Group Title: Physiological and Biochemical Zoology, 79 (5). pp. 847-856.
Title: Two closely related species of Desert Carpenter Ant differ in individual-level allocation to fat storage.
CITATION PDF VIEWER THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00083966/00001
 Material Information
Title: Two closely related species of Desert Carpenter Ant differ in individual-level allocation to fat storage.
Series Title: Physiological and Biochemical Zoology, 79 (5). pp. 847-856.
Physical Description: Book
Creator: Hahn, D.A.
Affiliation: University of Florida -- Entomology and Nematology Department
Publication Date: 2006
 Subjects
Subject: Hymenoptera   ( lcsh )
 Record Information
Bibliographic ID: UF00083966
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Downloads

This item has the following downloads:

UF00083966 ( PDF )


Full Text









Two Closely Related Species of Desert Carpenter Ant Differ in

Individual-Level Allocation to Fat Storage


Daniel A. Hahn*
Department of Entomology and Nematology, University of
Florida, P.O. Box 110620, Gainesville, Florida 32611-0620

Accepted 3/16/2006; Electronically Published 8/15/2006



ABSTRACT

Comparison of closely related species that differ in their life
histories is a powerful method for studying the underlying
physiological mechanisms contributing to life-historyvariation.
I investigated whether two closely related members of the Cam-
ponotus festinatus species complex of desert carpenter ants, C.
nr. festinatus Desert Light and C. nr. festinatus Desert Dark,
differed in their life-history tactics with respect to fat storage.
Newly mated queens were collected in the field, and colonies
were reared under common conditions in the laboratory for 2
yr before sampling. I show that the two species differ in fat
storage at the individual level. While the basic scaling relation-
ship between lean mass and fat content did not differ between
the two species, Dark workers and soldiers stored significantly
more fat per unit lean mass than Light workers or soldiers.
There were no significant demographic differences in the pro-
portions of workers or soldiers involved in fat storage between
the two species, although there was a trend toward Light col-
onies having a greater proportion of soldiers storing large
amounts of fat. There was also no significant difference in the
total amount of fat stored by the two species at the colony
level. The detection of strong individual-level effects but no
colony-level effects was likely due to the low statistical power
of colony-level analyses. Showing that these two closely related
species differ in fat storage at the individual level in a common
environment demonstrates their utility as a model for under-
standing the physiological and behavioral mechanisms regu-
lating life-history variation in fat storage in ants.



Introduction

A fundamental problem in physiological ecology is identifying
mechanisms that control variation in patterns of resource al-

* Email: dahahn@ufl.edu.

Physiological and Biochemical Zoology 79(5):847-856. 2006. 2006 by The
University of Chicago. All rights reserved. 1522-2152/2006/7905-5072$15.00


location to phenotypic and life-history traits (Roff 1992; Stearns
1992; West-Eberhard 2003). One approach to identifying mech-
anisms begins with comparing allocation patterns between
closely related species with differing life histories. Social insects
provide an interesting perspective on resource allocation be-
cause selection occurs at a number of levels in insect societies,
with the colony as an important unit of selection (Bourke and
Franks 1995). However, most colony-level traits are products
of integrating multiple individual-level traits (Bourke and
Franks 1995). Therefore, understanding resource allocation in
social insects necessitates studying traits at both individual and
colony levels.
Allocation of nutritional resources to storage is an important
physiological life-history trait in most organisms. Storage is
functionally important because it allows organisms to decouple
times of resource need from times of availability, which allows
organisms to optimally time their investments into various life-
history traits and survive periods of nutrient limitation. Among
social insect species, nutrients can be stored either internally
as fats, carbohydrates, and proteins or externally as honey, pol-
len, or seeds (H6lldobler and Wilson 1990; Hunt and Nalepa
1994; Martinez and Wheeler 1994). In the ants, the most com-
mon type of nutrient reserve is internal fat storage, whereby
workers accumulate triglycerides in the fat body during times
of excess nutrient availability and mobilize these stores from
the fat body during times of nutrient limitation, passing them
to colony members through either lipid-rich oral secretions
from the postpharyngeal gland or unfertilized trophic eggs from
queens and workers containing functional ovaries (Voss 1981;
Holldobler and Wilson 1990).
In social insects, nutrient storage is a central characteristic
of seasonal life cycles (Ricks and Vinson 1972; Jensen 1978;
Tschinkel 1987, 1993, 1998; Hunt and Nalepa 1994; Moritz
1994; Bourke and Franks 1995; Borgesen 2000). For example,
in many ant species, the timing of sexual production and max-
imal nutrient need occurs during the spring, preceding the peak
of nutritional resource availability, which usually occurs in the
summer (Ricks and Vinson 1972; Jensen 1978; Tschinkel 1987,
1993, 1998; Borgesen 2000). Several species of ant have been
shown to accumulate fat reserves during times of high resource
availability that are depleted coincident with sexual production
several months later (Ricks and Vinson 1972; Jensen 1978;
Tschinkel 1987, 1993, 1998; Borgesen 2000). In these cases,
nutrient reserves provide the resources necessary for colonies
to produce sexual at the most advantageous time, even if re-
source availability is low at that time. In addition, fat reserves









