johnson1974

Aggression and Competition among Stingless Bees: Field Studies
Author(s): Leslie K. Johnson and Stephen P. Hubbell
Source: Ecology, Vol. 55, No. 1 (Jan., 1974), pp. 120-127
Published by: Ecological Society of America
Stable URL: http://www.jstor.org/stable/1934624
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Ecology (1974) 55: pp. 120-127
AGGRESSION AND COMPETITION AMONG STINGLESS BEES:
FIELD STUDIES'
LESLIE
K.
JOHNSON
Department of Zoology, University of California,
Berkeley, California 94720
AND
STEPHEN
P.
HUBBELL
Department of Zoology, University of Michigan,
Ann Arbor, Michigan 48104
A bstract. Many species of stingless bees exhibit complex intraspecific and interspecific
aggressive behavior towards each other when they meet on flowers or artificial baits. Such
aggressive encounters significantlylower the amount of time that bees spend on food sources,
as well as the amount of nectar or pollen which they can gather per visit. In addition, the
intensityand duration of aggression at artificial baits rises sharply with increased sugar concentration. Differentspecies vary markedly in inherentaggressiveness. Learning and recruitment appear to reinforce the effects of aggression on the spatial separation of foraging in
competing colonies.
Key words: Aggression; bees, stingless; competition; Costa Rica; Trigona; tropical bees.
tinctivecoloration or size. Althoughthere are other
Stingless bees are a major biomass component of bees in Costa Rica withwhichtheycould be confused,
the nectar- and pollen-foraginginsect communityin theydid not occur in our study areas. C and M are
many tropical areas. At one dry forest study area both all-black bees that are somewhat difficultto
near Bagaces, Guanacaste Province,Costa Rica, sting- distinguishin flight;with practice they can be idenless bees constituteclose to 30% of this insect com- tified at rest on flowers or baits. M is a slightly
munity (Heithaus and Opler, personal communica- smallerbee and it moves more slowly and less jerkily
tion). Publishedstudieson foragingbehaviorin these as it feeds. Fortunately,the two species,for unknown
eusocial bees are few, dealing principallywith the reasons, rarely co-occurred in our experimentsat
communication of food location by odor trails or differentstudysites around Turrialba.
sound production (Esch 1967, Lindauer 1967, Kerr
METHODS
1969). Arnold (1966) and Johnson (1970) found
The
bees
were
observed
on flowers and artificial
aggression between species of stinglessbees visiting
artificialbaits; but, except for reportsof nest-robbing baits. The baits were designed to present a constant
behavior (Schwarz 1948, Nogueira-Neto 1949), there food supply of controlled accessibility and quality
have been virtuallyno accounts of bee aggression (sucrose concentration). Therefore any changes in
over naturalfood sources. Aggressionover resources the patternsof exploitationcould be attributedto the
is well known in some othergroups of Hymenoptera, bees and their interactions,and not to a changing
especially ants (e.g., Haskins and Haskins 1965, quality or quantity of the food supply. Each bait
Wilson 1971). We report here on several recent was a 10-cm-squareclear-plasticsandwich box on a
experimentaland descriptivefield studies of aggres- stake 80 cm tall. The box was half filled with sugar
sion and competitionamong stinglessbees, conducted syrup,on which floateda 4-mm-thickstyrofoamplatform on which the bees could land. The bees could
at Turrialba,Costa Rica.
This discussionwill primarilyconcern interactions drink the syrup through 16 half-inchholes drilled
betweenthe large (10-mm) bee, Trigona silvestriana through the platform. As the syrup was drunk by
the bees, the platform floated downwards, thereby
(S) and medium-sized(7-mm) bee, T. corvina (C)both aggressivespecies; the less aggressive,medium- maintaining a constant food availability (exposed
sized (7-mm) bees, T. testacea (T) and T. mexicana surface area of syrup) throughtime.
