Joan M. Herhers.
Social organisation in Leptothorax ants: within- and between- species patterns.
Psyche 90:361-386, 1983.

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SOCIAL ORGANIZATION IN LEPTOTHORAX ANTS: WITHIN- AND BETWEEN-SPECIES PATTERNS*
BY JOAN M. HERBERS
Department of Zoology, University of Vermont Burlington, Vermont 05405
Recent application of quantitative techniques to behavior (cf. Colgan 1978) has resulted in new approaches to undertanding social interactions among animals. A technique particularly widely-used for study of ant colonies is development of the colony ethogram, or behavioral profile. We now have ethogram information for a wide variety of species. Most reports in the literature focus on a single colony (Table 1); variation within a species is rarely discussed. In addition, the colony time budget, an important second class of information, is generally not reported (Table 1). The appropriate- ness of behavioral comparisons across species is thereby severely limited by availability of only one type of behavior frequency catalog, for only one colony per species. Caste complexity and division of labor related to morphological or age variation comprise another type of information contributing to an understanding of social organization. As a rule, queens have smaller repertoires than do workers; majors have different etho- grams than minors; and older workers display different behavior frequencies than do younger workers. Studies of morphology affect- ing behavior have concentrated on polymorphic species for which descrete worker castes can be distinquished; recent work has shown that, even for monomorphic species, worker size can bias behavior (Wilson 1978, Berbers and Cunningham 1983). A reasonably complete description of social organization for an ant species should treat ethograms, time budgets, and behavioral caste specialization, both within and between different colonies. Here I report such details for three colonies of Leptothorax ambi- guus. This information is then compared to data from the closely- related L. longispinosus to arrive at an understanding of between- and within- species variation in social behavior. Manuscript received by the editor July 22, 1983 36 1




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Table 1. Ethograms for many ant species have been published, but variation between colonies is rarely reported. Species
At~ibl~wpone pa1lipe.s
A I ta .se.~den.s
Campanom (Co1obop.si.s) s p .
Cepha1ote.s airatus
Formica perpilosa
Leptoihorax curvispinosiix
Oreclognothus versirolor
Pheidole dentata
Solenopsis gen~inata
Solenopsis invicta
Zarrvptocerus varians
Zarrj pmcerus varians
# of Colonies
Observed
# of Colony
Ethograms Reported
1
I
I
I
I
I
(pooled data)
4
1
(pooled data)
1
I
1
I
I
1
Time Budget
Reported'?
no
no
no
yes
no
no
no
yes
(pooled data)
no
no
no
no
no
no
Reference
Traniello 1982
Wilson 1980
Cole 1980
Corn 1980
Brandao 1978
Wilson & Fagen
1974
Herbers 1982
Herbers &
Cunningham 1983
Carlin 1982
Wilson 1976a
Wilson 1978
Wilson 1978
Wilson 1976b
Cole 1980




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Herbers - Social Organization in Leptothorax Colonies of L. ambiguus were collected in May 1982 from the E. N. Huyck Preserve (Albany County, NY). These colonies were settled in artificial nest boxes and maintained according to standard methods (Herbers and Cunningham 1983); in addition, frozen fruit- flies were provided as a food source.
For detailed observations, three colonies were chosen on the basis of queen and worker number to match earlier studies of L. longispi- nosus (Herbers 1982). All Leptothorax colonies studied were of approximately equal worker number, all had eggs and larvae, and all produced alates by summers' end; only queen number varied significantly (Table 2).
Behavioral observations were conducted June 9-August 24, 1982 through a Wild M5-A stereomicroscope as follows: a worker was chosen at random and all her actions were recorded over a
30-minute period. In addition, activities of individuals around her in the field of view were recorded. Head widths of the randomly-chosen ants were measured at a standard depth of field, by use of an ocular micrometer.
Data analysis followed methods outlined by Fagen and Goldman ( 1977) for behavior catalogs; Herbers and Cunningham ( 1983) for statistical evidence of polyethism and morphological bias; and Cole ( 1980) for producing dendrograms.
Table 2. Colony shes of Leptothorax used in this comparative study. Data on L. hgixpinosus were reported by Herbers ( 1982). Original
# of
Queens
3
I
0
1
1
5
4
Original
# of
Workers
27
28
20
30
3 1
28
36
Eggs
Laid'?
yes
yes
yes
yes
yes
yes
yes
Larvae Alates
Present'? Reared'?
yes yes
yes yes
yes yes
yes yes
yes yes
yes yes
yes yes