848 D.A. Hahn


are important for the survival of colonies in climates where
nutritional resources can be unavailable for months at a time,
such as north-temperate ants that experience long overwinter-
ing periods (Lachaud et al. 1992; Cushman et al. 1993; Kaspari
and Vargo 1995).
Nutrient storage also affects individual-level behavior in so-
cial insects and has been correlated with colony-founding strat-
egies. Social insect workers performing risky off-nest tasks, such
as foraging or defense, tend to be older and leaner (Porter and
Jorgensen 1981; O'Donnell and Jeanne 1995; Blanchard et al.
2000; Toth and Robinson 2005). The adaptive value of exposing
low-cost "disposable" workers to risky tasks has long been rec-
ognized, but it was unclear whether declining fat stores mo-
tivated individuals to forage or were a product of the high
energetic demands of foraging. Recent pharmacological ma-
nipulations of fat storage in honeybee workers has shown that,
among same-age individuals, leaner workers were more likely
to initiate foraging, demonstrating a causative effect of fat stores
on behavior (Toth et al. 2005). This rationale has been extended
by Hunt and Amdam (2005), who have developed a model
predicting that nutritional reserves affect foraging behavior and
dominance interactions during the founding phase of the col-
ony life cycle, thereby promoting division of labor in groups
of Polistes wasps, a model system for the evolution of social
behavior. Furthermore, nutrient reserves are generally corre-
lated with colony-founding strategies among ant species.
Queens of independent claustral-founding species, which seal
themselves in underground chambers and rear their first brood
without foraging, contain greater reserves than either species
in which queens forage during the founding period or species
that found colonies dependent either on workers from the natal
nest or as social parasites (Keller and Passera 1989; Passera and
Keller 1990; Johnson 2002; Hahn et al. 2004). Therefore, un-
derstanding the regulation of nutrient reserves, particularly fat
storage, at the individual and colony levels is critical to un-
derstanding both the division of labor characteristic of social
insect colonies and the evolution of important colony life-
history traits, such as the timing of reproduction, founding
mode, and overwintering behavior.
Like all social insects, ant colonies can increase their fat stores
through both individual- and colony-level tactics (Tschinkel
1993). First, individual workers can increase the amount of fat
they store per unit lean mass. Second, colony demography can
shift so that a greater proportion of individuals within the
colony can participate in storing significant amounts of fat.
Most ant species employ a combination of these two tactics
(Ricks and Vinson 1972; Jensen 1978; Tschinkel 1987, 1993,
1998; Hasegawa 1993). In some ant species with polymorphic
workers, fat storage scales allometrically with body size so that
larger individuals contain proportionally greater fat stores; as
a result, fat storage becomes a function of the larger soldier
caste in these species, with individuals storing vast quantities
of nutrients termed repletes (Wilson 1974; Porter and Tschinkel


1985; Tsuji 1990; Lachaud et al. 1992; Hasegawa 1993; Tschinkel
1993, 1998). Interestingly, internal storage of liquid foods in
the crops of the honey-pot ant Myrmecocystus mexicanus is also
size dependent, with the largest workers in the colonybecoming
repletes regardless of the body size distribution within the col-
ony, ...... ii-1 ii body size plays an important role in storage
capacity in ants in general (Rissing 1984). Larger colonies gen-
erally contain greater total fat stores because they have both
more individuals and a greater mean individual size resulting
from a higher proportion of large individuals (Jensen 1978;
Hasegawa 1993; Tschinkel 1993, 1998). Therefore, understand-
ing the regulation of fat storage in social insects, such as ants,
requires understanding both the physiological mechanisms un-
derlying liporegulation in individual colony members and the
demography of storage among individuals within colonies
through time.
To investigate the mechanisms underlying the regulation of
fat storage in ants, I quantified patterns of allocation to fat
storage at the individual, caste, and colony levels in two closely
related desert carpenter ants that appeared to differ in fat stor-
age. Members of the Camponotus festinatus (Buckley) species
complex are distributed throughout the southwestern United
States and northern Mexico, and the complex contains a wide
range of morphological, ecological, and molecular genetic var-
iation over its range (Creighton 1950; Goodisman and Hahn
2005; A. N. Lazarus, S. P. Cover, D. A. Hahn, and J. J. Wer-
negreen, unpublished data). In southern Arizona, where the
complex has been best studied, four distinct species have been
identified on the basis of consistent morphological and mo-
lecular variation (Goodisman and Hahn 2005; A. N. Lazarus,
S. P. Cover, D. A. Hahn, and J. J. Wernegreen, unpublished
data). Two of these species, which are genetically and mor-
phologically distinguishable-C. nr. festinatus Desert Light
(hereafter "Light") and C. nr. festinatus Desert Dark (hereafter
"Dark"), so termed for differences in cuticle coloration-occur
sympatrically in low-elevation desert areas in the Tucson Basin.
Observation of these two species in the field and laboratory
suggested that individuals differed in the amount of fat body
development, which is indicative of fat storage (Rosell and
Wheeler 1995).
To determine whether these two species differed in fat stor-
age, I performed a common-garden experiment wherein queens
were collected during mating flights and colonies were reared
under controlled conditions in the laboratory for 2 yr, after
which colonies were sampled to determine whether the two
species differed in their fat storage tactics. Specifically, I asked
the following questions. First, did individual workers or soldiers
differ in the amount of fat they stored between the two species?
Second, did colonies of the two species differ in the demography
of fat storage among individuals in the colony? Third, was there
caste-level specialization in nutrient storage in colonies of either
species, as has been observed in other ants? Finally, was there









Fat Storage in Carpenter Ants 849


any difference between the two species in the total amount of
fat stored by colonies?

Material and Methods

Insect (.. Il. t..,,; and Rearing
Queens of both species were collected at an ultraviolet light
trap located at ca. 1,000 m elevation on the University of Ar-
izona Santa Rita Experimental Range in Pima County, Arizona,
on July 14 and July 19, 1999. Queens were confined in 30-mL
glass test tubes filled with ca. 10 mL of deionized water and
plugged with cotton to provide moisture. Queens and resulting
colonies were maintained in an environmental chamber at 30C
for the duration of the experiment. Tubes were checked for
brood every 3-5 d. After 8 wk, queens of both species had
produced two to five pupae. Tubes containing pupae were
checked every second day for eclosion of the first worker. Once
the first worker had closed, tubes were unplugged and placed
in a 200-mL plastic petri dish containing fresh cotton-plugged
test tubes, which served as water sources and nesting substrates.
Colonies resided in petri dish nests for approximately 9 mo
while they grew in size. After 12 mo, colonies were moved to
33 x 18 x 10-cm plastic boxes containing more cotton-
plugged test tubes as water sources and nesting substrates. Col-
onies were fed ad lib. with a combination of frozen immatures
of the cockroach Nauphoeta cinera and the moth Manduca sexta
and a 1 : 1 v/v honey-water mixture supplemented with 0.5%
Vanderzant's vitamin mix and 0.5% Wesson's salts mix from
the time of first worker eclosion until sampling.


