Social waves in giant honeybees (Apis dorsata) elicit nest vibrations

Giant honeybees (Apis dorsata) nest in the open and have developed a wide array of strategies for colony defence, including the Mexican wave-like shimmering behaviour. In this collective response, the colony members perform upward flipping of their abdomens in coordinated cascades across the nest surface. The time–space properties of these emergent waves are response patterns which have become of adaptive significance for repelling enemies in the visual domain. We report for the first time that the mechanical impulse patterns provoked by these social waves and measured by laser Doppler vibrometry generate vibrations at the central comb of the nest at the basic (=‘natural’) frequency of 2.156 ± 0.042 Hz which is more than double the average repetition rate of the driving shimmering waves. Analysis of the Fourier spectra of the comb vibrations under quiescence and arousal conditions provoked by mass flight activity and shimmering waves gives rise to the proposal of two possible models for the compound physical system of the bee nest: According to the elastic oscillatory plate model, the comb vibrations deliver supra-threshold cues preferentially to those colony members positioned close to the comb. The mechanical pendulum model predicts that the comb vibrations are sensed by the members of the bee curtain in general, enabling mechanoreceptive signalling across the nest, also through the comb itself. The findings show that weak and stochastic forces, such as general quiescence or diffuse mass flight activity, cause a harmonic frequency spectrum of the comb, driving the comb as an elastic plate. However, shimmering waves provide sufficiently strong forces to move the nest as a mechanical pendulum. This vibratory behaviour may support the colony-intrinsic information hypothesis herein that the mechanical vibrations of the comb provoked by shimmering do have the potential to facilitate immediate communication of the momentary defensive state of the honeybee nest to the majority of its members. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00114-013-1056-z) contains supplementary material, which is available to authorized users

Stereoscopic motion analysis in densely packed clusters: 3D analysis of the shimmering behaviour in Giant honey bees

<p>Abstract</p> <p>Background</p> <p>The detailed interpretation of mass phenomena such as human escape panic or swarm behaviour in birds, fish and insects requires detailed analysis of the 3D movements of individual participants. Here, we describe the adaptation of a 3D stereoscopic imaging method to measure the positional coordinates of individual agents in densely packed clusters. The method was applied to study behavioural aspects of shimmering in Giant honeybees, a collective defence behaviour that deters predatory wasps by visual cues, whereby individual bees flip their abdomen upwards in a split second, producing Mexican wave-like patterns.</p> <p>Results</p> <p>Stereoscopic imaging provided non-invasive, automated, simultaneous, <it>in-situ </it>3D measurements of hundreds of bees on the nest surface regarding their thoracic position and orientation of the body length axis. <it>Segmentation </it>was the basis for the <it>stereo matching</it>, which defined correspondences of individual bees in pairs of stereo images. Stereo-matched "agent bees" were re-identified in subsequent frames by the <it>tracking </it>procedure and <it>triangulated </it>into real-world coordinates. These algorithms were required to calculate the three spatial motion components (dx: horizontal, dy: vertical and dz: towards and from the comb) of individual bees over time.</p> <p>Conclusions</p> <p>The method enables the assessment of the 3D positions of individual Giant honeybees, which is not possible with single-view cameras. The method can be applied to distinguish at the individual bee level active movements of the thoraces produced by abdominal flipping from passive motions generated by the moving bee curtain. The data provide evidence that the z-deflections of thoraces are potential cues for colony-intrinsic communication. The method helps to understand the phenomenon of collective decision-making through mechanoceptive synchronization and to associate shimmering with the principles of wave propagation. With further, minor modifications, the method could be used to study aspects of other mass phenomena that involve active and passive movements of individual agents in densely packed clusters.</p

How to Join a Wave: Decision-Making Processes in Shimmering Behavior of Giant Honeybees (Apis dorsata)

