Components of Feed Efficiency in Broiler Breeding Stock: Is Improved Feed Conversion Associated with Increased Docility and Lethargy in Broilers?

Components of Feed Efficiency in Broiler Breeding Stock: Is Improved Feed Conversion Associated with Increased Docility and Lethargy in Broilers? 1,2 D. O. Skinner-Noble,*,3 R. B. Jones, and R. G. Teeter*,4 *Department of Animal Science, Oklahoma State University, Stillwater, Oklahoma; and Welfare Biology Group, Roslin Institute (Edinburgh), Roslin, Midlothian EH25 9PS, Scotland, United Kingdom ABSTRACT Two trials were conducted to test the hypothesis pecking, preening, standing, and resting. Third, the 20 that broilers classified as good rather than poor feed converters (low vs. high feed-to-gain ratios, respectively) would show decreased activity (increased lethargy) birds showing the best FCR and the 20 showing the worst were induced into tonic immobility (TI). Feed conversion was not related to either the novel object or the TI mea- and reduced fear of human caretakers. In both trials sures of fearfulness. Neither was FCR related to feeding, birds were reared to juvenile selection age when the 192 drinking, walking, or pecking. However, FCR was positively correlated with both standing and preening and males with the best breast and leg conformation were negatively correlated with resting behavior. placed into individual bird cages (46 60 cm), and their In trial 2, the FCR test lasted for 11 d. The birds were feed conversion ratio (FCR) was measured. In trial 1, birds assessed for behavioral patterns (feeding, drinking, walking, pecking, preening, standing, and resting) as in the remained in the cages for 7 d for assessment of feed conversion. During the feed conversion testing period, previous trial. As in trial 1, FCR was positively correlated the birds were subjected to three behavioral tests. First, with standing and negatively correlated with resting behavior. Contrary to expectations, the results of these two approach-avoidance responses to a novel object placed in the feeder were observed when feeding would be expected. Second, behavior patterns of individual birds were assessed by recording feeding, drinking, walking, studies suggest that broilers with good FCR were actually less lethargic and no more fearful than those that showed poor FCR values. (Key words: broiler, feed conversion, fearfulness, behavior, activity) 2003 Poultry Science 82:532 537 INTRODUCTION The interface between well-being and production is not always well understood. Genetic selection of modern livestock species has improved economically important traits, but less attention has been paid to its effects on the animal behavior and well-being. Two approaches to addressing these questions are 1) to examine the behavior of selected genetic lines known to differ in production traits and 2) to perform correlation analyses in existing populations. 2003 Poultry Science Association, Inc. Received for publication August 29, 2002. Accepted for publication November 22, 2002. 1 Study 1 of this report was presented at the Annual Meeting of the Poultry Science Association, July 2001, Indianapolis, IN [D. O. Skinner- Noble, R. B. Jones, and R. G. Teeter, 2001. Is improved feed conversion associated with increased lethargy and docility in broiler chickens? Poult. Sci. 80(Suppl. 1):44 (Abstr.)]. 2 Submitted for publication with permission of the director, Oklahoma Agricultural Experiment Station, Stillwater, OK 74078. This research was supported under H-2025. 3 Formerly D. O. Noble. 4 To whom correspondence should be addressed: poultry@ okstate.edu. Selection experiments have demonstrated links between selection for production traits and behavior. Selection for increased 8-wk BW in chickens has led to an increased number of meals of similar size compared to that observed in a line selected for decreased BW (Barbato et al., 1980). Similarly, selection for increased 16-wk BW in turkeys increased the duration but not the number of feeding bouts when compared to a randombred control population (Noble et al., 1996). In addition, changes in BW were associated with alterations in taste preferences (Barbato et al., 1982) and specific amino acid appetites (Noble et al., 1993a,b). Data on the relationships between behavior and feed conversion, however, are generally lacking. Two behavioral states that may impact significantly on feed conversion in broilers are activity and fear, especially in response to human caretakers. For example, birds that are more active may have poorer feed conversion ratios (FCR), since energy would be expended on activity rather than allocated to growth and production. However, measuring activity can be problematic. For example, traditional Abbreviation Key: FCR = feed conversion ratio (g feed/g gain); RFC = residual feed consumption; TI = tonic immobility. 532

