Seasonal Variations in Heart Rate Variability as an Indicator of Stress in Free-Ranging Pregnant Przewalski's Horses (E. ferus przewalskii) within the Hortobágy National Park in Hungary

Stress-coping styles are associated with energy budgets and variability in energy management strategies in a capital breeder

Individuals vary in their stress-coping styles, characterized by specific behavioural and physiological traits that influence their response to stressors. Theory suggests that these traits are linked to underlying metabolic mechanisms that affect energy management strategies. Despite the potential of this powerful comparative approach, few studies have explored how stress-coping styles relate to energy management strategies. Using heart rate telemetry data from a large, capital-breeding pinniped, the grey seal (Halichoerus grypus), we sought to investigate the relationship that stress-coping styles (via individual resting heart rate variability, rHRV) may have on energy management strategies. Background energy expenditures, a proxy for metabolic rate and other background processes, and daily energy expenditures were found to be individually repeatable in grey seal mothers across successive breeding seasons. Proactive individuals (low rHRV) exhibited consistently higher background and daily energy expenditures than reactive females (high rHRV). However, reactive phenotypes were more variable overall in energy management strategy, highlighting greater flexibility in their energy management strategy. Our results highlight key energetic trade-offs associated with stress-coping styles in grey seal mothers during this short but critical life-history stage; proactive individuals tended to exhibit a single pattern of energy management, expending greater energy while incurring greater risk of over-spending, than those with a more reactive phenotype.

Physiological effects of research handling on the northern elephant seal (Mirounga angustirostris)

Wildlife researchers must balance the need to safely capture and handle their study animals to sample tissues, collect morphological measurements, and attach dataloggers while ensuring their results are not confounded by stress artifacts caused by handling. To determine the physiological effects of research activities including chemical immobilization, transport, instrumentation with biologgers, and overnight holding on a model marine mammal species, we collected hormone, blood chemistry, hematology, and heart rate data from 19 juvenile northern elephant seals (Mirounga angustirostris) throughout a translocation experiment. Across our six sampling timepoints, cortisol and aldosterone data revealed a moderate hormonal stress response to handling accompanied by minor changes in hematocrit and blood glucose, but not ketone bodies or erythrocyte sedimentation rate. We also examined heart rate as a stress indicator and found that interval heart rate, standard deviation of heart rate, and apnea-eupnea cycles were influenced by handling. However, when seals were recaptured after several days at sea, all hormonal and hematological parameters had returned to baseline levels. Furthermore, 100 % of study animals were resighted in the wild post-translocation, with some individuals observed over four years later. Together, these findings suggest that while northern elephant seals exhibit measurable physiological stress in response to handling, they recover rapidly and show no observable long-term deleterious effects, making them a robust species for ecological and physiological research.

Effects of different winter paddock management of Thoroughbred weanlings and yearlings in the cold region of Japan on physiological function, endocrine function and growth

Effects of different winter paddock management of Thoroughbred weanlings and yearlings in Hokkaido, Japan, which is extremely cold in winter, on physiological function, endocrine function and growth were investigated. They were divided into two groups; those kept outdoors for 22 hr in the paddock (22hr group) and those kept outdoors for 7 hr in daytime with walking exercise for 1 hr using the horse-walker (7hr+W group), and the changes in daily distance travelled, body temperature (BT), heart rate (HR), HR variability (HRV), endocrine function and growth parameters were compared between the two groups from November at the year of birth to January at 1 year of age. The 7hr+W group could travel almost the same distance as the 22hr group by using the horse-walker. The 22hr group had a lower rate of increase in body weight than the 7hr+W group in January. In addition, lower in BT and HR were observed, and HRV analysis showed an increase in high frequency power spectral density, indicating that parasympathetic nervous activity was dominant. And also, changes in circulating cortisol and thyroxine were not observed despite cold environment. On the other hand, the 7hr+W group had higher prolactin and insulin like growth factor than the 22hr group in January, and cortisol and thyroxine were also increased. Physiological and endocrinological findings from the present study indicate that the management of the 7hr+W group is effective in promoting growth and maintaining metabolism during the winter season.

