Species-specific patterns in the social activities of harvester ant

l n s e c t e s Sociaux, Paris
9
M a s s o n , P a r i s , 1984
1984, V o l u m e 31, n ~ 1, p p . 74-86
SPECIES-SPECIFIC PATTERNS IN THE SOCIAL ACTIVITIES
OF HARVESTER ANT COLONIES (POGONOMYRMEX)
Deborah M. GORDON
Department of Zoology, Duke University, Durham, NC 27706, U.S.A. ~'
Re~u l e 19 juillet 1983.
Accept6 le 19 d6cembre 1983.
SUMMARY
This field study examines the social behavior of five sympatric species of desert
seed-eating ants (Pogonomyrmex barbatus, P. rugosus, P. maricopa, P. desertorum,
P. californicus). The species differed significantly in measures of activity r h y t h m s in
various colony tasks, use of space around the nest yard, and reaction to disturbance.
Species differences were related to the typical size of a colony's outside work force.
The behavior of P. rugosus, P. barbatus, and P. maricopa, ,which had larger outside
work forces, emphasized territoriality and the acquisition of f o o d ; that of P. desertorum
and P. calffornicus, which had smaller outside ,work forces, emphasized the avoidance
of contact 'with o t h e r colonies. Examining the p a t t e r n s in colony behavior can illuminate
interspecific relationships in desert ant communities.
RESUME
Diffdrences spdcifiques dans le comportement social de cinq espSces
de fourmis moissonneuses CPogonomyrmex)
Cette ~tude faite sur le terrain dans le d~sert du sud-ouest des Etats,Unis diff~rencie
le c o m p o r t e m e n t social de cinq espbces de fourmis moissonneuses : Pogonomyrmex barbatus, P. rugosus, P. maricopa, P. desertorum, P. cal#ornicus. Les esp~ces different
significativement q u a n t ~t leurs rythmes d'activit6 pour plusieurs tgches (fourragement,
maintien du nid, surveillance, maintien des d6bris, r a s s e m b l e m e n t ) , ainsi que pour
l'utilisation de l'espace autour du nid et la r6action de la soci6t6 h u n d~rangement.
En g6n6ral, les diff6rences de c o m p o r t e m e n t peuvent s'organiser en fonction de la
quantit6 habituelle d'ouvri~res hors du nid. Chez les esp~ces ayant une plus grande
quantit6 d'ouvribres ~ l'ext6rieur du nid, l'organisation de la soci6t6 m e t plus en valeur
l'acquisition de la n o u r r i t u r e et la territorialit6. Par contre, les esp~ces ayant moins
d'ouvri6res h o r s du nid ont un c o m p o r t e m e n t social qui leur p e r m e t de r6duire leur
contact avec d ' a u t r e s soci6t6s. En examinant les r6gularit6s temporelles et spatiales du
c o m p o r t e m e n t social, on peut mieux c o m p r e n d r e les r a p p o r t s interspdcifiques chez les
c o m m u n a u t 6 s de fourmis.
* Present a d d r e s s : Museum of Comparative Zoology, Harvard
M A 02138. USA.
University, Cambridge,
S P E C I E S - S P E C I F I C P A T T E R N S I N POGONOMYRMEX
75
INTRODUCTION
Many P o g o n o m y r m e x species are parts of desert c o m m u n i t i e s containing
other species of the genus. The species are all granivorous and similar in
m o r p h o l o g y and nest site requirements.
Food appears to be a limiting
resource (DAVIDSON, 1977 a ; HANSEN, 1978). Behavioral differences a m o n g the
species are t h o u g h t to be the result of interspecific c o m p e t i t i o n (WmTFORD
and ETTERSHANK, 1975; DAVIDSON, 1980). There are suggestions that, in ants,
c o m p e t i t i o n leads to selection at the colony level (WILSON, 1971; CULVER,
1974), especially selection for differences in foraging strategy (CARROLLand
JANZEN, 1973; HANZEN, 1978). This study endeavors to find species-specific
differences in the colony-level (social) behavior of five s y m p a t r i c species of
desert seed-eating ants ( P o g o m y r m e x barbatus, P. maricopa, P. desertorum,
and P. californicus).
Previous c o m p a r i s o n s of the social behavior of P o g o n o m y r m e x species
have a t t e n d e d almost exclusively to foraging behavior. DAVIDSON (1977b)
c o m p a r e d species that use p e r m a n e n t t r u n k trails with species that appear
to forage randomly, as individuals.