Twenty-fivebaits were used in each experiment,
(M); and small (4- to 5-mm) T. latitarsus (L), a
timid species. With the exception of C and M, all arranged in five-by-fivegrids 12 m on a side at
the species can be easily recognized on sight in the several field sites always in cleared areas, but never
field, even in moderatelyfast flight,because of dis- more than 100 m fromlarge sections of undisturbed
forestalong the Rio Reventazon. With the exception
of one colony of C in a hive box, all the colonies
' ReceivedJuly10, 1972; acceptedMay 21, 1973.
INTRODUCTION
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Winter1974
AGGRESSION AND COMPETITION AMONG BEES
121
whose workers visited the baits were wild. Five
sucrose concentrations,0.0, 0.2, 0.4, 0.8, and 2.4 M,
were presented simultaneouslyto the bees, and the
five baits of the same concentrationwere staggered
on the grid rows in such a way that each row and
column had exactly one bait of each concentration.
This range of concentrationswas chosen so as to
span and somewhat exceed the range of concentrations found in natural nectar sources (0.4 to about
1.0 M). At these baits, we studied the interaction
betweenindividualworkerbees fromthe same colony,
fromdifferentcolonies of the same species, and from
colonies of differentspecies. An Esterline-Angus
events recorder was equipped with a remote keyboard; when a key was depressed it caused one of
twentypens to deflect,making a timingmark on the
movingpaper chart. Using this device, we timed the
duration of six behavior states: hovering,landing,
walking, feeding, interactingwith another bee, and
leaving, for over 2,000 individual bees including
several species. The "interaction"category includes
all interruptionsof the other behavior states caused
by the presence of anotherbee. When bees fromone
colony are foragingon a bait with no rival colonies
present, there is no fighting,but interruptionsof
feeding and walking are common when many bees
are present, simply due to jostling. In Fig. 2 the
interaction category is called "fight" even though
when T is alone only jostlingoccurs.
RESULTS
Levels of aggression increasing in intensityfrom
mere threatto mortal combat could be distinguished
in both of the highly aggressive species S and C
when visitingthe baits and flowers. At low intensities
(level 1) a bee threatensits opponents. The bee
spreads its mandibles,tiltingits head up so that they
point forward at a rival bee on the bait or flower.
The bee may also unfold its wings and hold them
at a wide angle to the body, a posture that is never
seen when the bee is alone or in the company of
otherbees fromthe same colony. When an individual
l
..''
...._
...
..
j.
....;~~
.
..
.
.
.
.
y.;....^;
...
FIG. 1. Top: Trigonasilvestriana
givesthe open wing
("v-wing") displaywhile simultaneously
feeding. Rival
bees of other species (particularlyT. testacea and T.
mexicana) are flyingor hoveringin the immediate
vicinity.Photoby L. K. Johnson.Middle:Threeworkers
of Trigonacorvinaattacka workerof the same species
froma rivalcolonyon a 2.4-Msucrosebait. A fourthbee
preparesto enterthe fray (upper right) while another
bee ignoresthe fightand preparesto flyoffto the nest
(lower left). Bottom:Two workersof Trigonacorvina
fromrival coloniesfighton an 0.8-M sucrosebait. rhe
bee at the upper left is chewingon the forelegof its
opponent,who in turn is displayinga typicallevel-1
aggressive
displaywithits head tiltedup, mandiblesopen
and thrustforward.Photo by L. Master.
............
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;
3:y'
^
:
.
122
LESLIE
from a rival colony or species hovers near the occupied bait or flower,the defendingbee will adopt
the outspread ("v-wing") display and hold the posture for a few seconds to several minutes,depending
upon the length of time the rival is present in the
vicinity(Fig. 1, top). In some cases the bee raises
its body as well, particularlyelevatingthe abdomen.
In this postureit may dart at adversarieson the bait
or floweror paw the air with its forelegsat a nearby
hovering bee. Flying bees swoop at other flying
bees, hover over an opponent already on bait or
flower,or face off in pairs and slowly rise as a twosome to heightsof 10 m or more.
At a more intenselevel (level 2) the rivals make
briefbodily contact. The attackermay land momentarily on the back of its opponent, often causing it
to drop offthe bait or flower. Bees in flightgrapple
brieflyand fall to the ground. On baits or flowers
bees sometimesdeliver a simple bite to legs, wings,
or head.
