W.G.P. Schouten

Individual behavioural characteristics in pigs

Abstract In two identical experiments a total of 218 piglets from 20 sows were used to test if consistent individual behavioural differences exist among pigs. At an age of 1–2 weeks piglets were divided between aggressive and non-aggressive individuals on the basis of their behaviour in two successive social confrontation tests (SC1 and SC2). Substantial agreement in this classification existed between the two observers and between SC1 and SC2. No significant sex and litter effects were found in the occurrences of aggressive behaviour. After mixing at 10 weeks and again at 15 weeks of age, aggressive behaviour was mainly shown by the aggressive individuals as classified in the social confrontation tests. In a non-social backtest piglets, restrained in a supine position, were classified as resistant (R; more than two escape attempts), intermediate (I; two escape attempts) or non-resistant (NR; less than two escape attempts). Based on the outcome of five successive backtests in Weeks 1, 2 and 3 piglets were classified eventually as R ( n =95), NR ( n =77) or Doubtful (D; n =46). R piglets had a shorter latency to first resistance, but a higher number of vocalizations than NR ones, while the behaviour of the D piglets was in between. Piglets classified as R in the backtest were mostly the aggressive individuals, while NR piglets were mostly the non-aggressive ones; D piglets were equally distributed over aggressive and non-aggressive individuals. This association in behaviour and its consistency over time strongly suggests the existence of behavioural strategies to cope with conflict situations that are typical of individual pigs and are measurable already in the very first weeks of their life.

Individual behavioral and physiological strategies in pigs.

Previous experiments demonstrated consistent individual behavioral differences in pigs. Some showed a more active behavioral response (so-called A/R pigs), others a more passive behavioral response (so-called NA/NR pigs). In the present study we selected 32 A/R and 32 NA/NR individuals and tested them individually in an open field at 3 (OF1) and 8 weeks of age (OF2). Individual response patterns were remarkably consistent between OF1 and OF2. While more A/R than NA/NR pigs made escape attempts, the A/R ones vocalized less, and were less inhibited to approach novel objects in OF1 and OF2, although they spent less time in exploring these objects than NA/NR pigs. Cortisol (CS) level after OF1 increased in A/R pigs but did not change in NA/NR ones, while CS level in OF2 remained constant in A/R pigs but decreased in NA/NR pigs. CS response to ACTH1-39 was measured at 3 and 8 weeks of age but did not differ between types. Basal CS level was higher in NA/NR than in A/R pigs and accompanied by adrenal hypertrophy. Mean heart rate (HR) was higher of A/R pigs compared to NA/NR ones in two backtests. HR of A/R pigs substantially increased (23.9 bpm = 15.5%) in reaction to the novel object in OF2, while HR of NA/NR ones only slightly increased (4.5 bpm = 2.9%), or even decreased (bradycardia). A/R pigs had more often heart deviations than NA/NR ones. The present study demonstrates that the two behavioral strategies of pigs are characterized by consistent differences in behavioral, physiological, and endocrine responses to conflict situations.

Can we predict behaviour in pigs?: Searching for consistency in behaviour over time and across situations

Individual differences in animal behaviour could elucidate the differences in stress coping style, which have consequences for production, health and welfare. Therefore, individual behavioural differences in pigs and consistency of responses in different test situations were studied. If differences in behaviour reflect coping characteristics, then behaviour in one situation should predict behavioural reactions in other situations and at other times. In this study, a backtest was performed on 315 Great Yorkshire∗ Dutch Landrace piglets at 3, 10 and 17 days of age. On day 3, groups of approximately 10 piglets per sow were formed, based on escape behaviour in the first backtest (backtest score): high resisting (HR, all scores >3), low resisting (LR, all scores <3), miscellaneous (MISC, various scores between 0 and 10) or original (OR) litters to determine if group composition would influence coping behaviour. In weeks 5–7 and/or 10–12, a human approach test (HAT), a novel object test (NOT), and an open door test (ODT) were performed with all pigs simultaneously, in the home pen. Pearson correlation coefficients were calculated between the test results and a factor analysis was performed. Furthermore, data were analysed on pen level, and within MISC- and OR-pens on animal level, using multivariate linear models. Significant correlations were found between the backtests and between HAT, NOT and ODT. Backtest results on three ages loaded on the same factor, and HAT, NOT and ODT at one age also loaded on one factor. No differences were found in HAT, NOT and ODT for the different pens (HR, LR, MISC and OR). On animal level, animals with higher backtest scores also had higher HAT scores at 5–7 weeks (P<0.05) within the MISC-pens. At 10–12 weeks, no differences were found. This study suggests that there are consistencies in behaviour of pigs over time and across situations, so coping can be regarded as a trait variable. However, since correlations are well below one, we suggest that other factors such as time (development) and (test) situation may also play an important role in determining an individual’s behavioural reaction. The absence of correlations between backtest and the group tests is explained by the theory that these different tests measure different aspects of the coping style.