Other work shows that aggressive
e n c o u n t e r s and the spacing of t r u n k trails play a role in intra- and interspecific c o m p e t i t i o n (H6LLDOBLER, 1976a; HARRISON and GENTRY, 1981).
However, some species use t r u n k trails only part of the time (e. g., on P .
rugosus : CHEW, 1976 ; BERNSTEIN, 1975 ; DAVIDSON, 1977 a ; Ht~LLDOBLER, 1976 a ;
RISSING, 1981). Aggressive encounters are reported to be frequent by some
a u t h o r s (DE VITA, 1979; H/JLLDOBLER, 1976 a), rare or nonexistent by others
(CHEw, 1976 ; WHITFORD et al., 1976). F u r t h e r examination of colony behavior
is needed to characterize foraging strategy, and thereby to u n d e r s t a n d b e t t e r
the ecology of desert ant communities.
Species-specific differences in the time of day when mating occurs
maintain r e p r o d u c t i v e isolation of the sympatric P o g o n o m y r m e x species
considered here (HSLLDOBLER, 1976 b). Except for one study of P. barbatus
(GORDON, 1983), previous field w o r k on P o g o m y r m e x activity r h y t h m s has
examined only overall activity level or mating behavior. The present study
investigates species-specific temporal and spatial patterns in a variety of
activities a r o u n d the nest.
METHODS
Field observations .were made of 37 colonies in July and August, 1982, in the
Chiricahuan Desert, near Rodeo, New Mexico. At one study site, 8 colonies of P. barbatus, 5 of P. desertorum, and 13 of P. maricopa 'were observed. P. imberbiculus ,workers
were seen, very rarely, at this first study site. A second study site contained 5 study
colonies of P. californicus and 1 of P. desertorum. P. barbatus foragers occasionally
entered the site, but it contained no nests of Pogonomyrmex species other than the
two mentioned. At the third study site, 5 P. rugosus colonies 'were observed. The
nearest nests of other species, P. maricopa and P. desertorum, 'were about 30 meters
D.M.
76
GORDON
away. All species ,were active in the m o r n i n g a n d inactive d u r i n g the a f t e r n o o n . S o m e
species r e e m e r g e in the late a f t e r n o o n . All d a t a 'were collected d u r i n g m o r n i n g activity
periods.
Table I. - - Classification of colony activities.
T a b l e a u I. - - Classification des activit6s des ouvri~res h o r s du nid.
Foraging
Nest m a i n t e n a n c e
A. A n t s travel directly a'way f r o m the n e s t e n t r a n c e , n o t c a r r y i n g
a n y t h i n g , o n f o r a g i n g trail if there is one.
B.
Ants travel directly to the n e s t e n t r a n c e c a r r y i n g a seed o r
insect bit, on f o r a g i n g trail if there is one.
A.
C a r r y i n g o u t : A n t s c o m e o u t of the n e s t e n t r a n c e c a r r y i n g
s o m e t h i n g , p u t it d o w n in the n e s t yard, a n d go b a c k into
the nest.
B.
Clearing v e g e t a t i o n : Ants climb in vegetation at edge of n e s t
yard, clip pieces of it off 'with m a n d i b l e s .
C. A n t s o p e n n e s t e n t r a n c e at the b e g i n n i n g of the activity period
by c a r r y i n g o u t soil.
D. A n t s close n e s t e n t r a n c e at the e n d of the activity period by
filling it ,with soil.
Patrolling
A. Ant ,walks w i t h f r e q u e n t s t o p s a n d c h a n g e s in direction (comp a r e d to foragers). A b d o m e n is often b e n t u n d e r n e a t h t h e
thorax. Objects a n d o t h e r a n t s e n c o u n t e r e d are f r e q u e n t l y
inspected 'with a n t e n m e .
B.
Midden 'work
Convening
At the site of a d i s t u r b a n c e , s u c h as a n e w object in t h e n e s t
y a r d n o t b r o u g h t in by ants, a n t s g a t h e r a n d s t a n d ~with m a n dibles open.
A. Ants s t a n d on the m i d d e n , repiling it or
antennae.
B. Ants m o v e o b j e c t s f r o m one m i d d e n to
n e s t yard.
C. Ants c o m e into the n e s t yard, n o t along
b r i n g i n g s m a l l pebbles, a n d p u t t h e m d o w n
Ants mill a r o u n d in n e s t e n t r a n c e .
bet'ween ,workers.
i n s p e c t i n g it ,with
a n o t h e r m i d d e n in
the foraging trail,
on the n e s t m o u n d .