Level 3 involves prolonged biting and pulling on
the mandibles and extremitiesof a rival bee. Two
bees interlockmandibles and tug for several minutes
to an hour or more. Other bees may join such fighting pairs and pull on unoccupied legs and wings
(Fig. 1, middle).
At the most intenselevel (level 4) two bees wrestle
with theirventralsurfacestogether,legs grippingthe
opponent,and mandibleslocked or chewingthe rival's
head or neck. Many wrestlingbees never separate
alive. The combatants give off a sticky substance,
apparentlysecreted by the mandibular gland, which
fouls wings and legs. They also release alarm pheromone, which disturbs nearby bees. One pair of
grapplingbees may cause 40 or 50 bees to fly up
from a bait suddenly, probably in response to the
alarm pheromone released. In the species studied,
C appears to be more easily alarmed by fightsthan
does S. When an S invades a bait occupied by several
C and begins a fight,very often the other C on the
bait suddenly leave, almost simultaneously,and fly
in agitatedcircles around the bait. Other S that may
be hoveringnearbythenland on the emptiedbait and
meet with minimum interferenceand aggression.
These displacementscan be very rapid, taking place
in less than a minute. Similar patternsare observed
when S displaces M and T frombaits,withthe exception that fightsare rare. M and T usually fly off
aftera level 1 interactionwith S, or aftervery brief
bodily contact (level 2).
S always displaces C, T, and M when there are
fewer than approximately50 to 60 bees on the bait.
However, on those occasions when a bait was heavily
visited by C or by T and M together,S was often
unable to displace the bees despite repeated attempts
by individual S over periods rangingup to 3 days.
It is not entirely clear why they cannot displace
Ecology, Vol. 55, No. 1
P. HUBBELL
K. JOHNSON AND STEPHEN
1. Observed and Poisson expected frequencies of
single and joined dead Trigona corvina associated with
bait Y3 (2.4 M sucrose). K equals the number of other
bees to which each dead bee is attached; 0 equals observed frequencyand E equals expected frequency
TABLE
K
0
1
2
3
>3
0
E
319
61
13
1
0
313.5
71.6
8.2
0.6
0.0+
crowded bees, but a possible explanation is that the
individual S bee cannot easily pick out for attack an
individual M or T from the background of frenzied
commotionand movementon the bait. When fewer
bees are present,it is an easier matter to fix on a
is supparticularmoving object. This interpretation
ported by observationsof individual S on crowded
baits. They rapidlyturnthis way and that,flash the
v-wing display over and over, then begin but rarely
complete a rush on another bee. They never feed
under these circumstances.
Aggression is most intense, measured both as
frequencyof aggressionlevels 3 and 4, and as frequencyof death,when theinterspecific
combatantsare
the species S and C. Aggressionby thesesame criteria
is more intense in these two species when the rivals
are colonies of the same species. Aftera 2-day battle
between three colonies of C over a grid of 25 baits,
1,812 dead bees were found. Associated with one
heavily contested 2.4 M bait, we found 158 injured
and 484 dead bees, and frequencies of single and
joined dead bees as shown in Table 1. A chi-squared
test
( X22f
-
4.74, 0.10 < p < 0.05, where values of
K > 2 were pooled) of a Poisson fit of these frequencies suggestsa nonrandomclusteringof bees in
death. It seems likely that larger fightinggroups
increase the per-individual risk of mortal injury.
Perhaps in groups there is a higher local titer of
attackpheromonethatcauses more intenseexpression
of aggression. Fig. 1, middle and bottom, illustrate
groups of fightingbees.
As mentioned,interspecificencounters rarely involve two species of equal aggressiveness,and brief
2. Probabilities of being threatened or attacked
by a given species, per bee-minute, for each species.
