Neural modulators of temperament: A multivariate approach to personality trait identification in the horse

An Observational Study of Wood-Chewing in Mares and Their Foals Kept in Fenced Outdoor Paddocks with Variable Browsing Access

The natural behaviour of wood-chewing (WC) in horses may, under certain conditions, become excessive or it may precede crib-biting . However, the potential causes of WC, including frustrated foraging motivation) or developmental exploratory or teething behavior are poorly understood. We studied WC in 31 mares and their foals, examining associations with browsing access, stocking density, foal sex and performance of other non-nutritive oral behaviors. Behaviors (counts/hour) were recorded by all-occurrences sampling during focal, continuous observations of grazing,resting and suckling for 20  ×  10 min periods, for three days when foals were 3-5 months of age and kept at pasture with their mares. 17/31 mares and 28/31 foals exhibited WC, with foals performing more WC than mares (Mdn (IQR): foals: 0.95 (0.42,2.26); mares: 0.01 (0,0.19)). Colts exhibited significantly more WC than fillies (F1,13.07 = 4.855, p = 0.046), possibly because of sex-based differences in exploratory tendencies. The amount of browsing access had a differential effect on mares and foals (F1,40.29 = 3.720, p = 0.033). However, the amount of browsing access was partially confounded with foal age. Overall, we suggest that WC is associated more with developmental exploration than frustrated foraging access. Experimental work is needed to confirm our findings.

Spontaneous eye blinks in horses (Equus caballus) are modulated by attention

Spontaneous eye blinks are brief closures of both eyelids. The spontaneous eye blink rate (SEBR) exceeds physiological corneal needs and is modulated by emotions and cognitive states, including vigilance and attention, in humans. In several animal species, the SEBR is modulated by stress and antipredator vigilance, which may limit the loss of visual information due to spontaneous eye closing. Here, we investigated whether the SEBR is modulated by attention in the domestic horse (Equus caballus). Our data supported previous studies indicating a tonic SEBR specific to each individual. We also found that, superimposed on a tonic SEBR, phasic changes were induced by cognitive processing. Attention downmodulated the SEBR, with the magnitude of blink inhibition proportional to the degree of attentional selectivity. On the other hand, reward anticipation upregulated the SEBR. Our data also suggested that horses possess the cognitive property of object permanence: they understand that an object that is no longer in their visual field has not ceased to exist. In conclusion, our results suggested that spontaneous eye blinks in horses are modulated by attentional cognitive processing.

Assessing the Peripheral Levels of the Neurotransmitters Noradrenaline, Dopamine and Serotonin and the Oxidant/Antioxidant Equilibrium in Circus Horses

Due to the paucity of information on circus management effects on the welfare of horses, this study investigated the plasma concentrations of noradrenaline, dopamine and serotonin, known to be indices of mental status, as well as the reactive oxygen metabolites (d-Roms) and the biological antioxidant potential (BAP), likely to denote the oxidant/antioxidant equilibrium of organisms, in horses managed in different Italian circuses. For the study, 56 circus horses of different breeds and ages were enrolled and divided into six groups according to the horses' management (circus management, groups G1-G5; classic riding management representing the control group, CG). From each horse, blood samples were collected in order to assess the concentration of selected parameters. One-way ANOVA showed no differences (p > 0.05) in serotonin, dopamine, noradrenaline, d-Roms and BAP values between circus and control horses. No differences related to the breed of the horses enrolled in the study were found in the values of all investigated parameters (p > 0.05). Furthermore, neurotransmitters showed overlapping levels between the different age classes of investigated horses (p > 0.05); contrariwise, the age of the horse displayed a significant effect on BAP values, with the oldest horses (16-21 age class) exhibiting lower BAP values compared to 4-5, 6-10 and 11-15 age classes (p < 0.05), whereas the d-Roms showed similar values in horses of different age classes (p > 0.05). The results gathered in the present study suggest that the mental status of horses under circus management was not compromised; however, better attention and care in the management of older horses is advocated, as they showed a lower biological antioxidant potential than younger horses; thus, they could be more susceptible to oxidative stress.

Does social motivation mitigate fear caused by a sudden sound in horses?

