Honey-crop Load of Honey Bee Undertakers as

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Thorax Temperature and Honey-crop Load of Honey Bee Undertakers as

Potential Constraints on Thsk Performance BY JOSEPH R. COELHO

Abstract In this study I examine the relationship berween physiological states of worker honey

bees

and the lift requirements of these tasks. Undertaking has the grearest lift constraint of any honey bee flight behavior, anl the maximal force production of a worker honey bee barely allows it to carry a bee corpse of typical mass. Undertakers carrying corpses had thorax remperatures (40.0 + 0.2"C) significantly higher than those of undertakers patrolling or bees of similar age performing other tasks in the same environment. More importantly, the thorax temperature of undertakers falls within a very narow range that provides near-maximal force production during flight. Undertakers, whether removing corpses or patrolling, had honey crop volumes that were very lorv, but similar to those of several other groups of bees of sirnilar age performing different tasks. The high temperatures of undertakers could result frorn tl-re salne motor program used by unloaded worker bees to achieve take.off, while crop volumes may be determined by energy requirements. Nonetheless, the temperature and mass allocation patterns ofundertakers provide them with the greatest load-lifting capacity. This level ofperformance is probably necessary to perform their task successfully.

Introduction The success of insects has often been attributed in part to their ability to fly (".g.Dudley l99l), which presumably conveys numerous survival advantages to such activities as dispersal, foraging, and escape. Whlle recent work has demonstrated the ability of many insecrs to elevare and regulate body temperature during flight (see Heinrich 1983), little is understood about the effects of body temperature on flight performance or ecologically relevar-rt flight activities. In this study I attempt to bridge this gap by examining the effects of temperature. and load-induced force production constraints on natural behavior in honey bees (Apis melliferaL.).

Thorax temperature (T,n) strongly influences force producrion in honey bees (Esch 1976; Coelho 1997q). The force produced during flight plotred as

[Jndertaker Bee Flight Physiology

/ l7

function of T* describes

a convex-upward shape (Coelho 1991a). The peak of the curve defines the optimum temperature for force production: 38.6"C in workers. The T* breadth for some activities can be predicted from the T,nforce curve (Coelho l99la). The minimum force required for take-off is equal to the sum of the masses of the bee and its load multiplied by the acceleration of gravity. The low and hlgh T,n producing this force set the range over

which the activity can be carried out. Hence, T* may put limits on the efficiency with which a bee can perform a given task. Force production is also linearly dependent on flight muscle mass (Marden 1987). Since force production determines the ability to lift loads, the allocation of body mass to flight muscle will affect worker honey bees' ability to perform flight-related tasks. Worker honey bees perform a great variety of tasks both inside and outside the hive (Lindauer 1953; Free 1965). Division of labor is based largely on a system of temporal polyethism, in which workers perform different tasks at different ages. Young bees carry out jobs that are largely confined to the hive, while older bees perform outdoor tasks such as foraging (Seeley 1985; Winston 1987). The age period that falls between these extremes, known as the transition to foraging, is accompanied by an acute increase in flight capacity

(Harrison 1986).

In this study, I focus primarily on undertaker bees and compare T* of undertakers to those of groups of bees of approximately the same age occupying the same environment. Undertaker bees perform necrophoresis, the removal of dead bees from the hive (Visscher 1983). This behavior is believed to decrease the spread of parasites and diseases harbored in dead bees, and prevents the accumulation of dead bees in the nest cavity, which would otherwise become filled with them. Undertakers are a well-defined caste of worker bees, composing 1-2 percent of the worker population (Visscher 1983). Undertakers exhibit strong task fidelity and appear to be the only bees whlch perform necrophoresis. A recent study demonstrated that marked undertakers do not carry foreign live bees out of the hive, and marked guard bees do not carry out dead bees (Breed, Smith, and Torres 1992). Hence, the two tasks are not interchangeable. !7'hen undertakers are removed from the colony, there is no shift of other workers to necrophoretic behavior, and corpse removal efficiency decreases (G. E. Robinson personal communication). These studies suggest a lack of plasticity for undertaking, which is consistent with the established genetic component to undertaking behavior (Robinson and Page 19BB).

