Edward O. Wilson.
Trail Sharing in Ants.
Psyche 72:2-7, 1965.

Full text (searchable PDF, 1236K)
Durable link: http://psyche.entclub.org/72/72-002.html

The following unprocessed text is extracted from the PDF file, and is likely to be both incomplete and full of errors. Please consult the PDF file for the complete article.

TRAIL SHARING IN ANTS
BY EDWARD 0. WILSON
Biological Laboratories, Harvard University INTRODUCTION : THE KINDS OF TRAIL SHARING A very few cases have been recorded of ant workers regularly utilizing the trails of other ant species. Forel (1898) designated as 'parabiosis" the following complex behavior that includes trail sharing. Colonies of the Neotropical rain forest species Crematogaster limata parabiotica Forel and Monacis debilis (Emery) [~Doli- choderus debilis var. parabiotica Forel] commonly nest in close association, with the nest chambers kept separate but interconnected by passable openings; while the workers forage along common odor trails. Wheeler ( 1921 ) confirmed the phenomenon and showed that, in the one instance where he observed food gathering, the two species were attending membracids together. Wheeler also discovered a similar association between Crematogaster parabiotica and Campo- notus fernoratus ( Fabricius) . Both species were observed utilizing common trails and gathering honeydew from jassids and membracids on the same plants, as well as nectar from the same extrafloral nectaries of Inga. Not only were the Crematogaster and Camponotus workers tolerant of each other in this potentially competitive situation, they were on quite intimate terms. They "greeted" each other with calm antennation on the trails, and on three occasions Wheeler observed Camponotus actually regurgitating to Crematogaster. It has not been established whether parabiosis is mutualistic or parasitic in nature. The distinction must be a subtle one in such a complicated relationship. The form "parabiotica" of Crematogaster limata is evidently always associated with other ants. If future taxonomic studies prove it to be a species distinct from limata, it is a likely parasite. It would then be shown to be dependent on its associ- ates, while the latter species often nest and forage by themselves. But the prima facie case for mutualism seems even stronger. The broods are never mixed, and as Weber (1943) points out on the basis of his own studies, all of the parabiotic species participate vigorously to- gether in nest defense. There is no evidence that the presence of the Crematogaster harms the other species, except possibly by competition for the same food resources. On the contrary, Camponotus fernoratus Manuscript received by the editor January 5, 1965, 2




================================================================================

19651 Wilson - Trail Sharing 3
maintains flourishing populations in localities where virtually every colony lives in parabiosis with Crematogaster. While the Neotropical parabionts are doubtfully mutualistic, the relationship of the European Camponotus lateralis (Olivier) and Crematogaster scutellaris (Olivier) can be classified as weakly parasitic. Goetsch ( 1953) and Kaudewitz ( 1955) have described instances in which Camponotus workers followed the Crematogaster trails in large numbers to the Crematogaster feeding grounds and exploited the same food resources during the same time of day. The Crematogaster were hostile to the Camponotus, which assumed a crouching, conciliatory "Wartestellung" on meeting the host workers. Unlike the Neotropical parabionts, the two species nest separately. Moreover, the relationship is not obligatory on the Camponotus lateralis, since the colonies of that species are often found far removed from Crematogaster colonies.
I will now describe a third example of trail sharing which I recently discovered between the dolichoderine Azteca chartifex Fore1 and formicine Camponotus beebei Wheeler. This case is of additional interest in that it seems to illustrate a close approach to the third or neutral class of symbiosis, namely commensalism. AZTECA CHARTIFEX AND CAMPONOTUS BEEBEI
During a trip to Trinidad, West Indies, in 1961, my attention was drawn to Camponotus beebei, a formicine ant previously known from only several specimens collected in Trinidad and British Guiana. On each of three occasions on which the species was encountered, twice at Spring Hill, Arima Valley, and once near Cumuto Village on the Aripo Savanna, workers were found running over tree trunks along the odor trails of the much more abundant and aggressive dolichode- rine Azteca chartifex. The Camponotus were never found away from the Azteca trails. Extended observations at Spring Hill revealed that the Camponotus always followed the Azteca trails for long distances with fidelity equal to that maintained by the Azteca them- selves. That this was true trail symbiosis was further evidenced by the fact that no other alien species remotely approximated such behavior.
Workers of several other arboreal species occasionally blundered into the same Azteca files but ran abruptly away without tracing the main route of the files.
One of the Spring Hill Camponotus nests was located. It was in a dead, hard branch of a mango tree that had fallen and lodged in the crown of a three-meter-tall grapefruit tree in a citrus plantation. The Camponotus workers were seen to emerge from their nest holes, run



