ACTA NEUROBIOL. EXP. 1916, 36: 545-559

THE FORAGING BEHAVIOR OF THE ANT MYRMIC.!A LAEVINODIS NYL.

Janina DOBRZANSKA and Jan DOBRZANSKI

Department of Neurophysiology, Nencki Institute of Experimental Biology Warsaw, Poland

Ab'stract. The foraging territory is not divided among particular foragers, a t least in the young, small families of M. laevinodis. The same individuals may be observed repeatedly only at the aphides over a period of several days, which may be explained by a transient me- morizing of the source of attractive food. Along with the faculty of these ants to inform their nestmates of the food source, a certain continuity in care of the aphides is ensured. Various forms of behavior among particular foragers coming in contact with the trophy exceeding their strength were noted. The less time the ant hesitates before to the nest, the sooner and more efficiently it recruits helpers and the smaller is its dependence on its own odor trails when moving on the terrain. Since there exist transitory forms between diametrically op- posite manifestations of such behavior, possibly the process of indivi- dual acquirement of capabilities necessary for fulfilling foraging func- tion occurs.

INTRODUCTION

Our observations of numerous nests of Myrmica laevinodis in various biotopes revealed a great dependence of the nests' structure on the variability of the biotopes. This is in accordance with the observations of Stawarski (16) who described plasticity in the adaptation of this species to different biotopic conditions: on the open and sunlit areas the nest structure penetrates into the ground to about 0.5 m, but i t becomes shallow as the number of trees and the density of the bushes

546 J. DOBRZANSKA AND J. DOBRZANSKI

rise. As we have observed, those adaptative changes are not limited exclusively to the depth of the nest. The greater the number of trees in a given area, the smaller the part of the nest's structure performed by the ants themselves: to a growing degree they make use of natural shelters, such as bedding and undergrowth. Eventually - in forests with very thick bedding or very rich undergrowth-. the nests become reduced to mere hiding-places under a layer of dried leaves or in the moss (various transitional forms of nests can, of course, be found). This observation was already made by Wengris (18). Such hiding-places do not seem to require of the ants any building operations. Needless to say that so far we referred only to summer vegetation, as winter nests are always dug deep in the ground.

Setting up a new nest without the necessity of performing any build- ing operations is presumably somehow related to a great tendency of M. laevinodis to leave its previous nest and move with its whole "belongings" to a new place. However, according to Pisarski (personal communication) the tendency to migrate and build nests in a sloppy way may appear only in young, small families of M. laevinodis. As the family grows nest-building may require greater efforts and, conse- quently, migratory tendencies may decrease. Therefore, our results obtained in this investigations may only be referred to small families and it is reasonable to assume that penetration of the terrain is diffe- rent in greater societies of the species.

In the small nests we examined, it was enough to apply a single negative stimulus (such as for example lifting up the bedding in search for a nest underneath) to make the ants leave for good. The activity of M. laevinodis foragers in the vicinity of the nest is usually so weak, that by merely watching the nest it is hardly possible to determine its precise locus. In order to make sure about the nest position it is inevi- table to lift the bedding or draw the moss aside a t least once; that is why, as a matter ot fact, each examined nest should be under constant round the clock observation. However, it is technically impossible and the experimenter is never sure that the nest has remained in its origi- nal place when he resumes his everyday observations. The experimenter refrains from looking deep into the area of the nest lest he should frighten the ants away. Thus, in effect, he may spend several days of observation in vain over abandoned nests. Since the removal is gene- rally limited to carrying the "belongings" to a new place, without any building or cleaning operations, it is a matter of pure chance to find the new location of the colony. In this way it is easy to lose the object which might have required of the experimenter weeks of preliminary work (including individual marking).

FORAGING BEHAVIOR I N ANTS ,547

This tendency to constant removals suggests that there is no division of territory among the foragers. The assumption required experimental verification.