FEED CONVERSION AND BEHAVIOR 533 methodology involving direct observation of individual birds is very time-consuming. Use of additional observers can allow for more data points, but this introduces potential observer effects that may serve to increase variation. Scan sampling of pens or cages of birds can allow for estimations of activity budgets by recording the numbers of birds eating, drinking, standing, resting, or moving. Such estimates, however, are made on a group basis, potentially precluding their use in individual bird evaluations. Electronic devices used to measure activity have ranged from strategically placed electric eye systems in floor pens (Siegel and Guhl, 1956) to Doppler radar systems used in metabolism chambers (MacLeod, 1991). However, none of these systems has gained widespread acceptance in poultry species. One method of circumventing behavioral observations of activity is to utilize residual feed consumption (RFC) to estimate wasted energy. The advantage of this method is that large numbers of observations can be used to develop population estimates, including estimates of genetic parameters. Estimates of RFC are made by accounting for the energy of basal metabolism and the energy in the product produced (meat or eggs) with the remaining portion being the RFC. In a review of RFC, Luiting (1990) concluded that most of the variation in RFC was related to activity, feathering, variation in basal metabolic rate not accounted for in the model, the area of denuded skin, body temperature, and composition. In light of this conclusion, RFC may not be a suitable estimate of the energy cost of activity. Estimates of the energy costs of activities are generally lacking. MacLeod (1991) measured ME and heat production (and determined net energy) in birds that were either allowed to eat or were force-fed the same amount of feed. While the ME values for birds eating were similar to those that were force-fed, the heat production was 30% lower for birds that were force-fed, resulting in higher net energy than measured in birds that ate their food. Thus, it may be possible to estimate the energy expended by measuring feeding behavior per se. Fear of humans, of inanimate stimuli, or of sudden changes in the environment can seriously damage productivity in layers and broilers (Hemsworth and Barnett, 1989; Hemsworth et al., 1994; Jones, 1996, 1997). Indeed, underlying fearfulness is negatively related with a number of productivity measures, including growth and FCR in poultry (Hemsworth et al., 1994; Jones, 1996, 1997). Fear of humans can also reduce growth rate and reproductive performance in pigs (Hemsworth and Coleman, 1998). Exposure to humans or other fear-eliciting disturbances during FCR testing could compromise the assessment of this important trait. It might also influence the assessment of many physiological variables. Fear levels can be measured by testing the birds in unfamiliar environments or by exposing them to potentially alarming stimuli while they remain in their home cages (Jones, 1996). Two methods were used to assess fearfulness in the current experiment. First, the birds tonic immobility (TI) responses to brief manual restraint were measured in a separate room. Tonic immobility is an antipredator reaction characterized by a state of reduced responsiveness, and the duration of the response is positively related to the antecedent fear state (Jones, 1986, 1996). Second, the broilers approach-avoidance responses to a conspicuous novel object placed at the front of the cage were measured. Here, because the degree of avoidance of a stimulus is considered to be a useful operational measure of the fear it induces, high avoidance scores are regarded as indicative of heightened fear (Jones, 1996; Hemsworth and Coleman, 1998). Potential relationships between docility, lethargy, and feed conversion are generally unknown. The objectives of the present study were to determine how selection for improved FCR could potentially lead to correlated responses in behaviors and to determine the relationships among feed conversion, lethargy, and docility in two populations of broiler chickens. MATERIALS AND METHODS Two trials were conducted in the present study. Differences between trials will be discussed in turn. In both trials, birds were reared to juvenile selection age (38 d in trial 1 and 45 d in trial 2) in floor pens. At the juvenile selection age, males were weighed and subjectively evaluated for breast conformation and leg deformities. Both the breast conformation score and the leg deformity scores were on a scale from 1 to 5 indicating poor to superior conformation, respectively. A selection index was constructed to select the best 192 birds for subsequent feed conversion testing. The index was twice the standardized breast deviation plus the standardized BW deviation plus the standardized leg score deviation. This juvenile selection procedure was done to simulate a preliminary selection in a pedigree broiler breeder line. This experimental design permitted inferences regarding potential selection effects on correlated traits. The person performing the selections had received training from a commercial broiler breeding company. While the training methodology was proprietary, training methods included tests of the selector s ability to consistently relate breast conformation scores with breast yield and consistently evaluate leg deformities. In both trials, scan sampling of selected behaviors was conducted on each of two mornings and two afternoons in each week of test (1 wk in trial 1 and 2 wk in trial 2). An observer walked past each cage 10 times each morning and each afternoon and classified the behavior shown by each bird into one of eight categories. A bird was considered to be eating if it was standing at the feeder manipulating feed with its beak, drinking if standing at the drinker manipulating the drinker system with its beak, standing if not moving and in an upright posture, walking if in an upright posture and moving from one place to another, resting if motionless and its body in contact with the cage floor, pecking if performing a nonconsummatory peck at the cage or the feeder, preening if manipulating its feathers with its beak, or other for any behavior not contained in the aforementioned descriptions.

