Transcript Document

Types of Social Interactions
• Cooperation = mutualism
– Fitness gains for both participants
• Altruism
– Fitness gain for recipient
– Cost for actor
• Selfishness
– Actor gains
– Recipient loses
• Spite
– Fitness loss for both participants
Prevalence of Altruism
• Appears to be common in nature
– Young macaws help parents raise their siblings
instead of reproducing themselves
– Human runs into a burning house to save a child
• Darwin mentioned that altruism was a
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“special difficulty” for his theory
How can an allele that codes for altruism
survive in the face of natural selection?
Kin Selection and the Evolution
of Altruism
• William Hamilton developed a genetic model
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showing how an allele for altruistic behavior
could persist
Coefficient of relationship, r
– Probability that homozygous alleles in two
individuals are identical by descent
• Hamilton’s Rule
– Altruistic behavior will spread if
• Br – C > 0
– B = benefit to recipient
– C = cost to actor
Kin Selection and the Evolution
of Altruism
• Altruism will spread when benefits to
recipient are great, cost to actor is
small, and participants are closely
related
• Inclusive Fitness
– Direct fitness results from personal
reproduction
– Indirect fitness results from reproduction
by relatives
Kin Selection and the Evolution
of Altruism
• Indirect fitness accrues when relatives
reproduce more than they would have
without aid by actor
• When natural selection favors the
spread of alleles that increase indirect
fitness, Kin Selection occurs
• Kin selection explains many cases of
apparent altruism
– True altruism does not exist in nature
Calculating Coefficients of
Relatedness
• Haldane was quoted in a bar: “I would
lay down my life for the sake of two
brothers or eight cousins”
• Perform path analysis to assess
relatedness
– Parents related to offspring 1/2
– Full siblings related 1/2
– Half siblings related 1/4
– Cousins related 1/8
Alarm Calling in Belding’s
Ground Squirrels
• Females are more likely to give alarm
calls than males
• Mothers, daughters, and sisters were
more likely to assist each other chasing
trespassers off their territories than
unrelated individuals
White-Fronted Bee Eaters
• Young adults forgo breeding to help
their parents raise their siblings
– Nest building, nest defense, food delivery,
incubation
• Helping at the nest usually found in
species where breeding opportunities
are limited
– Best of a bad job strategy
• Bee eater coefficient of relatedness
determines if they will help
White-Fronted Bee Eaters
White-Fronted Bee Eaters
• Presence of helpers increases parental
success by 0.47 fledglings
Evolution of Eusociality
• True sociality describes social systems
with three characteristics
– Overlap in generations
– Cooperative brood care
– Specialized castes of nonreproductive
individuals
• Will examine two groups
– Hymenoptera
– Naked mole rats
Haplodiploidy and Eusocial
Hymenoptera
• Hymenopterans exhibit most extreme
form of eusociality
• Millions of individuals per colony
– Very few reproduce
• How can this persist?
• Hamilton proposed that haplodiploidy
may be the reason
– Females grow from fertilized eggs
– Males grow from unfertilized eggs
Haplodiploidy and Eusocial
Hymenoptera
• Because of disparity in chromosome
number, sisters share 75% of genes
– Parents and offspring share 50%
– Females are better off rearing sisters than
offspring
• Testing haplodiploidy hypothesis
– Workers should prefer to invest in sisters
over brothers
• Related to sisters 3/4
• Related to brothers 1/4
Haplodiploidy and Eusocial
Hymenoptera
• Testing haplodiploidy hypothesis
– Workers should favor 3:1 sex ratio
– Queens are equally related to sons and
daughters and should favor 1:1 sex ratio
– Conflict of interest between queens and
workers
– Sudstrom found that wood ant queens lays
equal numbers of male and female eggs
– Workers selectively killed male eggs prior
to hatching
– Workers win sex ratio battle
Haplodiploidy and Eusocial
Hymenoptera
• Does haplodiploidy explain eusociality?
