Problem of Behavioral Plasticity in Slave-Making Amazon-Ant Polyergus Rufescens Latr

ACTA NEUROBIOL. EXP. 1978, 38: 113-132 PROBLEM OF BEHAVIORAL PLASTICITY IN SLAVE-MAKING AMAZON-ANT POLYERGUS RUFESCENS LATR. AND IN ITS SLAVE-ANTS FORMICA FUSCA L. AND FORMICA CINEREA MAYR Janina DOBRZARSKA Department of Neurophysiology, Nencki Institute of Experimental Biology Warsaw, Poland Abstract. Changes in the natural behavior, expressing adaptation to cohabitation in a community, were observed in the slave-making ants P. rufescens and the slave-species F. fusca and F. cinerea. After a few raids, the initially undirected arousal evoked in the slaves by the amazons' raids, begins to acquire attributes appropriate to the situation, but completely different in both studied slave-species. F. cinerea picks up the pupae abandoned on the nest by the slave-making ants and eventually begins to wrench them away from the amazons arriving with prey. F. fusca, whose nests have openings so narrow that it prevents the mass entry of amazons with prey, begin to enlarge those openings shortly after the amazons return. After a certain number of raids, F. fusca begin to enlarge the openings immediately after the departure of the amazons for the slave-raid. The amazons, on their side, adjust soon to the specific behavior of the given slave-species; in nests with F. fusca, they make use of enlarged openings, carrying their prey through them into the nest; when F. cinerea are the slaves, most of the amazons begin to drop the pupae on the nest, and later even surrender the prey to the slaves who meet them. It is supposed that in all three species, under the influence of specific conditions there occurs the process of learning of new forms of behavior. INTRODUCTION Social parasitism in ants is the only biological phenomenon of its kind and as such it can and should be a source of scientific inspiration not only for ethologists and evolutionists, but also for theorists of general biology. The importance of social parasitism to the process of theory building was already appreciated by Darwin. Unfortunately, this phenomenon has not been yet thoroughly investigated and almost every attempt lo describe it scientifically produces different and frequently contro- versial opinions. What is more, the opinions on some aspects of P. rufescens' behavior are contradictory (e.g., the issue of raid-making me- chanisms), and after many years of studies we have even begun having serious doubts about some opinions hitherto regarded as indisputable. It concerns, among others, the widely held opinion about the lack of behavioral plasticity in the amazon-ants. The only social function performed by P. rufescens ants is raiding the nests of the Serviformica species with the aim of abducting and bringing back new slaves in the form of pupae. The whole behavior of those ants is subordinated to this one goal. It is therefore understandable that the studies on P. rufescens have been primarily or exclusively con- cerned with the elements of behavior connected with the raids. They are characterized by an established, stereotyped manner of operations with hundreds of soldiers which enables the column to move at an extremely fast pace and which gives the invaders a great advantage over the enemy (5, 8). The stereotyped manner in which those raids are carried out has been influential in the shaping of the opinion that the amazons have little behavioral plasticity and operate on the basis of a rigid chain of reactions. Thus Fore1 wrote about "blind instict of Polyergus" (8, p. 189); also in one of our works (5) we spoke of the "rigity of behavior and lack of plasticity" in P. rufescens. But with more research being done on this species serious doubts arise about the correctness of such an one-sided view. In search of manifestations of plasticity in the slave-making ants we have analyzed those aspects of their life which can be regarded as demanding most plasticity - i.e., the aspects of their social coexistence, or specifically in our case, the cooperation with slaves during the raids. In order to distinguish the aquired adjustments from the inborn ones, we have searched for possible differentiation in the slave-making ants' behavior contingent upon the possession of a given species of slaves, i.e., for differences dependent on the reactions of nestmates with whom the amazons come into contact in various situations of social coexistence and cooperation. We have studied comparatively the behavior of P. rufescens living in colonies with one of two frequently occurring slave-species which differ sharply in their behavior: F. fusca L. and F. cinerea Mayr. Changes in slave-ants behavior were also simultaneously observed. The basic biological facts about the studied slave-species Formica fusca L. It is a timid ant