Rhythmic 24 h variation of core body temperature and locomotor activity in a subterranean rodent (Ctenomys aff. knighti), the tuco-tuco

A clock for all seasons in the subterranean

In 1976, Pittendrigh and Daan established a theoretical framework which has coordinated research on circadian clock entrainment and photoperiodism until today. The "wild clocks" approach, which concerns studying wild species in their natural habitats, has served to test their models, add new insights, and open new directions of research. Here, we review an integrated laboratory, field and modeling work conducted with subterranean rodents (Ctenomys sp.) living under an extreme pattern of natural daily light exposure. Tracking animal movement and light exposure with biologgers across seasons and performing laboratory experiments on running-wheel cages, we uncovered the mechanisms of day/night entrainment of the clock and of photoperiodic time measurement in this subterranean organism. We confirmed most of the features of Pittendrigh and Daan's models but highlighted the importance of integrating them with ecophysiological techniques, methodologies, and theories to get a full picture of the clock in the wild. This integration is essential to fully establish the importance of the temporal dimension in ecological studies and tackling relevant questions such as the role of the clock for all seasons in a changing planet.

An increased number of individuals of a potential host facilitates non-photic synchronisation in the haematophagous insect Triatoma infestans

BACKGROUND

Triatoma infestans (Kissing bug) is the main vector of the parasite causative of Chagas disease in Latin-America. This species shows clear activity rhythms easily synchronised to day-night cycles (photic cycle). The haematophagous nature of these insects lead us to think that they may temporally adapt to the particular activity rhythms of potential hosts (non-photic cycle). Our previous data showed that kissing bugs were weakly affected by the activity-inactivity rhythm of a single host.

OBJETIVE

To determine if by increasing the number of individuals of a potential host, T. infestans could increase the likelihood of synchronisation.

METHODS

Individual activity rhythms of experimental insects, maintained in constant darkness in light-tight cabinets, localised in a room with 24 rodents, were continuously monitored. Another insect group that served as control was maintained in the same conditions but in a room without rodents.

FINDINGS

Most of the experimental insects synchronised, expressing a 24 h period coincident with the activity-inactivity rhythms of the rodents, while the controls free ran with a period significantly longer than 24 h.

CONCLUSION

Analogous to what happens with high vs low light intensity in photic synchronisers, a high number of rodents, in contrast to the previous one-rodent experiment, increased the potency of this non-photic zeitgeber.

Temporal Dissociation Between Activity and Body Temperature Rhythms of a Subterranean Rodent (Ctenomys famosus) in Field Enclosures

Several wild rodents, such as the subterranean tuco-tucos (Ctenomys famosus), switch their time of activity from diurnal to nocturnal when they are transferred from field to the laboratory. Nevertheless, in most studies, different methods to measure activity in each of these conditions were used, which raised the question of whether the detected change in activity timing could be an artifact. Because locomotor activity and body temperature (Tb) rhythms in rodents are tightly synchronized and because abdominal Tb loggers can provide continuous measurements across field and laboratory, we monitored Tb as a proxy of activity in tuco-tucos transferred from a semi-field enclosure to constant lab conditions. In the first stage of this study ("Tb-only group," 2012-2016), we verified high incidence (55%, n = 20) of arrhythmicity, with no consistent diurnal Tb rhythms in tuco-tucos maintained under semi-field conditions. Because these results were discrepant from subsequent findings using miniature accelerometers (portable activity loggers), which showed diurnal activity patterns in natural conditions (n = 10, "Activity-only group," 2016-2017), we also investigated, in the present study, whether the tight association between activity and Tb would be sustained outside the lab. To verify this, we measured activity and Tb simultaneously across laboratory and semi-field deploying both accelerometers and Tb loggers to each animal. These measurements (n = 11, "Tb + activity group," 2019-2022) confirmed diurnality of locomotor activity and revealed an unexpected loosening of the temporal association between Tb and activity rhythms in the field enclosures, which is otherwise robustly tight in the laboratory.

Disentangling the taxonomic status of Ctenomys (Rodentia: Ctenomyidae) populations inhabiting northern areas of La Rioja Province, Argentina

The Ctenomys population from Anillaco, La Rioja Province, Argentina has been the subject of detailed studies of circadian rhythms, vocalization, ecology and spatial dynamics. However, the taxonomy and systematics of this population as well as populations of Ctenomys inhabiting Andean areas of this province is unknown. In this study, we present the phylogenetic relationship of the “Anillaco tuco-tuco” as well as of two other populations of Ctenomys, all distributed in the northern portion of La Rioja Province, Argentina. Based on molecular analyses and morphological comparisons, we reached to the conclusion that the “Anillaco tuco-tuco” population corresponds to the species Ctenomys famosus. In addition, an undescribed species, member of the “mendocinus” lineage of Ctenomys, is recognized in the Andean portions of the province.

