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Explore eusocial behavior in insects and animals, including worker sterility, kin selection, and ecological factors shaping cooperative societies. Investigate cycles of inbreeding and dispersal strategies for population survival. Discover the mechanisms influencing colony sex ratios and relatedness in eusocial species.
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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
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
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
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
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
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
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.
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
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.
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%).
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
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
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