534 SKINNER-NOBLE ET AL. Trial 1 Approximately 450 large-bodied randombred control broiler chicks were obtained from the University of Arkansas at hatching. This line is a composite of seven male lines and six female lines available in the early 1990s. The first two generations were spent making the crosses so that genes from both sexes of each line were incorporated into the randombred line. Each line s males were crossed onto the other line s females and vice versa so that all possible crosses were made. There have been five generations of full random mating since the final mixing generation (N. B. Anthony, 2001, University of Arkansas, Fayetteville, AR, personal communication). All birds were placed straightrun into pens and fed a single diet closely approximating a broiler grower diet (21% crude protein and 3,200 kcal ME per kilogram diet) from hatching to 36 d of age. Birds were fasted for 48 h prior to the start of FCR testing to stimulate appetite. The birds were then moved to individual FCR cages and allowed 2 d to acclimatize before behavioral testing began, and this feed conversion test lasted 1 wk. On the third day, all birds were tested for their approach-avoidance responses to a novel object placed in their feeder at the front of the cage by the observer who stood nearby (see Jones, 1996; Jones et al., 1996). In the test, the feed was stirred and a multicolored rod approximately 15 cm long was placed in the feeder. The observer stepped back 0.5 m and scored the bird s orientation and posture at 15 s intervals over 2 min. It scored 1 if it pecked the rod or fed around it, 2 if it faced the front of the cage but with its head inside, 3 if it faced the side of the cage, 4 if it faced the rear, and 5 if it showed escape behavior. Posture scores were either 1 for standing or 2 for crouching. After 7 d all the birds in the FCR cages, as well as their feeders, were reweighed. All the birds were ranked for FCR. The TI responses of those birds in the upper and lower 10% of the FCR ranking (20 birds each) were then measured. Tonic immobility was induced by manually restraining the bird on its side for 15 s (Jones, 1986). The number of inductions required to induce TI (up to three), lasting at least 10 s and the duration of the immobility response, i.e., till the bird righted itself, were recorded. If the bird failed to right itself after 3 min, it was given a maximum score of 180 s for TI duration. Trial 2 Birds (600 males of a commercial broiler strain) were reared to 42 d using one of three feeding programs: 1) starter diet throughout, 2) grower diet throughout, or 3) traditional starter, grower, and finisher feeding program. The starter, grower, and finisher diets contained 22.67, 20.20, and 17.33% crude protein, and 3,077, 3,154, and 3,229 kcal ME per kilogram diet, respectively. This trial used a longer period of FCR testing than the first trial and did not expose the birds to fasting prior to the start of the test. In contrast, birds were allowed a pretest adaptation period of 3 d prior to the start of the FCR test. Following the adaptation period, birds and feed were weighed, and the birds then remained on test for 11 d. Assessments of fear were not conducted in this trial. Birds were fed the finisher diet during the FCR test. In both trials correlation analyses were used to estimate relationships between performance and behavioral traits. The FCR was adjusted for differences in starting BW and subsequently included in the correlation analyses. In addition, results of the FCR test were used to classify birds into quartiles of the FCR distribution for subsequent analysis of variance with FCR quartile as the source of variation. The TI responses of the upper and lower FCR deciles were compared using the Mann-Whitney U test (two-tailed). Trial 1 RESULTS Of the behavioral responses observed, only standing, resting, and preening were correlated with feed conversion (Table 1). Unexpectedly, birds that had lower FCR (improved feed conversion) stood more, rested less, and showed more preening than did their poorer converting counterparts. Feed intake was only correlated with standing behavior. Feeding