H. Topoff, K. Lawson, and P. Richards.
Trail Following and Its Development in the Neotropical Army Ant Genus Eciton (Hymenoptera: Formicidae: Dorylinae).
Psyche 79(4):357-364, 1972.

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TRAIL FOLLOWING AND ITS DEVELOPMENT IN
THE NEOTROPICAL ARMY ANT GENUS ECITON
(HYMENOPTERA: FORMICIDAE: DORYLINAE)
INTRODUCTION
Results of numerous field studies on the neotropical army ants Eciton hamatum (Fabricius) and E. burchelli (Westwood) have shown that newly eclosed callow workers do not participate in adult activities such as raiding until they have matured in the col- ony environment for several days after the onset of the nomadic phase. Nevertheless, the callows of both species do emigrate along with the colony, beginning on the very first nomadic day (Schneirla, 1971). We have offered several hypotheses to account for the be- havioral deficit of the callows. One hypothesis proposes that the sensitivity of the callows to their colony's chemical trail is too low for them to be able to follow along it during the day's raid. By late afternoon, however, the strength of the trail may be sufficiently increased (as it is constantly reinforced by tens of thousands of ants running back and forth over it) so that the callows are able to par- ticipate in the emigra,tion. This paper presents the results of a study designed to compare the performance of callow and mature adult workers of E. hamatum and E. burchelli on their own colony's trail. METHODS
This study was conducted on Barro Colorado Island, Panama Canal Zone. Features of the Island that make it suitable for army ant research have been described by Rettenmeyer ( 1963). Field methods consisted of daily patrols of the Island's numerous trails to locate statary colonies. The first indication that a colony was about to enter a new nomadic phase was the appearance of empty pupal cases on the ground near the bivouac, and the presence of lightly-pigmented callow workers inside of the nest. The first nomad- ic day was defined as the day on which the first emigration of the entire colony took place.
Callow and mature adult ants were collected by collapsing a sec- 'Department of Psychology, Hunter College of the City University of New York, and Department of Animal Behavior, The American Museum of Natural History, New York, N.Y. 10024. Manuscript received by the editor November 20,1972



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3.58 Psyche [December
tion of the bivouac wall with a trowel, and allowing the ants to drop into a plastic box. When this method did not yield enough ants, we aspirated additional individuals from the area around the nest. In the laboratory the ants were kept in a glass aquarium. At the start of each day's test, a petroleum ether extract of whole ants was prepared by adding 60 ml of ether to a jar containing 300 intermediate-sized mature adult ants. A circular trail of the ether extract (63 cm in circumference) was deposited on discs of filter paper, by allowing the extract to flow out of a microburet suspended above a phonograph turntable which was spinning at 78 rpm. The ether evaporated in 5 seconds, and in order to observe whether the test ants were following the trail, the filter paper was placed upon a corresponding circular template. The template circle, as well as the lines dividing it into 16 sectors, were clearly visible through the discs of filter paper. A new disc of filter paper was used for each trial.
In order to insure uniform orientation of each test ant a,t the start of trail following, a plastic enclosure was used (Fig. I). It
VERTICAL PARTITION
A
CIRCULAR CHAMBER
- LOCATION OF TRAIL
DISC OF FILTER PAPER
Fig. 1. Apparatus used to insure that all test ants initially orient to the trail in an identical manner. For explanation, see text.



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19721 Topoff, Lawson ‰â Richards - Eciton 359 consisted of a circular chamber (5.0 cm in diameter, 1.4 cm high) which opened into a tunnel that was 7.9 cm long. The tunnel was the same height as the adjoining circular chamber, and was 1.2 cm wide. This width was chosen because it was approximately equal to the leg span of the intermediate-sized workers of E. hamaturn and E. burchelli (i.e., workers whose overall length was between 7.0 and 9.0 mm). The curvature of the tunnel was equal to that of the trail.
The circular chamber was divided approximately in half by a vertical partition attached to its lid. Each test ant entered the cham- ber through a hole in the lid, and the partition prevented it from contacting the trail during the 15 second period in which it was al- lowed to adapt to the chamber. At the end of this interval the lid with the partition was replaced with another lid without a partition, thus allowing the ant access to the trail. For each dilution of ether extract that was used, 60 intermediate- sized ants (30 callows and 30 mature adults) were selected at ran- dom from the mass of ants in the aquarium. The dilutions were
prepared by adding different amounts of pure ether to the extract. Ants were tested individually, and as indices of trail following we recorded the number of ants following, the distance travelled on the trail (maximum distance = 580 mm), and the speed of travel along the trail. If an ant did not leave the runway at the end of one minute, a negative response was recorded. RESULTS
The results of comparing trail-following performances between callow and mature adult worker ants from three colonies of E. hamaturn and one colony of E. burchelli amre shown in Table I. It is clear that when comparisons are made as to the relative ability of callow and mature adult ants to follow any portion of the trail, as well as comparisons of the number from both groups able to com- plete the trail, the differences are not significant in approximately two-thirds of the cases. The third measure of trail following we considered was the median distance (in mm) followed by callow
and mature adult ants in each test series. Again, these differences were usually not significant. The only measure of trail-following performance that consistently illustrated the behavioral immaturity of the callows was their speed of running over the trail. At all dilutions on all nomadic and statary days in which tests were con- ducted, the running speed of the callows of both species was slower than that of the mature adult ants.




