Deborah M. Gordon.
The persistence of role in exterior workers of the harvester ant, Pogonomyrmex badius.
Psyche 91:251-266, 1984.

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THE PERSISTENCE OF ROLE IN EXTERIOR
WORKERS OF THE HARVESTER ANT,
POGONOM YRMEX BADIVS
Department of Zoology, Duke University
Durham, NC 27706
The social organization of an ant colony channels individuals into particular tasks, so that the colony continues to perform its tasks in a regular way. In other words, the colony is organized so that certain roles (sensu Oster and Wilson 1978), vital to its function, continue to be filled. The present study reports on role behavior in exterior workers of the harvester ant Pogonomy~rmex badius. Three questions are addressed: 1) In P. badius, do particular groups of ants consistently do certain tasks? 2) Does the task per- formed by an ant depend on its age? Because P. badius workers live for about a year (Porter and Tschinkel 1982), marked individuals were observed for a longer time (up to four months) in the present study than in previous studies of Pogonomyrmex species (Holldobler 1976, Porter and Jorgensen 1981). 3) Does the task performed by an ant depend on its size? P. badius is the only polymorphic species in its genus. Since majors rarely emerged from the nest, only the role behavior of minor workers was considered in this study. However, even within the minor subcaste of P. badius, a greater size variation exists than in other Pogonomyrmex species. The relationship between size and role in P. badius minors is investigated. In the present study, the behavior of exterior workers is classified into a more detailed set of tasks than in previous studies of the genus. Foraging, i.e. food retrieval, is only one of five activities observed outside the nest. The classification of tasks used here is needed to explain other aspects of Pogonomyrmex behavior (Gordon 1983a and b).
*Current address: MCZ Laboratories, Harvard University, Cambridge, Mass. 02138. Manuscript received b!, the editor April 24, 1984



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All experiments were conducted in the laboratory with five queen- right colonies of P. badius, each containing about 650 workers. The presence of brood inside the nest could not be ascertained. Colonies were kept in soil-filled tanks (45 X 25 X 20 cm), and fed with Bhaktar-Whitcomb diet (1970), which was placed on the soil surface in a small dish. Individual ants were marked with Testers Gloss Enamel, each with a drop on the top of the head. Single ants were kept isolated overnight after being marked, and then put back into the home colony. No adverse reactions to the marking procedure were observed.
All tasks performed by ants outside the nest were classified into the following five activities (Table I): Foraging, Nest Maintenance, Patrolling, Midden Work, and Convening. Ants inside the nest were not considered in this study. No majors were marked, since they spent almost all of their time inside the nest, probably assisting in brood care (pers. obs.).
An observation record consisted of the numbers of marked and unmarked ants in each of the five categories of behavior. Observa- tions were made between 8:00 and 20:OO. Each observation lasted 5-10 minutes, depending onthe total number of ants outside the nest. At least one hour elapsed between successive observations. A. Continuity of role. In each of the five colonies, ants collected while performing one of the five categories of behavior were removed from the colony and marked. For example, in one colony only foragers were marked, in another only patrollers were marked, and so on. In each colony, five ants were marked every five days until 20 ants had been marked. Each group of five ants was marked with paint of a different color. Thus each colony was observed for a total of 35 days, which I call the first observation period. During this time, a total of 728 observations were made, about 145 observations on each colony. The data were analysed separately for each colony as follows. First, a two-way chi-squared procedure with four degrees of freedom (Sokal and Rohlf 198 1) was used to test whether the distribution of the ratio of marked to unmarked ants depends on activity. Next, another chi-squared test, this time a two-by-two test with one degree of freedom, was performed. The latter tested whether ants were significantly likely to continue doing the activity they were doing when marked, called the "tagged activity." Both tests used the total



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Table 1.
Classification of activities of exterior workers of P. basius in laboratory colonies Foraging A. Standing at the food dish, eating. B. Taking food items into nest.
C. Piling sand on food.
Nest Maintenance
A. Carrying sand out of nest, putting it down and going back into nest. B. Rearranging sand on terrarium surface. Patrolling
A. Walking around edge of terrarium. inspecting sides with antennae. B. Walking around with abdomen tucked under thorax, with frequent stops and changes of direction.
C. Pawing at sand with front legs and inspecting the resulting small depression with antennae. Midden Work A. Repiling midden.
B. Standing on midden.
C.
Carrying objects (dead ants, food bits, twigs, etc.) to midden. Convening A. Standing with a group, grooming each other. B.
Standing with a group, self-grooming.
C. With a group milling around slowly under the lamp, sometimes inspecting others of the group with antennae.




