Prassede Calahi, James F.A. Traniello, and Michael H. Werner.
Age polyethism: its occurrence in the ant. Pheidole hortensis, and some general considerations.
Psyche 90:395-412, 1983.

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AGE POLYETHISM: ITS OCCURRENCE IN THE ANT PHElDOLE HORTENSIS.
AND SOME GENERAL CONSIDERATIONS.
A main theme of eusociality is division of labor (Wilson 197 1 . 1975). which can be based on physiological differences (as in the case of the reproductive queen and sterile workers), morphological (size) differences among workers, or age differences within a physi- cal class. In social insects both age and physical classes can comprise castes, that is, groups of individuals which perform specialized labor for sustained periods of time (physical castes: Oster and Wilson. 1978; Wilson, 1980a.b; Herbers, 1980; age castes: Oster and Wilson. 1978; Porter and Jorgenson. 1981; Mirenda and Vinson, 1981; See- ley, 1982). We constructed an ethogram for the Indo-Australian ant Pheidole hortensis, and tested the general hypothesis of division of labor in the worker caste by seeking to answer these questions: 1. Is there division of labor between physical castes? 2. Is there division of labor among age classes within a physica 1 caste?
3. And if there is age polyethism, is it continuous or discrete? (See Wilson 1976a.)
We will consider and discuss each question separately, and then compare our results with those from other studies on social insects. In particular we will contrast age polyethism in Pheuiole horten.vis with that of a New World Pheidole species, P. deniaia. Data Collection
Three colonies of Pheidole hortensis were collected in July 1979 from virgin rainforest at Gilmalk, Sri Lanka by Anula Jayasuriya - Department of Biology. Boston University, Boston. MA 02215 ^Museum of Comparative Zoology Laboratories, Harvard University. Cambridge. MA 02138
Munust-rip! received hr /hi, edi~or Ai~.qii.s/ 8, I983



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396 Psyche [VOI. 85
The ants were identified by E. 0. Wilson, and voucher specimens deposited in the collection at the Harvard Museum of Comparative Zoology. Colonies thrived and produced brood in artificial nests made of glass tubing of 5 mm diameter (approximately that of twigs in which wild colonies have been found (Jayasuriya 1979), and fitted with moist cotton plugs. Colonies were maintained at 26O C while observations and experiments were carried out. As is typical of this genus, P. hortensis has a completely dimor- phic worker caste. And, as is true for many ant species in general, newly eclosed P. hortensis are quite light in color, and darken as they age. Using the method first described by Wilson (1976a), we found that based on these color differences and the degree of pig- mentation of body parts each physical caste could reliably be sepa- rated into five color or age classes (see Appendix I). Using the obvious size and color differences, ethogram data on workers of different ages was compiled from 24 hours of observation on one colony over a ten week period.
The nest tube and surrounding area were watched, and every observed act was noted along with the age class and physical caste of the ant performing it. During the 24 hours of observation 3,689 acts of 25 behaviors were recorded for minor workers, and 256 acts of six behaviors for majors. At the end of the study the colony consisted of 192 minors, 32 majors, brood, and the queen. Data Analysis
Completeness of the behavioral repertory was assessed by statisti- cal comparison with a lognormal Poisson distribution (Bulmer 1974, Fagen and Goldman 1977).
The hypothesis of age-based division of labor was tested with a standard \1 comparison between observed performance frequencies by each age class for behaviors, and expected frequencies generated with the following formula (Altmann and Altmann 1977): Eij = Expected frequency of Behavior, by age class, Bi = Observed frequency of Behavior, by all age classes = Number of ants in age class,
3 = Total number of ants in all age classes combined



