The Becker-DeGroot-Marschak method (BDM) is an auction-like mechanism widely used in behavioural economics, marketing research, and, more recently, in neuroimaging studies of human decision making. The BDM has never been used with animal subjects before, yet its application in monkeys would allow for comparison of studies across species while providing a direct measure of what a reward is worth to a monkey in a single experimental trial. In the BDM, a subject is given a budget with which they can place a bid for some reward, and a computer then randomly selects a competing bid. If the subject’s bid is higher than the computer’s bid then the subject pays an amount equal to the computer’s bid, receives the reward object, and gets to keep the remaining budget. If the subject’s bid is lower than the computer’s bid, the subject does not gain the reward object but retains the entire budget. To adapt the task for monkeys, two rhesus macaques were taught to use water as a budget, and to use a joystick to place a bid in terms of this budget for different volumes of fruit-juice reward. The BDM ensures that the subject’s optimal action is to place a bid equal to their value for the reward-object. This property of truthful value revelation is the BDM’s most important feature in the context of value-based decision making. Currently, the only method of eliciting a monkey’s value for one reward in terms of another depends upon inference of the magnitudes at which the two rewards are chosen with equal probability. Using this ‘binary-choice’ method, many trials are needed to infer a single value: pairwise comparisons of many different magnitudes must be made and choices of each pair must be repeated so that the probability of choosing a reward can be estimated. In contrast, the BDM provides a direct measure of the monkey’s value for the reward as they explicitly state this value on each trial by selecting an equivalent bid. Therefore, the BDM more efficiently utilises the limited time in which a monkey’s behaviour can be assessed in each experimental session, as animals lose the motivation to participate when they become sated. The thesis summarised here describes the training and performance of two rhesus macaques on a novel version of the BDM, specifically designed for a subject that cannot be instructed on the optimal strategy. The technical steps and intermediate tasks that are needed to train a monkey to flexibly place bids by operating a joystick are also detailed, as well as the development of different versions of the task over three years of testing. The results of the final version of the BDM are then presented for both monkeys, showing rational bidding behaviour consistent with an understanding of the method’s contingencies. Theoretical concerns and limitations of the BDM in such a setting are also discussed and the thesis outlines how future experiments can make use of and adapt this version of the BDM for neuronal recording experiments.

Made available in DSpace on 2014-06-11T19:25:25Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-07-21Bitstream added on 2014-06-13T19:47:32Z : No. of bitstreams: 1 soares_dr_me_jabo.pdf: 515920 bytes, checksum: f75f638c73fff3c5d1395d3d7b52804b (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O objetivo foi estudar o processo de adaptação ao confinamento avaliando se há variação entre diferentes grupos de bovinos e suas relações com temperamento. Foram avaliados animais em dois bancos de dados. No estudo 1: utilizados animais dos grupos genéticos Nelore Seleção (NeS), Nelore Controle (NeC) e Guzerá (Guz). No estudo 2: animais dos grupos genéticos Nelore e Cruzado. Indicadores de adaptação: peso vivo, ganho de peso, por período e total; tempo de permanência no cocho (TPC) e atos agressivos (AGR) (somente no estudo 2). Medidas de temperamento: Escore de movimentação e Tensão; Velocidade de saída, sendo que no estudo 2 foram incluídas duas medidas: Distância de fuga e Teste de Condução. Foram utilizadas análises de correlação residual GLM (Manova) para análise de associação entre as medidas, PROC MIXED do SAS para avaliar e quantificar os efeitos das medidas umas sobre as outras, além do cálculo de repetibilidade por meio dos componentes de variâncias dentro e entre animais. Constatou-se que animais considerados de melhores temperamentos possuem maiores TPC e maiores AGR e que é possível avaliar o processo de adaptação, verificado através das correlações significativas entre as medidas estudadas, melhoria no temperamento ao longo do tempo, aumento do TPC e redução da AGR. Sendo assim, conclui-se que NeS, no estudo 1, obteve melhor capacidade de adaptação, devido aos melhores resultados de desempenho, temperamento e comportamento alimentar, seguido de NeC e Guz. Em contrapartida, no estudo 2, não houve diferenças em desempenho e temperamento entre os dois grupos genéticos, entretanto, a maior freqüência de AGR foi no Cruzado, sugerindo provável dificuldade de adaptação a este grupo / The aim was to study the process of adaptation to confinement evaluating