Daylength Shapes Entrainment Patterns to Artificial Photoperiods in a Subterranean Rodent

Photoperiodism plays an important role in the synchronization of seasonal phenomena in various organisms. In mammals, photoperiod encoding is mediated by differential entrainment of the circadian system. The limits of daily light entrainment and photoperiodic time measurement can be verified in organisms that inhabit extreme photic environments, such as the subterranean. In this experimental study, we evaluated entrainment of circadian wheel-running rhythms in South American subterranean rodents, the Anillaco tuco-tucos ( Ctenomys aff. knighti), exposed to different artificial photoperiods, from extremely long to extremely short photophases (LD 21:3, LD 18:6, LD 15:9, LD 9:15, LD 6:18 and LD 3:21). Artificial photoperiods synchronized their activity/rest rhythms and clear differences occurred in (a) phase angles of entrainment relative to the LD cycle and (b) duration of the daily activity phase α. These photoperiod-dependent patterns of entrainment were similar to those reported for epigeous species. Release into constant darkness conditions revealed aftereffects of entrainment to different photoperiods, observed in α but not in the free-running period τ. We also verified if animals coming from summer and winter natural photoperiods entrained equally to the artificial photoperiods by evaluating their phase angle of entrainment, α and τ aftereffects. To this end, experimental animals were divided into “Matching” and “Mismatching” groups, based on whether the experimental photoperiod (short-day [L < 12 h] or long-day [L > 12 h]) matched or not the natural photoperiod to which they had been previously exposed. No significant differences were found in the phase angle of entrainment, α and τ aftereffects in each artificial photoperiod. Our results indicate that the circadian clocks of tuco-tucos are capable of photoperiodic time measurement despite their natural subterranean habits and that the final entrainment patterns achieved by the circadian clock do not depend on the photoperiodic history.

Telling the Seasons Underground: The Circadian Clock and Ambient Temperature Shape Light Exposure and Photoperiodism in a Subterranean Rodent

Living organisms anticipate the seasons by tracking the proportion of light and darkness hours within a day—photoperiod. The limits of photoperiod measurement can be investigated in the subterranean rodents tuco-tucos (Ctenomys aff. knighti), which inhabit dark underground tunnels. Their exposure to light is sporadic and, remarkably, results from their own behavior of surface emergence. Thus, we investigated the endogenous and exogenous regulation of this behavior and its consequences to photoperiod measurement. In the field, animals carrying biologgers displayed seasonal patterns of daily surface emergence, exogenously modulated by temperature. In the laboratory, experiments with constant lighting conditions revealed the endogenous regulation of seasonal activity by the circadian clock, which has a multi-oscillatory structure. Finally, mathematical modeling corroborated that tuco-tuco’s light exposure across the seasons is sufficient for photoperiod encoding. Together, our results elucidate the interrelationship between the circadian clock and temperature in shaping seasonal light exposure patterns that convey photoperiod information in an extreme photic environment.

Dynamic spatial overlap in a solitary subterranean rodent: the Anillaco tuco-tuco (Ctenomys sp.)

Despite striking diversity in mammalian social behavior, studies of social organization have often dichotomized species by identifying them as either solitary or social (i.e., group living). This tendency has been particularly pronounced for subterranean rodents, the majority of which have long been assumed to be solitary. As a result, variation in social organization has likely been underestimated for these animals, particularly for species in which patterns of space use suggest limited or temporally dynamic opportunities for interactions among conspecifics. Here, we characterize patterns of space use in a population of tuco-tucos (Ctenomys sp.) from Anillaco, La Rioja Province, Argentina. Although these animals have been the subject of extensive research regarding circadian patterns of activity, spatial and social relationships among free-living individuals have not been documented. Analyses of radiotelemetry data from 17 individuals monitored during the breeding season (December 2015) revealed that partial overlap of individual home ranges was common, occurring between male–female as well as female–female pairs of animals. Spatial relationships, however, were dynamic, with both home range sizes and overlap changing on a daily basis. Although members of the study population did not meet the criteria typically used to identify group living in subterranean species, they were not completely solitary. Instead, the animals displayed an intermediate form of social organization characterized by persistent partial overlap of the areas occupied by different adults. These data add to the growing comparative picture of social variation in Ctenomys and suggest that further studies of these animals should contribute to improved understanding of the factors underlying differences in mammalian social systems.