The paradox of spring: Thyroid and glucocorticoid responses to cold temperatures and food availability in free living Carneddau ponies

In seasonal environments, maintaining a constant body temperature poses challenges for endotherms. Cold winters at high latitudes, with limited food availability, create opposing demands on metabolism: upregulation preserves body temperature but depletes energy reserves. Examining endocrine profiles, such as thyroid hormone triiodothyronine (T3) and glucocorticoids (GCs), proxies for changes in metabolic rate and acute stressors, offer insights into physiological trade-offs. We evaluated how environmental conditions and gestation impact on faecal hormone metabolites (fT3Ms and fGCMs) from late winter to spring in a free-living population of Carneddau ponies. Faecal T3Ms were highest in late February and March, when temperatures were lowest. Then, fT3Ms concentrations decreased throughout April and were at the lowest in May before increasing towards the end of the study. The decline in fT3M levels in April and May was associated with warmer weather but poor food availability, diet diversity and diet composition. On the other hand, fGCM levels did not display a clear temporal pattern but were associated with reproductive status, where pregnant and lactating females had higher fGCM levels as compared to adult males and non-reproductive females. The temporal profile of fT3Ms levels highlights metabolic trade-offs in a changing environment. In contrast, the ephemeral but synchronous increase in fGCM concentrations across the population suggest a shared experience of acute stressors (i.e., weather, disturbance or social). This multi-biomarker approach can evaluate the role of acute stressors versus energy budgets in the context of interventions, reproduction, seasonality and environmental change, or across multiple scales from individuals to populations.

Providing baseline data for conservation-Heart rate monitoring in captive scimitar-horned oryx

Heart rate biologging has been successfully used to study wildlife responses to natural and human-caused stressors (e.g., hunting, landscape of fear). Although rarely deployed to inform conservation, heart rate biologging may be particularly valuable for assessing success in wildlife reintroductions. We conducted a case study for testing and validating the use of subcutaneous heart rate monitors in eight captive scimitar-horned oryx (Oryx dammah), a once-extinct species that is currently being restored to the wild. We evaluated biologger safety and accuracy while collecting long-term baseline data and assessing factors explaining variation in heart rate. None of the biologgers were rejected after implantation, with successful data capture for 16-21 months. Heart rate detection accuracy was high (83%-99%) for six of the individuals with left lateral placement of the biologgers. We excluded data from two individuals with a right lateral placement because accuracies were below 60%. Average heart rate for the six scimitar-horned oryx was 60.3 ± 12.7 bpm, and varied by about 12 bpm between individuals, with a minimum of 31 bpm and a maximum of 188 bpm across individuals. Scimitar-horned oryx displayed distinct circadian rhythms in heart rate and activity. Heart rate and activity were low early in the morning and peaked near dusk. Circadian rhythm in heart rate and activity were relatively unchanged across season, but hourly averages for heart rate and activity were higher in spring and summer, respectively. Variation in hourly heart rate averages was best explained by a combination of activity, hour, astronomical season, ambient temperature, and an interaction term for hour and season. Increases in activity appeared to result in the largest changes in heart rate. We concluded that biologgers are safe and accurate and can be deployed in free-ranging and reintroduced scimitar-horned oryx. In addition to current monitoring practices of reintroduced scimitar-horned oryx, the resulting biologging data could significantly aid in 1) evaluating care and management action prior to release, 2) characterizing different animal personalities and how these might affect reintroduction outcomes for individual animals, and 3) identifying stressors after release to determine their timing, duration, and impact on released animals. Heart rate monitoring in released scimitar-horned oryx may also aid in advancing our knowledge about how desert ungulates adapt to extreme environmental variation in their habitats (e.g., heat, drought).

