Chapter 13: Evolution of Social
Behavior
 Costs
 There
and benefits of Social Life.
are a large number of possible
costs and benefits associated with social
behavior.
Potential costs and benefits of
sociality
 Greater
conspicuousness to predators, but
also better defense against predators.
 Many
social behavior e.g. schooling by
fish seem to be primarily anti-predator
behaviors.
Fig 13.6
Schooling catfish
Potential costs and benefits of
sociality
 There
have been numerous studies that
have documented the anti-predator
benefits of social behavior.
 Groups
detect predators sooner and there
is also a dilution effect by being a member
of a group.
Potential costs and benefits of
sociality
 Group
defense also is a benefit.
 Colonially nesting gulls deter predatory
birds.
 Males in colonies of bluegill sunfish
collaborate to drive away egg-eating
predators.
Male bluegill sunfish nest colonially as defensive adaptation against
egg-eating predators.
Potential costs and benefits of
sociality

If bluegill sunfish have evolved colonial nesting
to deter predators, then we would expect
solitarily nesting related species to suffer less
from predators.

As predicted, solitarily nesting pumpkinseed
sunfish has powerful jaws with which it can deter
predators and so does not need to group nests
for protection.
Potential costs and benefits of
sociality
 More
rapid disease transmission is likely
among social organisms and parasites can
spread more readily.
 There may be some advantage is sociality
in that more grooming assistance may be
available, but on balance disease
transmission appears to be a clear cost of
sociality.
Potential costs and benefits of
sociality
 For
example, colonial cliff swallow
nestlings are much more affected by
swallow bugs than solitarily nesting birds.
 Nestlings
parasitized by bugs were
significantly smaller and less likely to
survive than unparasitized individuals.
Fig 13.5
Cliff swallow young exposed to parasites (left) and unexposed (right).
Potential costs and benefits of
sociality

More competition for food is a likely cost of
sociality.

Among lions females are forced to wait until the
males have fed before having a chance to eat.

In fieldfares (a European thrush) the larger the
colony, the lower the survival rate of nestlings
because starvation rates increase.
Fig 13.4
Potential costs and benefits of
sociality

However, for other birds (and bees), which feed
on spatially clumped, but unpredictable food
supplies, colonial breeding appears to
significantly improve foraging success.

