ACTA NEUROBIOL. EXP. 1973. 33: 597-622 ETHOLOGICAL STUDIES ON POLYCALIC COLONIES OF THE ANTS FORMICA EXSECTA NYL. Janina DOBRZAfiSKA Department of Neurophysiology, Nencki Institute of Experimental Biology Warszawa, Poland Abstract. The members of a polycalic colony of Formica exsecta freely walk over its various nests, while performing social functions. It seems that the external service in the species ceases to belong strictly to one nest and forms a supra-nest attachment. The probability of this relation is more credible in the light of the results of this research which disclosed a tendency to local migrations. For, both these ethological peculiarities require an inhibition of the attachment to the re- stricted habitat, which seems to be characteristic of the species of the genus Formica. It seems that we are dealing here with a process of colony integration, and that we witness the extention of the social unit from the nest into the whole colony. INTRODUCTION Many species of ants, including those of the genus Formica, build multi-nest colonies, knlown in myrmecological literature as polycalic ones. Such a colony is, as a rule, developed by splitting from a parent nest (9) and, therefore, all nests in the colony are of common origin. A com- plex of families of ants, which preserve their mutual ties by common feeding areas, common paths and common breeding (of aphids i,s named a multi-nest colony. This phenomenon has been confirmed in the case of the Formica by Kiil (14), Dlussky (4, 5) and Marikovsky (16). Mari- kovsky (16) and Dlussky (3, 5) consider the inter-nest exchange of brood, which in their opinion certainly takes place in the fall, as an indispens- able feature of the colony. Inside the colonies there also occurs an inter-nest exchange of adult individuals. It is continued often during the entire period of summer vegetation. This phenomenon was described by Huber (13) in F. pra- 598 J. DOBRZANSKA tensis and by Okknd (17) in F, rufa. okland was the first \to confirm that inter-nest exchange of the inhabitants must have an influence on integration ,of the colony, preserving the mutual ties between thfe families. Besides the exchange of individuals between the parallelly existing nests of the colony, the phenomenon of migration, i.e. removal to another habitat is also known. Bath the exchange and the removals are per- formed in the way of a mutual carrying, ,as was already described by Huber (13) and Adlerz (1). The up-toidate description [of this phenomenon is included in the monograph written by Wilson (19). The ant-workers engaged with removal carry in their mandibulae the brood, the sexual individuals and their own fellow-workers. Arnoldi (2) was the first who found that the consistent postures assumed by the ants being carried show differences characteristic of taxonomic groups. The formation of polycalic colonies is, as a matter of fact, the first example of the remwal since splitting from the nest is such a partial removal. During a period of prosperity and, consequently, an increase in population 'of the nest, a drive for establishing a new nest and oarry- ing to it some of their fellow-ants and their brood is developed in some workers. Transfe~redto a new place, the workers return, however, in great numbers to the parent nest, frequently carrying the brood. This was described by Huber (13), Fore1 (10) and Wheeler (18). The process of removal is, therefore, very long, ,sometimes continuing throughout the entire period of summer vegetation. It was already Huber, who with his great penetrating capability of observation, concluded that the ants which found a new nest and are first !to move from the old one, are the most excitable individuals, the ones which are most responsive to certain stimuli. In the case of splitting, these are probably the stimuli connected with the density of population which increases in the parent nest. The drive evoked by these stimuli has to (overcome the conservatism, that is, an attachment to the ants' old habitat which is characteristic of the species of the Formica (6, 7, 18). Precisely this conservatism compels less excitalble ants which were passively carried to a new nest to return to the parent nest. The phenomenon of complete migration is also known. It may be caused by factors which (are harmful to the community and which compel to migrate from the previous habitat. Such factors may occur iln the form of sudden cataclysms as, for instance, a flood, an abtack by strmger neighbors, etc. These situations compel the ants to perform an instant escape as they pose a mortal danger to the lingering members of the community. However, there also happen factors which force the ants to a complete