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Division of Labor

Division of Labor. Amy Toth 12 April 2007 amytoth@uiuc.edu. 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.

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Division of Labor

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  1. Division of Labor Amy Toth 12 April 2007 amytoth@uiuc.edu

  2. 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

  3. 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

  4. The evolution of complexity has involved cooperation and role specialization.

  5. 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

  6. 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

  7. 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)

  8. 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?

  9. Reproductive caste: fruiting body Non-reproductive caste: stalk Social amoeba, Dictyostelium http://dictybase.org/Multimedia/development/development.html

  10. Actually a Cnidarian colony: order Siphonophora Different individuals specialized as Gas-filled float Nectophores- jet propulsion Gastrozooids- ingestion and distribution of nutrients Sexual medusoids Portuguese Man-o-War, Physalia physalis

  11. 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

  12. African Wild Dogs • Some females (usually mother) remain with pups during a hunt • Hunters return and regurgitate food

  13. 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 Roles in vertebrate societies

  14. Vervet Monkeys Cercopithecus aethiops

  15. Gartlan, 1968

  16. Chimps Suggest females may have an important role in development of tools and spreading of technology.

  17. 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

  18. 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

  19. 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

  20. 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

  21. “A factory in a fortress” FORTRESS: Large, well-buffered against environment Guarded/protected FACTORY: Output is workers and reproductives All of this requires a sophisticated division of labor

  22. 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

  23. 3 forms of social insect DOL 1) Reproductive castes • Queens vs. workers (kings only in termites)

  24. 3 forms of social insect DOL 1) Reproductive castes • Queens vs. workers (kings in termites) 2) Behavioral castes • Task specialization among workers • Temporal polyethism

  25. Honey bee temporal polyethism

  26. 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!

  27. 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

  28. Major & minor coastal brown ants Camponotus truncatus minor worker and soldier Nasutitermes soldiers and workers

  29. How are different castes formed? Best studied in the social Hymenoptera (ants, bees, wasps)

  30. 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)

  31. 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 Soldier (maxim) minin medium

  32. 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

  33. Behavioral caste-related changes in the brain • Mushroom Body volume increases in foragers (Withers et al.) (Withers, Fahrbach, & Robinson)

  34. p=0.009, n=34 p= 0.008, n=38 Foraging-related changes in nutritional status Forager Nurse (Toth & Robinson, Animal Behaviour 2005)

  35. b n = 3 trials Overall ANOVA P <0.0001 a a Abdominal lipid (mg) a b n = 45 bees/trial Overall ANOVA P <0.0001 Experimental reduction of lipid stores causes foraging Proportion of bees foraging n = 3 trials c (Toth et al. 2005)

  36. How is a complex social insect DOL organized? Central control (e.g., by queen) is rare, especially in large colonies.

  37. 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)

  38. 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)

  39. 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

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