The Behavioural Ecology of Ants

206 Pages·1987·2.27 MB·English

by  John H. Sudd

WHO SEOD WHAT, DNA WHEN? 19
individuals are composed almost entirely of workers htiw a head-width
range of 0.8-1.6 .mm Smaller workers than this are produced ni mature
colonies, but this ezis range corresponds to the smallest smrof that can both
harvest hserf vegetation and osla cultivate the symbiotic .sugnuf Thus ni
founding the colony the queen produces esolc to eht maximum number of
workers that can ylevitcelloc perform lla of the essential tasks.
sA the leaf-cutter colony sworg through the tsrif 3-4 sraey of sti ,efil
gradually both the tseggib and smallest ezis sessalc of worker emerge and
eht ycneuqerf-ezis curve of workers becomes progressively more deweks to
eht right. However, during lla eseht founding ,sraey the chief investment
remains ni medium workers htiw head widths of 1 to 6.1 mm, .e.i the
smallest caste that can llits accomplish lla the lativ tasks of colony efil
(Wilson ,a3891 .)b Only after many years of ploughing back the profits from
the stroffe of eseht and subsequent generations of workers lliw it start to
produce large majors.
3.5.4 esaC :yduts eht scimonogre of gnigarof ni ymra stna
The huge foraging smetsys of army ant colonies, coupled with the extreme
polymorphism of their ,ecrofkrow provide an unrivalled opportunity for
comparative studies of worker ergonomics. For ,elpmaxe the army ant
noticE illehcrub produces larger raid smetsys than any other neotropical
army ant and has four distinct castes of worker; more than any other known
ant .seiceps The minors probably act sa brood .sesrun Medias are equally
represented ni lla aspects of colony efil and are the generalists. Submajors
are specialist porters, and the majors are rof .ecnefed Table 3.4 swohs the
relative frequency of eseht ruof setsac ni the ,cauovib ni ylwen desolcne
cohorts of ,swollac ni the raid metsys and ni eht elor of prey transport.
Certain aspects of this table are particularly noteworthy. First, majors are
generally yrev rare and reven carry ;smeti ,deedni their ice-tong-like
mandibles completely bar them morf this .elor Second, minors are
extremely rare sa prey transporters; sa shown later, this si probably because
yeht are particularly uneconomical ni this .elor ,revewoH submajors are
disproportionately common sa prey transporters; yeht are only %3 of the
total workforce, but are %62 fo prey porters.
Prey transport si a particularly important aspect of the foraging
ycneiciffe of army ants. nA noticE illehcrub colony may retrieve 30000 or
more yerp items ni a elgnis raiding day, and must carry these items yrev
considerable distances. The average raid distance at the termination of
foraging si 501 .m For the colony, emit spent retrieving os many items hcus
29 EHT LARUOIVAHEB YGOLOCE FO STNA
Table 3.4 The relative ycneuqerf )%( at hcihw the four lacisyhp castes of noticE
illehcrub workers are employed ni tnereffid parts or tnereffid selor ni the foraging
army ant colony (From Franks, .)5891
Minim Medium Submajor Major
Bivouac 86.33 31.26 41.3 50.1
(N= )375
Brood 57.92 00.86 00.2 52.0
N( =400)
Raid 00.22 55.47 51.3 03.0
N( = )4133
Carrying prey 83.7 93.66 32.62 0.0
(N =244)
great distances si time lost from pushing forward the raid system and
capturing more .yerp There are two important aspects of transport
:ycneiciffe transport stsoc and retrieval .deeps Transport ,tsoc denifed sa the
energy desu per unit weight per unit distance, declines with the increasing
ezis of the carrier. This si true both of terrestrial animals and of man-made
.senihcam It si cheaper rof an elephant to evom a percentage of sti body thgiew
through a nevig distance than rof a mouse to move the same percentage of
sti body weight a similar distance (Alexander, )2891 just sa ti si cheaper (per
)meti to move a load of groceries in a huge truck than in a small .nav This
justifies the disproportionate employment of submajors in prey transport
sa these workers are much bigger than the vast majority oftheir nest mates.