migration such as unfavorable meteorological conditions, a displacement from the feeding area by strmger neighbors, an invasion POLYCALIC COLONIES OF ANTS 590 of parasites, or other adverse conditions, which increase gradually. If such is the cause, in various individuals reactions may be released which depend on their individufal excitability much the same as on the exces- sive density of population. Then, the migration process resembles the first type of removal caused by an overcrowded nest, when there occurs a conflict of contradictory tendencies: some individuals are busy with the removal and some others attempt to return to the old habitat. A re- moval, revealing the existence of external phenomena harmful to the entire community and which cannot be overcome m the spot, may be recognized by, among other things, the o~bservation of the final effect: regardless of the duration of the removal it leads to the abandonment of the old nest and to the migration of the entire community to a new habitat. The described cases of a complete migratioln among ants of the genus Formica are rather rare. I. INTER-NEST RELATIONS IN A COLONY Material and method Mutual relations between particular families of a polydic colony of Formica exsecta were studied in 1967 in a 13-nest colony, henceforth called C13, the plan of which is given in Fig. 1. The colony covered about 300 m2 of a grass-covered clearing in the Solska Forest. The pre- liminary olbservations revealed that four nests (i.e. NV, NIX, NX and NXII) were uninhabited by ants at the beginning of the experiments. To study the movement of ants in the area of the colony, all indivi- duals, regardless of their functions, were first marked with dyes identic- NVl NIX N1@ NII. NY NX NU[@ NN* 25 Fig. 1. Sketch of colony C13. Dimensions of circles indicating the nests are in proportion to their actual size. ally on one of the nests (NVZ) as the were walking over it. Even pre- liminary results have revealed the necessity d an additional marking of the ants involved in definite social functiolns in particular nests (specific- ally in nests ZZZ and XIZZ). The building function has been chosen as most appriopriate fw this purpose for the following reasons: (i) it does not leave any doubts as to which of the nests is performed; (ii) the building activity of workers is confined to the dome of the nest where they are distinctly visible and, consequently, the possibility of com- mitting errors in determining the number of marked wo~kersis reduced to a ba~eminimum; (iii) the errors concenning quality are also ruled ~utin practice, that is, the building function cannot be ascribed to an ant which does any other work since building material transported by a worker is on the whole easily recognizable; in rare cases when the observer is in dmbt, it is quite enough to watch the use made by the ant of the object it is transporting to make sure that it is a building material. If necessary and practicable, the shape of the marks of some ants was recorded individually. In the present paper a nest oln which a definite group of ants has been marked is called an AMN (ants-marking nest) and the remaining ones - other nests (in relation to the former group). The observations discussed here were started withiln 2 days after ant-marking had been completed. Results According to the experimental method described above, 254 indi- viduals were marked, regardless of their functions, on nest AMNVZ, and 224 building ants on nest AMNZZI. In the next stage of experiments, all nests of the colony were observed and the presence of marked indi- viduals on them was recorded. Within a fortnight, of 203 recordings olb- tained from both groups 199 (98O/o!) came from other nests (in relation to each group). On om AMN's there were only four recordings of marked anlts. In view of the fact that all individuals met on AMNVI were marked, we can ascertain that those were ants performing various functions of external service. The numbers cited above give ample evidence that the ants marked on the two different nests, regardless of their functions, moved in large numbers to other nests of the colony. Figure 2 presents a comparison 01 the mobility of ants from the two AMN's: on the left - movement from AMNVI and on the right - from AMNIZZ. The situation of each nest in this Figure depends on its distance (see Fig. 1) fmthe AMN's (proportions have not been exactly pre- POLYCALIC COLONIES OF ANTS , , , , , , , , , , , , , I"""' F3l - 44 40 36 32 28 24 20 16 12 8 4 0 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 Observaflons Observof,brrs Fig. 2. The distribution of newcomers from the AMN's; those from AMNVI on the left, from AMNIII on the right.