Sampling
After 2 yr in the laboratory, five colonies of each species were
sampled during July 2001. Sampling consisted of separating all
workers and soldiers from the colony. Workers and soldiers of
both species are capable of accumulating significant stored fat
and protein reserves in their fat bodies, causing the gaster to
become distended (Martinez and Wheeler 1994; Rosell and
Wheeler 1995). In this study, gaster distension was used as a
visual proxy of fat body development and consequently fat
storage. To investigate the demography of fat storage in the
two species, workers and soldiers were each sorted into one of
two categories based on observable gaster distension: low or
high fat. Because the abdominal cuticle of these ants is very
light and the intersegmental membranes are completely trans-
lucent when the gaster is distended, it is possible to distinguish
between distension due to fat body development and distension
of the crop with liquid. Individuals whose gasters were clearly
distended because of full crops and had little fat body devel-
opment were placed into the low-fat group. The number of
workers and soldiers in each of the fat body groups was counted
for each colony. Twenty individuals from each fat body group
in each colony were haphazardly selected for further analyses.


Each individual was placed into a 1.5-mL plastic microcentri-
fuge tube and frozen at -200C until further analysis. Colonies
of each species were sampled alternately so that bias in sampling
time throughout was minimized (i.e., Light colony 1 was sam-
pled, then Dark colony 1 was sampled, etc.).


Fat Extraction and Quantification
Six to 12 individuals from each fat body development group
within each colony were selected for fat content analysis, yield-
ing 312 individuals in total. Samples were freeze-dried until
constant weight, weighed, and stored at -200C until fat anal-
ysis. Fat content was analyzed using a modification of Van
Handel's (1985) procedure. In brief, samples were homogenized
for 60-90 s at 250 rpm in 1: 1 (v/v) chloroform-methanol
solvent in 1.5-mL microcentrifuge tubes with a plastic pestle
and centrifuged at 12,000 g for 20 min at 40C. The supernatant
was removed, and the above procedure was repeated once with
the 1: 1 chloroform-methanol mixed solvent and once with
pure chloroform. To remove polar lipids, pooled supernatants
were subsequently run through a column containing 0.2 g of
100-mesh silicic acid that had been dried at 1000C overnight.
Columns were washed eight times with 1 mL pure chloroform
to elute nonpolar lipids, of which triglycerides represented more
than 95% of the total. The resulting solution was dried under
nitrogen gas and resuspended in a known amount of pure
chloroform. A subsample of this solution was used to spectro-
photometrically estimate fat mass against a range of known
triolein standards using the sulfophosphovannilin method.
Lean mass for each individual was calculated as total dry
mass minus fat mass. Total fat storage for each fat body group
within each caste and colony was estimated by taking the mean
fat storage value for the individuals analyzed from that fat body
group and multiplying it by the total number of individuals in
the group. Colony-level fat storage was calculated by taking the
sum of the total fat storage values for each fat body group
within a colony.

Results

Individual-Level .II \.l ..,,,; to Fat St, .,., between Species
Because there is often significant between-colony variation in
social insects (Bourke and Franks 1995), nested ANOVAs with
colony as a random factor nested within species were used for
individual-level analyses. There was no significant difference in
lean mass in either workers or soldiers between the two species
(workers, whole model: F = 1.60, df = 9, P = 0.117; workers,
species: F = 2.80, df = 1, P = 0.096; soldiers, whole model:
F = 2.19, df = 9, P = 0.028; soldiers, species: F = 0.15,
df = 1, P = 0.701; Fig. la). There were also no significant
colony-level effects on lean mass in workers, but there was a
significant effect of colony on lean mass in soldiers (workers,
colony [species]: F = 1.48, df = 8, P = 0.167; soldiers, colony









850 D. A. Hahn


la Desert Dark
6 0 Desert Light

5 -

4 4

3

2
0



Workers Soldiers

b8
6 lb Desert Dark
'4 o Desert Light


:0
8
6
4 -
2 -
*i


Workers


Soldiers


Figure 1. a, Individual-level comparisons of lean mass in workers and
soldiers of the two species, using means adjusted for colony-level ef
fects. b, Individual-level comparisons of fat content in workers and
soldiers of the two species, using means adjusted for effects of colony
and lean mass. Error bars represent 1 SE; bars are subsumed within
some points. An asterisk denotes a statistically significant difference.


[species]: F = 2.46, df = 8, P = 0.017). Certain colonies in
both species had larger soldiers than others.
Fat content was positively correlated with lean mass in both
workers and soldiers of both species (Pearson's correlations;
Dark workers: r = 0.54, n = 93, P<0.001; Light workers:
r = 0.60, n = 102, P< 0.001; Dark soldiers: r = 0.63, n = 55,
P< 0.001; Light soldiers: r = 0.58, n = 62, P< 0.001). There-
fore, lean mass was used as a covariate, along with colony nested
within species, in analyses of fat content. Dark workers and
soldiers stored significantly more fat than Light workers or
soldiers when lean mass and colony were accounted for (Table
1; Fig. lb). There were also significant colony-level effects on
fat content in both workers and soldiers when lean mass was
held constant (Table 1). Individuals from certain colonies in
both castes within both species stored more fat per unit lean
mass than those in other colonies. There was no clear rela-
tionship between colony size and fat content in either species.


