Activity of root voles (Microtus oeconomus) under snow: social encounters synchronize individual activity rhythms

Ambient Temperature Effects on the Spring and Autumn Somatic Growth Trajectory Show Plasticity in the Photoneuroendocrine Response Pathway in the Tundra Vole

Seasonal mammals register photoperiodic changes through the photoneuroendocrine system enabling them to time seasonal changes in growth, metabolism, and reproduction. To a varying extent, proximate environmental factors like ambient temperature (Ta) modulate timing of seasonal changes in physiology, conferring adaptive flexibility. While the molecular photoneuroendocrine pathway governing the seasonal responses is well defined, the mechanistic integration of nonphotoperiodic modulatory cues is poorly understood. Here, we explored the interaction between Ta and photoperiod in tundra voles, Microtus oeconomus, a boreal species in which the main impact of photoperiod is on postnatal somatic growth. We demonstrate that postweaning growth potential depends on both gestational and postweaning patterns of photoperiodic exposure, with the highest growth potential seen in voles experiencing short (8 h) gestational and long (16 h) postweaning photoperiods-corresponding to a spring growth program. Modulation by Ta was asymmetric: low Ta (10 °C) enhanced the growth potential of voles gestated on short photoperiods independent of postweaning photoperiod exposure, whereas in voles gestated on long photoperiods, showing a lower autumn-programmed growth potential, the effect of Ta was highly dependent on postweaning photoperiod. Analysis of the primary molecular elements involved in the expression of a neuroendocrine response to photoperiod, thyrotropin beta subunit (tshβ) in the pars tuberalis, somatostatin (srif) in the arcuate nucleus, and type 2/3 deiodinase (dio2/dio3) in the mediobasal hypothalamus identified dio2 as the most Ta-sensitive gene across the study, showing increased expression at higher Ta, while higher Ta reduced somatostatin expression. Contrastingly dio3 and tshβ were largely insensitive to Ta. Overall, these observations reveal a complex interplay between Ta and photoperiodic control of postnatal growth in M. oeconomus, and suggest that integration of Ta into the control of growth occurs downstream of the primary photoperiodic response cascade revealing potential adaptivity of small herbivores facing rising temperatures at high latitudes.

Accelerometer-based predictions of behaviour elucidate factors affecting the daily activity patterns of spotted hyenas

Animal activity patterns are highly variable and influenced by internal and external factors, including social processes. Quantifying activity patterns in natural settings can be challenging, as it is difficult to monitor animals over long time periods. Here, we developed and validated a machine-learning-based classifier to identify behavioural states from accelerometer data of wild spotted hyenas (Crocuta crocuta), social carnivores that live in large fission-fusion societies. By combining this classifier with continuous collar-based accelerometer data from five hyenas, we generated a complete record of activity patterns over more than one month. We used these continuous behavioural sequences to investigate how past activity, individual idiosyncrasies, and social synchronization influence hyena activity patterns. We found that hyenas exhibit characteristic crepuscular-nocturnal daily activity patterns. Time spent active was independent of activity level on previous days, suggesting that hyenas do not show activity compensation. We also found limited evidence for an effect of individual identity on activity, and showed that pairs of hyenas who synchronized their activity patterns must have spent more time together. This study sheds light on the patterns and drivers of activity in spotted hyena societies, and also provides a useful tool for quantifying behavioural sequences from accelerometer data.

Seasonal Changes in Locomotor Activity Patterns of Wild Social Natal Mole-Rats (Cryptomys hottentotus natalensis)

Differences in individual locomotor activity patterns may be linked to a number of ecological factors, such as changes in ambient temperature or photoperiod. Observations on subterranean mammals suggest that they exhibit diel rhythms despite the lack of visual cues in their underground burrows, but it is unknown how seasonality and individual characteristics affect their activity. In this study we use RFID technology to monitor daily activity patterns of wild, social Natal mole-rats (Cryptomys hottentotus natalensis) during the summer and winter to investigate how their activity varies with season and whether their activity depends on individual characteristics such as body mass, sex and reproductive status. We found that in winter, individuals were more active during the time with the highest soil temperatures, whereas in summer, they showed a bimodal activity pattern during early morning and late afternoon coinciding with cooler soil temperatures. Individual characteristics, including reproductive status, did not affect general activity indicating that reproductive and non-reproductive individuals contribute equally to cooperative behaviors. We suggest that the activity patterns may be a behavioral adaptation to avoid extreme burrow temperatures and a mechanism to maintain a stable core body temperature. We highlight the advantages of RFID technology to study wild small mammal movements.

