Collection of temperature data from a sequence of infrared images.

<p>The forceps were held open to encompass the heat-generating thorax of the bee (top) and camera software computed the average temperature within the selected region, represented by a circle in the inset image. For recording the surface temperature surrounding a bee (T_surr), forceps were removed (bottom) so as not to influence the measurement, taken as the average temperature within the larger circle (radius  =  bee body length). T_diff is the difference of the average temperature within the large circle (T_surr) from the average temperature in the smaller circle (T_th). Temperature scale values (°C) were adjusted for thermal camera settings.</p

Distance from the nest's perimeter with respect to behavior and caste.

<p>Younger castes (cell cleaners and nurse bees) slept closer to the center of the nest (i.e., farther from the perimeter) than older castes (food storers and foragers). Bees slept outside cells closer to the perimeter of the nest than when they slept inside cells or were awake, except in the case of foragers, who were active during the day near the nest entrance. Cell cleaners did not sleep outside cells and foragers did not sleep inside cells, hence the absence of relevant bars. These data represent averages for castes calculated from average values per bee (± s.e.m.). Asterisks signify statistically significant differences within castes.</p

Surface temperature surrounding foragers as a continuous predictor of foragers' behavior.

<p>When T_surr was lower, foragers tended to be asleep; when T_surr was higher, foragers tended to be awake. Temperatures represent bins of +/−0.25°C (e.g., 29.75°C = 29.50–29.99°C; the unlabeled 30.25°C = 30.00–30.49°C). All data were collected from foragers outside cells. Food storers also exhibited lower T_surr when asleep than when awake.</p

Infrared images revealing thermal activity across beehives.

<p>(A) Sequence of colony-scale changes across the entrance side of Colony 1. In clockwise order from the upper left corner, 1700, 0400, 0900 and 1500 h, respectively. Entrance/exit is in the lower left corner of the hive, leading out tube at left of each image. Brood comb is most easily seen as the glowing warm area at 0400 h. (B) Observation hive containing Colony 2, with filter-covered lamp at upper right, and bees visibly exiting hive tunnel at lower right. (C) Exposed nest composed of parallel sheets of comb, set up by Dirk Ahrens-Lagast to induce bees to construct a more natural nest architecture; not used in study. B.A.K. took all images with FLIR thermal cameras on non-experiment days under different ambient temperature conditions. Temperature scale values (°C) were adjusted for thermal camera settings (see Materials and Methods).</p

Caste-dependent maps of behavior with regard to comb cell contents.

<p>Cell cleaners (Colony 1 pictured) were awake (<b>o</b>) and asleep (inside cells; triangles) primarily in the brood comb area, spending more time asleep than awake with uncapped brood in their midst. Foragers, on the other hand, spent more time awake (<b>o</b>) than asleep (<b>x</b>  =  deep sleep, <b>+</b>  =  light sleep) near uncapped brood comb. White  =  uncapped brood, red  =  capped brood, yellow  =  pollen cells, blue  =  uncapped nectar, green  =  capped honey, brown  =  empty comb, gray  =  no comb. Nest entrance/exit is indicated by an arrowhead, and was restricted to one side of the nest.</p

Average thoracic surface temperature of worker honey bees (T_th) with regard to caste, behavior, day vs. night, or T_surr.

<p>All measurements were taken from worker bees outside of cells; cell cleaners slept exclusively inside cells, so are excluded. Data represent averages for castes (± s.e.m.). Asterisks signify statistically significant differences within castes. (A) Average thoracic surface temperature of bees (T_th) awake vs. asleep, (B) day vs. night, and (C) awake vs. asleep relative to surrounding surface temperatures (T_diff).</p

Sleep positions of individual foragers with respect to temperatures T_th and T_surr.

<p>Unique shapes (including uniquely oriented shapes) distinguish different foragers (e.g., every triangle facing down represents data from one individual at different times, while every triangle facing up represents data from a different individual, squares  =  a third individual, etc.). Shapes represent T_th (inner shape) and T_surr (outer shape) for each observation of a marked, sleeping forager over 24 h on the entrance side of the hive (she may have also slept on the reverse side of the hive, as pictured in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102316#pone.0102316.s003" target="_blank">Fig. S3</a>). Temperatures (°C) correspond with the color scale (white  =  no data, represented by diamond at lower left). Hive entrance/exit is indicated by arrowhead, and was restricted to one side of the hive. All relevant forager data are included in graph (<i>n</i> = 11 bees), but we treated forager as a random factor in mixed effects analyses to statistically cope with repeated measures of individuals. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102316#pone.0102316.s002" target="_blank">Figs. S2</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102316#pone.0102316.s003" target="_blank">S3</a> for differences across behaviors and worker castes.</p

Proportion of observations bees of different worker castes were asleep during the day and night.

<p>Younger castes slept with no distinction between day and night. The clearest statistical distinction (signified with an asterisk, applying a linear mixed-effects model) appeared in foragers, with more time spent asleep during the night.</p

Percent and total number of observations honey bees of different worker castes engaged in wakefulness and sleep.

<p>Percent and total number of observations honey bees of different worker castes engaged in wakefulness and sleep.</p

Average thoracic surface temperature of honey bee workers (T_th) over the course of 24 h with respect to caste and behavior.

<p>An observation of an awake bee is represented by o with dashed lines fitting the data; an observation of a sleeping bee is represented by x with solid lines fitting the data. Gray backdrop represents nighttime and colors correspond with the temperature scale on y-axes and with <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102316#pone-0102316-g003" target="_blank">Figs. 3</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102316#pone.0102316.s002" target="_blank">S2</a>, and S3. Average T_th is reported per bee per census period and, although all bee data are included in these graphs, we treated bee as a random factor in mixed effects analyses. All measures were taken from bees outside of cells; no data exist for cell cleaners sleeping outside cells, hence the absence of x in the cell cleaner graph.</p