Therefore, the observed colony-level effects were not mediated
by colony size.
Significantly greater fat storage per unit lean mass by Dark
workers and soldiers becomes apparent when the scaling re-
lationship between lean mass and fat mass is compared in each
caste of the two species. To hold colony-level effects constant,
the residuals of lean mass and fat mass from ANOVAs with
colony nested within species were plotted against each other
to determine whether the relationship between lean mass and
fat storage differed between the two species in either caste.
There was a significant positive linear relationship between lean
mass and fat mass in both castes of both species (Fig. 2). Re-
gression slopes did not differ between the two species in either
caste (workers: t = 0.32, df= 191, P>0.05; soldiers: t=
1.03, df = 113, P> 0.05). However, the elevations of the re-
gression lines were significantly greater for both Dark workers
and Dark soldiers (workers: t = 3.30, df = 192, P< 0.005; sol-
diers: t = 2.37, df = 114, P< 0.01). Therefore, the basic scaling
relationship between lean mass and fat storage did not differ
between the two species in either caste, but Dark workers and
soldiers contained more fat per unit lean mass throughout the
range of lean masses.


Demography of Fat S,..i, between Species

Individuals visually assigned to the high-fat group contained
significantly greater fat reserves than individuals in the low-fat
group for both castes in each species when colony-level effects
were held constant (Table 2). Therefore, visual estimation of
fat body development was a good proxy for individual fat con-
tent to assess fat storage demography within colonies. There
was no difference between the two species in the proportion

Table 1: Nested ANCOVA for the effects of species,
colony nested within species, and lean mass on lipid
content

Caste, Source df F P

Worker fat content:
Whole model 10 25.54 <.001
Species 1 14.42 <.001**
Colony (species) 8 13.69 <.001
Lean mass (mg) 1 149.72 <.001
Error 184
Total 194
Soldier fat content:
Whole model 10 13.30 <.001
Species 1 5.99 .016**
Colony (species) 8 4.60 <.001
Lean mass (mg) 1 62.69 <.001
Error 106
Total 116
** Significant species-level effect.










Fat Storage in Carpenter Ants 851


04

03

02
0
C-)



aa)
-01-
- -


U,



-02




0.6-


0 1
Residual Lean Mass


Residual Lean Mass


Figure 2. Plots of the relationships between colony-adjusted lean mass
and fat mass in workers (a) and soldiers (b) of the Desert Dark (solid
lines) and Light species (dashed lines). Dark workers fat mass =
0.011 + 0.067(lean mass), R2 = 0.39, df = 92, F = 59, P<0.001.
Light workers fat mass = -0.010 + 0.063(lean mass), R2 = 0.52,
df = 101, F = 109, P<0.001. Dark soldiers fat mass = 0.014 +
0.038(lean mass), R2 = 0.40, df = 61, F = 40, P< 0.001. Light soldiers
fat mass = -0.017 + 0.028(lean mass), R2 = 0.28, df = 54, F = 21,
P< 0.001.

of workers or soldiers in the low- and high-fat categories within
colonies, although there was a moderately significant trend to-
ward Light colonies containing a greater proportion of high-
fat soldiers (low-fat workers: t = 0.29, df = 8, P = 0.604; high-
fat workers: t = 0.36, df = 8, P = 0.726; low-fat soldiers:
t = 0.95, df = 8, P = 0.386; high-fat soldiers: t = 2.10, df =
8, P = 0.069; Fig. 3). This trend suggests that although Dark
workers and soldiers store more fat per unit lean mass than
Light workers and soldiers, Light colonies may involve a slightly
greater proportion of their soldiers in storing large quantities
of fat.

SiN.,. Attributes between Castes within Each Species
In both species, soldiers contained significantly greater total fat
mass than workers (Table 3; Fig. 4a). However, for both species,


2a Desert Dark Workers -
SDesert Light Workers --




*
0


^-^^8-^


Dark workers:
Whole model
Fat storage state
Colony
Error
Total
High-fat storage state
Low-fat storage state
Light workers:
Whole model
Fat storage state
Colony
Error
Total
High-fat storage state
Low-fat storage state
Dark soldiers:
Whole model
Fat storage state
Colony
Error
Total
High-fat storage state
Low-fat storage state
Light soldiers:
Whole model
Fat storage state
Colony
Error
Total
High-fat storage state
Low-fat storage state


18.55
62.50
8.02


<.0001
<.0001
<.0001


.177 .008
.098 + .006


21.92
47.80
16.52


<.0001
<.0001
<.0001


.149 .007
.092 .005


17.65
52.43
8.90


<.0001
<.0001
<.0001


.323 .020
.133 .015


14.42 <.0001
3.53 <.0001
16.26 <.0001


.193 .008
.107 .013


workers contained a significantly greater proportion of fat per
unit lean mass than soldiers (Table 4; Fig. 4b). There were
significant colony-level effects on both total fat mass and the
fat-to-lean-mass ratio in both species (Tables 3, 4).



