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Eusociality Reproductive division of labor (sterile workers). Overlapping generations. Cooperative brood care. Examples include: Hymenoptera (all ants, some bees & wasps; >12 origins) Isoptera (termites; 1 or 2 origins) aphids, ambrosia beetles, snapping shrimp, thrips, some naked mole rats

How can worker sterility spread, or be maintained, when w=0? How can altruism evolve? “But with the working ant we have an insect differing greatly from its parents, yet absolutely sterile; so that it could never have transmitted successively acquired modifications of structure or instinct to its progeny. It may well be asked how is it possible to reconcile this case with the theory of natural selection?” C.D.

diploid organisms ABAB CDCD ACACBDBD A DA D BCBC r between sisters = 0.50 r between mother & daughters = 0.50

The haplo-diploid Hymenoptera A B CB C BC A B A CA C

r between sisters = 0.75 r between mother & daughters = 0.50 Workers are more closely related to sisters than daughters (when singly mated, single queen colony). The haplo-diploid Hymenoptera

Potentially provides an explanation for: Kin selection within social groups why eusociality arose >12 times in Hymenoptera, but only 1-2 times in Isoptera. why workers are female in Hymenoptera, but both sexes in Isoptera. …but the principles of kin selection remain important and instructive. Potential problems: r among sisters often below 0.75 (multiple mating, multiple queens) reproductive value, sex ratios B can be very large

# of patrilines Sister Relatedness (%) …∞ # of matrilines Sister Relatedness (%) …∞ Potential problems: r among sisters often below 0.75 (multiple mating, multiple queens)

# of patrilines Sister Relatedness (%) …∞ # of matrilines Sister Relatedness (%) …∞ Altruism is favored when: rB - C > 0 when r is low, the benefits need to be really high!

r between sisters = 0.75 r between brothers & sisters = 0.25 For workers 3:1 sex ratio is optimal r between mothers & daughters = 0.5 r between mothers & sons = 0.5 For queens, 1:1 sex ratio is optimal. Conflict between workers & queens over the optimal sex ratio. Kin selection can also cause conflict within colonies

Sex ratio skew Is the ratio 1:3 or 1:1? (are workers in control, or are queens?) 1. Because males are haploid, queens control the primary sex ratio. 2. Because workers rear the young, they control the secondary sex ratio.

Sex ratio skew Ratios are female-biased in single-queen ant colonies. No relatedness asymmetry = less sex ratio bias: multiple-queen ant colonies (low r) solitary bees & wasps (only primary sex ratio) termites (diploid)

Sex ratio skew Within-species variation also matches expected patterns. Queller & Strassmann 1998

How direct altruism toward relatives? Many transitions were promoted by mechanisms that isolate particular groups.

Social Insect Recognition Systems Label Template AB EF AE

Label Template AB EF AE A B A B Social Insect Recognition Systems

Problem: imprinting decouples template from kinship. In cross-fostering experiments, callows imprint on the wrong odors. In some cases, things can go really awry…

Cal. Acad. Sci. When recognition systems break down: Polyergus breviceps Specialized mandibles: cannot hunt, eat, etc.

Polyergus breviceps; Alex Wild When recognition systems break down:

Many forms of social parasitism within ant colonies: reproductive skew slavemaking inquilinism cuckoos When recognition systems break down:

Kin selection not the only mechanism for the origin of eusociality. Bivoltine life cycle: first generation undergoes uninterrupted development and reproduction. Second (following) generation enters diapause to pass unfavorable season and reproduces the following year.

onset of winter Example: Polistes waspsHunt and Amdam 2005 Science

Promotes overlapping generations between reproductive and non- reproductive individuals and brood care. onset of winter Example: Polistes waspsHunt and Amdam 2005 Science

Characteristics of eusocial societies 1. Group Living 2. Overlap of Two or More Generations 3. Cooperative Care of the Young 4. Reproductive Division of Labor - reproductive skew

Reproductive Skew

Reproductive skew: a term to describe the distribution of direct reproduction among cooperating individuals

Alpha Beta Alpha Beta Low Skew High Skew Reproductive Output From Keller & Reeve 1994

Index of Reproductive Skew 0 1 The Eusociality Continuum Some ants Acorn woodpeckers Prairie dogs Etc. Dwarf mongooses Wild dogs Some termites Etc. Most ants, bees Yellow jackets Some aphids Etc. From Sherman et al \ 1995

Where has eusociality evolved (sterility)? Taxa number of times Insects: Hymenoptera Ants, bees, wasps11 Isoptera Termites 1 Homoptera Gall-forming aphids 1 Coleoptera Bark-nesting weavils 1 Thysanoptera Gall-forming thrips 1 Non-insects Snapping shrimps and naked mole rats 2 Total17