F r e q u e n t antennae c o n t a c t s
In this s t u d y , colony b e h a v i o r o u t s i d e the n e s t is divided into five c a t e g o r i e s :
foraging, n e s t m a i n t e n a n c e , patrolling, m i d d e n 'work, a n d convening, defined in table I.
I n the sequel, f o r a g i n g 'will be u s e d only in the s e n s e given in table I.
Activity rhythms
O b s e r v a t i o n s w e r e m a d e d u r i n g the m o r n i n g activity period to d e t e r m i n e ~vhether
there exist species-specific r h y t h m s in the five activities. The period ~vas divided into
6 o n e - h o u r slots, b e g i n n i n g at 6:00 a.m. E a c h of the colonies ,was o b s e r v e d 6 - 20 times,
at least once in e a c h t i m e slot, w i t h the exception of the P. rugosus colonies, ,which
w e r e n o t o b s e r v e d d u r i n g the last t i m e slot. At least one h o u r elapsed bet~veen
successive o b s e r v a t i o n s of t h e s a m e colony. An o b s e r v a t i o n w a s m a d e by r e c o r d i n g the
n u m b e r of a n t s in each of the five previously defined (table I) categories of behavior.
SPECIES-SPECIFIC P A TTE R NS IN P O G O N O M Y R M E X
77
Only ants within 1.3 m of the nest e n t r a n c e were counted. The total n u m b e r of ants
outside the nest b u t w i t h i n 1.3 m of it, or observation s u m (OS), is the s u m of the
five n u m b e r s (one for each activity). 654 observations ,were m a d e in 27 days.
The data were analysed using a multivariate analysis of variance (TIMM, 1975). For
each colony, the largest OS in any o b s e r v a t i o n 'was determined. Each observation,
consisting of c o u n t s of ants in each of the five activities, was c o n v e r t e d to 5 fractions
of the colony's largest OS, to normalize for differences in colony size. Normalized
n u m b e r s ,were t h e n s u b m i t t e d to an arcsin t r a n s f o r m a t i o n (SoKAL a n d ROHLF, 1981). For
each colony a n d time slot, the means of the 5 n u m b e r s 'were used as a 5-component
vector of responses. The factors considered ,were species and time slot as m a i n effects,
colony nested 'within species, a n d species • time-slot interaction, all fixed effects. For
each activity, multiple c o m p a r i s o n s of species m e a n s ~vere made using the B o n f e r r o n i
test (TIMM, 1975). The test for a species effect employed the colony-within-species effect
as its e r r o r term.
Reactions t o disturbance
To test ,whether species react differently to the presence of a ne",v object in the
nest yard, a small t'wig a b o u t 5 cm was placed on the m o u n d n e a t the nest e n t r a n c e
while the colony 'was active, away f r o m the trurtk trail if one existed. Smaller species
were given n a r r o w e r , lighter twigs. A total of 80 tests were p e r f o r m e d on 30 colonies,
including some colonies from each species. Each colony ,was tested at several different
times in the m o r n i n g activity period. The possible effect of colony activities at the
time of the test on colony reaction to the twig 'was not considered in the analysis
presented here.
After 6 minutes, colony reaction was scored a s : 1) twig :vas moved some~vhere
e l s e ; 2) twig ,was inspected b u t not m o v e d ; or 3) twig 'was inspected a n d the n u m b e r s
of ants emerging f r o m the nest decreased sharply. A Fisher exact probability test
(SPIEGEL, 1956) 'was used to d e t e r m i n e w h i c h species pairs reacted similarly to the twig.
Use of space around the nest yard
To c o m p a r e uses of space by the five species, a s h o r t (2-4 m i n u t e ) 16 m m film 'was
m a d e s h a w i n g activities in the nest yards of colonies of each species, d u r i n g t h e i r peaks
of foraging activity. Later, w i t h the aid of a stop-motion analysis projector, the p a t h s
of some ants 'were traced and, if possible, their activities identified.
RESULTS
Activity rhythms
Figure 1 s h o w s t h e t e m p o r a l p a t t e r n s o f t h e f i v e a c t i v i t i e s f o r e a c h
species.
Tables I I and I I I s h o w t h e r e s u l t s o f t h e m a n o v a .
I n table II,
significant values for time slot show that the normalized numbers in each
activity depend on the time.
T h a t is, t h e p e a k s s h o w n i n figure 1 a r e n o t
results of random
fluctuation.
Within a species, colonies show different
normalized numbers devoted to each task.
Only in the case of foraging do
colonies of a given species fail to differ significantly.