This can occur through the use of information
centers and more effective use of local
enhancement information because coloniality
clumps foragers in space.
Potential costs and benefits of
sociality
 Sociality
also increases opportunities for
reproductive interference, which can be
positive or negative depending on an
individual’s success.
Potential costs and benefits of
sociality
 For
social behavior to evolve the balance
of costs and benefits should overall favor
the behavior.
 A major
component of social behavior is
positive interactions with other individuals.
Two major factors will dictate whether
individuals are likely to behave
altruistically towards others.
Altruism
 Altruisitic
behavior is puzzling as it is
behavior that imposes a cost on the actor
for the benefit of another individual.
 It
should thus be selected against.
 However,
kin selection and the possibility
of reciprocal altruism can favor altruistic
behavior.
Coefficient of relatedness
 A key
parameter in understanding kin
selection is the coefficient of
relatedness: r.
r
is the probability that the homologous
alleles in two individuals are identical by
descent.
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.
Hamilton’s rule
 Given
r the coefficient of relatedness
between the actor and the recipient,
Hamilton’s rule states that an allele for
altruistic behavior will be favored and
spread if
 Br - C >0
 Where B is benefit to recipient and C is the
cost to the actor. Unit of measurement for
B and C is surviving offspring.
Hamilton’s rule
 Altruistic
behaviors are most likely to
spread when costs are low, benefits to
recipient are high, and the participants are
closely related.
Inclusive fitness
 Hamilton
invented the idea of inclusive
fitness. Fitness can be divided into two
components:
 Direct fitness results from personal
reproduction
 Indirect fitness results from additional
reproduction by relatives, that is made
possible by an individual’s actions.
Kin selection
 Natural
selection favoring the spread of
alleles that increase the indirect
component of fitness is called kin
selection.
Kin selection in Belding’s
Ground Squirrels
 Giving
alarm calls alerts other individuals
but may attract a predator’s attention.
 Belding’s
Ground Squirrels give two
different calls depending on whether
predator is a predatory mammal (trill) or a
hawk (whistle; Sherman 1985).
Is alarm calling altruistic?
 Sherman
and colleagues observed 256
natural predator attacks.
 In
hawk attacks whistling squirrel is killed
2% of the time whereas non-whistling
squirrels are killed 28% of the time.
 Calling squirrel appears to reduce its
chance of being killed.
Kin selection in Belding’s
Ground Squirrels
 In
predatory mammal attacks trilling
squirrel is killed 8% of the time and a nontrilling squirrel is killed 4% of the time.
 Calling
squirrel thus appears to increase
its risk of predation.
 Whistling appears to be selfish, but trilling
altruistic.
Kin selection in Belding’s
Ground Squirrels
 Belding’s
Ground Squirrels breed in
colonies in Alpine meadows.
 Males
disperse, but female offspring tend
to remain and breed close by. Thus,
females in colony tend to be related.
Kin selection in Belding’s
Ground Squirrels
 Sherman
had marked animals and had
pedigrees that showed relatedness among
study animals.
 Analysis
of who called showed that
females were much more likely to call than
males.
Kin selection in Belding’s
Ground Squirrels
 In
addition, females were more likely to
call when they had relatives within
earshot.
Kin selection in Belding’s
Ground Squirrels
 Relatives
also cooperated in behaviors
besides alarm calling.
 Females
were much more likely to join
close relatives in chasing away
trespassing ground squirrels than less
closely related kin and non-kin.
Kin selection in Belding’s
Ground Squirrels
 Overall,
data show that altruistic behavior
is not randomly directed. It is focused on
close relatives and should result in indirect
fitness gains.
Reciprocal Altruism
 The
second major way in which altruism
can be favored is if recipients repay
altruistic behavior in the future.
Reciprocal Altruism
 Some
animals occasionally behave
altruistically towards non-relatives.
 Such
behavior is adaptive if the recipient is
likely to return the favor in the future.
Reciprocal altruism
 Reciprocal
altruism most likely in social
animals where individuals interact
repeatedly because they are long-lived
and form groups, and also when
individuals have good memories.
Reciprocal altruism in Vampire bats
 E.g.
Vampire Bats. Feed on blood and
share communal roosts.
 Bats
may starve if they fail to feed several
nights in a row.
 However,
bats who have fed successfully
often regurgitate blood meals for
unsuccessful bats.
Reciprocal altruism in Vampire bats
 Cost
of sharing some blood is relatively
low for donor bat but very valuable for
recipient.
 Research
shows that Vampire bats share
with relatives, but also share with
individuals who have shared with them
previously and with whom they usually
share a roost.
Association is measure
of how frequently two
individuals associate
socially. Regurgitators
regurgitate to
individuals they
associate with regularly.
Helpers at the nest.
White-fronted Bee-eaters
 In
a large number of birds young that are
old enough to breed on their own instead
help their parents rear siblings.
 Helpers
assist in nest building, nest
defense and food delivery.
Helpers at the nest.
White-fronted Bee-eaters
 Helping
usually occurs in species where
breeding opportunities are limited:
territories or nest sites are hard to acquire.
 Young
make the best of a bad job by
remaining home to assist their parents.
Helpers at the nest.
White-fronted Bee-eaters
 Steve
Emlen et al. studied white-fronted
bee-eaters intensively in Kenya.
 Nest
in colonies of 40-450 individuals.
Groups of relatives (clans) defend feeding
territories in vicinity of colony.
Helpers at the nest.
White-fronted Bee-eaters
 First
year birds that opt to help can choose
among many relatives when deciding
whom to help.
 Bee-eaters
conform to predictions of
Hamilton’s rule.
Helpers at the nest.
White-fronted Bee-eaters
 Coefficient
of relatedness determines
whether a bee-eater helps or not.
 Also,
bee-eaters choose to help their
closest relatives.
Helpers at the nest.
White-fronted Bee-eaters
 Nonbreeders
in clan that are not relatives
(birds that have paired with members of
the clan) are not related to offspring being
reared and are much less likely to help
than relatives.
Helpers at the nest.
White-fronted Bee-eaters
 Assistance
of helpers is of enormous
benefit to parents. More than 50% of beeeater young starve before leaving the nest.
 On
average, presence of each helper
increases number of offspring successfully
reared to fledging by 0.47. Thus, there is
a clear inclusive fitness benefit.
Inclusive fitness and Pied
Kingfishers
 Another
example of a species with helpers
at the nest is the Pied Kingfisher.
 Pied
Kingfishers nest colonially in tunnels.
 Some
one-year old males may not be able
to find a mate and so become primary
helpers assisting their mother to feed
young and deter predators.
Inclusive fitness and Pied
Kingfishers
 Primary
helpers have alternatives. They
could choose not to help and delay
breeding until next year (delayer) or assist
at another unrelated nest (secondary
helper).
 What
are costs and benefits of being a
primary helper?
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.
Evolution of Eusociality
 Eusociality
(true sociality).
 Many
eusocial insects (bees, ants,
termites) do not reproduce.
 Instead
they act as helpers at parents
nests for their entire life.
Evolution of Eusociality

Sterility and obligate helping is an extreme type
of altruism that goes far beyond the helpers at
the nest behavior seen in birds.

Suicidal behavior in defense of the group is quite
common.

