Social interactions and the circadian rhythm in locomotor activity in the cockroach Leucophaea maderae

The presence/absence of conspecifics modulates the circadian locomotor activity and body mass in spotted munia (Lonchura punctulata)

Biological clocks regulate the behavior and physiology of animals by tracking the local time using diverse time cues. Social cues are relevant in studying the behavior of gregarious animals, but these cues have not been widely studied in birds. Temporal information for circadian timekeeping is socially communicated through visual, physical, olfactory, and auditory means. We examined the efficacy of pulsatile social interactions on locomotor activity and its associated characteristics such as distribution profile of rest and activity, total counts, activity duration, phase shift in activity onset, and circadian periodicity in spotted munia. Besides, we analyzed the effect of such social interactions on their body mass. Spotted munia exhibited phase shift in the onset of activity when subjected to social isolation, but these cues could not affect their circadian periodicity. In Pair as well as in Group, social isolation led to increased activity and activity duration, and decreased body mass in guests relative to the host bird. Our results suggest that the circadian rhythm of locomotor activity in spotted munia is quite sensitive to socialization and isolation, and isolation is detrimental for the birds. Consistent with these observations, the decline in body mass revealed the physiological consequences of social isolation on spotted munia.

Social synchronization of circadian rhythms with a focus on honeybees

Many animals benefit from synchronizing their daily activities with conspecifics. In this hybrid paper, we first review recent literature supporting and extending earlier evidence for a lack of clear relationship between the level of sociality and social entrainment of circadian rhythms. Social entrainment is specifically potent in social animals that live in constant environments in which some or all individuals do not experience the ambient day-night cycles. We next focus on highly social honeybees in which there is good evidence that social cues entrain the circadian clocks of nest bees and can override the influence of conflicting light-dark cycles. The current understanding of social synchronization in honeybees is consistent with self-organization models in which surrogates of forager activity, such as substrate-borne vibrations and colony volatiles, entrain the circadian clocks of bees dwelling in the dark cavity of the nest. Finally, we present original findings showing that social synchronization is effective even in an array of individually caged callow bees placed on the same substrate and is improved for bees in connected cages. These findings reveal remarkable sensitivity to social time-giving cues and show that bees with attenuated rhythms (weak oscillators) can nevertheless be socially synchronized to a common phase of activity. This article is part of the theme issue 'Synchrony and rhythm interaction: from the brain to behavioural ecology'.

Social modulation of ageing: mechanisms, ecology, evolution

Human life expectancy increases, but the disease-free part of lifespan (healthspan) and the quality of life in old people may not show the same development. The situation poses considerable challenges to healthcare systems and economies, and calls for new strategies to increase healthspan and for sustainable future approaches to elder care. This call has motivated innovative research on the role of social relationships during ageing. Correlative data from clinical surveys indicate that social contact promotes healthy ageing, and it is time to reveal the causal mechanisms through experimental research. The fruit fly Drosophila melanogaster is a prolific model animal, but insects with more developed social behaviour can be equally instrumental for this research. Here, we discuss the role of social contact in ageing, and identify lines of study where diverse insect models can help uncover the mechanisms that are involved. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

The Effect of Ambient Humidity on the Metabolic Rate and Respiratory Patterns of the Hissing Cockroach, Gromphadorhina portentosa (Blattodea: Blaberidae)

We examined the effects of humidity on the metabolic rates and respiratory patterns of Gromphadorhina portentosa (Schaum) (Blattodea: Blaberidae) to determine whether insects transition from continuous, cyclical, and discontinuous (DGC) respiration in response to water conservation. Eight male G. portentosa were placed under five different humidity treatments (0, 23, 40, 60, 80% RH). Using flow through respirometry we: (i) determined the effect of humidity on metabolic rate; (ii) observed if changes in metabolic rate were correlated with changes in closed/flutter (CF) or the open (O) phase of DGC; and (iii) determined whether increased spiracular closure was correlated with an increase in water retention. Although G. portentosa had similar rates of CO2 release when placed under 0, 40, 60, and 80% RH, cockroaches placed at 23% RH had a significantly higher metabolic rate. There was no effect of humidity on the duration of the CF phase of the DGC. However, the O phase of DGC was significantly longer when G. portentosa was placed at humidity levels above 23% RH. Interestingly, we saw that the average rate of mass lost to the environment did not change when cockroaches were placed at humidity levels ranging from 0 to 80% RH. This suggests that modulation of the spiracles allows G. portentosa to regulate the amount of water lost to the environment.

Peptidergic circadian clock circuits in the Madeira cockroach

Circadian clocks control physiology and behavior of organisms in synchrony with external light dark cycles in changing photoperiods. The Madeira cockroach Rhyparobia maderae was the first model organism in which an endogenous circadian clock in the brain was identified. About 240 neurons constitute the cockroach circadian pacemaker network in the accessory medulla. The expression of high concentrations of neuropeptides, among them the most prominent circadian coupling factor pigment-dispersing factor, as well as their ability to generate endogenous ultradian and circadian rhythms in electrical activity and clock gene expression distinguish these pacemaker neurons. We assume that entrainment to light-dark cycles and the control of 24h rest-activity rhythms is achieved via peptidergic circuits forming autoreceptive labeled lines.

