J. Anim Sci. 2007. 85:218-224. doi:10.2527/jas.2006-310
© 2007 American Society of Animal Science
Evidence of phenotypic relationships among behavioral characteristics of individual pigs and performance1
J. P. Cassady2
North Carolina State University, Raleigh, NC 27695-7627
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Abstract
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The objective of this research was to estimate phenotypic relationships among backtest scores (BTS), resident-intruder test scores (RIS), growth rate, LM area, and backfat in pigs. Little is known about the relationships among measures of behavioral characteristics of individual pigs and economically important traits. However, it may be expected that a pig’s behavior affects its performance and that of its pen mates. The backtest was used in this experiment because it was previously shown to be a measure of individual stress-coping behavior and was related to lean gain. The resident-intruder test was used because it is a measure of a pig’s tendency for aggressive behavior toward an unfamiliar pig. Each test was performed twice on pigs (n = 150) from 20 litters, and complete performance data was available on 140 pigs. Between 7 and 14 d of age, the backtest was performed by placing each pig in a supine position and gently restraining it for 60 s. The number of escape attempts (bouts of struggling) and total time spent struggling were recorded. The BTS was the summation of escape attempts during both tests. Resident intruder tests were assessed when pigs were between 30 and 50 d of age. A solid divider was placed in the resident pig’s pen. The resident was placed alone on 1 side of the divider, away from its penmates. An intruder pig of the same sex and smaller size was then placed into the pen. When an attack was initiated by the resident, the pigs were immediately separated, the test was terminated, and a score of 1 was recorded. If no attack occurred by 5 min, the test was terminated and was given a score of zero. The cumulative score from both tests was the RIS. Dam effects influenced BTS (P < 0.01) and RIS (P < 0.03). Preweaning ADG of pigs with a BTS of 8 was 120 g greater than that of pigs with a BTS of 2. However, ADG from 20 to 76 d of age was 131 g greater in pigs with BTS = 2 than in pigs with BTS = 8. Lean gain of pigs with RIS = 2 was 25 g/d greater than in those with RIS = 0 or 1. This resulted in pigs with RIS = 2 having 1.6 kg more acceptable, standardized, fat-free lean. Conflicting results were found when relating the BTS to performance. However, with the RIS, greater aggression toward other pigs was associated with better performance. It was concluded that an unfavorable phenotypic relationship existed between RIS and lean growth.
Key Words: backtest • behavioral trait • correlation • performance • pig • resident intruder test
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INTRODUCTION
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The pork industry must maintain profitability while responding to public pressure to address animal-welfare concerns. Production strategies that improve animal performance and well-being are especially desirable. Little is known about relationships between behavioral characteristics of individual pigs and performance. Past efforts to improve production efficiency have sometimes improved pig well-being by increasing survival rate.
One example is the ryanodine receptor (RYR1) genotype, which results in malignant hyperthermia, a major gene that affects response to stress (Weaver et al., 2000
), growth, meat quality, and survival. Pigs possessing the deleterious RYR1 allele have greater lean gain (growth); a higher incidence of pale, soft, and exudative pork (poor meat quality; Tor et al., 2001); and are more likely to die in response to stress (survival). Selection against this deleterious allele improved meat quality and pig survival but decreased rate of lean gain.
It is expected that other genes exist that influence both well-being and performance. Determining the relationships among pig behavior and important performance traits may reduce the costs of production and improve animal well-being. Relationships between behavioral measures and performance traits need to be estimated to better understand the consequences for well-being. The backtest (Hessing et al., 1993; van Erp-van der Kooij et al., 2001) and resident-intruder test (Erhard and Mendl, 1997) are accepted measures of behavioral characteristics of individual pigs.
The objective of this experiment was to estimate phenotypic relationships among the backtest, resident intruder test, growth, LM area, and backfat depth. If significant phenotypic relationships were found, then it might be useful to examine genetic relationships among these tests and production traits to determine opportunities for improving well-being through genetic selection.
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MATERIALS AND METHODS
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All procedures involving animals were approved by the North Carolina State University Institutional Animal Care and Use Committee.
