Contributions To the Biology of the Queenless Ponerine Ant Diacamma Ceylonense Emery (Formicidae)

11 Pages·2003·6.2 MB·

by  Vedham Karpakakunjaram

CONTRIBUTIONS TO THE BIOLOGY OF THE QUEENLESS PONERINE ANT
DIACAMMA CEYLONENSE EMERY (FORMICIDAE)
(Withfive text-figures)
Vedham Karpakakunjaram1 Padmini Nair1 Thresiamma Varghese1 George Royappa1
, , , ,
Milind Kolatkar2 and Raghavendra GADAGKAR1’2’3
Key words: Diacamma ceylonense queenless ant, Ponerinae, Formicidae, Hymenoptera
,
All ants are generally classified as highly eusocial. However, there are some 100 species ofants
belonging to the morphologically primitive subfamily Ponerinae, which lack a morphologically
differentiatedqueen. One orasmall numberofmated workers (gamergates) function as queens, i.e.
they produce male as well as female offspring. Such ant species are ofgreat interest as they provide
unique opportunities to understand the causes and consequences ofqueenlessness. This is the first
report ofa long-term field study we have initiated on a large population ofthe queenless ponerine
ant, Diacamma ceylonense Emery, on the campus ofthe Indian Institute of Science, Bangalore.
Data are provided on the numbers ofcolonies present during a 213-week period, from which the
probable time ofcolonisation ofthe site by D. ceylonense is suggested. Twenty-six entire colonies
were excavated to study the adult and brood composition. While some colonies did not appear to
have a gamergate at the time ofexcavation, others had only one gamergate per colony. Only the
gamergate was mated and had well developed ovaries, while all the workers were unmated and had
undeveloped ovaries. Excavated colonies had an average of230 adult females, 2 males and 80 items
of brood. The gamergates were indistinguishable from their workers in body size but there was
significant variation in the size ofworkers between colonies. On an average, colonies employed
24% oftheir workers outside the nest for foraging and other duties. The relative constancy ofthis
proportion permits estimation oftotal colony size by merely estimating the number ofextranidal
workers and thus without having to excavate the nests. In the five colonies studied, foragers spent
an average of 9 to 23 days oftheir lives in performing foraging duties. Most foragers remained
faithful to one oranarrow range ofcompass directions, although such directional preferences ofall
ofthem put together permitted colonies to exploit resources in all directions.
Introduction In this scale of social evolution, all ants are
generally classified as highly eusocial (Holldobler
Eusocial insects are defined as those and Wilson 1990, Bourke and Franks 1995).
insects whose colonymembers exhibit overlap of However, some 100 species ofants, belonging to
generations, cooperative brood care and the morphologically primitive subfamily
reproductive caste differentiation into a queen Ponerinae, lack amorphologically distinguishable
caste and a worker caste. Highly eusocial species queen. Their colonies consist only ofworkers (in
are those in which the queen and worker castes additionto males ofcourse), one or a few ofwhom
are morphologically differentiated (Wilson 1971). are mated and take on the function ofqueens, i.e.
production of male and female offspring. These
'Centre for Ecological Sciences,
mated, reproducingworkersaretermed gamergates
Indian Institute of Science,
Bangalore 560 012, Karnataka, India. (Peeters 1991). Queenless ants are ofgreat interest
Evolutionary and Organismal Biology Unit, for several reasons: (1) They provide an
Jawaharlal Nehru Centre for Advanced Scientific
opportunity to understand the conditions under
Research, Jakkur, Bangalore 560 064,
Karnataka, India. which the queen caste may be lost. (2) In the
Email: [email protected] absence ofwinged queens, new colonies have to
JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, 100(2&3), AUG.-DEC. 2003 533
BIOLOGY OF THE QUEENLESS P0NER1NE ANT
be necessarily founded by walking gamergates, Diacamma ceylonense Emery, which is
which must result in a rather unusual population abundantly distributed in and around Bangalore.