whether there is variation between different groups of cattle and their relationship to temperament. Animals were evaluated in two farms. In study 1, Nellore Selection (NES), Nelore Control (NEC) and Guzera (Guz). In study 2, Nellore and Crossbreed. Adaptation indicators: weight, weight gain per period and total time spent at the trough (TPT) and aggressive acts (AGR) (only in study 2). Measures of temperament, movement and tension score, flight speed and in study 2 included two measures: Flight distante and handling test. We used GLM residual correlation analysis (Manova) to analyze the association between the measures, PROC MIXED of SAS to evaluate and quantify the effects of measures on each other, and the calculation of repeatability through the components of variance within and between animals. It was found that animals which have better temperaments showed TPT larger and AGR larger and is possible to evaluate the adaptation process, verified by the significant correlations between the measures studied, improvement in mood over time, increase in TPT and reduction of AGR. Thus, we conclude that NeS, in study 1, had the best ability to adapt, due to best performance, temperament and feeding behavior, followed by NeC and Guz. In contrast, in study 2, there were no differences in performance and temperament between the two genetic groups, however, Crossbreed had greater frequency of AGR, suggesting a probable difficulty in adapting to this group

Blumetto Velazco, OR. (2012). Caracterización de sistemas de producción de porcino y vacuno de engorde en Uruguay [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16002 / Palancia

<p>  Broiler chickens are routinely exposed to various conditions, such as heat stress and stocking density, which may negatively influence their welfare status. This study examined the influence of a commercially available proprietary phytogenic feed additive (Probiotech International, Inc.) on welfare measures, environmental measures, productivity, body temperature, and behavior of commercial broiler chickens. Two flocks (Trials 1 & 2) utilized a total of 1,650 Ross 708 broilers housed in two separate rooms with floor pens at Purdue University’s ASREC Poultry Unit. </p> <p>  Trial 1 birds (n = 750) were divided into 20 separate pens (5 pens/treatment): PHD: phytogenic supplementation and raised at standard industry stocking density (37 kg/m²), PLD: phytogenic supplementation and raised at a lowered stocking density (27 kg/m²), CHD: control diet and raised at standard industry stocking density, and CLD: control diet and raised at a lowered stocking density. Due to the aromatic properties of the phytogenic additive, PHD and PLD birds were housed in one room and CHD and CLD birds were housed in another room. The feed supplement was mixed in at an inclusion rate of 0.25 g/kg. Phytogenic supplementation began at 15 d, coinciding with feeding the grower diet, and continued until 42 d when the study concluded. Heat stress was applied to all birds from 30 to 32 d, where the peak temperature did not exceed 34.4ºC. Welfare measures (gait, footpad dermatitis, hock burn, and feather cleanliness) and litter quality were assessed at 27 d and 39 d. Ammonia concentrations were measured at 35 d and 39 d. Productivity (body weight, feed intake, and feed conversion ratio) was measured weekly. Body temperature via thermography of the eye surface was collected at 29, 32, and 34 d. Behavior data were collected at the following periods for 2 continuous days: 23-24 d (Period 1), 31-32 d (Period 2), and 36-37 d (Period 3). </p> <p>  Trial 2 birds (n = 900) were assigned to 25 separate pens (5 pens/treatment) with the same groups as Trial 1, with an additional group (MHD): control diet, housed in the same room as PHD and PLD birds, and raised at standard industry stocking density. Phytogenic supplementation was provided as in Trial 1. Heat stress was again applied to all treatments from 30 to 32 d, where the peak temperature did not exceed 31ºC. Welfare measures and litter quality were collected at 27 d and 38 d. Ammonia concentrations were measured at 27, 31, and 38 d. Productivity was measured from 15 d to 27 d (grower phase), and 27 d to 38 d (finisher phase). Body temperature via cloacal temperature recording occurred at 29, 31, and 33 d. </p> <p>  Welfare data and litter quality were analyzed using PROC LOGISTIC (SAS 9.4); productivity data were analyzed using PROC MIXED (SAS 9.4); behavior data were analyzed using PROC GLIMMIX (SAS 9.4); and eye surface temperature, cloacal temperature, and ammonia concentration were analyzed using a nested mixed model found in the afex package using R (version 4.2.1) and R Studio (R Foundation for Statistical Computing). All statistical differences were considered significant when P < 0.05. Results of Trial 1 indicated that phytogenic supplementation significantly influenced hock burn and feather cleanliness, body weight and feed conversion ratio, ammonia concentration, as well as Period 1 sitting, Period 2 drinking, preening, standing, and wing spreading, and Period 3 drinking, preening, sitting, standing, and walking behaviors. Results of Trial 2 indicated that diet had no effect on any measured parameter. The variation in results suggests that factors such as stocking density or management strategies influenced the measured parameters, rather than diet alone. More research is needed to understand the specific effects of phytogenic feed additives, social and environmental stressors and whether phytogenic feed additives can improve bird performance and welfare under longer heat stress periods.</p>