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Psyche
[December
DISCUSSION
It is clear that the absence of callow army ants from raiding trails during the first few days of the nomadic ~hase can not be ex- plained on the basis of their inability to follow the chemical trail of their colony. Nevertheless, there are significant differences in trail- following performance between callow and mature adult ants, espe- cially in their speed of running along the trail. If the callows did attempt to participate in raiding activities, their deficit in locomotory ability could make them very inefficient predators. This, together with the observation that callow army ants placed in raiding columns are behaviorally disorganized (Schneirla, 1971 ) makes it likely that it is adaptive to the colony for the callows not to participate in raid- ing until they have matured. However, the fact that the behavior of the callows is adaptive to the colony still does not explain the mechanism that keeps them inside of the nest. Although there is relatively little data on the behavior of the cal- low workers in the nest immediately after their eclosion, Schneirla ( I 952) reported that they feed voraciously. Rettenmeyer ( I 963) also observed callows feeding on booty, but pointed out the difficulty of estimating the quantity of food consumed or the duration of the active feeding periods. If the callows are preoccupied in feeding intensively during the first few days after their emergence from the pupal stage of development, this could account for their absence from raiding columns.
Another hypothesis stems from the observation that callow workers tend to cluster tightly together, both in the bivouac and in emigra- tion columns ( Rettenmeyer, I 963 ; Schneirla, I 938). This intense
clustering may result from an extreme sensitivity of the callows to chemical and tactual stimuli. Inside the bivouac the intensity of tactual stimulation is high, as a result of the continual contact among callows and other ants. The nest is also saturated with trail sub- stance and other chemical stimuli (which arise from the workers, the queen, and even the booty). Accordingly, whenever a callow leaves the bivouac the intensity of stimuli impinging upon it decreases abruptly. Outside the nest the amount of tactual stimulation is de- creased as the adult ants fan out along the trail. The intensity of chemical stimuli is also decreased, as1 volatile chemicals diffuse and as the concentration of trail chemicals decreases from a dense sphere surrounding the ants inside the nest to only a narrow trail leading away from the nest. As a result, callows attempting to leave the nest would experience a sudden decrease in chemotactual stimulation, with the result that they would become disoriented. This, in turn,



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Topoff, Lawson ‰â Richards - Eciton
would cause the callows to reverse their direction of movement and return to the stimulus-saturated interior of the nest. By the end of the day's raid, and immediately preceding the start of an emigration, a large portion of the adult workers are out of the nest. Then, as the full-scale emigration progresses, the remaining adult population, together with the brood and queen, also leaves the nest. Thus, at some time between the beginning of an emigration and its peak, the relative concentration of chemical and tactual stimulation outside of the bivouac may be sufficient to attract the callows out of the nest. This may occur because the callows are particularly attracted to the chemical secretions and tactual stimuli that arise from one subpopu- lation of the entire colony (such as the adult workers, the brood, the queen, or even the booty).
Or, the stimulative effect on the
callows may be quantitative - that is the callows may leave the nest when the magnitude of stimulation outside the nest increases suffi- ciently, regardless from which particular segment of the colony the stimuli arise.
We have recently made an observation in the field that is con- sistent with the hypothesis that high concentrations of social stimu- lation arising inside the nest serve to keep callow army ants inside the nest during the first few days of the nomadic phase. During a recent study of the nocturnal army ant species Neivanzyrrnex nigre- scens (Cresson) in southeastern Arizona, we located a statary col- ony bivouacked beneath a rock in the bank of a creek. The statary raids were unusually weak, with only several dozen workers partici- pating. On subsequent nights, the raids never increased in strength. One night we were surprised to find several dozen newly eclosed callow ants running sluggishly over the entire raiding route (which extended up to 50 m from the nest). No emigration occurred that night. The next night the callows were again observed along with the adults on the raiding trails. No emigration occurred the second night either. We then proceeded to dig up the nest, and we found that the entire colony consisted of between 100 and 200 mature adults and approximately 50 callows. There was no large mature adult or callow population, no brood, and no queen. As far as I know, this is the first report of callow army ants participating in raiding immediately after their eclosion from the pupal stage, and it is significant that in this one case there was no colony to speak of, and hence no large source of social stimulation. In the army ants, as in social species representing many levels of invertebrate and vertebrate evolutionary history, immature individ- uals are not immediately integrated into the society of their group,



