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First published online September 5, 2008
Journal of Experimental Biology 211, 3028-3040 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.017426
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Caste-dependent sleep of worker honey bees

Barrett A. Klein1,*, Kathryn M. Olzsowy2, Arno Klein3, Katharine M. Saunders1 and Thomas D. Seeley2

1 Section of Integrative Biology, Department of Ecology, Evolution and Behavior, The University of Texas at Austin, TX 78712, USA,
2 Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA
3 Division of Molecular Imaging and Neuropathology, New York State Psychiatric Institute, Columbia University, New York, NY 10032, USA


Figure 1
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  		Fig. 1. Worker honey bees displaying typical sleep postures while relatively immobile, with limbs and body drooping in the direction of gravity. Bees exhibit a sleep state while (A) in groups, (B) isolated, (C) dangling motionless from tarsal claws, or (D) leaning against the observation hive wall or floor. Photographs taken with a Panasonic AG-DVC30 video camera in infrared-sensitive mode (A,B) and a Nikon D70 under red lamplight (C,D).

 

Figure 2
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  		Fig. 2. Timeline of each sampling method. (Scan:72 h) Scan sampling of behavior of ~40 worker bees every 30 min for 72 h. Enlarged view represents one of 144 consecutive 30 min periods, and each dark bar signifies a single observation of one of the ~40 bees. Bees were selected by the function they performed (or by evidence of their recent eclosion, in the case of cell cleaners) prior to the 72 h study. (Focal:life) Focal sampling of worker bees at important stages of their adult lives. Two groups of cell cleaners were introduced (represented by the subscripts 1 and 2) and two randomly selected bees were each observed for 15 min each hour (enlarged view) for 48 h periods. (Focal:24 h) Focal sampling of a representative of each worker caste. Each bee was observed for 30 min every hour for 24 h. Enlarged view represents 1 h of 24 consecutive hours during which a single food storer was observed for 30 min.

 

Figure 3
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  		Fig. 3. Relative immobility in relaxed state with respect to worker caste. Immobile bees were observed outside comb cells with motionless antennae (black bars), with antennae twitching or exhibiting larger motions (gray bars), or bees were observed inside cells and immobile (white bars). Patterns of sleep and immobility within cells remained fairly consistent across the studies, with older bees sleeping more outside cells and younger bees spending more time immobile inside cells. Error bars indicate s.e.m. associated with black bars, black+gray bars, or white bars. Different letters indicate statistically significant differences among the castes with respect to these measures. Castes are ordered by presumed (nurse bees, food stores and foragers in Scan:72 h study) or actual (cell cleaners in Scan:72 h study, and all bees in Focal:life and Focal:24 h study) age, from left to right. (Scan:72 h) percentage observations of ~40 randomly selected marked bees; scan sampling: 3–5 s per bee every 30 min for 72 h. (Focal:life) percentage time, four bees randomly selected from same age group; focal sampling: 15 min per bee per hour for 48 h every week for four weeks of their lives. (Focal:24 h) percentage time, one bee of each worker caste; focal sampling: 30 min per hour for 24 h.

 

Figure 4
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  		Fig. 4. Relative immobility in relaxed state during the day (sun) versus during the night (moon) with respect to worker caste. Food storers and foragers sleeping more, by any cumulative measure, during the night than during the day. Error bars indicate s.e.m. associated with black bars, black+gray bars, or black+gray+white bars. The asterisk signifies a statistically significant difference between daytime and nighttime measurements with respect to these measures. All worker castes spent the same amount of time immobile inside cells during the night as during the day, so measure of `in cell and immobile' did not alter night–day differences for any worker caste. (Scan:72 h), % observations. (Focal:life), % time.

 

Figure 5
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  		Fig. 5. Timing of sleep outside cells or immobility inside cells with respect to worker caste. Worker bees were observed in an immobile, relaxed state outside comb cells with motionless antennae (black bars), with antennae slightly twitching or antennae exhibiting larger motions (gray bars), or inside cells (white bars). Shaded backgrounds indicate nighttime. Castes are ordered by behavior/function (Scan:72 h study) or age (Focal:life study). (Top; Scan:72 h) Percentage of bees in sleep state outside cells or immobile inside cells. Scan sampling: each bee recorded (if found) every 30 min for 72 h. (Bottom; Focal:life) percentage of time bees (N=4 per hour) spent in sleep state outside cells or immobile inside cells. Error bars reflect s.e.m. margins associated with black+gray bars, or white bars.

 

Figure 6
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  		Fig. 6. Autocorrelations conducted on Scan:72 h data, assessing rhythmicity of worker castes in relaxed posture exhibiting relative immobility when (A) antennae were immobile, (B) antennae were variable, (C) immobile inside cells, and (D) the sum of A, B and C. Food storers exhibited a 24 h rhythm when antennae were variable, and foragers exhibited strongly significant 24 h rhythmicity when antennae were immobile, variable, and when all measures were summed. There was no evidence for ultradian or circadian rhythms (i.e. large oscillations were absent) for cell cleaners, nurse bees or any in-cell immobility. An asterisk indicates periodicity and shaded region represents the 95% confidence intervals. A peak at the center of each graph indicates zero lag (amount of shift) and therefore perfect correlation.

 

Figure 7
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  		Fig. 7. Uninterrupted bouts of relative immobility with respect to worker caste. (A) Outside cells: food storers and foragers exhibited longer unbroken sleep bouts during the night (moon) than during the day (sun), as measured by antennal immobility (black bars) or in combination with antennae slightly twitching or exhibiting larger motions (gray bars). (B) Inside cells (white bars): unbroken bouts of relative immobility decreased with age. No difference existed between night and day, so data were collapsed for each caste. Worker bees were observed for 48 h during each of four stages of their adult lives (cell cleaners to foragers, respectively). An asterisk indicates a significant difference between daytime and nighttime measurements of black and black+gray bars and different letters indicate statistically significant differences among white bars. Error bars indicate s.e.m. associated with black bars, black+gray bars, or white bars.

 

Figure 8
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  		Fig. 8. Activity of three bees, mapped on opposite sides of the bottom frame of the observation hive, indicating that the forager slept on the periphery of the hive, the cell cleaner visited cells within the brood comb area, and the food storer visited cells on the edge of the brood comb. White space roughly outlines brood comb on 13 August. Filled circles, cell cleaner's cell visits; open circles, food storer's cell visits; black ovals, 12 nighttime sleep sites of the forager, numbered chronologically. The three bees were observed on separate days continuously for 30 min per hour for 24 h.

 

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© The Company of Biologists Ltd 2008