An Ecological Perspective on Sleep Disruption

More sleep for behavioral ecologists

From jellyfish to parrot fish and roundworms to homeotherms, all animals are thought to sleep. Despite its presumed universality, sleep is a poorly understood behavior, varying significantly in its expression across, and even within, animal lineages. There is still no consensus about the origin, architecture, ecology of sleep, or even its defining characters. The field of behavioral ecology has the potential to extend our knowledge of sleep behavior to nontraditional models and in ecologically relevant settings. Here, we highlight current efforts in diversifying the field to generate stronger synergies between historically human-focused sleep research and behavioral ecology. Our primary aim is for behavioral ecology to enhance sleep research by contributing crucial observations as well as by creating novel comparative and evolutionary frameworks. At the same time, sleep research can enhance behavioral ecology by exposing the relevance of sleep to wakeful behaviors. Nikolaas Tinbergen's four levels of analysis have served as a foundation for comprehensively addressing questions in behavior, and we introduce some Tinbergian approaches to examine the interplay between sleep and wake under ecologically meaningful conditions.

Anthropogenic disturbance affects calling and collective behaviour in corvid roosts

Acoustic communication plays an important role in coordinating group dynamics and collective movements across a range of taxa. However, anthropogenic disturbance can inhibit the production or reception of acoustic signals. Here, we investigate the effects of noise and light pollution on the calling and collective behaviour of wild jackdaws (Corvus monedula), a highly social corvid species that uses vocalizations to coordinate collective movements at winter roosting sites. Using audio and video monitoring of roosts in areas with differing degrees of urbanization, we evaluate the influence of anthropogenic disturbance on vocalizations and collective movements. We found that when levels of background noise were higher, jackdaws took longer to settle following arrival at the roost in the evening and also called more during the night, suggesting that human disturbance may cause sleep disruption. High levels of overnight calling were, in turn, linked to disruption of vocal consensus decision-making and less cohesive group departures in the morning. These results raise the possibility that, by affecting cognitive and perceptual processes, human activities may interfere with animals' ability to coordinate collective behaviour. Understanding links between anthropogenic disturbance, communication, cognition and collective behaviour must be an important research priority in our increasingly urbanized world. This article is part of the theme issue 'The power of sound: unravelling how acoustic communication shapes group dynamics'.

Ecological responses of squamate reptiles to nocturnal warming

Nocturnal temperatures are increasing at a pace exceeding diurnal temperatures in most parts of the world. The role of warmer nocturnal temperatures in animal ecology has received scant attention and most studies focus on diurnal or daily descriptors of thermal environments' temporal trends. Yet, available evidence from plant and insect studies suggests that organisms can exhibit contrasting physiological responses to diurnal and nocturnal warming. Limiting studies to diurnal trends can thus result in incomplete and misleading interpretations of the ability of species to cope with global warming. Although they are expected to be impacted by warmer nocturnal temperatures, insufficient data are available regarding the night-time ecology of vertebrate ectotherms. Here, we illustrate the complex effects of nocturnal warming on squamate reptiles, a keystone group of vertebrate ectotherms. Our review includes discussion of diurnal and nocturnal ectotherms, but we mainly focus on diurnal species for which nocturnal warming affects a period dedicated to physiological recovery, and thus may perturb activity patterns and energy balance. We first summarise the physical consequences of nocturnal warming on habitats used by squamate reptiles. Second, we describe how such changes can alter the energy balance of diurnal species. We illustrate this with empirical data from the asp viper (Vipera aspis) and common wall lizard (Podarcis muralis), two diurnal species found throughout western Europe. Third, we make use of a mechanistic approach based on an energy-balance model to draw general conclusions about the effects of nocturnal temperatures. Fourth, we examine how warmer nights may affect squamates over their lifetime, with potential consequences on individual fitness and population dynamics. We review quantitative evidence for such lifetime effects using recent data derived from a range of studies on the European common lizard (Zootoca vivipara). Finally, we consider the broader eco-evolutionary ramifications of nocturnal warming and highlight several research questions that require future attention. Our work emphasises the importance of considering the joint influence of diurnal and nocturnal warming on the responses of vertebrate ectotherms to climate warming.