Potent social synchronization can override photic entrainment of circadian rhythms

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.

Socially synchronized circadian oscillators

Daily rhythms of physiology and behaviour are governed by an endogenous timekeeping mechanism (a circadian 'clock'). The alternation of environmental light and darkness synchronizes (entrains) these rhythms to the natural day-night cycle, and underlying mechanisms have been investigated using singly housed animals in the laboratory. But, most species ordinarily would not live out their lives in such seclusion; in their natural habitats, they interact with other individuals, and some live in colonies with highly developed social structures requiring temporal synchronization. Social cues may thus be critical to the adaptive function of the circadian system, but elucidating their role and the responsible mechanisms has proven elusive. Here, we highlight three model systems that are now being applied to understanding the biology of socially synchronized circadian oscillators: the fruitfly, with its powerful array of molecular genetic tools; the honeybee, with its complex natural society and clear division of labour; and, at a different level of biological organization, the rodent suprachiasmatic nucleus, site of the brain's circadian clock, with its network of mutually coupled single-cell oscillators. Analyses at the 'group' level of circadian organization will likely generate a more complex, but ultimately more comprehensive, view of clocks and rhythms and their contribution to fitness in nature.

Time of day changes in cyclic nucleotides are modified via octopamine and pheromone in antennae of the Madeira cockroach

The cockroach Rhyparobia (Leucophaea) maderae expresses a circadian rhythm in pheromone-dependent mating activity that peaks at the late day/early night. In contrast, the circadian rhythm in olfactory sensitivity of the Madeira cockroach is at its minimum during this time. Until now, the reasons for this obvious discrepancy in phase were not understood. Previously, it was shown that cyclic nucleotides modulate olfactory sensitivity in a zeitgeber time (ZT)-dependent manner. In moths' olfactory receptor neurons, adapting pheromone concentrations elevate cGMP levels, which decrease pheromone sensitivity. In contrast, cAMP elevations sensitized pheromone responses. Thus, with immunoassay kits, it was determined whether cAMP and cGMP baseline levels vary in a ZT-dependent manner in antennal lysates of female R. maderae, revealing underlying circadian rhythms in olfactory sensitivity. Furthermore, it was examined whether adapting pheromone exposure elevates cGMP levels in cockroach antennae, possibly overshadowing underlying circadian rhythms in sensitivity via sensory adaptation. It was shown for the first time that cAMP and cGMP baseline levels oscillate in antiphase in a ZT-dependent manner in an insect's antenna, with the maximum in cAMP concentrations coinciding with maximal mating activity during the late day. Moreover, the cAMP baseline level oscillation expressed a circadian rhythm since it persisted under constant darkness in contrast to cGMP baseline levels. Furthermore, while excess exposure to male pheromones increased cGMP and decreased cAMP baseline levels, the stress hormone octopamine increased adenylyl cyclase activity at all ZTs tested. Therefore, it is suggested that cyclic nucleotide-dependent modulation of olfactory sensitivity due to olfactory overstimulation and stress could be responsible for previously measured phase discrepancies between rhythms in mating behavior and pheromone sensitivity.

Sensory cues involved in social facilitation of reproduction in Blattella germanica females

Cockroaches, like many other animal species, form aggregations in which social stimuli from conspecifics can alter the physiology, morphology, or behavior of individuals. In adult females of the German cockroach, Blattella germanica, social isolation slows oocyte development, sexual maturation, and sexual receptivity, whereas social interactions as minimal as between just two females accelerate reproduction; however, the sensory modalities and pathways that mediate these physiological and behavioral changes are poorly understood. We explored the roles of visual, olfactory, and tactile cues in the reproductive physiology of German cockroach females, and whether their effects are species-specific and related to circadian time. Our results show that tactile cues are the primary sensory input associated with social conditions—with no evidence for involvement of the visual and olfactory systems—and that the antennae play an important role in the reception of these tactile cues. This conclusion is supported by the observation that interactions with other insect species of similar or larger size and with similar antennal morphology also stimulate oocyte development in B. germanica. Social facilitation of reproduction is expected to be influenced by the circadian timing system, as females engage in more social contact during the day when they shelter in aggregations with conspecifics. Surprisingly, however, the female's reproductive rate was unresponsive to social interactions during the photophase, whereas social interactions as short as two hours during the scotophase were sufficient to induce faster reproduction.

We discuss the adaptive significance of these sensory-neuroendocrine responses in the German cockroach.