Negative Energy Balance Enhances Ultradian Rhythmicity in Spring-Programmed Voles

Voles are small herbivorous rodents that can display both circadian activity rhythms (~24-h periodicity) and ultradian activity rhythms (~1- to 6-h periodicity). Ultradian rhythms are observed on an individual level, but also in synchronized populations. Ultradian rhythm period has been suggested to be influenced by energy balance, but the underlying mechanisms of ultradian rhythmicity are poorly understood. We manipulated energy balance by implementing the “work-for-food” paradigm, in which small rodents are exposed to increasing levels of food scarcity at different ambient temperatures in the laboratory. Photoperiodical spring-programmed voles on high workload changed their nocturnal circadian activity and body temperature rhythm to ultradian patterns, indicating that a negative energy balance induces ultradian rhythmicity. This interpretation was confirmed by the observation that ultradian patterns arose earlier at low temperatures. Interestingly, a positive relationship between ultradian period length and workload was observed in tundra voles. Spectral analysis revealed that the power of ultradian rhythmicity increased at high workload, whereas the circadian component of running wheel activity decreased. This study shows that the balance between circadian and ultradian rhythmicity is determined by energy balance, confirming flexible circadian and ultradian rhythms in females and males of 2 different vole species: the common vole (Microtus arvalis) and the tundra vole (Microtus oeconomus).

Gonads or body? Differences in gonadal and somatic photoperiodic growth response in two vole species

To optimally time reproduction, seasonal mammals use a photoperiodic neuroendocrine system (PNES) that measures photoperiod and subsequently drives reproduction. To adapt to late spring arrival at northern latitudes, a lower photoperiodic sensitivity and therefore a higher critical photoperiod for reproductive onset is necessary in northern species to arrest reproductive development until spring onset. Temperature-photoperiod relationships, and hence food availability-photoperiod relationships, are highly latitude dependent. Therefore, we predict PNES sensitivity characteristics to be latitude dependent. Here, we investigated photoperiodic responses at different times during development in northern (tundra or root vole, Microtus oeconomus) and southern vole species (common vole, Microtus arvalis) exposed to constant short (SP) or long photoperiod (LP). Although the tundra vole grows faster under LP, no photoperiodic effect on somatic growth is observed in the common vole. In contrast, gonadal growth is more sensitive to photoperiod in the common vole, suggesting that photoperiodic responses in somatic and gonadal growth can be plastic, and might be regulated through different mechanisms. In both species, thyroid-stimulating hormone β-subunit (Tshβ) and iodothyronine deiodinase 2 (Dio2) expression is highly increased under LP, whereas Tshr and Dio3 decrease under LP. High Tshr levels in voles raised under SP may lead to increased sensitivity to increasing photoperiods later in life. The higher photoperiodic-induced Tshr response in tundra voles suggests that the northern vole species might be more sensitive to thyroid-stimulating hormone when raised under SP. In conclusion, species differences in developmental programming of the PNES, which is dependent on photoperiod early in development, may form different breeding strategies as part of latitudinal adaptation.