Population Description
Pigs (n = 150; 79 gilts and 71 barrows) were from 20 litters born during a 4-d period. Litters were produced by mating Yorkshire x Landrace sows to Duroc boars at the North Carolina State University Swine Education Unit, Raleigh, NC. Sows were bred by natural service to more than 1 sire, so sire identification was unavailable. Sows farrowed in crates, and piglets were weaned at 20 ± 1 d. Pigs received no vaccinations. After weaning, pigs were moved to 1 of 2 nursery rooms and penned in groups of 10 by sex and weight. Nursery pens had nipple waters, self-feeders, and tribar flooring (0.33 m2/ pig). The nursery room was maintained at 28° C for the first 2 wk. The temperature was reduced by 0.6° C every other day until the temperature reached 19° C. Pigs were moved from the nursery to the finishing barn at 66 d of age. Upon entering the finishing unit, pigs were regrouped by sex and weight into pens of 5 (1.4 m2/ pig). Finishing pens had concrete, slatted floors, nipple waters, and self-feeders. Pigs were fed pelleted feed prepared by Southern States Cooperative Inc. (Richmond, VA), as described in Table 1.
Performance Measures
Pigs were weighed at birth and at 20 ± 1, 76 ± 1, and 153 ± 1 d of age. Preweaning ADG, ADG from 20 to 76 d of age, ADG from 76 to 153 d of age, and days to 100 kg were calculated. At 153 d of age, ultrasound measurements of last rib backfat depth and LM area were collected by a certified ultrasound technician.
Total kilograms of acceptable, standardized, fat-free lean (FFL) and kg of acceptable, standardized, fat-free lean per day on test (LG) were calculated by using a formula from the National Pork Board handbook (Berg, 2000) and substituting last rib backfat in place of tenth rib backfat:
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Backtest
Before weaning, each piglet was tested twice by the backtest, as described by Hessing et al. (1993) and van Erp-van der Kooij et al. (2001). Pigs were first tested between 6 and 10 d of age and again between 13 and 17 d of age. During the backtest, the piglet was placed in a supine position for 60 s. The researcher gently restrained the pig by placing one hand loosely over the neck while using the other hand to extend and loosely hold the hind legs. The total number of attempts to struggle and total time spent struggling were recorded. If a pig was struggling at the end of the testing period, the period was extended until the end of that attempt. The number of attempts to struggle is commonly referred to as the backtest score. The number of attempts during the 2 backtests were summed and analyzed as a single value, designated the backtest score (BTS). The BTS ranged from 1 to 10. Because few animals had a BTS of 1, 9, or 10, individuals with BTS of 1 or 2 were combined, as were individuals with BTS of 8, 9, or 10.
Total time spent struggling was recorded because some pigs had several struggling bouts of short duration, whereas others had fewer bouts of longer duration, resulting in different BTS but a similar total time spent struggling.
Resident Intruder Test
Each pig was subjected twice to the resident intruder test, as described by Erhard and Mendl (1997). The tests were done when pigs were 33 ± 1.5 and 44 ± 3.3 d of age. A solid divider (80-cm high) was placed in the center of the resident pen. A resident pig was selected for testing and placed on one side of the divider, away from its pen mates. An intruder pig of the same sex, smaller size, and from a different nursery room was then placed into the pen. Some test pigs (n = 111) were also used as intruders. Latency to attack was recorded as the time from introduction of the intruder pig until attack by the resident pig. When that first attack occurred, the pigs were immediately separated and the test was terminated [resident-intruder score (RIS) = 1]. Attacks by the intruder were rare. After 5 min with no attack, the test was terminated (RIS = 0). This test was repeated 14 d later. The RIS that was analyzed reflected the cumulative number of attacks during the 2 tests, with possible values of 0, 1, or 2.
Statistical Analysis
Backfat, ADG from 76 to 153 d of age, days to 100 kg, fat-free lean, lean gain, and LM area data from 10 pigs were excluded before analysis. Five pigs were removed before the end of the study because they were needed for another purpose. Four pigs weighing under 78 kg and 1 pig weighing 136 kg at the end of the study were excluded as being outliers. Dam and litter were confounded. Therefore the effect of dam included one-half the additive genetic variance, all the maternal variance, and variance due to litter environment. Repeatability reflected the proportion of variance attributable to the individual. This was estimated using the MIXED procedure (SAS Inst. Inc., Cary, NC). Fixed effects for the backtest included dam, sex, and the covariate of birth weight. Fixed effects for the resident intruder test included dam, sex, nursery pen, and the covariate of weaning weight. Pig was included as a random effect, and repeatability was estimated by dividing the variance due to pig by the total variance. Relationships between behavior traits and performance traits were tested by contrasting least squares means estimated using the GLM procedure of SAS.