genetic structure. (3) They can be used as model There is a large population of D. ceylonense in
systems to understand how one ora small number the so-called Jubilee Garden in the campus ofthe
of individuals can establish and maintain Indian Institute ofScience, Bangalore (13° 00' N
reproductive monopoly among a group of 77° 32' E). Such an easily accessible population
identical, ornearly identical, individuals. This is a facilitates observation ofcolonies in their natural
problem that has been extensively investigated habitats. Most previous work on Diacamma and
,
with bees and wasps (see Gadagkar 2001 and indeed on most ponerine ants, has been largely
references therein) but because queenlessness restricted to observation in artificial nests in the
in ponerine ants is evolutionarily derived from laboratory, ofants removed far from their natural
the queenright condition (Baroni Urbani et al. habitat.
1992), a new perspective is expected from a study For all these reasons, we have initiated a
of these ants. long-term study of the population of
In this context, the genus Diacamma D. ceylonense in the Jubilee Garden ofthe Indian
represents a most fascinating example. In Institute of Science, Bangalore. This is the first
Diacamma all females eclose with a pair ofclub- report of this ongoing study.
,
like thoracic appendages called gemmae. The
gamergate mutilates all workers who eclose after Material and Methods
her, by clipping their gemmae. Such mutilation
results in poorly understood neurological changes This study was conducted in the Jubilee
in the victims, making them incapable of sexual Garden, Indian Institute of Science, Bangalore.
calling and mating. This unique mechanism by The site is a plantation ofAcacia auriculiformis
,
which the gamergate maintains her reproductive with a few individuals of Polyalthia longifolia,
monopoly seems to have a built-in mechanism to Tamarindus indica and Ficus sp. The area (about
ensure that a colony does not necessarily die if two hectares) was divided into 60 line transects.
its gamergate dies. Mutilated workers do not Initially, an extensive search was conducted to
mutilate others so that, upon the death of a locate the nests ofD. ceylonense. All nests were
gamergate, the next individual to eclose retains marked with sequentially numbered aluminium
her gemmae, mutilates all those that eclose after tags and the tree nearest to each nest was marked
her and takes over as the next gamergate ofthe with red paint. Every Monday, the nests were
colony (Fukumoto et al. 1989, Peeters and monitored for activity and for the occurrence of
Higashi 1989). While this description fits all other males. Nests that appeared to be abandoned were
species ofDiacamma examined so far, there does checked by inserting a stick inside the nest
appear to be an exception. There is at least one entrance and waiting for the appearance of
species in which the gamergate seems to be workers. In addition, such nests were also
capable of maintaining reproductive monopoly monitored for activity during the subsequent 4-5
without mutilating her workers. This species, weeks, before confirmingthem as abandoned. On
which was first found at the foot of the Nilgiri the first Monday and Tuesday of every month,
hills, and mistakenly identified as Diacamma the entire study site was searched systematically
vagans Smith (Peeters et al. 1992), is yet to be to record any newly initiated nests. Data thus
described. We therefore refer to it here as obtained, during 213 weeks, on the numbers of
Diacamma sp. from Nilgiri. Diacamma sp. from nests, numbers of newly initiated nests,
Nilgiri is morphologically very similar to abandoned nests and number of males, were
534 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, I00(2&3), AUG.-DEC. 2003
BIOLOGY OF THE QUEENLESS PONERINE ANT
analysed in relation to abiotic factors such as observations and continuing throughout the
maximum and minimum temperatures and total period ofobservation. All extranidal behaviours
rainfall usingthe Kendall’s correlation coefficient. visible to the observer, positioned near the nest
Climatic data were obtained from the India mound, were recorded. The 852 hours of data
Meteorological Station, Bangalore. As there was a thus collected, were used to compute the number
monotonic increase in the number ofnests during the of extranidal ants active per day, the number of
first 120weeksofstudy, alinearregressionmodelwas days for which different ants were active, and