The normalized
numbers
of ants engaged in midden 'work, convening,
and foraging are significantly species-dependent.
Species differences in the
values for nest maintenance
a r e n o t s i g n i f i c a n t , b u t c l o s e t o t h e 0.05 l e v e l .
All f i v e s p e c i e s a p p e a r t o d e v o t e s i m i l a r n o r m a l i z e d
numbers
of ants to
patrolling.
The significant species x time-slot interaction shows that species
D. M. G O R D O N
78
9 FORAGING
O PATROLLING
9 CONVENING
9 MIDDENWORK
A NESTMAINTENANCE
50
R barbaras
40
20
lo
R morJcopa
3O
20
10
0
io
e ,,,g. . . .
~ 3o
Fi,j.
0
~ 2o
G
m
t? desertorum
3O
I. - - Activity r h y t h m s in m o r n i n g
activity period.
For each species, t h e m e a n n o r m a lized n u m b e r of the colony's o u t s i d e
'work force in e a c h activity is sho'wn
t h r o u g h o u t the m o r n i n g activity
period.
N o r m a l i z e d n u m b e r s are
s h a w n as p e r c e n t a g e s . E a c h line
shc'ws t h e t e m p o r a l p a t t e r n in one
of the five activities. T h e a b s c i s s a
is h o u r s e l a p s i n g since 6:00 a.m.
E r r o r b a r s s h o w the s t a n d a r d e r r o r
of the m e a n .
20
R ca/,~ormcus
40
30
20
10
TIME SLOT
Fig. 1. - - R y t h m e s d'activit6 p e n d a n t la
pdriode d'activit6 m a t i n a l e .
P o u r c h a q u e esp~ce, la figure m o n tre le n o m b r e n o r m a l i s 6 m o y e n d'ouvri~res h o r s d u nid p o u r c h a q u e
activit6 p e n d a n t la pOriode d'activi~6
m a t i n a l e . Les n o m b r e s normalisOs
s o n t donnOs en p o u r c e n t a g e s . Chaq u e ligne m o n t r e l'Ovolution temporelle d ' u n e des 5 activitOs. L'abscisse
reprOsente les h e u r e s d e p u i s 6 h du
m a t i n . L ' e r r e u r s t a n d a r d est indiquOe de p a r t et d ' a u t r e de c h a q u e
moyenne,
SPECIES-SPECIFIC PATTERNS IN POGONOMYRMEX
79
d i f f e r e n c e s in n o r m a l i z e d n u m b e r s d e v o t e d to e a c h a c t i v i t y d e p e n d o n t i m e ,
as s h o w n in Iigure 2. O n e m a y c o n c l u d e f r o m tables I I and I I I t h a t , a l t h o u g h
c o l o n i e s of a g i v e n s p e c i e s v a r y s i g n i f i c a n t l y , t h e r e a r e also s p e c i e s - s p e c i f i c
activity r h y t h m s that are significantly different f r o m one another.
Table II. - - P-values for multivariate analysis of variance.
Tableau II. - - Seuils de probabilit6 obtenus par l'analyse de la variance h plusieurs
variables.
Effect
Activity
Manova
Foraging
Nest
maintenance
Patrolling
Midden
,,.vor.k
Convening
0.0420
0.0662
0.8303
0.0028
0.0042
0.01
0.1680
0.0001
0.0063
0.0029
0.0119
0.6001
0.0001
0.0061
0.0095
0.0001
0.0001
0.0001
0.0001
0.0003
0.0001
0.0001
0.0002
0.0001
Species
Colony
within
species
Time slot
Time slot
•
species
Table III. - - Results of multivariate analysis of variance.
Type III sums of squares (SS) and F values (F) for all effects, by activity.
Tableau III. - - Rdsultats de l'analyse de la variance h plusieurs variables.
Degr6s de libert6 (DF), sommes de carr6s (SS) et coefficient F (F), pour tous les
effets, par activit6.
