Physiological and behavioural adjustment of a wild rodent to laboratory conditions.

Wild animals are brought to captivity for different reasons, for example to be kept in zoos and rehabilitation centres, but also for basic research. Such animals usually undergo a process of adjustment to captive conditions. While this adjustment occurs on the behavioural and the physiological level, those are usually studied separately. The aim of this study was to assess both the physiological and behavioural responses of wild wood mice, Apodemus sylvaticus, while adjusting to laboratory conditions. Over the course of four weeks, we measured in wild-caught mice brought to the laboratory faecal corticosterone metabolites and body mass as physiological parameters, stereotypic behaviour and nest-quality, as welfare-linked behavioural parameters, and four personality measures as additional behavioural parameters. The results of our study indicate that mice exhibited an adjustment in both behaviour and physiology over time in the laboratory. While the hormonal stress response decreased significantly, body mass and the proportion of stereotypic behaviours showed a tendency to increase over time. The slight increase of stereotypic behaviours, although not statistically significant, suggests the development of repetitive and non-functional behaviours as a response to laboratory conditions. However, we suggest that those behaviours might have been used by animals as a coping strategy to decrease the physiological stress response. Other behavioural parameters measured, such as boldness and nestbuilding behaviour were stable over time. The information obtained in the present study hints at a complex interplay between behavioural and physiological adjustments of wild animals to laboratory conditions, which should be considered when intending to use wild animals in experimental research.

Assessing Optimal Cell Counts in Sperm Shape Abnormality Assays in Rodents.

Rodents have been the preferred models for the evaluation of the toxicity of pollutants and drugs and their genotoxic effects, including sperm shape abnormalities. The scientific literature is dominated by studies conducted with model animals in laboratory conditions, but a generally accepted and standardized protocol addressing the optimal number of sperm cells to count is still lacking. In this study, we reviewed the literature regarding the number of counted sperm cells in such assessments, published from 1969 to 2023. To infer the number of counts providing the best cost/benefit regarding the robustness of the assay results, a new dataset involving the analysis of two populations of wild rodents was produced. We evaluated the frequency of sperm shape abnormalities in a total of 50 wild brown rats (Rattus norvegicus) captured in two port cities, aiming to detect the impact of differential sperm cell counts in the obtained results. During necropsy, the fresh epididymis tail of adult male rats was excised, and sperm cells were fixated in slides. For each animal, a total of 300, 500, 1000, and 2000 cells were sequentially counted, and head abnormalities were registered. Counting 300 sperm cells failed to detect significant differences between groups and 500 counts resulted in marginally significant differences. Only when 1000 or 2000 sperm cells were counted, significant differences emerged between groups. We propose that studies addressing sperm shape abnormalities should standardize counts to an optimal value of 1000 cells per animal, warranting robust statistical results while providing the best compromise concerning labor time.

Energetics and thermal adaptation in semifossorial pine-voles Microtus lusitanicus and Microtus duodecimcostatus.

Rodents colonising subterranean environments have developed several morphological, physiological and behaviour traits that promote the success of individuals in such demanding conditions. Resting metabolic rate, thermoregulation capacity and daily energy expenditure were analysed in two semi-fossorial pine-vole species Microtus lusitanicus and Microtus duodecimcostatus inhabiting distinct areas of the Iberian Peninsula. Individuals capture location varied in habitat and soil features, allowing the comparison of energetic parameters with ecological characteristics, that can help explain the use of the subterranean environment and dependence of the burrow system. Results showed that M. duodecimcostatus has lower mass independent resting metabolic rate when compared with M. lusitanicus, which may be a response to environmental features of their habitat, such as dryer soils and lower water availability. Thermal conductance increased with body mass and was dependent on the ambient temperature. No significant differences were observed in the daily energy expenditure, but water economy data demonstrated the influence of the water available in the habitat on the energetics of voles. These species may rely on behavioural adaptations and seasonal use of burrows to cope with thermal challenges of subterranean activity and soil constraints. We found strong evidence that M. lusitanicus is able to use torpor as a response to low ambient temperatures which is a new observation among Arvicolines.

Phenotypic flexibility in the energetic strategy of the greater white-toothed shrew, Crocidura russula.