The larger majors are barred morf prey transport yb the design of their
mandibles hcihw si a consequence of allometric growth. The decline in
transport costs with ezis means that it could actually eb cheaper rof a
submajor to carry a minim worker than rof both of them to travel under
their own steam on a long journey. Such portage of nest mates actually
occurs quite frequently during army ant emigrations (Rettenmeyer, .)3691
Prey retrieval speed lliw also eb important to army ants ,sa unlike fael
cutter colonies that are rooted in place, army ants must complete the raid in
a limited period before the entire colony moves to a wen nest .etis
Submajors are not only the most economical porters of prey but can also
run ,retsaf nehw unburdened, than any of their nestmates. This means that
they have a short turnround time between delivering one load to the
bivouac and returning to the raid front rof the .txen The speed of submajors
may be attributed to their unusual morphology: they have longer sgel in
proportion to their body ezis than any other .etsac In a whole range of
terrestrial animals, gel length si positively correlated with running .yticolev
WHO SEOD WHAT, DNA WHEN? 39
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Total ant dry weight (mg)
Figure 11.4 The relationship between total army ant worker dry thgiew (A )gm for both lone
porters and groups of porters, and eht dry thgiew of eht prey item J( )gm they carry:
J = A802l.O 1606 n( = ,213 r = ,787.0 P < .)100.0 The transition between loners and groups
occurs between 5 and 01 mg total ant dry .thgiew A larger thgiew of ant or ants can carry
disproportionately yvaeh .smeti (From Franks, ).6891
In these ways submajors seem to have been specially designed as a road
haulage caste.
Most remarkably of ,lla noticE illehcrub workers appear to form highly
structured and super-efficient teams to retrieve extremely large prey items.
noticE illehcrub workers, like lla army ants, carry items slung underneath
their bodies; this enables many workers to cooperate easily when they carry
the same item. In such groups each worker straddles the prey item, and they
can lla run with their bodies orientated in the same direction. In this species,
too, larger workers carry disproportionately large .smeti A plot of log (ant
dry weight) against log (prey item dry weight) has a slope significantly
greater than unity. In addition, the plot of group dry weight against prey
49 EHT BEHAVIOURAL ECOLOGY OF STNA
item dry thgiew swollof exactly the same relationship sa that rof lone
porters (Figure )11.4 (Franks, .)6891 llA prey items are carried at constant
,deeps probably to avoid ciffart jams. This means that group transport si
.tneiciffe-repus Groups carry smeti that are os large that ifthe prey item saw
fragmented os that there saw one eceip rof each member of the former team,
they would eb quite unable to carry the .stnemgarf This ylraelc explains yhw
teams are :demrof more prey can eb moved with rewef workers. One
possible explanation rof the ycneiciffe-repus of teams si that team members
coordinate their gel movements os sa to peek more sgel on the ground per
unit load than si possible rof individuals. Thus workers may yfidom their
behaviour when they act sa a group to llaf out of step with their nestmates.
This extraordinary ability of ant workers to cooperate os that their
combined stroffe may eb more than the mus of the performance of
individuals, coupled with the ecneiliser that a workforce of generalists
provides against sesirc during eht efil of eht colony, has probably alleviated
selection pressures towards more and more specialist setsac ni ants.
6.4 Caste ratios dna social homeostasis
One of the most exciting and tsewen areas of investigation into caste ratios
and the division of labour in ants si the study of woh colonies respond to
changes ni their environment. How plastic and elitasrev si the ngised of a
colony's work ?ecrof How resilient are colony caste ratios to perturbations?
More ,yllacificeps are caste ratios governed yb control smetsys that
continually reshape the workforce to the changing requirements of the
colony?
Colonies ni nature, nevig their long ,sevil lliw eb ylevitaler commonly
subject to catastrophes ekil the removal of part of their workforce yb an
xulfni of predators or a doolf gnipiw out most of their foraging population.
