Deborah M. Gordon.
Daily rhythms in social activities of the harvester ant. Pogonomyrmex badius.
Psyche 90(4):413-423, 1983.

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DAILY RHYTHMS IN SOCIAL ACTIVITIES
OF THE HARVESTER ANT, POGONOMYRMEX BA DIUS* BY DEBORAH M. GORDON
Department of Zoology,
Duke University.
Durham, N.C. 27706
Daily cycles in behavior are well known throughout the animal kingdom. There is some evidence that the activities of ant colonies are temporally organized so that, at a given time of day, a certain set of tasks is done. This study explores that possibility by examining temporal patterns in the social behavior of the harvester ant, Pogonomyrmex badius. Such patterns should be distinguished from circadian rhythms to which conform endogenous, physiological events exhibited by individual animals (e.g. McCluskey 1958 and 1965). The present study is concerned with daily rhythms in social activities performed by groups of ants. Two questions are ad- dressed: 1) Are certain tasks performed at characteristic times of day? 2) How do activity rhythms vary among different colonies? There have been many studies of daily temporal patterns in the overall activity levels of ant colonies, measured as the numbers of ants entering or leaving the nest (Levieux and Diomande 1978a, Hunt 1974, Hansen 1978, Van Pelt 1966), or the numbers of ants in certain areas for specified durations (Janzen 1967, Levieux and Diomande 1978b, Levieux 1979a and 1979b. Golley and Gentry 1964). Temporal patterns of overall activity level are well docu- mented for several Pogonomymex species (Holldobler 1970 and 1976a, Whitford and Ettershank 1975, Whitford et at. 1976). Some authors have described temporal patterns of selected social activities of various ant species (Moglich and Alpert 1979, Janzen 1967, Levieux and Diomande 1978a and 1978b), including Pogonomyr- mex (Willard and Crowell 1965, Holldobler 1976b). But, except for Holldobler's (1976b) study of mating activity, the cited work pre- sents no systematic data on temporal patterns in behavior other than entering and leaving the nest. In some recent field studies of *Manuscript received by the editor November 21, 1983



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414 Psyche PC
Pogononi.\wnex behavior, I found activity rhythms of vari - u s ony tasks (Gordon 1983b and 1983~).
P. badius has received much less attention than other 7-0 vrmex species, perhaps because it is geographically isolatons'd fi them. Nevertheless, the scanty literature on P. badius behav ÌÔm o r c tains some descriptive reports that suggest the existence of ^A. cti rhythms in this species as well (Van Pelt 1966, Hangartnc r et 1970). The present study was made in the laboratory. In tlÌÔå S s v activity rhythms could be investigated more systematical 1 ;y t would be possible in a field study.
Four queenright colonies (colonies 1,2,3, and 4), each COB t- air 300-600 workers, were observed for 30 days from March I , 1' through April 2, 1983. Colonies were kept in open, soil-filled tei ria, and fed Bhaktar-Whitcomb (1970) diet or chopped meal w o r The study colonies had all been kept in the laboratory for ab - u t year. They were chosen for the study because they had been c a n s ently active and healthy since being brought into the lab- ratc The laboratory temperature was maintained at 27OC (å±I0) Observations of each colony were made 5 times daily, ^K=> mce each of five 100-minute time periods, as follows: Time p^^s" rioi (TPl), 9:50-11:30; TP2, 11:30-13:20; TP3, 13:20-14:50 : T l4:5O- 16:3O; TP5, l6:3O- 18: 10, and usually in the middle of t :Ìö- e ti period at the following five times; 10:40, 12:20, 14:00, 15: 4 0 2 17:20. Overhead fluorescent lights in the laboratory were o- n fr 7:30 to 23:OO. A lamp with a 60 watt bulb was placed from ^Ì 0 to cm above each colony as a heat source. These lamps were i- llui nated daily from 1 1 :3O until 16:30. Thus, during the first a n d 1 observations, room lights but not individual lamps were on; d u r the 2nd-4th pde periods, individual lamps were on as well. 1 ^2- m p atures on the terraria surfaces, both under the individual l a m p s a at other points on the opposite side of the tanks, were m c a s u i with a thermistor (Yellow Springs Instrument Co. #408). Te -pe ture measurements were made in the terraria of the f o u r stu colonies and also in those of four other colonies maintained in identical manner. The colonies were fed every other day 5 m n diately after the 12:20 observation.