THE PROBLEM OF DIVISION OF TERRJTORY

Ants found in particular areas of the territory, mainly foragers, were individually marked with the use of wire rings. The method of marking numerous individuals, using one, two or three colored wires, is applicable to Myrmica species owing to a special structure of the petiolus in these ants (7). During 2 or 3 days all ants entering a freely selected area of the territory (15 X 35 cm) were marked. Next, the experimental area was observed for 2 to 3 wk and the vicinity of the examined nest was scoured (the experimenters chose nests in areas with rather scant undergrowth to facilitate observation). During three sum- mer seasons approximately 1,100 individuals were marked in that way. From among that number of ants only 56 were noted for the second time and still less of them (only 11) were found in the same area where they were originally marked. To make a comparison: it is worth-while to remind that with Formica rufa species, marking 10 workers on such an area is sufficient for the experimenter to find the marked ants within that area any time during several weeks (2).

After termination of a given series of experiments, we checked if the given nest not moved away in the mean-time because very few notes of formerly marked individuals could have resulted from the tendency of M. laevinodis to move from one nest to another. It was found that 73O/o of nests under control were left in their original locus. The marked foraging workers belonging to those nests totalled 719. The 11 individuals observed for the second time in the area form a negligible percentage of that number, which shows the accidental character of their staying on the territory.

I t was yet important to see whether wire-marking was or was not harmful or even lethal to the ants. Several nests were opened at the end of the observations. In every nest where a t least 20 ants have been marked, we found that marked individuals behaved during panic in the nest just like their non-marked nestmates. This fact was in agree- ment with our expectations, for Myrmica ants have a very hard chitin, and in our previous studies we managed to mark harmlessly not only Formica (4, 6) but also Leptothorax (5) and even the especially soft Lasius niger (8).

Thus, it seems justified to conclude that a very small individuals found in the same foraging area during observations undoubtedly points

548 J. DOBRZAmSKA AND J. DOBRZANSKI

out to the lack of division of territory between M. laevinodis workers. From studies of several species of ants it appears that individuals whjch take care of aphides are conservative: they are attached to their func- tion and they perform it always in the same place (2, 11-15). It was therefore important to see if there is such an attachment to place in aphides-rising M. laevinodis individuals. We marked the ants visiting the aphides and then watched them over a period of several days. Sometimes we managed to see the same ants again in the same place; that was the evidence of a slightly greater attachment to the place of work than in the foragers. However, in this respect the behavior of M. laevinodis cannot be compared -neither quantitatively nor qualita- tively- with the behavior of Formica species, where some individuals remain with the aphides for weeks in the same place (3). M . laevinodis ants can be found only for 3 to 4 days after marking, and then they vanish definitely. Table I shows the data obtained by marking the representatives of six colonies of M. laevinodis noted to visit the aphides.

TABLE I

Number of M. laevinodis workers visiting aphides

Successive days of observations

1 1 2 / 3 1 4 1 5 / 6 1 1 0 ;

Percentage of marked to 1 4 6 1 3 0 1 0 0 0 / 3.8 all ants

--

Colony

For the purpose of comparison we equalled the number of the marked M. laevinodis ants (by a suitable selection of colonies) with that of

!

1 20 7 2 5 0 5 1 6 0 4 0 4 0 5 0 1 2 4 0 5 1 7 0 4 0 3 0 5 0 2 0 3 14 3 0 4 0 6 0 3 0 4 0 3 0 4 0 4 11 5 15 6 16

Total 181 ; includ- lng 7 marked

Number cf marked

individuals

the formerly observed (unpublished data) F. truncicola species (see Ta-

Registered I I Total

8 3 % 2 * - 1

E -

FORAGING BEHAVIOR I N ANTS 549

ble 11). It can easily be noticed that F. truncicola, quite unlike M. laevi- nodis, were characterized by the stability of function and attachment to the place of work. It may only be right to say that M. laevinodis showed only a transient memory of the actual place where attractive

TABLE I1

Number of F. truncicola workers visiting aphides. Between 6th and 10th day of observations

there was a 3 day's interval

Marked

I marked

Observed

lo2

I Successive day of observations 1

All ants

Including

Total

1 70

Ratio of marked

food (i.e., aphides' excretion) was found without forming a stable habit, so typical for other species engaged in that function. This momentary recording of food placement, together with the tendency of the Myrmica workers to inform each other about the source of food and to follow the trail of nestmates that had brought the food- may be a sufficient guarantee of a certain continuity in the care of the aphides by a given colony. Yet the care is not performed by the same workers and has nothing in common with a stable division of labour and territory.