Undermkers respond to chemical cues produced by dead bees, and generally remove them from the hive in much iess than one hour (Visscher 1983; Breed et al. 1992), too rapidly for substantial evaporative weight loss from the corpses to occur. Hence, a bee corpse weighs about the same as a live bee: -85 mg. This represents a load well above those typically carried by nectar or

18

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pollen foragers ('S7inston 1987). lTorker bees produce 1.73 mN of force at the thermal optimum (Coelho 1991a). For an undertaker and its corpse ( 170 mg total), the force required for lift is equal to the sum of the gravitational force acting on each, or 1.65 mN. This represents 95 percent of the maximum force producible by the undertaker bee, leaving oniy 5 percent for thrust.

According to the worker bee force-temperature curve, undertaker activity should be limited to T,n ranging from 35.9 to 47.3"C. Undertakers should also avoid behaviors which increase non-flight-muscle mass, such as loading the honey crop. In contrast, bees of the same age performing other tasks bear relatively hght loads or none at all; therefore, they need not maximize lift, and should not need to optimize T,n.

Material and methods The study was conducted using managed colonies of honey bees of Italian origin in the summer months of 1990.1995. Thorax remperature was measured to within + 0.loc by grasping a bee by the legs with forceps, immobilizing it against a shaded polystyrene block, and inserting a 30 AWG copperconstantan thermocouple probe connected to thermocouple thermometer (Model HH-77T, Omega Engineering, Stamford, CT; time consrant = 0.29 s) into its thorax within 3 s. These methods minimize errors due to potentiaily rapid changes in T* after capture (Stone and !7i11mer 1989). Ambienr temperature (T") was taken with the same thermocouple (shaded and wiped off to remove moisture) shortly after T,n measurement, and in approximately the same area as the bee had been. Bees were stored individually in sealed 1.5 ml microcentrifuge tubes and frozen. Body mass (Mo) and thorax mass (M,n) were determined on an analytical balance accurate to + 0.1 mg, and were measured within 24 h o{ freezing in order to avoid effects of water sublimation. Thorax temperature was measured on undertakers carrying corpses, undertakers patrolling, fanners, guards, and bees applying propolis (gluers). Mo and M,n were measured on fanners, guards, gluers, and necrophore[ic undertakers.

Crop volume was determined by grasping a bee by the legs with forceps, cutting off the abdomen at the petiole with scissors, and expressing the contents of the crop into a volumetric glass capillary tube. The linear distance spanned by the liquid was measured and converted to volume (prl-). This procedure provided more accurate measurements on small crop volumes than other methods (see Sylvester, Rinderer, and Bolten 1983), but made the subjects unsuitable for measurements of Mo and M,n. Crop volume was measured on undertakers carrying corpses, undertakers patrolling, fanners, guards, and gluers.

Necrophoresis was elicited by dropping dead bees onto the hive enrrance

(Jndertaker Bee Flight Physiologt

/ 19

ramp. Generally, after a short time, an undertaker was observed attempting to carry off the corpse. Undertakers were identified as those bees which persistently dragged the corpse about and exhibited vigorous wing beating for at least 5 s in apparent attempts to lift off (see Visscher 1983). These bees were collected for measurements during actual necrophoresis or marked for later measurements.

