Abstract
Sleep appears to play an important role in the lives of honey bees, but to understand how and why, it is essential to accurately identify sleep, and to know when and where it occurs. Viewing normally obscured honey bees in their nests would be necessary to calculate the total quantity and quality of sleep and sleep’s relevance to the health and dynamics of a honey bee and its colony. Western honey bees (Apis mellifera) spend much of their time inside cells, and are visible only by the tips of their abdomens when viewed through the walls of an observation hive, or on frames pulled from a typical beehive. Prior studies have suggested that honey bees spend some of their time inside cells resting or sleeping, with ventilatory movements of the abdomen serving as a telltale sign distinguishing sleep from other behaviors. Bouts of abdominal pulses broken by extended pauses (discontinuous ventilation) in an otherwise relatively immobile bee appears to indicate sleep. Can viewing the tips of abdomens consistently and predictably indicate what is happening with the rest of a bee’s body when inserted deep inside a honeycomb cell? To distinguish a sleeping bee from a bee maintaining cells, eating, or heating developing brood, we used a miniature observation hive with slices of honeycomb turned in cross-section, and filmed the exposed cells with an infrared-sensitive video camera and a thermal camera. Thermal imaging helped us identify heating bees, but simply observing ventilatory movements, as well as larger motions of the posterior tip of a bee’s abdomen was sufficient to noninvasively and predictably distinguish heating and sleeping inside comb cells. Neither behavior is associated with large motions of the abdomen, but heating demands continuous (vs. discontinuous) ventilatory pulsing. Among the four behaviors observed inside cells, sleeping constituted 16.9% of observations. Accuracy of identifying sleep when restricted to viewing only the tip of an abdomen was 86.6%, and heating was 73.0%. Monitoring abdominal movements of honey bees offers anyone with a view of honeycomb the ability to more fully monitor when and where behaviors of interest are exhibited in a bustling nest.
Highlights
Sleep is a behavior steeped in mystery, yet it appears to offer essential benefits (Rattenborg et al, 2007; Cirelli & Tononi, 2008)
We report the difference of Tsurr from temperature of a bee’s thorax (Tth) to indicate the surface temperature of the bee relative to the surface temperature of her surroundings (Tdiff = Tth − Tsurr) (Klein et al, 2014)
When visibility was restricted, maintaining cells and eating were difficult to distinguish from each other, but sleeping and heating were identifiable based on ventilatory rates and lack of major body motions alone (86.6% and 73.0% of observations were correctly identified, respectively)
Summary
Sleep is a behavior steeped in mystery, yet it appears to offer essential benefits (Rattenborg et al, 2007; Cirelli & Tononi, 2008). Sleep may assist with honey bee communication (Klein et al, 2010, 2018) and memory (Zwaka et al, 2015), so accurately identifying sleep and knowing when and where it occurs is essential for further investigating sleep’s role in honey bee ecology. To better understand sleep’s benefits, or the detriments that come with sleep loss, it is essential to monitor sleep, including when it occurs in dark, hidden places. Several species of honey bees (Apis spp.) nest inside cavities, and all species of honey bees spend periods of their lives concealed inside honeycomb cells, within which much of the colony’s behaviors occur and without which the colony would inevitably perish. If significant periods of sleep occur within honeycomb cells, it would be wise to take inside-cell behavior into account
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