In The search for other mechanisms of penetration of the foraging territory in M. laevinodis-replacing the division of labor and terri- tory - we examined the way in which the ants find food and transport it to their nest.

INFORMING ABOUT FOOD

In order to check if M. laevinodis ants are able to inform their nest- mates about food they had found, we made a series of observations according to the model which has become popular in myrmecology (9).

At a distance of about 0.5-1.5 m from the nest we put a dead fly which served as a bait, because, according to Holldobler ( lo) , M. laevi- nodis is not a pure predatory ant and willingly picks up dead insects. The bait was pinned to the ground with pine needle to prevent the ant from lifting the fly. As it was pointed out before, M. laevinodis workers penetrate the environment sporadically, and that is why the bait may not be found by the foraging ants and, instead, taken by the ants of

550 J. D O B R Z A ~ S K A AND J. DOBRZAmSKI

another species if put in a quite random place. Therefore we tried to put the bait directly upon the way of one M. laevinodis ant which was seen penetrating the area. It should be added here that by doing so we could not be sure that the ant would become interested in the bait. The reason is that many a time upon touching the bait the ants walk away from i t and continue their chaotic penetration of the terrain. Some individuals escape from the bait after touching or noticing it (in the latter case they stop walking and direct their antennae towards the bait at a distance of several up to ten of millimeters). Such ants were disregarded as irrelevant to the investigated problem and only the ants that were undoubtedly attracted by the bait are referred to in this paper. Those ants were called primary foragers (PF) and marked indivi- dually with paint. The best moment for marking is when P F is beginn- ing to jerk the trophy in order to move it. At this very moment the ant is so busy with its job that it generally does not notice being touched with a blade of grass with some paint on it. When the workers' contact with the bait is of very short duration, they must be marked on the run, although it frightens them away for several seconds.

Most often PF quits the trophy and walks towards the nest after a few seconds or minutes of unsuccessful jerking of the bait. The route of the PF is marked with pine needles. A moment after the P F has entered its nest several workers can be seen to leave it and find (usu- ally immediately) the trace of the P F which they follow up to the trophy. These ants are called secondary foragers (SF). In some cases they are accompanied by PF. This is not indispensable' for the SFs, which easily manage to get to the trophy on their own. A good and simple test of what guides them may be obtained by sweeping the PF's way to the nest: the SFs seem to lose the trail whirl for a while in one spot and return to the nest. A guiding role of the odor trails left by PF on its way (1) may therefore be concluded.

It happens that an extremely excited P F calls out from the nest too many nestmates; those excess ants return home within a few minutes. Our records (see below) point clearly to some regularity in that beha- vior of the SFs': in dozens of our observations of M. laevinodis at the capture the number of individuals gathering around the capture was -- after a lapse of time -- always limited to about 10. This would occur regardless of the initial quantity of ants, no matter whether they were recruited or came upon the trophy by chance. One possible explanation is that a medium-size trophy is simply too small to offer room enough for more ants to gather around; the workers that failed to get close to the food must return home. This assumption was verified in some observations during which several flies, instead of a single one, were put one beside another, in a line 2.0-3.0 cm long. This pro-

FORAGING BEHAVIOR I N ANTS 5 5 1

cedure resulted in increasing the number of recruited foragers, which were sometimes called to the bait two or even three times. However, most of them only walked around the bait, as if explosing the surro- unding area in search of more food. Only 8 to 10 (12 at the most) ants got together directly at the trophy. Provided no other capture was found (as in Observation l ) , those supernumerary ants stopped walking back and forth within several minutes and returned to the nest. It was only once that the authors managed to bring to the food 18 ants, with a total of more than 40 individuals recruited.

The phenomenon described above seemed worth mentioning, despite the fact that so far no satisfactory explanation has been arrived at. Sudd (17) described a similar regularity in Fheidole pallidula. In that species the maximum number of workers a t the bait never exceeded 50. Maybe this regularity is biological or ethological in essence.