Activity
Foraging
Effect
Model
Error
Species
Colony
~.vithin
species
Time slot
Time slot
x
species
Nest
maintenance
Patrolling
Midden
~vork
Convening
(DF)
(60)
(142)
(4)
SS
F
SS
F
32695.79 5.39 9949.61 2.61
14352.07
8825.46
144..8.99 3.58 1149.46 4.62
SS
F
8673.73 3.16
6590.56
119.50 0.65
SS
F
6222.27 7.26
2027.60
1158.31 20.28
SS
F
4603.84 3.87
2812.49
680.13 8.58
(32)
(5)
4131.09 1.28 3752.29 1.89
16037.49 31.74 1188.89 3.83
2605.18 1.78
2327.15 10.18
1824.03 3.99
1117.09 15.65
1153.05 1.82
725.07 7.32
(19)
8894.45 4.63 3258.42 2.76
3190.18 3.67
1015.65 3.74
1091.38 2.90
I n s o m e w a y s , t h e five s p e c i e s b e h a v e s i m i l a r l y . T h e p e a k s in m i d d e n
w o r k a n d c o n v e n i n g u s u a l l y o c c u r at t h e b e g i n n i n g of t h e a c t i v i t y p e r i o d ;
p e a k s o f n e s t m a i n t e n a n c e o c c u r e i t h e r at t h e b e g i n n i n g o r a t t h e e n d o f
t h e p e r i o d ; a n d f o r a g i n g p e a k s in t h e m i d d l e (fig. 2). I n g e n e r a l , a c o l o n y
precedes and follows excursions beyond the nest yard (foraging) with
D. M. GORDON
80
50 --
FORAGING
R
40
30
20
10
30
PATROLLING
It
20
j~o[;/
~
20
N,,,
8
AINrENANC
~t
C
,
20I
R = r u g o s u s , M = m a r i c o p a , C = californicus, D = desertorum).
The abscis-
~
MIDDEN W O R K
CONVENING
O
Fig. 2. - - Species c o m p a r i s o n s of t e m p o r a l
p a t t e r n s in each activity.
For each activity, the species mean normalized n u m b e r is sho'wn t h r o u g h o u t
the m o r n i n g activity period. Normalized n u m b e r s are s h o w n as percentages. Each line r e p r e s e n t s the activity
r h y t h m of one species. The peak for
each species is labelled ( B = b a r b a t u s ,
R
sa is hours elapsing since 6:00 a.m.
Fig. 2. - - Comparaisons des esp~ces pour la
rdpartition temporelle de chaque activit6.
Chaque ligne reprdsente les n o m b r e s
normalisds moyens d'ouvri~res obtenus
et d6s lors le r y t h m e d'activitd observ~ - - p o u r une esp~ce et une activitY,
au cours de la p6riode d'activit6 matinale. Les n o m b r e s normalis6s sont
donn6s en pourcentages. Le m a x i m u m
d'activit6 est libell6 p o u r chaque esp6ce
(B = barbatus, R = rugosus, M = maricopa, C = californicus, D = desertor u m ) . L'abscisse repr6sente les heures
TIME SLOT
depuis 6 h du matin.
SPECIES-SPECIFIC PATTERNS IN POGONOMYRMEX
81
activities in or near the nest yard (nest maintenance, midden work, patrolling,
convening).
Each species devotes the largest number of the workers outside
t h e n e s t t o f o r a g i n g , a n d t h e l o w e s t n u m b e r to m i d d e n w o r k a n d c o n v e n i n g .
The results of the Bonferroni comparisons of species means are shown
i n table IV. Table V s h o w s t h e m e a n a n d m a x i m u m O S f o r e a c h s p e c i e s .
P. barbatus, P. rugosus, a n d P. maricopa h a v e t h e l a r g e s t n u m b e r s o f a n t s
o u t s i d e t h e n e s t (table V).
These three species devote especially large
n u m b e r s o f w o r k e r s t o f o r a g i n g (table IV).
Table IV. - - Comparison of species mean proportions.in each activity.
For each activity, species are listed in decreasing order, top to bottom, of the mean
proportion of the colony's outside 'work force in that activity. Species with the
same letter were not significantly different (Bonferroni test, p > 0.05).
Tableau IV. - - Comparaisons entre esp~ces des proportions moyennes de chaque activit6.
Pour chaque activit6, les esp6ces sont rang6es en ordre d6croissant, de haut en
bas, de la proportion movenne d'ouvri6res hors du nid qui font cette activit6. Les
esp6ces suivies de la m6me lettre ne sont pas significativement diff6rentes (test
de Bonferroni, p > 0.05)
Foraging
Maricopa
Barbatus
Rugosus
Desertorum
Californicus
Nest
maintenance
A
A B
A B
A B
B
Maricopa
Rugosus
Barbatus
Californicus
Desertorum
A
A B
A B
A B
B
Activity
Patrolling
Rugosus
Maricopa
Barbatus
Desertorum
Californicus
Midden
work
A
A
A
A
A
Convening
Maricopa A
Barbatus A B
Rugosus A B C
Desertorum B C
Californicus
C
Barbatus A
Maricopa
Rugosus
Desertorum
Californicus
B
B
B
B
Table V. - - Comparison of species activity levels.