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19831 Calabi, Traniello, and Werner - Pheidole 397 We excluded from analysis behaviors with frequencies < 1% of all behaviors performed by that physical caste; for P. hortensis that gives a possible frequency per behavior of about 25, or five occur- rences per age class, under the null hypothesis. Associations between age class and behavior were assessed by a relative performance measure (RPM). We calculated the probability that ants of a particular age class will perform a given behavior, and divided those ratios by the highest such probability for that behav- ior. Thus
x = frequency of behavior, performance by ants of age class,, y = frequency of all behaviors by age classj, z = highest such frequency for that behavior,, and RPM = (x + y) + 7.
Finally, any attempt at an ergonomic assessment requires that one distinguish between task and non-task behaviors. We use the terms as follows. "Behavior" means a logical unit like grooming, made up of one or more physical acts, such as drawing the tibia1 comb over the antennae. "Task" is used in the sense of Oster and Wilson (1978) to denote a set of acts which achieve some purpose of the colony. Thus there are task and non-task behaviors, and though all tasks are behaviors, not all behaviors are tasks. 1. Completeness of Repertory.
The repertory of each physical caste separately and of the species as a whole was judged complete, based on statistical comparison with a lognormal Poisson distribution (Bulmer 1974, Fagen and Goldman 1977). For minors, the observed repertory size is 25 behaviors, and the estimated size is 26, with a 95% confidence interval of [23, 291. For majors, six behaviors were observed, and six estimated, with a 95% confidence interval of [5, 71. For P. hortensis the observed repertory includes 31 behaviors, with 33 estimated, and a 95% confidence interval of [30, 361.
2. Division of labor by physical castes. Comparison of the behavioral repertories of the two physical castes shows that there is, with the exception of trophallaxis, no overlap in task performance (Table 1). Of the tasks carried out,



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398 Psyche [VOI. 85
brood care, food acquisition, and allogrooming are performed by minors and defensive tasks by majors. Defense by minors was seen only when the colony was experimentally submitted to attack by other ant species, and even then the two physical castes performed different tasks: minors pinioned foreign ants, making it easier for majors to snip them up.
Selfgroom
Allogroom Minor
Allogroom Ma.jor
Allogroom Queen
Carry Egg
Carry Larva
Carry Pupa
Groom Egg
Groom Larva
Groom Pupa
Assist Larval Eclosion
Assist Pupal Eclosion
Trophallaxis w
Trophallaxis w
Trophallaxis w
Trophallaxis w
Retrieve Food
Forage
1 Larva
/ Minor
/ Major
/Queen
Eat Brood/ Exuvia
Eat Dead Adult
Carry Brood / Exuvia
Carry Dead Adult
Carry Meconium
Carry Nest Material
Eat Solid Food in Nest
Patrol at Food
Patrol Arena
Guard
To1~l.Y
Table 1. Ethogram of Pheidole hor/ensis. Observed frequencies are followed by values in parentheses indicating the frequency of each act relative to the total number of behaviors performed by a physical caste.



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19831 Calabi, Traniello, and Werner - Pheidole 399 Both castes exchange food with minors, but trophallaxis is rela- tively more important in the majors' repertory. Though its actual frequency is low, it constitutes their only non-defense task. and it comprises 5% of the tasks they perform versus only 1% for minors. In fact, when all categories of trophallaxis by minors are combined (with majors, larvae, and the queen as well as with minors), trophal- laxis still comprises only 2% of the minors' task repertory. This relative frequency of the behavior by majors led us to ask whether majors serve as a replete or "cask" caste, as in Camponotusfraxi- nicola (Wilson 1974). Pheidole hortensis majors with full gasters show a three-fold weight increase, but we were unable to perform the critical experiments and test for proportionate weight gain. However, in random surveys of the colony, replete majors (those with distended gasters) performed virtually none of the behaviors typcial of majors (Table 2). During experimentally induced attack (assault with sympatric Tetramorium spp.), replete majors engaged in defense only if the attack was severe (many ants) or extended in time. How much of this inactivity by "storage" majors is due to protecting the food supply and how much to relative inability to move is not clear.
Another potential set of caste differences relates to the granivo- rous habits of many Pheidole species in which minors harvest and majors mill seeds. In an attempt to observe such caste differences in P. hor~ensis, we offered grass, vegetable, and bird seeds of various sizes and fat contents. All were ignored by both physical castes. 3. Division of labor by age classes within a physical caste. To answer this question, we tested the null hypothesis that each age class should perform a given behavior in proportion to the number of ants in that age class. Thus, in a colony with three age "Replete" Ma.jors Non-"Repletes"
Patrol at Food
12 66
Patrol Arena
0 S
Attack Intruder
1 IS
Guard Nest Entrance
0 20
Table 2. Behavioral differences within the major worker caste of Phl~doic hor~m'iix. The numbers of individuals observed performing various acts during random surveys of the colony are presented.