Colony- and Caste-Level Attributes between Species

Although individual colonies ranged between 540 and 1,221
individuals among the five colonies sampled from each species,
there was no difference in total number of individuals, total
number of workers, or total number of soldiers between the


Table 2: Two-way ANOVA for the effects of visual fat storage
state and colony on fat content for each caste within each
species and the resulting adjusted means for fat content for
high- and low-fat-content individuals within each caste and
species

Adjusted Mean
Species, Caste, Source df F P Fat SE (mg)









852 D. A. Hahn


U .0 --- -------------------------------
3a 0* Desert Dark

0 o Desert Light
06 L


04 -


02 -


00 -
Low Fat Workers High Fat Workers


012
3b
Desert Dark
0.10 o Desert Light

008

0.06 -

004 -

0 02 -

0.00


Low Fat Soldiers


High Fat Soldiers


Figure 3. a, Comparisons of the proportion of the total number of
individuals in the colony accounted for by high and low-fat workers
of the two species, b, Comparisons of the proportion of the total
number of individuals in the colony accounted for by high and low
fat soldiers of the two species. Error bars represent 1 SE.


two species (total individuals: t = 0.71, df = 8, P = 0.498; total
workers: t = 0.73, df = 8, P = 0.485; total soldiers: t = 0.12,
df = 8, P = 0.907; Fig. 5a). In addition to being the same size,
on average, colonies of the two species did not differ signifi-
cantly in either overall total fat storage or the total amount of
fat stored by workers or soldiers, although there was a slight
trend toward Dark colonies storing more fat overall (total fat
content: t = 0.74, df = 8, P = 0.483; worker fat content: t =
0.76, df = 8, P = 0.471; soldier fat content: t = 0.06, df = 8,
P = 0.957, respectively; Fig. 5b).


Discussion

I have shown that two species within the Camponotusfestinatus
species complex differ in individual-level fat storage tactics in
a common environment. Dark workers and soldiers stored sig-
nificantly more fat per unit lean mass than Light workers and
soldiers. There was no detectable significant demographic dif-


ference in the proportions of workers or soldiers with different
levels of fat body development between the two species, al-
though Light colonies contained a noticeably greater propor-
tion of high-fat body distension soldiers. Despite the differences
in individual-level fat storage tactics between the two species,
the total amount of fat stored by colonies of the two species
did not differ.
Soldiers of both C. festinatus species stored significantly
greater total amounts of fat than workers, but soldiers stored
proportionally less fat than workers per unit lean mass in both
species, which contrasts with species containing repletes (Wil-
son 1974; Porter and Tschinkel 1985; Tsuji 1990; Lachaud et
al. 1992; Hasegawa 1993; Tschinkel 1993, 1998). Greater pro-
portional fat storage in workers rather than soldiers, combined
with much greater numbers of small workers in the colonies
of both species, led to workers containing 87% and 84% of the
total colony fat load in Dark and Light colonies, respectively,
highlighting the importance of small workers relative to soldiers
in colony-level fat storage in these two species. Many ant species
with worker size variation contain a greater proportion of work-
ers than soldiers, and the role of small workers in internal
nutrient storage may be more important than is currently rec-
ognized across the ants.
Storing more fat per unit lean mass has been well docu-
mented as a tactic for increasing fat storage during ontogeny
among colonies of a number of ant species (Jensen 1978; Has-
egawa 1993; Tschinkel 1993, 1998) and now has been shown
to contribute to between-species differences as well. Interest-
ingly, neither mean individual size nor the basic scaling rela-
tionship between lean mass and fat storage differed in either
caste of the two species (i.e., a positive linear relationship, with
slopes that did not differ significantly). However, the elevation
of the relationship was significantly greater in both Dark work-
ers and Dark soldiers, reinforcing that the Dark species stores



Table 3: ANCOVA for caste differences in the natural
log of total fat mass in the two species

Form, Source df F P

Desert Dark:
Whole model 9 9.50 <.001
Colony 4 7.77 <.001
Caste (colony) 5 10.64 <.001**
Error 145
Total 154
Desert Light:
Whole model 9 15.02 <.001
Colony 4 16.57 <.001
Caste (colony) 5 12.26 <.001**
Error 147
Total 156
Significant caste-level effect.










Fat Storage in Carpenter Ants 853


Worker Soldier


Worker Soldier


-2 Desert Dark
-20
0 Desert Light
-22

-2 4
*
-26 -

-28 -

-30 -

-3 2 -

-3 4


Worker Soldier


Worker Soldier


Figure 4. a, Comparisons of the natural log of total fat storage between
workers and soldiers in each species, b, Comparisons of the natural
log of the fat mass to lean mass ratio between workers and soldiers
in each species. Symbols represent adjusted means from the nested
ANOVAs in Table 3. Error bars represent one standard error. An as
terisk denotes a statistically significant difference.


more fat per unit lean mass throughout its body size range. If
fat storage in individual ants is maintained around an internally
determined liporegulatory set point, as it is in mammals (Mer-
cer 1998; Woods and Seeley 2000), these results suggest that
Dark individuals have a higher liporegulatory set point than
Light individuals over their entire range of body sizes and that
liporegulatory set points are more labile than the basic scaling
relationship between body size and fat storage in these ants.
Showing that Dark individuals store significantly more fat in
a common environment validates the use of this pair of species
as a comparative model for understanding the physiological
and behavioral mechanisms regulating fat storage in ants. Fu-
ture work should focus on comparing behavioral mechanisms
of fattening (e.g., the distribution of food among colony mem-
bers) and physiological mechanisms of fattening (e.g., lipid
synthesis and mobilization responses to nutrition and activity)
in individuals of both species in a common environment.