This thesis focuses on conflict resolution and collective decision-making in co-navigating pigeons, Columba livia. These birds have a remarkable homing ability and frequently fly in flocks. Group navigation demands that group members reach consensus on which path to follow, but the mechanisms by which they do so remain largely unexplored. Pigeons are particularly suitable for studying these mechanisms, due to their sociality and the fact that their possession of information can easily be altered and quantified. I present the results of a series of experiments that manipulated the experience of homing pigeons in various ways so as to observe the effect of information they had previously gathered on their group behaviour. Key findings were: Previous navigational experience contributes to the establishment of leader-follower relationships. The larger the difference in experience between two co-navigating pigeons, the higher the likelihood the more experienced bird will emerge as leader. Shared homing experience through repeated joint flights can allow two pigeons to develop into a “behavioural unit”. They form spatial sub-groups when flying with less familiar birds, and perform a similar transition between compromise- and leadership-dominated flights as single birds, although they are more likely to accept compromise routes. Such previous association histories between birds can thus affect collective decision-making in larger flocks. There is a trade-off between the amount of spatial information handled and the efficiency with which such information can be applied during homing. Leading/following behaviour is influenced by the recency of the route memories. Leadership hierarchies in pigeon flocks appear resistant to changes in the navigational knowledge of a subset of their members, at least when these changes are relatively small in magnitude. The stability of the hierarchical structure might be beneficial during decision-making. Mathematical modelling suggests that underlying hierarchical social structures can increase navigational accuracy. Hierarchically organised groups with the smallest number of strong connections achieve highest accuracy. Group leader-follower dynamics resemble the underlying social structure.

The behaviour of zoo animals can come to anticipate temporally predictable feeding times. However, there is a lack of consensus over the effects of such routines on behaviour and welfare. Few studies have been published in this area, perhaps in part due to methodological challenges. The current programme of work therefore aimed to extend knowledge on the effect of predictable feeding routines, and to develop a suitable methodology to overcome challenges. Prior to predictable feeding, tigers showed patterns of increased ‘active’ behaviours such as locomotion and pacing, and decreased inactivity. Geckos also showed increased ‘active’ behaviours such as locomotion and ‘stationary but moving head’ prior to predictable, compared to unpredictable feeding times, with significant differences between conditions (RSS = 0.059 and 0.047, p < .05). However, anticipatory patterns were not demonstrated for meerkats, which highlighted that other environmental factors may affect an animal’s response to temporal predictability (such as obtaining other food during the day, or signalled predictability). Studying patterns of behavioural change over time requires long periods of observation which is often not possible for researchers. The current programme of work argues that a measure which can be reliably used by many, relatively untrained observers is necessary to study predictability. The measure of ‘busyness’, a subjective rating of animal behaviour, was tested for reliability and validity. Busyness ratings showed good inter-observer reliability (ICC > .72) and correlated with traditional measures of behaviour. Busyness ratings demonstrated clear patterns related to feeding time and gave a useful compound measure of behavioural change. The use of multiple observers was extended to a citizen science approach, where useful data on anticipation in fish were obtained from aquarium visitors using a touch screen. The current programme of work successfully investigated the effects of predictable feeding routines on patterns of animal behaviour, alongside the development of suitable methods. The qualitative techniques developed here offer potential to increase the data obtained in future research into predictability and many other topics.

