Submission for Sigma Xi Student Research Showcase

1. Why are there
“Lazy” ants?
Explaining the occurrence of seemingly
unproductive workers in social insects
D Charbonneau A Dornhaus

2. Most animals are “lazy”
“All cold-blooded animals and a large
number of warm-blooded ones spend
an unexpectedly large proportion of
their time doing nothing at all, or at any
rate, nothing in particular.”
-Elton 1927

3. Most animals are “lazy”
Animals across a wide range of taxa, life histories and ecologies
have been reported of spending >50% of their time resting
Solitary animals
>50% 85%
Harvestmen Hummingbirds
(Williams 1962) (Wolf et al. 1975)
Social animals
75% 70%
Lions Howling monkeys
(Schaller 1972) (Smith 1977)
Eusocial insects
80% 60%
Honey bees Temnothorax ants
(Schmid-Hempel 1990) (Dornhaus et al. 2009)

4. “Laziness” in social animals
Inactivity in social animals is further complicated by social
behaviors, such as division of labor and specialization
In groups, the amount of
work required for the
survival of an individual
is dependent upon the
amount of work done by
others in the group.
division of labor

5. “Laziness” in social insects
Inactivity in social animals is
particularly interesting because
the colony can regulate the
number of workers it produces
Workers are expensive to
produce and maintain, so
there is a real cost to having
“too many”
If >50% of the colony is inactive at any time, why
would the colony produce so many apparently
extraneous workers?

6. Why are there
“Lazy” ants?

7. Model system used to answer
this question:
The ant: Temnothorax rugatulus
Worker
ant
Queen ant

8. Recording ant behavior
Sample unit: 5 minute HD videos
Food “Stone” wall
Water Ants

9. Behaviors and activity
Breaking down observable behavior
into categories of activity
Observable task Activity category
Building
Foraging
Wandering outside nest
Brood care
Self-Grooming Active
Groom other
Be groomed
Trophallaxis
Eating
Moving but not active Wandering Inside
Stationary and not active Inactive
Similar to: Cole, BES 1986

10. Defining “inactivity”
Individual-level inactivity:
Mean proportion of time an
individual worker spends inactive
during one video
i.e. % of worker time inactive
Colony-level inactivity:
Mean individual-level inactivity
within a colony
or
∑ time spent inactive by all ants in colony
= % of colony
∑ of individual observation times
time inactive

11. Inactivity varies
At the individual-level
Colony 2 Colony 3
And at the colony-level

12. Inactivity varies
My work focuses on individual-level variation
Colony 2 Colony 3
And at the colony-level

13. Main question:
How can we explain high levels of worker inactivity?
Inactivity in the field / Lab artifact
Does colony-level inactivity occur in the field at levels
comparable to those observed in the lab?
Consistency of worker inactivity / Shift-work
Can apparent individual-level variation in inactivity be
explained by a bias in observation timescale?
Selfish worker reproduction
Is worker ovary development positively
correlated to individual-level inactivity?
Inactive workers as a reserve labor force
Do highly inactive workers become more active when
the colony is presented with more work?
Inactive workers facilitate communication
Are highly inactive workers actually performing a
communication task?

14. Main question:
How can we explain high levels of worker inactivity?
Inactivity in the field / Lab artifact
Does colony-level inactivity occur in the field at levels
Ecologically comparable to those observed in the lab?
non-relevant
explanations Consistency of worker inactivity / Shift-work
Can apparent individual-level variation in inactivity be
explained by a bias in observation timescale?
Selfish worker reproduction
Is worker ovary development positively
correlated to individual-level inactivity?
Inactive workers as a reserve labor force
Do highly inactive workers become more active when
the colony is presented with more work?
Inactive workers facilitate communication
Are highly inactive workers actually performing a
communication task?

15. Main question:
How can we explain high levels of worker inactivity?
Inactivity in the field / Lab artifact
Does colony-level inactivity occur in the field at levels
comparable to those observed in the lab?
Consistency of worker inactivity / Shift-work
Can apparent individual-level variation in inactivity be
explained by a bias in observation timescale?
Functions of
individual
inactivity that
Selfish worker reproduction
Is worker ovary development positively
are detrimental correlated to individual-level inactivity?
to colony fitness
Inactive workers as a reserve labor force
Do highly inactive workers become more active when
the colony is presented with more work?
Inactive workers facilitate communication
Are highly inactive workers actually performing a
communication task?

16. Main question:
How can we explain high levels of worker inactivity?
Inactivity in the field / Lab artifact
Does colony-level inactivity occur in the field at levels
comparable to those observed in the lab?
Consistency of worker inactivity / Shift-work
Can apparent individual-level variation in inactivity be
explained by a bias in observation timescale?
Selfish worker reproduction
Is worker ovary development positively
correlated to individual-level inactivity?
Functions of Inactive workers as a reserve labor force
Do highly inactive workers become more active when
individual the colony is presented with more work?
inactivity that
are beneficial to
colony fitness
Inactive workers facilitate communication
Are highly inactive workers actually performing a
communication task?