The model used to test BTS and total time spent struggling included fixed effects of sex and dam, and birth weight as a covariate. Some pigs (n = 111) were used as intruders before being tested for the RIS. To determine if this prior experience affected the results of the RIS, pigs were coded as 0 if they did not have prior experience as an intruder and 1 if they did have prior experience as an intruder. Fixed effects of sex, dam, nursery pen, prior experience, and the covariate of weaning weight were included in the models for RIS and latency to attack (Table 2). The main effect of prior experience on RIS (P = 0.77) and latency to attack (P = 0.59) was dropped from the final model. The model for production traits included the fixed effects of sex, dam, BTS, RIS, and the covariates total time spent struggling and latency to attack. An effect of latency to attack was not detected for any trait and was subsequently dropped from all models. Fixed effects of nursery pen and finishing pen were included where appropriate to adjust for the environmental effect of pen.
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Table 2. P-values of fixed effects of sex, dam, birth weight, weaning weight, and nursery pen on behavioral traits
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Preliminary analyses were performed with backfat depth and LM area, including BW, as a covariate when ultrasound measurements were recorded. Both linear and quadratic effects of BW on backfat and LM area were estimated. Significant linear effects were detected, and the regression coefficient was estimated. Those regression coefficients were then used to adjust backfat depth and LM area to 100 kg. Further analyses of back-fat depth and LM area were conducted using the adjusted values.
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RESULTS
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Repeatability of BTS, total time spent struggling, RIS, and latency to attack were 0.49, 0.38, 0.28, and 0.18, respectively. Thus, the tests were low to moderately repeatable. The BTS and total time spent struggling were highly correlated (r = 0.83). Phenotypic correlation between backtest and resident intruder test (r = 0.10) did not differ from zero. Based on the lack of a significant phenotypic correlation between BTS and RIS, it was concluded that the backtest and resident intruder test measured different behavioral characteristics. Fixed effect of dam affected BTS and total time spent struggling (Table 2
). No association was found for BTS or RIS with sex or birth weight (Table 2).
The main effect of BTS was significant for preweaning ADG, weaning weight, and ADG from 20 to 76 d of age (Table 3). Pigs differing in RIS (P = 0.01) had different fat-free lean and lean gain and tended to differ for days to 100 kg (P = 0.10) and backfat (P = 0.08; Table 4). Both BTS and RIS were associated with ADG during the nursery phase (P = 0.02). Contrasts among least squares means for all performance traits are in Tables 3 and 4. As BTS score increased, preweaning ADG increased. Pigs with a BTS of 8 gained 120 g/d more that did pigs with a BTS of 2 (P < 0.01). This difference in preweaning ADG resulted in pigs with greater BTS having greater weaning weights (P < 0.01). However, in the nursery, pigs with a BTS of 2 gained 131 g/d more than pigs with BTS of 8 (P < 0.01), whereas pigs differing in BTS grew at similar rates in the finishing phase. The net effect was no difference in days to 100 kg among pigs differing in BTS.
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Table 3. Effect of backtest score (BTS) and resident-intruder test score (RIS) on economically important traits, and regression coefficients for total time spent struggling
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Table 4. Effect of backtest score (BTS) and resident-intruder test score (RIS) on economically important traits, and regression coefficients for total time spent struggling
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For the resident intruder test pigs (Figure 1), those attacking only during the first test (n = 39) and those attacking only during the second test (n = 17) were given the same RIS score of 1. A preliminary analysis was done in order to determine if order in which a pig attacked affected performance. No statistically significant difference in performance was found among pigs due to order of attack. Therefore, these 2 groups were pooled. In the nursery, pigs with RIS = 2 gained 44 g/ d more than pigs with RIS = 1 (P < 0.01), resulting in significantly fewer days to 100 kg. Pigs with RIS = 1 had 1.6 mm more backfat than did pigs with RIS = 2 (P < 0.05). These differences in growth rate and backfat resulted in pigs with RIS = 2 having greater lean gain (P < 0.01) and kilograms of fat free lean (P < 0.01).
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Figure 1. A schematic representation of resident intruder score (RIS) vs. lean gain ( = an individual pig) showing that overlap exists in lean gain among pigs with varying RIS scores.