The application of allostasis and allostatic load in animal species: A scoping review

Principles of allostasis and allostatic load have been widely applied in human research to assess the impacts of chronic stress on physiological dysregulation. Over the last few decades, researchers have also applied these concepts to non-human animals. However, there is a lack of uniformity in how the concept of allostasis is described and assessed in animals. The objectives of this review were to: 1) describe the extent to which the concepts of allostasis and allostatic load are applied theoretically to animals, with a focus on which taxa and species are represented; 2) identify when direct assessments of allostasis or allostatic load are made, which species and contexts are represented, what biomarkers are used, and if an allostatic load index was constructed; and 3) detect gaps in the literature and identify areas for future research. A search was conducted using CABI, PubMed, Agricola, and BIOSIS databases, in addition to a complementary hand-search of 14 peer-reviewed journals. Search results were screened, and articles that included non-human animals, as well as the terms "allostasis" or "allostatic" in the full text, were included. A total of 572 articles met the inclusion criteria (108 reviews and 464 peer-reviewed original research). Species were represented across all taxa. A subset of 63 publications made direct assessments of allostatic load. Glucocorticoids were the most commonly used biomarker, and were the only biomarker measured in 25 publications. Only six of 63 publications (9.5%) constructed an allostatic load index, which is the preferred methodology in human research. Although concepts of allostasis and allostatic load are being applied broadly across animal species, most publications use single biomarkers that are more likely indicative of short-term rather than chronic stress. Researchers are encouraged to adopt methodologies used in human research, including the construction of species-specific allostatic load indexes.

Effect of integrated management bundle on 1-year overall survival outcomes and perioperative outcomes in super elderly patients aged 90 and over with hip fracture: non-concurrent cohort study

BACKGROUND

Due to concomitant factors like frailty and comorbidity, super elderly (≥90 years) patients with hip fracture differ from patients aged 65-89 years in perioperative complications and mortality. The integrated management bundle referred to bundled application of multiple clinical measures. The aim of this study was to analyze effect of integrated management bundle on 1-year overall survival and perioperative outcomes in super elderly patients with hip fracture, with multidisciplinary management group serving as the control group.

METHODS

In this retrospective cohort study, super elderly patients with hip fracture were included from Jan 2017 to Nov 2020. Patients were retrospectively divided to multidisciplinary management group and integrated management bundle group. The primary outcome was 1- year overall survival, and the secondary outcome was perioperative outcomes. Kaplan-Meier methods was used to compare survival probability. Multivariable Cox's modeling was used to explain the effect of integrated bundle on 1-year overall survival adjusted for confounders. The perioperative outcomes including complications and in-hospital data of two groups were compared. The multivariable logistic regression was used to explain the effect of integrated bundle on the occurrence of perioperative complications adjusted for confounders. Prognostic factors related to survival was identified by multivariable Cox's regression analysis.

RESULTS

Ninety-seven patients comprised multidisciplinary management group, and 83 comprised integrated management bundle group. The Kaplan-Meier plots showed that the survival probability of integrated management bundle group was significantly better than multidisciplinary management group (HR:0.435, 95%CI:0.207-0.914, P = 0.039). Multivariable analysis after adjustment for confounders showed a 42.8% lower incidence of mortality integrated management bundle group than multidisciplinary management group (HR:0.428, 95%CI:0.186-0.986, P = 0.046). Incidence of hypoproteinemia, and electrolyte disturbance in integrated management bundle group was significantly lower than multidisciplinary management group (all P < 0.05). In addition, significant reduction was observed in length of stay (P < 0.05) in integrated management bundle group. Multivariable logistic regression showed integrated management bundle was independent protective factor of hypoproteinemia, and electrolyte disturbance. mECM score ≥ 6 and ASA score > 2 were independent risk factors of overall survival (HR: 1.940, 95%CI: 1.067-3.525,P = 0.030; HR: 2.281, 95%CI: 1.113-4.678,P = 0.024).