Time management in a co-housed social rodent species (Arvicanthis niloticus)

Sociality has beneficial effects on fitness, and timing the activities of animals may be critical. Social cues could influence daily rhythmic activities via direct effects on the circadian clock or on processes that bypass it (masking), but these possibilities remain incompletely addressed. We investigated the effects of social cues on the circadian body temperature (Tb) rhythms in pairs of co-housed and isolated grass rats, Arvicanthis niloticus (a social species), in constant darkness (DD). Cohabitation did not induce synchronization of circadian Tb rhythms. However, socio-sexual history did affect circadian properties: accelerating the clock in sexually experienced males and females in DD and advancing rhythm phase in the females in a light-dark cycle. To address whether synchronization occurs at an ultradian scale, we analyzed Tb and activity rhythms in pairs of co-housed sisters or couples in DD. Regardless of pair type, co-housing doubled the percentage of time individuals were simultaneously active without increasing individual activity levels, suggesting that activity bouts were synchronized by redistribution over 24 h. Together, our laboratory findings show that social cues affect individual "time allocation" budgets via mechanisms at multiple levels of biological organization. We speculate that in natural settings these effects could be adaptive, especially for group-living animals.

Male prairie voles display cardiovascular dipping associated with an ultradian activity cycle

Mammals typically display alternating active and resting phases and, in most species, these rhythms follow a circadian pattern. The active and resting phases often are accompanied by corresponding physiological changes. In humans, blood pressure decreases during the resting phase of the activity cycle, and the magnitude of that "nocturnal dipping" has been used to stratify patients according to the risk for cardiovascular disease. However, in contrast to most mammals, prairie voles (Microtus ochrogaster) have periods of activity and rest that follow an ultradian rhythm with period lengths significantly <24h. While rhythmic changes in blood pressure across a circadian activity cycle have been well-documented, blood pressure patterns in species that display ultradian rhythms in activity are less well-studied. In the current study, we implanted pressure-sensitive radiotelemetry devices in male prairie voles and recorded activity, mean arterial pressure (MAP), and heart rate (HR) continuously for 3days. Visualization of the ultradian rhythms was enhanced using a 1h running average to filter the dataset. Positive correlations were found between activity and MAP and between activity and HR. During the inactive period of the ultradian cycle, blood pressure decreased by about 15%, which parallels the nocturnal dipping pattern seen in healthy humans. Further, the duration of inactivity did not affect any of the cardiovascular measures, so the differences in blood pressure values between the active and inactive periods are likely driven by ultradian oscillations in hormones and autonomic function. Finally, specific behavioral patterns also were examined. Both the instrumented animal and his non-instrumented cagemate appeared to show synchronized activity patterns, with both animals displaying sleep-like behavior for more than 90% of the inactive period. We propose that the prairie vole ultradian rhythm in blood pressure is an analogue for circadian blood pressure variability and can be used to study the long-term effects of commonly prescribed drugs on blood pressure dipping.

When the sun never sets: diverse activity rhythms under continuous daylight in free-living arctic-breeding birds

Circadian clocks are centrally involved in the regulation of daily behavioural and physiological processes. These clocks are synchronized to the 24 h day by external cues (Zeitgeber), the most important of which is the light–dark cycle. In polar environments, however, the strength of the Zeitgeber is greatly reduced around the summer and winter solstices (continuous daylight or continuous darkness). How animals time their behaviour under such conditions has rarely been studied in the wild. Using a radio-telemetry-based system, we investigated daily activity rhythms under continuous daylight in Barrow, Alaska, throughout the breeding season in four bird species that differ in mating system and parental behaviour. We found substantial diversity in daily activity rhythms depending on species, sex and breeding stage. Individuals exhibited either robust, entrained 24 h activity cycles, were continuously active (arrhythmic) or showed ‘free-running’ activity cycles. In semipalmated sandpipers, a shorebird with biparental incubation, we show that the free-running rhythm is synchronized between pair mates. The diversity of diel time-keeping under continuous daylight emphasizes the plasticity of the circadian system, and the importance of the social and life-history context. Our results support the idea that circadian behaviour can be adaptively modified to enable species-specific time-keeping under polar conditions.