Below are shown some examples of records made during observa- tions of information behavior in M. laevinodis (in each observation by the term "PF" only the relevant primary forager was labelled).

Observation 1 h o u r s : 10:44 P F approaches the bait and immediately leaves it without even

trying to lift the capture. 10:46 P F enters the nest. 10:47 A group of 15 workers leave the nest. They walk in file

towards the bait following the trail left by PF, which is absent in the group.

10:48 First recruited SFs reach the bait. 10:51 SFs immediately quit the bait, 12 remain. P F is walking at

a distance of 10 cm from the bait. I t came to the area of the bait by a roundabout way, i.e., it did not follow its own trail.

10:55 P F approaches the bait and goes away right afterwards. 11:OO 'There are 11 ants around the bait; about 40 are walking in the

neighborhood besides those marching along the trail. 1l:Ol Another bait has been put at a distance of 40 cm from the

first one. I t was immediately approached by a nearby walking ant - PF,.

11:02 PF, called out 3 workers from the circling crowd and brought them to the second bait.

Observation 2

17:33 P F approaches the bait. 17:48 P F walks back to the nest. 18:03 P F returns to the nest-alone.

J. D O B R Z A ~ S K A AND J. DOBRZARSKI

Observation 3

PF approaches the bait and begins to jerk it. PF keep on jerking the bait until 18:55. In the meantime 4 other ants pass by. They seem to have come across the bait by chance.

Observation 4

PF scents out the bait which is 5 cm away, approaches it and examines it with its antennae. Another P F comes along. One of the PFs is frightened away at the attempt of marking it. No other ant has come to the bait so far. Ten workers come within several seconds. It is possible that they were called by an unidentified ant.

Observation 5

Two baits have been placed at a distance of 10 cm from one another. PF1 escaped from the first bait for several minutes when being marked. PF, comes to the second bait. A moment later it goes to the nest and returns to the bait bringing about 20 SFs. PF1 finds again the first bait and rushes to the nest at once. PF, comes across the SFs recruited to the second bait and joins them. Before coming to the second bait PF, turns to the first one. PF, does not reach its bait and starts walking around.

Observation 6

Primary forager jerks the fly until 13:02.

Observation 7

PF approaches the bait and leaves it shortly afterwards. lev en workers leave the nest and walk toward the bait. PF is not present in this group. Four SFs of the group reach the bait (found in the meantime by another group of 7 ants) and 3 others keep on walking to the fly. They are 15 cm away from the bait. PF returns to the fly. It has not accompanied the SFs nor followed its own traces. The total number of ants at the bait is now 10 (not counting PF). Two more are walking nearby.

FORAGING BEHAVIOR I N ANTS 553

There are 17 ants at the bait and 3 more are coming up. The actual number of ants is 22 and 4 more are approaching. The ants cluster densely around the fly; some of them walk away. Eight ants remain at the bait; the rest go back towards the nest.

Observation 8

The bait is approached by the ant which was marked as P F the day before. Two more ants come up. Four ants are gathered at the bait. They tear the fly away from the needle and carry it to the nest.

Observation 9

PF approaches the bait and goes back to the nest immediately afterwards. Four ants set out of the nest and follow PF's trail. Seven workers are already at the bait to which came PF- by another roundabout way SFs recruited by the P F followed its original trace (from bait to nest).

Observation 10

P F comes up to the bait. P F returns to its home nest. Five workers walk to the bait following precisely the PF's trail. SFs reach the bait. They are closely followed by PF, moving along the original way.

Observation 1 I

P F reaches the bait. P F returns to its home nest. Eleven workers leave the nest; PF is with them. Sixteen ants are gathered at the bait. Among them is the P F marked in previous observations. A few ants come back to the nest bringing nothing. Twelve SFs are still at the fly. Among them are both the PFs: from the present and former observations.

654 J. DOBRZANSKA AND J. DOBRZANSKI

Observation 12

16:05 P F approaches the bait, jerks i t for a while and walks back to the nest situated at a distance of 1.5 m from the pinned fly.

