Societies - groups of conspecifics organized in a cooperative manner Sociobiology - study of the biological basis of soc

1. Societies - groups of conspecifics organized in a cooperative manner • Sociobiology - study of the biological basis of social behavior • Eusociality • Overlapping generations • Reproductive division of labor including nonreproductive (or sterile) workers • Communal care of young

2. Hymenoptera • Ants - all species, although there is a parthenogenetic species with no queens! Many show evidence of caste polymorphism - different morphological forms perform different tasks within the colony. • Bees - some, may have evolved multiple times • Wasps - only in Vespids, variable expression

3. Termites - all species are eusocial with cellulose-digesting bacteria, and exhibit caste polymorphisms • Naked mole-rats - any worker can eventually reproduce, food is advertised with calls and chemical trails • Caribbearn shrimp in sponges - colonies of up to 300 shrimp in one sponge, single reproductive pair, larger workers defend sponge against other shrimp

4. Spiders - no caste polymorphism, group living on a common web has evolved multiple times, sterility not yet demonstrated • Aphids - a few species, have soldier forms, and are periodically asexual • Beetle - one Australian species that builds colonial burrows in eucalyptus trees

5. Possible explanations for worker sterility • Kin selection – haplodiploidy • However, female workers are more closely related to their own male offspring (r = 1/2) and their nephews (r = 3/8) than their brothers (r = 1/4). Therefore, expect workers to lay unfertilized eggs

6. If mothers are polyandrous (mate multiple times), then workers may be more closely related to their brothers than to half-nephews (r=1/8). • Expect workers to kill unfertilized eggs laid by other workers. Example: honeybees and yellowjackets are polyandrous and have low levels of worker reproduction

7. What about diploid eusocial animals (e.g. termites, naked mole rats)? • One proposed hypothesis is that these populations undergo cycles of inbreeding. With high levels of inbreeding-mother-son and sister brother can rapid approach r>3/4 for both males and females.

8. But high levels of inbreeding can lead to inbreeding depression • Thus inbreeding might alternate with some dispersal. • A rare disperser morph is found in mole rats: it is fatter, attempts to disperse in captive settings, solicits mating with non-colony members. Once settled reverts to xenophobia and loses fat stores

9. Parental manipulation (Trivers and Hare) - colony sex ratios of reproductives. • Monogynous species - 3:1 investment ratio in females as would be predicted if the offspring control reproduction • Polygynous species and slave-making ants - 1:1 sex ratios as predicted.

10. When queens from a monogynous eusocial bee were removed, a daughter took over reproduction removing the asymmetry in relatedness • her sisters are helping now to produce nieces and nephews, both related to the sisters by 3/8, rather than to rear sisters with r=0.75 and brothers with r=0.25. • The daughter queen colonies did produce more males (63%) than unmanipulated queen colonies (43%).

11. Subsocial beginning: single females with offspring remaining to help • Single foundress colonies such as honey bees, some ants, e.g. Solenopsis, and wasps • Predict high levels of relatedness, which occurs in some species, although in some paper wasps relatedness varies due to multiple mating by queens or multiple females reproducing

12. Parasocial beginning: joint-nest founding • Semisocial when some foundresses stop reproducing and just help, e.g some polistine wasps, harvester ants, some termites • Predict low levels of relatedness, e.g. in swarm-founding wasps r ~= 0.2 • Requires some other reason for eusociality than kin selection, e.g. mutualism over founding a nest

13. Ecological factors impose mutualism • Nests may be essential resource that is very costly to build, maintain, or defend. Cost of building own nest may outweigh cost of staying in established nest • Nest success depends on the number of foundresses and colony size (as in eusocial shrimp). Benefit of staying versus leaving increases as more animals stay