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ANIMAL PRODUCTION |
* Animal Sciences Group, Wageningen UR, Department of Nutrition and Food Edelhertweg 15, P.O. Box 65, 8200 AB Lelystad, The Netherlands;
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Department of Animal Sciences, Louisiana State University Agricultural Center, Baton Rouge 70803-4210;
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USDA-ARS-SOMMRU, 2167 NSRIC, Ames, IA 50011-3310
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: behavior • cortisol • intestinal integrity • pig • tryptophan
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Pluske et al., 1997; Jensen and Yngvesson, 1998; Ruis et al., 2000). Stress in pigs in response to mixing is assumed to affect the pig’s endocrinology, physiology, and metabolism, which result in changes in the body’s homeostasis. A relief of the negative aspects of stress after weaning and mixing may be obtained by dietary measures. It has been shown that the dietary Trp may reduce aggression and alleviate stress in many species such as pigs (Sève et al., 1991), rats (Leathwood, 1987), and chickens (Laycock and Ball, 1990; Shea-Moore et al., 1996; Rosebrough, 1996; van Hierden et al., 2004). As such, dietary Trp may improve animal welfare and overall performance of piglets after weaning and mixing.
Tryptophan has been shown to affect brain and nervous system function through interference with serotonergic neurotransmission (Adeola and Ball, 1992; Pethick et al., 1997; Huether et al., 1999). Tryptophan serves as the immediate precursor for serotonin synthesis, and Trp-induced serotonergic activity in the brain has been implicated in the regulation of many behavioral and physiological processes such as mood, aggression, susceptibility to stress, sleep patterns, and feed intake (Leathwood, 1987; Baranyiova, 1991; Huether et al., 1999; Sève, 1999; Markus et al., 2000). In theory, increased dietary concentrations of Trp would elevate brain serotonin concentration and reduce susceptibility to stress. If dietary Trp is able to reduce stress, it may thereby increase feed intake, gastrointestinal integrity, and overall growth performance in nursery pigs.
The aim of the current study was to investigate the effects of supplemental dietary Trp as-fed on behavior, neuroendocrinology, intestinal integrity, and growth performance in nursery pigs subsequent to weaning and mixing.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Animals
Eight sows (Dutch Landrace x Great Yorkshire) were housed in individual farrowing crates 18 d before farrowing, and were fed a conventional gestation diet until parturition. Nursery pigs were cross-fostered and balanced for gender after farrowing to ensure that each sow had 9 pigs. Pigs had access to nipple waterers (1 per farrowing crate) and were checked twice daily by animal care technicians. No creep feed was supplied.
Preweaning
During the lactation period, each pig within each crate was characterized based on a backtest and teat order (Ruis et al., 1997). Generally, pigs with the highest backtest score usually suckled the anterior teats. Using the backtest score and an average of the day-today teat order, pigs were characterized at weaning. Backtests were performed on each pig 2 d after farrowing. Backtests were conducted by removing each pig from its pen and placing it in the supine position on a flat hard surface for a period of 1 min. The pig was restrained manually following the technique described by Hessing et al. (1993). Each pig was given a resistance score that ranged from 0 (no resistance) to 6 (resisted 6 times during the minute) for each pig. The backtest scores were used for allotting during the mixing period. The same scientist performed all tests.
The teat order was determined daily. Pigs were marked with paint sticks in order to distinguish them from one another during teat order observations. The first teat (anterior) on the right and left sides of each sow was usually suckled by the resistant pigs in the crate, whereas the last teat (posterior) was usually suckled by the least resistant pigs in the crate. Recordings of teat order were consistent within pen over time.
Pigs were weighed twice a week for determination of BW gain. Pigs were handled daily (15 min/pen) and habituated to the cotton swabs (daily insertion of a cotton swab in the back of the pig’s mouth for several minutes) that would later be used for salivary cortisol collection. This handling was done to get the pigs accustomed to people, thus minimizing stress the days that evaluations were conducted. The same scientist performed all of the behavioral records and handling to ensure consistency. Pigs were weaned at 25 d of age.
Postweaning
After 25 d, sows were removed, and the piglets were left in their respective crates. Bars used in the farrowing crates also were removed so the piglets had an open crate in which to move around. At weaning (d 0), pens (n = 4 per dietary treatment) were randomly allotted to diets (Table 1) containing either 0 or 5 g of supplemental Trp/kg (as-fed basis). Diets were offered ad libitum and provided in pelleted form. Body weights and feed intake were recorded daily.
