Honeybee nestmate recognition: the thermal behaviour of guards and their examinees
Institut für Zoologie, Universität Graz, Universitätsplatz 2, A-8010 Graz, Austria
* e-mail: anton.stabentheiner{at}uni-graz.at
Accepted 30 May 2002
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Summary |
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Key words: honeybee, Apis mellifera carnica, guard bee, examined bees, nestmate recognition, thermoregulation, thermal behaviour, thermography
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Introduction |
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The observation of unexpected thoracic heating phases in the examinees led
to several hypotheses about their biological significance. One suggestion was
that examinees heat up to get rid of guards' pheromones, which marks them as
possible intruders. One substance that is known to have pheromonal function in
honeybees and is suspected to be a marking pheromone is 2-heptanone from the
mandibular glands (Shearer and Boch,
1965; Free, 1987
).
We tested 2-heptanone for its effect on the thorax temperature of bees.
Guards examine other bees not only at the nest entrance but also inside the
nest (Butler and Free, 1952;
Stabentheiner, 1994
). The
behaviour of a guard examining a bee resembles to some extent the behaviour of
grooming bees during social grooming interactions inside the colony. In order
to see whether or not the thoracic heating phases of the examined bees also
exist in groomed bees, that is, whether they are required for grooming, we
measured the thorax temperature of bees performing grooming dances and of
groomed and grooming bees.
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Materials and methods |
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To test for the effect of 2-heptanone on thoracic heating, we applied 0.9
or 9 µg of 2-heptanone dissolved in small droplets (100 nl) of paraffin oil
to bees in an observation hive and measured their body temperature
thermographically. These amounts were chosen on the basis of total content of
2-heptanone in honeybee heads (Boch and
Shearer, 1967; Sakamoto et
al., 1990
). For comparison, the same amount of pure paraffin oil
was applied.
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Results |
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Especially during the longer lasting examinations, one examinee was usually inspected by several guards, who took turns to examine them (Fig. 2). A surprising finding was that the examinees showed a much higher variation in Tths than the guards (Table 1A). Though some examinees decreased their Tths during the initial phase of examination (Fig. 2), the high variation resulted for the greater part from phases of intense thoracic heating (Figs 1A1, A2, 2). This way, 21 out of 100 examinees that were examined for periods of more than 30 s reached a Tths as high as 43-48.5 °C! One or several heating phases of more than 2 °C was observed in 64 % of these long lasting examinations (Fig. 3). The delay between the first contact of the first guard with an examinee and the beginning of the first heating phase ranged from approximately 1 s to several minutes. Because of the heating phases, the total mean of the examinees' Tths (36.1 °C) was higher than their Tths at the time when the first guard made contact with them (35.1 °C; Table 1A). The mean increase of Tths during heating phases >2 °C amounted to 4.9±2.48 °C (mean ± S.D.) (maximum: 13.5 °C), and the mean heating rate was 7.0±2.72 °C min-1 (range: 2.8-15.5 °C min-1; N=49). During shorter examinations of 10-30 s duration, only in 3 % of the examinees were heating phases of more than 2 °C observed, but 91 % kept their thorax temperature within ±2 °C (Fig. 3).
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Since it is impossible for the guards to examine all bees at the nest
entrance, many returning foragers enter the colony without having contact with
them. Consequently, guards are also active inside the nest
(Butler and Free, 1952;
Stabentheiner, 1994
). Because
of the higher air temperature inside the colony, the lower limits of thorax
temperature were at a higher level, with Tths ranging from
33.5 to 39.4°C in the guards and from 35.4 to 46.7°C in the examinees.
Like at the hive entrance, the guards' mean Tths
(36.6°C) was below that of the examinees (42.2°C;
Table 1B). The examinees showed
similar heating phases to those at the nest entrance
(Fig. 4). In contrast to
examinees and guards, the grooming dancers, groomed and grooming bees
increased their thorax temperature only slightly above ambient temperature
(Table 2). Heating phases of
groomed bees comparable to those of examinees were never observed.
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Examinees intercepted by a guard for thorough examination exhibited a
typical submissive posture, with bended abdomen to expose their tergal glands,
and after some time, they stood on their four hindlegs to display
self-grooming of the head and tongue stropping with the forelegs (cf.
Butler and Free, 1952).
Thermography showed in many of these examinees that the proboscis was
similarly warm as the head when they started tongue stropping, but after some
time it became visible as a cold spot (Fig.
1B). We conclude that it cooled down via evaporation
because it was wet. This interpretation is supported by the observation that
when some bees touched the hive wall with their proboscis they left behind a
cool spot. Head surface temperature during tongue stropping, as measured
between the ocelli, remained below 43°C even in the most intensely heating
bee with a Tths of 48.5°C.
In the experiments where 2-heptanone was applied to hive bees, the largest
increase of Tths within a period of 2 min was evaluated.