fitted to these datawhich was then used to predictthe the rate per hour at which they performed
probable time of colonisation of Jubilee Garden by extranidal activities. When the ants left the
Diacamma ceylonense. vicinity ofthe nest mound, the compass direction
Twenty-six colonies with all individuals and in which they departed was recorded in eight
brood were collected from within the Indian discrete directions namelyNorth, Northeast, East,
Institute of Science campus, but outside the Southeast, South, Southwest, West and
Jubilee Garden. Nest characteristics such as height Northwest. When the foragers returned, what
ofthe mound, number ofbrood chambers, depth they brought back (food, building material or
of the nest and commensals encountered, were nothing) was noted. The proportion ofsuccessful
recorded for all these nests. Head width and and unsuccessful trips made by foragers who
alitrunk length ofall individuals were measured in had made at least three trips were then computed
21 of these colonies. Data on head width and and compared. A successful trip was defined as
alitrunk length were subjected to one-way analysis the act ofreturning to the nest with either food or
of variance to partition variance within and building material. An unsuccessful foraging trip
between colonies. In the nine colonies that had a was one in which the ants returned with neither
gamergate, one-sample t-tests were performed to food nor building material. Data on foraging
compare each gamergate with her workers. In 15 directions of all active foragers were subjected
ofthese 2 colonies (including the nine that had a to Rayleigh’s test (Batschelet 1981) of
1
gamergate), the numbers of mature eggs, width randomness to determine the degree of
and length ofproximal oocytes and the presence randomness in the foraging paths, both for
(or absence) of sperm in the spermatheca, were individual ants as well as for colonies as a whole.
recorded for all individuals by dissecting their Another eleven colonies were excavated in
gaster. an attempt to predict the colony size from the
Five other colonies were chosen for numbers ofants engaged in extranidal tasks. Over
observation of the extranidal behaviours. Four a period of three days, all ants seen outside the
ofthese colonies were observed for four weeks nest were marked. At least two persons spent 3
each while one was observed for 55 weeks. Each hours and 30 minutes each, per day (30 minutes at
observation session was of4-hour duration and the beginning ofevery hour from 0600 hrs-1800
was conducted either in the morning (0600 to hrs) for three consecutive days, marking the ants.
1000 hrs), midday (1000 to 1400 hrs) or afternoon Pilot experiments had indicated that this effort
(1400 to 1800 hrs). One each of the morning, was morethan adequate to markall extranidal ants.
midday and afternoon sessions were completed On the fourth day the nest was excavated to
in each week, thus yielding 12 hours ofdata per determine the colony size (these nests were then
week, per nest. All new workers seen outside the used for other experiments not reported here) so
nest were uniquely marked with quick-drying that the relationship between the number of
coloured paint on the thorax or abdomen, starting extranidal ants and the total colony size could be
two weeks before the commencement of ascertained.
JOURNAL BOMBAY NATURAL HISTORY SOCIETY, I00(2&3), AUG.-DEC. 2003 535
BIOLOGY OF THE QUEENLESS P0NER1NE ANT
Results and Discussion Table 2: Co-inhabitants observed in
Diacamma ceylonense nests
Diacamma ceylonense nests were found Commensal No. of nests in which
to occur quite commonly in the study area. The observed
nests are subterranean, with a distinct mound at Cockroaches 12
the entry of the nest. The mounds are often Crickets 11
decoratedwith drytwigs, leaves, insect and spider Pseudoscorpions 9
Bagworms 3
exuviae, and occasionally, bird feathers. Moffett
Grubs
1
(1985) observed that these decorations are used
Earthworms
1
to harvest dewdrops, which the ants drink in the Isopods 14
mornings. The excavated nests contained 230.5 Other ant species 1
±108.6 adultfemaleants, 2.3 ±5.3 males, 18.5 ±19.4
eggs, 22.6±18.7 larvae and40.0 ±36.6 pupae. The yet complete. Of the 15 colonies used for
nests were 63.9 ± 20.6 cm deep under the ground dissection, 9 had one gamergate each, while the
with 4.1 ±1.2 brood chambers and the mounds remaining 6 colonies did not have a gamergate.