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400 Psyche [VOI. 85
classes comprising 30, 20, and 50% of the total colony, there is no age polyethism if those age classes perform 30, 20, and 50% respectively of any task. The data show that the age classes of both physical castes do not perform tasks in proportion to their numbers (Table 3). On the basis of y2 comparisons, for most tasks with adequate sample sizes the null hypothesis can be rejected because there are significant differences between the observed and expected frequencies. This indicates that there is age-based division of labor in both physical castes (Table 4). Four tasks by minors (assist ecol- sion, allogroom majors, trophallaxis with minors, and carry exu- viae) are performed without apparent age bias, and eight behaviors by minors and one by majors were observed too rarely to permit assessment.
Thus of behaviors with an adequate sample size, for P. hortensis minors 13 of 17, and for majors four or five, behaviors show age- based division of labor.
4. Continuous versus discrete age castes. Wilson (1976a) states that division of labor is discretized if there is an exclusive association between (sets of) tasks and age class(es) and that it is continuous under all other conditions of age classltask association. The general question of age polyethism has two parts. Given that some tasks are performed more or less often by certain age classes, can adjacent age classes be combined because they show similar performance patterns? And second, are such associations between age class(es) and tasks exclusive? To test for associations, we calculated relative performance measures (RPM) for each age class by behavior. This descriptive way of treating the data controls for variation in age class size and in number of performances observed per age class, and it permits comparison between fre- quently and rarely occurring behaviors, as well as comparisons of age class performances within and between behaviors. Figure 1 shows that there are no consistent similarities between the relative performance probabilities for any pairs of adjacent age classes. This implies that no pair of age classes can be combined, and that these age classes do differ behaviorally, representing real castes. It is also clear that the associations between age castes and tasks or groups of tasks are not exclusive: the age-based division of labor is continuous rather than discretized in both the minor and



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19831
Calabi, Traniello, and Werner - Pheidole ROW
MINOR WORKERS TOTA L
AGE CLASS l(15) 11(27) 111(45) lV(24) V(81) 1 92 Selfgroom
Allogroom Minor
Allogroom Major
Carry Egg
Carry Larva
Carry Pupa
Groom Egg
Groom Larva
Groom Pupa
Assist Eclosion
Trophallaxis w; Larva
Trophallaxis w/ Minor
Forage
Retrieve Food
Eat Brood ! Exuvia
Eat Dead Adult
Carry Exuvia
COLUMN TOTAL
MAJOR WORKERS
AGE CLASS l(5) 11(15) 111(5) lV(3) V(4) 32 Selfgroom
Guard
Patrol at Food
Patrol Arena
COLUMN TOTAL 16 62 46 3 2 9 1 247
Table 3. Observed frequencies with which each age class (1 through V) performed various acts. Values in parentheses indicate the number of individuals in each age class.




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Psyche
[Vol. 85
MINORS
Selfgroom
Allogroom Minor
Allogroom Ma.jor
Carry Egg
Carry Larva
Carry Pupa
Groom Egg
Groom Larva
Groom Pupa
Assist Eclosion
Trophallaxis w Larva
Trophallaxis w Minor
Forage
Retrieve Food
Eat Brood Exuvia
Eat Dead Adult
Carry Exuvia
MAJORS
Selfgroom 110 23.7 **
Guard
Patrol at Food
Patrol Arena
Table 4. X2 values and significance levels for differences between observed and expected behavior frequencies for the five age classes within each physical caste. ** indicates that the P values were so small they do not appear in the X2 table and are highly significant. NS. no significant difference. Only behaviors with frequencies $: 1% are included. N = total number of acts observed.



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Calabi, Traniello, and Werner - Pheidole -a03
Figure 1. Relative performance measures (RPM) of various behaviors in the iepertories of ma.jor (MJ 1-MJ4) and minor (A-Q) workers of Pheidole horn4.s. Roman numerals 1-V correspond to worker age classes. 4dditional details in Materials and Methods.



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Psyche [VOI. :
Figure
I . (Continued)




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19831 Calabi, Traniello, and Werner - Pheidole 405 MINORS
Groom Egg
Sel f groom
Allogr. Minor
Allogr. Major
Assist Eclosion
F Carry Egg
G Groom Pupa
MJ-3
H Eat ~rood/Skin
I Troph w/ Larva
J Troph w/ Minor X
K Groom Larva
0.4 -
IZ
L Carry Larva
M Carry BrdlSkin
N Eat Dead Adult
0 Carry Pupa
P Retrieve Food MJ-4
Q Forage
MAJORS
-
MJ-1 Selfgroom
MJ-2 Guard
MJ-3 Patrol @ Food
MJ-4 Patrol Arena
Figure I. (Continued)