We were not able to elicit thoracic heating of more than 1°C at higher
frequency in bees treated with 0.9 or 9 µg 2-heptanone/100 nl paraffin oil
(26.7 %, N=15, with both concentrations) in comparison with bees
treated with pure paraffin oil (30 %, N=10; 2=2.619,
d.f.=1).
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Discussion |
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In many of the examined bees, their mouthparts were warm in the beginning
and then cooled down, very probably because they became wet during
examination. This resembles the situation in hive bees receiving food from
foragers. The recipient's proboscis becomes warmer when the warm nectar of the
donors reaches it, and it cools down afterwards because of evaporation
(Stabentheiner et al., 1995;
Farina and Wainselboim, 2001
).
Regurgitated droplets of fluid have a significant cooling effect not only on
the mouthparts but also on the whole head and even the thorax
(Heinrich, 1979b
). Therefore,
our data suggest that evaporative cooling via regurgitated fluid
helped the examinees to prevent their brain from overheating during intense
thoracic heating. However, because tongue stropping with a wet proboscis in
many examinees preceded thoracic heating and was accompanied by self-grooming,
which distributes the fluid from the mouthparts over the head, its exact role
in the identification process remains to be investigated.
Several hypotheses for the biological significance of the examinees' phases
of thoracic heat-up have to be considered: (1) Although honeybees increase the
temperature of their thoracic flight muscles to facilitate take-off
(Esch, 1960; Heinrich,
1979a
,
1993
;
Schmaranzer and Stabentheiner,
1988
), it is improbable that the heating phases serve as a
preparation for easier escape because the heated examinees remained in the
typical submissive posture and did not attempt to fly away. Inside the nest on
the combs, escape by flight is prohibited by the narrow space anyway. Both
outside and inside the nest, most examinees that had heated up did not even
run away immediately after all guards had terminated examination. Many of them
showed phases of auto-grooming after all guards had left
(Fig. 4). (2) We exclude the
hypothesis that the heating phases are a request for grooming, because the
interaction between grooming and groomed bees was observed to be more
leisurely than between guards and examinees, and because the interaction
started before the thorax temperature increased. The thorax temperature of
grooming dancers, groomed and grooming bees increased only slightly above
ambient temperature (Table 2).
The examinees never showed an active behavioural pattern such as grooming
dances, which stimulate the grooming behaviour of hivemates
(Haydak, 1945
). (3) The
remaining hypothesis is that the heating phases play a role in the
identification process. The examinees might heat up because they want to get
rid of (3a) pheromones from the guards that mark them as possible intruders or
(3b) volatiles that disturb identification or (3c) because they want to
increase evaporation of substances that help guards to identify them as
hivemates. 2-heptanone from the mandibular glands has a pheromonal function in
honeybees (Shearer and Boch,
1965
), and it is under suspicion to be used as a marking pheromone
(Free, 1987
). However, because
we were not able to elicit an increased frequency of thoracic heating through
treatment with 2-heptanone we do not attribute hypothesis (3a) a high
priority, though we cannot completely disprove it at present.
Some support for the hypothesis (3b) that the examinees want to get rid of
chemicals that disturb recognition comes from a recent observation that it
takes longer for guards to reject examinees that are treated with floral oils
(Downs et al., 2000). This,
however, does not contradict the hypothesis (3c) that the examinees enhance
their chemical signalling by increasing the evaporation of compounds from
their surface by increasing its temperature. From the whole blend of odours of
a honeybee that may play a role in nestmate recognition (e.g.
Page et al., 1991
;
Breed and Julian, 1992
;
Arnold et al., 1996
;
Breed, 1998
;
Fröhlich et al., 2000
),
it is some of the fatty acids or their esters and some of the short chained
alkanes and alkenes that gave positive results in recognition bioassays
(Breed and Julian, 1992
;
Breed, 1998
). Honeybees are
also especially sensitive to fractions containing the more polar components of
comb waxes (hydroxy alkyl esters, acids and primary alcohols) in tests using
the proboscis extension reflex
(Fröhlich et al., 2000
).
The vapour pressure of these substances at the thorax temperature of returning
foragers and examinees (28-48 °C) is low but depends on temperature. In
the liquid phase of cis-9-octadecenoic acid (oleic acid), for
instance, it increased by a factor of about 5.9 between 30 and 40 °C
(approx. 3.63x10-5 to 2.16x10-4 Pa)
(Dykyj et al., 2000
). In an
alkane that yielded a positive bioassay result (hexadecane), the vapour
pressure of the liquid phase increases by 2.7 times in the same temperature
interval (0.374-1 Pa) (Dykyj et al.,
2000
). An increase of this magnitude may enhance detectability for
the guards, all the more so as many of the substances that play a role in
nestmate recognition are acquired in minute quantities from the environment
inside the colony (Breed,
1998
). Although it is clear that substances other than those
tested by Breed and coworkers (1992, 1998) play a role in nestmate
recognition, our results show that body temperature is a parameter that has to
be considered in research on nestmate recognition in honeybees.
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Acknowledgments |
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References |
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