4a
Desert Dark
o Desert Light


-1 2

14

0)
E -16

- -18-
0
0 -20
U_
22-
--
C
_J
-24

-26


It may seem contradictory that I found strong support for
a difference in allocation tactics at the individual level between
the two species and weak to no support for species differences
in colony-level traits, such as total fat storage or proportion of
the colony accounted for by various fat body development
groups. If Dark workers and soldiers store more fat per unit
lean mass, either Dark colonies should have contained signif-
icantly greater fat stores or the two species could have achieved
the same colony-level fat stores through Light colonies con-
taining a greater proportion of individuals storing significant
amounts of fat (e.g., a greater proportion of high-fat individ-
uals). I found strong support for neither. However, this was
likely due to a lack of statistical power to detect colony-level
differences, compared to my ability to detect individual-level
differences between the two species, rather than to a true lack
of colony-level differences. Post hoc power analyses of all the
colony-level comparisons in this study revealed that statistical
power was low for all comparisons, with no comparison having
greater than 50% power, which is much lower than the 80%
95% power levels suggested as benchmarks by some authors
(Lougheed et al. 1999; Walsh et al. 1999; Di Stefano 2003).
Such low power estimates make it impractical to detect differ-
ences at the colony level with laboratory-reared colonies such
as these. For example, one would have to sample 58 colonies
between the two species to approximate 80% power and 71
colonies to approximate 95% power for the observed levels of
variance in the colony-wide fat comparison, unrealistic goals
considering the workload and space necessary to rear laboratory
colonies. Although this study does not provide conclusive evi-
dence for differences in colony-level traits between the two
species, the strong evidence for individual-level differences in
fat storage suggests that they must exist, and the trend toward
Light colonies containing a greater proportion of high-fat sol-
diers merits further study.



Table 4: ANCOVA for caste differences in natural log
of fat mass/lean mass ratio in the two species

Form, Source df F P

Desert Dark:
Whole model 9 19.28 <.001
Colony 4 9.98 <.001
Caste (colony) 5 20.64 <.001**
Error 145
Total 154
Desert Light:
Whole model 9 29.66 <.001
Colony 4 18.51 <.001
Caste (colony) 5 31.42 <.001**
Error 147
Total 156
** Significant caste-level effect.









854 D. A. Hahn


1400


16n -


E 100

U- 80
-0
U
S60

40

20

0


All
Individuals


Workers


Soldiers


Figure 5. a, Colony-level comparisons between species of the total
number of individuals, total number of workers, and total number of
soldiers between species, b, Colony-level comparisons between species
in total amount of fat stored by all individuals in the colony, byworkers,
and by soldiers. Error bars represent 1 SE.


Another explanation for why no significant differences in
colony-level traits were found between the two species could
be the young age of the colonies. Because colony-level fat stor-
age is known to increase with colony age and size in several
species of ants, the young age of the laboratory colonies used
in this study may have contributed to the lack of detectable
colony-level differences (Jensen 1978; Tschinkel 1993, 1998).
These 2-yr-old colonies were smaller than their peak age and
size in the lab and were clearly still in the ergonomic growth
phase of their life cycle. Differences in fat storage at the colony
level between the two species could become more pronounced
with time and more easily detected as the colonies move from
the ergonomic growth stage into the reproductive stage of their
life cycle, where they will reach peak size, raising the question
of whether the ontogeny of fat storage differs between the two
species (Oster and Wilson 1978). Future efforts to assess
colony-level fat storage should focus on mature colonies sam-
pled from both the laboratory and the field.
There was significant variation among colonies within each


species. Individuals from certain colonies within each species
had consistently higher lean masses or fat stores, although in-
dividuals from colonies with the greatest average lean masses
did not always contain the greatest fat stores and fat storage
was not related to colony size. This suggests that intraspecific
variation in fat storage tactics may exist. Significant variation
between colonies is a well-known feature of social insects that
can be caused by numerous factors, including colony genotype,
maternal, environmental, size, and age-related effects (H6ll-
dobler and Wilson 1990; Bourke and Franks 1995). Because
environment, age, and size were kept the same among colonies
in this study, it seems likely that the strong observed colony-
level effects were due to genetic effects, maternal effects, or
perhaps demographic stochasticity.
Strong differences in fat storage tactics at the individual level
prompt the question of why these two closely related species
have evolved different fat storage tactics. Variation in resource
availability has been implicated as a cause of increased nutrient
storage in a number of animals, including insects, birds, and
mammals (Boggs 1981; Houston and McNamara 1993; Perrin
and Sibly 1993; Rogers et al. 1993; Bednekoff and Houston
1994; Diamond 2003). Deserts are notoriously variable places
in terms of environmental conditions, particularly rainfall,
which is directly related to productivity and resource availability
in these moisture-limited systems (Inouye 1991; Pake and Ven-
able 1996). For perennial organisms, such as ant colonies, in-
vesting heavily in nutrient stores when nutrient availability is
high is a potential bet-hedging strategy for dealing with times
of reduced resource availability (Rogers 1987; Philippi and Se-
ger 1989; Rogers et al. 1993; Hopper 1999). Observations of
colonies of these two species in areas of sympatry in the Tucson
Basin suggest no significant differences in reproductive timing,
foraging patterns, or habitat/microhabitat selection, making the
observed differences in fat storage all the more interesting (D.
A. Hahn, unpublished data). Further study of the two species
in areas of allopatry and sympatry over a wider geographic
range would be useful in determining whether ecological factors
such as microhabitat selection differ between the two species
in a way that could influence the costs and benefits of different
fat storage patterns.
Differences in individual-level fat storage tactics may also
have consequences for behavior in the two species. Studies in
ants, bees, and wasps have shown that individuals performing
risky off-nest behaviors, such as foraging or defense, contain
less fat stores than those performing less risky in-nest behaviors,
such as nursing larvae (Kondoh 1968; Porter and Jorgenson
1981; MacKay 1983; Lachaud et al. 1992; O'Donnell and Jeanne
1995; Blanchard et al. 2000; Toth and Robinson 2005). This
correlation has led several authors to It. _. l 11 i fat stores may
influence the repertoire of behaviors an individual will perform
and that this correlation has evolved because of the adaptive
value of individuals with less energetic content, which would
be less costly for the colony to lose, performing risky tasks


