The Evolution of Social Behavior

Chapter 13

Or

Why can’t we all just get along?

Costs and BenefitsCosts

Increased competition within the group for food, mates, or other limited resourcesIncreased risk of infection by contagious diseases or parasitesIncreased risk of exploitation of or interference with parental care by other group members

BenefitsReduction in predator pressure by improved detection or repulsion of enemies or by the dilution of riskImproved efficiency in foraging for large or evasive preyImproved defense of key spatial or food resources against other groups of conspecificsImproved care of offspring through communal feeding and protection

ExamplesCliff swallows

Cost – ectoparasite infection of Oeciacus vicarius Benefit – anti-predator mobbing behavior Bluegills

Cost – fungal infectionsBenefit – anti-predator against bass and catfish

Optimal Sociality

Optimal group size, colony size, etc.

Maximizes fitness of the group members

predatorspoor food

diseasecompetition for food

G*Group size

Fitn

ess

Is this Real?

We should never see optimal group sizes (G*) in nature

Not stable

Expect to see Group sizes > G*When would you expect to see optimal group sizes?When a social system enforces G* and kicks others out / limits entry to groupExample - Chickadee and titmice flocksAnti-Example - Starlings

“Helpful behavior”

Shared mating effort

Mutualism: cooperative behavior with payoff

Long-tailed manikins

•Males work in pairs to attract females

•Dominant male receives all mating benefits

•Subordinate becomes dominant when the other male dies/disappears

Reciprocal altruism: One organism provides a benefit to another in the expectation of future reciprocation

•Assumes that cheaters can be identified/punished•Assumes that the giver incurs a short-term personal cost•Assumes that receivers are not preferentially related to givers

Meerkats: Reciprocal altruists?If true: meerkats should:

•Share sentinel duties equally•Sentinels should be at greater risk

Welcome to the Game

Situation

2 players looking for a “nash equilibrium”, or selfish equilibrium, that is stable.

If fitness of both players are equal at nash equilibrium, have ESS.

Evolutionary Stable Strategy

On to the Prisoner’s Dilemma

The Dilemma

You are one of two prisoners with limited ratio of daily food.If you take it all (cheat) you are better off than sharing (coop).If you both cheat, you get little ration (food lost)

How do you ration/partition the food to survive?

Payoff Matrix – to cheat or cooperate

(R, R) (S, T)

(T, S) (P, P)

Cooperate Cheat

Cooperate

Cheat

Pla

ye

r 1

Player 2

T>R>P>S (payoff player 1, payoff player 2)

T – temptation to cheat (15 pt)R – reward for cooperation (10 pt)P – penalty for cheating (5 pt)S – suckers payoff (2 pt)

Payoff Matrix – to cheat or cooperate

Conditions under which altruistic behavior should could occur:•When there is opportunity for repeated opportunities to give/receive by the same unrelated players: tit for tat•When players are related

The Dilemma

Mutual coop gives the highest payoff for both players, but cheating is the ESS.

Discussed by Axelrod and Hamilton Science 211(27 Mar 1981) 1390-1396

A possible situation to achieve cooperation is tit-for-tat

Tit-for-Tat (TFT)

“Do unto another as they do unto you”

Initially cooperate with a stranger

If stranger cooperates, keep cooperating in following exchanges

If stranger cheats, you CHEAT!

TFT is a possible ESS.

Hard to see how it starts, Why?

Requirements

Must be able to remember actions of othersMust be able to recognize others (individuals)No fixed end to the interactions between players

With a fixed end point, cheating the last round is favored

Overall payoff is a sum of all individual payoffs/interactions

Possible Examples

Vampire bats

Baboons

Lions

Problems w/ TFT-based cooperation

Can’t invade a population of cheatersPayoff’s to players are unlikely to be “additive”Payoff’s are likely to be multiplicative over time.

Ex. 10 plays of the gameMust survive all 10 playsLet p = prob of surviving each playp10 = prob of surviving all ten plays

Effect of this

Your own personal fitness is likely to be influenced by the fitness of the other player.

When fitnesses are linked in a multiplicative way, you will cooperate in your own self interest until the end

A Problematic Case of Apparent Altruism

Giving up reproduction to help others reproduce

“Altruism” and indirect selection

There has to be something in it for the helper, otherwise it is not adaptive.

Calculating r

Need a pedigree to calculate r that includes both the actor and recipient and that shows all possible direct routes of connection between the two.

Because parents contribute half their genes to each offspring, the probability that genes are identical by descent for each step is 50% or 0.5.

Calculating r

To calculate r one should trace each path between the two individuals and count the number of steps needed. Then for this path r = 0.5 (number of steps)

Thus, if two steps r for this path = 0.5 (2) = 0.25.To calculate final value of r one adds together the r values calculated from each path.