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On d 4, behavior was recorded from 0800 to 0900, and saliva was collected from 1000 to 1100. Subsequently, 1 pig per pen was sampled for blood via the jugular vein and subsequently killed by pentobarbital overdosing and decapitation to obtain brain and small intestine. Pigs selected to be killed were chosen so that they were balanced by BW and were considered intermediate resistant pigs in the crate.
At mixing on d 5, at 0500, pigs were mixed based on the back-test and teat order, BW, gender, and dietary treatment. Within each crate, the 2 most resistant and the 2 least resistant pigs were established. Using back test scores, teat suckling order, BW, and gender, the most resistant pig was paired with another most resistant pig from another crate, and these 2 pigs were placed in the same crate. This process was used for the 2 most resistant and 2 least resistant pigs within each crate. This pairing process was continued until all pigs had been moved and mixed with other pigs. The 4 intermediate resistant pigs within each crate were moved and mixed based on BW and gender. Each crate contained 8 pigs: 2 most resistant pigs, 2 least resistant pigs, and 4 intermediate pigs. Behavior was recorded from 0600 to 0700 and from 0800 to 0900, and saliva was collected from 1000 to 1100. Subsequently, 1 pig per pen was sampled for blood via the jugular vein and subsequently killed by pentobarbital overdosing and decapitation to obtain brain and small intestine. Pigs chosen to be killed were pigs that fought but were defeated.
On d 6, behavior was recorded from 0800 to 0900, and saliva was collected from 1000 to 1100. Subsequently, 1 pig per pen was sampled for blood via the jugular vein and subsequently killed by pentobarbital overdosing and decapitation to obtain brain and small intestine. Pigs chosen to be killed were pigs that fought on d 5 but were defeated, as recorded by behavioral analysis during mixing on d 5.
On d 10, behavior was recorded from 0800 to 0900, and saliva was collected from 1000 to 1100. At 1100 all pigs were fed the basal (0 g of Trp/kg supplemented) diet. The experiment was terminated on d 20.
Plasma Tryptophan and Cortisol
Venous blood was collected in heparinized tubes by jugular vein puncture. Tubes were immediately chilled to 0°C on ice and centrifuged at 4°C for 10 min at 2,000 x g. Plasma was stored at –20°C. Plasma Trp concentrations were measured by reversed-phase liquid chromatography using a C18 (Thermo Hypersil, Runcorn, UK) column and detected with a fluorescence detector at 217 nm (de Jonge and Breuer, 1994). The inter- and intraassay CV for the plasma Trp assay were < 10%. Plasma cortisol concentration was measured using a time-resolved fluoroimmunoassay (Ruis et al., 2001). Cortisol-binding plasma proteins were inactivated by incubating the plasma at a temperature of 95°C for 30 min. A low and a high control sample and a 50/50 (vol/vol, %) mixture of the low and high samples were analyzed to check for linearity and reproducibility. The mean concentrations and the interassay CV were 19.7, 62.8, and 43.9 ng/mL and 11.3, 6.2, and 6.9% for low, high, and 50/50 control samples, respectively. The intraassay CV (n = 14) of 4 samples with cortisol concentrations ranging between 20 and 70 ng/mL was 6.5%. Recovery of standard cortisol from plasma samples was 95%. The minimal detectable dose, or sensitivity, of the assay was 1.6 ng of cortisol per mL.
Salivary Cortisol
Pigs were introduced and habituated to cotton swabs early in the experiment to ensure stress-free sampling during the collection at crucial times. At the time of collection, pigs chewed on 2 cotton swabs simultaneously until thoroughly moistened (usually 1 min). The swabs were then placed in centrifuge tubes and placed on ice until centrifugation for 10 min at 400 x g to remove the saliva from the swabs. The swabs were removed from the tubes, and saliva samples were stored at –80°C until analyses. Cortisol content in saliva was analyzed with a solid-phase radioimmunoassay kit (Coat-a-count cortisol TKCO, Diagnostic Products Corporation, Apeldoorn, The Netherlands) modified for pig cortisol (Ruis et al., 1997). Measurement of sextuplicates of pooled saliva samples in 4 assays with mean cortisol concentrations of 18.8, 5.9, and 0.8 ng/mL resulted in mean intraassay CV of 3.6, 3.8, and 9.1%, respectively. The interassay CV for the same saliva samples in these 4 assays were 7.8, 9.8, and 7.4%, respectively. Recovery of calibrator cortisol, added to saliva, was estimated to be 104%. According to the manufacturer, cross reactivities of the assay for corticosterone, cortisone, 11-deoxycorticosterone, and 11-deoxycortisol were 0.9, 1.0, 0.3, and 11.4%, respectively. The minimal detectable dose or sensitivity of the assay was 0.13 ng of cortisol per mL.
