Division of Labor

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

Maynard Smith & Szathmáry 1995

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)

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

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

Role Specialization and the Major Transitions

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?

http://dictybase.org/Multimedia/development/development.html

Social amoeba, Dictyostelium

• Reproductive caste: fruiting body

• Non-reproductive caste: stalk

Portuguese Man-o-War,

Physalia physalis

• Actually a Cnidarian colony: order Siphonophora

• Different individuals specialized as– Gas-filled float– Nectophores- jet

propulsion– Gastrozooids- ingestion

and distribution of nutrients

– Sexual medusoids

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

Behavior

Age-Sex ClassAdult males

Adult females

Juvenile males

Subadult females

Infants

Territorial display .66 0 .33 0 0Vigilance, look-out behavior

.35 .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

Punish intragroup aggression

1.00 0 0 0 0

Lead group movement

.32 .49 0 .16 0

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

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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 sex-based 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”

All of this requires a sophisticated division of labor

FACTORY:

Output is workers and reproductives

FORTRESS:

Large, well-buffered against environment

Guarded/protected

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)

1) Reproductive castes• Queens vs. workers (kings in termites)

2) Behavioral castes• Task specialization among workers• Temporal polyethism

3 forms of social insect DOL

Honey bee temporal polyethism

NURSING FOOD STORING

GUARDING

UNDERTAKINGFORAGING

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!

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

3 forms of social insect DOL

Major & minor coastal brown ants

Nasutitermes soldiers and workers

Camponotus truncatus minor worker and soldier

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)

0

0.5

1

1.5

2

2.5

3

Trial 1 Trial 2

mg lipid

Forager

Nursep= 0.008, n=38 p=0.009,

n=34

(Toth & Robinson, Animal Behaviour 2005)

Foraging-related changes in nutritional status

Forager Nurse

Experimental reduction of lipid stores causes foraging

(Toth et al. 2005)

Pro

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Overall ANOVAP <0.0001

n = 3 trials

0

0.08

0.16

0.24

1 2 3

aa

b

n = 3 trials

Ab

do

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pid

(m

g)

0

0.4

0.8

1.2

Control No Pollen TOFA

Overall ANOVAP <0.0001

n = 45 bees/trial

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bc

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

(Beshers, Robinson, Page, Bonabeau, Theraulaz & colleagues)

-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

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