CONCLUSIONS

The integrated management bundle improved 1-year overall survival and played positive effects in improving perioperative outcomes. It might be a more suitable management modality for super elderly patients with hip fracture.

Retention and loss of PIT tags and surgically implanted devices in the Eurasian beaver

BACKGROUND

Passive integrated transponder devices (PIT tags) are a valuable tool for individual identification of animals. Similarly, the surgical implantation of transmitters and bio-loggers can provide useful data on animal location, physiology and behavior. However, to avoid unnecessary recapture and related stress of study animals, PIT tags and bio-loggers should function reliably for long periods of time. Here, we evaluated the retention of PIT tags, and of very high frequency (VHF) transmitters and bio-loggers that were either implanted subcutaneously or into the peritoneal cavity of Eurasian beavers (Castor fiber).

RESULTS

Over a 21-year period, we implanted PIT tags in 456 individuals and failed to detect a PIT tag at recapture in 30 cases, consisting of 26 individuals (6% of individuals). In all instances, we were still able to identify the individual due to the presence of unique ear tag numbers and tail scars. Moreover, we implanted 6 VHFs, 36 body temperature loggers and 21 heart rate loggers in 28 individuals, and experienced frequent loss of temperature loggers (at least 6 of 23 recaptured beavers) and heart rate loggers (10 of 18 recaptured beavers). No VHFs were lost in 2 recaptured beavers.

CONCLUSIONS

Possible causes for PIT tag loss (or non-detection) were incorrect implantation, migration of the tag within the body, a foreign body reaction leading to ejection, or malfunctioning of the tag. We speculate that logger loss was related to a foreign body reaction, and that loggers were either rejected through the incision wound or, in the case of temperature loggers, possibly adhered and encapsulated to intestines, and then engulfed by the gastro-intestinal tract and ejected. We discuss animal welfare implications and give recommendations for future studies implanting bio-loggers into wildlife.

Effect of season and diet on heart rate and blood pressure in female red deer (Cervus elaphus) anaesthetised with medetomidine-tiletamine-zolazepam

Temperate zone ungulates like red deer (Cervus elaphus) show pronounced seasonal acclimatisation. Hypometabolism during winter is associated with cardiovascular changes, including a reduction in heart rate (fH) and temporal peripheral vasoconstriction. How anaesthesia with vasoactive substances such as medetomidine affect the seasonally acclimatised cardiovascular system is not yet known. We anaesthetised eleven healthy female red deer with medetomidine (0.1 mg/kg) and tiletamine/zolazepam (3 mg/kg) twice in winter (ad libitum and restricted feed) and in summer (ad libitum and restricted feed), with a two-week washout-period in-between, to test for the effect of season, food availability and supplementation with omega-3 or omega-6 polyunsaturated fatty acid (PUFA) on fH and arterial blood pressure (ABP) during anaesthesia. Six animals received pellets enriched with omega-6 fatty acids (FA), and five animals with omega-3 FA. Anaesthesia significantly decreased fH in summer but not in winter and ABP was lower in winter (p < 0.05). The combination of omega-6 FA enriched pellets and food restriction resulted in a lower fH and higher ABP during anaesthesia with more pronounced changes in winter (p < 0.001). Our results demonstrate that season, food availability and type of PUFA supplementation in red deer affect the cardiovascular system during anaesthesia.

Inside out: heart rate monitoring to advance the welfare and conservation of maned wolves (Chrysocyon brachyurus)