Text gives species symbols. Zero entriesmean no cases
were observed
TABLE
Aggressed
Aggressor
S
T
M
W
A
S
T
M
W
A
0.0
.0519
.0199
.0833 .0317
0.0
.0248 0.0
.0295 .0260
.0214 .0033 0.0
0.0
0.0
0.0
.0017 .0273 0.0
0.0
0.0
0.0
0.0
0.0
0.0
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Winter 1974
AGGRESSION
AND COMPETITION
3. Relationshipbetweenintensity
of aggression
and molarsucroseconcentration
of baitduringa 2-day
intraspecific
battlebetweenthreecolonies of Trigona
corvina. The numberof fightswas recordedbetween
1000 and 1100 hr on the 1stday of theencounter.The
numberof deathsis the totalforeach bait concentrationafterthebattlewas over
TABLE
Sucrose
Number
Number
concentration of fightsPercent of deaths Percent
(molar)
(F)
ofF
(D)
ofD
0.0
0.2
0.4
0.8
2.4
35
26
37
94
176
9.5
7.1
10.1
25.5
47.8
82
140
111
338
1,141
4.5
7.7
6.1
18.7
63.0
encounters generally suffice to establish which bee
is dominant and will control the resource. For example, the interspecificencounterswe observed on
one 2.4 M bait at another field site involved only
aggression levels 1 and 2 and were countable as
instantaneousevents. Table 2 shows minimumestimates of the probability of being threatened or
attacked per bee-minuteon this bait for the species
S, T, and M, and fortwo othervisitors,Apis mellifera
(A) and several species of polybiine wasps (W).
They are minimal estimates because events happen
veryquicklyon thebaits and an occasional interaction
was missed. Of these species, S is clearly the most
aggressive interspecifically,followed by T, M, W,
and A. At this bait no intraspecificfightswere seen,
which suggeststhat only one colony of each species
was involved.
Greater aggression occurs on the more concentratedfood sources,whethermeasuredby the percent
of all fightingbees that are on baits of a given
AMONG
BEES
123
concentration,or by the total number of deaths
associated with each concentration(Table 3). These
data were obtained on the same 2-day battle between
three colonies of C mentionedearlier. Fights were
counted on each bait between 1000 and 1100 hr on
the firstday of the battle, and deaths were counted
at the end of each day and summed for the 2-day
period. Early in the experiment,fightingoccurred
on all the baits, in spite of concentrationdifferences.
That deaths occurred on the water baits (0.0 M
sucrose) at all is remarkable. We hypothesizethat
early visitorsto concentratedbaits initiallyassociate
all baits with food, and on early returnflightswill
defend any bait even before sampling its concentrais supportedby the fact that
tion. This interpretation
the percentageof deaths over the total 2-day period
is higherthan the percentageof early fightson the
most concentratedbaits. Also the percentageof bees
fightingon the water baits had declined to zero by
the end ( 1700 hr) of the 1st day. Differences in
aggressionwith changes in food quality are, in fact,
greaterthan early-fightfigureswould indicate since
these figuresare overestimatesof the average level
of aggressionon the weaker concentrations.
Aggressive species noticeably diminish the gridforagingsuccess of less aggressiverivals,whetherone
measures individual or group effort of the rivals.
Fig. 2 shows that the aggressivespecies S affectsthe
way individualsof the more timidspecies T distribute
theirtime among six behavior states while visitinga
2.4 M bait. T hovers longer before landing with S
presentthan withoutS, and feeds only 28% as long.
