APS 323 Social Insects: Lecture 15

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APS 323 Social Insects Lecture 15
Francis L. W. Ratnieks Laboratory of Apiculture
Social Insects
Department of Animal Plant Sciences University
of Sheffield
Lecture 15 Task Partitioning
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Aims Objectives
Aims 1. To provide information on the following
topics in task partitioning definition,
relationship to division of labour, benefits and
costs, taxonomic occurrence, patterns of material
transfer, etc. Objectives 1. Understand the
relationship of task partitioning to division of
labour in the organization of work in insect
societies 2. Learn specific examples
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Task Partitioning Basic Information
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Direct Nectar Transfer in Honey Bee
receiver
forager
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Definition of Task Partitioning
TP is the division of a task into 2 or more
sub-tasks e.g. nectar collection storage
versus collection, storage
Division of Labour Workers / Tasks Task
Partitioning Task / Workers
Honey bees and stingless bees have task
partitioning as above. Bumble bees so not. A
nectar forager bumble bee also stores the
nectar. Ratnieks Anderson. 1999. Insectes
Sociaux
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Factory Production Line
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Task Partitioning
Task partitioning is the division of a task into
two or more sub-tasks. If a load of forage is
passed from one worker to another this is task
partitioning. In honey bees and stingless bees, a
nectar forager transfers her nectar to one or
more nectar receiver bees in the nest. This is
similar to a bucket brigade or assembly
line. All known examples of TP involve the
handling of material. The two sub-tasks are
connected by the flow of material between them.
Many materials are transferred, especially
forage and waste. Transfer can be direct, as in
the honey bee nectar example, or indirect, as in
Atta cephalotes waste disposal. Here, waste is
transferred via a cache in the tunnel connecting
a garden chamber to a dump chamber. Ratnieks
Anderson. 1999. Insectes Sociaux
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TP and DoL
Task Partitioning (TP) and Division of Labour
(DoL) are two of the main ways that work is
divided in an insect society. DoL has been
studied more but recent study shows that task
partitioning is very common foraging and waste
disposal. TP and DoL are not mutually exclusive
alternatives. It is not TP or DoL. Frequently it
is TP and DoL. TP can actually cause greater DoL.
In the honey bee nectar transfer example, the
fact that nectar collection and storage are
separate sub-tasks means that there can also be
DoL, with a group of forager bees and a group of
receiver bees. The receiver bees are younger
bees. Later they will become foragers. Ratnieks
Anderson. 1999. Insectes Sociaux
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Task Partitioning Possible Benefits
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Wood Pulp Collections Building in Wasps
Polistinae Polybia TP 1 forager transfers to
2-4 builders
Vespinae -TP Same wasp forages builds
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Wood Pulp Collections Building in Wasps
Wasps can carry more wood pulp than they can
actually build with. Having TP decouples foraging
and building. As a result, wood pulp foragers can
bring back a larger load of pulp, which they can
transfer to several builders. TP provides a clear
benefit. Fewer foraging trips are needed.
Foraging trips are energetically costly and also
dangerous.
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Honeydew Collection Transport in Ants
1. Smaller workers milk honeydew secreting
Homoptera
3. Larger workers transport honey-dew back to nest
2. Direct transfer of honeydew to larger workers
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Honeydew Collection Transport in Ants
Many Hemiptera (e.g. aphids, scale insects, plant
hoppers) feed on plant phloem and excrete
honeydew, which is rich in sugars. This is
collected by ants, bees, wasps and some other
organisms. Ants must both collect honeydew from
the insects and transport it. In Oecophylla
weaver ants Formica wood ants smaller workers
tend to milk the insects and then transfer the
honeydew to larger workers for transport.
Presumably it is energetically cheaper to do it
this way. A small ant can probably milk an insect
as well as a big ant but would not be as
efficient at transporting liquid. Here TP is
again combined with DoL and makes use of the size
differences among the workers.
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Leaf Collection in Atta
Cutters cut the stems of whole leaves which drop
to the ground
Cutters
Carriers cut the leaves on the ground up and
transport the pieces back to the nest
indirect transfer
carriers
nest
Atta will cut leaves in trees and allow them to
fall to the ground, where they are cut up and
transported to the nest. By doing this a lot
less walking, up and down trees, is needed. In
one study on Atta sexdens, only about 50 of the
leaves were retrieved. It is easy to cut leaves
and leaves are abundant, so cutting excess leaves
is a good strategy.
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Leaf Collection in Atta
Leaves cut by Atta laevigata at Fazenda
Aretuzina, Brazil, and which have fallen to the
ground where they will be cut up into small
pieces and taken back to the nest. Many cut
leaves are wasted as they are not used.