were 3.2 ±4.1 cm above the ground (Table 1). The number of mature eggs in the gamergates
Males were usually found in the first chamber, ranged from 0-24 with amean of8.78 ±10.28 eggs.
closest to the entrance. The next 2-3 chambers None ofthe workers had developed ovaries and
had brood and the last chamber at the bottom none ofthem were mated in any ofthe colonies,
contained refuse. We often found cockroaches, including those without a gamergate (Table 1).
isopods, crickets, pseudoscorpions, bagworms, Thus D. ceylonense colonies are monogynous
grubs, earthworms and a few other ant species in and recent molecular studies suggest that they
the nests; we presume that these were commensals are also monoandrous but with some serial
(Table 2). Nine out of the twenty-one colonies polygyny (gamergate turnover) (Gopinath 2002).
excavated, seemed to lack a gamergate; the For both head width and alitrunk length,
possibility that the gamergate was lost during there was a significant added variance between
excavation seems unlikely but cannot, ofcourse, colonies. In the case of head width, this added
be entirely ruled out. The remaining 12 colonies variance between colonies was significantly
invariably had only one gamergate each. In two higher than the variance within colonies, while in
nests, one or two individuals other than the the case of alitrunk length, this variance was
gamergate also had gemmae, but only on one side significantly less than the variance within
oftheir thorax; perhaps their mutilation was not colonies (Table 3). Variance between colonies is
Table 3: One-way analysis of variance of head width and alitrunk length of individuals
of Diacamma ceylonense from 21 colonies
Source DF SS MS F
P
Head width3 Among colonies 20 1811.07 90.55 791.42 0.001
Within colonies 4731 541.31 0.11
Total 4751 2352.38
Alitrunk lengthb Among colonies 20 1151.20 57.96 94.61 0.001
Within Colonies 4683 2849.00 0.61
Total 4703 4000.19
3Variance between colonies significantly higher than variance within colonies
bVariance between colonies significantly lesser than variance within colonies
536 JOURNAL BOMBAY NATURAL HISTORY SOCIETY, I00(2&3), AUG.-DEC. 2003
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JOURNAL BOMBAY NATURAL HISTORY SOCIETY, 100(2&3), AUG.-DEC. 2003 537
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1
BIOLOGY OF THE OUEENLESS PONERINE ANT
200
likely to have a genetic basis and variance within
colonies, a nutritional basis. As expected from
the factthatthe gamergate is simplyan unmutilated
worker, in 17 out of18 tests (9 colonies, eachtested
for headwidth and alitrunk length), the gamergate
was not significantly different in body size as
comparedtothe workers in the colony (one sample
t-test; Table 3).
The number of active nests steadily
increased from the commencement ofthis study
(December 1993) up until 120 weeks. After that
the number ofactive nests decreased to an extent
(Fig. 1a). In our213 weeks ofstudy, we recorded a
total of 1 17 nests, active at one time or another.
Although new nests were initiated and abandoned
throughout the study period, initiations usually
outnumbered abandonings during the first 175
weeks, while abandonings outnumbered
initiations during the next 38 weeks (Fig. lb,c).
