Transcript 31_DivisionofLabor

Division of Labor
Amy Toth
12 April 2007
[email protected]
Outline
• Specialized roles aid cooperation
• Benefits of division of labor (DOL)
• Examples
– Protozoa
– Invertebrate
– Vertebrate societies
• Social insect DOL
– Forms of DOL
– Mechanisms of DOL
– Models of DOL
The Major Transitions
1. Replicating molecules

Molecules in protocells
2. Independent replicators 
Chromosomes
3. RNA as gene & enzyme 
DNA genes, protein enzymes
4. Bacteria (prokaryotes)

Eukaryotes (organelles)
5. Asexual clones

Sexual populations
6. Single-celled organisms 
Multicellularity
7. Solitary individuals

Eusocial colonies
8. Primate societies

Human societies (language)
Maynard Smith & Szathmáry 1995
The evolution of complexity has
involved cooperation and role
specialization.
Role Specialization and
the Major Transitions
Genes: chromosomes  genes take on
specific functions
Eukaryotes: organelles  e.g.
mitochondria and chloroplast are
specialized “organs” that generate
energy
Sex: sperm and egg  gametes
specialize on either motility or nutrient
storage
Role Specialization and
the Major Transitions
Multicellularity: cell types, tissues,
organs  e.g. gametes vs. somatic
cells, liver vs. brain tissue, etc.
Eusociality: castes  queen-worker,
worker subtypes
Human society: gender roles, social
rank, employment specializations
• Karl Marx talks at length about division of
labor in human society increasing as a
result of industrialization
Useful Definitions
Role: pattern of behavior that
appears repeatedly in
different societies belonging
to the same species.
Caste: a set of individuals,
smaller than the society itself,
which is limited more or less
strictly to one or more roles.
Polyethism: the differentiation of
behavior among categories of
individuals within the society,
especially age and sex
classes and castes.
(Edward O. Wilson,
Sociobiology)
Benefits of division of labor
–
Improved society functioning: all jobs get
done
Improved performance by specialization
–
•
•
–
Evolved morphological or behavioral
specializations
Learned specializations
Allows for an increasingly large and
complex social system
 The challenge is: how to organize division of
labor?
Social amoeba, Dictyostelium
• Reproductive
caste: fruiting
body
• Non-reproductive
caste: stalk
http://dictybase.org/Multimedia/development/development.html
• Actually a Cnidarian
colony: order
Siphonophora
• Different individuals
specialized as
– Gas-filled float
– Nectophores- jet
propulsion
– Gastrozooidsingestion and
distribution of nutrients
– Sexual medusoids
Portuguese Man-o-War,
Physalia physalis
Behavioral Roles in Vertebrate
Societies
• Highly structured division of labor not found in
vertebrate societies
– Exceptions: naked mole rats & humans
• Direct roles: a behavior displayed by a subgroup
that benefits other subgroups, thus the group as
a whole
• Indirect roles: a selfish behavior that is neutral or
destructive to other subgroups
– Indirect roles more common in most vertebrate
societies
African Wild Dogs
• Some females (usually mother) remain with pups
during a hunt
• Hunters return and regurgitate food
Roles in vertebrate societies
• Leadership
– “Leader of the pack” in
wolves: dominant
males during chases
– Herds of Red Deer,
African Elephants,
Mountain sheep led by
a fertile hind (female)
– Dominance
hierarchies
widespread: hyenas,
zebras, etc.
• Control
– Intervention in aggressive
episodes
– Not necessarily dominant
– E.g. Japanese macaque,
vervet monkey
Vervet Monkeys
Cercopithecus aethiops
Age-Sex Class
Behavior
Adult
males
Adult
Juvenile Subadult Infants
females males
females
Territorial display
.66
0
.33
0
0
Vigilance, look-out
behavior
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.38
.03
.12
.12
Receive friendly
approach
.12
.46
.04
.27
.12
Friendly approach
to others
.03
.32
0
.47
.15
Chase territorial
intruders
.66
0
.33
0
0
1.00
0
0
0
0
.32
.49
0
.16
0
Punish intragroup
aggression
Lead group
movement
Gartlan, 1968
Chimps
Suggest females may
have an important role
in development of
tools and spreading of
technology.
Naked Mole Rat DOL
• Queen
– Behaviorally dominant, sole
reproductive
• Male harem
• Workers (male and female)
– Small
• foraging, nest-building
– Medium
• digging and colony defense
– Largest
• Young care, work little and may
inherit colony
Human Societies
• “Human societies … have equaled and in
many cases far exceeded insect societies
in the amount of division of labor they
contain.” -EO Wilson
• However, choice of role is often for selfish,
not altruistic reasons
– E.g. the state of Maine needs pharmacists
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Neanderthals, Homo sapiens neanderthalensis,
