Seasonality of breeding in wild tuco-tucosCtenomys talarum in relation to climate and food availability

Growing underground: Development of thermogenesis in pups of the fossorial rodent Ctenomys talarum

In mammals, during the pup’s development and adult life, integrated requirements of all activities of the individual must conform to a sustained rate of metabolism. Thus, partitioning the available energy according to short-term priorities at a specific moment allows animals to survive and optimize long-term reproductive success. In altricial rodents, thermal balance is a key factor for survival. When no exogenous source of heat is present, altricial pups rapidly lose heat, reaching ambient temperature (Ta). Fossorial rodents showed a strong dependence on burrows, where Ta remains relatively stable within narrow ranges. Pups of the fossorial rodent Ctenomys talarum are altricial, making them an excellent model to evaluate the development of thermogenic capacity. In this study, the ontogeny of the thermogenic capacity of pups of C. talarum was evaluated. Using respirometry techniques, non-shivering thermogenesis (NST), total thermogenic capacity (cold-induced maximum metabolic rate, MMR), and resting metabolic rate (RMR) in pups until post-weaning age (day 60) were analyzed. No NST was present in pups until day 60 despite the presence of molecular markers for NST in brown adipose tissue deposits, which became functional in adults. Although pups are altricial at birth, they maintain their thermal balance behaviorally during lactation. Total thermogenic capacity became fixed at an early age, indicating an improvement in shivering thermogenesis (ST) efficiency after day 10, which might be related to the development of musculature related to digging. Before the aboveground dispersal period (~day 60), pups gradually reached adult Tb by improving ST and thermal isolation, allowing them to confront climatic fluctuations on the surface.

Clocks Ticking in the Dark: A Review of Biological Rhythms in Subterranean African Mole-Rats

Biological rhythms are rhythmic fluctuations of biological functions that occur in almost all organisms and on several time scales. These rhythms are generated endogenously and entail the coordination of physiological and behavioural processes to predictable, external environmental rhythms. The light-dark cycle is usually the most prominent environmental cue to which animals synchronise their rhythms. Biological rhythms are believed to provide an adaptive advantage to organisms. In the present review, we will examine the occurrence of circadian and seasonal rhythms in African mole-rats (family Bathyergidae). African mole-rats are strictly subterranean, they very rarely emerge aboveground and therefore, do not have regular access to environmental light. A key adaptation to their specialised habitat is a reduction in the visual system. Mole-rats exhibit both daily and seasonal rhythmicity in a range of behaviours and physiological variables, albeit to different degrees and with large variability. We review previous research on the entire circadian system of African mole-rats and discuss output rhythms in detail. Laboratory experiments imply that light remains the strongest zeitgeber for entrainment but in the absence of light, animals can entrain to ambient temperature rhythms. Field studies report that rhythmic daily and seasonal behaviour is displayed in their natural habitat. We suggest that ambient temperature and rainfall play an important role in the timing of rhythmic behaviour in mole-rats, and that they likely respond directly to these zeitgebers in the field rather than exhibit robust endogenous rhythms. In the light of climate change, these subterranean animals are buffered from the direct and immediate effects of changes in temperature and rainfall, partly because they do not have robust circadian rhythms, however, on a longer term they are vulnerable to changes in their food sources and dispersal abilities.

Diet effect on osmoregulation in the subterranean rodent Ctenomys talarum

Water conservation requires osmoregulatory skills, sometimes limited by the environment and/or physiological and behavioral characteristics acquired along the evolutionary history of the species. Fossoriality had probably emerged as a survival mechanism to face increasing aridity, as suggested for Ctenomys, a genus that radiated to different environments. Ctenomys talarum (tuco-tuco) is an herbivorous subterranean rodent that lives in coastal grasslands inside humid burrows that reduce evaporation. However, their osmoregulatory mechanisms may be challenged by atmospheric variations when foraging aboveground and by the annual variability in dietary water and salt content. Then, it is of great interest to identify how much of this flexibility of C .talarum is attributed to physiological regulation. We analyzed the effect of water and salt content of diet on urinary, plasmatic, fecal and respiratory parameters. Tuco-tucos were not able to maintain their body weight under the offered monodiet, especially under the low hydrated diet, which explains its generalist and opportunistic foraging behavior. C. talarum mainly obtained water through food, whereas water metabolic production was negligible. Evaporative water loss did not vary between diets, but individuals under water restriction showed decreased fecal water loss and urine volume, high urine concentration but stable plasmatic osmolality and ionic concentration values. Under salt stress, urinary parameters remained relatively stable and high plasmatic osmolality was detected. Despite C. talarum produced more diluted urine than rodents from xeric environments, it is able to concentrate it 4 times above than the required at field even under the lowest water availability. This may be a characteristic associated with the evolutionary history of the species, which evolved in an arid context.