Coefficient of Relatedness “r”r = proportion of alleles shared between 2

individuals by common decent.Full siblings and parent-offspring = 0.5

½ siblings & aunt/uncle-niece/nephew = 0.25

Grandparent-grandchild = 0.25

Cousin-cousin = 0.125

Mother Sister

Offspring Offspring

0.5 0.5

0.5

0.5 * 0.5 * 0.5

0.125

Key Question

Can an animal leave more copies of itself (it’s own genes) by reproducing itself or by helping a relative reproduce?

Inclusive fitness

Hamilton invented the idea of inclusive fitness. Fitness can be divided into two components: Direct fitness results from personal reproductionIndirect fitness results from additional reproduction by relatives, that is made possible by an individual’s actions.

Terms

Inclusive fitness (IF) = direct fitness + indirect fitness

Direct fitness = personal reproduction

Indirect fitness = reproduction through relatives (helping)

Hamilton’s Rule for giving “altruistic aid”

r = coefficient of relatednessc = cost of giving help (in terms of reproduction) b = reproductive benefit realized by helping relativeHelp when rb-c > 0 or rb > c

T or F An animal can realize new indirect fitness if it has closely-related living relatives that reproduce.

You can only recognize indirect fitness by helping (ie. you have to do something to acquire fitness)

F

Direct selection: traits producing altruistic acts that directly influences your/offspring personal fitness Indirect selection: traits producing altruistic acts that directly influences fitness of your genetic relatives Kin selection: traits producing altruistic acts directed at both offspring and closely related individuals

How altruistic traits could spread in a population

Direct selection: altruistic acts that directly influences your personal fitness Indirect selection: altruistic acts that directly influences fitness of close relatives Kin selection: altruistic acts directed at both offspring and closely related individuals

Belding’s Ground squirrels: •Alarm callers are far more likely to be captured than alarm call receivers •Females are twice as likely to give alarm calls

How altruistic traits could spread in a population

Belding’s Ground squirrels

Helpers in the Nest

Over 400 species of bird practice helping other reproduce and raise offspring

Based on four questions:

Are they related to the parents of the offspring?

Do they really help?

Does Hamilton’s Rule actually predict what’s going on?

What is restricting helpers from breeding?

Are they related to the parents of the offspring

Or is r > 0?

Most times, helpers are related, r > 0Full sibs (r=0.5) or step kids (r=0.25) are the most common.

If more than 0.5, then due to twins or inbreeding.

r = 0 can happen, due to “adopted” orphans helping a family

Do they actually help

Or is b > 0?

Yes

Helpers can feed offspring and protect young from predators.

Does Hamilton’s Rule actually predict what’s going on

Or is rb-c > 0 hold for helpers?Some cases in the tropics hold with rb-c > 0.

Most of the time rb-c < 0, better of reproducing yourself.

So why not reproduce?c is defined as lost reproduction assuming that helpers had the options of breeding on their own.

In reality, c can be really low with no opportunity to breed elsewhere.

B.B.S. – Making the best of a bad situation

What is restricting helpers from breeding?

Lack of breeding sites, resources, and experience

Pied Kingfisher

Altruism and inclusive fitness in the Pied Kingfisher

Four yearling male phenotypes:1. Those who find mates2. Those who don’t find a mate but help their parents (mother) raise

siblings (primary helpers)3. Those who don’t find a mate but help strangers raise siblings

(secondary helpers)4. Those who don’t find a mate and wait until next year (delayers)

Primary helpers work harder

What are costs and benefits of being a primary helper?

Three strategies with different fitness payouts

Inclusive fitness and Pied Kingfishers

Primary helpers work harder than delayers and secondary helpers so they have a lower chance of surviving to breed the next year (54%) than secondary helpers (74%) or delayers (70%).

Also only 66% of primary helpers attract mates, but 91% of secondary helpers do (in 10 of 27 cases with the female they helped the previous year). Delayers have only a 33% chance.

Inclusive fitness and Pied Kingfishers

To determine payoffs need to add the reproductive success of each approach over the two years.Calculate payoffs by multiplying probability of survival times number of offspring produced times probability of survival times probability finding a mate times relatedness to offspring.

Inclusive fitness and Pied Kingfishers

Primary helpers gain reproductive benefit in both years (0.58 [indirect fitness]+ 0.41 young [direct fitness]= 0.99).Secondary helpers obtain a second year payoff of 0.84 young and delayers only 0.29.Primary helpers have lower RS is year 2, but this is more than compensated for by indirect fitness benefit from year 1.

Eusociality

Three characteristics of eusocial systems:1. The mother, along with individuals (related or not), conduct cooperative

care of young. 2. A reproductive division of labor emerges from sterile castes which often

have certain morphological/behavioral enhancements for conducting specific tasks.