Light-dark cycle inversion effect on food intake and body weight in rats

Introduction

Background: most organisms inhabiting this planet have rhythmic functions in cycles that approximate 24 hours as a result of evolutionary adaptation. Disruption of these rhythms causes disruption in many bodily functions, including energy expenditure and consumption, and lipid and glucose metabolism, in addition to altering several biochemical parameters. Objective: the aim of this study was to determine the effect of altering the light-dark cycle on diurnal and nocturnal food consumption and body weight in rats. Material and methods: three experiments were carried out with an experimental group and a control group in each one. The groups included six males with an age of four months at the beginning of the experiment. Each experiment was 30 days long, starting with a baseline of 10 days and then inverting the light-dark cycle for another 20 days. In the first experiment the inversion took place at the end of the baseline period; in the second, the inversion was performed on days 10 and 20; in the third experiment inversions occurred every five days following the initial 10 days of baseline. Results: our results show a lower body weight gain in the experimental groups when compared to the control groups. Conclusions: significant differences in total consumption of food were not found, but were seen in the patterns of day and night consumption, along with a tendency to develop alterations characteristic of metabolic syndrome, which increased with the frequency of light-dark cycle inversion.

Temporal flexibility in activity rhythms of a diurnal rodent, the ice rat (Otomys sloggetti)

Diurnality in rodents is relatively rare and occurs primarily in areas with low nighttime temperatures such as at high altitudes and desert areas. However, many factors can influence temporal activity rhythms of animals, both in the field and the laboratory. The temporal activity patterns of the diurnal ice rat were investigated in the laboratory with, and without, access to running wheels, and in constant conditions with running wheels. Ice rats appeared to be fundamentally diurnal but used their running wheels during the night. In constant conditions, general activity remained predominantly diurnal while wheel running was either nocturnal or diurnal. In some animals, entrainment of the wheel running rhythm was evident, as demonstrated by free-running periods that were different from 24 h. In other animals, the wheel running activity abruptly switched from nocturnal to subjective day as soon as the animals entered DD, and reverted back to nocturnal once returned to LD, suggesting the rhythms were masked by light. Wheel running rhythms appears to be less robust and more affected by light compared to general activity rhythms. In view of present and future environmental changes, the existence of more unstable activity rhythms that can readily switch between temporal niches might be crucial for the survival of the species.

Circadian rhythmicity of body temperature and metabolism

This article reviews the literature on the circadian rhythms of body temperature and whole-organism metabolism. The two rhythms are first described separately, each description preceded by a review of research methods. Both rhythms are generated endogenously but can be affected by exogenous factors. The relationship between the two rhythms is discussed next. In endothermic animals, modulation of metabolic activity can affect body temperature, but the rhythm of body temperature is not a mere side effect of the rhythm of metabolic thermogenesis associated with general activity. The circadian system modulates metabolic heat production to generate the body temperature rhythm, which challenges homeothermy but does not abolish it. Individual cells do not regulate their own temperature, but the relationship between circadian rhythms and metabolism at the cellular level is also discussed. Metabolism is both an output of and an input to the circadian clock, meaning that circadian rhythmicity and metabolism are intertwined in the cell.

Day and night in the subterranean: measuring daily activity patterns of subterranean rodents (Ctenomys aff. knighti) using bio-logging

While most studies of the impacts of climate change have investigated shifts in the spatial distribution of organisms, temporal shifts in the time of activity is another important adjustment made by animals in a changing world. Due to the importance of light and temperature cycles in shaping activity patterns, studies of activity patterns of organisms that inhabit extreme environments with respect to the 24-hour cyclicity of Earth have the potential to provide important insights into the interrelationships among abiotic variables, behaviour and physiology. Our previous laboratory studies with Argentinean tuco-tucos from the Monte desert (Ctenomys aff. knighti) show that these subterranean rodents display circadian activity/rest rhythms that can be synchronized by artificial light/dark cycles. Direct observations indicate that tuco-tucos emerge mainly for foraging and for removal of soil from their burrows. Here we used bio-logging devices for individual, long-term recording of daily activity/rest (accelerometry) and time on surface (light-loggers) of six tuco-tucos maintained in outdoor semi-natural enclosures. Environmental variables were measured simultaneously. Activity bouts were detected both during day and night but 77% of the highest values happened during the daytime and 47% of them coincided with time on surface. Statistical analyses indicate time of day and temperature as the main environmental factors modulating time on surface. In this context, the total duration that these subterranean animals spent on surface was high during the winter, averaging 3 h per day and time on surface occurred when underground temperature was lowest. Finally, transport of these animals to the indoor laboratory and subsequent assessment of their activity rhythms under constant darkness revealed a switch in the timing of activity. Plasticity of activity timing is not uncommon among desert rodents and may be adaptive in changing environments, such as the desert where this species lives.