E.g. honey bee stings are barbed so that when a
bee stings it leaves its poison sac behind and
fatally injures itself. One species of ant has
grenade soldiers that burst an abdominal gland
and spray glue on enemies.
Evolution of Eusociality
 Eusociality
describes social systems with
three characteristics:



Overlap in generations between parents and
offspring.
Cooperative brood care.
Specialist castes of non-reproductive
individuals.
Haplodiploidy and eusocial
Hymenoptera
 One
idea advanced to explain eusociality
is the unusual genetic system
(Haplodiploidy) of the Hymenoptera (ants,
wasps, bees, etc.).
 Males
 Males
are haploid and females diploid.
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.
Haplodiploidy and eusocial
Hymenoptera
 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 eusocial
Hymenoptera
 Further
evidence that workers manipulate
sex ratios in their favor comes from
studies in which queen and worker
relatedness is altered.
Haplodiploidy and eusocial
Hymenoptera

Mueller studying a eusocial bee removed the
foundress queen from some nests, but not
others. When a queen is removed a daughter
takes over as queen.

Workers whose queen was removed are now
helping queen produce nieces and nephews (r =
0.375 for both) and are equally related to both.
Colonies where queens replaced produced far
more males than colonies where original queen
was left in place.
Haplodiploidy and eusocial
Hymenoptera
 In
a species of Formica ant colonies
queens may be monogamous or
polyandrous.
 Daughters
of single-mating mothers
heavily biased investment towards
daughters, but daughters of polyandrous
queens did not bias reproduction towards
females.
Haplodiploidy and eusocial
Hymenoptera
 Haplodiploidy
appears to influence worker
behavior, but consensus today is that it
alone cannot explain evolution of eusocial
behavior in Hymenoptera.
 There
are several reasons why.
Haplodiploidy and eusociality
 First,
haplodiploid explanation assumes all
workers have the same father. However,
honeybee queens mate with more than 17
males on average.
 As
a result relatedness between worker
honeybees often below 1/3.
Haplodiploidy and eusociality
 Second,
in many species, more than one
female founds a nest. In this case workers
may be completely unrelated.
Haplodiploidy and eusociality
 Third,
many eusocial species are not
haploid (e.g. termites) and many
haplodiploid species are not eusocial.
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.
Haplodiploidy and eusociality
 Nest
building and need to supply offspring
with a steady stream of food make it
impossible or very difficult for a female to
breed alone.
 Also,
if predation rates are high, solitary
breeding individuals may not live long
enough to raise their young.
Facultative strategies in paper
wasps.
 Paper
wasps (Polistes) are not sterile
(unlike ant and bee workers). Females
can remain at a nest with their mother,
nest with other females or establish their
own nest.
 Paper
wasp queens produce daughters
early in the reproduction cycle because
many stay at home to help rear siblings.
Facultative strategies in paper
wasps.

Daughters who help their mother could lay
unfertilized eggs to produce male offspring, but
usually don’t (when they do the queen [the eggs
grandmother] often eats them).

Benefits to daughters are in indirect fitness.
They enhance the success of nests by deterring
nest predators . Nests that have helpers
removed are more likely to fail.
Facultative strategies in paper
wasps.
 Not
all species of paper wasp follow the
mother-daughter model and some nests
may be made up of a mixture of relatives
and non-relatives or entirely of nonrelatives.
Facultative strategies in paper
wasps.
 Nonacs
and Reeve (1995) found in
Polistes dominulus that females of this
species follow one of three strategies.
 Initiate
own nest
 Join nest as a helper
 Wait for a nest to become available
Facultative strategies in paper
wasps.
 Individuals
founding their own nest are
very likely to fail because adult mortality is
high.
 Multiple
foundresses, however, can keep
the nest going.
Facultative strategies in paper
wasps.
 However,
in multifoundress nests there
may be frequent conflict.
 The
nests that did best were those where
one female was markedly bigger than the
others, which reduced fighting.
Facultative strategies in paper
wasps.
 Usually,
the queen dominates egg laying
(95% of eggs being hers: she generally
eats other females’ eggs).
 Helpers
who are relatives gain indirect
fitness benefits, but why do non-relatives
help?
Facultative strategies in paper
wasps.
 Often,
they can inherit the role of queen if
the queen dies.
 In
one study of 28 nests there were 13
changes of ownership in a season and 10
were achieved by resident helpers.
Facultative strategies in paper
wasps.
 In
some paper wasps, the queen cedes
some reproduction to non-relatives to
persuade them to stay as helpers.
 For
example, in Polistes fuscatus there is
a clear correlation between the proportion
of reproduction by the queen and her
relatedness to other foundresses.
Facultative strategies in paper
wasps.
 Some
individuals in Polistes dominulus
choose not to join an established nest as a
helper.
 This
“sit-and-wait” strategy also can pay
off because a female often can adopt an
orphaned nest or take one over late in the
season.
Facultative strategies in paper
wasps.
 Overall,
in paper wasps an individual’s
decision whether to be a helper or not is
affected by her relative size, relatedness
to other females, and the availability of
unoccupied nests.
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.
Naked Mole Rats
Fig 51.33
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 molerats 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.