3. Generations are overlapped allowing older generations to help younger generations.

Insects:1. All known ant species2. Most bees3. Many wasps4. Termites (diploid) 5. Aphids and thrips (relatively simple guard caste only)

Crustaceans 1. Some shrimp

Mammals1. Naked mole rats

Social Behavior in Eusocial Insects

Extreme cases of apparent altruism

Sterile helpers/workers

Sterile soldiers – self-sacrificing

Highly cooperative workers

Ex. Termites (Isoptera), ants, wasps, bees (Hymenoptera), some aphids, naked mole rats.

Haplodiploidy and eusocial Hymenoptera

One idea advanced to explain eusociality is the unusual genetic system (Haplodiploidy) of the Hymenoptera (ants, wasps, bees, etc.).

Males are haploid and females diploid.

Males develop from unfertilized eggs and females from fertilized eggs.

Haplodiploidy and eusocial Hymenoptera

Daughters receive all of their fathers genes and half of their mothers genes. Thus, daughters share ¾ of their genes.

This suggests females would be better off if they favored the production of reproductive sisters rather than their own offspring.

Haplodiploidy and eusocial Hymenoptera

Queens are equally related to all offspring and so should prefer a 1:1 ratio of sons to daughters among reproductives.

Females workers however should prefer a 1:3 ratio of brothers to sisters among reproductives.

Eusociality and Indirect selection

Can kin selection explain eusociality?

Basic fact for hymenoptera = r sister-sister = 0.75

The larger the r, the more likely to be helpful and “altruistic”.

Haploid-Diploid Inheritance

Eusocial math

Does r = 0.75 explain eusocial hymenopterans?

Maybe? What is b?, c?

Eusocial aphid are direct clones of their mothers

r = 1.0

Genetic ConflictR queen-worker = 0.5R worker-worker = 0.75R worker-male = 0.25

Workers favor production of female reproductives at a 3:1 ratio.Queens prefer a 1:1 ratio of reproductives

Fact: queens produce 1:1 sex ratio in eggsWorkers produce, by feeding, a 3:1 ratio

It has been shown in wood ants that queens produce equal numbers of male and female eggs, but the hatching ratio is heavily female biased. Workers apparently selectively destroy male eggs.

Haplodiploidy and eusociality

Phylogenetic analysis of Hymenoptera by Hunt (1999) emphasizes that eusociality relatively rare even though haplodiploidy occurs in all groups.

Eusociality occurs in only a few families which are scattered around the tree, which suggests eusociality has evolved independently multiple times.

Haplodiploidy and eusociality

Hunt also points out that eusociality has only evolved in groups that build complex nests, and care for young for a long time.

Association between nest building, long term care and eusociality suggests main driving force for eusociality is ecological not genetic.

Naked Mole-rats

Naked mole-rats are highly unusual mammals.

They are nearly hairless and ectothermic. They are eusocial and, like termites, can digest cellulose with the help of bacteria in their gut.

Fig 51.33

Naked Mole Rats

Naked Mole-rats

The behavior of naked mole-rats is similar to that of termites.

Like termites both males and females are diploid (unlike the Hymenoptera).

Naked Mole-rats

Colony may include as many as 200 individuals but there is only a single reproductive female (queen) and 1-3 reproductive males.

Remaining individuals act as workers. They dig tunnels to find food, defend the tunnel system from other mole-rats, and tend the young.

Naked Mole-rats

Leading hypothesis for why naked mole-rats are eusocial is inbreeding.

Average coefficient of relatedness is 0.81 and about 85% of matings are between parents and offspring or between full siblings.

Naked Mole-rats

Despite high level of relatedness conflicts still occur because reproductive interests of workers and reproductives are not identical.

Naked Mole-rats

Queens maintain control through physical dominance. Queen aggressively shoves workers who do not work hard enough and shoves are mainly directly towards less closely related individuals. Workers double their work rate after being shoved.

Naked Mole-rats

In addition to inbreeding, ecological factors such as severely limited alternative breeding opportunities and group defense appear to contribute to eusociality in naked mole-rats.

Naked Mole-rats

In related Damaraland mole rat there does not appear to be inbreeding and reproductives have a mean r of 0.02.

Mean relatedness of colony members is close to 0.5 so in this species ecological factors may be main driver of eusociality.

Who should a female worker bee help first?a) The queenb) A male offspring c) A female offspring

What if your mother mated with 2 or more males not 1?•In this case females average .5 relatedness•Male female relatedness becomes equal •Resources are more evenly distributed.

•If your more related to your sister than you’re your mother or brother then you’ll help your sister •In haplodiploid systems there is a 3:1 relatedness ratio between females and males and resources in these systems are disproportionatly distributed by females to females

Problems with indirective selection and eusociaity

Termites are diploidr worker-worker = 0.5

Queen determines sterility (queen bee with royal jelly)

Worker caste does not have a choice to reproduceOnly option is to help

Queens may mate more than once, and hold sperm from several males

r < 0.75 if multiple fathers.Colonies sometimes have several unrelated queens

r is < 0.75, in fact r < 0.5 in cases.