The balance between energetic acquisition and expenditure depends on the amount of energy allocated to biological functions such as thermoregulation, growth, reproduction and behavior. Ambient temperature has a profound effect on this balance, with species inhabiting colder climates often needing to invest more energy in thermoregulation to maintain body temperature. This leads to local behavioral and physiological adaptations that increase energetic efficiency. In this study, we investigated the role of activity, behavior and thermogenic capacity in the ability of the greater white-toothed shrew, Crocidura russula, to cope with seasonal changes. Individuals were captured in the Sintra-Cascais Natural Park, a Mediterranean region, and separated into three experimental groups: a control group, acclimated to a 12L:12D photoperiod and temperature of 18-20°C; a winter group, acclimatized to natural winter fluctuations of light and temperature; and a summer group, acclimatized to natural summer fluctuations of light and temperature. No differences were found in resting metabolic rate and nonshivering thermogenesis between the three groups. However, winter shrews significantly reduced their activity, particularly at night, compared to the control and summer groups. Differences in torpor use were also found between groups, with winter shrews entering torpor more frequently and during shorter periods of time than summer and control shrews. Our results indicate C. russula from Sintra relies on the flexibility of energy saving mechanisms, namely daily activity level and torpor use, to cope with seasonal changes in a Mediterranean climate, rather than mechanisms involving body heat production.

Predation risk modulates diet-induced obesity in male C57BL/6 mice.

OBJECTIVE: In this study, the behavioral and physiological changes induced by experimentally varying the risk of predation in male mice fed a high-fat diet were examined. In particular, the study aimed to assess whether the risk of being predated modulates the body weight gain, providing an ecological context for the obesity resistance observed in many species of small mammals. METHODS: Body weight, food intake, physical activity, and core body temperature of 35 male C57BL/6 mice were monitored for 20 days, while feeding a high-fat diet. A third of the animals were exposed to elevated risk of predation through exposure to the sounds of nocturnal predatory birds, and these were compared to animals exposed to a neutral noise or silence. RESULTS: Male mice exposed to predation risk had significantly lower weight gain than the neutral or silent groups. Reduced food intake and increased physical activity were the main proximal factors explaining this effect. The risk of predation also induced changes in boldness. CONCLUSIONS: This study provides evidence supporting the role of predation risk on body weight gain of small mammals.

Behavioural and physiological responses of wood mice (Apodemus sylvaticus) to experimental manipulations of predation and starvation risk.

Body weight and the levels of stored body fat have fitness consequences. Greater levels of fat may provide protection against catastrophic failures in the food supply, but they may also increase the risk of predation. Animals may therefore regulate their fatness according to their perceived risks of predation and starvation: the starvation-predation trade-off model. We tested the predictions of this model in wood mice (Apodemus sylvaticus) by experimentally manipulating predation risk and starvation risk. We predicted that under increased predation risk individuals would lose weight and under increased starvation risk they would gain it. We simulated increased predation risk by playing the calls made by predatory birds (owls: Tyto alba and Bubo bubo) to the mice. Control groups included exposure to calls of a non-predatory bird (blackbird: Turdus merula) or silence. Mice exposed to owl calls at night lost weight relative to the silence group, mediated via reduced food intake, but exposure to owl calls in the day had no significant effect. Exposure to blackbird calls at night also resulted in weight loss, but blackbird calls in the day had no effect. Mice seemed to have a generalised response to bird calls at night irrespective of their actual source. This could be because in the wild any bird calling at night will be a predation risk, and any bird calling in the day would not be, because at that time the mice would normally be resting, and hence not exposed to avian predators. Consequently, mice have not evolved to distinguish different types of call but only to respond to the time of day that they occur. Mice exposed to stochastic 24h starvation events altered their behaviour (reduced activity) during the refeeding days that followed the deprivation periods to regain the lost mass. However, they only marginally elevated their food intake and consequently had reduced body weight/fat storage compared to that of the control unstarved group. This response may have been constrained by physiological factors (alimentary tract absorption capacity) or behavioural factors (perceived risk of predation). Overall the responses of the mice appeared to provide limited support for the starvation-predation trade-off model, and suggest that wood mice are much more sensitive to predation risk than they are to starvation risk.