Juvenile hormone (JH) is a key regulator of insect development and reproduction. Given that JH commonly affects adult insect fertility, it has been hypothesized to also regulate behaviors such as dominance and aggression that are associated with reproduction. We tested this hypothesis in the bumblebee Bombus terrestris for which JH has been shown to be the major gonadotropin. We used the allatoxin precocene-I (P-I) to reduce hemolymph JH titers and replacement therapy with JH-III to revert this effect. In small orphan groups of workers with similar body size but mixed treatment, P-I treated bees showed lower aggressiveness, oogenesis, and dominance rank compared with control and replacement therapy treated bees. In similar groups in which all bees were treated similarly, there was a clear dominance hierarchy, even in P-I and replacement therapy groups in which the bees showed similar levels of ovarian activation. In a similar experiment in which bees differed in body size, larger bees were more likely to be dominant despite their similar JH treatment and ovarian state. In the last experiment, we show that JH manipulation does not affect dominance rank in groups that had already established a stable dominance hierarchy. These findings solve previous ambiguities concerning whether or not JH affects dominance in bumblebees. JH positively affects dominance, but bees with similar levels of JH can nevertheless establish dominance hierarchies. Thus, multiple factors including JH, body size, and previous experience affect dominance and aggression in social bumblebees.
Internal circadian clocks organize animal behavior and physiology and are entrained by ecologically-relevant external time-givers such as light and temperature cycles. In the highly social honey bee, social time-givers are important and can override photic entrainment, but the cues mediating social synchronization are unknown. Here we tested whether substrate-borne vibrations and hive volatiles can mediate social synchronization in honey bees. We first placed newly-emerged worker bees on the same or on a different substrate on which we placed cages with foragers entrained to ambient day- night cycles, while minimizing transfer of volatiles between cages. In the second experiment, we exposed young bees to constant airflow coming from either a free-foraging colony or a similar size control hive containing only empty combs, while minimizing transfer of substrate-borne vibrations between cages. After five days, we individually isolated each focal bee in an individual cage in an environmental chamber, and monitored locomotor activity. We repeated each experiment five times, each trail with bees from a different source colony, monitoring a total of more than 1000 bees representing diverse genotypes. We found that bees placed on the same substrate as foragers showed a stronger phase coherence; and in 3 of 5 trials their phase was more similar to that of foragers, compared to bees placed on a different substrate. In the second experiment, bees exposed to air from a colony showed a stronger phase coherence, and in 4 out of 5 trial their phase was more similar to that of foragers, compared to control bees exposed to air from an empty hive. These findings lend credence to the hypothesis that surrogates of activity such as substrate-borne vibrations, and volatile cues entrain circadian rhythms in natural free-foraging honey bee colonies.
Size polymorphism is common in bees, and is determined by environmental factors such as temperature, brood cell size, and the diet provided to developing larvae. In social bees, these factors are further influenced by intricate interactions between the queen, workers, and the developing brood which eventually determine the final size and caste of developing larvae. Environmental and social factors act in part on juvenile hormone and ecdysteroids, which are key hormonal regulators of body size and caste determination. In some social bees, body size variation is central for social organization because it structures reproductive division of labor, task allocation among workers, or both. At ecological scales, body size also impacts bee-mediated pollination services in solitary and social species by influencing floral visitation and pollination efficacy.
We studied phtototaxis, the directional movement relative to light in the bumblebee Bombus terrestris. We first developed and validated a MATLAB based system enabling reliable high-resolution tracking of a bumblebee relative to a changing LED light source. Our tracking protocol enables us to separate the phototaxis response from simple directional movement, overall levels of locomotor activity, or arousal state. We next used this system to compare the phototactic response of workers varying in their body size, age, or task performance. In all our experiments the bees showed a positive phototaxis. The strength of the phototactic response was influenced by body size but not age, and this effect was significant when the light source was weak. In one of two trials foragers that were larger showed stronger phototactic response than nurses when tested with the weak light source. Taken together, the evidence that phototaxis is associated with size-based division of labor in the bumblebee and with age-related division of labor in the honeybee, lend credence to response threshold models implicating the response to light in the organization of division of labor in cavity dwelling social insect.