Testing the Accuracy of Wearable Technology to Assess Sleep Behaviour in Domestic Dogs: A Prospective Tool for Animal Welfare Assessment in Kennels

Sleep is a physiological process that has been shown to impact both physical and psychological heath of individuals when compromised; hence, it has the potential to be used as an indicator of animal welfare. Nonetheless, evaluating sleep in non-human species normally involves manipulation of the subjects (i.e., placement of electrodes on the cranium), and most studies are conducted in a laboratory setting, which limits the generalisability of information obtained, and the species investigated. In this study, we evaluated an alternative method of assessing sleep behaviour in domestic dogs, using a wearable sensor, and compared the measurements obtained to behavioural observations to evaluate accuracy. Differences between methods ranged from 0.13% to 59.3% for diurnal observations and 0.1% to 95.9% for nocturnal observations for point-by-point observations. Comparisons between methods showed significant differences in certain behaviours, such as inactivity and activity for diurnal recordings. However, total activity and total sleep recorded did not differ statistically between methods. Overall, the wearable technology tested was found to be a useful, and a less-time consuming, tool in comparison to direct behavioural observations for the evaluation of behaviours and their indication of wellbeing in dogs. The agreement between the wearable technology and directly observed data ranged from 75% to 99% for recorded behaviours, and these results are similar to previous findings in the literature.

Artificial diet alters activity and rest patterns in the olive fruit fly

Olive fruit flies, Bactrocera oleae (Diptera: Tephritidae) reared in the laboratory on an artificial diet are essential for the genetic control techniques against this pest. However, the colony's laboratory adaptation can affect the quality of the reared flies. We used the Locomotor Activity Monitor to track the activity and rest patterns of adult olive fruit flies reared as immatures in olives (F2-F3 generation) and in artificial diet (>300 generations). Counts of beam breaks caused by the adult fly activity were used as an estimation of its locomotor activity levels during the light and dark period. Bouts of inactivity with duration longer than five minutes were considered a rest episode. Locomotor activity and rest parameters were found to be dependent on sex, mating status and rearing history. In virgin flies reared on olives, males were more active than females and increased their locomotor activity towards the end of the light period. Mating decreased the locomotor activity levels of males, but not of female olive-reared flies. Laboratory flies reared on artificial diet had lower locomotor activity levels during the light period and more rest episodes of shorter duration during the dark period compared to flies reared on olives. We describe the diurnal locomotor activity patterns of B. oleae adults reared on olive fruit and on artificial diet. We discuss how locomotor activity and rest pattern differences may affect the laboratory flies' ability to compete with wild males in the field.

Listening to Their Nights: Sleep Disruptions in Captive Housed Chimpanzees Affect Their Daytime Behavior

Chimpanzee's nocturnal sleep is a dynamic and complex process, still not fully understood. As in humans, not getting enough quality sleep due to frequent or lasting disruptions may affect their physical and mental health, hence wellbeing, which may be reflected in their daytime behavior. This study aims to understand the impact of abiotic factors, such as temperature and humidity on the nocturnal activity as well as the impact of nocturnal awakening events on daytime behavior in sanctuary-housed chimpanzees. We monitored noisy nocturnal activity through audio recordings for one year, documenting the number and duration of sound events produced by chimpanzees to indicate sleep fragmentation and disruption intensity, respectively. Our results indicate that indoor temperature and humidity indeed influence the chimpanzee's nocturnal activity. Furthermore, sleep fragmentation and intensity of nocturnal events significantly influenced the following day's behavior. After nights marked by frequent and/or intense sleep disruptions, higher levels of inactivity, and abnormal and self-directed behaviors were observed, and chimpanzees spent more time on affiliative interactions and in social proximity. These findings highlight the importance of controlling factors influencing nocturnal sleep quality. Furthermore, we demonstrated that economic audio recordings used to monitor nocturnal activity, provide insights into the chimpanzee's behavior and wellbeing.