This thesis investigates how collective decisions in bird flocks arise from simple rules, the factors that give some birds more influence over a flock's direction, and how travelling as a flock affects spatial learning. I used GPS loggers to track pigeons homing alone and in flocks, and applied mathematical modelling to explore the mechanisms underlying group decisions. Across several experiments, the key results were as follows: Flying home with a more experienced individual not only gave a pigeon an immediate advantage in terms of taking a more direct route, but the followers also learned homing routes just as accurately as pigeons flying alone. This shows that using social cues did not interfere with learning about the landscape during a series of paired flights. Pigeons that were faster during solo homing flights also tended to fly at the front of flocks, where they had more influence over the direction taken. Analysis of momentary interactions during paired flights and simulations of pair trajectories support the conclusion that speed increases the likelihood of leading. A pigeon's solo homing efficiency before flock flights did not correlate with leadership in flocks of ten, but leaders did have more efficient solo tracks when tested after a series of flock flights. A possible explanation is that leaders attended more to the landscape and therefore learned faster. Flocks took straighter routes than pigeons flying alone, as would be expected if they effectively pooled information. In addition, pigeons responded more strongly to the direction of several neighbours, during flock flights, than to a single neighbour during paired flights. This behaviour makes sense adaptively because social information will be more reliable when following several conspecifics compared to one. Through a combination of high-resolution tracking and mathematical modelling, this thesis sheds light on the mechanisms of flocking and its navigational consequences.

A dieta sólida é fator determinante para o desenvolvimento ruminal de bezerros. No entanto ainda não está claro qual o teor de fibra ideal na dieta durante os primeiros meses de vida. O presente trabalho avaliou 35 animais da raça holandesa alojados em gaiolas individuais suspensas, os quais foram submetidos a três tratamentos: I - concentrado convencional contendo 22,0 de FDN, II - concentrado com 31% de FDN, substituindo parte do milho por casca de soja e III - concentrado convencional com 22,0% de FDN mais feno de coast-cross a vontade. Todos os animais receberam 4 litros de sucedâneo lácteo por dia com 12,5% de sólidos, divididos em duas refeições. O ganho de peso foi acompanhado semanalmente, também foram aferidas medidas corporais, indicadores metabólicos para desenvolvimento ruminal e metabolismo intermediário, bem como avaliação de parâmetros ruminais e comportamentais. Os animais foram avaliados até a oitava semana de vida, quando foram desaleitados. A elevação do teor de FDN no concentrado até 31% com a substituição de milho por casca de soja, ou oferta de feno, não alterou o consumo de concentrado e desempenho. A dieta 31FDN estimulou precocemente a diferenciação do epitélio ruminal pela maior concentração e AGCC em idades mais jovens. Sendo a casca de soja considerada fonte energética passível de ser utilizada na formulação de concentrados iniciais para bovinos em aleitamento nas proporções utilizadas nesse estudo. No entanto a oferta de feno se mostra mais interessante para a redução de comportamentos não desejados. O teor de 22% de FDN no concentrado inicial não ocasionou menor pH ruminal e fecal em comparação com dietas de FDN elevado, mostrando que concentrados com teor de FDN e distribuição de partículas semelhantes ao deste estudo podem ser ofertados com segurança. / The solid diet is a determinant factor for the ruminal development. However it is still unclear what the optimal fiber content in the solid diet during the first few months of life. The present work evaluated 35 Holstein animals housed in individual suspended cages, which were submitted to three treatments: I - conventional starter containing 22.0 NDF, II - starter with 31% NDF, replacing part of the corn by soybean hull and III - conventional concentrate with 22.0% NDF plus coast-cross hay ad libitum. All animals received 4L of milk replacer with 12.5% solids, divided into two meals daily. Weight gain was monitored weekly, body measurements, metabolic indicators for ruminal development and intermediate metabolism, as well as ruminal and behavioral parameters were also evaluated. The animals were evaluated until the eighth week of life, when they were weaned. The increase of the NDF content in the starter up to 31% with the replacement of corn per soybean hull or hay supply did not change the starter consumption and performance. The 31FDN diet early stimulated the differentiation of the ruminal epithelium by the higher concentration of SCFA at younger ages. Soybean hulls are considered an energy source that can be used in the formulation of starter for pre-weaned calves in the proportions used in this study. However, the supply of hay is more interesting for the reduction of unwanted behaviors. The 22% NDF content in the initial concentrate did not cause lower ruminal and fecal pH in comparison with high NDF diets, showing that NDF starter and particle distribution similar to this study can be safely offered.