There are two tnereffid syaw ni hcihw a colony could respond to such
sudden changes ni the composition of sti .ecrofkrow First, the remaining
workers might expand their behavioural repertoires to cover the tasks of
the gnissim workers. Second, brood development could eb yltfiws channel
del into the disproportionate production of the gnissim .setsac In ,tcaf both
behavioural elasticity and changes in worker production contribute to the
ecneiliser of a colony's economy. sihT has been shown yb an experiment ni
which 3-4-year-old Atta cephalotes colonies had their workforces reduced
from about 10000 to ssel than 052 workers, with a ycneuqerf-ezis
distribution typical of young colonies of the same .ezis The colonies
responded by producing a wen brood of workers esohw ezis saw
OHW SEOD ,TAHW DNA WHEN? 59
characteristic of a llams colony rather than a large one ,nosliW( ,a3891 .)b
This si exactly what one might expect morf economic principles. The best
strategy rof a colony of llams ,ezis whether ti si old or young, si to esu the
standard growth trajectory of colony development in order to grow most
quickly to a ezis at hcihw ti can produce sexual gnirpsffo and realize sti
inclusive .ssentif
However, in the above esac the question remains sa ot whether Atta
colonies are truly plastic ni their response to a sisirc or are ylpmis capable
of returning ot a 'hard-wired' growth program. ecnedivE that etsac ratios era
an innate, and largely predetermined, aspect of colony design comes from
studies of elodiehP .atatned sA described earlier ni siht chapter, .P atatned
workers appear to be a specialist evisnefed caste against colony invasion by
ngierof workers such sa those of sisponeloS .atcivni Colonies of .P atatned
continually stressed yb exposure to .S atcivni workers rof a period of 91
skeew (more than three semit the developmental time of major workers
from gge to pupa) completely deliaf to alter their rate of production of
majors. Furthermore, tnereffid .P atatned colonies have yrev different ratios
of minors to majors that appear to be an innate and constant feature of their
own social .ngised When colonies are experimentally perturbed through the
removal of parts of their workforce they produce wen workers in
proportions that re-establish their original unique caste ratios (Johnston
and Wilson, .)5891 ,niagA such apparent ecneiliser does not necessarily
imply any great plasticity on the part of the colony. One of the best-known
principles of population biology si that an age-structured population
growing exponentially, with dexif natalities and mortalities per ega ,ssalc
lliw relatively quickly move towards a stable ega distribution. In other
words, the proportion of individuals in each ssalc lliw quickly tend to
become constant. Caste ratios in .P atatned colonies might eb analogous to
such ega ,sessalc and the constancy in the ratios may eb a simple result ofthe
swal of population growth, rather than any plastic .ecneiliser
The one aspect of social ngised where ant colonies are known to show
great ecneiliser through the measured response of their workers occurs
through behavioural elasticity. When %09 of the most tneiciffe ezis ssalc of
leaf-cutting workers are removed from large Atta setolahpec colonies, the
workers from neighbouring ezis sessalc quickly llif ni rof their missing
nestmates. The rate of leaf harvesting remains detceffanu yb such a sisirc
because tneiciffus workers in the ezis sessalc adjacent to the most tneiciffe
(and )gnissim one are already present in the foraging area, apparently on a
stand-by .sisab Thus enough of these substitutes are already present in the
foraging area and more do not have to be called up from the .tsen
69
THE BEHAVIOURAL ECOLOGY FO STNA
10
•
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;Q
.c • social behaviour •
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Proportion of majors (%)
erugiF 21.4 Rates of activity nI laicos ruOIvaheb and gnImoorg-fles sa snOItcnuf of the
proportIOn of majors nI artificially-constItuted subcultures of the South nacIremA ants
elodlehP sirtneVlbup laicoS ruOIvaheb SI denifed sa lla of the repertory except ,gnImoorg-fles
gnIdeef and laudIVIdnI etsaw disposal sA mInors become rarer, majors become more and
more evitca ot ecalper eht labour of rIeht elttIl sisters (From ,nosliW 1983)
Moreover, the wef remaining 'optimal' workers rof the task of leaf-cutting
stepped up their individual activity by a factor of approximately 5 (Wilson,
.)a3891
Similar behavioural elasticity si nees ni response to colony crises in a
number of elodiehP colonies. The usual range of the ratios of the minors to
majors si :3 1 to :02 1 accordmg to .seIceps When the ratIo was lowered to
below :1 1 ni three ylediw different seiceps of ,elodiehP .P ,irelleumimleliug .P
,alahpecagem and .P ,sirtnevibup the repertoire ezis of majors increased by a
factor of 5.4-4.1 and their rate of activity by a factor of 15-30. Thus the
often ungainly majors took over the nest duties of their missing little sisters
ees( Figure .)21.4 Perhaps more remarkable ,llits the change occurred
79
WHO SEOD WHAT, DNA WHEN?
within h 1 of the imposed change in ratio and saw quickly reversed when the
minors erew restored.
