to them.
  • If difficult to breed elsewhere, subordinates will require fewer reproductive concessions at home.

Taxa Other than Hymenoptera

• The question is posed whether haplodiploidy and eusociality are just unrelated quirks of the Hymenoptera.
• Gall-forming thrips (Thysanoptera) on acacias have soldier castes and they are haplodiploid. A foundress female mates with only one male and soldiers have a high coefficient of relatedness with their reproductive siblings.

Does Eusociality Require Haplodiploidy?

• Naked mole rats are diploid, but in many colonies, individuals are highly related because of inbreeding.

Cooperative, Eusocial Naked Mole Rats

• Naked mole-rat colonies are highly cooperative.

Kin Selection

• Kin selection is natural selection that favours altruistic behaviour by enhancing reproductive success of relatives, thus increasing the inclusive fitness of the altruist.
• This provides an explanation for cooperative breeding.