Social Behaviour
Chapter 19 (& 15: p. 275-276)
Evolution of Social Behaviour
Altruism
Kin Selection
Mutualism
Cooperation
Helpers
Alarm Calls
Eusociality
Evolution
Haplodiploidy
Environment
Read Pg.
Evolution
Reciprocal
altruism
Evolution
Prisoner’s
Dilemma model
Advantages of Group-living
Protection from predators
Improved food search/hunting
Easier location of mates
Physical warmth
Resource defense
Richer learning environment
Increased ability to modify environment
Costs of Group-living




Increased competition for resources
Increased exposure to disease/parasites
Increased conspicuousness to predators/ prey
Interference with reproduction
Types of Groups



Social vs. Non-Social (may be a continuum)
Evolution of simple aggregations
How might non-social groups have evolved?


Hamilton’s “Selfish Herd” hypothesis (Ch.15)
Complex social behaviours (altruism)
The Evolution of Complex Social
Behaviours

Evolution of Altruism


Degree of relatedness (r)



Altruism can increase fitness if individuals helped are
kin (relatives) = kin selection
r is the probability that alleles sampled from 2
individuals are identical by descent
Hamilton’s rule: rB>C
Kin Selection Theory

If you help your relatives to survive and reproduce,
than you are helping to pass on the genes that both
you and your relatives have in common.
Gr M
Gr F
Gr M
r=1/2
r=1/2
uncle r=1/2 mom
dad r=1/2 aunt
r=1/4
you
uncle
r=1/2
r=1/2
r=1/4
Gr F
r=1/2
r=1/2
sib
cous
r=1/8
Degrees of relatedness between:
You and mom, dad, or sib = ½ = 0.5
You and uncle = ½ * ½ = ¼ = 0.25
You and grandparents = ½ * ½ = ¼ = 0.25
You and cousin = ½ * ½ * ½ = 1/8 = 0.125
The Evolution of Complex Social
Behaviours

Kin Selection Theory


If you help your relatives to survive and
reproduce, than you are helping to pass on
the genes that both you and your relatives
have in common.
e.g. Alarm calls, Eusociality
Eusociality
Eusociality describes social systems with 3 key
characteristics:
(1) overlap in generations between parents and
offspring
(2) cooperative brood care
(3) non-reproductive worker caste

Eusociality in Social Insects


Haplodiploidy results in unusual
coefficients of relatedness (r)
males develop from unfertilized eggs,



haploid
do not have fathers, only mothers
females develop from fertilized eggs, are diploid


Sisters get the same set of chromosomes from their
father, (daughter to father r = 1)
but have a 50% chance of getting the same allele
from their mother (daughter to mother r = 1/2)
Sex determination in social insects
•The unusual genetic system of social insects makes
eusociality a likely consequence of kin selection
•In haplodiploidy, sex is determined by chromosome
number
-males develop from unfertilized eggs, are haploid
-females develop from fertilized eggs, are diploid
•Sons do not have fathers, only mothers
•Sisters get the same set of chromosomes from their
father, but have a 50% chance of getting the same allele
from their mother
Sex determination in social insects
Sisters are highly related to each other in haplodiploidy
Path #1
mother
father
(diploid)
(haploid)
.
Path #2
mother
father
(diploid)
(haploid)
1/2
1/2
1/2
.
1
X
sister A
sister B
sister
sister
Odds that one of sister A’s alleles came from mom = 1/2
Odds that mom gave the same allele to sister B = 1/2
-odds of identical-by-descent allele Path 1 =(½) (½) = ¼
Sex determination in social insects
Sisters are highly related to each other in haplodiploidy
Path #1
mother
father
(diploid)
(haploid)
.
Path #2
mother
father
(diploid)
(haploid)
1/2
1/2
1/2
.
1
X
sister
sister
sister A
sister B
Odds that one of sister A’s alleles came from dad = 1/2
Odds that dad gave same allele to sister B = 1
(his complete haploid genome)
-odds of identical-by-descent allele Path 2 = (½) (1) = ½
Sex determination in social insects
mother
(diploid)
father
(haploid)
mother
(diploid)
1/2
1/2
1/2
1
X
X
sister
father
(haploid)
sister
sister
sister
Combined odds of sisters sharing identical alleles:
(Path #1 odds) + (Path #2 odds) = (1/4) + (1/2) = 3/4
Because of this system, females are more related to their
sisters (r = ¾) than they are to their
own offspring (r = ½)
Sex determination in social insects
mother
(diploid)
father
(haploid)
mother
(diploid)
1/2
1/2
1
1/2
1/2
1/2
X
X
sister
father
(haploid)
sister
sister
brother
Sisters are only distantly related to their brothers:
(1/2)(1/2) = 1/4 (only one path links sisters and brothers)
In haplodiploidy, females maximize their inclusive fitness
by investing in the production of reproductive sisters, who
are closer relatives than their own offspring or brothers.
Haplodiploidy



females are more related to their sisters (r = ¾)
than they are to their own offspring (r = ½)
females are only distantly related to their
brothers (r = 1/4) [only one path links brothers
and sisters]
In haplodiploidy, females maximize their
inclusive fitness by investing in the
production of reproductive sisters, who
are closer relatives than their own
offspring or brothers.
Haplodiploidy predisposes Hymenoptera
to become eusocial
But…
haplodiploidy does not cause eusociality
Because…