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Nectar Collection in Honey Bees
-TP
Find new patch
Find new patch
Patch 1
Patch 2
Patch 3
TP
Successful foraging trip
Forager storage of nectar
Transfer to receiver
Dance following
Unsuccessful foraging trip
With TP, a forager can make more trips to a patch
of flowers that is in bloom for just a few days
and collect more nectar over her foraging career.
In the example above, both bees make 22 foraging
trips in their career before wearing out. But the
bee with TP foraged more intensely and only
worked two patches, whereas the bee without TP
worked three.
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Nectar Collection in Honey Bees
The advantage of TP in the previous examples is
obvious. But it is not obvious why honey bees
have TP in nectar collection. One hypothesis is
that it results in more successful foraging trips
per foraging career. Honeybees have a maximum
flight duration and wear out. Most flower
patches last less time that the foraging lifetime
of a bee When a foraging patch is no longer
rewarding, a forager has to find a new patch.
This can take a long time. When an unemployed
forager is directed to a patch of flowers by a
waggle dancer, it only finds the flowers one time
in four. Nectar transfer takes less time than
nectar storage, so with TP a forager can get back
to the flowers more quickly. With TP, foragers
will probably have a shorter but more intense
foraging career. They will not need to work as
many patches of flowers and so will collect more
nectar. TP may also allow more efficient storage.
Receivers may help in the evaporation of excess
water in nectar by placing the nectar in small
drops in cells, or by manipulating drops of
nectar in the mouthparts. These are time
consuming activities that would waste valuable
foraging time for an active forager.
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Task Partitioning Possible Benefits Hygiene in
Leafcutter Ants
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Atta Leafcutter Ants Public Health
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Fungus Garden
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Weed Fungus Escovopsis
To the symbiotic fungus
Pathogen of fungus Virulent Destroys colonies
Weeding concentrates disease
Escovopsis growing on waste dumped from Atta nest
Escovopsis fungus killer Ascomycotina
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Effect of Weed Fungus, Escovopsis
Healthy fungus garden
Infected fungus garden
Currie et al. 1999 Nature
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Streptomyces Antibiotic
left lower surface of Atta worker
right upper surface of Atta worker
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Atta Farming 4-Way Symbiosis
Farmers
Crop
Weedkilling antibiotic
From Schultz, 1999
Weed
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Hygiene in Atta Leafcutter Ants
Atta colonies face the challenge of keeping their
colony and fungus gardens healthy. The garden
fungus is susceptible to the weed fungus
Escovopsis. This can kill the fungus garden which
will kill the colony or entail very costly
remedies such as building a new nest. The ants
have a whole series of defences against
Escovopsis. They carefully lick and clean leaves
brought into the nest. They weed the fungus
garden and even groom individual strands of
mycelium. They can direct more weeding to areas
with Escovopsis. The worker ants also have an
antibiotic bacterium growing on their bodies
which kills Escovopsis. This gives the ants a
whitish appearance. The workers also have a
special gland, called the metapleural gland,
which secretes disinfectant chemicals. Sheffield
research has shown that Atta colonies also have
hygienic adaptations in the disposal of garbage.
These include several behavioural and
organizational features.
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Task Partitioning Possible Benefits Hygiene in
Leafcutter Ants Atta cephalotes
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Leafcutter Ant Nest
Garden chambers
Waste chambers
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Internal Waste Disposal
garbage chamber
fungus garden
garbage cache
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Adaptive Significance of Atta Organization
Combination of TP DoL isolates dump from main
nest
Task Partitioning Division Of Labour
yes yes
yes no
no yes
Fungus Garden
cache
cache
Garbage Dump
Hart Ratnieks 2001a Behavioral Ecology
Sociobiology
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Hygienic Features in Internal Dumping
Separate garbage and garden chambers Task
partitioning Division of labour Exclusion of
garbage workers from garden
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Wascomat Barrier Washer
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Barrier Laundry Room Design
Clean side
Dirty side
Washing machine
In
Out
Barrier wall
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TP Hygiene in Atta cephalotes
Atta cephalotes can have massive colonies with
many garden chambers in which their fungus crop
is grown. These are connected to underground
waste chambers in which the spent compost from
the fungus garden and other waste is
placed. Workers working in the fungus garden take
waste towards the garbage chambers. Few of them
enter the garbage chamber. Instead, the waste is
placed in a cache in the connecting tunnel.