Subterranean Desert Rodents (Genus Ctenomys) Create Soil Patches Enriched in Root Endophytic Fungal Propagules

Subterranean rodents are considered major soil engineers, as they can locally modify soil properties by their burrowing activities. In this study, the effect of a subterranean rodent of the genus Ctenomys on soil properties and root endophytic fungal propagules in a shrub desert of northwest Argentina was examined. Our main goal was to include among root endophytic fungi not only arbuscular mycorrhiza but also the dark septate endophytes. We compared the abundance of fungal propagules as well as several microbiological and physicochemical parameters between soils from burrows and those from the surrounding landscape. Our results show that food haulage, the deposition of excretions, and soil mixing by rodents' burrowing promote soil patchiness by (1) the enrichment in both types of root endophytic fungal propagules; (2) the increase in organic matter and nutrients; and (3) changes in soil edaphic properties including moisture, field capacity, and texture. These patches may play a critical role as a source of soil heterogeneity in desert ecosystems, where burrows constructed in interpatches of bare soil can act, once abandoned, as "islands of fertility," promoting the establishment of plants in an otherwise hostile environment.

Diurnally active rodents for laboratory research

Although inbred domesticated strains of rats and mice serve as traditional mammalian animal models in biomedical research, the nocturnal habits of these rodents make them inappropriate for research that requires a model with human-like diurnal activity rhythms. We conducted a literature review and recorded locomotor activity data from four rodent species that are generally considered to be diurnally active, the Mongolian gerbil ( Meriones unguiculatus), the degu ( Octodon degus), the African (Nile) grass rat ( Arvicanthis niloticus), and the antelope ground squirrel ( Ammospermophilus leucurus). Our data collected under 12-hour light/dark cycles confirmed and expanded the existing literature in showing that the activity rhythms of antelope ground squirrels and African grass rats are stronger and more concentrated in the light phase of the light/dark cycle than the activity rhythms of Mongolian gerbils and degus, making the former two species preferable and more reliable as models of consistent diurnal activity in the laboratory. Among the two more strongly diurnal species, antelope ground squirrels are more exclusively diurnal and have more robust activity rhythms than African grass rats. Although animals of these two species are not currently available from commercial suppliers, African grass rats are indigenous to a wide area across the north of Africa and thus available to researchers in the eastern hemisphere, whereas antelope ground squirrels can be found throughout much of western North America's desert country and, therefore, are more easily accessible to North American researchers.

Ontogeny of long-range vocalizations in a Neotropical fossorial rodent: the Anillaco Tuco-Tuco (Ctenomys sp.)

Tuco-tucos (Ctenomys spp.) are subterranean rodents that produce territorial, high intensity long-range vocalizations (LRVs) of broadband and low frequency that are essential for long-distance communication between individuals in different tunnel systems. Despite their importance, the development of LRVs remains poorly understood. In adult Anillaco Tuco-Tucos (Ctenomys sp.) the LRV is composed by two types of syllables (series and individual notes) that are repeated a variable number of times. We studied the development of the LRVs in eight juveniles of the Anillaco Tuco-Tuco ranging from 14-28 to 104-118 days after birth. We (1) tested whether the syllables followed any of three alternative developmental modes (retention of juvenile vocalizations, modification of juvenile precursors or de novo appearance in adults), (2) evaluated the development of structural and acoustic features of syllables, and (3) tested the prediction that juveniles should produce a greater proportion of atypical series in precursors of the LRV than adults, due to lack maturation and/or precise coupling of neuromuscular and anatomical structures. The LRV of the Anillaco Tuco-Tuco exhibited a mixed developmental mode: while series developed from juvenile precursors whose acoustic features gradually approached those of adults, individual notes appeared later in the ontogeny and de novo with acoustic features indistinguishable from those of adults. The number of series per vocalization increased through development and varied from one to 25 in juvenile males and from one to six in juvenile females. The structure of the most common series type (triad) did not exhibit ontogenetic changes and was present as such at the onset of the emission of vocalizations. On the contrary, acoustic features of juvenile triad notes changed with age in both sexes (duration 90% increased through development, while bandwidth 90% and peak frequency decreased). Furthermore, juveniles emitted a higher proportion of atypical series than adults (7.4% vs. 0.3%), as expected in the development of any complex behavior that requires practice to be mastered. The maturation of the LRV occurred well before the sexual maturation, presumably due to the protracted time needed to acquire or build a burrow system long before mating is possible. We propose that protracted vocal development is another component in the slow developmental strategy of Ctenomys and subterranean rodents in general.