Sleep is ubiquitous in vertebrates and invertebrates and its loss is typically associated with reduced performance, health, or survival, for reasons that are yet unclear [1—3]. Nevertheless, some animals can reduce sleep for increasing foraging time , under predation risk [5—8], during seasonal migration [9—11], or for having greater mating opportunities [12,13]. Here we tested the hypothesis that social bumble bee (Bombus terrestris) workers give-up sleep for improving brood-care. We combined video- recordings, detailed behavioral analyses, sleep-deprivation experiments, and response-threshold assessments, to characterize the sleep behavior of worker bees and showed that immobility bouts of ≥ 5' provide a reliable proxy for sleep. We next used this index to study sleep with an automated video-based activity monitoring system. We found that isolated workers severely reduce sleep time in the presence of both larvae that need to be fed, or pupae that do not. Reduced sleep was also correlated with around-the-clock activity and wax-pot building, which are typical for nest-founding mother queens. Cocoons, from which we removed the pupae, elicited a similar but transient sleep-loss in tending workers, suggesting that the pupa effect on sleep is mediated by pheromonal signals. Sleep time increased following brood removal, but remained lower compared to control bees, suggesting that the brood modulated sleep-need. This first evidence for brood modulation of sleep in an insect suggests that plasticity in sleep can evolve as a mechanism to improve care for dependent juveniles, even in social insect workers that do not care for their own offspring.
Colonies of the bumblebee Bombus terrestris are characterized by wide phenotypic variability among genetically similar full-sister workers, suggesting a major role for epigenetic processes. Here, we report a high level of ADAR-mediated RNA editing in the bumblebee, despite the lack of an ADAR1-homolog. We identify 1.15 million unique genomic sites, and 164 recoding sites residing in 100 protein coding genes, including ion channels, transporters, and receptors predicted to affect brain function and behavior. Some edited sites are similarly edited in other insects, cephalopods and even mammals. The global editing level of protein coding and non-coding transcripts weakly correlates with task performance (brood care vs. foraging), but not affected by dominance rank or juvenile hormone known to influence physiology and behavior. Taken together, our findings show that brain editing levels are high in naturally behaving bees, and may be regulated by relatively short-term effects associated with brood care or foraging activities.
Inferring connections forms a critical step toward understanding large and diverse complex networks. To date, reliable and efficient methods for the reconstruction of network topology from measurement data remain a challenge due to the high complexity and nonlinearity of the system dynamics. These obstacles also form a bottleneck for analyzing and controlling the dynamic structures (e.g., synchrony) and collective behavior in such complex networks. The novel contribution of this work is to develop a unified data-driven approach to reliably and efficiently reveal the dynamic topology of complex networks in different scales—from cells to societies. The developed technique provides guidelines for the refinement of experimental designs toward a comprehensive understanding of complex heterogeneous networks.Extracting complex interactions (i.e., dynamic topologies) has been an essential, but difficult, step toward understanding large, complex, and diverse systems including biological, financial, and electrical networks. However, reliable and efficient methods for the recovery or estimation of network topology remain a challenge due to the tremendous scale of emerging systems (e.g., brain and social networks) and the inherent nonlinearity within and between individual units. We develop a unified, data-driven approach to efficiently infer connections of networks (ICON). We apply ICON to determine topology of networks of oscillators with different periodicities, degree nodes, coupling functions, and time scales, arising in silico, and in electrochemistry, neuronal networks, and groups of mice. This method enables the formulation of these large-scale, nonlinear estimation problems as a linear inverse problem that can be solved using parallel computing. Working with data from networks, ICON is robust and versatile enough to reliably reveal full and partial resonance among fast chemical oscillators, coherent circadian rhythms among hundreds of cells, and functional connectivity mediating social synchronization of circadian rhythmicity among mice over weeks.
Abstract Several related and complementary theoretical frameworks have been proposed to explain the existence of prosocial behavior, despite its potential fitness cost to the individual. These include kin selection theory, proposing that organisms have a propensity to help those to whom they are genetically related, and reciprocity, referring to the benefit of being prosocial, depending on past and future mutual interactions. A useful paradigm to examine prosociality is to compare mean levels of this behavior between monozygotic (MZ) and dizygotic (DZ) twins. Here, we examined the performance of 883 6.5‐year‐old twins (139 MZ and 302 DZ same‐sex 6.5‐year‐old full twin pairs) in the Differential Productivity Task. In this task, the twins’ behaviors were observed under two conditions: working for themselves vs. working for their co‐twin. There were no significant differences between the performances of MZ and DZ twins in the prosocial condition of the task. Correlations within the twin dyads were significantly higher in MZ than DZ twins in the self‐interested condition. However, similar MZ and DZ correlations were found in the prosocial condition, supporting the role of reciprocity in twins’ prosociality towards each other.