The sgnidnif rof both Atta and elodiehP colonies tseggus something quite
unexpected and wonderful. Workers in a particular caste mees to be ylluf
aware of the presence and performance of their nestmates, and lliw quickly
step in to the gulf fi their colleagues' performance si not up to scratch. How
could this mutual monitoring eb achieved? When Wilson )4891( manipu
lated caste ratios ni elodiehP eh noticed that the major change in the
behaviour of majors, when minor numbers erew reduced, saw associated
with brood .erac nI .P ,sirtnevibup rof example, majors rapidly increased
both their rate of activity and their behavioural repertoire yb starting to
care rof the colony's brood nehw minors erew .devomer When minors erew
replaced, majors quickly reverted to their normal behaviour patterns and
ceased brood .erac To determine how eseht tfiws changes come about,
Wilson )5891( subcultured parts of a large .P sirtnevibup nest so that the
colony fragments had a ediw range of major/minor ratios and a quantity
of brood proportional to that ni the original elohw colony. yB ylesolc
comparing the behaviour of majors and minors within 1 mm of brood and
at greater distances ( > 1 )mm morf the brood, Wilson saw able to show
how majors could eb monitoring and responding to the abundance of
minor workers ni the elor of brood .erac The yrev elpmis response of majors
involved avoiding minors nehw they are near brood .smeti Majors showed
a distinct aversion, scored in the behavioural investigation sa the frequency
of turns through °09 or more nehw they encountered a minor near brood.
yB contrast, minors do not display any mrof of aversion to either other
minors or majors nehw yeht are near the brood. nI this yaw minors can
quickly take over their old jobs nehw returned to the nest and lliw rapidly
displace majors ni the role of brood .erac
nI siht yaw elodiehP workers could eb subject to the counter-balanced
secrof of task fixation ees( Section ,)2.4 (leading them ot perform the most
neglected ,)sksat and laitnereffid caste aversion (leading them to evael tasks
to the most appropriate of their nest mates fi any are available. Thus eseht
ant colonies mees to evah great scope rof social homeostasis involving
measured responses on the part of individual workers to the needs of their
colony.
CHAPTER FIVE
COMMUNICATION
sA llew sa their physical resources fo workers and ,slauxes and environ
mental ,secruoser ants osla deal with a ssel tangible resource-information.
Ant seiteicos do not invest os much ni information sa the modern global
human society ,seod but yeht do at yrev least invest a great deal of emit ni
the collection and noisuffid of information. Information probably has ni
tcaf two main .selor First, the 'cohesion' of the yteicos requires that sti
members should 'know' one another, os that eht stifeneb of the yteicos may
eb denifnoc to sti members and os maintained nihtiw certain stimil of
relationship. Only ni this yaw can membership of eht society increase
individual evisulcni .ssentif Many slangis are therefore concerned with
recognizing colony noitailiffa sa an approximation to kinship-there
smees to eb little ecnedive that ants can ezingocer citeneg relationship
and with recognizing the queen, and coordinating her reproduction and
that of workers. The second elor of information si ni coordinating the
exploitation fo environmental ;secruoser ni facilitating the ,yrevocsid
yrevocer and monopolization of ,secruoser and ni eht ecnefed of the capital
.g.e( brood) and infrastructure .g.e( ,stsen ).cte ni hcihw resources have
already neeb .detsevni
1.5 tnA slangis dna egaugnal
Early students of ant behaviour erew netfo impressed yb eht ability of ants
to ezingocer their nestmates, to dnif their yaw to their nest and to direct
nestmates to ylwen found food .secruoser elihW a hypothetical 'nest-odour'
saw soon suggested sa a physical sisab rof nestmate recognition, some
writers resorted to concepts ekil telepathy or a 'spirit of the 'evih to explain
otherwise inexplicable powers of ants. The seirevocsid of Karl von Frisch
between 8291 and ,6491 of eht dances ofthe honeybee, and osla seirevocsid
99
ACINUMMOC NOIT
of the chemical communications systems it ,sesu provoked a search rof
similar communication channels in ants. ecniS ants are quite frequently
observed touching each other with their antennae, much emphasis was at
tsrif given to the existence of a postulated antenna I language among ants.