Different regulation of cortisol and corticosterone in the subterranean rodent Ctenomys talarum: Responses to dexamethasone, angiotensin II, potassium, and diet

When harmful environmental stimuli occur, glucocorticoids (GCs), cortisol and corticosterone are currently used to evaluate stress status in vertebrates, since their secretions are primarily associated to an increased activity of the hypothalamic-pituitaryadrenal (HPA) axis. To advance in our comprehension about GCs regulation, we evaluated the subterranean rodent Ctenomys talarum to assess cortisol and corticosterone response to (1) the negative feedback of the HPA axis using the dexamethasone (DEX) suppression test, (2) angiotensin II (Ang II), (3) potassium (K⁺) intake, and (4) different diets (vegetables, grasses, acute fasting). Concomitantly, several indicators of individual condition (body mass, neutrophil to lymphocyte ratio, blood glucose, triglycerides and hematocrit) were measured for diet treatments. Results confirm the effect of DEX on cortisol and corticosterone in recently captured animals in the field but not on corticosterone in captive animals. Data suggest that Ang II is capable of stimulating corticosterone, but not cortisol, secretion. Neither cortisol nor corticosterone were responsive to K⁺ intake. Cortisol levels increased in animals fed with grasses in comparison to those fed with vegetables while corticosterone levels were unaffected by diet type. Moreover, only cortisol responded to fasting. Overall, these results confirm that cortisol and corticosterone are not interchangeable hormones in C. talarum.

Food Restriction Affects Inflammatory Response and Nutritional State in Tuco-tucos (Ctenomys talarum)

Insufficient or unbalanced food intake typically has a negative impact on immune responses. The understanding of this effect is, however, hampered by the effect that food has on general condition, which, in turn, affects immunity, and the interaction among general condition, immunocompetence, and concurrent infections. The goal of this study was to determine the effects of food restriction and methionine supplementation on immunity in tuco-tucos (Ctenomys talarum). Effects of diet manipulations on nutritional state, inflammatory response to phytohemagglutinin (PHA), and other immune parameters (bacterial killing capacity, natural antibodies, and leukocyte profile) were evaluated. Health and stress parameters and endoparasite loads were assessed to understand more deeply potential effects of treatments on immune status. Individuals under food restriction presented an altered nutritional state as well as increased stress levels (higher N: L ratios) compared with individuals fed ad libitum, and a marked reduction in the inflammatory response to PHA. Supplementation with methionine did not affect any of the parameters analyzed. Endoparasite loads were not affected by treatments. Our results support the idea that food insufficiency can modulate the individual's immune responsiveness through the lack of adequate essential nutrients, metabolic fuel and energetic reserves, or by a detrimental effect of the stress caused by nutrient limitation. We show that the response to PHA previously reported as nonenergetically costly for C. talarum, implies a nutritional cost; an opposite pattern to that previously found for the adaptive antibody response to sheep red blood cells in the same species.