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Psyche
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Social Organization of L. ambiguus colonies A total of 60 hours were recorded over the three colonies for a grand total of 3145 observations. Ethograms for the three colonies of L. ambiguus are reported in Table 3. A total of 46 behaviors were recorded for workers and 13 for queens. Despite the large catalog size, no behavior was unique to L. ambiguus; all in Table 3 are relatively common to many species included in Table 1. As expected, queens were much less active than workers (Table 3). Their behavior was almost exclusively directed towards the brood; the exceptional occasion for colony La-B occurred when a queen was observed walking outside the nest and taking a drink; she later returned inside. Because of the paucity of data on queen behav- ior, analyses below concern only worker behavior. Frequencies of observations for behaviors over three colonies are given in Figure 1. Sample coverages were uniformly greater than 99% (Figure 1). Consequently inferences about the true colony repertoire can safely be made. There was considerable variation among colonies (Figure I), yet the distributions were not signifi- cantly different from each other (x2 = 13,80, 14 df, P > .05). Thus distributions of observations over all behavior categories were roughly equivalent.
Comparisons of absolute frequencies among colonies showed that many worker behaviors were observed in all colonies (Table 3). For some, ethogram frequencies were nearly equal (IL, CP, ATW, RW) whereas for others the correspondences were not good (RE, IE, ALW). A third class of behaviors included those observed for only one or two nests (CE, FLD, AAE, CR). Thus considerable inter- nest variation existed. For a given colony, some behaviors known to occur in the species were missing from the ethogram, some were common or rare relative to other colonies, and some were equally frequent to others. Despite apparent discrepancies in absolute fre- quencies, the rankings of behaviors by frequency were similar over the three colonies (KendalPs coefficient of concordance; W = .864, 27 df, P < .001). That is, behaviors commonly observed in one col- ony were also common in others whereas those rare in one tended to be rare in all. In particular, behaviors missing from one colony's ethogram were generally rare in others. Therefore, although abso- lute frequency varied from nest to nest, relative frequencies were similar.




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831 Herbers - Social Organization in Leptothorax 365 NO. OF ACTS
(OCTAVE)
Figure I. Abundance histograms for three colonies of L. amhiguu'i and for pooled data. The abscissa gives the number of observations per behavior. to the base 2: thus octave 0 indicates behaviors observed once. octave I refers to exactly 2 observations. octave 2: 3 or 4 observations, octave 3: 5 through 8. and so on. The largest octave. 10 refers to behaviors observed 513-1024 times. The value of 6 given for each data set in- dicates the sample coverage, as described by Fagen and Goldman (1977). TOTAL
983986




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Psyche
[Vol. 85
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368 Psyche
Table 4. Time budgets for L. a ~ u u s workers. Behavior
RE
SG
MO
I E
GE
CE
1 L
GL
CL
RL
FLD
ALE
I P
GP
CP
AAE
ATW
ATB
RW
ALW
BG
ATQ
ALQ
RQ
ATM
ALM
CM
RM
ALF
BC
LN
LOT
I Ex
CEx
EEx
HM
CR
FF
MOT
I Dr
Proportion of Time (p)
[Vol. 85
Total




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19831 Herbers - Social Organization in Leptothorax Table 4. Time budgets for L. amhiguus workers. (Continued) Behavior Proportion of Time (p)
1.a-A l.ii-B 1.a-C Total
CDr - 0042
-
00 139
EDr 029 1 - - 01021
Fd N - 0065 ,0004 00229
FdO 0005 -
-
000 16
I Fd 0005
- -
000 1 7
DR 0008 - 00027
-
Time budgets for workers of the three colonies are given in Table 4. The largest elements in Table 4 correspond to resting; overall, workers spent 68% of the time motionless. It is interesting to note that the most sedentary colony was La-C, which had no queen; perhaps the high rate of inactivity was related to a lower rate of egg production or overall lack of queen stimulation. Even so, resting was predominant for all nests. The second dominant behavior was moving inside the nest, on average accounting for 13.8% of the worker time budget. In addition, self grooming was a large contribu- tor in all colonies observed, consuming on average 5% of worker's time. Thus personal behavior accounted for the vast majority of the time budget; activities which can be called "social" consumed less than 15% of the workers' time.
Among social behaviors, time budget variation among colonies was minimal for some types (GP, ATW, ATB). Proportions of time spent in other behaviors were quite different among colonies; the most extreme case was LOT, which varied by two orders of magni- tude (pi = .0100 for La-B and .0001 for La-C). Kendall's test for concordance showed that, despite differences in absolute proportion of time, rankings of behaviors by relative proportions were similar over all colonies (W = 0.763, 13 df, P < .005). That is, behaviors consuming a large portion of the time budget in one colony tended to be important for other colonies, and behaviors rare in one colony were usually rare in all. Despite quantitative differences in specific types of activity, overall qualitative agreement in time budgets was strong.




