ACTA NEUROBIOL. EXP. 1978, 38: 353-359
Short communication
SOME QUESTIONS RELATED TO MECHANISMS O F SLAVE-RAIDS
IN AMAZON-ANT POLYERGUS RUFESCENS LATR.
Jan DOBRZAmSICI and Janina DOBRZAflSKA
Department of Neurophysiology, Nencki Institute of Experimental Biology
Warsaw, Poland
Abstract. A new hypothesis concerning the mechanism of the
P. rufescens' slave-raids, is proposed. (i) The so-called pseudo-scouting
does not contribute to the slave-raids. (ii) The direction of the raid js
determined by the random movement of the most easily aroused
individuals - the so-called activists. (iii) Once the direction has been
set the army marches straight ahead and returns by the same road
orienting itself by visual stimuli; single individuals who lag behind can
however follow the scent-trial which had been left by the army. (iv) The
slave-capturing instinct of the amazons is not directed at the abduction
of pupae alone but also and even perhaps primarily at the abduction of
mature individuals belonging to the slave-species; that the amazons
bring almost only pupae is caused by the fact that the mature individuals
affer resistance and are consequently a more troublesome prey.
The data taken from observations and studies carried out over
a period of many years on the raiding-behavior of the slave-making ant
P. rufescens, indicate a need for a revision of the opinions expressed in
our earlier studies (6) and in the studies of other authors. It has become
necessary since the publications contain opinions and generalizations
which are almost always different and often contradict each other.
This study is an attempt to confront and discuss certain aspects of those
opinions on the basis of the published material and some new data which
we obtained an accurate picture of the behavior of those social-parasitic
ants could have important implications for many general considerations
in biology.
5
- Acta
Neurobiologiae E x p e r i m e n t a l i s
Direction of movement of an army-column in amazons. To this day,
many authors accept with outdoubt the opinion, expressed by Fore1
(8, vol. 4, p. 127) that there exist scout-ants and that P. rufescens' army
is commanded by "officer" ants. Wilson i n his latest monograph about
social insects (14, p. 368) adopts a similar position by an a priori
extrapolation onto the whole Polyergus genus, the results obtained by
Talbot (12) who has discovered the existance of scouts in American
Polyergus lucidus Mayr - and does so without taking into consideration
the data obtained by Dobrzanska. and Dobrzanski (6) and Koehler (9)
(qithuzgh he quotes both articles). However, even if there exists a very
close aifinity between two species in the same geographical zone, the
possibility of great behavioral differences can not be excluded. It is
possible to enumerate a great many examples, but it should suffice to
mention only the very closely related species of F. fusca L. and
F. cinerea Mayr. which have been the subjects of the previous work (4).
On the other hand, already i n Brun's paper (1, p. 137) it is possibls
to find doubts concerning the validity of Forel's thesis, as for example
when: "Oft (the authors' remark) ist die Armee von vornherein mit
bemerkenswerter Genauigkeit auf ein bestimmtes Nest der Sklavenart
orientiert". He assumes, therefore, that is not a general rule that
P. rufescens march out without being previously given information about
a specific goal for attack. Later on, Brun states clearly: "Die Vorhut der
Armee ist nicht konstant, sondern fortwahrenden Wechsel unterworfen;
jedenfalls wird sie nicht etwa durch ein einzelnes Individuum 'angefiihrt' ".
Thus, our thesis that the so-called scouts (namdd by us pseudo-scouts)
do not play a role in the finding of slave-species' nests and in the
directing of amazons' armies towards them (6), was preceded by Brun's
observations (1). We have introduced, however, a new opinion, that is,
the mass of slave-making ants is stimulated for marching out by a group
of the most excitable individuals (activists), and that the final directior
of attack is random, with the activists moving in a circular, chaotic way.