From the individual to species level, it is common for animals to have connections with one another. These connections can exist in a variety of forms; from the social relationships within an animal society, to hybridisation between species. The structure of these connections in animal systems can be depicted using networks, often revealing non-trivial structure which can be biologically informative. Understanding the factors which drive the structure of animal networks can help us understand the costs and benefits of forming and maintaining relationships. Multivariate modelling provides a means to evaluate the relative contributions of a set of explanatory factors to a response variable. However, conventional modelling approaches use statistical tests which are unsuitable for the dependencies inherent in network and relational data. A solution to this problem is to use specialised models developed in the social sciences, which have a long history in modelling human social networks. Taking predictive multivariate models from the social sciences and applying them to animal networks is attractive given that current analytical approaches are predominantly descriptive. However, these models were developed for human social networks, where participants can self-identify relationships. In contrast, relationships between animals have to be inferred through observations of associations or interactions, which can introduce sampling bias and uncertainty to the data. Without appropriate care, these issues could lead us to make incorrect or overconfident conclusions about our data. In this thesis, we use an established network model, the multiple regression quadratic assignment procedure (MRQAP), and propose approaches to facilitate the application of this model in animal network studies. Through demonstrating these approaches on three animal systems, we make new biological findings and highlight the importance of considering data-sampling issues when analysing networks. Additionally, our approaches have wider applications to animal network studies where relationships are inferred through observing dyadic interactions.

<p>Heart rate variability (<b>HRV</b>), or the variation in time between adjacent heart beats over time, is a non-invasive proxy measure of autonomic nervous system (<b>ANS</b>) function that has been used regularly in studies focused on evaluating livestock stress and welfare. The autonomic nervous system regulates involuntary physiological processes (<i>e.g.</i> respiration and heart rate) and consists of two main components, the parasympathetic (<b>PNS</b>), and sympathetic (<b>SNS</b>) branches, which act to maintain bodily homeostasis (PNS) or stimulate the “fight-or-flight” response after exposure to a stressor (SNS). Traditional linear HRV measures provide an estimation of overall autonomic activity or changes to the balance between the PNS and SNS branches by evaluating changes to the mean, variance, or frequency spectra of the R-R intervals. </p><p>To interpret HRV data obtained via linear HRV measures, particularly spectral HRV analysis, a linear assumption has to be assumed where SNS and PNS activity act in a purely antagonistic manner. However, this assumption is not always met. In many cases, ANS activity is altered in a nonlinear manner, which is reflected to some degree in the variability of heart rate output. Therefore, HRV measures that evaluate nonlinear changes to organizational or structural aspects of the R-R interval variability may be a useful compliment to traditional linear HRV measures for distinguishing between stressed and non-stressed states. The purpose of this dissertation was to evaluate the use of nonlinear HRV measures for evaluating dairy calf disbudding pain, piglet castration pain, growing pig heat stress, and as potential indicators of the subsequent immune response to a lipopolysaccharide (<b>LPS</b>) challenge in growing pigs.</p><p>Chapter 1 provides a knowledge base for understanding HRV and its use as a measure of autonomic stress in studies with livestock species. A brief explanation of animal welfare science, measures used to evaluate an animal’s welfare, and a demonstration of need for non-invasive physiological measures is provided before discussing the physiological basis of HRV. Relevant linear and nonlinear HRV measures are explained and examples of their use in livestock stress research are provided. Finally, a rationale for the studies conducted in this dissertation is presented.