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254 Psyche [vo~. 91
numbers of marked and unmarked ants observed in each activity, during the entire observation period. The latter, two-by-two test used the total numbers of marked and unmarked ants observed in the tagged activity, and the two summed values for the other four activities.
B. Death rates ofmarked ants. To determine whether death rate differed according to activity, the colony middens were inspected daily during the first observation period, and dead marked ants were counted.
C. Continuit-r of role after a two-month interval. The observa-
tions described in part A were ended on July 12, 1982. On September 12, 1982, observations of the same marked individuals in the same five colonies were begun again and continued for 25 days, which I call the second observation period. A total of 193 observations were made, and the data were analysed as before. D. Behavior of callow workers.
Adult ants that have recently
emerged from the pupal stage, or callows, are lighter in pigment than older adults. Their behavior was examined in two ways. First, for 25 days during the second observation period, the number of callows in each of the five activities was recorded, as well as the numbers of all other ants. Records were obtained on only two colonies, since the other three colonies had but few callows outside the nest. These data, consisting of 8 1 observation records, were analysed as in Part A with a two-way chi-squared procedure. This tested whether the distribu- tion of the observed number of callows depended significantly on activity.
Second, 20 convening callows in one colony were marked, using the procedure described in part A. They were observed for 40 days, beginning with the day on which the first group of five callows was marked. Convening callows were chosen because it was rare for a callow to be seen doing any of the other four activities. A total of 149 observations were made in this procedure. The data were analysed as in parts A and C.
E. Relation ofbody size to activity. To determine whether body size is correlated with activity, the head widths of 25 minors in each of 7 colonies, a total of 175 ants, were measured. Each group of 25 ants consisted of 5 ants from each of the five categories of behavior, or activities. Head width was measured at right angles to the line between the clypeus and occiput, immediately posterior to the eyes,



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19841 Gordon - Pugon~rmex badius 255
using an ocular micrometer calibrated in 0.1 mm. Measurements were estimated to the nearest 0.01 mm. The data were subjected to a two-way analysis of variance (Sokal and Rohlf 198 I), which tested for the effect of activity and colony of origin on head width and for a colony-activity interaction.
A-C. Continuity of rule.
In both observation periods, the ratio
of marked to unmarked ants depended highly significantly on activity in all five colonies (two-way chi-squared test, p < 0.000 1 for all tests). Table 2A shows the data from the first, 35-day observation period (part A). The deviations shown are based on the expected values derived in the two-way test for each colony. The significance values along the diagonal of the table are the result of the two-by-two chi-squared tests of whether marked ants continued to do the tagged activity. These results all show highly significant, positive deviations from the expected values. All groups of ants continued doing the activities that they were doing when marked. Thus, the data of Table 2A show that groups of ants do specialize in certain activities. Cells not on the diagonal in Table 2.4 show the data for ants in the four activities other than the tagged one. In some cases, the data show a large positive deviation from the expected value in an activity other than the tagged one. This indicates that marked ants were likely to perform a second activity as well as the tagged one. The results shown in Table 2A, and summarized in Table 2C, lead to the following conclusions.
Ants marked while doing midden work tended to be later engaged in patrolling, and vice versa, indicating that midden work and patrol- ling are done by the same ants. Ants marked while convening later did midden work as well. But ants marked doing midden work did not later do convening. Similarly, ants marked while doing nest mainte- nance later did patrolling as well. But ants marked while patrolling did not later do nest maintenance. These results suggest that ants marked while convening were in transition to midden work, and that ants marked while doing nest maintenance were in transition to patrolling. In these two colonies, I examined the proportions of marked ants doing each activity as a function of time in the course of the first observation period, but saw no interesting or possibly signifi- cant changes in these proportions. The possibility of later role