Neurotransmitters in the Hypothalamus
Pigs were euthanized by pentobarbital overdosing via the jugular vein, and subsequently the hypothalamus was extracted from the brain within 5 to 10 min. The hypothalamus was immediately frozen using dry ice in a precooled beaker containing n-heptane and stored at –80°C until analyses. The indolamines serotonin (5-hydroxytryptamine; 5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA), and the catecholamines dopamine and its metabolites 3,4-dihydroxyphenyl acetic acid and homovanillic acid were analyzed by reverse-phase/ion-pair HPLC with electrochemical detection for the measurement of neurotransmitters as previously described by Barf et al. (1996). The limit of detection (signal/noise ratio of 3:1) was 9.5 fmol/100 µL.
Intestinal Morphology and Permeability
After euthanization, intestinal tissue was removed for determination of morphology and permeability. For intestinal morphology, 5 samples of intestinal tissue were taken from different sites (10, 25, 50, 75, and 95% of the length) along the small intestine. Tissue was cut on the mesenteric side and pinned with the serosal side to a piece of cork. The same scientist performed the tissue extraction and pinning to eliminate variation. Samples were then placed in a neutral buffered formalin (4%) with the mucosal side down so the villi were fixed vertically. After fixation, a piece of each sample was embedded in paraffin wax by standard techniques. Each sample was subsequently stained (hematoxylin and eosin) and examined under a microscope using an ocular micrometer. The same scientist executed all measuring to reduce variation. Villus height was measured from the tip of the villi to the villous-crypt junction, whereas crypt depth was measured from this junction to the base of the crypt (Nabuurs et al., 1993). Ten measurements were taken from each slide.
To measure macromolecular transport (intestinal permeability; Bijlsma et al., 1996) an intestinal tissue sample was taken from about 50% of the length of the small intestine. Afterwards, it was cut along the mesenteric border and stripped of the muscle layers. Flat sheets of tissue free Peyer’s patches were mounted in duplicate in Ussing-chambers. The exposed area of intestine was 0.7 cm2. Both sides of epithelium were perfused with Ringer’s solution (142.6 mM Na+, 5.0 mM K+, 1.25 mM Ca2+, 1.1 mM Mg2+, 123.7 mM Cl–, 25 mM, HCO3–, 1.65 mM, HPO42– 0.3 mM H2PO4–) and gassed with 5% CO2 and 95% O2. Solutions were maintained at 37°C with water jackets and circulated (total volume 4.7 mL on each side). Mucosal-to-serosal paracellular transport of macromolecules was determined using horseradish peroxidase (HRP, 40 kD, type VI, Sigma, St. Louis, MO) as a model molecule, and transcellular uptake was determined using sodium fluorescein isothiocyanate (FITC) as a model molecule. Horseradish peroxidase and sodium fluorescein isothiocyanate dissolved in Ringer’s solution were added mucosally at a final concentration of 10–5 M and 10–3 M, respectively. Serosal samples of 400 µL were taken at 30, 60, 90, 120, and 180 min and were replaced by 400 µL of fresh Ringer’s to keep the volume constant. An arithmetic correction for the dilution of the buffer was applied to calculate the amount of HRP and FITC at the serosal side.
The HRP in the serosal samples was measured enzymatically. Briefly, 200 µL of phosphate buffer (0.1 M, pH 6.0) containing 0.003% H2O2 and 0.008% o-dianisidine dihydrochloric acid was added to a 30-µL sample of the test solution and mixed. The linear, HRP concentration-dependent rate of increase in optical absorption at 460 nm was determined with a Biorad Microplate Reader, Model 550 (Biorad Laboratories, Hercules, CA). Both the interassay and intraassay CV in 6 samples were <10% for HRP concentrations. The FITC in the serosal samples was determined by measuring the fluorescence in 50 uL samples in a 96-well microplate with a Wallac VICTOR 1420 Multilabel Counter (Perkin Elmer, Wellesley, MA. Both the interassay and intraassay CV in 4 samples were <10% for FITC concentrations.