Dynamic group coordination facilitates adaptive division of labor in response to group-level changes. Yet, little is known about how it can be operationalized in online collaborations among individuals with limited information about each other. We hypothesized that simple social information about the task distribution of others can elicit emergent task allocation. We conducted an online experiment where participants analyze images of a polluted canal by freely switching between two tasks: creating keyword-based tags for images and categorizing existing tags. During the task execution, we presented experimentally manipulated information about the contrasting group-level task distributions. Participants did not change the effort allocation between the tasks when they were notified that the group deficits workers in the task they intrinsically prefer. By contrast, they allocated more effort to the less preferred task than they would intrinsically do when their intrinsic effort allocation counterbalances the current distribution of workers in the group. Such behavioral changes were observed more strongly among those with higher skills in the less preferred task. Our results demonstrate the possibility of optimizing group coordination through design interventions at the individual level that lead to spontaneous adaption of division of labor at the group level. When participants were provided information about the group-level task distribution, they tend to allocate more effort to the task against their intrinsic preference.
Pigment-Dispersing Factor (PDF) is an important neuropeptide in the brain circadian network of Drosophila and other insects, but its role in bees in which the circadian clock influences complex behaviour is not well understood. We combined high-resolution neuroanatomical characterizations, quantification of PDF levels over the day and brain injections of synthetic PDF peptide to study the role of PDF in the honey bee Apis mellifera. We show that PDF co-localizes with the clock protein Period (PER) in a cluster of laterally located neurons and that the widespread arborizations of these PER/PDF neurons are in close vicinity to other PER-positive cells (neurons and glia). PDF-immunostaining intensity oscillates in a diurnal and circadian manner with possible influences for age or worker task on synchrony of oscillations in different brain areas. Finally, PDF injection into the area between optic lobes and the central brain at the end of the subjective day produced a consistent trend of phase-delayed circadian rhythms in locomotor activity. Altogether, these results are consistent with the hypothesis that PDF is a neuromodulator that conveys circadian information from pacemaker cells to brain centres involved in diverse functions including locomotion, time memory and sun-compass orientation.
Most processes within organisms, and most interactions between organisms and their environment, have distinct time profiles. The temporal coordination of such processes is crucial across levels of biological organization, but diferent disciplines differ widely in their approaches to study timing. Such differences are accentuated between ecologists, who are centrally concerned with a holistic view of an organism in relation to its external environment, and chronobiologists, who emphasize internal timekeeping within an organism and the mechanisms of its adjustment to the environment. We argue that ecological and chronobiological perspectives are complementary, and that studies at the intersection will enable both fields to jointly overcome obstacles that currently hinder progress. However, to achieve this integration, we first have to cross some conceptual barriers, clarifying prohibitively inaccessible terminologies. We critically assess main assumptions and concepts in either field, as well as their common interests. Both approaches intersect in their need to understand the extent and regulation of temporal plasticity, and in the concept of ‘chronotype’, i.e. the characteristic temporal properties of individuals which are the targets of natural and sexual selection. We then highlight promising developments, point out open questions, acknowledge difficulties and propose directions for further integration of ecological and chronobiological perspectives through Wild Clock research. This article is part of the themed issue ‘Wild Clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals’.
“Nurse” honeybees tend brood around-the-clock with attenuated or no circadian rhythms, but the brood signals inducing this behavior remain elusive. We first tested the hypothesis that worker circadian rhythms are regulated by brood pheromones. We monitored locomotor activity of individually isolated nurse bees that were either exposed to various doses of larval extracts or synthetic brood ester pheromone (BEP). Bees orally treated with larvae extracts showed attenuated circadian rhythms in one of four tested trials; a similar but statistically non-significant trend was seen in two an additional trial. Nurse bees treated with synthetic BEP showed rhythm attenuation in one of three trials. Next, we tested the hypothesis that capped brood, which does not require feeding, is nevertheless tended around-the-clock by nurse. By combining a new protocol that enables brood care by individually isolated nurse bees, detailed behavioral observations, and automatic high resolution monitoring of locomotor activity, we found that isolated nurses tended capped brood around-the-clock with attenuated circadian rhythms. Bees individually isolated in similar cages but without brood, showed strong circadian rhythms in locomotor activity and rest. This study shows for the first time that the need to feed hungry larvae is not the only factor accounting for around-the-clock activity in nurse bees. Our results further suggest that the transition between activity with and without circadian rhythms is not a simple switch triggered by brood pheromones. Around-the-clock tending may enhance brood development and health in multiple ways that may include improved larval feeding, thermoregulation and hygienic behavior.