Shimmering is a collective defence behaviour in Giant honeybees (Apis dorsata) whereby individual bees flip their abdomen upwards, producing Mexican wave-like patterns on the nest surface. Bucket bridging has been used to explain the spread of information in a chain of members including three testable concepts: first, linearity assumes that individual “agent bees” that participate in the wave will be affected preferentially from the side of wave origin. The directed-trigger hypothesis addresses the coincidence of the individual property of trigger direction with the collective property of wave direction. Second, continuity describes the transfer of information without being stopped, delayed or re-routed. The active-neighbours hypothesis assumes coincidence between the direction of the majority of shimmering-active neighbours and the trigger direction of the agents. Third, the graduality hypothesis refers to the interaction between an agent and her active neighbours, assuming a proportional relationship in the strength of abdomen flipping of the agent and her previously active neighbours. Shimmering waves provoked by dummy wasps were recorded with high-resolution video cameras. Individual bees were identified by 3D-image analysis, and their strength of abdominal flipping was assessed by pixel-based luminance changes in sequential frames. For each agent, the directedness of wave propagation was based on wave direction, trigger direction, and the direction of the majority of shimmering-active neighbours. The data supported the bucket bridging hypothesis, but only for a small proportion of agents: linearity was confirmed for 2.5%, continuity for 11.3% and graduality for 0.4% of surface bees (but in 2.6% of those agents with high wave-strength levels). The complimentary part of 90% of surface bees did not conform to bucket bridging. This fuzziness is discussed in terms of self-organisation and evolutionary adaptedness in Giant honeybee colonies to respond to rapidly changing threats such as predatory wasps scanning in front of the nest

Experimental setup.

<p>The experimental <i>Apis dorsata</i> nest in a bay at the shaded rear side of the second floor of a hotel in Chitwan (Nepal); built at a traditional nesting site, which can be discerned by the wax traces from previous years. In front of the nest on the unsecured balcony the recording devices were placed: IR, infrared camera; HD, high definition camera; chr1 & chr2, two additional high resolution cameras for other purposes [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.ref028" target="_blank">28</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.ref031" target="_blank">31</a>]; LDV, laser Doppler vibrometer [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.ref030" target="_blank">30</a>]. The yellow arrow marks the spot of the red LDV light reflected at the end plane of a rod, which had been stuck through the comb (see also schematics in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.s003" target="_blank">S3 Fig</a>).</p

Differential size (ΔA_CNR /Δt)—temperature (ΔT̃_sink /Δt) values of six selected CNRs (“nest 02”).

<p>Correlation of the summarized values ΔT̃_sink and ΔA_CNR under checked fanning conditions; for the number of cases (= pairs of images), see panel B): blue colour codes refer to the “presence of actively fanning bees”; red codes refer to the state “without fanning”. Differential values (ΔT̃_mid /Δt, ΔA_mid /Δt) were assessed frame by frame over time intervals of 100 frames corresponding to Δt <b>=</b> 21.16 s. (A-B) A_CNR data were uniformly distributed, and ΔA_CNR data were gauss distributed. (C) Regression functions (a: blue-coded, “fanning” state with R² = 0.9353; b: red coded, “non-fanning” state with R² = 0.0617) concern the central region of both correlations. One-sided arrows on the left side of correlation symbolize “closing” reactions (data for fanning state are here outside the scope); double-sided arrows on the right side give “inhalation” activities (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#sec020" target="_blank">Discussion</a> for ventilation hypothesis). Vertical blue and red bars in panel C give the range of data within ± standard error.</p

Comb dislocations in a giant honeybee nest during shimmering and quiescence.