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DISCUSSION
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A number of standardized tests have been developed to quantify behavioral characteristics of individual pigs in an effort to gain a better understanding of the pig’s nature, with the expectation that this will lead to improved production efficiency and state of being (S. Curtis, University of Illinois, Urbana, personal communication). When pigs experience stress, their endocrine profile is altered (Koolhaas et al., 1999). The impact of altered hormone concentrations in response to stress on production traits is not yet fully understood. Extensive efforts have been made to develop pig accommodations that maximize performance. Conversely, little effort has been made to understand why pigs respond differently to a common environment.
Relationships between BTS and pre and postweaning growth were inconsistent. Pigs were penned by sex and weight in the nursery. Any effects of this management system on the results of this experiment are unknown. Pigs with RIS = 2 also had the least backfat, resulting in the greatest lean gain and fat-free lean. After weaning, pigs with RIS = 2 consistently grew fastest (Table 3 and Table 4). They also had the greatest fat-free lean and lean gain and the least backfat. This is an unfavorable relationship between behavioral and production traits. There were 24 pigs with RIS = 0 and lean gain greater than the population average (Figure 1). Thus, the opportunity exists to select pigs above average lean gain and RIS = 0.
Several behavioral tests and measurements have been evaluated. Collectively, these tests are believed to measure several different individual behavioral characteristics of the pig. These include the open door test (van Erp-van der Kooij et al., 2002), novel object test (Jensen et al., 1995), novel environment test (Ruis et al., 2000), open field test (Jensen et al., 1995; Giroux et al., 2000), group feeding competition test (Ruis et al., 2000), human approach test (Hemsworth et al., 1981; Giroux et al., 2000), backtest (Hessing et al., 1993; van Erp-van der Kooij et al., 2001), resident intruder test (Erhard and Mendl, 1997), social confrontation test (Hessing et al., 1993), and competition effects (Muir, 2005; Van Vleck and Cassady, 2005). In the current study, pigs were assessed using the backtest and resident-intruder test. The backtest was selected because it had previously been associated with individual stress-coping behavior (van Erp-van der Kooij et al., 2003) and lean gain (van Erp-van der Kooij et al., 2000). The resident intruder test was chosen because it has been associated with aggressiveness among pigs, which has welfare implications (D’Eath, 2004).
The backtest has proved to be moderately repeatable over time. Phenotypic correlations of 0.42, 0.47, and 0.48 between repeated backtest scores at 10 and 17 d of age have been reported (van Erp-van der Kooij et al., 2000; van Erp-van der Kooij et al., 2002). This agrees with a correlation of 0.49 between repeated backtest scores in the current study. Moreover, greater correlations have been reported between repeated backtest scores than between repeated records of the human approach test, novel object test, and open door test scores, which are alternative measures of coping style (van Erp-van der Kooij et al., 2002). This indicates that the backtest measures a behavioral characteristic, which is less situation specific. Ruis et al. (2000) concluded that cortisol response to weighing at 25 wk of age was associated with backtest scores. The same authors also found that both peak and area under the curve of cortisol plots after ACTH injection were associated with backtest score. Geverink et al. (2002) reported an association among backtest scores at early ages and response to acute stressors in adult nulliparous sows. These findings further support the theory that backtest scores are indicative of true physiological differences in individual stress-coping behavior, which is measurable throughout a pig’s life.
The backtest score has been associated with response of adult pigs to acute stress. Increased BTS has been associated with increased aggression (Hessing et al., 1993; Ruis et al., 2000), greater lean-meat percentage (van Erp-van der Kooij et al., 2000), and better carcass grade at slaughter (van Erp-van der Kooij et al., 2000). Results from the current study do not support an association between backtest score and aggression. A possible reason for these differing results is that Hessing et al. (1993) used a social confrontation test when pigs were 1 wk old as a measure of aggression. Not only does the procedure for the social confrontation test differ from that of the resident-intruder test, but also pigs were tested at different ages. van Erp-van der Kooij et al. (2000) suggested that animals with greater backtest scores are more active, which may explain their leaner carcasses. Hartsock et al. (1977) associated increased aggression with higher protein percentage at slaughter. This is consistent with the positive relationships found between RIS and lean gain in the current study. Increased backtest scores also have been associated with a reduced ability to cope with stress (Ruis et al., 2001; Geverink et al., 2002). The results differ from those of van Erp-van der Kooij et al. (2000), who found that increasing BTS was associated with increased lean meat percent.