Not all haplodiploid species have sterile
castes e.g. honeybees
Some diploid species have sterile castes
e.g. diploid termites
Honeybee
• Queen mates with up to 20 males
•Queens mate multiple times
•reduces relatedness among sisters –do not share father
•r no longer significant
• workers able to discriminate between sister that
are more or less related
Conflict between queen and workers
Conflict of interest between queen and non-reproductive workers
Queen is equally related to sons and daughters (r = 1/2)
-she will favor a 1:1 sex ratio (equal # of daughters and sons)
Workers have r = 3/4 with sisters, but only r = 1/4 with brothers
-their fitness will be maximized when the queen produces a
3:1 sex ratio (more daughters than sons)
Who wins the conflict?
-in one species of ant, the queen laid eggs in a 1:1 ratio, but at
hatching the sex ratio was biased towards many more females
-workers selectively destroyed male larvae
-assert their own reproductive agenda over the queen’s
Bumble Bee
•Queen controls sex of egg (fertilize or
not)
•Workers control sex by
• provisioning
•lay male eggs
Eusociality without Haplodiploidy


The naked mole rat - 2 castes:
 “workers” Non-reproductive adults:
dig tunnels, find food
 “non-workers” Reproductive female (queen)
and several reproductive males (breed, keep
young warm)
Native to Africa, droughts common, live in
underground tunnels and eat tubers, can only
dig when wet, need many individuals to dig to
find enough food – ECOLOGY promotes
eusociality (Fig. 19.18)
Helpers at the Nest




In some bird species (e.g., Florida scrub jays,
pied kingfishers), offspring from previous years
help their parents – feed and protect younger
siblings instead of reproducing
Beneficial: Number of young fledged drops if
helpers removed
Altruism through kin selection?
Or, beneficial to individual (selfish)?
Helping at the Nest






Why help vs. have your own offspring?
Habitat “saturated” with breeders (no room)
May be better to wait for a high quality territory
(inherit it), than leave for a low quality one
May not be able to leave group location (e.g.,
limited food resources elsewhere)
No mates available
Life history characteristics: Small clutch size and
low adult mortality
How Do Helpers Benefit?





Increase inclusive fitness if helping kin
Enhance likelihood of future breeding (gain mate
if primary male dies - unrelated males)
May increase own survivorship (access to
resources, lower risk of predation in a group)
May gain useful reproductive experience (care of
young) & may be reciprocated in future
So, kin selection may explain helping
through increased inclusive fitness, but
many other factors
Evolution of Human Social Systems
Read Pgs. 348 – 349
 Possible evidence for kin selection

Cooperation in Non-Kin


Types:
Reciprocal altruism (reciprocity)



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Recipient benefits, donor’s fitness
decreased; later, roles reversed
e.g., vampire bats
Model: Prisoner’s Dilemma
Mutualism (and/or symbiosis)


Mutually beneficial
e.g., symbiotic fish
Reciprocal Altruism
(Wilkinson, G.W. (1984) Reciprocal food sharing in
the vampire bat. Nature. 308:181-184)
•33% of young bats (<2 yrs) fail to get blood on any
particular evening versus just 7% of adults
•chronic threat of starvation among vampire bats can
only survive 3 days without a meal
•successful bats regurgitate part of their blood meal
for group members that were not successful – but
they do not do this randomly, they only give to those
from whom they have received blood in the past
Altruistic acts dispensed primarily to
relatives and frequent roostmates
•Bats more likely to regurgitate blood meals to
other bats they frequently roost with
•Bats more likely to regurgitate blood meals to close
relatives
Why is this altruistic behaviour
favored?
Benefits of act to recipient (R) exceed cost of act to
donor (D)
Primates
Alliances in primates seen in:
 Grooming behaviour
 Fighting behaviour

most grooming and fighting alliances between
close relatives but not always
Rhesus macaques:
•Kin intervene more, both to help if kin are the
recipient or if kin are the the aggressors.
•Grooming rates highest between kin.
Japanese macaques:
•Agonistic aid was 81% between kin, primarily mothers and
grandmothers.
•Kin spend more time together (in proximity) than expected
by chance.
•Grooming higher between kin than non-kin.
•Severe aggression only occurred between non-kin (19
incidents). (Kurland)
Grooming and food sharing in
chimpanzees
•Chimpanzees in captive colony more likely to
share food with individuals who had groomed
them in previous 2 hours
•Resisted approaches by individuals who had
not groomed them
•Reciprocity depends on history of interactions
between two individuals
Reciprocal Altruism
• Kin selection cannot account for cooperation
in non-kin…
• How could RA in non-kin have evolved?
• A “cheater” could beg blood, then refuse to
return the favour = donor may die (unless it
finds another donor), cheater lives
• Why don’t individuals evolve to act selfishly?
Reciprocal Altruism (RA)

Axelrod & Hamilton (1981), others…
•
•
•
“Game theory” – two players interact
with goal of maximum individual gains
Model for evolution of reciprocal
altruism = “Prisoner’s Dilemma”
“Tit-for-tat” strategy
READ 338 -339
Evolution of Reciprocal Altruism

Requires these conditions:



Longer-lived animals, such that future
opportunity for repayment likely (multiple
encounters)
Altruist and recipient must be able to
recognize one another; identify and refrain
from helping cheaters
Benefit to recipient greater than cost to
altruist (but both individuals benefit in the
long run)
Mutualism (a form of symbiosis)

Cooperation between 2 different species
 Both benefit, neither harmed, therefore,
not considered altruism (fitness of both
organisms is increased)






Clownfish (genus Amphiprion) dwell among the
tentacles of tropical sea anemones.
Protects anemone from anemone-eating fish
In turn, stinging tentacles of anemone protect
fish from its predators (a special mucus on fish
protects it from getting stung).
Honeypot ants feed and care for aphids, “milk”
them for their honeydew secretions (by
stroking them with antennae)
Ants protect aphids, aphids feed ants
Mutualism:


Goby Fish + Shrimp
Plover + Crocodile
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