</p><p>Chapter 2 evaluates the use of HRV as an indicator of castration pain in 9-d-old piglets over a 3-d experimental period. Compared to sham castrated piglets, surgically castrated piglets exhibited greater low frequency to high frequency ratios (<b>LF/HF</b>), reduced sample entropy (<b>SampEn</b>), and greater percent determinism (<b>ÞT</b>) during the post-castration period. However, postural behavior was not different between treatments and serum cortisol concentrations only tended to differ between treatments at 1 and 24 h post-castration treatment, with surgically castrated pigs having numerically greater serum cortisol concentrations at both timepoints. These results demonstrate the ability of nonlinear HRV measures (SampEn and ÞT) to complement the physiological interpretation of linear HRV measures (LF/HF) in response to castration. Specifically, pigs who were surgically castrated exhibited more regularity (SampEn) and periodicity (ÞT) in their HRV data, and potentially more sympathetic activity (LF/HF) compared to sham castrated piglets, indicating greater pain-related stress. Additionally, HRV was a more sensitive measure of the stress response to castration than readily identifiable behaviors such as posture and the serum cortisol response.</p><p>Chapter 3 evaluates the use of HRV as an indicator of disbudding pain in dairy heifer calves (4 to 7-wk of age) over a 5-d experimental period. Calves who were given lidocaine and meloxicam prior to disbudding exhibited lower mean R-R interval (<b>RR</b>) values and a greater short-term detrended fluctuation analysis scaling exponent (<b>DFAα₁</b>) than sham disbudded calves. Together, these results indicate that calves who received pain mitigation exhibited greater pain-related stress (RR) and reduced physiological complexity in their heart rate signal (DFAα₁). Calves who were disbudded without pain mitigation had an intermediate response compared to sham disbudded calves and calves provided lidocaine and meloxicam. However, their numerical values closely followed those of calves provided lidocaine and meloxicam. These results demonstrate the usefulness of nonlinear HRV measures (DFAα₁) for evaluating nonlinear and correlational aspects of physiological complexity in response to disbudding. Additionally, the HRV results suggest that the provision of meloxicam does not reduce the amount of pain-related stress experienced by calves following disbudding.</p><p>Chapter 4 evaluates the use of HRV as an indicator of heat stress in growing pigs exposed to an acute heat episode. Heat stressed pigs exhibited greater body temperatures and spent less time in an active position compared to thermoneutral control pigs. Additionally, heat stressed pigs displayed an altered nonlinear HRV response to the acute heat phase compared to non-heat stressed control pigs. Specifically, heat stressed pigs exhibited lower SampEn and tended to exhibit greater ÞT, but no alterations to linear measures were observed in response to the acute heat episode. The low frequency to high frequency ratio was higher in heat stressed pigs during the period following the acute heat phase. Therefore, nonlinear HRV measures (particularly SampEn) may be more sensitive to the immediate physiological stress response to increased environmental temperature than traditional linear HRV measures.</p><p>Chapter 5 evaluates the use of baseline HRV as a potential indicator of the subsequent cortisol and pro-inflammatory cytokine response to an LPS challenge in growing pigs. The time for a pig to approach a human (<b>approach time)</b> prior to LPS administration was inversely related to baseline standard deviation of the R-R intervals (<b>SDNN</b>), and directly related to RR and the mean length of diagonal lines in a recurrence plot (<b>Lmean</b>). This result may have implications for the use of HRV as a measure of temperament in livestock species, since pigs with lower baseline SDNN (<i>i.e.</i> greater stress) and greater baseline Lmean (<i>i.e.</i> increased periodicity length in HRV data; greater stress) values took longer to approach a human observer before LPS administration (which occurred 1 d after HRV measurement). Area under the curve values for approach time following LPS administration were inversely related to high frequency spectral power (<b>HF</b>) and directly related to body weight, where pigs with low baseline HF values (<i>i.e. </i>lower parasympathetic activity) and higher body weights were slower to approach a human observer following LPS administration. Additionally, pigs with greater Lmean values had a greater change in body temperature following LPS administration. In conclusion, while baseline HRV measures were not directly representative of the cortisol or cytokine response following an LPS challenge, HF and Lmean may be useful indicators for evaluating certain aspects (sickness behavior and fever) of the innate immune response to an LPS challenge. <b></b></p><p> In conclusion, these studies demonstrate the usefulness of nonlinear HRV measures for evaluating livestock stress. Measures such as sample entropy and those derived from recurrence quantification analysis (ÞT, Lmean) seem to be particularly useful for complementing traditional linear HRV measures and, in some cases, are more sensitive measures of the physiological stress response (see chapter 4). Therefore, their inclusion in future studies on livestock HRV is warranted. However, further work is needed to fully elucidate the physiological significance of nonlinear HRV measures and their response to stress.</p>