Eusociality

• Conflicts over reproduction

• Definition and occurrence

• Explanations for worker sterility

• Routes to sociality

Definition

• Overlapping generations

• Reproductive division of labor including nonreproductive (or sterile) workers– Caste polymorphism (in some cases)

• Parental care in a permanent nest

Hymenoptera:ants, bees and wasps

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C E

Ants show caste polymorphism

Isoptera - all termites

Social aphids

Gall-forming thrips

Carribean snapping shrimp

Naked mole rats

Social spiders

Communal, but no sterile caste

Why worker sterility?

• Kin selection

• Parental manipulation

Haplo-diploidy and relatedness

Why worker sterility? Kin selection

• Because of haplodiploidy, workers are more closely related to sisters (r = 3/4) than to offspring (r = 1/2), assuming females mate once

• But, they are more closely related to male offspring (r = 1/2) and nephews (r = 3/8) than to brothers (r = 1/4). Therefore, workers should lay unfertilized eggs if mothers mate singly (bumblebees, stingless bees)

• If mothers are polyandrous, then female workers may be more closely related to brothers than half-nephews (r = 1/8). Expect workers to kill unfertilized eggs laid by other workers in polyandrous species, e.g. honeybees.

Worker policing in honeybees

“Queenright”

“Queenless”

Why worker sterility? Parental manipulation

• If parents can prevent their young from reproducing, then it may be better to help

• A mother that traded eggs with her daughters would trade grandkids (r = 1/4) for offspring (r = 1/2) and gain a 2-fold advantage. Daughter trades offspring for siblings and loses nothing (r =1/2 for both).

Sex ratio investment• Offspring control: expect 3:1

investment sex ratios because females are related to sisters by 3/4 and to brothers by 1/4

• Parental control: expect 1:1 sex ratios

• Data fit 3:1, rejects parental manipulation

• Only slave-makers are 1:1

Factors promoting eusociality

• Genetics– High relatedness of workers to reproductives

• Ecology– High cost of dispersal– Need for assistance in nest construction– Reproductive success is proportionately higher

for family than individual– Need to defend a multi-generation nest

Routes to sociality

Predict r < 0.5 Predict r > 0.5

Paper wasps, often single foundress

Swarm founding wasps

Inbreeding-outbreeding cycles in termites

Termites are diploid.Both sexes help.In some species, multiplereproductives occur.If these are produced byinbreeding over multiplegenerations, then termiteswithin a colony couldbe very closely related

Inbreeding in mole rats?