Table 4 is a matrix of transition probabilities
between behavior states for individual T recurrently
visitinga 2.4 M bait, before and afterthe arrival of
4. Matrix of transitionprobabilities between behavior states for individual Trigona testacea recurrentlyvisiting a 2.4-M bait, before and after the discovery of the bait by T. silvestriana. Numbers nonitalicized are the probabilities for T. testacea alone, and italicized probabilities are for T. testacea in the presence of T. sill'estriana. Probabilities were estimated from the event recordings as the fractionalnumber of times a given behavior state changed
to each new behavior state per 0.5-second interval
TABLE
From: (previous
behavior state)
Hover
Land
Walk
Feed
Fight
Leave
To: new behavior state
Hover
Land
Walk
Feed
Fight
Leave
0.8919
0.9067
0.0
0.2500
0.0189
0.0213
0.0
0.0
0.0
0.2593
0.0028b
0.1081
0.0671
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0370
0.0
0.0
0.0
0.0
0.9622
0.5417
0.8868
0.8511
0.0274
0.0294
0.0
0.1852
0.0
0.0
0.0
0.0
0.0378
0.0416
0.0565
0.0426
0.9315
0.8618
0.1053
0.0741
0.0
0.0
0.0
0.0087
0.0
0.1667
0.0189'
0.0638
0.0274'
0.0490
0.8421'
0.2222
0.0
0.0
0.0
0.0175
0.0
0.0
0.0189
0.0213
0.0137
0.0598
0.0526
0.2222
0.9972
0.9987
0.0013b
These bees were not fighting(T. testacea alone) but jostling one another as they moved on the bait.
These bees are returningfrom round-tripflightsto the nest; these probabilities were estimated from round-trip
flighttimes for several individually marked bees.
b
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LESLIE
124
K. JOHNSON AND STEPHEN
Ecology, Vol. 55, No. 1
P. HUBBELL
60
14
(r-)
13
12
50
40
CD
11
30
<)r2
LIJ
~20
1l1.I
0t
10
0.0
10
0.5
2.0
1.0 1. 5
(M sucrose)
2. 5
V)20
30
HOVER
LAND
WAL
K
FEED
FIGHT
LEAVE
BEHAV
IOR STATE
FIG. 2. The effectof the presenceof the aggressive
species,Trigonzasihvestriana,on the timespentper visit
by individualsof the less aggressivespecies,T. testacea,
in six behavior states. The bait has a 2.4-M sucrose
solution.Open bars: mean durationsof the statesbefore
T. silvestrianabegan visitingthe bait; solid bars: mean
()-40
VC)
ULJLUJ
~50
1
60i
FIG. 3.
of visitsby Trigona
Percentagedistribution
durations
inthepresence
ofT. ksili'cstrian~a. Theverticallatitarsitsand
bracketsmark the 95%0 confidencelimitsabout mean
values. The "fighting"
stateis nonzeroin the absenceof
T. silvestrianaS becauseofaccidental
interference
between
theworkers( jostling). AfterT. sill'estrianlaarrived,fighting mainlyinvolvedthreat-posturing.
species S. The entries in the matrix representthe
probability of changing from the behavior state
listed in the vertical column at the left to the state
listed in the row across the top during any given
half-second interval,as estimated from the charted
event recordings. The assumption is made in these
calculations that the bee's behavior can be approximatedreasonably well by a stationaryMarkov
process. For purposes of describing the expected
duration of each behavior state, this assumption is
adequate. However, the model is inadequate for
predictingthe observed variance in duration of each
behavior state,which is consistentlygreaterthan the
mean duration of each behavior state, indicatinga
violation of the Poisson assumption underlyingthe
Markov model. The model is ergotic because the
individual I do returnto the bait after leaving the
bait for the nest.
Significant changes occur in these probabilities
after species S discovers the bait. A hoveringT is
much less likely to land per unit time, and more
T. sillestriana to baits of five sucrose con-
0.0, 0.2, 0.4, 0.8, and 2.4 M. Open bars: T.
centrations:
latitarsus in the absence of T. silvestriana; solid bars: T.
latitarsitsin the presence of T. silvestrianza(below hori-
zontalaxis).
likely to resume hoveringfrom landing, walking, or
fighting.(The "fighting"categoryfor T alone refers
to jostlingin a nonaggressivemanner that interrupts
the bee's feeding or walking behavior states.) T is
less likelyto continue feedingwhen S is present.