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19831 Gordon - Pogonom.\~rmex badius 415 All behavior observed taking place outside the nest was classified as one of five activities: Foraging, Nest Maintenance, Patrolling, Midden Work, and Convening (Table 1). For each nest, observa- tions noted the numbers of ants in each of the five activities. The sum of the five numbers is the total number of ants observed outside the nest. A total of 600 observations were made on the four colonies. The data were analysed by profile analysis (Timm 1975) to deter- mine whether the numbers of ants engaged in particular activities depend significantly on both activity and time of day. Since the times of foraging corresponded so obviously to the time the ants were fed, foraging was not considered in the analysis. Profile analysis is a series of 3 multivariate analyses of variance (manova), described in detail below. Factors considered were col- ony and date as main effects. The hypothesis that the intercept was significantly greater than zero was also considered as a main effect. Date was considered to be a random effect. Each analysis was made using the data from all 4 colonies, then repeated for each colony separately. Data were log-transformed to ensure that ratios, not numbers of ants, were used in the analysis, making it possible to compare colonies of different sizes.
The first multivariate analysis of variance tested for significant differences in overall activity among time periods. For each day of observation of a given colony, a new variable was created for each time period by adding the (log-transformed) numbers of ants doing nest maintenance, midden work, convening and patrolling. Four differences between time periods were used as observation variables (Table 2, top).
The results (Table 2) show that colonies are significantly more active in TP2 than in TP1, in TP3 than in TP4, and in TP4 than in TP5. The overall activity level of the colony has a peak in the middle of the day.
The next manova tested for significant differences in the numbers of ants engaged in each activity, summed over all time periods. A new variable was created by adding, over all five time periods for each day of observation of a particular colony, the (log- transformed) number of ants doing each activity. Three differences



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Table 1. Classification of activities of exterior workers of P. badius in laboratory colonies Foraging A.
B.
c.
Nest Maintenance A.
B.
Patrolling A.
B.
Midden Work
Convening
Standing at the food dish, eating.
Taking food items into nest.
Piling sand on food.
Carrying sand out of nest, putting it down and going back into nest. Rearranging sand on terrarium surface.
Walking around edge of terrarium. inspecting sides with antennae. "0
.b
E
Walking around with abdomen tucked under thorax, with frequent stops and changes of direction. %
Pawing at sand with front legs and inspecting the resulting small depression with antennae. Repiling midden.
Standing on midden.
Carrying objects (dead ants, food bits, twigs, etc.) to midden Standing with a group, grooming each other. Standing with a group, self-grooming.
With a group milling around slowly under the lamp, sometimes inspecting others of the group with antennae.




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19831 Gordon - Pogonomyrmex badius 417
between activities were used as observation variables (Table 2, middle). The results (Figure 1 for foraging, and Table 2 for the 0th e- r activities) show that the activities may be ranked as follows, accord- ing to the numbers of ants engaged in each one: Midden work > patrolling > convening > nest maintenance > foraging. Figure 1 shows the activity rhythms of each of the four s t u d y colonies. The third manova tested whether some pairs of activity ^.es are performed by significantly different numbers of ants when t- he activities are compared at particular times (Table 2, bottom 3. Significant differences mean that the rate at which the cola n y invests workers in a particular task depends both on the task and o n the time of day. The results (Table 2) may be best understood 'fcÌ y inspecting Figure 1. For example, keeping in mind that the data ^a. re log-transformed, activity-time period difference number 4 (Table? 2) can be stated as follows: The ratio of number of ants doing mid& en work to number convening in time period 1 is significantly grea "fcer than the same ratio in time period 3. In other words, from TP 1 t- o TP3 convening increases faster, or has a steeper slope, than d o e s midden work. This difference is especially clear in the graph =or colony 4.
The overall results in Table 2 lead to the following conclusion n s about slope differences in Figure 1: Convening rises to a peak % n TP2, increasing more rapidly than midden work, then declines m o re rapidly than either midden work or patrolling. In general, patrol1 Ìö n g declines throughout the day while nest maintenance increases. T h e fact that activity-time period differences 7, 8, and 9 are not sign S-fi- cant indicates that all 4 activities change at about the same rate f r o xn TP3 to TP4.
The colony main effect was significant (p > 0.05) for time per% o d differences 1, 2, and 4, for activity differences 1, 2, and 3, and *or activity-time period differences 1, 4, 5, 6, and 10. The date m d n effect was significant for time period difference 3, activity difference 3, and activity-time period differences 3, 7, 8, and 1 1. Mean temperatures of the terraria surfaces are shown in Figure 2, as a function of the time of day.
The behavior of a colony clearly is temporally patterned. It h a s frequently been suggested that, in harvester ants, overall activ%ty



