Social Behaviors: Altruism to Spite

Social Behaviors

• Animals often cooperate with each other– But in most cases this cooperation is mutually

beneficial and is explained by self interest

Classification of Social Behaviors

Effect on actor Effect on Recipient

+ -

+ Mutually beneficial Selfish

- Altruistic Spiteful

Altruism

• A behavior is altruistic if it is costly to the personal reproduction of the actor who performs the behavior, but beneficial to other individuals

• Was initially a puzzle to behavioral ecologists since evolution acts on individuals

• Dawkins The Selfish Gene

Kin Selection and Inclusive Fitness

• Animals often cooperate with each other because they are related to each other- kin selection

• The likelihood that individuals share a same gene that is identical by descent provides a coefficient of relatedness or r

Coefficient of Relatedness

• How to calculate r– r = Σ (0.5) L

– At each generation link there is a 0.5 probability that a copy of a gene will get passed on

– For L generation links the probability is (0.5) L

Parent and offspring

R=1(0.5)1=0.5

Grandparent and grandchild

R=1(0.5)2=0.25

• How to calculate r– r = Σ (0.5) L

– At each generation link there is a 0.5 probability that a copy of a gene will get passed on

– For L generation links the probability is (0.5) L

Full siblings

R=2(0.5)2=0.5

Grandparent and grandchild

R=1(0.5)2=0.25

Coefficient of Relatedness

• Or just memorize it

r Descendant Kin Non-descendant Kin

0.5 Offspring Full siblings

0.25 Grandchildren Half-Siblings

Nephews and Nieces

0.125 Great-grandchildren Cousins

Coefficient of Relatedness

Inclusive Fitness

• From the view of a selfish gene– It can increase its transmission to the next

generation by increasing the reproductive success of the individual in which it is in (direct fitness)

– Or by increasing the reproductive success of other individuals who carry that gene (indirect fitness)

• Inclusive fitness is the sum of direct and indirect fitness

Inclusive Fitness

Hamilton’s Rule

• An altruistic act will spread due to kin selection if

• Altruistic cooperation can be favored if the benefits to the recipient (B) weighted by the genetic relatedness of the recipient to the actor (r) , outweigh the costs to the actor (C)

• Benefits are usually measured in the offspring you can produce by helping, and the costs are the number of offspring you lose by helping

BC

1r

> or rB-C >0

Hamilton’s Rule

• Imagine an individual can a) rear its own offspring or b) help its mother produce offspring– If full sibling, the individuals own offspring and its

mothers offspring have r=0.5, so B/C>1• Helping will be favored if you can help your mother rear

more full siblings than you can produce

Altruism

• The most extreme example of altruism is suicide and sterility in social insects– Worker bees sting predators and die as a result– Also do not reproduce themselves and instead

facilitate others to produce offspring

Altruism

• The marine insect equivalent: the snapping shrimp- Synalpheus regalis

– colonies contain over 300 individuals, but only one reproductive female

– kin-structured aggregations may promote evolution of specialized defenders that raise their own and the breeders' inclusive fitness by defending the colony.

Altruism

• The marine insect equivalent: the snapping shrimp- Synalpheus regalis– Because few predators can enter the narrow canals of

the sponges, the greatest competition comes from enemies that are of the same or closely related species

– Since most of the defenders do not breed, the only way to secure their genes in future generations is to protect their juvenile siblings, allowing them to grow to adulthood free from predation and survive long enough to reproduce.

–The only example of marine eusociality

Eusociality

• Eusociality- the highest level of social behavior and is defined by cooperative brood care (including brood care of offspring from other individuals), overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups

• Belding’s ground squirrels– Males leave the nest leaving their mother and

female siblings to stay in a closely related cohort– Females cooperate to defend each other’s young

against other infanticidal ground squirrels– Also give off alarm calls when predators are

nearby

Altruism

Kin discrimination

• In order for their to be kin selection, animals should have some mechanism to recognize their kin

• How might this occur?

• 1) Greenbeard allele– Recognition allele that expresses itself

phenotypically, enabling those possessing the gene to recognize that gene in others

• A single allele that a) causes the signal, b) causes it to be recognized in others, and c) produces cooperative behaviors• Is highly subject to cheating!