For measurements on undertakers patrolling (i.e. not carrying out necrophoresis), bees in the act of necrophoresis were grasped by the legs with forceps, quickly marked on the dorsum of the thorax with a small spot of model paint, and released. Later, when marked bees were observed patrolling the hive entrance, they were captured and measurements were made. This technique provided a minimum of disruption ro rhe acrivity of the undertakers. Indeed, many never released their corpses and continued to carry them away after marking was complete. This level of purposefulness is also noted by Visscher (1983). Several marked bees were later observed carrying a second dead bee out of the hive. The few bees which exhibited obvious abnormal behavior (persistent grooming, inability to rerurn ro rhe hive entrance) after marking were not used in the study. Patrolling undertakers were not weighed because of the unknown mass of the marking paint. Fanner bees were collected from the hive entrance ramp while they were actually beating their wings. Fanners inside the hive were not collected, nor were Nasonov scent-fanning bees, which are easily distinguishable (see !Uinston 1987). Guard bees were identified by their "greeting behavior" (sensu

Moore, Breed, and Moor 1987) and collected from the hive entrance. Another group of bees, which I call "gluers," was collected while they manipulated propolis (bee glue) and deposited ir on a hole in rhe comer of a hive. Undertaking, fanning, guarding, and applying propolis are all behaviors that are performed at the age of the transition to foraging (Gary 1975; \Tinston 1987). All animals were collected on the entrance ramp or external walls of rhe hive. Results Temperau,re

Necrophoretic undertakers exhibited significantly higher T,n than those of any of the other castes (Thble 1). The T,n of fanners alone was dependent on T^ (Thble 1). All bees measured had T.n within limirs sufficient to lift their own body mass (27.9-46.6"C; Coelho 1991a). However, the T* of 3lo/o o{ necrophoretic undertakers were within 1.0"C or less of the optimum T,n (38.6'C;Coelho 1991a). By comparisor',ZZ percent of patrolling undertakers, 5 percent of fanners, 4 percent of guards, and 0 percent of gluers had T,n within these limits. Figure 1 demonstrates the clustering of the distribution of necrophoretic undertaker T* under the peak of the T,n-force curve.

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Figure 1. The distribution of thorax temperatures of undertaker honey bees while performing necrophoresis (bars) as compared to force production (line) as a function of thorax temperature in worker honeybees (force curve from Coelho, l99la). Mass allocation

Undertakers, whether performing necrophoresis or patrolling, had honey crop volumes that were significantly lower than those of fanners, but similar to those of guards and gluers (Thble 1). Undertakers with intact honey crops had Mo and M,n that were indistinguishable from those of orher castes (Table 1). Discussion

\7hen attempting to carry off corpses, undertakers appear to operare at Trh that is nearly optimal for lift. While patrolling, they have T* that is slightly less optimal. The narrow range of T,n, independent of T^, demonstrated by necrophoretic undertakers while active outside rhe hive is striking. Although it is tempting to speculate that the T* of necrophoretic undertakers might represent a finely regulated state, it is more likely that they simply warm up to near-optimal T,n using essentially the same, simple motor program described by Esch (1976) for take-offin an unloaded worker honey bee. The bee beats its wings, trying repeatedly to take off. Thorax temperature increases until the lift force is equal to body mass (or in this case, the sum of two body masses),

then lift-off occurs. The heat balance during this period probably faciiitates the process. During the initial struggle to carry off the colpse, convective heat loss is probably low, even though the wings are beating, because the bee is not moving forward ar an appreciable speed (Digby 1955; Chappell 1982). At the same

Undertaker Bee Flight Physiologt / 21

time, metabolic heat production

T*

is

high; therefore,

a

net heat gain results and

At some point T* probably reaches a level sufficient to provide enough lift for take-off, and the undertaker flies away increases (Coelho 1991b).

with the corpse. Once aloft, the elevated metabolic rate associated with loadcarrying (Wolf et al. 1989) as well as low flight speed (Visscher 1983) should help to mainrain a high T* (Disbv 1955; Chappell 1982; Coelho 1991b), at least for the brief period required to complete the flight. While performing a task that also requires vigorous wing-beating, fanners have temperatures that are not matched to the requirements of maximal force production. In contrast, fanners are subject to environmental influences T,n is partly dependent on T". Fanner T,n is quite variable, and often strays well beyond the limits that generate maximal force production. The predicted suboptimal performance of fanners at temperature extremes is not a great hindrance to their function. The task continues to be performed, but presumably with a lower volume of air moved per unit time per bee. This effect can be compensated at the coiony level by the addition of more fanner bees when needed.