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370 Psyche [Val. 85
To sum, no significant differences in relative importance of behaviors were observed over the three colonies. Histograms of behavior frequency (Fig. I), rankings of ethogram frequency (Table 3) and relative time budget frequencies (Table 4) were not signifi- cantly different. Therefore, in subsequent analyses of within-species social organization below, data were pooled over three colonies. I will return to consideration of between-colony variation below. Division of labor among workers was investigated by considering the matrix of transition frequencies among behaviors listed in the ethogram. Within this 46 x 46 matrix, elements indicate how often each behavior followed and preceded every other behavior. To sim- plify presentation of the results, transitions among behaviors listed in the ethogram are synopsized in Table 5. Diagonal elements represent the frequencies by which behaviors in the same categories followed each other whereas off-diagonal elements represent transi- tion frequencies between behaviors in different groups (Herbers and Cunningham 1983). Division of labor is implicated if nonzero tran- sition frequencies are clustered in diagonal blocks of the matrix and zeroes occur in off-diagonal blocks.
Examination of Table 5 shows that transitions from (column 1) and into (row 1) Personal Behavior commonly occurred. This is no surprise, since virtually all workers displayed a form of personal behavior. However, among social behaviors, the overall distribution of nonzero transitions deviated strongly from random expectation (G = 55.24, 16 df, P<.001), in a pattern consistent with organization of behaviors into roles: behaviors within the group Brood Care were positively correlated in time, as were those within the groups Social Interactions, Physical Maintenance, and Colony Provisioning. Between these groups, there were significantly fewer transitions than random expectation (Table 5). The pattern of overabundance of nonzero transitions in diagonal blocks, and under-representation in off-diagonal blocks was absolutely consistent with expectation. Overall, nonzero transitions clustered in diagonal blocks, thereby providing statistical evidence of polyethism. Worker behavior can be provisionally categorized into four roles: brood care, social interactions, physical nest maintenance, and pro- visioning, since transitions among behaviors within a role occurred more often than random expectation whereas links between roles occurred less often than by chance. Information from the single-step



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19831 Herbers - Social Organization in Leptothorax 37 3 Table 5. Synopsis of one-step transition probabilities among worker behaviors - Entries indicate the number of nonzero transitions from Behavior i to Behavior 2 observed over three colonies.
Behavior .i
Category
. -
Personal Behavior
g Brood Care
. -
Social Interactions
Physical Maintenance
ffi
Provisioning
Personal
Behavior
9
16
14
7
8
Brood Social Physical
Care Inter- Main-
actions tenance
Provision -
ing
+ more than expected by chance
- fewer than expected by chance
G = 55.24, 16 df. P < .001
transition matrix therefore gave important insights to the nature - 3? polyethism in L. ambiguus. However, Table 5 must be interpreted with caution, since it does not report linkage over several act: - Analysis of per-second transition probabilities cannot detect trans= - tions between behaviors intervened by other acts. That is, over relatively long period, workers may switch roles, which would n< be disclosed by single-step transition analysis (Herbers and C u i - ningham 1983). While single-step transitions suggest patterns - S polyethism, inferences must be corroborated by long-term observa - tions of behavior.
Information on worker behavior over 30-minute periods is give n in Table 6. There is indicated the number of ants (out of a total - ‰ 57) that executed two behaviors within a 30-minute period. It is cle; Y- from Table 6 that behaviors provisionally assigned to different role s were often displayed by one worker over 30-minutes' time. Resuli s of statistical testing of Table 6 are summarized by Venn diagrams i n Figure 2. Behaviors intersecting in this figure co-occurred m o r e often than expected by chance (x2 tests) whereas sets not intersec~ - ing were observed for the same individuals at a rate no differel from chance expectation. Thus behaviors involved in egg care w e r e related, as were those concerning care of larvae and those related t



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372 Psyche [VOI. 85
care of pupae. However, no greater proportion of egg-workers also tended for larvae and pupae than chance expectation. That is, indi- viduals specializing on eggs were not necessarily those specializing on pupae or larvae. The final set of behaviors is a large but loosely-connected cluster. Regurgitation, grooming, and antenna- tion were closely interconnected (Figure 2); these were also peripherally connected with inspecting the nest exterior and moving outside, since individuals returning from an outside foray elicited interest and grooming from nestmates. Similarly, LN and HM do not directly intersect, since they co-occurred no more often than chance expectation; these two behaviors were indirectly linked through ATW and RW (Figure 2). Behaviors in the roles of social interactions, physical maintenance, and provisioning were therefore