Anthropogenic change is a major threat to individual species and biodiversity. Yet the behavioral and physiological responses of animals to these changes remain understudied. This is due to the technological challenges in assessing these effects in situ. Using captive maned wolves (Chrysocyon brachyurus, n = 6) as a model, we deployed implantable biologgers and collected physiological data on heart rate (HR) and heart rate variability (HRV) over a 1-year period. To test for links between HR and changes in the environment we analysed HR daily rhythms and responses to potential stressors (e.g. physical restraint, change in housing conditions, short-distance transportation and unfamiliar human presence). The 2-min HR averages ranged from 33 to 250 bpm, with an overall rest average of 73 bpm and a maximum of 296 bpm. On average, HRV was higher in females (227 ± 51 ms) than in males (151 ± 51 ms). As expected, HR increased at dusk and night when animals were more active and in response to stressors. Sudden decreases in HR were observed during transportation in three wolves, suggestive of fear bradycardia. We provide the first non-anesthetic HR values for the species and confirm that behaviour does not always reflect the shifts in autonomic tone in response to perceived threats. Because strong HR responses often were not revealed by observable changes in behaviour, our findings suggest that the number and variety of stressors in ex situ or in situ environments for maned wolves and most wildlife species may be underestimated. Our study also shows that integrating biologging with behavioral observations can provide vital information to guide captive management. Similar technology can be used to advance in situ research for developing more effective welfare, management and conservation plans for the species.

An external telemetry system for recording resting heart rate variability and heart rate in free-ranging large wild mammals

Measures of heart rate variability (and heart rate more generally) are providing powerful insights into the physiological drivers of behaviour. Resting heart rate variability (HRV) can be used as an indicator of individual differences in temperament and reactivity to physical and psychological stress. There is increasing interest in deriving such measures from free ranging wild animals, where individuals are exposed to the natural and anthropogenic stressors of life. We describe a robust, externally mounted heart rate monitor for use in wild mammals, deployed here on wild breeding adult female grey seals (Halichoerus grypus), that delivers millisecond precise measures of inter beat intervals (IBIs), allowing computation of resting HRV parameters. Based on Firstbeat™ heart rate belts, our system allows for remote, continuous recording of IBI data from over 30 individuals simultaneously at ranges of up to 200m. We assessed the accuracy of the IBI data provided by the Firstbeat™ system using concurrent IBI data derived from in-field electrocardiogram (ECG) recordings. Bland-Altmann analyses demonstrated high correspondence between the two sets of IBI data, with a mean difference of 0.87±0.16ms. We used generalized additive mixed-effects models to examine the impact of the default Firstbeat™ software artefact correction procedure upon the generation of anomalous data (flats and stairs). Artefact correction and individual activity were major causes of flats and stairs. We used simulations and models to assess the impact of these errors on estimates of resting HRV and to inform criteria for subsampling relatively error free IBI traces. These analyses allowed us to establish stringent filtering procedures to remove traces with excessive numbers of artefacts, including flats and stairs. Even with strict criteria for removing potentially erroneous data, the abundance of data yielded by the Firstbeat™ system provides the potential to extract robust estimates of resting HRV. We discuss the advantages and limitations of our system for applications beyond the study system described here.

Quantifying the Autonomic Response to Stressors-One Way to Expand the Definition of "Stress" in Animals

Quantifying how whole organisms respond to challenges in the external and internal environment ("stressors") is difficult. To date, physiological ecologists have mostly used measures of glucocorticoids (GCs) to assess the impact of stressors on animals. This is of course too simplistic as Hans Seyle himself characterized the response of organisms to "noxious stimuli" using multiple physiological responses. Possible solutions include increasing the number of biomarkers to more accurately characterize the "stress state" of animal or just measuring different biomarkers to more accurately characterize the degree of acute or chronic stressors an animal is experiencing. We focus on the latter and discuss how heart rate (HR) and heart rate variability (HRV) may be better predictors of the degree of activation of the sympathetic-adrenal-medullary system and complement or even replace measures of GCs as indicators of animal health, welfare, fitness, or their level of exposure to stressors. The miniaturization of biological sensor technology ("bio-sensors" or "bio-loggers") presents an opportunity to reassess measures of stress state and develop new approaches. We describe some modern approaches to gathering these HR and HRV data in free-living animals with the aim that heart dynamics will be more integrated with measures of GCs as bio-markers of stress state and predictors of fitness in free-living animals.