The interactions between flowering plants and insect pollinators shape eco- logical communities and provide one of the best examples of coevolution. Although these interactions have received much attention in both ecology and evolution, their temporal aspects are little explored. Here we review studies on the circadian organization of pollination-related traits in bees and flowers. Research, mostly with the honeybee, Apis mellifera, has impli- cated the circadian clock in key aspects of their foraging for flower rewards. These include anticipation, timing of visits to flowers at specified locations and time-compensated sun-compass orientation. Floral rhythms in traits such as petal opening, scent release and reward availability also show robust daily rhythms. However, in only few studies it was possible to adequately determine whether these oscillations are driven by external time givers such as light and temperature cycles, or endogenous circadian clocks. The interplay between the timing of flowers and pollinators rhythms may be ecologically significant. Circadian regulation of pollination-related traits in only few species may influence the entire pollination network and thus affect community structure and local biodiversity. We speculate that these intricate chronobiological interactions may be vulnerable to anthropo- genic effects such as the introduction of alien invasive species, pesticides or environmental pollutants Q1 . This article is part of the themed issue ‘Wild clocks: integrating chrono- biology and ecology to understand timekeeping in free-living animals’.
The insect antennae receive olfactory information from the environment. In some insects it was shown that the antennal responsiveness is dynamically regulated by circadian clocks. However, it is unknown how general this phenomenon is and what functions it serves. Circadian regulation in honeybee workers is particularly interesting in this regard because they show natural task-related chronobiological plasticity. Forager bees show strong circadian rhythms in behavior and brain gene expression, whereas nurse bees tend brood around-the-clock and have attenuated circadian rhythms in activity and whole brain gene expression. Here we tested the hypothesis that there is task-related plasticity in circadian rhythms of antennal responsiveness to odorants in worker honeybees. We used electroantennogram (EAG) to measure the antennal responsiveness of nurses and foragers to general odorants and pheromones around the day. The capacity to track 10 Hz odorant pulses varied with time-of-day for both task-groups, but with different phases. The antennal pulse-tracking capacity was higher during the subjective day for the day-active foragers whereas it was better during the night for around-the-clock active nurses. The task-related phases of pulse-tracking rhythms were similar for all the tested stimuli. We also found evidence for circadian rhythms in the EAG response magnitude of foragers, but not of nurses. To the best of our knowledge, these results provide the first evidence for circadian regulation of antennal olfactory responsiveness and odorant pulse tracking capacity in bees, or any other hymenopteran insect. Importantly, our study shows for the first time that the circadian phase of olfactory responsiveness may be socially regulated.
Internal clocks driving rhythms of about a day (circadian) are ubiquitous in animals, allowing them to anticipate environmental changes. Genetic or environmental disturbances to circadian clocks or the rhythms they produce are commonly associated with illness, compromised performance or reduced survival. Nevertheless, some animals including Arctic mammals, open sea fish and social insects such as honeybees are active around-the-clock with no apparent ill effects. The mechanisms allowing this remarkable natural plasticity are unknown. We generated and validated a new and specific antibody against the clock protein PERIOD of the honeybee Apis mellifera (amPER) and used it to characterize the circadian network in the honeybee brain. We found many similarities to Drosophila melanogaster and other insects, suggesting common anatomical organization principles in the insect clock that have not been appreciated before. Time course analyses revealed strong daily oscillations in amPER levels in foragers, which show circadian rhythms, and also in nurses that do not, although the latter have attenuated oscillations in brain mRNA clock gene levels. The oscillations in nurses show that activity can be uncoupled from the circadian network and support the hypothesis that a ticking circadian clock is essential even in around-the-clock active animals in a constant physical environment.