Undertakers, however, perform an individual task that may succeed or fail based on small variations in T,n. Gluers probably seldom need to fly, and thus should not require a high degree of flight performance. Guards fly when it is necessary for hive defense, and their T,n then appears to increase substantially. Attacking bees (presumably mostly guards) have relatively high T,n (37.7"C; Heinrich 7979), which confers greater maneuverability and, presumably, more effective defense. Returning nectar foragers carry loads weighing {rom 72 to 40 mg, as reported by various investigators (see Winston 1987). An average of 25 mg is realistic (Sylvester et al. 1983). Pollen loads are usually even lighter (9-29 mg; Winston 1987). Hence, the lift demands of foraging are not great, and it should only be constrained at the most extreme T" they encounter (see discussion by Coelho 1991a). In fact, foragers retuming to the hive do not optimize T,n as well as undertakers. Returning foragers are exposed to the ambient thermal environment for relatively long periods. Even though evaporative cooling at high T, through regurgitation is available (Heinrich 1980a; 1980b; Cooper, Schaffer, and Buchmann 1985; Coelho L99lb), workers exhibit a broad range of T,n that is partly dependent on T, (Cooper et al. 1985;

Coelho 1991a). One would predict that, under the proper conditions, T* would affect foraging ability through effects on force production, with more nectar being collected at the most favorable T,n, but this has not been tested. The T,n exhibited by necrophoretic undertakers confirms predictions based upon laboratory measurements of honey bee flight performance. Similar predictions were not confirmed in measurements of drone flight speed as a function of T,n, which is positive and linear, rather than a convex-upward curve (Ross and Coelho, L995). Hence, there may be limitations to applying T,n-

22 / Ex Scientia

force curves to behavioral phenomena. Since flight muscle mass composes approximately 95 percent of thorax mass (Marden 1987), the flight muscle masses of bees performing different tasks were not different. Hence, the demands of undertaking are not met by an increase in flight muscle mass. They could be met, conversely, by a reduction in non-flight-muscle mass, and the data might lend themselves to such an interpretation since undertakers have low crop volumes. However, the honey crop volumes observed may result from energetic, rather than load-lifting constraints. Flying honey bees utilize carbohydrate fuel taken

directly from the midgut into the hemolymph (Crailsheim 19BBa; 1gBBb). Fanners generally remain in one place facing the nest entrance and bear their wings vigorously, though with different wing-stroke amplitude and frequency from those of flight (Seeley and Heinrich 1981). Although they do not need to lift off, they are performing an energetically expensive activity for a lengthy period of time. Hence, it makes energetic sense that fanners carry more fuel on board to perform their task without interruption. Guarding and applying propolis are not likely to be as energetically taxing because they generally require much less flight activity. Though undertaking is probably very energetically expensive (Wolf et al. 1989), the task of an undertaker requires only a few minutes (Visscher 1983; Breed et al.7992), thus requiring a small amount of fuel for each bout. Nonetheless, the relatively low crop volume of undertakers may be a significant advantage to performing their task. For an individual undertaker, a honey crop load that is 2 mg lower (as compared to a fanner) may mean the difference between success and failure in taking off with a corpse. Given a finite maximum force production, every mg an undertaker carries in its crop represents 1 mg less it can carry in corpse mass. It is likely that worker honey bees are capable of sensing their honey crop volume via stretch receptors in the honey crop wall (see Neese 1988), as is the case in many other insects (Gelperin l97l). Perhaps this capacity is what allows foragers departing the hive to carry a crop volume proportional to the disance to their foraging area (Beutler 1950). Undertakers may use this ability to minimize their internal loads, which improves ability to carry external loads in the form of corpses. In summary, the favorable T,n and mass allocation patterns of undertakers, the workers with the greatest lift constraints, provide them with the greatest lift production. Because the maximal force production of an average worker honey bee barely allows it to carry a corpse of typical mass, these states are