Attemptswere made to measure the effectof an
aggressivespecies on the total group foragingeffort
and success of a less aggressivespecies; for example,
the contest between the aggressive S and the nonaggressive L for another grid of 25 baits of five
concentrations.On February 19, 1972, beforespecies
S had discovered the grid, 1,245 visits by L were
counted duringa 3-hr period. During a comparable
period (same time of day and weather) 11 days later
with S present,only 252 L visits were counted-a
79.8% reductionin numberof visits. There was also
a pronouncedreductionin thequalityof baits at which
L could successfullyfeed: S partially excluded L
from the preferred0.8 and 2.4 M baits (Fig. 3). If
we make the simplifying (but sometimes false)
assumption that each individual L fed if it was
recorded on a bait, we can estimate the relative
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Winter 1974
AGGRESSION
AND COMPETITION
AMONG
BEES
125
nearer the colony, the more of the baits it would
control;however,this also did not occur. Rather,the
firstcolony to discover the grid controlledthe most
0.0 M
2.4 M
0.4 M
0.2 M
baits, and the last to discoverit controlledthe fewest;
0.8 M
this order it so happened, was inverselyrelated to
colony distance. Both of these observationssuggest
0.2 M
0.8 M
0.0 M
2.4 M
0.4 M East colony
that "first discoverer take all" is the dominant
(>150m)
*
principlegoverningforagingsuccess between equally
D
D
*
0
matched competingcolonies. A complicatingfactor
West \ 2.4 M
0.4 M
0.2 M
0.8 M
0.0 M
051
in interpreting
our resultsis that we have only crude
colon
(-240 m)l?
estimates of the relative size of the foragingforce
of the three competing colonies, based on visual
0.8 M
2.4 M
0.0 M
0.4 M
0.2 M
estimatesof the dimensionsof the nests. The nest at
240 m had the biggest circumference,and the nest
at 23 m the smallest. If nest size is related to size of
0.4 M
0.2 M
0.8 M
0.0 M
2.4 M
foragingforce,which is probably generallytrue,our
results could be explained solely by the increased
South colony
Bait controlled by:
chance of discoveringthe gridwithmore bees search(23 m)
0 No colony *East colony
ing, therebynegatingthe distance effect. The roles
of early discovery, size of foraging force, scentWest colony
*South
colony
marking,visual memory,facilitationof aggressionby
FIG.4. Outcomeof 2-daybattlebetweenthreecolonies the presenceof nestmates,flightdirection,and chance
of Trigona corvina fora gridof 25 baits. Concentrations in determining
whichcolonies controlwhich resources
of baits indicatedby numbersnextto bait symbols.The all remainto be elucidated.
approximate
distancesto therespectivecoloniesfromthe
centerof thegridare givenin meters.
N
Q
00
O0-0O-
IMPLICATIONS
amounts of sucrose hauled by L in the presence and
absence of S. We furtherassume that the amount
of syrup drunk by each bee is constantregardlessof
sugar concentration,given that the bee drinks anythingat all. This was shown to be truein honeybees
over a wide range of sugar concentrations (Wells
and Giacchino 1968). Multiplyingbee visits,v, by
the correspondingsucrose concentration,M, we obtain 1,644.6 vm withoutS and only 201.6 VM with S.
Hence, the foragingsuccess of L, measured in vM
units,is reduced 87.7% by the presence of S. Most
of the reduction (90.9% ) can be attributedto a
lowered number of visits.
The outcome of competitionin terms of settling
who controls the high-qualityfood sources is less
predictablewhen the contestis between more nearly
equal competitors,such as rival colonies of the same
species. In the battle between three colonies of C
discussed earlier, the relativelystable outcome after
2 days of fightingis shown in Fig. 4. The results
are fairlytypical of our other experimentson intraspecific fights in that, by the end, each colony
exclusively controlled a set of quality baits (0.4 M
sucrose or better). We expected the colonies to
control those quality baits nearest their respective
hives, but this proved not to be the case: in general
there was no obvious relationship. We suspect that
this is because the grid is small relative to the total
potentialforagingrange of the species (our estimate
is at least 400 m). We had also expected that the
FOR NATURAL
FOOD
SOURCES
The observed aggression is not an artifact of
baiting, since we have observed similarly intense
levels of aggressionat naturalfood sources,especially
at inflorescencesthat are rich in nectar and pollen
and/or are spatially compact, such as Bactrix palm
and banana inflorescences,and the large flowers of
Bixa, waterlilies,and some curcurbids. Under one
pollen-laden Bactrix inflorescence open for just 2
hours,we found 44 dead bees: 5 S, 26 C, two pairs
of interlockedC, and 3 S attachedto 1, 2, and 3 C.