behavior was positively correlated with end BW and weight gain on test. Also of note, standing was negatively correlated with start and with end BW values as well as feed intake. As with most behavior traits, assessments of fearfulness were not associated with feed conversion (Table 1). Neither location in the cage nor posture score in response to the novel object were correlated with any of the performance traits measured. Similarly, birds from the top and bottom 10% of the population showed similar median numbers of inductions (2.85 vs. 2.71) and similar median durations (18.15 s vs. 23.29 s) of TI. An analysis of variance using quartiles of the feed conversion distribution yielded similar results to those observed using correlation analysis (Table 2). When birds from the FCR quartiles differed in behaviors, the top 25% of the population rested less, preened more, and were more active than those in the other quartiles. Birds from the top quartile of the population for feed conversion started the FCR test lighter than the rest of the population. These superior birds gained more weight on similar amounts of feed than other birds in the population. Trial 2 As in trial 1, resting and standing were positively and negatively correlated with feed conversion, respectively (Table 3). In addition, drinking was negatively correlated with FCR. In contrast to trial 1, preening was not correlated with FCR. Preening was, however, negatively correlated with BW at 48 and 59 d of age. As with trial 1, feeding behavior was positively correlated with weight gain. When quartiles of the FCR distribution were used as the source of variation, birds from the top quartile stood most, and birds from the bottom quartile rested more than other

FEED CONVERSION AND BEHAVIOR 535 TABLE 1. Correlation coefficients between performance traits and behaviors, trial 1 Behavior trait Performance NO 2 NO 2 trait 1 Eating Drinking Standing Resting Walking Pecking Preening location posture BW40 0.052 0.034 0.195** 0.106* 0.112** 0.008 0.054 0.030 0.060 FI4047 0.063 0.070 0.109** 0.031 0.046 0.071 0.025 0.003 0.042 BW47 0.078* 0.061 0.170** 0.062 0.106 0.019 0.024 0.014 0.018 WG4047 0.117** 0.066 0.055 0.040 0.027 0.035 0.017 0.079 0.062 FCR4047 0.060 0.015 0.089* 0.122** 0.028 0.061 0.089* 0.104 0.034 ADJFCR 0.098* 0.013 0.058 0.029 0.046 0.036 0.023 0.057 0.056 1 BW40 = BW at the start of the feed conversion ratio (FCR) test; FI4047 = feed intake from 40 to 47 d of age (the length of the FCR test); BW47 = BW at 47 d of age (the end of the FCR test); WG4047 = weight gained from 40 to 47 d of age (weight gained on test); FCR4047 = FCR (grams of feed per gram of gain) from 40 to 47 d of age; and ADJFCR = FCR from 40 to 47 d adjusted for 40 d BW. 2 NO = novel object test, location and posture in response to a novel object placed into the feeder. *P 0.05. **P 0.01. TABLE 2. Behavior traits by feed conversion ratio (FCR) quartile, trial 1 75th to 100th 50th to 75th 25th to 50th 0 to 25th Pooled Behaviors 1 percentile percentile percentile percentile Mean SEM Eating 1.11 0.87 0.88 0.93 0.91 0.04 Drinking 0.85 0.75 0.86 0.80 0.82 0.04 Standing 1.01 0.96 0.85 0.78 0.93 0.04 Resting 6.29 b 6.89 a 6.85 a 6.94 a 6.75 0.07 Walking 0.10 0.10 0.08 0.08 0.10 0.01 Pecking 0.05 0.04 0.02 0.03 0.04 0.01 Preening 0.58 a 0.39 b 0.44 ab 0.40 b 0.42 0.02 Active behaviors 2.62 a 2.12 b 2.26 b 2.21 b 2.26 0.06 Passive behaviors 7.35 b 7.88 a 7.72 a 7.75 a 7.72 0.06 Injestive behaviors 1.95 1.62 1.74 1.73 1.74 0.05 Noninjestive behaviors 8.03 8.38 8.24 8.24 8.24 0.05 Novel object test Novel object location 2 3.14 3.18 3.28 3.22 3.21 0.03 Novel object posture 3 1.21 1.17 1.21 1.21 1.19 0.02 FCR test results BW at start of test (g) 1,810.3 b 1,892.0 a 1,920.7 a 1,928.2 a 1,887.9 12.79 Feed consumed on test (g) 1,282.2 1,290.8 1,330.9 1,334.3 1,309.5 12.55 BW at end of test (g) 2,638.4 2,662.0 2,679.7 2,629.5 2,652.2 16.54 Weight gained on test (g) 828.1 a 770.0 b 759.0 b 701.3 c 764.3 7.86 FCR on test (g:g) 1.55 d 1.68 c 1.75 b 1.91 a 1.72 0.01 a d Means within a row with no common superscripts differ (P < 0.05). 1 Number of activities recorded per 10 observations. 