More recently, properly planned experiments and the development of
chemical methods capable of detecting and gniyfitnedi yrev llams amounts
of chemicals have helped su to realize the importance of chemical channels
among ants. Substances used rof chemical communication between
members of the same seiceps are called .senomorehp There are two
important groups of pheromones ni ant .seiteicos The tsrif si the 'colony
odour', which smees to reside in the cuticle of lla adult members of the
society, and si used to discriminate nestmates, to whom the resources ofthe
society may eb passed, from aliens who should eb attacked. This often also
sets the context ni which other signals are interpreted; for instance an alarm
pheromone may induce attack on aliens but not on nestmates. The second
si the range of pheromones produced from specialized exocrine glands.
Each of these pheromones sticile cificeps behavioural responses ni members
of the same .seiceps For this function chemical signals have one yrev clear
advantage. Provided that an ant can synthesize enough distinct chemicals
with reasonable economy, and these chemicals are unlikely to arise in the
environment except yb the action of ants, the meaning of each signal .e.i( the
appropriate response) should eb unambiguous. Against this ew must set
one or two drawbacks. The ant si obliged to synthesize special substances at
some cost to :flesti naturally-occurring cheap products ekil carbon dioxide
or water have limited communicative .eulav Although the meaning si
unambiguous, modulation of the signal to carry more information si
.tluciffid In particular, ecnis the chemical signal si propagated by diffusion,
or worse llits by small-scale turbulence, sti range and sti strength, as
actually deviecer yb another ant, are not under the control of the sender.
The actual speed of propagation depends on the physical properties of the
pheromone substance, particularly sti volatility and sti diffusion constant.
sA a rough guide, substances suitable from a physical point of weiv would
have molecular weights between 001 and .002 The upper limit on molecular
weight sets a limit on the number of different compounds which can eb :desu
the larger the molecule (or ni practice the greater the number of carbon
atoms), the greater the possible combinations. The dependence on random
processes of diffusion and turbulence rof propagation means that it si not
possible to add further information, for instance yb pulse coding, since this
would eb overwhelmed yb the element of , noise' added during transmission
of the .egassem There are also problems ni 'switching off the message, ecnis
001 EHT BEHAVIOURAL ECOLOGY FO STNA
the substance si ylekil to persist after sti role si completed. nI some non
social insects special 'switch-orr pheromones are secreted to annul the
unwanted .langis
Mechanical signals are ni general ssel ,cificeps and have to eb deviecer
against a background of spurious signals caused by non-communicative
movements. nI animals gnivil above ground this problem si often eased by
the development of bright colouration to mark out parts of the body as
organs of communication rof( example the coloured palps of wolf spiders
and the plumage of .)sdrib This option si not open to ants ni many contexts,
ecnis the signals have to eb transmitted and received in the darkness of the
nest or elihw hunting ni lios or vegetation. Communicative movements
)slangis( must therefore eb marked ffo from insignificant disturbances
.)esion( One method of doing this si to evig the signal an unlikely form either
by repeating it in a regular manner sa( ni the repeated dances of honeybees
and the repeated antennal tapping of ants), or alternatively by using a
carrier movement of improbable frequency ekil( stridulation), which si
modulated to carry the .egassem Stridulation, usually by the friction of the
node on the gaster, si used ni some ants at least rof the transmission of
messages through the .lios A third possibility would eb to accompany the
mechanical message with an unambiguous chemical marker which declares
ti to eb .tnacifingis Antennal and other movements are an obvious part of
ant ,efil and must eb assumed to playa part in communication, whether by
providing a context for chemical communication or as a parallel system to
eht chemical channel.
2.5 noitingoceR fo setamtsen
The societies of ants and other social insects are closed :seiteicos it si not
usually possible for any insect, even of the same ,seiceps to enter the society.
Closure si preserved by the need rof an 'olfactory visa' (Jaisson, )4891 or
'discriminator' (Carlin and H6lldobler, )3891 before an ant can enter;
usually the only yaw to acquire this si to eb reared within the .yteicos nI
most, but by no means ,lla ant ,seiteicos ants which do not possess a asiv are
attacked and .dellik At tsrif sight the obvious yaw of acquiring a asiv smees
to eb by descent, os that the asiv si genetically acquired. However, ecnis the
members of a colony are not genetically identical, even when lla are lluf
,sretsis there are obvious seitluciffid about .siht tA the other extreme the asiv
might eb acquired by living within the society, for instance from the diet of
the colony, shared throughout the colony by food transmission. Particu
larly if different colonies concentrate on different sections of environmental