Chapter 11
Kin Selection and
Social Behavior
Types of Social Interactions
z Cooperation = mutualism
{Fitness gains for both participants
z Altruism
{Fitness gain for recipient
{Cost for actor
z Selfishness
{Actor gains
{Recipient loses
z Spite
{Fitness loss for both participants
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Prevalence of Altruism
z 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
z Darwin mentioned that altruism was a
“special difficulty” for his theory
z How can an allele that codes for
altruism survive in the face of natural
selection?
Kin Selection and the Evolution of
Altruism
z William Hamilton developed a genetic model
showing how an allele for altruistic behavior
could persist
z Coefficient of relationship, r
{Probability that homozygous alleles in two
individuals are identical by descent
z Hamilton’s Rule
{Altruistic behavior will spread if
z Br – C > 0
• B = benefit to recipient
• C = cost to actor
Kin Selection and the Evolution of
Altruism
z Altruism will spread when benefits to
recipient are great, cost to actor is
small, and participants are closely
related
z Inclusive Fitness
{Direct fitness results from personal
reproduction
{Indirect fitness results from reproduction
by relatives
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Kin Selection and the Evolution of
Altruism
z Indirect fitness accrues when relatives
reproduce more than they would have
without aid by actor
z When natural selection favors the
spread of alleles that increase indirect
fitness, Kin Selection occurs
z Kin selection explains many cases of
apparent altruism
{True altruism does not exist in nature
Calculating Coefficients of
Relatedness
z Haldane was quoted in a bar: “I would
lay down my life for the sake of two
brothers or eight cousins”
z 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
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Alarm Calling in Belding’s Ground
Squirrels
z When predators approach individuals give an
alarm call.
z A trill in response to mammals and a whistle
in response to hawks
z 2% the whistler is captured while nonnonwhistlers are captured 28%
z The squirrel raising the alarm reduces its
own chances of capture by calling
z Whereas 8% are killed when trill is giving
compared to 4% nonnon-trill.
z Whistles are selfish, trills are genuinely
altruistic
Alarm Calling in Belding’s Ground
Squirrels
z Females are more likely to give alarm
calls than males
z Mothers, daughters, and sisters were
more likely to assist each other chasing
trespassers off their territories than
unrelated individuals
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WhiteWhite-Fronted Bee Eaters
z Young adults forgo breeding to help
their parents raise their siblings
{Nest building, nest defense, food delivery,
incubation
z Helping at the nest usually found in
species where breeding opportunities
are limited
{Best of a bad job strategy
z Bee eater coefficient of relatedness
determines if they will help
WhiteWhite-Fronted Bee Eaters
WhiteWhite-Fronted Bee Eaters
z Presence of helpers increases parental
success by 0.47 fledglings
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Kin Selection in Other Contexts
z Cannibalistic tadpoles (Spadefoots
(Spadefoots
toads)
{2 morphs
zNormal (omnivorous, generally eat plant
mat.
zCannibalistic (enlarged jaw muscles)
{28 Can in sep. containers
zShared with 2 omniv.
omniv. Never seen before (1
full sib, 1 not related. (should eat randomly)
z6 out of 28 ate siblings
Kin Selection in Other Contexts
Kin Selection in Other Contexts
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Altruistic Sperm
Evolution of Eusociality
z True sociality describes social systems
with three characteristics
{Overlap in generations
{Cooperative brood care
{Specialized castes of nonreproductive
individuals
z Will examine two groups
{Hymenoptera
{Naked mole rats
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Haplodiploidy and Eusocial
Hymenoptera
z Hymenopterans exhibit most extreme
form of eusociality
z Millions of individuals per colony
{Very few reproduce
z How can this persist?
z Hamilton proposed that haplodiploidy
may be the reason
{Females grow from fertilized eggs
{Males grow from unfertilized eggs
Haplodiploidy and Eusocial
Hymenoptera
z Because of disparity in chromosome
number, sisters share 75% of genes
{Parents and offspring share 50%
{Females are better off rearing sisters than
offspring
z Testing haplodiploidy hypothesis
{Workers should prefer to invest in sisters
over brothers
zRelated to sisters 3/4
zRelated to brothers 1/4
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Haplodiploidy and Eusocial
Hymenoptera
z 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
z Does haplodiploidy explain eusociality?
eusociality?