Workers living in the garbage chamber then take
it the rest of the way. This is another example
in which TP and DoL are combined. The use of
separate chambers, TP, and DoL effectively
isolate the garbage chambers from the garden
chambers. This is important as the waste may
contain Escovopis and other pathogenic
micro-organisms. Ants working in the garbage
chamber are not allowed back into the garden
chamber. Ants contaminated with garbage odour are
aggressed or killed if they try to enter the
garden chamber. Hart Ratnieks 2001a Behavioral
Ecology Sociobiology
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Task Partitioning Possible Benefits Hygiene in
Leafcutter Ants Atta colombica
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External Waste Disposal
Waste Transporters
Heap Workers
Hart Ratnieks 2002 Behavioural Ecology
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External Waste Disposal
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Downhill Waste Dumps
1 entrance hole nests
2 entrance hole nests

Distance to heap, m
gt2 entrance hole nests
3 holes
4 holes
0
20
40
60
80
Angle of slope (degrees)
In Atta colombica the waste dumps are always
downhill from the nest entrance. In addition, if
the slope is steeper the dump may be closer to
the entrance. Nests have 1-4 entrance holes.
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Division of Labour in Above Ground Workers
FORAGING
88.8
WORKERS RECRUITING TO EXTERIOR TASKS
0.04
7.4
11.2
GARBAGE TRANSPORT
HEAP WORKING
Foragers and garbage transporters are separate
groups of workers. In other words there is
division of labour. Most heap workers are former
garbage transporters. Heap workers stay at this
work.
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Hygienic Features in External Dumping
Dumps downhill from forage entrances Elevated
dumping sites Dumping into water Opposite
orientation of garbage and foraging
trails Division of labour Task partitioning
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Relative Orientation of Garbage Foraging Trails
Garbage dump
nest
Grey foraging trail orientation
A. colombica foraging trail is never in the same
direction as garbage trail.
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TP Hygiene in Atta colombica
Atta colombica also has large colonies with many
garden chambers in which their fungus crop is
grown. But the waste is dumped externally. There
is division of labour between above ground
workers. Foraging and garbage dumping are
separate careers. Workers do not switch between
these two activities. In addition, the garbage
trail never leads in the same direction as the
main foraging trail. The workers who work on the
garbage heap itself are former garbage
dumpers. The garbage dump is often below a
convenient tree trunk or log to dump off. In this
way the garbage dumping ants do not have to walk
on the garbage dump itself. The garbage dump is
always downhill to the nest entrance. Garbage is
also dumped into a stream if one is nearby.
Hart Ratnieks 2002 Behavioural Ecology
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Hygiene Insight from Atta Ants
Combine defence mechanism Antibiotics
behaviour system organization physical layout
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Independent Newspaper
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Task Partitioning Costs
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Costs of Task Partitioning
TP involves material transfer. Therefore, time is
wasted in transferring material looking for a
transfer partner queuing for partner delays
occur even if balance is perfect because of
stochastic variation in arrival
rates signalling to balance system (e.g.,
tremble dance) Loss of material during
transfer Increased inter-individual
contact increased disease transmission?
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Costs of Task Partitioning
Transfer costs can be reduced by caching Caches
can even out supply and demand Problems Can
t cache liquids easily (although could have a
wax nectar trough in bees?) Cached materials
might degrade (leaves dry out) Cached materials
might get lost or stolen Still need balance
between cachers and receivers
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Costs of Task Partitioning
Task partitioning results in a more complex
arrangement of work and introduces the problem of
organizing the system to balance the work
capacities of the various sub-tasks. That task
partitioning occurs at all indicates that it must
have substantial benefits, that are large enough
to overcome these costs and more.
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Task Partitioning Occurence
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TP is Taxonomically Widespread
TP occurs in Ants, Bees, Wasps, Termites It must
have evolved many times independently TP can vary
within a species for different forage
materials e.g. honeybees (nectar TP, pollen
TP) TP can vary between species for the same
material e.g. nectar honeybee TP, bumblebee
-TP
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Material Handling
Into nest Liquids nectar, honey dew,
water Solids seeds, leaves, prey, wood pulp,
propolis Out of nest Solids garbage, excavated
soil Inside nest Solids leaf pieces (in Atta,
Acromyrmex)
Ratnieks Anderson,1999, Insectes
Sociaux Anderson Ratnieks, 2000, Insectes
Sociaux Hart Ratnieks, 2001, Behavioral Ecology
Sociobiology
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Task Partitioning Types Patterns of Transfer
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Types of Transfer
Direct solids liquids Indirect
solids Direct indirect solids Always at
nest flying workers (bees, wasps) Also at
food/on trail non-flying (ants, termites)
Ratnieks Anderson,1999, Insectes Sociaux
Non-flying social insects may have foraging
trails and so are more likely to meet each other
outside the nest. It is probably for this reason
that they may also transfer materials away from
the nest.