================================================================================

4 Psyche [March




================================================================================

19651 Wzlson - Trail Sharing 5
down the mango branch to the branches of the grapefruit tree, which held an Azteca colony, and follow the Azteca trails to the ground. The Azteca workers seldom ventured up to the Camponotus nest. The Camponotus occupied scattered flat galleries in the mango branch. When cut apart the nest yielded 2 winged queens, 16 males, 6 major workers, 36 minor workers, and several larvae and pupae in various stages of development. The mango tree, from which the Camponotus colony fragment had evidently recently fallen, was also occupied by Azteca chartifex. In a second locality at Spring Hill, Camponotus workers were tracked up into the foliage of a tonka bean tree (Dipteryx sp.) beyond a large Azteca nest, but the Camponotus nest was not found. Nevertheless, it was evidently separate from the Azteca nest.
Both the Azteca and Camponotus followed the Azteca trails to the bases of the nest trees. Presumably both foraged extensively on the herbaceous ground vegetation, but their diets were not determined. Regardless of the nature of the diets, competition between the two species was reduced by the existence of opposite die1 schedules. The Camponotus foraged apparently exclusively during the day, at the time the Azteca files were at their lowest ebb. In the early evening the number of Azteca workers on the trails were seen to increase by as much as a hundred-fold, but not a single Camponotus worker was found through several hours of searching during this time. The Camponotus workers, then, "borrow" the Azteca trails when the owners put them to minimal use. The Azteca workers on the Spring Hill trails were hostile to the Camponotus workers and attacked them on the rare occasions when the latter slowed in their running, but the Camponotus were larger and faster and usually easily avoided their hosts without causing any visible disturbance. The Camponotus were never observed to interfere with the Azteca in any other way.
On the basis of the first observations it could still be legitimately asked whether the Camponotus were merely using the same visual or tactile "landmarks" on the tree trunks as the Azteca, rather than following their odor trails. This possibility was eliminated by the following experimental result. A freshly killed insect was pinned to the trunk of a tree one meter beneath the trail along which both species were running but within the range of occasional Azteca scouts. Within ten minutes, two Azteca workers had found the insect and laid odor trails from it back to the main trail. In the next five minutes over 100 Azteca workers moved back and forth along the new trail to the insect. In the same interval three Camponotus



================================================================================

Psyche
[March
workers, a major and two minors, approached along the main trail and, on reaching the junctures of the new trails, departed down than for various distances. The major went all the way to the insect and prowled around it for several minutes before returning to the main trail. In two subsequent replications of the experiment, two of thirteen and one of five Campowhas workers passing along the main trail were deflected onto the Azteca side trails during the period of peak Axteca response to the baits. Such deviations from the main trail were never observed except at this time. It was concluded that the Camponofus respond to the Axteca communication. The following observation led to the further conclusion that the Cmponotus were tracking the Axteca oifactorially rather than visually. Occasionally around midday the Azteca were unusually scarce on the main trail, while the Campnnotus remained moderately common. Stretches of 30 to 50 on. of the trail were often bare of Azteca, but many individual Camponotus followed the established track just as well. On dose examination I found no alterations in the surface structure of the main trail, other than the postulated chemical one, that could have supplied the Camjonotw with a clue. Although the Camponotus beebei utilize Azteca trails extensively, the following observation shows that they have maintained their own, private trail system. On a single occasion in February a line of seven Campmotus were seen moving along the main Azteca trail. Four of the workers ran in a tight group directly behind the leader, frequently advancing enough to touch the abdomen of the ant ahead. When the leader was touched, it dashed forward at a faster pace over a short distance. This part of the behavior was typical of communication by s i
tandem running", which I have described earlier in a paper on the genera Cardicondyla and Camponotus (Wilson, 1959). The re- maining two workers followed at a greater distance, tracing each twist and turn taken by the leader. During the next 15 minutes several other Camponofus workers passed the same way, again tracing parts of the route of the leader with dose fidelity. After that time, new Camponotus workers continued to run on the Azteca trail but ignored the Camponotus trail.
There could be no doubt that the
lead ant had secreted an odor trail of the recruitment type (see Wilson, 1963). It was laid on top of the Axteca trunk trail, which for most of its length was about 10 centimeters wide. Equally interesting was the fact that only the Cmnponotus responded to it. The Asteca workers continued to pass along their own trail during the episode but failed to orient to the inner track followed so closely



================================================================================

19651 Wilson - Trail Sharing 7
by the Camponotus. Thus the Camponotus workers appear to respond to two odor trails, while the host Azteca respond only to one. This study was supported by a grant from the National Science Foundation. The figure was prepared by Mrs. H. C. Lyman. ABSTRACT
Trail sharing is a rare event in ants. Of two previously described cases, one is interpreted as part of a relationship that is either mutu- alistic or weakly parasitic, probably the former, and the other as part of a weakly parasitic relationship.
A third, new case has been discovered which appears to be co'm- mensalistic. On Trinidad, West Indies, workers of the rather scarce formicine Canzponotus beebei utilize the arboreal odor trails of the abundant dolichoderine, Azteca chartifex. The Camponotus "borrow" the latter's trails during the day, when Azteca foraging is at a low ebb. The Camponotus workers are treated hostilely by the Azteca workers but are too swift and agile to be caught; their presence does not disturb the Azteca seriously. On a single occasion Camponotus workers were observed to lay their own private recruitment odor trail on top of the Azteca trails. The Camponotus trail lasted for about fifteen minutes and had no visible effect on the Azteca. FOREL, A.
1898. La Parabiose chez les Fourmis. Bull. Soc. Vaud. Sci. Nat., 34: 380-384.
GOETSCH, W.
1953. Vergleichende Biologic der Insekt-Staaten. Geest und Portig K.-G., Leipzig. 482 pp.
KAUDEWITZ, F.
1955. Zum Gastverhiiltnis zwischen Crematogaster scutellaris 01. mit Crematogaster lateralis bicolor 01. Biol. Zentralbl., 74: 69-87. WEBER, N. A.
1943. Parabiosis in Neotropical "ant gardens." Ecology, 24: 400-404. WHEELER, W. M.
1921. A new case of parabiosis and the "ant gardens" of British Guiana. Ecology, 2 :89-103.
WILSON, E. 0.
1959. Communication by tandem running in the ant genus Cardio- condyla. Psyche, 66 : 29-34.
1963. The social biology of ants. Ann. Rev. Entomol, 8:345-368.



================================================================================