The mean and maximum numbers of ants observed outside the nest within 1.3 m
of the nest entrance are shown for each species.
Tableau V. - - Comparaison des niveaux d'activit~ par esp~ce.
Le tableau montre pour chaque esp6ce le nombre moyen et le maximum de fourmis
hors du nid h moins de 1,3 m de l'entr6e du nid.
P.
P.
P.
P.
P.
Mean
78.22
48.19
41.14
17.80
11.38
barbatus
rugosus
maricopa
desertorum
californicus
Maximum
364
162
118
96
80
Rc~actions to disturbance
Table V I s h o w s t h a t P. barbatus a n d P. rugosus u s u a l l y r e a c t e d a c t i v e l y
to t h e t w i g a n d m o v e d it a w a y . P. maricopa a n d P. californicus w e r e l i k e l y
e i t h e r t o m o v e t h e t w i g o r i g n o r e it. T h e r e a c t i o n
o f P. d e s e r t o r u m w a s
different : colonies would ignore the twig or stop emerging from the nest.
Use of space around the nest yard
Figure 3 s h o w s r e p r e s e n t a t i v e
yard.
The implications
patterns in use of space around
of these results are discussed below.
the nest
D. M. GORDON
82
IA.j
,)
,
~21 ~
~
:.:,~ ..-"~'ll' "~l
I
I(l......~'..:'~..~/
,...'l ,','./k'~1~
._
)t f
"t
--,,'
I >,ii'<\
,?
/i/ll '~-~"
',,
Rrugosus(r
:
' II,~'
--
~"L...s,f~t/lll l lt'i
II I ) 17t \X~,',~:
\
))1 ftll I ".",, _.
''a"a' stt /I
f"
f
"'I
t
~'~,R c a l l f o r n l c u s
,/
\
r
maricopa
"
r,,~
I
P. d e s e r t o r u m
SPECIES-SPECIFIC
PATTERNS
IN POGONOMYRMEX
83
Table VI. - - Comparison by species of r e a c t i o n to twig placed in nest yard.
The table shows the outcome of all tests. Species 'with the same letter ~vere not
significantly different (p > 0.05).
" The difference bet*,veen P. rugosus and P. maricopa ~vas almost significant
(p > 0.0584).
Tableau VI. - - Comparaison par esp~ee des r6actions ~t la brindille plac6e pros de
l'entr6e du nid.
Le tableau m o n t r e les r6sultats de toutes les exp6riences. Les esp6ces suivies de
la m~me lettre ne sont pas significativement diff6rentes (test exact de Fisher,
p > 0.05).
* La difference entre P. rugosus et P. maricopa est presque significative
(p > 0.0584).
Colony reaction
Moved
twig
P.
P.
P.
P.
P.
barbatus
rugosus
maricopa
desertorum
californicus
Inspected
then ignored
t~,vig
Inspected twig,
then stopped
coming
out of nest
Results
of Fisher's
exact test
16
6
0
A
8
2
0
A
10
12
1
A*
3
4
7
4
4
0
B
B
DISCUSSION
W h e n all of t h e r e s u l t s a r e c o n s i d e r e d , e a c h s p e c i e s h a s u n i q u e b e h a v i o r
patterns.
However, the five species can be divided into two groups according
t o t y p i c a l s i z e o f t h e o u t s i d e w o r k f o r c e . P. b a r b a t u s , P. r u g o s u s ; a n d P.
m a r i c o p a h a v e a l a r g e r o u t s i d e w o r k f o r c e ; P. d e s e r t o r u m a n d P. c a t i f o r n i c u s
Fig. 3. - - Use of space in the nest yard.
Each figure shows the use of space by vcorkers in and near the nest yard. Figures
'were derived by tracing the p a t h s of ants visible in a four-minute film of the
nest yard. Not all paths are shown. Open circles represent nest e n t r a n c e s ; the
shaded area a Swiss Army knife, 8.4 cm long, for scale, or, in the P. barbatus
figure, a plastic ruler 15 cm long. Paths of foragers are represented by thin solid
lines, those of nest maintenance 'workers by dotted lines, those of patrollers by
dashed lines, and those of m i d d e n 'workers by thick solid lines.
Fig. 3. - - L'emploi de l'espace autour du nid.
Chaque dessin m o n t r e l'emploi de l'espace par les ouvri~res autour du nid. Ils
furent o b t e n u s en m a r q u a n t les chemins des fourmis visibles sur un film, de
4 min., des alentours du hid. Tous les chemins existants ne sont pas repr6sent4s.