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bJ
Table I.
Significance of difference in four measures of trail-following performance between callow and mature ts) a\
adult workers of Eciton.
Colony
E.h.-6
E.h.-7
E.h.-10
Phase
day
N- 1
N-2
N-1
N -2
S-20
Dilution
0
5
10
0
5
10
15
0
s
10
0
5
10
0
5
10
No. of ants
following
C A Pf
30 29 NS
2226 NS
11 15 NS
26 26 NS
19 18 NS
21 15 NS
5 5 NS
29 29 NS
9 19 <0.05
5 12 NS
2929 NS
2427 NS
19 26 NS
29 27 NS
16 24 NS
8 19 <0.05
No. of ants Median distance Mean running completed
C A Ff
30 27 NS
16 22 NS
1 9 NS
21 22 NS
12 13 NS
13 8 NS
0 3 NS
2424 NS
2 14 <0.01
2 6 NS
2928 NS
24 26 NS
12 21 <0.05
29 26 NS
12 23 <0.05
1 14 <0.01
followed
c
48 0
472
84
476
468
468
148
476
172
200
48 0
480
468
48 0
472
280
(mm per ant) speed (mm per s)




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Phase
Colony . day Dilution
E.h. indicates Eciton hamaturn.
No. of ants
following
2929 NS
9 26 <0.01
3 13 <0.05
29 29 NS
17 24 NS
10 13 NS
18 16 NS
13 15 NS
4 13 <0.05
4 7 NS
30 23 NS
14 22 NS
7 12 NS
2 5 NS
E.b. indicates
No. of ants
completed
2928 NS
7 26 <0.001
1 7 NS
28 28 NS
17 24 NS
7 6 NS
11 8 NS
8 9 NS
0 3 NS
0 0 NS
27 16 <0.01
8 16 NS
2 4 NS
0 0 NS
Eciton bu rchelli.
Median distance
followed
48 0
47 6
400
48 0
48 0
472
468
472
120
240
460
464
292
200
(mm per ant)
N - nomadic.
Mean running
speed (mm per s)
64 103 <0.001
39 68 <0.001
22 35 NS
68 116 <0.001
47 61 <0.001
30 40 <0.005
34 60 <0.001
26 52 <0.001
18 29 <0.02
22 44 <0.02
47
72 <0.001
36 56
<0.001
22 47
<0.001
34
34 NS
S - statary. C -callow. A -
Determination of P value for
adult.
NS - not significant. f -P value determined by chi square test. median distance followed and for mean running speed includes only those ants that followed some portion of the trail.




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364 Psyche
[December
but become so gradually, as a result of numerous maturational and experiential effects that occur within the colony environment. Al- though the literature on the social behavior of ants is vast, espe- cially studies concerning the pheromonal basis of sociality, beliavior- ists have largely ignored the developmental aspects of such studies. We hope that our contributions will stimulate more research on the development of social behavior in these insects. ACKNOWLEDGEMENTS
This work was supported by a grant (GB-14724) from the Na- tional Science Foundation and by a grant ( # 1375) from the Re- search Foundation of the City University of New York. SUMMARY
Comparisons were made between the trail-following performance of callow and mature adult ants representing two species of the neotropical genus Eciton. The behavioral immaturity of the callows was evidenced by the fact that their running speeds over the trails were significantly slower than those of the older ants. Nevertheless, the number of callows able to follow along any portion of the trail, as well as the number able to complete the trail, was not signifi- cantly different from the older ants. Accordingly, we can not ac- count for the absence of callow ants from raiding columns during the first few days after their emergence from the pupal stage of de- velopment as due to their inability to follow the chemical trail. We hypothesize that the callows' sensitivity to chemical and tactual stim- ulation inside of the nest may serve to prevent them from participat- ing in raiding activities.
LITERATURE CITED
RETTENMEYER, C. W.
1963. Behavioral studies of army ants. Univ. Kansas Sci. Bull., 44: 28 1-465.
SCHNEIRLA, T. C.
1938. A theory of army-ant behavior based upon the analysis of activ- ities in a representative species. Jour. Comp. Psychol., 25: 51-90. 1952. Basic correlations and coordinations in insect s'ocieties with spe- cial reference to ants. Colloq. Internatl. Cent. Nat. Rech. Sci., 34: 247-269.
1971. Army Ants: A Study in Social Organization. (W. H. Freeman & Co., San Francisco).




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