High-resolution tracking of hyrax social interactions highlights nighttime drivers of animal sociality

Network structure is a key driver of animal fitness, pathogen transmission, information spread, and population demographics in the wild. Although a considerable body of research applied network analysis to animal societies, only little effort has been devoted to separate daytime and nighttime sociality and explicitly test working hypotheses on social structures emerging at night. Here, we investigated the nighttime sociality of a wild population of rock hyraxes (Procavia capensis) and its relation to daytime social structure. We recorded nearly 15,000 encounters over 27 consecutive days and nights using proximity loggers. Overall, we show that hyraxes are more selective of their social affiliates at night compared to daytime. We also show that hyraxes maintain their overall network topology while reallocating the weights of social relationships at the daily and monthly scales, which could help hyraxes maintain their social structure over long periods while adapting to local constraints and generate complex social dynamics. These results suggest that complex network dynamics can be a by-product of simple daily social tactics and do not require high cognitive abilities. Our work sheds light on the function of nighttime social interactions in diurnal social species.

Cascading effects of caffeine intake by primary consumers to the upper trophic level

Secondary metabolites are central to understanding the evolution of plant-animal interactions. Direct effects on phytophagous animals are well-known, but how secondary consumers adjust their behavioural and physiological responses to the herbivore's diet remains more scarcely explored for some metabolites. Caffeine is a neuroactive compound that affects both the behaviour and physiology of several animal species, from humans to insects. It is an alkaloid present in nectar, leaves and even sap of numerous species of plants where it plays a role in chemical defences against herbivores and pathogens. Caffeine effects have been overlooked in generalist herbivores that are not specialized in coffee or tea plants. Using a host-parasitoid system, we show that caffeine intake at a relatively low dose affects longevity and fecundity of the primary consumer, but also indirectly of the secondary one, suggesting that this alkaloid and/or its effects can be transmitted through trophic levels and persist in the food chain. Parasitism success was lowered by ≈16% on hosts fed with caffeine, and parasitoids of the next generation that have developed in hosts fed on caffeine showed a reduced longevity, but no differences in mass and size were found. This study helps at better understanding how plant secondary metabolites, such as caffeine involved in plant-animal interactions, could affect primary consumers, could have knock-on effects on upper trophic levels over generations, and could modify interspecific interactions in multitrophic systems.

The cyclic interaction between daytime behavior and the sleep behavior of laboratory dogs

Sleep deprivation has been found to negatively affect an individual´s physical and psychological health. Sleep loss affects activity patterns, increases anxiety-like behaviors, decreases cognitive performance and is associated with depressive states. The activity/rest cycle of dogs has been investigated before, but little is known about the effects of sleep loss on the behavior of the species. Dogs are polyphasic sleepers, meaning the behavior is most observed at night, but bouts are also present during the day. However, sleep can vary with ecological and biological factors, such as age, sex, fitness, and even human presence. In this study, kennelled laboratory adult dogs' sleep and diurnal behavior were recorded during 24-h, five-day assessment periods to investigate sleep quality and its effect on daily behavior. In total, 1560 h of data were analyzed, and sleep metrics and diurnal behavior were quantified. The relationship between sleeping patterns and behavior and the effect of age and sex were evaluated using non-parametric statistical tests and GLMM modelling. Dogs in our study slept substantially less than previously reported and presented a modified sleep architecture with fewer awakenings during the night and almost no sleep during the day. Sleep loss increased inactivity, decreased play and alert behaviors, while increased time spent eating during the day. Males appeared to be more affected by sleep fragmentation than females. Different age groups also experienced different effects of sleep loss. Overall, dogs appear to compensate for the lack of sleep during the night by remaining inactive during the day. With further investigations, the relationship between sleep loss and behavior has the potential to be used as a measure of animal welfare.