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418 Psyche [Vol. 85
Table 2.
Results of profile analysis.
Results of test for differences in activity level by time period are shown at the top of the table: for test for differences in number of ants in each activity. middle of the table: for test for activity-time period differences (parallelism test). bottom of the table. Data are log-transformed. Symbols used are ** p > 0.01; *, p > 0.05; #, marginal significance. TP = time period; MW = midden work; CN = Convening; PT = patrolling: NM = nest maintenance.
Source:
Intercept as main effect
(DF = 29)
TPI - TP2
TP2 - TP3
TP3 - TP4
TP4 - TP5
Midden Work - Convening
Convening - Patrolling
Patrolling - Nest Maintenance
- TPI .CN - TP5.M W+ TP5,CN
- TP I .PT - TP5.CN + TP5.PT
- TP1.NM - TP5.PT
+ TP5.NM
- TP l .CN - TP3.M W+ TP3.CN
- TP1 .PT - TP3,CN + TP3,PT
- TP1 .NM - TP3.PT + TP3.NM
- TP3.CN - TP4,MW+ TP4.PT
- TP3.PT - TP4.CN + TP4.PT
- TP3.NM - TP4.PT + TP4.NM
- TP4.CN - TP2.MW+ TP2.CN
- TP4.PT - TP2.CN + TP2.PT
- TP4,NM - TP2.PT 4- TP2,NM
- TP4.NM - TP2.MW+ TP2.NM
- TP3.NM - TP5.PT + TP5.NM
- TP2.NM - TP5,PT + TP5.NM
Overall
(All 4 colonies)
Mean
Difference
-1.24
-0.07
0.19
1.19
1.43
-1.37
2.55
-0.19
0.17
0.25
0.59
-0.60
0.1 1
-0.06
0.03
0.06
0.21
-0.2 1
-0.12
-0.12
0.14
0.19
level (sometimes called foraging activity) is related to temperature (Rogers 1974, Whitford and Ettershank 1975, Bernstein 1979). My results support this suggestion. During TP2 through TP4, when the individual lamps were on and the soil temperatures were highest (Figure 2), colonies were significantly more active than they were during time periods 1 and 5. Temperatures in the field often become so high that ants are inactive from midday until early evening. In the



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19831 Gordon - Pogonomyrmex badius
Table 2. (continued).
By Colony
14 16
Mean Mean
Mean Mean
Difference
Difference
Difference Difference
laboratory conditions of the present study, temperatures never became that high. Time of food availability has also been suggested as a factor regulating activity rhythms of foraging ants (Hansen 1978, Hunt 1974, Levieux 1979a and 1979b, Levieux and Diomande 1978a and 1978b), and, in fact, all four study colonies foraged pri- marily at the time of peak food availability. However, fluctuations of temperature and of food availability may not account completely for the activity rhythms observed here. For example, the results show that rates of change in the numbers of ants in each activity vary with the time of day. This means that ants doing different tasks respond differently to environmental cues such



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420 Psyche [VOI. 85
I
COLONY 1
0 PATRO[UNG
å FORAGING
A NEST MAINTENANCE
A CONVENING
å MIDDEN
10 40 12.20 14.00 15.40 17 20
TIME PERIOD
I
COLONY 3
Figure 1. Daily activity rhythms.
The mean numbers of ants engaged in each activity are plotted as a function of time of day. Times shown are the usual observation times. Error bars show standard error of the mean. No error bar is present when the size of the error bar was smaller than that of the symbol for a point in the graph. as temperature and food availability. In P. badius, the five activities described here are performed by four distinct groups of ants (mid- den work and patrolling are done by the same individuals) (Gordon 1983d). Whether there are intrinsic physiological rhythms causing these different groups to be active outside the nest in different numbers, depending on time of day, is a question still to be explored.
Temporal patterns in overall activity level are known to exist in many species of ants. Further research on such species may reveal more detailed patterns of particular activities. Cognizance of such patterns is relevant to the design of further behavioral experiments,



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Gordon - Pogonomjv-mex badius
_QUNDER LIGHT
\
\
\
\
\
\
\
0
1 2 3 4 5
TIME PERIOD
Figure 2.
Mean temperature on the soil surface, both directly under the lamp and on the opposite side of the tank, plotted as a function of the times of measurement. because results may be affected by the times of day at which data are collected (e.g., Gordon 1983d and 1983b). The results show that, though colonies are similar to one another, distinct colonies have distinct activity rhythms (Figure 1). Thus, intercolony variation should also be taken into account when designing experiments. The main point of this study, then, is not that the activity rhythms of P. badius are those shown in Figure 1, but that each colony exhibits some temporal pattern of activities. In every colony, certain tasks are undertaken at characteristic times. Clearly, we need to consider temporal patterns when we endeavor to understand the social organization of the ant colony.



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Psyche
[Vol. 85
I am grateful to R. Shaw and D. S. Burdick for statistical advice, to T. Williams for valuable discussions, to B. Holldobler for com- ments on the manuscript, and to J. Gregg and R. Palmer for help with all stages of the project.
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