<p>(A-B) Two LDV records of comb dislocations (Z_comb; cf [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.ref030" target="_blank">30</a>], scaled in mm/s; see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#sec003" target="_blank">Methods</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.s003" target="_blank">S3 Fig</a>). Black curves, unfiltered data; white superposed curves give low-frequency components defined as moving average values (±50 frames with interframe intervals of Δt_ff = 20 ms); yellow background marks the first 60 s of observation, in which shimmering waves had been provoked by dummy wasp presentation [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.ref030" target="_blank">30</a>]. (C) FFT-diagram: abscissa, the periods of partial frequency components of comb dislocations p {Z_comb} in s; ordinate, the relative amplitude of these frequency components rA_f {Z_comb}. The period of the main low-frequency component presumably caused by “inhalation / exhalation cycling” (IEC) was assessed at p_f {Z_comb} = p_IEC = 109.96 s (marked by the black arrow in panel C), as compared to p_sh, the period of oscillation provoked by shimmering (cf. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.ref030" target="_blank">30</a>]); (D-F) Mathematical model estimating the hourly change in volume of gas exchange affected by IEC according to the ventilation hypothesis (ordinate: ΔV_IEC [dm³ h^-1]). In this model, the size of the nest area affected by ventilation was chosen as 1 000 cm² corresponding to an area with a radius of 17.85 cm around the respective CNR funnel. The volume of gas exchange is dependent on three mass parameters of the bee curtain (number of bee layers N_layers: panel D; individual weight of a honeybee W_bee in mg: panel E) and of the comb (the relative weight of the comb area rW_comb: panel F). Red curves, mean values; black curves, range of mean errors; n_LDV = 23 LDV episodes. Normative base data (see Tables <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.t001" target="_blank">1</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.t003" target="_blank">3</a>): density of bees in the bee curtain = 1 bee per cm³; density of cells at both comb sides = 787 / dm² [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.ref072" target="_blank">72</a>]; mass of the wax = 24.11 g dm^-2 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157882#pone.0157882.ref072" target="_blank">72</a>]; this adds up to a mass of W_comb = 134.29 g /dm² comb area with larvae-containing cells (corresponding with rW_comb = 1.0 in panel F).</p

Evidence for Ventilation through Collective Respiratory Movements in Giant Honeybee (<i>Apis dorsata</i>) Nests

<div><p>The Asian giant honeybees (<i>Apis dorsata</i>) build single-comb nests in the open, which makes this species particularly susceptible to environmental strains. Long-term infrared (IR) records documented cool nest regions (CNR) at the bee curtain (n_CNR = 207, n_nests > 20) distinguished by marked negative gradients (ΔT_CNR/d < -3°C / 5 cm) at their margins. CNRs develop and recede within minutes, predominantly at higher ambient temperatures in the early afternoon. The differential size (ΔA_CNR) and temperature (ΔT_CNR) values per time unit correlated mostly positively (R_AT > 0) displaying the Venturi effect, which evidences funnel properties of CNRs. The air flows inwards through CNRs, which is verified by the negative spatial gradient ΔT_CNR/d, by the positive grading of T_CNR with T_amb and lastly by fanners which have directed their abdomens towards CNRs. Rare cases of R_AT < 0 (< 3%) document closing processes (for ΔA_CNR/Δt < -0.4 cm²/s) but also suggest ventilation of the bee curtain (for ΔA_CNR/Δt > +0.4 cm²/s) displaying “inhalation” and “exhalation” cycling. “Inhalation” could be boosted by bees at the inner curtain layers, which stretch their extremities against the comb enlarging the inner nest lumen and thus causing a pressure fall which drives ambient air inwards through CNR funnels. The relaxing of the formerly “activated” bees could then trigger the “exhalation” process, which brings the bee curtain, passively by gravity, close to the comb again. That way, warm, CO₂-enriched nest-borne air is pressed outwards through the leaking mesh of the bee curtain. This ventilation hypothesis is supported by IR imaging and laser vibrometry depicting CNRs in at least four aspects as low-resistance convection funnels for maintaining thermoregulation and restoring fresh air in the nest.</p></div

The occurrence of CNRs.

<p>(A) Plot of ambient temperature in °C (ordinate: T_amb) over daytime in hours (abscissa). Pooled data from “nest 02” assessed in steps of 30 min (n = 209 measurements; n_exp = 11 experiments; Nov. 2010, Chitwan, Nepal); grey line, regression of arithmetical means: R² = 0.9785. (B-C) Occurrence of CNRs (total N_CNR = 208) per 30 min over daytime (panel B: R² = 0.9631) and T_amb (panel C: R² = 0.9754). Stars give significant differences between adjacent pairs of data (*, P < 0.05; **, P < 0.01; ***, P < 0.001; t-test). Full circles, means; vertical bars, mean errors. The background colours symbolize the temperature gradient from cool (blue) to warm (red). (D-G) IR images at different T_amb and day times (see white text insets); scale bar (inset D), temperature range in °C.</p

Benchmark properties of the comb of a giant honeybee nest under normative conditions.

<p>Benchmark properties of the comb of a giant honeybee nest under normative conditions.</p