who seldom fights, usually reacts to danger by flight, and when raided in its own nest it carries its offspring away. A proportion of the fleeing workers and nurses with the offspring escape in the upward direction, taking refuge on any protruding objects, mostly en grass blades. In the case of attack by the ants P. rufescens it is a very effective form of defence because the amazons never pursue the escaping ants. Those attributes of F. fusca are also reflected in -the structure of their nests. The nests are carefully concealed and have one or, at the most, two to three exit, holes placed close to each other, hidden in the grass and scrupulously covered with grass blades and twigs. In this way there are only narrow slits left on the outside through which only a single ant can pass. This species is not very predacious, the ants do not help each other in transporting the prey and the foragers usually being in only a small prey, which does not interfere with squeezing through the narrow entry hole. There is only a sporadic movement of workers around the nest; for most of the time all is quiet. Under the circumstances, the nest of F. fusca is virtually indetectable by sight. The entrance holes are full open only once in the year: during swarming, when the workers uncover and extend them. Formica cinerea Mayr. Both the biological characteristics and the behavioral attributes of this species add up to a completely different picture. It is a polycalic ant, which means that it builds multi-nest corn- plexes. Numerous daughter-nests are built around the original mother- nest linked with it and among themselves by strong community ties. The inter-nest relationships have not been thoroughly studied in F. cinerea, but only in other polycalic species of the same genus: F. rufa group (14), and F. exsecta (4, 10). On the basis of numerous observations we have reasons to believe that in principle they resemble those found in E'. cinerea. It means, simplifying slightly, that all nests of the colony have a common, interchangeable working caste. The nests are not made up of permament set of individuals; the worker can work in any nest and is treated eveEywhere in the colony as an equal nestmate. Probably the polycalic way of life is responsible for the sense of security in those ants. Their colonies occupy sometimes very large areas, completely controlled by their inhabitants. There is a continuous movement throughout the entire area of the colony. F. cinerea know how to fight when raided in their own nests and being excellent buildng workers, they frequently employ a defensive tactics of barricading themselves inside their nests. Of course, with barricading tactics it is possible to achieve certain results in the form of saving a )part of the nest only when raided by F. sanguinea. This form of defense cannot be employed against the raid by P. rufescens because they. seize a nest in a few seconds, which does not allow the raided ants enough time to dig themselves in. METHODS Marking. The most active ants in a given nest were initially marked with dye. It was impossible to mark the ants with wire rings during the observation because with this method it is necessary to catch the ants which in the circumstances inevitably raises alarm in the nest and upsets the whole situation. Marking with dye, on the other hand, has the advantage that it can be done while the ant is running; but in our case it proved particularly difficult, so, as a result, only a limited number of individuals were marked in every nest. The difficulty was mainly due to the fact that the individuals to be marked were highly aroused and that all three studied species move very fast, particularly F. cinerea, which belongs to the fastest Polish ants. When there are a great many ants moving around at a fast pace, the marking is further limited by the fact that it is possible to touch a given ant with a grass blade (used as a paint-brush) only once; if the ant is not painted at the first try, the attempts to mark it should be abandoned because of the possibility of making a mistake in a dense crowd and also because chasing after the ant always causes alarm, and aggression and upsets the situation. Whe- reas a quick, single touch, is not noticed by an aroused ant. As far as the circumstance permited (i.e., in cases when the same ant was found in another situation before the dye had come off) the marking was suplemented by a wire ring. The marking of the slave-making ants with rings is more difficult and time-consuming than the marking of other species. The trouble and the loss of time is caused by the fact that the captured amazon-ant drives its thin mandibles into the experimenter's finger and it is impossible to pull it off without damaging it.

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