– Workers should favor production of sisters if
all have the same father
• Honeybee queens mate over 17 times when
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founding a colony
Average r for workers is 1/3
In this case, workers are not more closely
related to sisters than offspring
Sometimes more than one queen founds nest
Some workers may not be related at all
Many eusocial species are not haplodiploid
Using Phylogenies to Analyze
Social Evolution
• Hunt reconstructed a phylogeny of
hymenopterans
– All are haplodiploid
– Few families are eusocial
– Eusocial families not closely related
– Eusociality must have evolved multiple
times
– Evolved in groups that build complex
nests and have extended care for larvae
Using Phylogenies to Analyze
Social Evolution
• Phylogeny suggests that the primary
agent favoring eusociality is not
genetic
• Best of a bad job hypothesis
– Building a complex nest and caring for
many larvae would be impossible for a
female to do by herself
– Must examine factors that affect B and C
as well as r
Facultative Strategies in
Paper Wasps
• Polistes paper wasps are not
completely eusocial
• Workers are not sterile
• Females may reproduce on their own
• Females pursue one of three strategies
– Initiate own nest
– Join a nest as a helper
– Wait for a breeding opportunity
Facultative Strategies in
Paper Wasps
• Examine costs and benefits of each
strategy
• Nests founded by single females or
multifemale groups
– Single foundress nests less successful
– Multifoundress nests more likely to be rebuilt
if destroyed
– Fights among foundresses determined by
body size
– Multifoundress nests grew fastest if large size
difference of dominant female and
subordinate helpers
Facultative Strategies in
Paper Wasps
• Why would females join a coalition and
help rear offspring that are not theirs?
– Indirect fitness gains because usually
related to foundress
– Direct fitness gains if foundress dies and a
subordinate inherits the nest
– Costs and benefits of this strategy depend
on female’s body size and coefficient of
relatedness
Facultative Strategies in
Paper Wasps
• If females do not help found nests and
wait, they may be able to adopt an
already-constructed nest
– Sit-and-wait tactic
– They leave their nest in spring and enter a
dormant state until the following season to
try to take over a new nest
Facultative Strategies in
Paper Wasps
• Sociality is facultative in Polistes
• Adaptive response to environmental
conditions
• Important conditions are female body
size relative to competitors, coefficient
of relatedness, and availability of nest
sites
• Genetic, social, and ecological factors
important
Naked Mole Rats
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Live underground in huge nests in Africa
Colonies of 70-80 members
Hairless, ectothermic, digest cellulose
All species are eusocial
– Single queen
– 2-3 reproductive males
– Workers are males and females
• Castes change with age
– First they tend young
– Later they excavate tunnels
– The oldest defend the nest
Naked Mole Rats
• Not haplodiploid
• Why are they eusocial?
– Highly inbred
• Average r for siblings is 0.81
• Highest coefficient of relatedness ever
recorded in mammals
– Still conflict among group members
• Workers more closely related to offspring
than half-siblings
Naked Mole Rats
• Conflict among group members
– Queens maintain control by physical dominance
– If the workers slow their work pace the queen
shoves them with her head
– Afterwards the workers increase their work pace
– Shoves are directed more often towards more
distant relatives
– Queen maintains eusociality by intimidation
Parent-Offspring Conflict
• Parental care is a special case of kin
selection
• Even parents and offspring can have
conflicts in costs and benefits
• Weaning Conflict
– Mothers begin to ignore or push young
away near end of weaning period
– Offspring will scream or attack mother
– Fitness interests are not symmetrical
Parent-Offspring Conflict
• Weaning Conflict
– Offspring are related to themselves r = 1
– Parents are related to offspring r = 0.5
– Parents are equally related to all offspring
and should optimize their investment in
each
– Offspring demand unequal amount of
parental investment
Parent-Offspring Conflict
• Weaning Conflict
– At start of nursing benefit of offspring
relative to cost of parent
– Ratio declines with time
– Young demand more milk which increases
parental cost
– Young can start finding own food which
decreases benefit
– Mothers should stop producing milk when
benefit to cost ratio reaches 1
Parent-Offspring Conflict
• Weaning Conflict
– By continuing to nurse, offspring devalue
mother’s cost of care
– Offspring should continue to try to nurse until
benefit-cost ratio is 1/2
– Period between these stages is weaning
conflict
– Avoidance and aggressive behavior
throughout this period
– If half-siblings are produced then ratio should
be extended to 1/4
Parent-Offspring Conflict
• White-fronted Bee Eaters
– Sons may set up territory or may help at
their parental nest
– Fathers coerce sons into helping by
harassing sons as they attempt to set up
territories
• Fathers prevent courtship feeding
• Harassment is preferentially directed at
sons to prevent them from breeding and
coerce them to help at the nest
Parent-Offspring Conflict
• White-fronted Bee Eaters
– 16 of 47 observed harassment events resulted
in successful recruitment to help
– Why don’t sons resist more effectively?