Socially adjusted synchrony in the activity profiles of common marmosets in light-dark conditions

Synchronized state of activity and rest might be attained by mechanisms of entrainment and masking. Most zeitgebers not only act to entrain but also to mask circadian rhythms. Although the light-dark (LD) cycle is the main zeitgeber of circadian rhythms in marmosets, social cues can act as weaker zeitgebers. Evidence on the effects of social entrainment in marmosets has been collected in isolated animals or in pairs where activity is not individually recorded. To characterize the synchronization between the daily activity profiles of individuals in groups under LD conditions, the motor activity of animals from five groups was continuously monitored using actiwatches for 15 days during the 5th, 8th, and 11th months of life of juveniles. Families consisting of twins (4 ♂♀/1 ♂♂) and their parents were maintained under controlled lighting (LD 12:12 h), temperature, and humidity conditions. Synchronization was evaluated through the synchrony between the circadian activity profiles obtained from the Pearson correlation index between possible pairs of activity profiles in the light and dark phases. We also calculated the phase-angle differences between the activity onset of one animal in relation to the activity onset of each animal in the group (ψ(on)). A similar procedure was performed for activity offset (ψ(off)). By visual analysis, the correlation between the activity profiles of individuals within each family was stronger than that of individuals from different families. A mixed-model analysis showed that within the group, the correlation was stronger between twins than between twins and their parents in all families, except for the family in which both juveniles were males. Because a twin is an important social partner for juveniles, a sibling is likely to have a stronger influence on its twin's activity rhythm than other family members. Considering only the light phase, the second strongest correlation was observed between the activity profiles of the individuals in the reproductive pair. Regarding the parameters ψ(on) and ψ(off), the juvenile/juvenile dyad had lower values than the other dyads, but these differences did not reach statistical significance in relation to all dyads. Comparing the results of the ψ(on) and ψ(off), and correlation indices, we suggest that the latter could detect differences between the animals that were not observed in the results of the phase-angle differences. These differences could be related to changes that occur during the active phase but not only in a particular phase, such as the temporal changes during the activity phase that characterize unimodal or bimodal patterns. Based on the differences in the correlations between individuals subjected to the same LD routine, we suggest that social cues modulate the circadian activity profiles of marmosets as a result of interactions between the animals within each group. Future studies are necessary to characterize the mechanisms of synchronization that are involved in this social modulation.

In search of a temporal niche: social interactions

Circadian rhythms can be entrained to periodic cues in the environment including the solar day, food resources, and temperature. Work on a variety of organisms has suggested that social interactions within and between species may also influence circadian rhythmicity, but conceptual and technical difficulties relating to animal models, housing environments, rhythm assays, and experimental design have complicated mechanistic investigations in the laboratory. We review these issues and introduce the gregarious Nile grass rat, Arvicanthis niloticus, as a suitable model for research on this problem. Understanding social influences on temporal organization at this supra-organismal, community level is of considerable translational value, as its implications range from conservation biology to human health.

Differential entrainment of a social rhythm in adolescent mice

Daily routines in animal activities range from sleep-wake cycles, to foraging bouts, to social interactions. Among animals living within groups, it is unclear whether the motivations that underlie social interactions respond to daily light-dark (LD) cycles or endogenous circadian rhythms. Employing two mouse strains (BALB/cJ [BALB] and C57BL/6J [B6]) with genetically based differences in social affect and circadian rhythms, we examined how social investigation (SI) is modulated by social deprivation and circadian factors. We found a genetic influence on SI that was moderated by the preceding duration of social deprivation, requiring 3-6 h of social isolation prior to testing. Following 6h of social deprivation, the SI responses of adolescent B6 mice were greater than those of BALB mice only when the isolation period was imposed during the dark phase of the LD cycle. When B6 mice were weaned into conditions of constant darkness, a novel, endogenous social rhythm emerged, which was characterized by two pronounced peaks of social responsiveness (relative to one peak under LD entrainment) that were separated by 12-h intervals. Irrespective of the lighting conditions during social isolation, the SI responses of adolescent BALB mice did not oscillate across the day. Similar strain-dependent patterns of sociability were evident within groups of mice that were left undisturbed in their home cage under LD entrainment or constant darkness. Overall, genetic influences on the social phenotypes of adolescent mice are thus moderated by an interaction between social deprivation and oscillations of an endogenous social rhythm that entrains to the LD cycle.