The ecology of sleep in non-avian reptiles

Sleep is ubiquitous in the animal kingdom and yet displays considerable variation in its extent and form in the wild. Ecological factors, such as predation, competition, and microclimate, therefore are likely to play a strong role in shaping characteristics of sleep. Despite the potential for ecological factors to influence various aspects of sleep, the ecological context of sleep in non-avian reptiles remains understudied and without systematic direction. In this review, we examine multiple aspects of reptilian sleep, including (i) habitat selection (sleep sites and their spatio-temporal distribution), (ii) individual-level traits, such as behaviour (sleep postures), morphology (limb morphometrics and body colour), and physiology (sleep architecture), as well as (iii) inter-individual interactions (intra- and inter-specific). Throughout, we discuss the evidence of predation, competition, and thermoregulation in influencing sleep traits and the possible evolutionary consequences of these sleep traits for reptile sociality, morphological specialisation, and habitat partitioning. We also review the ways in which sleep ecology interacts with urbanisation, biological invasions, and climate change. Overall, we not only provide a systematic evaluation of the conceptual and taxonomic biases in the existing literature on reptilian sleep, but also use this opportunity to organise the various ecological hypotheses for sleep characteristics. By highlighting the gaps and providing a prospectus of research directions, our review sets the stage for understanding sleep ecology in the natural world.

Sleep contributes to preference for novel food odours in Drosophila melanogaster

The importance of sleep in maintaining cognitive functions such as learning and memory has been reported in both vertebrates and invertebrates. Previous studies demonstrated that sleep deprivation impaired the olfactory memory retention of fruit flies as described in the classical conditioning paradigm. Here, we show that sleep deprivation leads to a preference for the odours of the rearing environment in Drosophila melanogaster. Flies whose sleep had been disturbed with periodic rotation stimuli during night-time preferred apple cider vinegar (ACV) to broth, while this preference was lower in flies without sleep deprivation and those rotated during daytime. Experiments using single odours showed an increase in responses to ACV due to sleep deprivation. These results suggest that sleep functions in food odour preference. Flies grown on medium supplemented with ACV showed greater preference for ACV, and those grown with broth supplementation showed a greater preference for broth under sleep-deprived conditions. These results suggest that flies with night-time sleep deprivation become attached to the environment on which they have developed, and that sleep contributes to preference for novel food odours. This study offers an approach to investigating the interaction between sleep and neural disorders concerning cognitive deficits towards novel stimuli.

Sleep in honey bees is affected by the herbicide glyphosate

Sleep plays an essential role in both neural and energetic homeostasis of animals. Honey bees (Apis mellifera) manifest the sleep state as a reduction in muscle tone and antennal movements, which is susceptible to physical or chemical disturbances. This social insect is one of the most important pollinators in agricultural ecosystems, being exposed to a great variety of agrochemicals, which might affect its sleep behaviour. The intake of glyphosate (GLY), the herbicide most widely used worldwide, impairs learning, gustatory responsiveness and navigation in honey bees. In general, these cognitive abilities are linked with the amount and quality of sleep. Furthermore, it has been reported that animals exposed to sleep disturbances show impairments in both metabolism and memory consolidation. Consequently, we assessed the sleep pattern of bees fed with a sugar solution containing GLY (0, 25, 50 and 100 ng) by quantifying their antennal activity during the scotophase. We found that the ingestion of 50 ng of GLY decreased both antennal activity and sleep bout frequency. This sleep deepening after GLY intake could be explained as a consequence of the regenerative function of sleep and the metabolic stress induced by the herbicide.

Drosophila insulin-like peptide 2 mediates dietary regulation of sleep intensity

Sleep is a nearly universal behavior that is regulated by diverse environmental stimuli and physiological states. A defining feature of sleep is a homeostatic rebound following deprivation, where animals compensate for lost sleep by increasing sleep duration and/or sleep depth. The fruit fly, Drosophila melanogaster, exhibits robust recovery sleep following deprivation and represents a powerful model to study neural circuits regulating sleep homeostasis. Numerous neuronal populations have been identified in modulating sleep homeostasis as well as depth, raising the possibility that the duration and quality of recovery sleep is dependent on the environmental or physiological processes that induce sleep deprivation. Here, we find that unlike most pharmacological and environmental manipulations commonly used to restrict sleep, starvation potently induces sleep loss without a subsequent rebound in sleep duration or depth. Both starvation and a sucrose-only diet result in increased sleep depth, suggesting that dietary protein is essential for normal sleep depth and homeostasis. Finally, we find that Drosophila insulin like peptide 2 (Dilp2) is acutely required for starvation-induced changes in sleep depth without regulating the duration of sleep. Flies lacking Dilp2 exhibit a compensatory sleep rebound following starvation-induced sleep deprivation, suggesting Dilp2 promotes resiliency to sleep loss. Together, these findings reveal innate resilience to starvation-induced sleep loss and identify distinct mechanisms that underlie starvation-induced changes in sleep duration and depth.