Kin discrimination

• Most greenbeard alleles are in microbes• But one is in fire ants- Gp-9 locus– New queens recruit into nests after dominant

queens mate– Workers with a b allele at the G9 locus use odor to

determine if recruiting queens possess the allele• And kill them if they don't

Kin discrimination

• 2) genetic kin discrimination- shared genes are recognized phenotypically– Chemical cues or odours- be altruistic to neighbors

who smell like you- ‘armpit effect’• 3) Environmental cues– Simple rules: Treat anyone at home as your kin– Learning: imprinting

Kin discrimination

• Beldings ground squirrells– Recognize kin by learning• Unrelated ground squirrels raised together are not

treated aggressively– There is also phenotype matching amongst

females– Odors from squirrel glands also give cues of

relatedness

Kin discrimination

Kin Selection and discrimination

• But kin discrimination isn't necessarily needed for kin selection– Populations with limited dispersal could lead to

high relatedness between interacting individuals, favoring indiscriminate altruism

Kin selection in oysters???

• Oyster broadcast their gametes and larvae spend a few weeks in the plankton before settling on other oysters– Oyster populations taken from different bays and

clumped together attract more spat than monocultures of oysters from one bay• Genetics being worked on…so stay tuned

Spite

• Hamilton’s rule also shows that natural selection can favor spiteful harming behaviors– rB-C>0………. • C is + behavior is costly to the actor, • B is - ; behavior is harmful to the recipient• R is -; recipeint is less related to the actor than an

average member of the population– Spite is favored when it is directed at non-relatives

and spiteful behavior frees up resources for relatives of the actor

Spite

• But many behaviors thought to be spiteful were actually just selfish as the behaviors provided a benefit in the long term

• Table 11.3

Spite

• Some wasp larvae develop soldiers to attack other larvae– Female soldier larvae attack brother larvae to free

up resources for the other females

Spite

• Only spiteful if • 1) Behavior is really costly and doesn’t give some

benefit – Soldier larvae aren't reproductive

• 2) Harming behaviors are directed towards relatively unrelated individuals– Soldiers preferentially attack larvae to which they are less

related• 3) There is strong competition for local resources and

variability in relatedness

Cooperation

• But what about those animals that cooperate that aren't kin?– How do we explain them?

Cooperation

• A behavior is cooperative if it provides a benefit to another individual and has been selected for because of its benefit to the recipient

• Why should an individual carry a behavior that benefits another individual?– Especially if that cooperation is cheated– The prisoners dilemma

Cooperation

• The prisoners dilemma is an illustration of how difficult a problem cooperation is…..

• Both individuals would benefit from mutual cooperation, but both are tempted to cheat

• So cooperation is not an necessarily an ESS, because a mutant cheater can spread

Cooperation

Hypothesis Description

Kin selection Helping individuals that share copies of the same gene

By-product benefits Cooperation acts as a byproduct of an otherwise selfish act

Reciprocity Helping another individual because the individual will help them back; tit for tat

Enforcement Rewarding cooperation and/or punishment for free riding

Cooperation: By-product benefit

• Foraging cooperatively is an example of by-product benefit– Animals can catch and consume more prey when

they work together– Everybody wins – Humpback whale feed cooperatively

Cooperation: Reciprocity

• Short term costs of helping another individual can be more than repaid by the help that is exchanged at a later point

• But how is it stable and not cheated?

• Animals must not only meet once– In terms of the prisoner’s dilemma, they must be

repeat offenders!• Tit-For-Tat Strategy– cooperate on the first move, and thereafter do

whatever your opponent did on a previous move• Start off nice and cooperate, punish defection, and

forgive (respond to cooperation of others, even if they initially defected)

Cooperation: Reciprocity

Cooperation: Enforcement

• Cooperation is much more likely if free-riders are punished

• Cleaners– Cleaner fish will eat parasites,but prefer to eat the

mucous or tissue of fish• If the fish bites its client, the larger fish aggressively

chases away the cleaner fish, making it less likely to take advantage in the future

The Ultimate Cooperation

• Interspecific Mutualism- a +/+ interaction in which each has behaviors that benefit the other species, but beneficial behaviors from the other species outweigh the costs

• Snapping Shrimp and Gobies• Many sessile species have mutualistic guards• Crab and Corals– The enemy of my enemy is my friend!