{Workers should favor production of sisters if
all have the same father
zHoneybee queens mate over 17 times when
founding a colony
zAverage r for workers is 1/3
zIn this case, workers are not more closely
related to sisters than offspring
zSometimes more than one queen founds nest
zSome workers may not be related at all
zMany eusocial species are not haplodiploid
Using Phylogenies to Analyze
Social Evolution
z 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
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Using Phylogenies to Analyze
Social Evolution
z Phylogeny suggests that the primary
agent favoring eusociality is not
genetic
z 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
z Polistes paper wasps are not
completely eusocial
z Workers are not sterile
z Females may reproduce on their own
z Females pursue one of three strategies
{Initiate own nest
{Join a nest as a helper
{Wait for a breeding opportunity
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Facultative Strategies in Paper
Wasps
z Examine costs and benefits of each
strategy
z 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
z 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
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Facultative Strategies in Paper
Wasps
z If females do not help found nests and
wait, they may be able to adopt an
alreadyalready-constructed nest
{SitSit-andand-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
z Sociality is facultative in Polistes
z Adaptive response to environmental
conditions
z Important conditions are female body
size relative to competitors, coefficient
of relatedness, and availability of nest
sites
z Genetic, social, and ecological factors
important
Naked Mole Rats
z Live underground in huge nests in Africa
z Colonies of 7070-80 members
z Hairless, ectothermic, digest cellulose
z All species are eusocial
{Single queen
{2-3 reproductive males
{Workers are males and females
z Castes change with age
• First they tend young
• Later they excavate tunnels
• The oldest defend the nest
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Naked Mole Rats
z Not haplodiploid
z Why are they eusocial?
eusocial?
{Highly inbred
zAverage r for siblings is 0.81
zHighest coefficient of relatedness ever
recorded in mammals
{Still conflict among group members
zWorkers more closely related to offspring
than halfhalf-siblings
Naked Mole Rats
z 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
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ParentParent-Offspring Conflict
z Parental care is a special case of kin
selection
z Even parents and offspring can have
conflicts in costs and benefits
z 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
ParentParent-Offspring Conflict
z 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
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ParentParent-Offspring Conflict
z 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
ParentParent-Offspring Conflict
z Weaning Conflict
{By continuing to nurse, offspring devalue
mother’s cost of care
{Offspring should continue to try to nurse until
benefitbenefit-cost ratio is 1/2
{Period between these stages is weaning
conflict
{Avoidance and aggressive behavior
throughout this period
{If halfhalf-siblings are produced then ratio should
be extended to 1/4
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ParentParent-Offspring Conflict
z WhiteWhite-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
zFathers prevent courtship feeding
zHarassment is preferentially directed at
sons to prevent them from breeding and
coerce them to help at the nest
ParentParent-Offspring Conflict
z WhiteWhite-fronted Bee Eaters
{16 of 47 observed harassment events resulted
in successful recruitment to help
{Why don’t sons resist more effectively?
zSons are equally related to siblings and own
offspring
zParents are more closely related to each
offspring than to grandchildren
zHelpers add 0.47 offspring to parental success
zNearly same as own offspring
• Worth it to save harassment from father
ParentParent-Offspring Conflict
z Siblicide
{In birds and mammals it is common to kill
siblings
{Seems maladaptive since r = 1/2
{Lougheed and Anderson studied boobies
in Galá
Galápagos Islands
{Lay clutch of two eggs separated by 22-10
days
{First chick often pushes younger from
nest
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ParentParent-Offspring Conflict
z Siblicide
{Masked boobies push second egg from
nest immediately
{BlueBlue-footed booby older chicks may
reduce food intake during short food
shortages to provide extra for sibling
zDuring long food shortages they kill their
siblings
{What role do parents play?
zParents should intervene to prevent death
of any of their chicks
ParentParent-Offspring Conflict
z 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)
{BlueBlue-footed booby parents intervene but
masked do not
{Why there is a difference among species
is not known
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Reciprocal Altruism
z What about cooperation among
unrelated individuals?
z Trivers proposed individuals will act
altruistically if favor is later returned
z 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
z 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
z Roughly equal benefits and costs
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Reciprocal Altruism
z Will evolve in longlong-lived, intelligent,
social species with small group size,
low dispersal rates, and mutual
dependence in activities
z Less likely to evolve in species with
dominance hierarchies
z Difficult to observe and quantify in
nature
Reciprocal Altruism
z BloodBlood-Sharing in Vampire Bats
{Social group of 88-12 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 6767-93% of time
zPrey are wary
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Reciprocal Altruism
z BloodBlood-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
{BloodBlood-sharing is not random but based on
relatedness and hope of future reciprocity
Reciprocal Altruism
z 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
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Reciprocal Altruism
z 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
z Social interactions are very complex
and much needs to be learned about
them to fully understand their natural
selection
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