2 Scored as eating = 1; facing forward = 2; facing sideways = 3; facing backwards = 4; and attempting to escape = 5. 3 Scored as standing = 1; and crouching = 2. TABLE 3. Correlations coefficients between performance traits and behaviors, trial 2 Behavior trait Performance trait 1 Eating Drinking Standing Resting Walking Pecking Preening BW48 0.084** 0.060* 0.191** 0.101** 0.024 0.001 0.095* FI4859 0.044 0.036 0.073** 0.041 0.018 0.029 0.044 BW59 0.109** 0.032 0.130** 0.042 0.011 0.006 0.104** WG4859 0.057* 0.036 0.071* 0.082** 0.018 0.008 0.031 FCR4859 0.030 0.070* 0.153** 0.136** 0.011 0.036 0.020 ADJFCR 0.027 0.053 0.155* 0.124 0.008 0.044 0.014 1 BW48 = BW at the start of the feed conversion ratio (FCR) test; FI4859 = feed intake from 48 to 59 d of age (the length of the FCR test); BW59 = BW at 59 d of age (the end of the FCR test); WG4859 = weight gained from 48 to 59 d of age (weight gained on test); FCR4859 = FCR (grams of feed per gram of gain) from 48 to 59 d of age; and ADJFCR = FCR from 48 to 59 d of age adjusted for 48 d BW. *P 0.05. **P 0.01.

536 SKINNER-NOBLE ET AL. TABLE 4. Behavior traits by feed conversion ratio (FCR) quartile, trial 2 75th to 100th 50th to 75th 25th to 50th 0 to 25th Pooled Behaviors 1 percentile percentile percentile percentile Mean SEM Eating 0.93 1.01 0.99 0.87 0.93 0.03 Drinking 0.85 0.73 0.69 0.67 0.73 0.03 Standing 0.94 a 0.79 ab 0.70 ab 0.51 c 0.79 0.03 Resting 6.90 b 7.00 b 7.23 b 7.58 a 7.12 0.06 Walking 0.16 0.16 0.16 0.11 0.15 0.01 Pecking 0.03 0.03 0.02 0.01 0.02 0.01 Preening 0.19 0.27 0.26 0.19 0.24 0.01 Active behaviors 2.13 2.17 2.05 1.88 2.05 0.05 Passive behaviors 7.87 7.83 7.95 8.12 7.95 0.05 FCR test results BW at start of test (g) 2,371 d 2,504 c 2,633 b 2,772 a 2,549.6 22.50 Feed consumed on test (g) 1,651 1,786 1,763 1,796 1,750.1 25.50 BW at end of test (g) 3,296 3,351 3,374 3,376 3,311.4 29.71 Weight gained on test (g) 925 a 847 b 741 c 603 d 774.7 14.94 FCR on test (g:g) 1.79 d 2.11 c 2.38 b 3.02 a 2.34 0.04 a d Means within a row with no common superscripts differ (P < 0.05). 1 Number of activities recorded per 10 observations. quartiles (Table 4). As in trial 1, birds with superior feed conversion started the test lighter than their poorer converting counterparts but gained more weight on similar amounts of feed. DISCUSSION Correlated responses to selection have been well documented in meat-type poultry (Siegel and Dunnington, 1987; Nestor et al., 1996). Selection experiments for improved feed conversion (Pym and Nicholls, 1979; Pym et al., 1984; Leenstra and Pit, 1987, 1988; Buyse et al., 1998) are less common than those for increased BW (see reviews by Anthony et al., 1996; Dunnington and Siegel, 1996; Marks, 1996; Nestor et al., 1996), and they offer relatively little insight into potential consequences for behavioral traits. Feed conversion can be improved in a limited number of ways. One of those ways would be to reallocate energy from extraneous activity into gain of muscle and fat. The hypothesis that this may be occurring with continued improvement in feed conversion was not supported by the data. Instead, it was contradicted in both the present trials; these showed that broiler chickens that showed good FCR were actually less lethargic and no more fearful than those showing poor FCR values. The similar magnitudes of the correlation coefficients obtained in the two trials, in spite of the use of different strains (experimental control strain vs. commercial broilers) at different ages (40 to 47 d vs. 48 to 59 d of age) and in different feed conversion testing conditions (fasted pretest vs. no pretest fasting, 7-d testing period vs. 11-d testing period) support the robust nature of the findings. An alternative hypothesis based on the present data is that birds with poor FCR are less active and therefore may be able to deposit more fat than more active superior converters of feed into gain. Differences between good and poor converters in activity would permit dietary energy to be deposited as fat. The energy costs of fat deposition would be greater than those of muscle deposition. This hypothesis is supported by selection experiments indicating