16:08 P F leaves the nest after 15 s stay in it. It returns to the bait by the original route and is followed by 23 workers and their queen, going one behind another. The train of the marching ants extends. Some of the ants return, so does the queen. The remaining SFs keep on walking along PF's trail but at some distance from their predecessor.

16:l l P F starts to circle in the neighborhood of the bait before com- ing to it. In the meantime nine SFa reach the bait following PF's original trace.

16:28 P F comes up to the fly for a moment and then resumes explo- ration of the environment.

A total of 180 observations of that sort have been performed. I t is obvious that there is a great differentiation of workers' beha-

vior in relation to food which is so heavy that the ants are unable to move it. There is no stable pattern of behavior of the primary foragers whose task is to lift too heavy load. From the standpoint of forms of behavior two different basic groups of PFs can roughly be distinguished. (i) ants in the first group grapple with the bait for a long time despite its resistance, (ii) the second group is represented by workers who call for help (recruitment) of their nestmates as soon as they realize that they are incapable of lifting the trophy themselves. Extreme represen- tatives of the two groups behave in an entirely different manner. On the one hand there a r t foragers who jerk the trophy all alone until they manage to tear it up i;nd carry the pieces to the nest (this happens rather rarely), or - usually - until other foragers or even alien ants come by chance and take the bait with them. On the other hand there are individuals setting about recruitment right after they have touched the bait with their antennae. These ants don't return to the bait and, instead, they go another way and get busy with penetration of the terrain.

These differences are exposed in Table I11 which, by necessity, in- cludes only a few typical observations. The observations are arranged according to the time spent by P F a t the bait without calling for help (see h'orizontal column A). On the left of Table 111 (Observations 3 and 6 ) there are the longest times recorded, whereas on the right of the Table (Observations 1 and 13) there are opposite cases, in which the ants immediately ran for help without even trying to move the capture. I t is interesting to note that with such an arrangement of the

a S 0,

Dependence of PFs' behavior on the duration of the primary visit to the bait --

Co- lumn Behavior of PFs and SFs

Successive numbers of observations

42 rnin

2 1 37 --

3 2 min min

15 rnin

12 min

duration of primary visit of PF to the bait

PFs did re- turn to nest

, 0.5 0.5 a few left min min se- imrne- rnin

yes PFs did not return to nest

presence of P F among SFs walking to bait initially yes I yes

but come back to bait with- 1 1 1 1 out SFs

- I - bait destination of PF after leaving nest

conds

PF's route after leaving nest

dia- tely

no

fol- lowed

its own trail

no no no

at first followed its own trail

round -about

round -about

followed its own

trail

556 J. DOBRZANSKA AND J. DOBRZAN'SKI

data in column A, the data in the remaining columns (B-D) appeared to be automatically arranged in a logical sequence. Such an arrange- ment of the data enabled us to note the dependence of the PF's beha- vior after recruiting its nestmates on the period of time spent by this P F alone at the bait, prior to the recruitment. It appeared that: the shorter the PF's prerecruitment time (col. A), the smaller its tendency to return to the bait with other ants (col. B); the quicker and more efficient its recruitment action, the less dependent is the P F from its own odor trails when it moves in the area, no matter whether it goes back to the bait or not (col. C and D). The second finding speaks either for the PF's very good knowledge of the foraging territory, or for its ability to move in this territory using its own traces and those of the nestmates. Thus the workers of the first group (those grappling with the bait for a long time) were not only unable to estimate the weight of the trophy (col. A) and recruit their nestmates (col. B), but also lacked good orientation in the territory (col. D). On the other hand, the workers who recruited immediately (second group) had all these abilities. There was a number of transitional forms between the two groups. For the total number of 180 observations, 118 were complete, i.e., covered all the stages of the foragers' behavior. Table IV shows

Division of all PFs according to their behavioral groups

Percentage of the I I

total 1 17 48

of successful

observations

the numerical distribution of ants representing different forms of re- cruiting behavior.