Individual differences in behavioral and physiological responses to restraint stress in pigs

Several recent studies on pigs have demonstrated a relationship between the degree of resistance displayed early in life in a so-called backtest and a variety of behavioural and physiological responses in piglets and young fattening pigs. To study whether pigs with diverging responses in the backtest, i.e., high-resisting (HR) and low-resisting (LR) pigs, differ also in adulthood in their responses to an acute stressor, adult nulliparous HR (n=36) and LR gilts (n=36) housed in groups or stalls were challenged by 5-min fixation with a nose sling. During the first minute of restraint, HR gilts vocalised significantly more than LR gilts. Over the whole 5-min period, HR gilts tended to vocalise more than LR gilts. Housing or backtest type did not affect immediate cortisol increase (Delta(-5 min; 15 min)). At t=45 min, cortisol concentrations in HR gilts but not in LR gilts were still higher than at t=-5 min. Heart rate quickly decreased during the first min of restraint and remained fairly constant thereafter. Estimated heart rate after 5 min of nose sling was significantly lower in HR gilts compared to LR gilts. Housing or backtest type did not affect heart rate variability indices. The results support the idea that the backtest relates to individual characteristics, which at an adult age also seem to play a role in the regulation of certain behavioral and physiological responses to short-term stress.

Stereotypic behavior, endogenous opioids, and postfeeding hypoalgesia in pigs ☆

Tethered sows, some of which performed marked behavioral stereotypies after feeding, were injected IM with 1 mg/kg of naloxone 30 min before feeding (with 2 saline control days). Tail-flick latencies on a pain-sensitivity test were recorded before and after feeding. On control days, tail-flick latencies after feeding were longer than those before feeding, and this effect was abolished by naloxone pretreatment. Thus, there is an opioid-based hypoalgesia after feeding. However, sows with marked behavioral stereotypies had shorter tail-flick latencies after feeding. Thus, we have no evidence that performance of behavioral stereotypies results in increased opioid activity. Naloxone reduced the time spent in behavioral stereotypies by approximately 30% but this may be due to a reduction in time spent active. Naloxone increased the frequency and reduced the mean duration of bouts of chain manipulating, operating the drinker and rooting. We suggest that endogenous opioids are involved in the positive feedback that maintains the persistence of behavior and inhibits switching between different activities.

The effects of mixing on behavior and circadian parameters of salivary cortisol in pigs.

The present study describes an experiment that was carried out to study the effects of mixing pigs once at 25 kg, preceded by transportation for 1.5 h, on the behavior and the circadian rhythmicity of salivary cortisol. The frequency of agonistic interactions was higher for mixed pigs. This was not only the case immediately after mixing, when pigs started to fight to establish a new social rank (p < 0.05), but also 5 to 6 weeks later; still more headknocks and bites towards other pigs were seen at that time among mixed pigs (p < 0.01). However, neither the basal cortisol concentration, assessed as the MESOR of the circadian rhythm of salivary cortisol, nor the amplitude of that rhythm was different between the groups.

Stereotypic behavior and heart rate in pigs

Heart rate responses to feeding of both loose-housed and tethered sows, some of which engaged in persistent behavioral stereotypies, were recorded telemetrically in order to determine if stereotypic behavior occurs in response to, and serves to reduce increased sympathetic nervous activity. Heart rates rose in response to feeding (after which stereotypic behavior is most common) and this was greater for tethered sows. This response was partly blocked by a beta-adrenoreceptor antagonist (carazolol), but not by naloxone, although the latter tended to lead to overall increases in heart rate. Thus long-term tethering of sows leads to greater sympathetic nervous responses to feeding. No consistent relationships were found between heart rate and the performance of stereotypic behavior. In Experiment 1, using sows tethered for 6-8 months, neither basal heart rates nor heart rates following naloxone or carazolol differed between high- and low-stereotyping sows. In Experiment 2, using sows tethered for 1-2 months, high-stereotyping sows had lower basal heart rates than low-stereotyping sows. Marked reductions in heart rate caused by a beta-adrenoreceptor blocker did not lead to any change in stereotypic behavior, and preventing stereotypic behavior led to a reduction not an increase in heart rates. The results suggest that stereotypies are performed in situations where heart rate is high, but they provide no evidence that stereotypies reduce this heart rate.

Relationship between plasma cortisol and stereotypic activities in pigs.

