Circadian rhythms of about a day are ubiquitous in animals and considered functionally significant. Honey bees show remarkable circadian plasticity that is related to the complex social organization of their societies. Forager bees show robust circadian rhythms that support time-compensated sun-compass navigation, dance communication and timing visits to flowers. Nest-dwelling nurse bees care for the young brood around the clock. Here, we review our current understanding of the molecular and neuroanatomical mechanisms underlying this remarkable natural plasticity in circadian rhythms.

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 potent and can override photic entrainment, but the cues mediating social entrainment 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 the spread of volatiles between cages. In the second experiment, we exposed young bees to constant airflow drawn from either a free-foraging colony or a similar-size control hive containing only heated empty honeycombs, while minimizing transfer of substrate-borne vibrations between cages. After 6 days, we isolated each focal bee in an individual cage in an environmental chamber and monitored her locomotor activity. We repeated each experiment 5 times, each trial 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 a phase more similar to that of foragers compared with bees placed on a different substrate. In the second experiment, bees exposed to air drawn from a colony showed a stronger phase coherence and a phase more similar to that of foragers compared with bees exposed to air from an empty hive. These findings lend credence to the hypothesis that surrogates of activity entrain circadian rhythms and suggest that multiple social cues can act in concert to entrain social insect colonies to a common phase.

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.

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.

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.

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 [4], 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.

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.

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.

Chapter Outline

30.1 Introduction

30.2 Overview of Division of Labor in Insect Societies

                30.2.1 Division of Labor for Reproduction

                30.2.2 Division of Labor among Workers

                30.2.3 Primitive and Advanced Eusociality

30.3 Insect Hormones That Influence Division of Labor

30.4 Endocrine Signaling in Reproductive Division of Labor

                30.4.1 JH and Reproductive Division of Labor

                30.4.2 Ecdysone and Reproductive Division of Labor

                30.4.3 Nutrition and Metabolic Factors Influencing Reproductive Division of Labor

                30.4.4 Biogenic Amines and Reproductive Division of Labor

30.5 Endocrine Influences on Division of Labor Among Workers: Behavioral Maturation

                30.5.1 Juvenile Hormone and Behavioral Maturation

                30.5.2 Ecdysone and Behavioral Maturation

                30.5.3 Nutrition and Metabolic Factors Influencing Behavioral Maturation

                30.5.4 Biogenic Amines and Behavioral Maturation

30.6 Endocrine Influences on Division of Labor Among Workers: Morphologically Distinct Castes

                30.6.1 Juvenile Hormone and Worker Caste Differentiation  

30.7 The Transcriptomic Architecture Linking Endocrine Signaling and Behavioral State

30.8 Evolutionary Perspectives

                30.8.1 Endocrine-Related Signatures of Selection

                30.8.2 Speculation on the Evolution of Division of Labor: A Neuroendocrine Perspective

                30.8.3 Level One: Incipient Societies and Endocrine-Mediated Social Inhibition among Adults

                30.8.4 Level Two: Pre-Adult Endocrine-Mediated Social Inhibition

30.8.5 Level Three: Pre-Adult, Endocrine-Mediated Social Inhibition Enhanced by Disruptive

Selection

                30.8.6 Level Four: Division of Labor among Adult Workers and its Regulation by Endocrine-

Mediated Social Inhibition

30.8.7 Level Five: Division of Labor among Morphologically Distinct Adult Workers and Its

Regulation by Pre-Adult, Endocrine-Mediated Social Inhibition

30.8.8 Concluding Remarks