Living in a herd has multiple advantages for social species and is a primary survival strategy for prey. The presence of conspecifics, identified as a social buffer, may mitigate the individual stress response. Social isolation is, therefore, particularly stressful for horses, which are gregarious animals. However, they are not equally vulnerable to separation from the group. We tested whether more and less socially dependent horses and independent individuals would differ in their responses to novel and sudden sounds occurring in two contexts: non-social and social motivation. Twenty warmblood horses were first exposed to two social tests: to evaluate the level of social dependence (rate of restless behaviour; social isolation) and the quantity and the quality of interactions in which they were involved (stay on a paddock). Two fear audio tests were then performed to compare the responses to sudden sounds while feeding (non-social motivation; control trial) and while moving towards the herd (social motivation; experimental trial). Socially dependent horses showed more pronounced avoidance behaviour and needed much more time to resume feeding during the control trial. Hence, dependent individuals appeared to be more fearful. However, during an experimental trial, horses of both groups tended to ignore the sound or paid only limited attention to the stimulus, continuing to move forward towards their conspecifics. Thus, social motivation may mitigate fear caused by a frightening stimulus and make fearful and dependent horses more prone to face a potentially stressful event. This finding should be taken into account in horse training and management.

Horse behavior and facial movements in relation to food rewards

Food rewards are believed to have a positive valence in horses. The aim of this study was to assess the effect of food rewards on horse behavior before entering a horse chute, and behavior and facial movements while restrained in it. Thirteen female adult horses were brought once daily to an animal handling facility for three weeks. In week 1, baseline period, no reinforcement was applied. In weeks 2 and 3, experimental phase, half of the horses received positive reinforcement treatment after entering and remaining in the chute; the remaining horses were considered as controls (no positive reinforcement applied). There was a cross-over between the groups during the experimental phase. Horses were individually brought to the restraining chute and videos recorded during 60-sec. The duration and number of entries into the area close to the gate leading to the chute were measured before restraining and body posture, neck position, and tail swinging were recorded in the chute. Facial movements were also recorded and scored using EquiFACS methodology. Multilevel linear and logistic models were built to assess behavioral changes from baseline to the treatment phase and between phases (control and positively reinforced). Horses did not change their body posture or tail swings across the different phases (P > 0.1) and were less likely to show lowered neck during the positively reinforced phase (OR: 0.05; CI95%: 0.00-0.56; P = 0.05) compared to baseline. The likelihood of a lowered neck did not differ between the positive reinforcement and control phases (P = 0.11). In the positively reinforced phase, horses seemed to be more attentive (ears forward) and active (less eye closures, more nose movements) than in the control phase. A three-day positive reinforcement phase did not elicit major changes in body behavior in the chute but affected the facial movements of group-housed mares.

Relationship between plasma dopamine concentration and temperament in horses

Dopamine (DA) is a neurotransmitter associated with animal behaviors. Along with other neurotransmitters such as oxytocin (OXT) and serotonin (5-HT), DA is also involved in determining the temperament of animals. However, the involvement of DA in horse temperament has not been well elucidated. Therefore, in this study, we aimed to determine the correlation between plasma DA concentration and OXT and 5-HT concentrations and behavioral temperament (eg, docility and friendliness, fearfulness, dominance, and trainability) of horses. Blood samples were collected from 31 horses and the concentrations of DA, OXT, and 5-HT were measured using enzyme-linked immunosorbent assay. The temperament of horses was assessed and scored by 3 researchers. The correlation between the plasma concentration of DA and OXT or 5-HT was statistically analyzed using SPSS software and linear regression analysis was performed to determine the association between DA concentration and OXT and 5-HT concentrations. Meanwhile, the DA concentration associated with each type of temperament was analyzed via one-way analysis of variance with LSD post hoc analysis as well as Student's t-test (for trainability). Plasma DA concentration was not found to be correlated with either OXT or 5-HT concentrations. Furthermore, we found no correlation between plasma DA concentration and dominance and trainability. However, our results suggest the possibility of predicting the degree of fearfulness of horses using plasma DA concentrations. We conclude that plasma DA concentration has a potentiality to be used as a biomarker to predict the fearfulness of horses.

Identification of Personality-Related Candidate Genes in Thoroughbred Racehorses Using a Bioinformatics-Based Approach Involving Functionally Annotated Human Genes

Considering the personality traits of racehorses (e.g., flightiness, anxiety, and affability) is considered essential to improve training efficiency and decrease accident frequency, especially when retraining for a second career that may involve contact with inexperienced personnel after retiring from racing. Studies on human personality-related genes are frequently conducted; however, such studies are rare in horses because a consistent methodology for personality evaluation is lacking. Using the recently published whole genome variant database of 101 Thoroughbred horses, we compared horse genes orthologous to human genes related to the Big Five personality traits, and identified 18 personality-related candidate genes in horses. These genes include 55 variants that involve non-synonymous substitutions that highly impact the encoded protein. Moreover, we evaluated the allele frequencies and functional impact on the proteins in terms of the difference in molecular weights and hydrophobicity levels between reference and altered amino acids. We identified 15 newly discovered genes that may affect equine personality, but their associations with personality are still unclear. Although more studies are required to compare genetic and behavioral information to validate this approach, it may be useful under limited conditions for personality evaluation.