• Sons are equally related to siblings and own
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offspring
Parents are more closely related to each
offspring than to grandchildren
Helpers add 0.47 offspring to parental success
Nearly same as own offspring
– Worth it to save harassment from father
Parent-Offspring Conflict
• Siblicide
– In birds and mammals it is common to kill
siblings
– Seems maladaptive since r = 1/2
– Lougheed and Anderson studied boobies
in Galápagos Islands
– Lay clutch of two eggs separated by 2-10
days
– First chick often pushes younger from
nest
Parent-Offspring Conflict
• Siblicide
– Masked boobies push second egg from
nest immediately
– Blue-footed booby older chicks may
reduce food intake during short food
shortages to provide extra for sibling
• During long food shortages they kill their
siblings
– What role do parents play?
• Parents should intervene to prevent death
of any of their chicks
Parent-Offspring Conflict
• Siblicide
– Reciprocal transplant experiment
– Chicks more likely to die with masked
booby nestmate
– Chicks more likely to die with masked
booby parents (surrogate or real)
– Blue-footed booby parents intervene but
masked do not
– Why there is a difference among species is
not known
Reciprocal Altruism
• What about cooperation among
unrelated individuals?
• Trivers proposed individuals will act
altruistically if favor is later returned
• Two conditions for reciprocal alturism
to evolve:
– Cost to actor must be smaller than or
equal to benefit to recipient
– Individuals that fail to reciprocate must be
punished
Reciprocal Altruism
• Most likely to evolve when:
– Each individual repeatedly interacts with
same set of individuals
– Many opportunities for altruism in an
individual’s lifetime
– Individuals have good memories
– Potential altruists interact in symmetrical
situations
• Roughly equal benefits and costs
Reciprocal Altruism
• Will evolve in long-lived, intelligent,
social species with small group size,
low dispersal rates, and mutual
dependence in activities
• Less likely to evolve in species with
dominance hierarchies
• Difficult to observe and quantify in
nature
Reciprocal Altruism
• Blood-Sharing in Vampire Bats
– Social group of 8-12 females and their dependent
offspring
– Roost together and associate with each other
daily
– Average r between individuals in study group in
Costa Rica was 0.11
– Vampire bats share blood meals
– Hunting is difficult and individuals are only
successful 67-93% of time
• Prey are wary
Reciprocal Altruism
• Blood-Sharing in Vampire Bats
– Without eating for three nights, a bat will
starve to death
– Both degree of relatedness and degree of
association were significantly related to
probability of regurgitating blood
– Blood-sharing is not random but based on
relatedness and hope of future reciprocity
Reciprocal Altruism
• Territory Defense in Lions
– Females cooperate in defending young
against infanticidal males, hunting prey,
and defending pride’s territory
– When females hear the roar of a female of
another territory, they head to the area to
attack
– Some individuals always tend to lead the
attack while others always seem to lag
behind
Reciprocal Altruism
• Territory Defense in Lions
– Why are laggards tolerated?
– It is not known but perhaps the laggards
make up later by being exceptional
hunters or good milk producers
• Social interactions are very complex
and much needs to be learned about
them to fully understand their natural
selection