Des cercles ouverts repr6sentent les entr6es des n i d s ; l'objet obscur repr4sente
un canif, 8,4 cm de longueur, pour m o n t r e r l'4chelle (remplac6 darts le dessin de
P. barbatus par une r~gle plastique de 15 cm de long). Les lignes fines et continues
repr6sentent les routes des f o u r r a g e u s e s ; les lignes pointill6es, celles des fourmis
qui assurent le maintien du h i d ; les lignes discontinues, celles des fourmis qui
surveillent; et les lignes 6paisses et continues, celles des ouvrihres s'occupant des
d6bris.
84
D . M . GORDON
have a s m a l l e r one (table V). Several aspects of the b e h a v i o r of P. barbatus,
P. rugosus a n d P. maricopa can be i n t e r p r e t e d as specializations for m o r e
efficient foraging. The three species devote especially large n u m b e r s of
w o r k e r s to foraging (table IV). In P. maricopa and P. barbatus, the p e a k
in p a t r o l l i n g o c c u r s b e f o r e the p e a k in foraging. F o r P. barbatus, this
sequence 'was m o r e p r o n o u n c e d the previous y e a r (GORDON, 1983). Since it is
patrollers, not foragers, t h a t r e c r u i t o t h e r ants to food, at least in P. barbatus
(GORDON, 1983), the activity sequence m a y allcyw m o r e f o r a g e r s to be r e c r u i t e d
to n e w f o o d sources. The use of t r u n k trails, especially consistent in P.
barbatus, m a y help the colony to h a r v e s t rich food sources m o r e quickly
(DAvIDsoN, 1977 b). Figure 3 shows t h a t even 'when P. maricopa does not use
t r u n k trails, foragers effectively cover the a r e a a r o u n d the nest.
The b e h a v i o r p a t t e r n s of these three species s e e m to e m p h a s i z e territoriality. I t has been suggested that Pogonomyrmex species defend their nest
y a r d s m o r e actively t h a n they do the. rest of the foraging ranges (HARRISON
and GENTRY, 1981; HOLL~)OBLER,1976h). Patrollers of b o t h P. maricopa and
P. barbatus h a d nearly covered the p e r i m e t e r s of the nest y a r d s during the
few m i n u t e s of filming (fig. 3). Both P. rugosus and P. barbatus r e a c t e d
actively to the p l a c e m e n t of twigs in the nest yards, as did P. maricopa to
a lesser extent.
The b e h a v i o r p a t t e r n s of P. rugos~,s are unusual. Peaks in patrolling,
foraging a n d nest m a i n t e n a n c e all o c c u r s i m u l t a n e o u s l y (fig. 1). The activities
of m i d d e n w o r k e r s and nest m a i n t e n a n c e w o r k e r s c a r r y i n g sand out of the
nest are not spatially segregated in the nest yards, as they a r e in o t h e r species
(fig. 3). P. rugosus is k n o w n to v a r y greatly in the times it is active a n d
in the types o f forage it takes, depending on seasonal v a r i a t i o n s in food
a b u n d a n c e (WHITFORD, 1978). Such flexibility m a y r e q u i r e a specialized
patrolling system. Foragers in P. rugosvs m a k e m o r e f r e q u e n t t u r n s t h a n
do those of o t h e r species (fig. 3), a p p a r e n t l y covering m o r e ground. Even
when using t r u n k trails, P. rugosus s e e m s r e a d y to switch to an individual
foraging strategy. Also, the nest y a r d s are p a t r o l l e d m o r e exhaustively t h a n
those of the o t h e r species (fig. 3). Thus P. rugosus s e e m s to use an all or
none a p p r o a c h to locate and h a r v e s t food, and to m a i n t a i n the t e r r i t o r i a l
integrity cf the nest yard.
In c o n t r a s t to the three species discussed above, the b e h a v i o r p a t t e r n s
of P. descrtorum and P. catifornicus seem to be specialized for avoiding
contact 'with n o n - m e m b e r s of the colony, r a t h e r t h a n f o r the acquisition of
food a n d territoriality. The'~ have the smallest n u m b e r s of foraging w o r k e r s
(table 1V). The p e r i m e t e r s of their nest yards are not c o v e r e d as f r e q u e n t l y
by p a t r o l l e r s as in the o t h e r three species (fig. 3). T h e i r p e a k s of nest
m a i n t e n a n c e c o m e at the ends of the activity periods, in a sequence t h a t m a y
help the colony to avoid e n c o u n t e r s w i t h o t h e r ant species, already foraging
w h e n P. calilornicus and P. desertorum are j u s t beginning to be active.