Breakthroughs and new horizons in reproductive biology of rare and endangered animal species

Because of higher extinction rates due to human and natural factors, more basic and applied research in reproductive biology is required to preserve wild species and design proper strategies leading to sustainable populations. The objective of the review is to highlight recent, inspiring breakthroughs in wildlife reproduction science that will set directions for future research and lead to more successes in conservation biology. Despite new tools and approaches allowing a better and faster understanding of key mechanisms, we still know little about reproduction in endangered species. Recently, the most striking advances have been obtained in nonmammalian species (fish, birds, amphibians, or corals) with the development of alternative solutions to preserve fertility or new information about parental nutritional influence on embryo development. A novel way has also been explored to consider the impact of environmental changes on reproduction-the allostatic load-in a vast array of species (from primates to fish). On the horizon, genomic tools are expected to considerably change the way we study wildlife reproduction and develop a concept of "precision conservation breeding." When basic studies in organismal physiology are conducted in parallel, new approaches using stem cells to create artificial gametes and gonads, innovations in germplasm storage, and more research on reproductive microbiomes will help to make a difference. Lastly, multiple challenges (for instance, poor integration of new tools in conservation programs, limited access to study animals, or few publication options) will have to be addressed if we want reproductive biology to positively impact conservation of biodiversity.

Reactive stress-coping styles show more variable reproductive expenditure and fitness outcomes

Stress-coping styles dictate how individuals react to stimuli and can be measured by the integrative physiological parameter of resting heart-rate variability (HRV); low resting HRV indicating proactive coping styles, while high resting HRV typifies reactive individuals. Over 5 successive breeding seasons we measured resting HRV of 57 lactating grey seals. Mothers showed consistent individual differences in resting HRV across years. We asked whether proactive and reactive mothers differed in their patterns of maternal expenditure and short-term fitness outcomes within seasons, using maternal daily mass loss rate to indicate expenditure, and pup daily mass gain to indicate within season fitness outcomes. We found no difference in average rates of maternal daily mass loss or pup daily mass gain between proactive and reactive mothers. However, reactive mothers deviated more from the sample mean for maternal daily mass and pup daily mass gain than proactive mothers. Thus, while proactive mothers exhibit average expenditure strategies with average outcomes, expenditure varies much more among reactive mothers with more variable outcomes. Overall, however, mean fitness was equal across coping styles, providing a mechanism for maintaining coping style diversity within populations. Variability in reactive mothers’ expenditures and success is likely a product of their attempts to match phenotype to prevailing environmental conditions, achieved with varying degrees of success.

Coping styles in capital breeders modulate behavioural trade-offs in time allocation: assessing fine-scale activity budgets in lactating grey seals (Halichoerus grypus) using accelerometry and heart rate variability

Abstract

Balancing time allocation among competing behaviours is an essential part of energy management for all animals. However, trade-offs in time allocation may vary according to the sex of the individual, their age, and even underlying physiology. During reproduction, higher energetic demands and constrained internal resources place greater demand on optimizing these trade-offs insofar that small adjustments in time-activity may lead to substantial effects on an individual’s limited energy budget. The most extreme case is found in animals that undergo capital breeding, where individuals fast for the duration of each reproductive episode. We investigated potential underlying drivers of time-activity and describe aspects of trade-offs in time-activity in a wild, capital breeding pinniped, the grey seal Halichoerus grypus, during the lactation period. For the first time, we were able to access full 24-h activity budgets across the core duration of lactation as well as characterize how aspects of stress-coping styles influence time allocation through the use of animal-borne accelerometers and heart rate monitors in situ. We found that there was a distinct trade-off in time activity between time spent Resting and Alert (vigilance). This trade-off varied with the pup’s development, date, and maternal stress-coping style as indicated by a measure of heart rate variability, rMSSD. In contrast, time spent Presenting/Nursing did not vary across the duration of lactation given the variables tested. We suggest that while mothers balance time spent conserving resources (Resting) against time expending energy (Alert), they are also influenced by the inherent physiological drivers of stress-coping styles.