Hartsock et al. (1977) documented and analyzed data associated with aggression during establishment of suckling order and concluded that pigs having greater fighting success selected the more anterior teats, had greater birth weights, had greater weight gain from birth to 7 wk, and were leaner at slaughter. This is consistent with the findings of Giroux et al. (2000), who concluded that pigs of higher rank order had greater weight gain before 4 wk of age. Those results agree with the current finding that pigs with RIS = 2 had greater lean gain.
In the current study, BTS and RIS were associated with economically important traits, although the phenotypic correlation between BTS and RIS did not differ from zero. This is consistent with the theory that the backtest is a measure of individual stress-coping behavior (van Erp-van der Kooij et al., 2003) and the resident-intruder test is a measure of a pig’s tendency for aggressive behavior toward an unfamiliar pig (Erhard and Mendl, 1997). The effect of dam was highly significant for BTS, total time spent struggling, and RIS. Because dam effects included 1-half the additive genetic variance, all the maternal variance, and variance due to litter environment, it is not possible to conclude whether a genetic component exists for BTS, total time spent struggling, and RIS.
Whereas studies employing various methods have addressed pig behavior in recent years (Forkman et al., 1995; Koolhaas et al., 1999; D’Eath, 2004), in only a few have correlations among measures of behavior and performance traits been studied (van Erp-van der Kooij et al., 2000; Ruis et al., 2002). Future research should be focused on determining the heritability of measures of behavior and testing for consistency of the relationship among behavior and performance across diverse genetic populations and environments. There apparently has been no study specifically designed to estimate the heritability of measures of behavior or genetic correlations among behavior and economically important performance traits. Therefore, these traits will have to be recorded in populations of known pedigree in order to estimate genetic (co)variances.
Relationships among behavioral characteristics of individual pigs, growth, and backfat were found. Many of these appear to be unfavorable because improved performance was associated with increased aggression toward other pigs. Thus, past selection in pigs to improve the economic efficiency of pork production may have resulted in unintended changes in pig behavior. Whereas improved lean gain tended to be associated with greater RIS, there certainly were individual pigs with lower RIS and excellent lean growth. If these measures of behavior are heritable, it should be possible to identify and select pigs that have favorable performance and behavior. The next step will be to estimate the heritability of various measures of pig behavior. This is essential to determining if pig behavior can be altered by direct genetic selection.
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Footnotes
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1 I would like to thank O. W. Robison, Jack Odle, and Stanley Curtis for their helpful comments and suggestions during the preparation of this manuscript. I would also like to thank Peggy Overton for her work on this project. 
2 Corresponding author: joe_cassady{at}ncsu.edu
Received for publication May 13, 2006.
Accepted for publication August 4, 2006.
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LITERATURE CITED
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|---|
Berg, E., editor. 2000. Live animal evaluation. Pork composition and quality assessment procedures. National Pork Board, Des Moines, IA.