probably required for the undertaker to perform its task successfully. If it cannot lift off with the corpse, it may only be able to drag it a short distance from the hive (as is the case with drone corpses), depending on the location of the hive and the nature of the substrate. Undertakers begin necrophoresis after taking their first "play" flights, but before foraging (Sakagami 1953). At this age worker bees undergo a host of

(Jndertaker Bee Flight Physiologt / 23

physiological changes which enhance flight capacity, including a 40 percent reduction in body mass (mostly lost abdominal fecal mass) and an increase in maximal oxygen consumption (Harrison 1986). Though these changes undoubtedly increase foraging capacity, they are probably essential for worker bees to perform necrophoresis at all. Acknowledgments This study was made possible through the assistance of the following: Salma Aziz, Sharon Bringer, Kevin Harvey, Jennifer Hoagland, Scott Luft, Allan Ross, and Joseph Signorelli.

Literature Cited Beutler, R. 1950. Zeit und Raum im Leben der Sammelbiene. Naturwissenschaften 37:102105. Breed, M. D.,

T A. Srnith, and A. Torres. 1992. Role of guard honey bees (Hymenoptera: Apidae) in nestmate discrimination and replacement of removed guards. Annals of the Entomological Society of America 85:633-637. Chappell, M. A. 1982. Temperature regulation of carpenter bees (Xylocopa caLifornica) foraging in the Coiorado desert of Southern California. Physiological Zoology 55:267 280.

Coelho, J. R. 1991a. The effect of thorax temperature on force production during tethered flight in honeybee (Apk mellifera) drones, workers, and queens. Physiological Zoology 64:823-835. Coelho, J. R. 1991b. Heat transfer and body temperature in honey bee drones and workers (Hyrnenoptera: Apidae). Environmental Entomology 70:1627 -1635. Cooper, P. D., \7. M. Schaffer, and S. L. Buchmann. 1985. Temperature regulation of honey bees (Apis melLifera) foraging in the Sonoran desert. Journal of Experimental Biology

114:l-15. Crailsheim, K. 19BBa. Regulation of food passage in the intestine of the honeybee (Apis mellifera L.). Journal of Insect Physiology 34:85-90. Crailsheim, K. 19881r. Intestinal transport of glucose solution during honeybee flight. Pages ll9-128 in !7. Nachtigall, ed. BIONA report 6: The flying honeybee; aspects of energetics. Gustav Fischer Verlag, New York. Digbn P. S. B. 1955. Factors affecting the temperature excess of insects in sunshine. Journal of Experimental Biobgy 32:27 9 -298. Dudley, R. 1991. Comparative biomechanics and the evolutionary diversification of flying insect mrrrphology. Pages 503-514 in E. Dudley, ed. The unity of evolutionary biology. Dioscorides Press, Portland. Esch, H. 1976. Body telnperarure and fllght performance of honey bees in a servomechanically controlled wind tunnel. Journal of Comparative Physiology

lA9A:765-277. Free, J. B. 1965. The allocation ofduties among worker honeybees. Symp. Zool. Soc.

London 14:39-59. E. 1975. Activities and behavior of honey bees. Pages 185-264 in Dadant & Sons, eds. The hive and the honeybee. Dadant & Sons, Hamilton, ILlinois. Gelperin, A. 1971. Regulation of feeding. Annual Review of Entomology 16:365-378. Harrison, J. M. 1986. Caste-specific changes in honeybee flight capacity. Physiological Zoology 59:175-187. Gary, N.