Significance statement

How animals apportion their time among different behaviours is key to their success. These trade-offs should be finely balanced to avoid unnecessary energy expenditure. Here, we examine how grey seal mothers balance their activity patterns during the short, but energetically demanding, period of pup-rearing. Animal-borne accelerometers provided a uniquely detailed and continuous record of activity during pup-rearing for 38 mothers. We also used heart rate monitors to provide measures of each individual’s stress-coping style. We found that mothers balance time Resting against remaining Alert while time Presenting/Nursing was largely independent of all factors measured. Stress-coping styles were found to drive the balancing and variation of all behaviours. This novel indication that differences in personality-like traits may drive whole activity budgets should be considered when assessing trade-offs in time allocation across a much wider variety of species.

Season's Effects on Some Clinical, Hematological Parameters and Blood Cortisol Level in Sedated Arabian Horses With Xylazine

Influence of heat or cold stress in sedated animals is unclear and requires further investigations. The present study aimed to evaluate the season's effects on some clinical, hematological parameters and blood cortisol level in sedated Arabian horses with xylazine. Therefore, seven Arabian horses were used to investigate heart and respiratory rates, and capillary refill time and serum cortisol level were recorded before (0) and at 5, 15, 60, and 180 minutes postsedation. Heparinized venous samples were collected before (0) and 3 hours postsedation for analysis of hematological analysis. Arterial blood samples were collected before and 1 hour postsedation for arterial blood gases and electrolytes analysis. Repeated analysis of variance was performed (P < .05). Significant decreases have been observed in heart and respiratory rates at 5, 15, and 60 minutes postsedation in summer and only at 5 minutes postsedation in winter. Arterial oxygen pressure and arterial carbon dioxide pressure showed a significant decrease and increase, respectively at 1 hour postsedation in summer and winter. The serum cortisol levels were significantly higher in summer than in winter at 5, 15, and 60 minutes postsedation. In summer, the postsedation concentrations of cortisol did not change significantly than its values before sedation. However, in winter, the cortisol concentration decreased significantly at 5, 15, and 60 minutes postsedation compared with their value before sedation. The present study suggests that these season's effects on the sedated Arabian horses could take into consideration in xylazine-sedated Arabian horses.

Correcting the Activity-Specific Component of Heart Rate Variability Using Dynamic Body Acceleration Under Free-Moving Conditions

Heart rate variability (HRV) analysis is a widely used technique to assess sympatho-vagal regulation in response to various internal or external stressors. However, HRV measurements under free-moving conditions are highly susceptible to subjects’ physical activity levels because physical activity alters energy metabolism, which inevitably modulates the cardiorespiratory system and thereby changes the sympatho-vagal balance, regardless of stressors. Thus, researchers must simultaneously quantify the effect of physical activity on HRV to reliably assess sympatho-vagal balance under free-moving conditions. In the present study, dynamic body acceleration (DBA), which was developed in the field of animal ecology as a quantitative proxy for activity-specific energy expenditure, was used as a factor to correct for physical activity when evaluating HRV in freely moving subjects. Body acceleration and heart inter-beat intervals were simultaneously measured in cattle and sheep, and the vectorial DBA and HRV parameters were evaluated at 5-min intervals. Next, the effects of DBA on the HRV parameters were statistically analyzed. The heart rate (HR) and most of the HRV parameters were affected by DBA in both animal species, and the inclusion of the effect of DBA in the HRV analysis greatly influenced the frequency domain and nonlinear HRV parameters. By removing the effect of physical activity quantified using DBA, we could fairly compare the stress levels of animals with different physical activity levels under different management conditions. Moreover, we analyzed and compared the HRV parameters before and after correcting for the mean HR, with and without inclusion of DBA. The results were somewhat unexpected, as the effect of DBA was a highly significant source of HRV also in parameters corrected for mean HR. In conclusion, the inclusion of DBA as a physical activity index is a simple and useful method for correcting the activity-specific component of HRV under free-moving conditions.