that improved feed conversion results in birds with less fat (Thomas et al., 1958; Chambers and Gavora, 1982; Leenstra and Pit, 1987; Buyse et al., 1998). Leenstra and Pit (1988) concluded that the genetic correlation between feed conversion and abdominal fat was such that selection for either one should lead to substantial changes in the other trait. These hypothesized differences in deposition of fat are conceivable in view of the lack of differences between FCR quartiles in feed intake with increased gain of superior converters. Decreased starting weights of superior feed converters may also reduce maintenance requirements, allowing more dietary energy to be available for growth. Reduced starting weights of good converters of feed to gain, however, may lead to increased days to similar market weights. Given average daily gain to start of the FCR test, the birds classified in the best quartile would require two to five additional days to reach a final BW of 1,816 g. Resulting differences in BW at 42 d would be from 120 to 350 g. It is also possible that differences in feed conversion may be due to individual differences in the shape of the growth curve. In spite of differences present among FCR quartiles in BW at the start of the feed conversion test, these differences are generally lacking by the completion of the test. When FCR data were adjusted for differences in starting BW, changes in correlation were only in magnitude and not direction. The lack of a BW effect on correlations between FCR and behavior indicate that BW is not an overriding factor affecting the relationship between FCR and behavior. At first glance, the present finding that broilers categorized as either good or poor converters showed similar levels of underlying fearfulness is inconsistent with previous reports that fear of humans was negatively associated with feed conversion in laying hens and broiler chickens in Australian laboratories and farms (Hemsworth and Barnett, 1989; Jones et al., 1993; Hemsworth et al., 1994; Jones, 1996). However, this apparent inconsistency may simply reflect genetic stock effects and methodological differences between the studies. Whereas the birds were caged individually in the present study, the laying hens referred to above

FEED CONVERSION AND BEHAVIOR 537 were housed in pairs (Hemsworth and Barnett, 1989), and the broilers were reared on the floor in commercial flocks of several thousands (Jones et al., 1993; Hemsworth et al., 1994). Increased group size and stocking density are thought to be positively related to fearfulness (Hansen, 1976; Sefton and Crober, 1976; Jones, 1997). It is also likely that the present birds received more contact with people and were thereby less fearful of human beings than those used in the Australian studies (Eddy and Gallup, 1994; Jones, 1995). In conclusion, broiler chickens that showed good FCR were actually less lethargic and no more fearful than poor feed converters. Selection for improved feed conversion in broilers, therefore, should not lead to either increased fearfulness or increased lethargy. Potential changes in the shape of the growth curve and its impact on BW at different ages, however, may complicate interpretation of feed conversion results. ACKNOWLEDGMENTS The contribution made by R. B. Jones was supported by the Roslin Institute and the Biotechnology and Biological Sciences Research Council, UK. Birds used in the first trial were supplied by N. B. Anthony of the Center of Excellence for Poultry Science at the University of Arkansas, USA. REFERENCES Anthony, N. B., K. E. Nestor, and H. L. Marks. 1996. Short-term selection for four-week body weight in Japanese quail. Poult. Sci. 75:1192 1197. Barbato, G. F., J. A. Cherry, P. B. Siegel, and H. P. Van Krey. 1980. Quantitative analysis of feeding behavior of four populations of chickens. Physiol. Behav. 25:885 891. Barbato, G. F., P. B. Siegel, and J. A. Cherry. 1982. Genetic analysis of gustation in the fowl. Physiol. Behav. 29:29 33. Buyse, J., H. Michels, J. Vloeberghs, P. Saevels, J. M. Aerts, B. Ducro, D. Berckmans, and E. Decuypere. 1998. 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