24 / Ex Scientia Heinrich, B. 1979. Thermoregulation of African and European honeybees during foraging, attack, hive exits and returns. Joumal of Experimental Biology 80:217 .229. Heinrich, B. 1980a. Mechanisms of body-temperature regulation in honeybees, Apis mellifera I. Regulation of head temperature. Journal of Experimental Biology 85:61-72 Heinrich, B. 1980b. Mechanisms of body-temperature regulation in honeybees, Apismellifera II. Regulation of thoracic temperature ar high air remperarures. Joumal of Experimental Biology 85:73-87.

B. 1993. The hot-blooded insects. Harvard University Press, Cambridge. 601 pp. Lindauer, M. 1953. Division of labour in the honeybee colony. Bee World 34:63-73, 85-91. Marden, J. H. 1987. Maximum lift production during takeoff in flying animals. Joumal of Heinrich,

Experimental Biology 130 :23 5 -258. Moore, A. j., M. D. Breed, and M. J. Moore. 1987. The guard honey bee: ontogeny and behavioral variability of workers performing a specialized task. Animal Behavior 35:11591167. Neese, V. 1988. Die Entfernungsmessung der Sammelbiene: Ein energetisches und zugleich sensorisches Problem. Pages 1-15 in W. Nachtigall, ed. BIONA report 6: The Flying Honeybee; Aspects of Energetics. Gustav Fischer Verlag, New York. Robinson, G. E., and R. E. Page, Jr. 1988. Genetic determination of guarding and undertaking in honeybee colonies. Nature (London) 333:356-358. Ross, A. J., and J. R. Coelho. 1995. The effect of thorax temperature on flight speed in honey bee drones (Apis mellifera). Pages 987-991 in R. D. Yearout, ed. Proceedings of the Ninth National Conference on Undergraduate Research, Volume IIL University of North

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York. Stone, C. N., and P. G. Willmer. 1989. Endothermy and temperature regulation in bees: critique of 'grab and stab' measurement of body temperature. Joumal of Experimental

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Visscher, P. K. 1983. The honeybee way of death: necrophoric behaviour

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colonies. Animal Behavior 3i:1070-1076. !7inston, M. L. 198i. The blology of the honey bee. Harvard University Press, Cambridge, Mass. 281 pp.

\Uinston, M. L., and E. N. Punnett. 1982. Factors determining temporal division of labor in bees. Canadian Journal of Zool ogy 60.29 47 -29 52. P. Schmid-Hempel, C. P. Ellington, and R. D. Stevenson. 1989. Physiologicai correlates of foraging efforts in honeybees: oxygen consumption and nectar load. Functional Ecology 3:417 -424.

Woll T.,

Thble 1. Temperature, crop volume and

Caste

Activity

undertaker necrophoresis undertaker patrolling

fanner guard gluer

T,n ('C)

mass

allocation of worker honey

r

bees performing

different tasks.

volume Body mass (-e) (UL)

Crop

40.0 +

0.2(55)^ 0.265 3.84 + 0.40(19)" 87.8 + 1.3(78)

36.5 +

0.3(27)b 0.114 3.78 + 0.47(33)^

0.6(53) 33.1 + 0.5(10)d 0.002 3.37 + 0.37(10)' 86.8 + 4.5(10) applying propolis 28.7 + 0.9(10)' 0.001 3.91 + 1.09(10)''t 94.7 + 3.8(10)

fanning guarding

38.8 +

0.4(54)' 0.369.

5.79 +

0.59(22)b

87.7 +

Thorax

mass

(mg)

30.4 + 0.3

29.7 + 0.2

30.7 + 0.5 30.2 + 0.3

e N

Data are reported as mean + SEM(sample size), r is the correlation of thorax temperature (T,n) with ambient temperature. Groups not sharing the same superscripts are statistically different (P < 0.05, T:-test). *r is statistically significant, P < 0.01.

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