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256 Psyche [vo~. 91
changes of ants marked while convening or doing nest maintenance is discussed below.
The numbers (each out of 20) of dead marked ants in the first observation period were as follows: midden workers, 1 1; patrollers, 16; foragers, 2; conveners, 1 ; nest maintenance workers, 2. Table 2B shows the data from the second, 25-day observation period, begun two months later. Several obvious differences between the data of Table 2A and that of Table 2B should be noted. Fewer observations (149) were made in the second observation period than in the first one (728). The colony in which patrollers were marked was barely active during the second observation period, for unknown reasons. Colony inactivity was not accompanied by any obvious increase in the number of dead workers visible in the midden. Because no marked ants were observed, it was not possible to analyse the data from this colony. As noted above, many midden workers and patrollers died during the first observation period. Though only two marked foragers died during this first 35-day period, the number of marked foragers had decreased two months later, indicating that many foragers died during the interval between the first and second observation periods.
The two-by-two tests for the second observation period show that marked ants were no longer likely to do the tagged activity. All marked ants had either died or changed activity after a two-month Table 2(A-D). Behavior of marked ants
(Pages 000-000)
In Tables 2A and 29. each row of the matrix shows the results from one colony, in which ants were marked doing the indicated activity. The first five columns show the subsequent activities of marked ants. In each cell are shown the total numbers of marked and unmarked ants observed during the entire observation period, the devia- tion from the expected value, and the percent deviation (of the expected value). **means p < 0.001: NS means "not significant." The last column shows the mean number of marked ants per observation.
Table 2C summarizes Tables 2A and 2B. It shows the results of the two-by-two chi-squared tests of whether marked ants continue doing the tagged activity. Also shown are strong positivedeviations from expected values in other activities. Plus and minus signs indicate the sign of the deviations from expected values. As in Tables 2A and 2B, each row of the matrix shows the results from one colony, in which ants were marked doing the indicated activity.
Table 2D is similar to Tables 2A and 2B. It shows the data from part D, in which callow ants were marked when convening.




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Gordon - Pogono~rmex badius
Table 2A. Behavior of marked ants during first observation period. Mean no.
Subsequent A ctivitys
marked ants
Acti\*it.~ When seen per
Marked F NM PT MW CN observation
* *
Marked 47 126 26 16 29
Unmarked
309 1397 281 373 568
2.28
Dev.forExp.
t19.6 +8.8 +2.4
-13.9 -16.9
åÁ/ Dev. 72
8 10 46 36
**
Marked 37 436 116 134 184
NM Unmarked 623 4755 11 12 2764 2915
5.60
Dev. for Exp.
-8.8
+75.9
+30.8 -67.0 -31.0
% Dev.
19 2 1
3 6 33 14
-
* *
Marked
23 34 123 173 48
,,T Unmarked
397 2776
904 1736 1309
2.18
Dev. for Exp.
+0.6
-1 15.8
+68.3 +71.2 -24.3
% Dev.
3 77 125 69 34
- --
- -
**
Marked
9 67 102 343 276
Unmarked
242 1514
819
1840 4688
7.05
Dev. for Exp. - 1 1.2 -60.3 +27.9 + 167.3 - 123.6 5% Dev,
5 5 47
3 8
9 5 30
**
Marked 50 325 131 245 313
cN Unmarked 404 2481 701 1124 I362
6.57
Dev.forExp. -17.7 -93.4 +6.9
+40.9 +63.3
% Dev. 26 22 6
20 25
F= Foraging; NM = Nest Maintenance; PT= Patrolling; MW = Midden Work; CN = Convening.




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Table 2B. Behavior of marked ants during second observation period NS
Marked 1 I 3 29 3 2 9 1
Unmarked 130 458 566 557 721
0.64
Dev. for Exp. +2.0 -26.5 -9.0 -5.6 +39.1 % Dev. 22
89 24
15 7 5
Marked 11 8 133 4 1 5
NM Unmarked 264 925 1289 994 937
3.94
Dev. for Exp. -0.8
-32.1
+71.9
-3.5 -35.5
% Dev. 7 80
118 8 8 8
Marked 0 0 0 0 0
pT Unmarked 7 12 13 15 0
0
Dev. for Exp. - - -
% Dev. -
- - -
Marked 4 2 4 1 11 67
Mw Unmarked 76 136 360 417 165
0.74
Dev. for Exp. -3.8 -1 1.5 +I .8 -30.8 +44.3 % Dev. 48 8 5
5 74 195
NS
Marked 23 4 46 40 12
cN Unmarked 128 375 488 529 192
2.23
Dev. for Exp. +12.7 -21.8 +9.7
+1.3 -1.9
% Dev. 5 5
84 26 3 14
F= Foraging; NM = Nest Maintenance; PT= Patrolling; M W = Midden Work; CN = Convening.