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Patterns of Transfer
Linear
transfer
collection
use/storage
Apis
Atta
Interlocking
Polybia
Ratnieks Anderson, 1999 , Insectes Sociaux
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Patterns of Transfer
collection
use/storage
direct
Apis nectar
collection
use/storage
indirect
collection
use/storage
direct or indirect
collection
use/storage
direct or no transfer
direct or indirect or no transfer
collection
use/storage
Atta leaf pieces
Ratnieks Anderson,1999, Insectes Sociaux
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Atta Ants Material Transfer
main trail
side trail
fungus garden
internal dump
external dump
external dump
Hart, Ratnieks Anderson, 2002
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Three Interlocking Cycles Polybia wasps
water collectors
builders
wood pulp collectors
The most complex pattern known involves three
interlocking cycles. It occurs in Polybia
occidentalis wasps (Vespidae Polistinae
Epiponini) and probably in other epiponines. It
also occurs in the British ant Lasius fuliginosus
which builds a nest out of material it has
collected. There are cycles of honeydew foragers,
building material foragers, and builders.
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Patterns of Transfer Apis Foraging
-TP Pollen placed directly into cells by
forager TP Nectar transferred to storer bee in
nest Water transferred to storer bee in
nest Propolis transferred to builder bee in
nest occasionally, a nectar forager does not
transfer (c. 1 foragers)
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Why Handle Pollen Without TP?
Pollen foragers collect pollen in the pollen
baskets. They are not unloaded by another bee but
put the pollen directly into a cell by placing
their legs in the cell and kicking the pollen
off. Transfer of pollen would probably be
inefficient as it would take two receivers to
unload one forager. That is, one to take each of
the two pollen loads. This would presumably be
carried in the receivers mandibles after
transfer. It is not possible to transfer a pollen
load from one pollen basket to another. A pollen
load is fragile and could easily break apart or
be dropped during transfer. This is a second
possible reason why pollen is not transferred.
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Why Handle Propolis With TP?
Water is a liquid that is held in the foragers
crop and is transferred like nectar. A receiver
extends her tongue and drinks from the foragers
mouth. Propolis is a solid like pollen and is
held in the pollen basket. However, it is subject
to TP. A builder unloads a forager by taking the
foragers propolis in her mandibles. Why are
pollen and propolis handled differently whereas
water and nectar are not? It is suggested that
propolis is directly transferred because a
forager cannot unload herself because the
propolis is sticky. Propolis foragers can
sometimes wait hours to be unloaded, if there is
little demand for propolis.
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Simulation Modelling of 2-Cycle Task Partitioning
With Direct Transfer
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Computer Simulation of 2-Cycle TP
Cycle 1
Cycle 2
Direct transfer
A computer simulation can be set up to
investigate queueing delays. It is possible to
vary the work capacities of the foragers and
receivers, and the mean and variance in the
durations of the foraging and receiving trips.
The simulation generates the unloading delays for
both receivers and foragers for either serve in
random order or first come first served.
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Ergonomics Colony Size Queueing Delay
Even when the work capacities are balanced there
are still delays. This is because of stochastic
variation in the arrival rates of foragers and
receivers into the transfer area. The average
stochastic delay in the larger colonies is
smaller because large size lessens the effect.
These results suggest that TP is more likely in
larger colonies because the delay cost will be
lower. Anderson Ratnieks. 1999. American
Naturalist
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Hypothesis Colony Size Task Partitioning
Nectar Collection Storage TP Apis -
TP Bombus swarm-founded colonies queen-founded
colonies Wood Pulp Collection Building
TP Polybia -TP Vespula swarm-founded
colonies queen-founded colonies
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Yucatan, Mexico
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Dept. Apiculture, University of Yucatan
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Meliponary Shelter
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Meliponine Research in Mexico
Adam Hart
M. beecheii nest
A. mellifera M. beecheii
Studied 5 species of stingless bees. All of them
had TP. This supports the idea that TP is
favoured by large colony size as all stingless
bees have swarm-founded nests. That is, nests are
always large. Hart Ratnieks. 2002. Ecological
Entomology
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Colony Size TP
The simulation model shows that the stochastic
delays in finding a transfer partner will be
smaller in larger colonies. This suggests the
testable hypothesis that TP will be more common
in large-colony species. The data support this
prediction. Swarm-founding wasps and honey bees
have TP in the handling of wood pulp and nectar,
but Vespinae wasps and bumble bees do not. These
species have colonies founded by a lone queen, so
their colonies start small and often do not grow
to large sizes. In Vespula wasps there are also
data to show that nectar is handled via TP in
larger population colonies. A trip to Merida,
Yucatan, Mexico to study stingless bees provided
additional support for the hypothesis. Five
species were studied and all had TP in nectar
collection. Stingless bees have swarm-founded
colonies with colony sizes typically in excess of
1000 workers.
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END OF COURSE