In our investigations of information among ants we did not record or analyse the instances in which some individuals were indifferent to the bait. This indifference could sometimes have resulted from giving the ants an improper species of fly as a bait. Most certainly that could not occur too often, since other, more active workers would be lured to the fly. Unfortunately we did not mark individuals uninterested in the bait which soon proved attractive to their nestmates, For that

PFs not informing nestmates (group 1)

I

Transitional forms of behavior I PFs informing - ' nestmates immedia-

tely and not return- Calling for help Bringing help by ing with them to the

unsuccessfully bait (group 2)

FORAGING BEHAVIOR IN ANTS 5.57

reason, until further observations are carried out, we can only suppose that those ants were beginners in the external service. Moreover, their fear of unknown environment might have been strengthened by a typi- cal food (flies in the stage of imago). The use of a typical food is, on the other hand, advantageous to our investigations by making the observations of the behavior at the bait (described in Table I11 and IV) still more significant. Both the long-lasting jerking of the bait (until it is moved) and the recruiting of nestmates to the bait, are undoubtedly good evidence of the fact that the ants are actively interested in food and in various modes of carrying it to the nest. No objective external reason can be found to account for so different forms of behavior among various individuals in quite similar situations as those presented in Table I11 and IV. Therefore a subjective inherent reason must be sought. Thus the question arises whether the Tables reflect some static, indi- vidual differences in the behavior of the ants or a dynamic process of acquiring the above cited abilities by each ant as they gain experience.

In favour of the existence of such a dynamic process speak the transifional forms of behavior of such foragers (Observations 2 and 37, Table IV, col. 3) that set off for help but failed to recruit their nest- mates, and other foragers (Observations 5, 10 and 11, Table IV, col. 4) which succeeded in recruitment but then strictly depended on their own traces when moving in the territory and finding the bait again. All those transitional forms of behavior are, once more, arranged logi- cally, in accordance with column A of Table 111, i.e., with the duration of PF's visit to the bait.

It seems reasonable to treat the above described process as a process of individual acquisition of skill and experience, so indispensable for the role of a foraging ant. If this assumption were acceptable then Tables I11 and IV would nicely illustrate the growing experience of the foragers and show how individual ants have increased their abilities to move in the environment, recognize the trophy and recruit the nest- mates. It must be emphasized here that the way in which those func- tions are performed is not stereotyped but situation-dependent. An appropriate example is provided by the forager which in one case (Observation 7) was seen to recruit its nestmates just after finding the trophy and the very next day (see Observation 8) did not go to the home nest at all. This sudden change in behavior was caused by the fact that three other active ants came spontaneously to the bait within a minute or so after PF's arrival at the fly. With the help of the new- comers the bait could eventually be torn to pieces. On the other hand, PF, in Observation 1 brought helpers not from the nest but directly from the territory where too many ants had gathered at another bait.

558 J. DOBRZARSKA AND J. DOBRZANSKI

In the third case (Observation 5) a forager was observed to return to the bait (from its way home) with the nestmates that had already been called by another ant.

Also the presence of strange ants, especially T. caespitum, in the vicinity of the food, has a great impact on the behavior of M. laevinodis foragers: the latter never call for help in the presence of other species of ants. In such a case some of M. laevinodis ants jerk the bait for a while and then escape and never come back. Some of the others succeed in cutting off a piece of food and bringing it to the nest. The PF would never return to the bait alone or even with the nestmates if it encountered some T. caespitunl ants close to the bait. Neither would it send its nestmates there. We have had no exception to this rule in dozens of observations. That is why only areas free of T. caespitum nests should be chosen for investigation of the mechanisms of informa- tion. The presence of those tiny ants may be the source of a misleading assumption that NI. laevinodis is incapable of recruiting to food, whereas, in the circumstances, the lack of recruiting should be interpreted as a sign of great plasticity of behavior. T. caespitum ants have such a pre- cise mechanism of communication and answer the call in crowds, that they just swarm around the trophy without letting another ant come near and are so numerous as to be dangerous even for the large-sized enemies.

Hence, behavioral differences among various foragers of M. laevino- dis may result both from acquired experience and (certaisly) from individual ability of adaptation to various circumstances.

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Accepted 20 March 1976

Janina D O B R Z A ~ S R A and J a n DOBRZARSKI, Nencki Institute of Experimental Biology, Pasteura 3, 02-093 Warsaw, Poland.