Personality traits affect learning performance in dwarf goats (Capra hircus)

A wide range of species exhibit time- and context-consistent interindividual variation in a number of specific behaviors related to an individual's personality. Several studies have shown that individual differences in personality-associated behavioral traits have an impact on cognitive abilities. The aim of this study was to investigate the relationship between personality traits and learning abilities in dwarf goats. The behavior of 95 goats during a repeated open field (OF) and novel object test (NO) was analyzed, and two main components were identified using principal component analysis: boldness and activity. In parallel, the goats learned a 4-choice visual initial discrimination task (ID) and three subsequent reversal learning (RL) tasks. The number of animals that reached the learning criterion and the number of trials needed (TTC) in each task were calculated. Our results show that goats with the lowest learning performance in ID needed more TTC in RL1 and reached the learning criterion less frequently in RL2 and RL3 compared to animals with better learning performance in ID. This suggests a close relationship between initial learning and flexibility in learning behavior. To study the link between personality and learning, we conducted two analyses, one using only data from the first OF- and NO-test (momentary personality traits), while the other included both tests integrating only animals that were stable for their specific trait (stable personality traits). No relationship between personality and learning was found using data from only the first OF- and NO-test. However, stability in the trait boldness was found to have an effect on learning. Unbold goats outperformed bold goats in RL1. This finding supports the general hypothesis that bold animals tend to develop routines and show less flexibility in the context of learning than unbold individuals. Understanding how individual personality traits can affect cognitive abilities will help us gain insight into mechanisms that can constrain cognitive processing and adaptive behavioral responses.

The impulsive horse: comparing genetic, physiological and behavioral indicators to that of human addiction

Stress and genotype elicit changes in impulse control in a range of species that are attributable to adaptations in both the central and peripheral nervous system. We examined aspects of this mechanism in the horse by assessing the effect of a dopamine receptor genotype (DRD4) and central dopaminergic tone (measured via spontaneous blink rate [SBR] and behavioral initiation rate [BIR]), on measures of impulsivity, compulsivity (3-choice serial reaction time task) and sympathetic/ parasympathetic system balance (heart rate variability [HRV]). Genotype did not have a significant effect on any of the parameters measured. SBR but not BIR correlated significantly with levels of impulsivity. There was no clear association of HRV parameters with either measures of central dopaminergic activity or impulsivity/compulsivity. Overall, some elements of the data suggest that the horse may be a useful animal model for assessing the genetic and environmental factors that lead to the physiological and behavioral phenotype of human addiction, particularly when considering the relationship between central dopaminergic tone and impulsivity.

Effect of transportation and social isolation on facial expressions of healthy horses

Horses have the ability to generate a remarkable repertoire of facial expressions, some of which have been linked to the affective component of pain. This study describes the facial expressions in healthy horses free of pain before and during transportation and social isolation, which are putatively stressful but ordinary management procedures. Transportation was performed in 28 horses by subjecting them to short-term road transport in a horse trailer. A subgroup (n = 10) of these horses was also subjected to short-term social isolation. During all procedures, a body-mounted, remote-controlled heart rate monitor provided continuous heart rate measurements. The horses' heads were video-recorded during the interventions. An exhaustive dataset was generated from the selected video clips of all possible facial action units and action descriptors, time of emergency, duration, and frequency according to the Equine Facial Action Coding System (EquiFACS). Heart rate increased during both interventions (p<0.01), confirming that they caused disruption in sympato-vagal balance. Using the current method for ascribing certain action units (AUs) to specific emotional states in humans and a novel data-driven co-occurrence method, the following facial traits were observed during both interventions: eye white increase (p<0.001), nostril dilator (p<0.001), upper eyelid raiser (p<0.001), inner brow raiser (p = 0.042), tongue show (p<0.001). Increases in 'ear flicker' (p<0.001) and blink frequency (p<0.001) were also seen. These facial actions were used to train a machine-learning classifier to discriminate between the high-arousal interventions and calm horses, which achieved at most 79% accuracy. Most facial features identified correspond well with previous findings on behaviors of stressed horses, for example flared nostrils, repetitive mouth behaviors, increased eye white, tongue show, and ear movements. Several features identified in this study of pain-free horses, such as dilated nostrils, eye white increase, and inner brow raiser, are used as indicators of pain in some face-based pain assessment tools. In order to increase performance parameters in pain assessment tools, the relations between facial expressions of stress and pain should be studied further.