Twenty-one out of 117 nests were active
throughout the 2 3 weeks ofobservations. Given
1
that they were already present at the beginning
of the study, they must have been older than
213 weeks. Males were few in number but
were recorded more or less throughout the
year (Fig. Id). Climatic data are depicted in
Fig. 1e. There was a positive correlation between
minimum temperature and number of nests
initiated (x = 0.26 at p<0.008) and abandoned 150 200
Week
(i = 0.38, at/?<0.000 ). There was also a positive 40 450
1
correlation between rainfall and number ofnests
initiated (x = 0.43 at /?«0.01) and abandoned O 300
o
(t = 0.39, at p<0.0001) and number of males A~\A
CL 20 JvT Rainfall
(x = 0.29, at p<0.005). Maximum temperature £
- 150
showed no such correlation. j
UL
uliJl
Since the number ofnests increased steadily
,
duringthe first 120 weeks, a linearregression model Dec Dec Dec Dec
1093 1994 1995 1996 1997
was fitted to the data (Fig. 2). The model is highly
Month
significant. Extrapolatingbackwards in time, using
the slope and its 95% confidence interval, we may Fig. 1 : Thenumberof(a) active nests, (b) nests initiated,
(c) nest abandoned, (d) males and (e) mean monthly
hypothesize that the Jubilee Garden was colonised rainfall (bars), mean monthly maximum temperature
by D. ceylonense some 85 to 1 13 weeks before the (upper line) and mean monthly minimum temperature
beginningofthe study, i.e. between September 199 (lower line), during the 213 weeks of census of the
and May 1992 (Fig. 2). Jubilee Garden population ofDiacamma ceylonense
538 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, 100(2&3), AUG.-DEC. 2003
BIOLOGY OF THE QUEENLESS P0NER1NE ANT
Fig. 2: Number ofactive nests from the 120-week data was used to fit a linear regression model which was
extrapolated backwards using the slope and its 95% confidence intervals, to estimate the probable time of
colonisation ofJubilee Garden by Diacamma ceylonense
Diacamma ceylonense workers are extranidal tasks like nest cleaning and shifting of
individual, diurnal foragerswith directional fidelity materials around the nest increased soon after
within a narrow range ofdirections (as seen from rainfall. On finding an item of prey, the forager
our observations given below). Their major food paralysed it by stinging and returned with it to
consisted of termites, although bugs, the nest quickly and in a short, more linear path
cockroaches, grasshoppers, crickets, spiders and as compared to the often long-winding path it
other species of ants were also observed being had followed during its outwardjourney. When a
brought to the nest on some occasions. Prey, termite nest or trail was discovered, the successful
however large, was never retrieved in groups, but forager made repeated trips to the same source.
there was some cooperation among the workers The mean number ofactive individuals seen
near the nest to manoeuvre it through the nest outside the nest during any 4-hour observation
entrance. Foraging continued for most ofthe day session varied from about 6 to 32 and the mean
on colder days, but was restricted to the mornings foraging life span ofworkers ranged from 9 to 23
and evenings only, on warmer days. Foraging was days (Table 4). Foraging was the most frequently
suspended during heavy rains although some performed extranidal activity. In four out offive
Table 4: Number of active animals per observation session, and foraging life span in five colonies
Nests No. of sessions & No. of active animals/session3 Foraging life span in days
No. of days observed Mean ±SD (Range) Mean ±SD (Range)
071 165 sessions; 385 days 6.7 ±3.6 (1-17) 23.3 ±22.2 (1-154)
025 12 sessions; 26 days 17.2 ±2.2 (11-21) 13.2 ±10.2 (2-26)
056 12 sessions; 27 days 32.3 ±8.0 (22-51) 9.0 ±7.6 (2-27)
0117 12 sessions; 26 days 12.9 ±3.7 (5-18) 12.9 ±8.7 (2-26)
0134 12 sessions; 23 days 16.2 ±5.5 (4-22) 15.5 ±6.1 (2-23)
a session = 4 hours
1
JOURNAL BOMBAY NATURAL HISTORY SOCIETY, 100(2&3), AUG.-DEC. 2003 539
BIOLOGY OF THE QUEENLESS PONERINE ANT
Fig. 3: Proportions ofsuccessful (shaded bars) and unsuccessful (blank bars) trips made by foragers in five
colonies (numbers above bars are the numbers ofsuccessful/unsuccessful trips)
colonies (Fig. 3; Mann Whitney U test; Colony individual foragers were non-random with respect
71, 56and 134atp<0.001,andColony 1 17atp<0.05), to the directions in which they made foragingtrips,
the number of successful foraging trips was butthat the colony as a whole (with all its foragers
significantly less than the number ofunsuccessful put together) was random in this regard. In each
trips. In one colony (Fig. 3, Colony 25), foragers colony, even the most random forager was less
were somewhatmore successful in retrieving prey so than the colony as a whole (according to
items, but the number of successful and Rayleigh’s test of randomness, p<0.05, see Fig.