5b Desert Dark
0 Desert Light









Fat Storage in Carpenter Ants 855


(Porter and Jorgenson 1981; MacKay 1983; O'Donnell and
Jeanne 1995; Blanchard et al. 2000). Furthermore, recent work
in honeybees has shown that manipulating fat reserves has a
causative effect on foraging behavior, with leaner individuals
more readily engaging in risky tasks such as foraging. If fat
storage influences ant behavior in a similar way, differences in
individual-level storage tactics between the two C. nr. festinatus
desert species could lead to significant behavioral differences,
perhaps in the rate that individuals progress through behavioral
development during their lifetimes, or differences in their mo-
tivation to forage or defend their nests. In addition, Dark work-
ers and soldiers containing more fat per unit dry mass through-
out their body size range than Light workers and soldiers
suggests that losing a worker or soldier may be more costly to
Dark colonies, which could have implications for the evolution
of colony-level traits, such as collective foraging and nest de-
fense, colony growth rates, and sexual production. Clearly, fur-
ther studies of colonies over a greater portion of the colony
life cycle are needed in the two species to determine whether
there are individual- or colony-level behavioral or life-history
differences associated with fat storage. By demonstrating sig-
nificant individual-level differences in fat storage among work-
ers and soldiers of these two closely related desert carpenter
ant species from an area of sympatry reared in a common
environment, this study has laid the foundation for using the
C. festinatus species complex as a model for understanding both
the proximate physiological and behavioral mechanisms and
the ultimate evolutionary mechanisms regulating nutrient stor-
age at the individual and colony levels in social insects.


Acknowledgments

Foremost, I would like to thank Diana Wheeler for introducing
me to the Camponotus festinatus species complex, pointing out
that there might be a difference in fat storage between the two
desert species, and providing me with many years of guidance
and encouragement. Norm Buck provided invaluable training
and assistance in the lab. Paul Matson, Amanda Jaksha, and
Julio Loya all provided able assistance in various stages of this
project. Diana Wheeler, Reg Chapman, Leticia Aviles, Allen
Gibbs, Mike Wells, Lisa Nagy, Bob Johnson, Mike Scharf, and
Andy Yang provided thoughtful comments that improved the
study or manuscript. As always, Jennifer Weeks provided critical
logistical support. This project was supported by a National
Science Foundation Research Training Grant in the Analysis of
Biological Diversification to the University of Arizona (DBI-
9602246) and small grants from the University of Arizona Cen-
ter for Insect Science and Sigma Xi.

Literature Cited

Bednekoff P.A. and A.I. Houston. 1994. Optimizing fat reserves
over the entire winter: a dynamic model. Oikos 71:408-415.


Blanchard G.B., G.M. Orledge, S.E. Reynolds, and N.R. Franks.
2000. Division of labour and seasonality in the ant Lepto-
thorax Ill'ip'iiin, : worker corpulence and its influence on
behavior. Anim Behav 59:723-738.
Boggs C.L. 1981. Nutritional and life-history determinants of
resource allocation in holometabolous insects. Am Nat 117:
692-709.
Bergesen L.W. 2000. Nutritional function of replete workers in
the pharaoh's ant, Monomorium pharaonis (L.). Insectes Soc
47:141-146.
Bourke A.F.G. and N.R. Franks. 1995. Social Evolution in Ants.
Princeton University Press, Princeton, NJ.
Creighton W.S. 1950. Ants of North America. Bull Mus Comp
Zool Harv Univ 104:1-585.
Cushman J.H., J.H. Lawton, and B.F.J. Manly. 1993. Latitudinal
patterns in European ant assemblages: variation in species
richness and body size. Oecologia 95:30 37.
Diamond J. 2003. The double puzzle of diabetes. Nature 423:
599-602.
Di Stefano J. 2003. How much power is enough? against the
development of an arbitrary convention for statistical power
calculations. Funct Ecol 17:707-709.
Goodisman M.A.D. and D.A. Hahn. 2005. Breeding system,
colony structure, and genetic differentiation in the Campo-
notus festinatus species complex of carpenter ants. Evolution
59:2185-2199.
Hahn D.A., R.A. Johnson, N.A. Buck, and D.E. Wheeler. 2004.
Storage protein content as a functional marker for colony-
founding strategies: a comparative study within the harvester
ant genus Pogonomyrmex. Physiol Biochem Zool 77:100-108.
Hasegawa E. 1993. Caste specialization in food storage in the
dimorphic ant Colobopsis nipponicus (Wheeler). Insectes Soc
40:261-271.
Holldobler B. and E.O. Wilson. 1990. The Ants. Harvard Uni-
versity Press, Cambridge, MA.
Hopper K.R. 1999. Risk-spreading and bet-hedging in insect
population biology. Annu Rev Entomol 44:535-560.
Houston A.I. and J.M. McNamara. 1993. A theoretical inves-
tigation of the fat reserves and mortality levels of small birds
in winter. Ornis Scand 24:205-219.
Hunt J.H. and G.V. Amdam. 2005. Bivoltinism as an antecedent
to sociality in the paper wasp genus Polistes. Science 308:
264-267.
Hunt J.H. and C.A. Nalepa. 1994. Nourishment, evolution and
insect sociality. Pp. 1-19 in J.H. Hunt and C.A. Nalepa, eds.
Nourishment and Evolution in Insect Societies. Westview,
Boulder, CO.
Inouye R.S. 1991. Population biology of desert annual plants.
Pp. 27-54 in G.A. Polis, ed. The Ecology of Desert Com-
munities. University of Arizona Press, Tucson.
Jensen T.F. 1978. An energy budget for a field population of
Formica pratensis. Nat Jutl 20:203-206.
Johnson R.A. 2002. Semi-claustral colony founding in the seed