The fact that the direction of expeditions is a matter of chance has been
confirmed by Koehler's studies (9). Koehler laid down pupae belonging
to the Serviformica species in a straight line from the nest and in the
direction different from the one which had already been marked out
by the activists. In this way he cou-ld influence the direction of ihe
expedition. He had obtained, with the use of very weak stimuli (he laid
out only 25 pupae), an effect similar to the one which we have obtained
by passing over the activists from one place to another (6). The influence,
that even such weak stimuli can have on the direction of the attack,
constitutes an indirect proof that a choice of direction is not determined
by the previously collected information about a specific goal but is
induced by quite accidental stimuli. Very clear evidence for this was
provided by Koehler's comparison between the number of pupae captured
in naturally occurring attacks with the gains of attacks artificially
steared in a direction chosen a t random. The numbers turned out to be
similar which definitely indicates that prey-finding in nature is also
a matter of chance. The probability of finding prey, by an army directed
towards nests previously discovered by scouts, would be greatly
increased. It. should therefore be accepted that our conclusions (6) which
were hased on a rather indirect evidence, have been confirmed by
Koehler's studies (9).
A few other observations of ours could be added in this connection.
At a distance of 4 m away from the studied P , rufescens' nest there was
a big road belonging to F. rufa. From time to time, the slave-making
ants went on an raid across that road which resulted every time
in an attack by F . rufa ants who defend their roads with an almost equal
ferocity to that with which they defend their nests. Although those ants
cannot match the amazons in a duel, and thus many more of them were
killed, their practically unlimited quantitative superiority resulted in the
slave-making ants being decimated each time. The continual occurrence
of such fight led by the end of the summer, to such a decrease in the
amazons' numerical force that the colony ceased to make raids and, in
the following year, ceased to exist at all. It is abolutely clear, that the
direction across the F. rufa path could not have been indicated to
the amazons by a scout, because it could not have had a chance
to go and return across the enemy's busy road after having made
a reconnaissance trip. More important, they would have lacked the
incentive to proceed in the very direction where they would encounter
such a powerful and dangerous obstacle. It is beyond doubt that the
direction of those expeditions was not predetermined but was chosen by
an army unaquainted with the potential danger. This evidences the
completely spontaneous character of the proces by which an army of
the amazons moves in a randomly chosen direction. Also there do
exist normally functioning P. rufescens' nests which are completely or
almost completely deprived of pseudo-scouts, i.e., of individuals who
move around a territory giving the impression of making a reconnaissance
which has given a rise to the theory about scouts. The existance of
nests without pseudo-scouts is an additional argument in support of our
hypothesis according to which they are an atavistic group of individuals
who no longer play a part in the life of a community. Their movements
around a territory superficially resemble foraging behavior. Although
foraging has completely fallen out of the P. rufescens' model of behavior,
the pseudo-scouts not only do not bring anything but also do not even
show sings of trying to find prey.
Raignier and Boven (11) have found a similar mechanism of the
direction choosing in the hunting raids of African Dorylinae ants.
However, the African Dorylus have nothing taxonomically in common
with Polyergus, moreover, its extremely predatory nomadic way of life
is quite opposite to the slave-making behavior of Polyergus. Thus,
similarity in the direction findings stems solely from the similar structure
of raids. On the other hand, slave-making ants, Polyergus lucidus, which
are close relative to Polyergus rufescens differ considerably as to the
army behavior, since, according to Talbot (12) they have scouts
responsible for direction of the expeditions.
Directional orientation. It is worthwhile to add a few words
concerning the problem of orientation in the slave-making ants. Our
thesis ( 6 ) that the amazons' army does not follow a trail, but having
once chosen a direction moves straight ahead and returns by the same
route by visually finding the direction, appears to be contradicted by
some observations. For instance, it has been observed that a singular
ant runs out of a nest at a time when a column has already moved
a long distance away, beyond its range of vision. Usually, such stragglers
do not show any hesitation and run in the direction taken by the
expedition but only begin to lose their way when the scent-trail left
by the column has erased. The erasure of their own trail does not,
however, make any impression on the returning army: burdened with
prey, the slave-making ants make their way back to'the nest along their
own road, even if in their absence the road had been dug over (6). The
thesis about the visual orientation of an army should be thus supplemented by saying that single amazons who have not been directed
during the general incitement can use the scent-trails left by the
F , rufescens' column.