Circadian rhythms in behaviour and physiology are important for animal health and survival. Studies with individually isolated animals in the laboratory have consistently emphasized the dominant role of light for the entrainment of circadian rhythms to relevant environmental cycles. Although in nature interactions with conspecifics are functionally significant, social signals are typically not considered important time-givers for the animal circadian clock. Our results challenge this view. By studying honeybees in an ecologically relevant context and using a massive data set, we demonstrate that social entrainment can be potent, may act without direct contact with other individuals and does not rely on gating the exposure to light. We show for the first time that social time cues stably entrain the clock, even in animals experiencing conflicting photic and social environmental cycles. These findings add to the growing appreciation for the importance of studying circadian rhythms in ecologically relevant contexts.
A hallmark of insect societies is a division of labor among workers specializing in different tasks. In bumblebees the division of labor is related to body size; relatively small workers are more likely to stay inside the nest and tend (''nurse'') brood, whereas their larger sisters are more likely to forage. Despite their ecological and economic importance, very little is known about the endocrine regulation of division of labor in bumblebees. We studied the influence of juvenile hormone OH) on task performance in the bumblebee Bombus terrestris. We first used a radioimmunoassay to measure circulating JH titers in workers specializing in nursing and foraging activities. Next, we developed new protocols for manipulating JH titers by combining a size-adjusted topical treatment with the allatotoxin Precocene-I and replacement therapy with JH-III. Finally, we used this protocol to test the influence of JH on task performance. JH levels were either similar for nurses and foragers (three colonies), or higher in nurses (two colonies). Nurses had better developed ovaries and JH levels were typically positively correlated with ovarian state. Manipulation of JH titers influenced ovarian development and wax secretion, consistent with earlier allatectomy studies. These manipulations however, did not affect nursing or foraging activity, or the likelihood to specialize in nursing or foraging activity. These findings contrast with honeybees in which JH influences age -related division of labor but not adult female fertility. Thus, the evolution of complex societies in bees was associated with modifications in the way JH influences social behavior. (C) 2016 Elsevier Inc. All rights reserved.
The emergence of eusociality (“true sociality”) in several insect lineages represents one of the most successful evolutionary adaptations in the animal kingdom in terms of species richness and global biomass. In contrast to solitary insects, eusocial insects evolved a set of unique behavioral and physiological traits such as reproductive division of labor and cooperative brood care, which likely played a major role in their ecological success. The molecular mechanisms that support the social regulation of behavior in eusocial insects, and their evolution, are mostly unknown. The recent whole-genome sequencing of several eusocial insect species set the stage for deciphering the molecular and genetic bases of eusociality, and the possible evolutionary modifications that led to it. Studies of mRNA expression patterns in the brains of diverse eusocial insect species have indicated that specific social behavioral states of individual workers and queens are often associated with particular tissue-specific transcriptional profiles. Here we discuss recent findings that highlight the role of non-coding microRNAs (miRNAs) in modulating traits associated with reproductive and behavioral divisions of labor in eusocial insects. We provide bioinformatic and phylogenetic data, which suggest that some Hymenoptera-specific miRNA may have contributed to the evolution of traits important for the evolution of eusociality in this group.
One of the most important and evolutionarily conserved roles of sleep is the processing and consolidation of information acquired during wakefulness. In both insects and mammals, environmental and social stimuli can modify sleep physiology and behavior, yet relatively little is known about the specifics of the wake experiences and their relative contribution to experience-dependent modulation of sleep. Honey bees provide an excellent model system in this regard because their behavioral repertoire is well characterized and the environment they experience during the day can be manipulated while keeping an ecologically and sociobiologically relevant context. We examined whether social experience modulates sleep in honey bees, and evaluated the relative contribution of different social signals. We exposed newly emerged bees to different components of their natural social environment and then monitored their sleep behavior in individual cages in a constant lab environment. We found that rich waking experience modulates subsequent sleep. Bees that experienced the colony environment for 1 or 2 days slept more than same-age sister bees that were caged individually or in small groups in the lab. Furthermore, bees placed in mesh-enclosures in the colony, that prevented direct contact with nestmates, slept similarly to bees freely moving in the colony. These results suggest that social signals that do not require direct or close distance interactions between bees are sufficiently rich to encompass almost the entire effect of the colony on sleep. Our findings provide a remarkable example of social experience- dependent modulation of an essential biological process.