In anothercase, species S occupied the same banana
inflorescencesin August 1971 and in February 1972.
During those months S could predictablybe found
there threateningand attackingbees and wasps that
hovered and triedto land. S also attacked ants,flies,
and even an occasional chrysomelidor curculionid
beetle.
Such rich, spatially compact resources are usually
controlledby an aggressivespecies. For these species
the advantagesof gainingcontrolof high-qualityfood
sources presumablyoutweighthe costs to the parent
colony as a whole. At present it is impossible to
estimatethe daily losses to a colony fromaggression,
but the proportion of bees that die from injuries
sustained in such agonistic encounters is probably
quite low. Except on the very richestresources,the
process of competitivedisplacementor exclusion can
be deceptivelyundramaticyet very effective. Occasional threat displays, mid-air collisions, and brief
grapples with arrivingscout bees from rival colonies
can prevent the scout from landing and feeding at
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126
LESLIE
K. JOHNSON AND STEPHEN
P. HUBBELL
Ecology, Vol. 55, No. 1
the food source. We have observed, for example, of rival colonies of stinglessbees. It also demonstrates
lone scouts of Trigona fulviventris(F) circling but that agonistic behavior increases with increasing
not landing on bushes of Cassia biflora and Ardisia quality of the food resource. Quality is defined in
revoluta that were occupied by threat-posturing terms of concentrationsand amounts of nectar and
Trigona fuscipennis(FP) individuals or by workers pollen, as well as by the spatial compactness of the
from other colonies of F. Many workers of the resource. This dependence of the intensityof aggresscout's species and/or colony could be foundon other sion upon the quality of the food source is not
Cassia and Ardisia in the vicinity. Exclusion of lone particularlysurprising. But if the phenomenon is
scouts is significantbecause it preventsrecruitment widespread, it is remarkablehow rarely it has been
of workers from the scout's colony. Scouts report described in the ecological or behavioral literature.
The importanceof this resultis that nonaggressive
only the location of food sources at which theyhave
successfullyfed (Kerr 1959). Individual bees learn species are excluded from compact, high-quality
visual landmarksnear the location of food resources resourcesby aggressivespecies. We can only specuthathave providedthemwithtake-homereward;after late about its implicationsfor competitivecoexistence.
making the trip several times they fly to and from At least two types of mechanisms could promote
the nest without laying odor trails. If the food coexistence between aggressive and nonaggressive
source is reasonablylong-lasting,the bees will return species. The firstis specialization on differentfood
to the site for days (Colwell and McCafferty1969), sources. Whateverthe selectiveforces that produced
continuingto forage there until the old site is ex- large stinglessbees, it is likelythat the larger species
hausted of food before they scout for new sources. cannot afford the energetic cost of visiting small,
This behavior tends to reinforcethe foragingsepara- widely spaced plants with flowers providing slight
amounts of pollen and nectar (Heinrich and Raven
tion that has resultedfrom aggression.
In some cases lone scout bees do not land on a 1972). Our impressionsare thatsmall bees do indeed
food source even though no overt agonisticbehavior specialize on small-flowered,over-dispersedplants.
is exhibited by the first occupant. There is some Moreover, on baits we consistentlyobserved the
evidence that the scout is activelyavoiding the food lowest intensityof aggression between the largest
source, probably because of the odor of the foreign species S and the smallest species L, which suggests
bee's markingpheromoneon the food source. Scouts thatin naturethesespecies may encountereach other
of F are reluctantto land on, and will not recruitto, more rarelythan do species more nearlyequal in size.