Behavioral and cardiac responses in mature horses exposed to a novel object

This study aimed to investigate whether breed, sex, and age affected temperament differently (more or less neophobic) in mature horses during a novel object test. The study included Jeju crossbred (n = 12, age = 9.42 ± 4.57 y), Thoroughbred (n = 15, age = 10.73 ± 3.09 y), and Warmblood horses (n = 12, age = 13.08 ± 3.55 y) with the females (n = 22, age = 11.36 ± 4.24 y) and geldings (n = 17, age = 10.65 ± 3.66 y). Jeju crossbreds (Jeju horse × Thoroughbred) are valuable considering their popular usage in Korea, but limited studies have explored temperament of Jeju crossbred horses. A trained experimenter touched the left side of the neck with a white plastic bag (novel object). The test ended when the horse stopped escape response and heart rate (HR) dropped to baseline. Behavioral score and escape duration were measured as behavioral variables. Multiple variables related to HR and heart rate variability (HRV) were measured to reflect emotional state. These included basal HR (BHR), maximum HR (MHR), delay to reach maximum heart rate (Time to MHR), standard deviation of beat-to-beat intervals (SDNN), root mean square of successive differences (RMSSD), and ratio of low to high frequency components of a continuous series of heartbeats (LF/HF). Statistics revealed that Thoroughbreds had significantly higher behavioral scores, and lower RMSSD than Jeju crossbreds (p < 0.05), suggesting greater excitement and fear to the novel object in Thoroughbreds. None of the behavioral or cardiac parameters exhibited sex differences (p < 0.05). Age was negatively correlated with SDNN and RMSSD (p < 0.05), indicating that older horses felt more anxiety to the novelty than younger horses. Thoroughbreds and females had distinct correlations between behavioral and HRV variables in comparison with other groups (p < 0.05), implying that escape duration might be a good indicator of stress, especially in these two groups. These results are expected to improve equine welfare, safety and utility, by providing insights into the temperament of particular horse groups, to better match reactivity levels with specific functions.

Blink rate as a measure of stress and attention in the domestic horse (Equus caballus)

Measuring animal stress is fundamentally important for assessing animal emotional state and welfare. Conventional methods of quantifying stress (cortisol levels, heart rate/heart rate variability) require specialist equipment and are not instantly available. Spontaneous blink rate (SBR) has previously been used to measure stress responses in humans and may provide a non-invasive method for measuring stress in other animal species. Here we investigated the use of SBR as a measure of stress in the domestic horse. SBR was measured before and during a low-stress event (sham clipping) and compared with heart rate variability and salivary cortisol. For the entire sample, there was a reduction in SBR (startle response) during the first minute of clipping. For horses reactive to clipping, the initial reduction in SBR was followed by an increase above baseline whereas the SBR of the non-reactive horses quickly returned to baseline. For the entire sample, SBR correlated with heart rate variability and salivary cortisol. We have demonstrated that SBR is a valid fast alternative measure of stress in horses, but the initial 'startle' response must be considered when using this parameter as a measure of animal stress.

Reported Behavioural Differences between Geldings and Mares Challenge Sex-Driven Stereotypes in Ridden Equine Behaviour

Simple Summary

It has been shown that people within the horse industry have preconceived ideas about horse behaviour, temperament and rideability, based solely on the sex of the horse. Such ideas can have welfare implications, if personnel allow bias to affect their interactions with particular horses. Such welfare implications include employment of harsher training methods, and increased horse wastage. The current study explored data on riders’ and trainers’ reports of ridden horse behaviour. Reported sex-related behavioural differences were evaluated based on 1233 responses from the pilot study of the Equine Behaviour and Research Questionnaire (E-BARQ) survey. Results from the study suggest there are some sex-related differences in behaviour between male and female horses; geldings are more likely to chew on rugs and lead ropes when tied, and mares are more likely to move away when being caught in paddock. However, there was no evidence of sex-related differences associated with behaviour when ridden which may warrant further investigation. Findings from this study may be used to educate riders and trainers about the need to regard behaviour and motivation in ridden horses as sex-neutral.