went extinct about 30,000 years ago,
Homo sapiens sapiens (us) did not. Why?
Neanderthal women and children may have
participated in hunting (no division of labor!)
Hunter-gatherer societies have age- and sexbased division of labor
Social Insect Division of Labor
• Complex and well-defined systems of
division of labor
• Organization shaped by colony-level
selection
• Common features in several
independently evolved eusocial lineages
“A factory in a fortress”
FORTRESS:
Large, wellbuffered against
environment
Guarded/protected
FACTORY:
Output is
workers and
reproductives
All of this requires a sophisticated
division of labor
Insect societies as
“Superorganisms”
• The colony has germ line (queen) and somatic
(worker) components
• The colony has its own physiology
– Temperature tightly regulated
– Nutrient flow carefully controlled
– Communication between individuals analogous to a
nervous system (but not centralized)
• Colonies reproduce themselves
– Either by “budding” = swarm founding, or independent
founding
3 forms of social insect DOL
1) Reproductive castes
•
Queens vs. workers (kings only in
termites)
3 forms of social insect DOL
1) Reproductive castes
•
Queens vs. workers (kings in termites)
2) Behavioral castes
•
•
Task specialization among workers
Temporal polyethism
Honey bee temporal polyethism
NURSING
FOOD STORING
GUARDING
FORAGING
UNDERTAKING
Temporal polyethism
• Behavioral division of labor associated with
worker age
– Young  old
– Inside  outside
– Safer  more dangerous
• Characteristic of many highly eusocial insects:
–
–
–
–
Honey bees, some bumble bees
Some ants
Some wasps
Even some termites!
3 forms of social insect DOL
1) Reproductive castes
•
Queens vs. workers (kings only in
termites)
2) Behavioral castes
•
•
Task specialization among workers
Temporal polyethism
3) Morphological castes
•
•
Morphologically & behaviorally
specialized workers
E.g. ants, termites, aphids
Major & minor coastal brown ants
Camponotus truncatus minor worker and soldier
Nasutitermes soldiers and workers
How are different castes
formed?
Best studied in the social
Hymenoptera (ants, bees, wasps)
Mechanisms of reproductive caste
differentiation
•
Larval
– “Nutritional castration” to make workers
» E.g. royal jelly given only to honey bee queen
larvae, not worker larvae
– Hormonal influences
– Gene expression differences
– Genetic influences
» E.g. in some harvester ants, the exception not
the rule
•
Adult
– Dominance interactions (e.g. paper wasps)
– Other social influences (e.g. queen pheromones)
Mechanisms of morphological
caste differentiation
• Larval
– Social control of developmental rate
– Environmental influences
– Nutritional differences
» Can lead to size differences
» Allometric size changes
Leafcutter ant castes
minin
medium
Soldier (maxim)
Mechanisms of behavioral caste
differentiation
• Adult
– Social regulation
• Pheromones (honey
bees)
• Dominance interactions
(wasps)
–
–
–
–
Genetic influences
Hormonal
Gene expression
Neurobiological
• Neurochemicals
• Brain structure
– Nutritional– my thesis
work!
Especially well-studied in
honey bees
Behavioral caste-related changes
in the brain
• Mushroom Body volume increases in foragers
(Withers et al.)
(Withers, Fahrbach, & Robinson)
Foraging-related changes in
nutritional status
3
mg lipid
2.5
2
p= 0.008, n=38
1.5
p=0.009,
n=34
Forager
Nurse
1
0.5
0
Trial 1
Trial 2
(Toth & Robinson, Animal Behaviour 2005)
Forager
Nurse
Proportion of
bees foraging
Experimental reduction of lipid stores causes
foraging
0.24
b
n = 3 trials
Overall ANOVA
P <0.0001
0.16
a
0.08
a
Abdominal lipid (mg)
0
1
1.2
2
3
a
b
0.8
c
0.4
n = 3 trials
n = 45 bees/trial
Overall ANOVA
P <0.0001
0
Control
No Pollen
TOFA
(Toth et al. 2005)
How is a complex social insect
DOL organized?
Central control (e.g., by queen) is
rare, especially in large colonies.
Theoretical Models of DOL
• Response threshold model
-Variation in worker thresholds to a stimulus: e.g. # of corpses
- Specialization arises because stimulus kept at low level, few
workers ever perform a given task
(Beshers, Robinson, Page, Bonabeau, Theraulaz & colleagues)
Theoretical Models of DOL
• Foraging for work
– Where a worker is in nest depends on age
• Young, near center of nest (brood)  old, pushed
to periphery
– Task need associated with nest position
– Worker fills need
– Not well supported empirically
(Franks & colleagues)
Theoretical Models of DOL
• Self-organization, emergent properties
– Individuals respond to local cues: social and
environmental
– May be a few “key” individuals-- e.g. honey
bee dancers within foraging task groups
– The social phenotype is greater than the
summation of the individual behaviors
– E.g. honey bee swarming behavior