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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 halfnephews (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 noncolony 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