Abstract

Horse trainers and riders may have preconceived ideas of horse temperament based solely on the sex of the horse. A study (n = 1233) of horse enthusiasts (75% of whom had more than 8 years of riding experience) revealed that riders prefer geldings over mares and stallions. While these data may reflect different sex preferences in horses used for sport, they may also reduce the chances of some horses reaching their performance potential. Further, an unfounded sex prejudice is likely to contribute to unconscious bias when perceiving unwanted behaviours, simplistically attributing them to demographic characteristics rather than more complex legacies of training and prior learning. The current study analysed reported sex-related behavioural differences in ridden and non-ridden horses using data from responses to the pilot study of the Equine Behaviour Assessment and Research Questionnaire (E-BARQ) survey. Respondents (n = 1233) reported on the behaviour of their horse using a 151-item questionnaire. Data were searched for responses relating to mares and geldings, and 110 traits with the greatest percentage difference scores between mares and geldings were selected were tested for univariate significance at p < 0.2. Multivariable modelling of the effect of sex (mare or gelding) on remaining traits was assessed by ordinal logistic regression, using a cumulative proportional log odds model. Results revealed mares were significantly more likely to move away when being caught compared to geldings (p = 0.003). Geldings were significantly more likely to chew on lead ropes when tied (p = 0.003) and to chew on rugs (p = 0.024). However, despite sex-related differences in these non-ridden behaviours, there was no evidence of any significant sex-related differences in the behaviours of the horses when ridden. This finding suggests that ridden horse behaviour is not sexually dimorphic or that particular horse sports variously favour one sex over another.

Eye Blink Rates and Eyelid Twitches as a Non-Invasive Measure of Stress in the Domestic Horse

Simple Summary

Eye blink rate has been used as an indicator of stress in humans and, due to its non-invasive nature, could be useful to measure stress in horses. Horses exhibit both full and half blinks as well as eyelid twitches. We exposed 33 horses to stressful situations such as separation from herdmates, denied access to feed and sudden introduction of a novel object, and determined that full and half eye blinks decrease in these situations. Feed restriction was the most stressful for the horse as indicated by increased heart rate, restless behaviour and high head position. The decrease in eye blink rate during feed restriction was paralleled with an increase in eyelid twitches. There was no increase in eyelid twitches or heart rate with the other treatments indicating that the horses did not find these overly stressful, but they did focus their attention more during these situations. Observation of eye blinks and eyelid twitches can provide important information on the stress level of horses with a decrease in eye blinks and an increase in eyelid twitches in stressful environments.

Abstract

Physiological changes provide indices of stress responses, however, behavioural measures may be easier to determine. Spontaneous eye blink rate has potential as a non-invasive indicator of stress. Eyelid movements, along with heart rate (HR) and behaviour, from 33 horses were evaluated over four treatments: (1) control—horse in its normal paddock environment; (2) feed restriction—feed was withheld at regular feeding time; (3) separation—horse was removed from visual contact with their paddock mates; and (4) startle test—a ball was suddenly thrown on the ground in front of the horse. HR data was collected every five s throughout each three min test. Eyelid movements and behaviours were retrospectively determined from video recordings. A generalized linear mixed model (GLIMMIX) procedure with Sidak’s multiple comparisons of least squares means demonstrated that both full blinks (16 ± 12^b vs. 15 ± 15^b vs. 13 ± 11^b vs. 26 ± 20^a full blinks/3 min ± SEM; a,b differ p < 0.006) and half blinks (34 ± 15^ab vs. 27 ± 14^bc vs. 25 ± 13^c vs. 42 ± 22^a half blinks/3 min ± SEM; a,b,c differ p < 0.0001) decreased during feed restriction, separation and the startle test compared to the control, respectively. Eyelid twitches occurred more frequently in feed restriction (p < 0.0001) along with an increased HR (p < 0.0001). This study demonstrates that spontaneous blink rate decreases while eyelid twitches increase when the horse experiences a stressful situation.