The relationship between the performance of post-feeding stereotypic behaviour and plasma cortisol levels and the effect of prevention of stereotypic behaviour on plasma cortisol levels were studied in chronically restrained sows. In experiment 1 sows classified as high or low stereotypers on the basis of time spent manipulating chain and behaviour directed to the nipple drinker were found not to differ in plasma cortisol levels. Removing the chain and nipple drinkers also did not affect cortisol concentrations of either group. In experiment 2 detailed recordings were made by an automatic logging system of two major categories of stereotypy; chain manipulation and drinking. There were large individual differences in levels of both chain manipulation and drinking. Both activities showed a similar diurnal pattern, but average levels across individuals were not correlated. Both activities showed a continuous distribution across individuals and therefore a correlational rather than factorial analysis was used. Chain manipulation was not correlated to cortisol concentrations on any of the control days and removing the chain for one day (experiment 2A) or for seven days (experiment 2B) was also not accompanied by increased cortisol levels. However individuals showed a consistent cortisol response to chain removal as indicated by correlations between treatment days. Drinking showed a tendency to be negatively correlated to cortisol concentrations on control days. Long-term chain removal was accompanied by an increase in average drinking that showed a non-significant tendency to be negatively correlated to plasma cortisol on treatment days and thus may explain part of the consistency of response to chain removal. The lack of correlation between chain manipulation and plasma cortisol suggests that this activity does not serve to reduce plasma cortisol in chronically restrained sows. The relationship between drinking and cortisol need not necessarily reflect the coping capacity of this behaviour but may be a physiological consequence of the large amounts of water ingested.

Oestrus, ovulation and peri-ovulatory hormone profiles in tethered and loose-housed sows

Multiparous sows that had been tethered during lactation were put in two different housing conditions after weaning (Day 0); the sows were either tethered by neck chain, or individually housed in a pen of approximately 6 m2. After two months, ten tethered and eleven loose housed sows were used to assess stress and reproductive parameters. Stereotypic behaviour after the afternoon feeding was assessed from Day 18 onwards; at Day 53 stereotypic behaviour tended to occur during a higher percentage of time in the tethered sows (P = 0.11) and at Day 66, the differences were significant (tethered, 78 +/- 5 vs. loose-housed, 40 +/- 10% of time (mean +/- sem); P = 0.03). At Day 35 and 55, cortisol profiles after afternoon feeding were similar for the two groups of sows (P > 0.10). Around oestrus (approximately Day 64), the profiles of oestradiol-17 beta, luteinizing hormone and progesterone were measured and proved to be similar for both treatment groups (P > 0.10). The duration of oestrus was shorter in the tethered sows (42 +/- 4 vs. 63 +/- 2 h; P < 0.001) and, consequently, the timing of ovulation during oestrus (h after onset of oestrus) was advanced in the tethered sows (28 +/- 2 vs. 41 +/- 2; P < 0.001). The duration of ovulation did not differ (tethered, 2.9 +/- 0.5 vs. loose-housed, 2.1 +/- 0.2 h; P = 0.16). The sows were sacrificed at Day 5 after ovulation; ovulation rate, fertilization rate, embryo development and embryo diversity were similar for the two groups, as were adrenal weight and size of adrenal cortex. Duration of oestrus and the levels of stereotypies at Day 60 tended to be negatively related in the tethered sows (P = 0.07), but not in the loose-housed sows (P = 0.65). In conclusion, sows that had been tethered during pregnancy and lactation, and were housed loose or were tethered again at weaning within two months differed both in stereotypic behaviour and in duration of oestrus, without apparent effects on reproductive hormones.

Influences of intermittent daily draught on the behaviour of weaned pigs

The influence of time-unpredictable and uncontrollable draught (forced cold air) on the behaviour of pigs was observed in a climate-controlled pig house with two identical rooms each with five pens. Two days after farrowing, pigs were matched pairwise to correct for genetic, weight and sex differences, and weaned at an average age of 35 days. From then on, the pigs in the experimental room were submitted to draught in a time-unpredictable way. Days with time-unpredictable draught were followed by days without draught. Behavioural studies started on Day 35 and ended on Day 75 of the experiment. The total activity of the pigs was higher during draught (P<0.005). Explorative behaviour was four times higher during draught periods than during non-draught periods. Redirected explorative behaviour on penmates, including earbiting, occurred more during draught periods (P<0.05). Agonistic behaviour increased strongly during draught periods (P<0.005); headknocks with biting as an excessive form of aggression occurred only during these periods. Even in periods without draught, pigs in the experimental room had a sternum: recumbent lying ratio which was higher that that of pigs in the control room and lay in contact with penmates more than did pigs in the control room. Unpredictable and uncontrollable draught as a climatic stressor had enormous effects on the behaviour of pigs; redirected explorative behaviour on penmates and excessive aggression could be detrimental for health and the performance of pigs.