The Interplay of Energy Balance and Daily Timing of Activity in a Subterranean Rodent: A Laboratory and Field Approach

The tuco-tuco (Ctenomys aff. knighti) is among the rodent species known to be nocturnal under standard laboratory conditions and diurnal under natural conditions. The circadian thermoenergetics (CTE) hypothesis postulates that switches in activity timing are a response to energetic challenges; daytime activity reduces thermoregulatory costs by consolidating activity to the warmest part of the day. Studying wild animals under both captive and natural conditions can increase understanding of how temporal activity patterns are shaped by the environment and could serve as a test of the CTE hypothesis. We estimated the effects of activity timing on energy expenditure for the tuco-tuco by combining laboratory measurements of metabolic rate with environmental temperature records in both winter and summer. We showed that, in winter, there would be considerable energy savings if activity is allocated at least partially during daylight, lending support to the CTE hypothesis. In summer, the impact of activity timing on energy expenditure is small, suggesting that during this season other factors, such as predation risk, water balance, and social interaction, may have more important roles than energetics in the determination of activity time.

The Animal Kingdom Is Also a Bioengineering Field: Exploring the Art and Science of Vetinary Medicine [Retrospectroscope]

Medical science developed in tandem with the evolution of biological species and their associated diseases. Because of the close interaction between humans and other animals, even those in the wild, taking care of the former also means caring for the latter. Several scientific forerunners delved into animals' anatomical and physiological secrets in their quest to better understand animal biology and functions, thereby laying the foundation for animal medicine. Here, I briefly explore the long and complex road that led to the current state of veterinary science and provide a few examples of its present standing. (Contributions from the ancient world and eastern countries are not considered, as they represent a different area of interest.).

Locomotor Activity and Body Temperature Patterns over a Temperature Gradient in the Highveld Mole-Rat (Cryptomys hottentotus pretoriae)

African mole-rats are strictly subterranean mammals that live in extensive burrow systems. High humidity levels in the burrows prevent mole-rats from thermoregulating using evaporative cooling. However, the relatively stable environment of the burrows promotes moderate temperatures and small daily temperature fluctuations. Mole-rats therefore display a relatively wide range of thermoregulation abilities. Some species cannot maintain their body temperatures at a constant level, whereas others employ behavioural thermoregulation. Here we test the effect of ambient temperature on locomotor activity and body temperature, and the relationship between the two parameters, in the highveld mole-rat. We exposed mole-rats to a 12L:12D and a DD light cycle at ambient temperatures of 30°C, 25°C and 20°C while locomotor activity and body temperature were measured simultaneously. In addition, we investigated the endogenous rhythms of locomotor activity and body temperature at different ambient temperatures. Mole-rats displayed nocturnal activity at all three ambient temperatures and were most active at 20°C, but least active at 30°C. Body temperature was highest at 30°C and lowest at 20°C, and the daily cycle was highly correlated with locomotor activity. We show that the mole-rats have endogenous rhythms for both locomotor activity and body temperature. However, the endogenous body temperature rhythm appears to be less robust compared to the locomotor activity rhythm. Female mole-rats appear to be more sensitive to temperature changes than males, increased heterothermy is evident at lower ambient temperatures, whilst males show smaller variation in their body temperatures with changing ambient temperatures. Mole-rats may rely more heavily on behavioural thermoregulation as it is more energy efficient in an already challenging environment.

Form and function of long-range vocalizations in a Neotropical fossorial rodent: the Anillaco Tuco-Tuco (Ctenomys sp.)