Elevated Sensitivity to Tactile Stimuli in Stereotypic Horses

Although stereotypic behaviors are a common problem in captive animals, why certain individuals are more prone to develop them remains elusive. In horses, individuals show considerable differences in how they perceive and react to external events, suggesting that this may partially account for the emergence of stereotypies in this species. In this study, we focused on crib-biting, the most common stereotypy displayed by horses. We compared how established crib-biters (“CB” = 19) and normal controls (“C” = 18) differed in response to a standard “personality” assessment test battery, i.e., reactivity to humans, tactile sensitivity, social reactivity, locomotor activity, and curiosity vs. fearfulness (both in novel and suddenness situations). Our analyses showed that crib-biters only differed from control horses in their tactile sensitivity, suggesting an elevated sensitivity to tactile stimuli. We suggest that this higher tactile sensitivity could be due to altered dopamine or endogenous opioid physiology, resulting from chronic stress exposition. We discuss these findings in relation to the hypothesis that there may be a genetic predisposition for stereotypic behavior in horses, and in relation to current animal husbandry and management practices.

Equine Personality: Association With Breed, Use, and Husbandry Factors

Temperament can be defined as innate properties of the nervous system, whereas personality includes the complex behavioral traits acquired through life. Association between personality and behavior is important for breeding, selection, and training of horses. For the first time, we evaluated if equine personality components previously identified in Japan and Europe were consistent when applied to American horses. We examined the association of personality with breed, age, sex, management, training, stereotypies, and misbehaviors. The owner directed personality survey consisted of 25 questions. An online version of the survey was created. The principal component analysis method was used to associate behavioral traits with personality components. Factor analysis with orthogonal transformation was performed on scores for personality-related questions. A total of 847 survey responses were used. Quarter Horses, "other" breed, and Thoroughbred were the most common breeds. Three principal personality components were extracted as each behavioral trait belonged to one of these three components. Arabians, Thoroughbreds, Saddlebreds, and Walking horses were the most nervous and Quarter Horses, and Paints, Appaloosas, and Drafts were the least nervous. No trained discipline was significantly associated with any personality component. There were no significant associations between stereotypies and misbehaviors and nervous or curious personality. For the first time in predominantly American horses, we have evaluated personality components and their association with breed, age, sex, training discipline, and stereotypies. We refute links between personality and trained discipline and confirm the lack of association between nervous personality and stereotypies and misbehaviors.

Personality Research in Mammalian Farm Animals: Concepts, Measures, and Relationship to Welfare

Measuring and understanding personality in animals is a rising scientific field. Much research has been conducted to assess distinctive individual differences in behavior in a large number of species in the past few decades, and increasing numbers of studies include farm animals. Nevertheless, the terminology and definitions used in this broad scientific field are often confusing because different concepts and methods are used to explain often synonymously applied terms, such as personality, temperament and coping style. In the present review we give a comprehensive overview of the concepts and terms currently used in animal personality research and critically reveal how they are defined and what they measure. First, we shortly introduce concepts describing human personality and how these concepts are used to explain animal personality. Second, we present which concepts, methods and measures are applied in farm animal personality research and show that the terminology used seems to be somehow species-related. Finally, we discuss some findings on the possible impact of personality on the welfare of farm animals. The assessment of personality in farm animals is of growing scientific and practical interest. Differences in theoretical frameworks and methodological approaches may also entail the diverse use of the different concepts between basic and applied research approaches. We conclude that more consistency is needed in using different theoretical concepts, terms and measures, especially in farm animal personality research. The terms coping style and temperament, which are used in different ways, should not be examined as independent concepts, but rather should be considered as different aspects of the whole personality concept. Farm animal personality should be increasingly considered for the improvement of animal housing, management, breeding and welfare.

Effect of transportation on the sympatho-adrenal system responses in horses

The objective of current study was to evaluate the effect of transportation stress on the circulating adrenaline (A), noradrenaline (NA) and dopamine (DA) responses of stallions, according to the different distances. Forty-two stallions were studied before and after road transportation of 100, 200 and 300km, for a period of 1-3h. An increase in plasma A after 100km (P<0.001) was observed. A similar result was seen in plasma NA after 100km (P<0.001), and 300km (P<0.001). Increases in plasma DA after 100 and 200km (P<0.0001) were observed, with a decrease after 300km (P<0.0001). Significant interactions among groups and times for A (P<0.0001), NA (P<0.0006) and DA (P<0.0001) changes were observed. These results indicate that the sympatho-adrenal system response of horses was greater after short (100km), than medium-longer period of transportation (200 and 300km). This may indicate an adaptation process during transport, considering A and NA as the primary candidate neurotransmitters for the maintenance of homeostatic process to alleviate the effects of the perceived transportation stress.