When aggressive and nonaggressive species do
a bait recentlyexploited and marked by FP, even
when the bait is moved to a site where only F has overlap in naturalfood sources, a second mechanism
access to the bait. This phenomenon cannot be mightforestallexclusion for relativelylong periods,
attributed to aggression. Perhaps it is due to a even where these shared resourcesare limitingto the
combinationof (a) avoidance of a potential attack, species. Spatial heterogeneitycoupled with differsignaled by the odor of a rival colony or species, and ences in mode and efficiencyof foragingmay produce
(b) preventionof timeand energywasted in foraging such delayed exclusion, even though by themselves
at a food source that has recentlybeen or is being they cannot stabilize an otherwiseunstable competiexploited by other bees. The second explanation is tive interaction. It was the rule rather than the
plausible in situationswhen the firstcolony to dis- exception that species such as L and T found the
grids of baits before (rarely after) the aggressive
cover a food source is likely to get it all.
Less aggressive species have other tactics for species S and C. In otherwork we have seen earlier
procuringresources. Some utilize spatiallydispersed, discovery of baits by subgenus Plebeia than by
less defendableresources. Small species, particularly colonies of S or F present in the same community.
of the subgenus Plebeia, have been seen to land on If it is true that most nectar sources are quickly
baits occupied by aggressivespecies such as S, feed exhaustible,periodicallyrenewedfood supplies, nonfor very brief periods of time, and then fly up at aggressivespecies might persist for very long times
the slightestthreat or hint of movement in their simply by being more efficientat discovering and
direction by S. Rather than flyingaway, however, exploiting shared resources than their aggressive
theytypicallyhover near the bait in a dancing flight, counterparts.Aggressivenessperhaps has evolved in
and reland at the firstclear opportunity.It was by larger species in part as compensation for longer
this method that some L were still able to visit the discoverytimes. However, in our opinion it is much
2.4 M baits in the experimentdiscussed above, even more likely to have arisen from intense intraspecific
competitionover the controlof high-quality,compact
though the baits were heavily visited by S.
food sources. Insofar as the outcome of aggressive
DISCUSSION
encounters also depends upon the relative size of
This study shows that aggression can produce workers of the rival colonies (the bigger in general
major changes in the foragingpatternsand success being more likely to win), we also suggestthat such
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Winter 1974
AGGRESSION
AND COMPETITION
intraspecificcompetitionselected for increased body
size, up to the limit set by the energetic costs of
foragingfor the food resourcesin question.
This study also suggests how intraspecificcompetition operates in stinglessbees. Colonies of the
same species do not escape competitionby specialization, but they do reduce competition by foraging
more efficientlynear their own nest. If the "first
discoverertake all" principleis of overridingimportance in foragingsuccess, each colony should have
the advantageover competingcolonies when the food
source is nearby,both in termsof probabilityof first
discoveryand in termsof the energeticcost of collecting the food. If species of stinglessbees are foodlimited, we might then expect the colonies to be
relativelyuniformlyspaced in theirenvironmentwith
minimallyoverlappingfeedingareas. In at least some
species of bees, notablyS, C, and F, we have indeed
found uniformspacing patternsbetweenthe colonies.
Thus far, detailed foraging informationin these
species has not been obtained,but we have observed
aggressionbetween adjacent colonies of F over food
sources located betweenthe nest sites. The resultsof
the grid experimentsreported here do not test the
hypothesisthatthereis an inverserelationshipbetween
probability of discovery and distance to the food
source in a single colony. In our intraspecificstudy,
three nests of C were involved, which confounded
distance effectson time to firstdiscovery,with the
effectsdue to differencesin the size of the colony
and its foragingforce.
ACKNOWLEDGMENTS
We thankR. Caldwelland an anonymousreviewerfor
criticallyreadingthe manuscript,and the staffof the
de CienciasAgricolas,Turrialba,
Instituto
Interamericano
Costa Rica for their generouscooperation. We have
benefited
withmanypeople,in particular
fromdiscussions
G. Pyke,P. Opler,D. Titman,and L. Master. The work
was supportedby NSF grantGB-33324 and by a grant
fromthe RackhamSchool of GraduateStudiesand the
AMONG
BEES
127
Committee on Tropical Studies of the University of
Michigan.
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