Monika Budzyńska
Behavioural and physiological mechanisms of reactions to stressful stimuli in Arab horses
Aim of work. The aim of this study was to establish the relationships between behavioural and physiological mechanisms of Arab horses’ response to stressful stimuli. The effect of genetic factors (8 sire groups) on changes of behavioural and physiological indicators during stressful situation was considered as well. 

Material and methods. In the years 2006–2008, the test results were collected using a total of 81 Purebred Arab mares maintained at one of the Polish State Arab breeding stud farms. Mares’ behavioural reactivity was assessed in two novel stimuli tests: fearfulness test and coloured mats’ alley-way test. Visual and auditory stimuli, used in the tests, were acting as a potential stressful source for studied animals. According to the index of behavioural reactivity (IRB), based on the both tests’ scores, the mares were classified into three groups: fearful (n = 14), medium-fearful (n = 48) and fearless (n = 19). HR level was recorded in particular stages of the study: before behavioural tests, during both tests and after the tests. For every horse, hormones (ACTH, cortisol) and haematological indicators (HCT, HGB, RBC, MCV, MCH, MCHC, WBC, LYM, NEU, MON, BAS, EOS, PLT) in the blood samples collected before tests and immediately after them, were measured. 
Results and discussion. The results showed that there was a relationship between behavioural and physiological mechanisms in horses exposed to stressful stimuli. Both tests were useful to provoke stress response in Arab mares. Fearful horses were characterized by predominance of sympathetic-adrenomedullary system (SAS) and they showed higher behavioural activity and HR but lower ACTH increase in the response to the stressful stimuli. On the other hand, fearless horses reacted by higher reactivity of the hypothalamus-pituitary-adrenal (HPA) axis and showed lower behavioural activity and HR but significantly higher ACTH increase during stress-induced response. Significant relations between horses’ behavioural and physiological mechanisms indicate that behavioural parameters recorded during fearfulness test and coloured mats’ alley-way test, as well as hormones (ACTH, cortisol) and HR, can be used to assess reactivity level to stressful stimuli. The identification of relations and changes in some haematological indicators (HGB, HCT, RBC, MCV, WBC, LYM, BAS, EOS, PLT) can provide a complementary method of stress assessment in horses. Moreover, it was stated that genetic factors, based on mares’ origin from eight sire groups, can also effect the level of behavioural and physiological reactions to stressors. 

Conclusions. The present work revealed that fearful horses tended to represent the proactive style (“active copers”) as their coping strategy, mainly showing behavioural response, whereas fearless horses tended to demonstrate a reactive style (“passive copers”), i.e. reacting passively to stressful stimuli and showing higher HPA-axis reactivity. The changes of behavioural and physiological measures indicate their usefulness as reliable markers of stress reaction’s assessment in horses considering coping response type according to predominance of the SAS or the HPA axis. Measures of fearfulness, heart rate, hormones and the other underlying characteristics can be used to predict horses’ strategies to react in stressful situation and to identify selection criteria for breeding programmes.