Solitary and Gregarious Locusts Differ in Circadian Rhythmicity of a Visual Output Neuron

Locusts demonstrate remarkable phenotypic plasticity driven by changes in population density. This density dependent phase polyphenism is associated with many physiological, behavioral, and morphological changes, including observations that cryptic solitarious (solitary-reared) individuals start to fly at dusk, whereas gregarious (crowd-reared) individuals are day-active. We have recorded for 24-36 h, from an identified visual output neuron, the descending contralateral movement detector (DCMD) of Schistocerca gregaria in solitarious and gregarious animals. DCMD signals impending collision and participates in flight avoidance maneuvers. The strength of DCMD’s response to looming stimuli, characterized by the number of evoked spikes and peak firing rate, varies approximately sinusoidally with a period close to 24 h under constant light in solitarious locusts. In gregarious individuals the 24-h pattern is more complex, being modified by secondary ultradian rhythms. DCMD’s strongest responses occur around expected dusk in solitarious locusts but up to 6 h earlier in gregarious locusts, matching the times of day at which locusts of each type are most active. We thus demonstrate a neuronal correlate of a temporal shift in behavior that is observed in gregarious locusts. Our ability to alter the nature of a circadian rhythm by manipulating the rearing density of locusts under identical light-dark cycles may provide important tools to investigate further the mechanisms underlying diurnal rhythmicity.

Timekeeping through social contacts: social synchronization of circadian locomotor activity rhythm in the carpenter ant Camponotus paria

In ant colonies a large proportion of individuals remain inside nests for most of their lives and come out only when necessary. It is not clear how, in a nest of several thousand individuals, information about local time is communicated among members of the colony. Central to this seem to be circadian clocks, which have an intrinsic ability to keep track of local time by entraining to environmental light-dark, temperature, and social cycles. Here, the authors report the results of their study aimed at understanding the role of cyclic social interactions in circadian timekeeping of a day-active species of carpenter ant Camponotus paria. The authors found that daily social interactions with visitors (worker ants) was able to synchronize the circadian locomotor activity rhythm of host worker ants and queens, in one-on-one (pair-wise) and multi-individual (group-wise) interactions. Interestingly, the outcome of cyclic social interactions was context specific; when visitor workers socially interacted with host workers one-on-one, host workers considered the time of interaction as subjective day, but when visitor workers interacted with a group of workers and queens, the hosts considered the time of interaction as subjective night. These results can be taken to suggest that members of the ant species C. paria keep track of local time by socially interacting with workers (foragers) who shuttle in and out of the colony in search of food. (Author correspondence: vsharma@jncasr.ac.in ).

Social influences on circadian rhythms and sleep in insects

The diverse social lifestyle and the small and accessible nervous system of insects make them valuable for research on the adaptive value and the organization principles of circadian rhythms and sleep. We focus on two complementary model insects, the fruit fly Drosophila melanogaster, which is amenable to extensive transgenic manipulations, and the honey bee Apis mellifera, which has rich and well-studied social behaviors. Social entrainment of activity rhythms (social synchronization) has been studied in many animals. Social time givers appear to be specifically important in dark cavity-dwelling social animals, but here there are no other clear relationships between the degree of sociality and the effectiveness of social entrainment. The olfactory system is important for social entrainment in insects. Little is known, however, about the molecular and neuronal pathways linking olfactory neurons to the central clock. In the honey bee, the expression, phase, and development of circadian rhythms are socially regulated, apparently by different signals. Peripheral clocks regulating pheromone synthesis and the olfactory system have been implicated in social influences on circadian rhythms in the fruit fly. An enriched social environment increases the total amount of sleep in both fruit flies and honey bees. In fruit flies, these changes have been linked to molecular and neuronal processes involved in learning, memory, and synaptic plasticity. The studies on insects suggest that social influences on the clock are richer than previously appreciated and have led to important breakthroughs in our understanding of the mechanisms underlying social influences on sleep and circadian rhythms.

Molecular dynamics and social regulation of context-dependent plasticity in the circadian clockwork of the honey bee

The social environment influences the circadian clock of diverse animals, but little is known about the functional significance, the specifics of the social signals, or the dynamics of socially mediated changes in the clock. Honey bees switch between activities with and without circadian rhythms according to their social task. Forager bees have strong circadian rhythms, whereas "nurse" bees typically care for the brood around-the-clock with no circadian rhythms in behavior or clock gene expression. Here we show that nurse-age bees that were restricted to a broodless comb inside or outside the hive showed robust behavioral and molecular circadian rhythms. By contrast, young nurses tended brood with no circadian rhythms in behavior or clock gene expression, even under a light-dark illumination regime or when placed with brood--but no queen--in a small cage outside the hive. This behavior is context-dependent because nurses showed circadian rhythms in locomotor activity shortly after removal from the hive, and in clock gene expression after ∼16 h. These findings suggest that direct interaction with the brood modulates the circadian system of honey bees. The dynamics of rhythm development best fit models positing that at least some pacemakers continue to oscillate and be entrained by the environment in nurses that are active around the clock. These cells set the phase to the clock network when the nurse is removed from the hive. These findings suggest that despite its robustness, the circadian system exhibits profound plasticity, enabling adjustment to rapid changes in the social environment.