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Gordon - Pogonomvrmex badius
Table 2C. Summary of results from first and second observation periods Firs I Observation Period
Activity Sub,wquent Activit.1,
when
Marked F NM PT MW CN
Second Observation Period
Activity Siihxequeiil Acl/\~//,~~
when
Marked F NM PT MW CN
F= Foraging: NM = Nest Maintenance: PT= Patrolling: MW = Midden Work; CN = Convening.




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260 Psyche [vo~. 91
interval. Ants originally marked while convening now appeared to do foraging instead. Ants marked while doing midden work now did convening, as did those marked while foraging. Finally, ants marked while doing nest maintenance now tended to do patrolling. These results are summarized in Table 2C.
D. Behavior of callow workers. The two-way chi-squared test on the data on callow workers shows that the ratio of callows to other ants depends highly significantly on activity (p < 0.0001). In both colonies in which callows were observed, the callows were most often observed convening.
Callows that were marked while convening and observed for 35 days were significantly likely to continue to convene (p < 0.000 1 for two-way chi-squared test, p < 0.00 1 for two-by-two chi-squared test) (Table 2D). Although dark-pigmented, older ants marked while con- vening were also likely to do midden work during the first 35 days (Table 2A), it appears that convening callows were not likely to do midden work (Table 2D).
The results of these two procedures show that callow ants convene for at least 35 days. Although in the field P. badius callows acquire darker, adult coloration within 25 days (Gentry 1974), only 6 of the 20 callows marked while convening were as dark as non-callow adults at the end of 40 days. The others were still of intermediate coloration. E. Body size.
There was no significant relationship between head width and type of activity in minor workers. The two-way anova showed no interaction between colony of origin and type of activity (DF= 24, SS = 0.44, F= 0.64, p > 0.8993), no effect of type of activity on head width (DF = 4, SS = 0.09, F = 0.77, p > 0.5492), and a significant effect of colony of origin on head width (DF = 6, SS = 1.30, F = 7.53, p > 0.001). Mean head size for all 25 workers in all five activities ranged from 1.53 mm in one colony to 1.78 mm in another of the seven colonies. Mean worker size increases with colony age (Oster and Wilson 1978). Differences in age among the seven colonies probably account for the effect of colony of origin on head width. At any one time, exterior workers in the P. badius colony can be divided into four groups that consistently perform certain roles for at least 35 days: 1) midden work and patrolling, 2) nest maintenance, 3) foraging, and 4) convening.




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Gordon - Pogonom~~~rmex badius
Table 2D. Behavior of callows while convening Activity Subsequent Ac~i\if.r
when
Marked F NM PT MW CN
* *
Marked
26 17
6 3 74 385
Unmarked
2290 14382 4664 973 1 1560 1
CN
Dev. for Exp.
- 1.7 - 155.2 -6.5 -43.3 4-193.8
% Dev.
6 90 12 37 101
--
F= Foraging; NM = Nest Maintenance; PT= Patrolling; MW = Midden Work; CN= Convening.
Convening of exterior workers is observed much more often in the laboratory than in the field. Convening includes resting and mutual grooming. It probably serves a thermoregulatory function as well, because convening ants in laboratory colonies always gather in the warmest place on the terrarium surface, directly underneath a lamp. Convening has been observed in field colonies of Pogonomyrmex, but only rarely (MacKay 1981; Gordon 1983b, 1984a, b). It is possible that, in the field, convening is usually done inside the nest where conveners would be less subject to predation. The results of this study, summarized in Table 2C, suggest the diagram depicted in Figure 1. The diagram shows how role may depend on worker age in exterior workers of P. badius. It should be emphasized that the diagram is hypothetical, pending further investi- gation, and that it rests on two assumptions. In keeping with results on many other species (Wilson 197 l), it is assumed that younger ants first work inside the nest, then work outside the nest for the remainder of their lives. Another assumption is that in the laboratory experiments reported on here, the death rate of ants in a particular activity depends on their age rather than on the hazards or energetic costs associated with their activity. Convening ants are clearly less active than ants in the other four activities (see Table 1). However, there is at present no empirical basis for distinguishing the other four activities in terms of the energy expended performing them. Ants doing midden work and patrolling are shown as the oldest in Figure 1, because ants marked while doing these activities died within



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DEATH
PATROLLING
+ CONVENING
MIDDEN WORK
t FORAGING