unsuccessful trips did not differ significantly from 5). The mechanism by which the foragers achieve
each other (Mann Whitney U test, p>0.05). such collective randomness, in spite ofindividual
Foragers who had made at least three trips were non-randomness, remains to be investigated.
used for this analysis. The proportions of Experiments in which all the ants seen
successful and unsuccessful trips were computed outside the nests were marked before nest
for each worker and then averaged across workers excavation demonstrate that the number of ants
in each colony. seen outside the nest is a reasonably constant
Many foragers seemed to be faithful to proportion ofthe total colony (24.39% ±9.63%)
certain foraging directions (Fig. 4), although the (Table 5). This information can be used to
colony itself seemed to have foragers covering approximately infer colony size without
all directions. Rayleigh’s test of randomness necessarily having to excavate nests. Besides
confirms that in all five colonies studied, helping to avoid unnecessary excavation ofnests,
540 JOURNAL BOMBAY NATURAL HISTORY SOCIETY, 100(2&3), AUG.-DEC. 2003
BIOLOGY OF THE QUEENLESS PONERINE ANT
Table 5: Colony size and percentage
of extranidal ants
Colony No. of Colony Percentage
code extranidal size of foragers
Colony 71
ants marked
n=96; T=1830;
DC28 69 457 15.10
DC29 89 376 23.67
100n DC30 135 593 22.77
80- DC31 23 192 11.98
DC42 23 121 19.00
60 -
Colony 25
DC47 47 145 32.41
n=28; T=265;
DC52 47 271 17.34
DC53 24 137 17.51
DC64 60 145 41.38
DC65 39 103 37.86
DC66 12 41 29.26
Colony 56
Mean ±SD 51.6 ±35.9 234.6 ±171.6 24.39 ±9.63
n=65; 1=231;
the proportion of foragers is in itself a useful
parameter in many kinds of studies (see for e.g.
Andre et al. 200 Gopinath 2002). Investigations
1 ,
to determine intra-colony genetic relatedness
using microsatellite markers, population genetic
Colony 117 structure using both nuclear and mitochondrial
n=23; T=94; markers and the mechanism of mutilation of the
gemmae are in progress and would form the
subjects of future publications.
Acknowledgements
We thank the Ministry ofEnvironment and
Colony 134
Forests, Department ofScience and Technology,
n=19; 1=66;
Council for Scientific and Industrial Research,
Department of Biotechnology, Government of
India and the Jawaharlal Nehru Centre for
4 5 6 7 8
Advanced Scientific Research, Bangalore, for
Number of directions foraged financial assistance, and several student trainees
n = number of foragers observed and visitors, including Karthik, Nutan, Sumana,
T = total number of trips observed Vidya, Yuvana, Andre, J.-B. and Ravi Kumar for
help with marking ants in the field, for various
Fig. 4: Frequency distribution ofnumbers offoragers experiments. We also thank Christian Peeters for
who foraged in different numbers ofdirection
helpful comments on an early draft ofthis paper.
in five colonies
JOURNAL BOMBAY NATURAL HISTORY SOCIETY, 100(2&3), AUG.-DEC. 2003 541
,
BIOLOGY OF THE QUEENLESS PONERINE ANT
N
(a)
(b)
(c)
T
239 15
W
244 „ . 268
h>-
c
332
o %
W^
104
166
206
(n=96; r=0.13)
lO 11
CM
11
c
o 21
o
O
25
(n=28; r=0.59)
25
CO
to
>
c
o
o
O
53
(n=65; p=0.11)
(r=0.36)
n = no. of foragers for the colony as a whole; r = randomness
Bars are proportions of number of trips made by forager(s) in different directions. Values beside each bar
indicate the number of trips made by one or more foragers (in a) and one forager (in b and c).
Fig. 5: Foraging direction for (a) colony as a whole, (b) the most non-random forager and (c) for the relatively
most random forager, in the five colonies studied.
542 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, I00(2&3), AUG.-DEC. 2003