When allowed, females prefer novel males in the polygynous subterranean rodent Ctenomys talarum (tuco-tuco)

The ability to recognize familiar conspecifics plays an important role at the time of choosing a mating partner in rodents. A laboratory study using preference test was used in order to test the hypothesis that, in the polygynous subterranean rodent Ctenomys talarum, females prefer novel males when offered two odors, or individuals (one familiar and one novel) limited in their movements so that male aggression is prevented. Our findings show that females prefer novel tuco-tucos at three levels of male assessment: odor samples (consisting of shavings soiled with urine, feces, and presumably, other body secretions collected from the male home cage), confined males behind a wire mesh, and full contact with tethered males. Previous studies of this species demonstrated that in the wild, male-male competition and male coercion severely limit the possibility of females mating non-neighbors, i.e. novel males. Females mating neighbors to whom they are familiarized, obtain high quality mating since they are territorial, highly competitive males. Nonetheless, when females have the opportunity, as shown in the two-choice experiments, they choose novel males, probably benefiting their progeny from novel genetic combinations. Hence, combining evidence from laboratory and field studies in C. talarum, it is possible to better understand female preferences in modeling individual reproductive strategies.

Digestive flexibility in females of the subterranean rodent ctenomys talarum in their natural habitat

We studied the occurrence of digestive strategies at different levels in females of the subterranean herbivorous rodent Ctenomys talarum living in their natural habitat. We determined the dimensions of different parts of the gastrointestinal tract and organs along as the activity of key digestive enzymes(sucrase, maltase and N-aminopeptidase) in small intestine in females seasonally caught. Females of C. talarum did not show seasonal variations in the mass of the different parts of the gastrointestinal tract. In nonreproductive females large intestine was longer in autumn, whereas reproductive females did not show seasonal variations in the length of the different parts of the gut. Females of C. talarum exhibited a high sucrose, maltase and N-aminopeptidase activity in small intestine, although these activities were higher in small intestine of females caught in autumn (nonreproductive) than in females caught in winter (reproductive). The results show that C. talarum females exhibit characteristics in the gut at the morphological and biochemical level, which could represent digestive strategies to face the constraints imposed by their costly particular way of life.

Cortisol and corticosterone exhibit different seasonal variation and responses to acute stress and captivity in tuco-tucos (Ctenomys talarum)

In this work we aimed to evaluate variations in plasma glucocorticoids (GCs, cortisol and corticosterone) levels throughout an annual cycle in free-living male tuco-tucos (Ctenomys talarum) and compare their responses to acute and chronic stressors (trapping, manipulation, immobilization, confinement in a novel environment, transference to captivity). In addition, we used leukocyte profiles to allow discrimination between basal and stress-induced seasonal changes in GC concentrations. Our results showed that cortisol and corticosterone are differently affected by environmental stimuli in C. talarum. Both hormones showed different patterns of variation in the field and responses to captivity. Moreover, only cortisol was responsive to acute stressors. Leukocyte profiles indicated that animals were unstressed in the field and therefore, that we were able to measure basal, stress-independent, fluctuations in GC levels. GC concentrations were low in comparison to values frequently reported for other mammals. Our results suggest differentiated physiological roles for cortisol and corticosterone in our study species and further emphasize the complexity of GC physiology in wild mammals.

Mounting a specific immune response increases energy expenditure of the subterranean rodent Ctenomys talarum (tuco-tuco): implications for intraspecific and interspecific variation in immunological traits

It was recently hypothesised that specific induced defences, which require substantial time and resources and are mostly beneficial against repeated infections, are more likely to be favoured in ‘slow-living-pace’ species. Therefore, understanding how different types of immune defences might vary with life history requires knowledge of the costs and benefits of defence components. Studies that have explored the energetic costs of immunity in vertebrates have done so with a focus primarily on birds and less so on mammals, particularly surface-dwelling rodents. In this study, we evaluated whether an experimental induction of the immune system with a non-pathogenic antigen elevates the energetic expenditure of a subterranean rodent: Ctenomys talarum (tuco-tucos). In both seasons studied, a significant increase in oxygen consumption was verified in immune-challenged tuco-tucos injected with sheep red blood cells (SRBC) compared with control animals. The increase in oxygen consumption 10 days after the exposure to SRBC was lower for female tuco-tucos monitored in the breeding season compared with females in the non-breeding season. Interestingly, antibody titres of female tuco-tucos did not decrease during the breeding season. Our results add new insight into the role of other factors such as basal metabolic rate or degree of parasite exposure besides ‘pace of life’ in modulating the interspecific immunological variation observed in natural populations of mammals.