1) The research aims to determine the relative contributions of the central and peripheral nervous systems in creating response thresholds in Temnothorax rugatulus ants. 2) Preliminary work found variation in the number of antennal sensory hairs independent of antenna size, suggesting a potential role of the peripheral nervous system. 3) The study will test whether response thresholds correlate more strongly with number of sensory hairs or brain region morphology to identify the dominant source of behavioral variation.

1. N I C O L E F I S C H E R , P H . D .
A D V I S O R : A N N A D O R N H A U S , P H . D .
R E S E A R C H A S S I S T A N T : J E N P I M E N T E L , U G
Neural Mechanisms of Behavioral
Variation in Temnothorax Ants

2. Introduction
— Evolutionary success of social
insects can be attributed to their
division of labor.
— The response threshold
model proposes this behavioral
variation as emergent of internal
thresholds for task-associated
stimuli2.
1. Weidenmüller, A (2004) The control of nest climate in bumblebee (Bombus terrestris) colonies: interindividual variability and self
reinforcement in fanning response. Behav Ecol 15(1): 120-8.
2. Wilson, EO (1976) Behavioral discretization and the number of castes in an ant species. Behav Ecol Sociobiol 1:141-54.
Figure taken from cis.arl.arizona.edu, an image
of “Temnothorax rugatulus workers, individually
marked, tending brood”
PC: Alex Wild

3. Background
— Temnothorax rugatulus
— The neural mechanisms under-
lying response thresholds can be
centrally and/or peripherally-
derived.
— Preliminary work on T. rugatulus suggests
significant variation in the number of antennal
sensory hairs, sensilla (Figure 1), which is also
independent of antenna size (n = 25, p = 0.376, R2 =
-0.01)2.
1. Riveros, A, Gronenberg, W (2010) Sensory allometry, foraging task specialization and resource exploitation in honeybees. Behav Ecol
Sociobiol 64: 955-67
2. Fischer, NE, Charbonneau, D, Gronenberg, W, Dornhaus, A (2013) Neural mechanisms of behavioral variation in Temnothorax ants.
University of Arizona. Tucson, AZ.

4. The Problem
— What are the relative
contributions of the CNS
and PNS in creating
response thresholds?
— How do response
thresholds arise from the
peripheral and central
nervous systems to give
rise to intra-specific
variation? Figure taken from Sites.middlebury.edu, a diagram of body
parts included in the CNS and PNS

5. Hypothesis
— Ha: Behavioral variation is dependent on
differences in sensory thresholds.
¡ Ha1: Worker response thresholds emerge
from the PNS.
¡ Ha2: Worker response thresholds are
regulated by the CNS.
— H0: Thresholds are independent of
structural properties.

6. Methods and Approaches
— To test Ha, individual
responses to varying
concentrations of
2-heptanone were
measured.
— Quantified according to
3 seemingly-induced
behaviors:
÷ Walking velocity
÷ Antennal drumming rate
÷ Proportion of time with mandibles
open
Image taken from Phys.org,
a photograph of the Monomorium
destructor’s open mandibles,
PC: Eli Sarnat
Image taken from
Alexanderwild.com,
T. rugatulus ants
painted with patterns
of color to help track
their behaviors

7. Methods and Approaches
— To test Ha1, counted the number
of sensilla on the first segment of
the antenna
— To test Ha2, olfactory input and
processing brain regions are
morphometrically analyzed
¡ Dissection
¡ Histological processing
¡ Confocal microscopy Image taken from Nicole-elise-
fischer.weebly.com, a cross section of a T.
rugatulus brain, emphasizing the mushroom
body (in teal), a region involved in sensory
integration

8. Predictions
— If Ha, workers will react at lower odor
stimulus levels.
¡ If Ha1, workers of similar behavioral
threshold levels will exhibit similarity
in peripheral sensory structures.
¡ If Ha2, workers of similar behavioral
thresholds will exhibit similar
neuroanatomical phenotypes.

9. Directions for this Lab
— Determine whether sensilla number, relative
regional brain volume, both, or neither(H0) are
significant for explaining threshold variation.
¡ Linear stepwise model using sensilla number, regional brain
volume, and their interactions to explain response to 2-
heptanone
— Explore the basis for differential regional brain
volume
¡ Staining and quantifying brain synapses, knowing synaptic
density and cortical thickness makes up the volume of regions
of the brain

10. Directions for Me
— Working with Nicholas
DiRienzo, Ph.D., under the
same advisor
¡ Recent research on individual
differences in web structure
and aggression in black widows
— Testing effects of biogenic
amines and topical solvent types on the behaviors of two
web-building spiders
— Studying the effects of brain hormone levels on behavior
and web structure in Phidippus johnsoni (Red-back spider)
Image taken from Museumvictoria.com.au, a
photograph of a female red-back spider in her web

11. Acknowledgments
— Anna Dornhaus, Ph.D., advisor
— Nicole Fischer, Ph.D., supervisor
— Colin Lynch, B.S.
— Nicholas DiRienzo, Ph.D., future
supervisor
— Arizona Science, Engineering, and
Mathematics Scholars (ASEMS) staff and
faculty