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406 Psyche [VOI. 85
major physical castes. For instance, in Figure I examine the minors' behaviors F, G, and E, and the series H through N, The relative performance measures for tasks F and G are highest for age class 11. yet age classes I1 and I have similarly high RPM for task E. Or, in the second case, for all the tasks H through N, age class 111 shows very high RPM, but the tasklage class association is not exclusive. For K, M, and N at least, other, but not necessarily adjacent age classes, show similarly high RPM.
P. hortensis exhibits both physical and age castes, and the latter show continuous rather than discretized polyethism. We will com- pare these results with results from other species, and consider some of their general implications for the study of age polyethism. I. Repertory size and numerical considerations. Both repertory size and the proportion of rarely occurring behav- iors in P. hortensis are in the same range as those of other species. Numbers of behaviors in repertories judged complete by Fagen and Goldman analysis are: 27 for workers of monomorphic Leptozhorax species (Wilson and Fagen 1974) and for minor workers of Orectognathus versicolor (Carlin 19821, and 28 each for minor Pheidole dentata (Wilson 1976a), Formica perpilosa (Brandgo l979), and Camponotus sericeiventris (Busher 1982). Extremes may be represented by minor repertories of Solenopsis geminata, S. invicta, and Zacrjptocerus varians: 17, 20, and 38, respectively (Wilson, l976b, 1978). Repertories for majors range from two (Solenopsis geminata, Wilson 1978) to 24 (Orectognathus versi- color, Carlin 1982); for the dimorphic 2. varians and P. dentata, major repertories are I1 and nine (Wilson 1976a and b). Total repertories for all these species range from 20 to 40 behaviors. Given the similarity in repertory size for minor workers of both P. hortensis and P. dentata, it may seem odd that in P. hortensis considerably fewer behaviors (I 3 vs 23) are performed with age bias. The difference results from the respective criteria used to reject rarely occurring behaviors from analysis. Because he does not make statistical comparisons, Wilson rejects only 2 of 28 behaviors on grounds of insufficient data. However, when the cut-off criterion



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19831 Calabi, Traniello, and Werner - Pheidole 407 used for P. hortensis is applied (rejected behaviors with frequencies < 1%) of those performed by that physical caste) the number of behaviors rejected for P. denfata increases from two to seven of 28 (N= 1.222; Wilson 1976a). Thus the appropriate comparison of age- biased behaviors among behaviors with frequencies 2 1% shows a similar situation for the two: 13 of 17 behaviors for P. horfensis, and I5 of 19 for P. dentata. Of behaviors performed without apparent age bias, allogroom majors and trophallaxis with ma-jors, are in common to the two species. (In general ant repertories include high proportions of infrequent behaviors: for minors I0 of 28 (P. dentala -Wilson f976a), 27 of 38 (Zacr.~pfocerus varians-Wilson 1976b) 7 of 28 (Formica perpilosa-Brand50 1979). 14 of 27 (Orectognathus versicolor-Carlin 1982) and 9 of 28 (Campono~us sericeivenfris- Busher 1982). Although some castes have small repertories (majors of P. dentata and Solenopsis geminata, -Wilson I 976a, 1 W8), many other castes show similar proportions of infrequent to fre- quent behaviors (Wilson and Fagen 1974, Traniello 1978, Brand50 1979. Carlin 1982).
2. Age-based division of labor.
It is a virtual truism that among social insects older workers forage and have little or nothing to do with brood care. Yet in P. hortensis older workers, in addition to performing all foraging and food retrieval (P and Q, Figure I), show high RPM for several brood-care tasks (K, L, M, and 0). We suggest that these represent labor cohorts, based on ant experience or colony need. They could arise via the mechanism of task fixation, a feedback-based task stabilizing mechanism documented in wasps (Forsyth 1978) and suggested for the ant An~bl.wpone pallipes (Traniello 1978). An individual performs some task (e.g., trophallaxis with larvae), receives positive feedback (continually finds hungry larvae), and over time does not switch to other tasks because the positive feedback does not cease. The susceptibility of individuals to task fixation could vary so that even in a system with age-based task- switching, task fixation might override age-based behavioral change.
Although it remains to be demonstrated whether such fixation occurs in P. hortensis, we wish to point out one possible con- sequence of task fixation and the resultant caste bbatypical" be-



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havior. A colony labor profile by age class could be an artifact of previous colony needs and activities, specifically for the period previous to the study by just less than an average worker life span. For instance, suppose that a colony engaged in high brood production several weeks previous to observations. There would have been both need and opportunity for much brood care, and also opportunity for fixation on brood tasks. Further imagine that the food supply then dwindled and brood production decreased. Some