D’Eath, R. B. 2004. Consistency of aggressive temperament in domestic pigs: The effects of social experience and social disruption. Aggress. Behav. 30:435–448.[CrossRef]
Erhard, H. W., and M. Mendl. 1997. Measuring aggressiveness in growing pigs in a resident-intruder situation. Appl. Anim. Behav. Sci. 54:123–136.[CrossRef]
Forkman, B., I. L. Furuhayg, and P. Jensen. 1995. Personality, coping patterns, and aggression in piglets. Appl. Anim. Behav. Sci. 45:31–42.[CrossRef]
Geverink, N. A., W. G. Schouten, G. Gort, and V. M. Wiegant. 2002. Individual differences in behavioral and physiological responses to restraint stress in pigs. Physiol. Behav. 77:451–457.[CrossRef][Medline]
Giroux, S., G. Martineau, and S. Robert. 2000. Relationships between individual behavioural traits and post-weaning growth in segregated early-weaned piglets. Appl. Anim. Behav. Sci. 70:41–48.[CrossRef][Medline]
Hartsock, T. G., H. B. Graves, and B. R. Baumgardt. 1977. Agonistic behavior and the nursing order in suckling piglets: Relationships with survival, growth, and body composition. J. Anim. Sci. 44:320–331.[Medline]
Hemsworth, P. H., J. L. Barnett, and C. Hansen. 1981. The influence of handling by humans on the behavior, growth, and corticosteroids in the juvenile female pig. Horm. Behav. 15:396–403.[CrossRef][Medline]
Hessing, M. J. C., A. M. Hagelso, J. A. Van Beek, P. R. Wiepkema, W. G. Schouten, and R. Krukow. 1993. Individual behavioural characteristics in pigs. Appl. Anim. Behav. Sci. 37:285–295.[CrossRef]
Jensen, F. B., K. Thodberg, and E. Köster. 1995. Individual variation and consistency in piglet behaviour. Appl. Anim. Behav. Sci. 45:43–52.[CrossRef]
Koolhaas, J. M., S. M. Korte, S. F. De Boer, B. J. Van Der Vegt, C. G. Van Reenen, H. Hopster, I. C. De Jong, M. A. Ruis, and H. J. Blokhuis. 1999. Coping styles in animals: Current status in behavior and stress-physiology. Neurosci. Biobehav. Rev. 23:925–935.[CrossRef][Medline]
Muir, W. M. 2005. Incorporation of competitive effects in breeding programs. Genetics 170:1247–1259.[Abstract/Free Full Text]
Ruis, M. A. W., J. H. A. te Brake, B. Engel, W. G. Buist, H. J. Blokhuis, and J. M. Koolhaas. 2001. Adaptation to social isolation. Acute and long-term stress responses of growing gilts with different coping characteristics. Physiol. Behav. 73:541–551.[CrossRef][Medline]
Ruis, M. A. W., J. H. A. te Brake, B. Engel, W. G. Buist, H. J. Blokhuis, and M. Koolhaas. 2002. Implications of coping characteristics and social status for welfare and production of paired growing gilts. Appl. Anim. Behav. Sci. 75:207–231.[CrossRef]
Ruis, M. A. W., J. H. A. te Brake, J. A. Van de Burgwal, I. C. de Jong, H. J. Blokhuis, and J. M. Koolhaas. 2000. Personalities in female domesticated pigs behavioural and physiological indications. Appl. Anim. Behav. Sci. 66:31–47.[CrossRef]
Tor, M., J. Estany, D. Villalba, D. Cubilo, J. Tibau, J. Soler, A. Sanchez, and J. L. Noguera. 2001. A within-breed comparison of RYR1 pig genotypes for performance, feed behaviour, and carcass, meat and fat quality traits. J. Anim. Breed. Genet. 118:417–427.[Medline]
van Erp-van der Kooij, E. V., A. H. Kuijpers, J. W. Schrama, E. D. Ekkel, and M. J. Tielen. 2000. Individual behavioural characteristics in pigs and their impact on production. Appl. Anim. Behav. Sci. 66:171–185.[CrossRef]
van Erp-van der Kooij, E. V., A. H. Kuijpers, J. W. Schrama, F. J. C. M. van Eerdenburg, W. G. P. Schouten, and M. J. M. Tielen. 2002. Can we predict behaviour in pigs? Searching for consistency in behaviour over time and across situations. Appl. Anim. Behav. Sci. 75:293–305.[CrossRef]
van Erp-van der Kooij, E. V., A. H. Kuijpers, F. J. van Eerdenburg, S. J. Dieleman, D. M. Blankenstein, and M. J. Tielen. 2003. Individual behavioural characteristics in pigs–influences of group composition but no differences in cortisol responses. Physiol. Behav. 78:479–488.[CrossRef][Medline]
van Erp-van der Kooij, E. V., A. H. Kuijpers, F. J. van Eerdenburg, and M. J. Tielen. 2001. A note on the influence of starting position, time of testing and test order on the backtest in pigs. Appl. Anim. Behav. Sci. 73:263–266.[CrossRef][Medline]
Van Vleck, L. D., and J. P. Cassady. 2005. Unexpected estimates of variance components with a true model containing genetic competition effects. J. Anim. Sci. 83:68–74.[Abstract/Free Full Text]
Weaver, S. A., A. L. Schaefer, and W. T. Dixon. 2000. The effects of mutated skeletal ryanodine receptors on calreticulin and calsequestrin expression in the brain and pituitary gland of boars. Brain Res. Mol. Brain Res. 75:46–53.[Medline]
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Abstract
INTRODUCTION