17. Inactivity in the field / Lab artifact
Does colony-level inactivity occur in the field at levels
comparable to those observed in the lab?
We recorded behavior in
the field and in the lab
and compared overall
inactivity levels
There were no significant
differences in inactivity
between observations in
the field and in the lab
p=0.38
Linear Mixed-Effects Model - Fixed-effects: Treatment
(lab or field); Random effects: Colony/Date/Timepoint

18. Consistency of worker inactivity
Can apparent individual-level variation in inactivity
be explained by a bias in observation timescale?
Observations of variation in
individual inactivity are just a
snapshot in time
Snapshot Multiple
observations
(large variation) (little variation) It’s possible that
over longer
timescales, indiv
idual variation
would disappear
Theoretical frequency distributions

19. Consistency of worker inactivity
Can apparent individual-level variation in inactivity
be explained by a bias in observation timescale?
Recording schedule
8 am – 08:00h
5 minute HD videos, 12 pm – 12:00h Day
every 4 hours, 4 pm – 16:00h
over 24 hours 8 pm – 20:00h
12 am – 24:00h Night
4 am – 28:00h
5 Colonies recorded
on 3 separate days,
within a 2 week period
Individually marked ants
(Temnothorax rugatulus)
Individual marking allows workers to be
tracked across multiple time points and days

20. Consistency of worker inactivity
Can apparent individual-level variation in inactivity
be explained by a bias in observation timescale?
Overall freq. distributions of inactivity for each colony
Even when
observed over
longer time periods
and at different
times of the day,
there are
consistent
differences in
individual
inactivity levels
p-value
Ant <0.0001
Time period 0.24
Ant x Time period 0.66
Linear Mixed-Effects Model - Fixed-effects: Time Step, Unique Ant
ID, Time Step x Unique Ant ID; Random effects:

21. Inactivity can’t be explained by
Inactivity in the field / Lab artifact
Does colony-level inactivity occur in the field at levels
Ecologically comparable to those observed in the lab?
non-relevant
explanations Consistency of worker inactivity / Shift-work
Can apparent individual-level variation in inactivity be
explained by a bias in observation timescale?
Selfish worker reproduction
Is worker ovary development positively
correlated to individual-level inactivity?
Inactive workers as a reserve labor force
Do highly inactive workers become more active when
the colony is presented with more work?
Inactive workers facilitate communication
Are highly inactive workers actually performing a
communication task?

22. Future work will look at:
Inactivity in the field / Lab artifact
Does colony-level inactivity occur in the field at levels
comparable to those observed in the lab?
Consistency of worker inactivity / Shift-work
Can apparent individual-level variation in inactivity be
Functions of explained by a bias in observation timescale?
individual
inactivity that Selfish worker reproduction
are detrimental Is worker ovary development positively
correlated to individual-level inactivity?
to colony fitness
- and - Inactive workers as a reserve labor force
Do highly inactive workers become more active when
Functions of the colony is presented with more work?
individual
inactivity that
are beneficial to
Inactive workers facilitate communication
Are highly inactive workers actually performing a
colony fitness communication task?

23. Selfish worker reproduction
Is worker ovary development positively correlated
to individual-level inactivity?
To test this
hypothesis, we will
measure individual
inactivity and then dissect
worker ovaries.
A positive relationship
Inactivity
between inactivity and
measures of ovary
development (e.g. number
of ovarioles, ovary
length, diameter of largest
oocyte) would suggest a Oocyte diam.
reproductive conflict where
highly inactive workers are
conserving energy by
avoiding work and putting
that energy toward their
own reproduction
Ovary dissections
by undergraduate
student Neil Hillis

24. Inactive workers as a reserve
labor force
Can apparent individual-level variation in inactivity
be explained by a bias in observation timescale?
We have preliminary
evidence that inactivity
might be linked to the
amount of work
There is an apparent
relationship between
colonies, their
brood/worker
ratios, and colony-
level inactivity.
Future work will involve manipulating
“available work”:
- Starvation
- Increased brood
- Predator
- Wall destruction
- Emigration
Linear Model - Fixed-effects: Brood/Worker
ratio, Season of observation, and Colony

25. Inactive workers facilitate
communication
Are highly inactive workers actually performing a
communication task?
Automated tracking developed
Looking at network topology, we by Dr. Shin at UNCC
can determine whether highly
inactive workers hold a highly
central position
Inactivity
Deg. centrality
We will also do a removal
experiment where the 20% laziest
workers and 20% random workers
Information
are removed from colonies and
measure whether information flow
flow
(Blonder & Dornhaus 2011) is lower
in colonies with fewer inactive
workers
20% laziest 20% random
removed removed

26. Broader impacts
Ecological trade-offs
Knowledge about how animals allocate their time between
inactivity, or rest, and other more active tasks provides insight into
ecological trade-offs
Division of labor / work allocation
Social insect colonies are evolved (and thus optimized)
and highly successful organized complex systems.
Our findings will be applicable to improving management
strategies and information flow in human-made and
biological networks (e.g. the internet, traffic, social
groups, and neural networks).
Emerging technologies
Automated tracking allows us to gather large amounts of
network data quickly. These can be used to investigate network
topologies of social insect interaction networks. We can also
look beyond interaction networks and answer novel
questions, by creating innovative network types such as time-
ordered and bipartite task networks (Charbonneau et al. 2013)

27. Thank you!
Dornhaus Lab
University of Arizona
GIDP-EIS
Center for Insect Science
IOS-0841756
Undergraduate helpers (Andrew Scott, Alex Down, Matt IOS-1045239
Velazquez, Nicole Fischer, and Mary Levandowski)
Nicole Fischer, Mary Levandowski
Folks at UNCC (Min Shin, Corey Poff and Hoan Nguyen)