Sleep is nearly universal throughout the animal kingdom and homeostatic regulation represents a defining feature of sleep, where animals compensate for lost sleep by increasing sleep over subsequent time periods. Despite the robustness of this feature, the neural mechanisms regulating recovery from different types of sleep deprivation are not fully understood. Fruit flies provide a powerful model for investigating the genetic regulation of sleep, and like mammals, display robust recovery sleep following deprivation. Here, we find that unlike most stimuli that suppress sleep, sleep deprivation by starvation does not require a homeostatic rebound. These findings are likely due to flies engaging in deeper sleep during the period of partial sleep deprivation, suggesting a natural resilience to starvation-induced sleep loss. This unique resilience to starvation-induced sleep loss is dependent on Drosophila insulin-like peptide 2, revealing a critical role for insulin signaling in regulating interactions between diet and sleep homeostasis.

Personality and artificial light at night in a semi-urban songbird population: No evidence for personality-dependent sampling bias, avoidance or disruptive effects on sleep behaviour

Light pollution or artificial light at night (ALAN) is an increasing, worldwide challenge that affects many aspects of animal behaviour. Interestingly, the response to ALAN varies widely among individuals within a population and variation in personality (consistent individual differences in behaviour) may be an important factor explaining this variation. Consistent individual differences in exploration behaviour in particular may relate to the response to ALAN, as increasing evidence indicates its relation with how individuals respond to novelty and how they cope with anthropogenic modifications of the environment. Here, we assayed exploration behaviour in a novel environment as a proxy for personality variation in great tits (Parus major). We observed individual sleep behaviour over two consecutive nights, with birds sleeping under natural dark conditions the first night and confronted with ALAN inside the nest box on the second night, representing a modified and novel roosting environment. We examined whether roosting decisions when confronted with a camera (novel object), and subsequently with ALAN, were personality-dependent, as this could potentially create sampling bias. Finally, we assessed whether experimentally challenging individuals with ALAN induced personality-dependent changes in sleep behaviour. Slow and fast explorers were equally likely to roost in a nest box when confronted with either a camera or artificial light inside, indicating the absence of personality-dependent sampling bias or avoidance of exposure to ALAN. Moreover, slow and fast explorers were equally disrupted in their sleep behaviour when challenged with ALAN. Whether other behavioural and physiological effects of ALAN are personality-dependent remains to be determined. Moreover, the sensitivity to disturbance of different behavioural types might depend on the behavioural context and the specific type of challenge in question. In our increasingly urbanized world, determining whether the effects of anthropogenic stressors depend on personality type will be of paramount importance as it may affect population dynamics.

The origins and evolution of sleep

Sleep is nearly ubiquitous throughout the animal kingdom, yet little is known about how ecological factors or perturbations to the environment shape the duration and timing of sleep. In diverse animal taxa, poor sleep negatively impacts development, cognitive abilities and longevity. In addition to mammals, sleep has been characterized in genetic model organisms, ranging from the nematode worm to zebrafish, and, more recently, in emergent models with simplified nervous systems such as Aplysia and jellyfish. In addition, evolutionary models ranging from fruit flies to cavefish have leveraged natural genetic variation to investigate the relationship between ecology and sleep. Here, we describe the contributions of classical and emergent genetic model systems to investigate mechanisms underlying sleep regulation. These studies highlight fundamental interactions between sleep and sensory processing, as well as a remarkable plasticity of sleep in response to environmental changes. Understanding how sleep varies throughout the animal kingdom will provide critical insight into fundamental functions and conserved genetic mechanisms underlying sleep regulation. Furthermore, identification of naturally occurring genetic variation regulating sleep may provide novel drug targets and approaches to treat sleep-related diseases.