SPECIES-SPECIFIC PATTERNS IN POGONOMYRMEX
85
I n P. desertorum, a t w i g p l a c e d o n t h e n e s t m o u n d w a s s o m e t i m e s e n o u g h
to c a u s e a d e c r e a s e in t h e n u m b e r s of w o r k e r s e m e r g i n g f r o m t h e n e s t
(table VI). DAVIDSON's (1980) r e p o r t , t h a t f o o d i t e m s t a k e n b y P. desertorum
v a r i e d so as to a v o i d t h e s e e d s p r e f e r r e d b y P. rugosus, s u p p o r t s t h e
c h a r a c t e r i z a t i o n o f P. desertorum as a t i m i d , b e h a v i o r a l l y s u b o r d i n a t e s p e c i e s .
T h e e n t i r e a c t i v i t y p e r i o d o f P. caIifornicus is l a t e r t h a n t h a t o f o t h e r
s p e c i e s (fig. 1), w h i c h t e n d s to d i m i n i s h i t s c o n t a c t w i t h o t h e r s p e c i e s ,
U n l i k e t h e o t h e r s p e c i e s , it k e e p s i t s m i d d e n i n s i d e t h e n e s t (DE VITA, 1979 ;
RISSINC, 1981), r e d u c i n g t h e n u m b e r o f w o r k e r s n e e d e d o u t s i d e t h e n e s t .
(However, a few midden workers were seen piling seed husks outside the nests).
F u r t h e r m o r e , i t s c o l o n i e s r e l o c a t e f r e q u e n t l y , a p p a r e n t l y to a v o i d b e i n g t o o
c l o s e to o t h e r c o l o n i e s (DE VITA, 1979).
In general, foraging strategy and territorial behavior are elements of
the c o l o n y b e h a v i o r p a t t e r n s d e s c r i b e d h e r e . T h e n u m b e r s a c o l o n y d e v o t e s
to f o r a g i n g , t h e t i m e w h e n w o r k e r s f o r a g e , t h e s e q u e n c e in w h i c h p a t r o l l i n g
a n d f o r a g i n g o c c u r , t h e s p a t i a l a r r a n g e m e n t o f f o r a g e r s - a l l a r e a s p e c t s of
f o r a g i n g s t r a t e g y . T e r r i t o r i a l b e h a v i o r i n c l u d e s t h e t i m i n g a n d l o c a t i o n of
e x p o s u r e to c o n t a c t s w i t h n o n - m e m b e r s o f t h e c o l o n y , t h e t i m i n g o f e x c u r s i o n s
into the home range of other colonies, and the extent to which the nest yards
are patrolled and defended.
To investigate how contact between species
is m e d i a t e d , a n d h o w f o o d r e s o u r c e s a r e p a r t i t i o n e d , it is u s e f u l f i r s t to
u n d e r s t a n d t h e p a t t e r n s of c o l o n y b e h a v i o r .
If we understood completely the spatial and temporal regularities in the
behavior of Pogonomyrmex
colonies, we
could predict
interspecific
r e l a t i o n s h i p s ~ S u c h a full u n d e r s t a n d i n g h a s n o t y e t b e e n a c h i e v e d . F o r
example, the variation with habitat of species-specific behavior will need to
be investigated.
Characterizing species by colony behavior patterns can
i l l u m i n a t e i n t e r s p e c i f i c r e l a t i o n s h i p s in d e s e r t a n t c o m m u n i t i e s .
ACKNOWLEDGMENTS.- - The study was done at the Southwestern Research Station in Portal,
Arizona, and was supported by a grant from the Theodore Roosevelt Memorial Fund. I
thank Mr and Mrs Finn RICHARDSfor allowing me to use their land as a study site. K. ROT~I
and B. BAILEYprovided invaluable assistance in the field? and discussions ,with G. ALPERT,
R. CHEW, B. HOLLDOBLERand D. D~WIBSON'were very he!pful. The Insect Identification
Institute of the U.S.D.A., Beltsville, MD, confirmed the identification of the ants. I am
grateful to R. SHAW and C Bg_~Xt)T for statistical advice; to J. GRECC, P. KLOPFER,
P. JAIssos and an anonymous reviewer for their comments on the manuscript ; and to
J. GORI)ON, R. PALMER and J. GREGG for their help throughout the project.
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86
D.M.
GORDON
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