The underground environment poses particular communication challenges for subterranean rodents. Some loud and low-pitched acoustic signals that can travel long distances are appropriate for long-range underground communication and have been suggested to be territorial signals. Long-range vocalizations (LRVs) are important in long-distance communication in Ctenomys tuco-tucos. We characterized the LRV of the Anillaco Tuco-Tuco (Ctenomys sp.) using recordings from free-living individuals and described the behavioral context in which this vocalization was produced during laboratory staged encounters between individuals of both sexes. Long-range calls of Anillaco tuco-tucos are low-frequency, broad-band, loud, and long sounds composed by the repetition of two syllable types: series (formed by notes and soft-notes) and individual notes. All vocalizations were initiated with series, but not all had individual notes. Males were heavier than females and gave significantly lower-pitched vocalizations, but acoustic features were independent of body mass in males. The pronounced variation among individuals in the arrangement and number of syllables and the existence of three types of series (dyads, triads, and tetrads), created a diverse collection of syntactic patterns in vocalizations that would provide the opportunity to encode multiple types of information. The existence of complex syntactic patterns and the description of soft-notes represent new aspects of the vocal communication of Ctenomys. Long-distance vocalizations by Anillaco Tuco-Tucos appear to be territorial signals used mostly in male-male interactions. First, emission of LRVs resulted in de-escalation or space-keeping in male-male and male-female encounters in laboratory experiments. Second, these vocalizations were produced most frequently (in the field and in the lab) by males in our study population. Third, males produced LRVs with greater frequency during male-male encounters compared to male-female encounters. Finally, males appear to have larger home ranges that were more spatially segregated than those of females, suggesting that males may have greater need for long-distance signals that advertise their presence. Due to their apparent rarity, the function and acoustic features of LRV in female tuco-tucos remain inadequately known.

Social modulation of the daily activity rhythm in a solitary subterranean rodent, the tuco-tuco (Ctenomys sp)

South American subterranean rodents are mainly described as solitary and mutual synchronization was never observed among individuals maintained together in laboratory. We report that a single birth event was capable of disrupting the robust nocturnal activity rhythm of singly housed tuco-tucos from north-west Argentina. "Around-the-clock activity" was displayed by 8 out of 13 animals whose cages were closer to the newborn pups. However, experimental exposure to a pup vocalization did not produce a similar effect on the rhythms of adult animals. Our results indicate an effect of social interaction in the expression of biological rhythms even in solitary animals.

Vitrified canine testicular cells allow the formation of spermatogonial stem cells and seminiferous tubules following their xenotransplantation into nude mice

Belgian Malinois (BM), one of the excellent military dog breeds in South Korea, is usually castrated before sexual maturation. Therefore, the transfer of their genetic features to the next generation is difficult. To overcome this, testicular cells from 4-month-old BMs were frozen. Testicular cells were thawed after 3 months and cultured in StemPro-34 medium. Spermatogonial stem cell (SSC) characteristics were determined by the transplantation of the cultured germ cell-derived colonies (GDCs) into empty testes, containing only several endogenous SSCs and Sertoli cells, of immunodeficient mice, 4 weeks after busulfan treatment. Following the implantation, the transplanted cells localized in the basement membrane of the seminiferous tubules, and ultimately colonized the recipient testes. Xenotransplantation of GDCs together with testicular somatic cells conjugated with extracellular matrix (ECM), led to the formation of de novo seminiferous tubules. These seminiferous tubules were mostly composed of Sertoli cells. Some germ cells were localized in the basement membrane of seminiferous tubules. This study revealed that BM-derived SSCs, obtained from the castrated testes, might be a valuable tool for the transfer of BM genetic features to the next generation.

Nocturnal to Diurnal Switches with Spontaneous Suppression of Wheel-Running Behavior in a Subterranean Rodent

Several rodent species that are diurnal in the field become nocturnal in the lab. It has been suggested that the use of running-wheels in the lab might contribute to this timing switch. This proposition is based on studies that indicate feed-back of vigorous wheel-running on the period and phase of circadian clocks that time daily activity rhythms. Tuco-tucos (Ctenomys aff. knighti) are subterranean rodents that are diurnal in the field but are robustly nocturnal in laboratory, with or without access to running wheels. We assessed their energy metabolism by continuously and simultaneously monitoring rates of oxygen consumption, body temperature, general motor and wheel running activity for several days in the presence and absence of wheels. Surprisingly, some individuals spontaneously suppressed running-wheel activity and switched to diurnality in the respirometry chamber, whereas the remaining animals continued to be nocturnal even after wheel removal. This is the first report of timing switches that occur with spontaneous wheel-running suppression and which are not replicated by removal of the wheel.