A new look at the slave-capturing instinct. It is generally thought
that the slave-capturing instinct is ~reciselydirected toward the robbing
of pupae only, and that it is for the lack of pupae that the amazons
take larvae and even as a last resort, freshly hatched ants. In
contradistinction to this view, might be our observations performed on
P. rufescens with slave-ants F. cinerea.
F. cinerea Mayr., one of the more frequent slaves, is a difinitely
polycalic species. When one of the nests is entered by a P, rufescens
female who sets up there a family on the basis of socially-parasitic
relations, the nest remains a part of the colony and, through its slaves,
maintains indirectly social links with the other nests inhabited by
normal F. cinerea families. The existence of such social links is
confirmed also by Czechowski's report (2). It follows, from his and our
observations, that the slave-making ants - in their raids - do not
attack nests belonging to the same colony. This is
perfectly
understandable because, as the result, of constant interactions, all ants
inhabiting a colony treat each other as equal nestmates, together with
th slave-making ants living in that colony.
Pisarski's (10) and Dobrzaliska's (3) studies on F. exsecta Nyl. indicate
that there exist very close ties between the nests of a polycalic colony
but that those ties are not uniform, with not all the nests necessarily
communicating with each other and exchanging workers. However, fcr
as long as there exists an indirect social link between them, it prevents
alienation from the colony. We have every reason to assume that
similar relations occur in polycalic colonies of F. cinerea (4).
The P. rufescens are, however, in contrast to F. cinerea - monocalic
ants. Treating as their nestmates those inhabitants of the colony who
constantly come into contact with them, they can lose the persona!
scent-link (weaker in them than in polycalic ants) with a nest that they
do not have a direct contact with. This kind of relative isolation can
undoubtedly explain the mistaken raid of amazons on a nest belonging
to their own colony. A slave-making ants' army, by their normal
procedure, forced their way into a nest positioned 1.5 m away from
their own, and carried out only several pupae and many mature workers
who were marked with dyes when they were being cerried by the
amazons. The carried F. cinerea did not of course offer any resistance
because they were caught by their nestmates from the same colony.
They folded up in a specific position for carrying and passively let
themselves be transported to the nest inhabited by the slave-making
ants.
Most of the F. cinerea who had been brought as trophy very soon
.went calmly back to their own nest although not all the individuals
marked during the transportation were noticed on the return trip. It
would be quite normal if some of them were too passive to go
self-dependently out from the nest and some of them stayed behind
having found actual stimuli for taking up some activity in the new nest,
similarly to what takes place in F , exsecta colonies (3).
This event throws some new light onto the issue of the slavecapturing instinct in the slave-making ants. It should be remembered
that the amazons during their attacks on strange nests do not display
active aggression or a hostile attitude to the inhabitants, fighting and
killing only when attacked themselves. It is also characteristic that the
easy to carry larvae are brought relatively infrequently and only when
pupae cannot be found; but it does happen that the amazons bring
young white ants who struggle and run away, are troublesome in every
respect and at the same time do not evoke in the carrying soldiers
them, any expressions of aggression (5).
A compfetely different behavior is displayed by F. sanguinea Latr.
the another slave-making species. Being, unlike the P. rufescens, a
facultative parasite, they treat their prey also, or even primarily, as
a source of nourishment (7, 13). This is reflected in the F. sanguinea's
behavior during raids: they actively attack the inhabitants of raided
nests, killing them and bringing them back as quarry, together with the
pupae into their own nest.
The comparison between the t w o very different behaviors suggests
that the bringing back of pupae into the nest without aggressive or
food-consummatory reflexes must be connected in the P. rufescens with
the loss of the negative reaction to strangers. Every worker belonging
to the slave-species could constitute a potential nestmate to the amazons
with their slave-capturing instinct being reduced to a tendency for
bringing every one of them into their own nest. It could even be infered,
from analyzing the known facts, that the amazons are predisposed
towards catching the mature individuals but the mature individuals run
away or fight and the very young ~truggleand escape thus significantly
slowing down the speed of action which is the necessary condition for
the success of a raid. The P. rufescens' slave-capturing instinct which
demands thousands of captures is most easily satisfied by pupae.
It is to be expected that described attitude to the individuals in the
form of imago is operating only in relation to a given species of slaves
which actually inhabits the slave-making ants' nest.
This investigation was supported by Project 10.4.1.01 of the Polish Academy
of Sciences.
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Accepted 26 June 1978
J. DOBRZANSKIa n d J. DOBRZARSKA, Nencki Institute of Experimental Biology, Pasteura 3,
02-093 Warsaw. Poland.