856 D. A. Hahn


harvester ant Pogonomyrmex californicus: a comparative anal-
ysis of colony-founding strategies. Oecologia 132:60-67.
Kaspari M. and E.L. Vargo. 1995. Colony size as a buffer against
seasonality: Bergmann's rule in social insects. Am Nat 145:
610-632.
Keller L. and L. Passera. 1989. Size and fat content of gynes in
relation to the mode of colony founding in ants. Oecologia
80:236-240.
Kondoh M. 1968. Bioeconomic studies on the colony of an
ant species, Formica japonica Motschulsky. II. Allometric
study of the body mass and corpulence relating to body size
of workers. Jpn J Ecol 64:370-392.
Lachaud J.P., L. Passera, A. Grimal, C. Detrain, and G. Beugnon.
1992. Lipid storage by major workers and starvation resis-
tance in the ant Pheidole pillduila. Pp. 153-160 in J. Billen,
ed. Biology and Evolution of Social Insects. Leuven Univer-
sity Press, Leuven.
Lougheed L.W., A. Breault, and D.B. Lank. 1999. Estimating
statistical power to evaluate ongoing waterfowl population
monitoring. J Wildl Manag 63:1359-1363.
MacKay W.P. 1983. Stratification of workers in harvester ant
nests (Hymenoptera: Formicidae). J Kans Entomol Soc 56:
538-542.
Martinez T. and D.E. Wheeler. 1994. Storage proteins in adult
ants (Camponotus festinatus): roles in colony founding by
queens and in larval rearing by workers. J Insect Physiol 40:
723-729.
Mercer J.G. 1998. Regulation of appetite and body weight in
seasonal mammals. Comp Biochem Physiol C 119:295-303.
Moritz R.FA. 1994. Nourishment and sociality in honeybees.
Pp. 345-389 in J.H. Hunt and C.A. Nalepa, eds. Nourishment
and Evolution in Insect Societies. Westview, Boulder, CO.
O'Donnell S. and R.L. Jeanne. 1995. Worker lipid stores de-
crease with outside-nest task performance in wasps: impli-
cations for the evolution of age polyethism. Experientia 51:
749-752.
Oster G.F and E.O. Wilson. 1978. Caste and ecology in the
social insects. Princeton University Press, Princeton, NJ.
Pake C.E. and D.L. Venable. 1996. Seed banks in desert annuals:
implications for persistence and coexistence in variable en-
vironments. Ecology 77:1427-1435.
Passera L. and L. Keller. 1990. Loss of mating flight and shift
in the pattern of carbohydrate storage in sexual of ants. J
Comp Physiol B 160:207-211.
Perrin N. and R.M. Sibly. 1993. Dynamic models of energy
allocation and investment. Annu Rev Ecol Syst 24:379-410.
Philippi T. and J. Seger. 1989. Hedging one's evolutionarybets,
revisited. Trends Ecol Evol 4:41-44.
Porter S.D. and C.D. Jorgensen. 1981. Foragers of the harvester
ant, Pogonomyrmex owyheei: a disposable caste. Behav Ecol
Sociobiol 9:247-256.
Porter S.D. and W.R. Tschinkel. 1985. Fire ant polymorphism:


the ergonomics of brood production. Behav Ecol Sociobiol
16:323-336.
Ricks B.L. and S.B. Vinson. 1972. Changes in nutrient content
during one year in workers of the imported fire ant. Ann
Entomol Soc Am 65:135-138.
Rissing S.W. 1984. Replete caste production and allometry in
the honey ant, Myrmecocystus mexicanus Wesmael. J Kans
Entomol Soc 57:347-350.
Roff D.A. 1992. The Evolution of Life Histories: Theory and
Analysis. Chapman & Hall, New York.
Rogers C.M. 1987. Predation risk and fasting capacity: do win-
tering birds maintain optimal body mass? Ecology 68:1051-
1061.
Rogers C.M., V. Nolan Jr., and E.D. Ketterson. 1993. Geo-
graphic variation in winter fat of dark-eyed juncos: displace-
ment to a common environment. Ecology 74:1183-1190.
Rosell R.C. and D.E. Wheeler. 1995. Storage function and ul-
trastructure of the adult fat body in workers of the ant Cam-
ponotus festinatus (Buckley). Int J Insect Morphol 24:413
426.
Stearns S.C. 1992. The Evolution of Life Histories. Oxford Uni-
versity Press, Oxford.
Toth A.L., S. Kantarovich, A.F Meisel, and G.E. Robinson. 2005.
Nutritional status influences socially regulated foraging on-
togeny in honeybees. J Exp Biol 208:4641-4649.
Toth A.L. and G.E. Robinson. 2005. Worker nutrition and di-
vision of labor in honeybees. Anim Behav 69:427-435.
Tschinkel W.R. 1987. Seasonal life history and nest architecture
of the winter-active ant, Prenolepis imparis. Insectes Soc 34:
143-164.
1993. Sociometry and sociogenesis of colonies of
the fire ant Solenopsis invicta during one annual cycle. Ecol
Monogr 63:425-457.
1998. Sociometry and sociogenesis of colonies of the
harvester ant, Pogonomyrmex badius. I. Worker characteris-
tics in relation to colony size and season. Insectes Soc 45:
385-410.
Tsuji K. 1990. Nutrient storage in the major workers of Pheidole
ryukyuensis. Appl Entomol Zool 25:283-287.
Van Handel E. 1985. Rapid determination of total lipids in
mosquitoes. J Am Mosq Control Assoc 1:302-304.
Voss S.H. 1981. Trophic egg production in virgin fire ant
queens. J Ga Entomol Soc 16:437-440.
Walsh H.E., M.G. Kidd, T. Moum, and V.L. Friesen. 1999. Po-
lytomies and the power of phylogenetic inference. Evolution
53:932-937.
West-Eberhard M.J. 2003. Developmental Plasticity and Evo-
lution. Oxford University Press, Oxford.
Wilson E.O. 1974. The soldier of the ant Camponotus (Colo-
bopsis) fraxinicola as a trophic caste. Psyche 81:182-188.
Woods S. and R. Seeley. 2000. Adiposity signals and the control
of energy homeostasis. Nutrition 16:894-902.




University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - - mvs