Statistical Analysis
In all analyses, the pens of pigs served as the experimental units. The analyses of data that did not include repeated measures (growth data per period, brain metabolites, and intestinal permeability) were done using a completely randomized design, where treatments (0 and 5 g L-Trp/kg diet) and treatments x periods were the experimental factors. The percentages of a given type of behavior were fitted to the following logistic regression: log(p%[100% –p]) = treatments, in which p% was the expected percentage. The repeated measurements data on saliva and plasma cortisol, and the means of villus height, crypt depth, and their ratio across 10, 25, 50, 75, 95% of the small intestine were analyzed per day with ANOVA and the Student’s 1-sided t-test, respectively. Effects and differences were declared significant when their P values were less than 0.05; an effect with its P value between 0.05 and 0.10 was considered a trend. All analyses were performed with GENSTAT software (Payne et al., 1987). The results are expressed as means +/–SEM.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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).
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). In contrast, plasma cortisol concentrations were not different between the dietary groups, whereas mixing induced a significant (P < 0.05) plasma cortisol response in both groups (d 4, before mixing, 11.7 ± 1.8 vs. 11.3 ± 2.9; d 5, at mixing, 35.6 ± 10.4 vs. 41.7 ± 19.9; and d 6, after mixing, 17.4 ± 7.9 vs. 34.7 ± 15.7 ng/mL; in control vs. Trp-supplemented pigs, respectively).
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). In addition, when intestinal morphology was analyzed by orthogonal polynomial contrasts, it seemed that villus height was greater, crypt depth was smaller, and VH:CD was greater in pigs fed supplemental Trp relative to those fed the basal diet. More specifically, villus height at 10, 25, 50, 75, and 95% of the length of the small intestine differed (P < 0.01) among the dietary treatments, as indicated by cubic polynomial contrasts; villus height at 25, 50, 75, and 95% differed (P = 0.06), as indicated by quadratic polynomial contrasts; villus height at 50, 75, and 95% differed (P = 0.04), as indicated by quadratic polynomial contrasts; crypt depth at 10, 25, 50, 75, and 95% differed (P = 0.08), as indicated by cubic polynomial contrasts; crypt depth at 25, 50, 75, and 95% differed (P = 0.04), as indicated by quadratic polynomial contrasts; and VH:CD at 10, 25, 50, 75 and 95% differed (P < 0.05), as indicated by mean polynomial contrasts. Intestinal permeability was not affected by dietary treatment (Figure 2).
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). Average daily gain, ADFI, and G:F were not affected by Trp supplementation (Table 5).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Dietary and Plasma Tryptophan Concentrations
Increasing dietary Trp from 2 g of true digestible Trp/kg diet to 7 g of true digestible Trp/kg diet through supplementation with 5 g of Trp/kg increased dietary Trp availability by 3.5-fold, which resulted in a 2-fold increase in the plasma Trp concentrations 4 to 6 d after diet introduction.
Serotonergic Activity in the Hypothalamus
Dietary Trp supplementation increased both 5-HT and 5-HIAA concentrations in the hypothalamus of piglets. The turnover of serotonin or the activity of the serotonergic nervous system is generally expressed as the ratio between 5-HT and its direct metabolite 5-HIAA (van Hierden et al., 2002). Our study indicated that feeding pigs diets supplemented with 5 g of Trp/kg resulted in a 38% increase in this ratio. The catecholamines were not as affected as the indolamines by Trp supplementation. In general, our results are in agreement with previous studies in pigs. Brain indolamines and catecholamines were evaluated in pigs fed diets in combination with deficient, adequate, or excess Trp. The addition of Trp increased 5-HT, 5-HIAA, and total hydroxyindole concentration in many regions of the brain (Meunier-Salaün et al., 1991; Adeola and Ball, 1992; Henry et al., 1996). Passage of Trp through the blood-brain barrier occurs within the physiological range of blood Trp concentrations. Large neutral AA like Ile, Leu, Val, Phe, and Tyr compete with Trp for passage through the blood-brain barrier. Thus, there is no absolute threshold for blood Trp concentrations to enter the brain (Sève, 1999).
Salivary and Plasma Cortisol Concentrations
The hypothalamic-pituitary-adrenal axis is activated during stress, resulting in the release of glucocorticoids from the adrenal cortex in blood, and subsequently the appearance in saliva (Korte, 2001). In our study, salivary but not plasma cortisol concentrations were reduced approximately 2-fold before and at mixing in Trp-supplemented pigs compared with control pigs. The reason for this discrepancy between salivary and plasma cortisol concentrations could be the fact that the cortisol concentration in saliva is mainly in a free, biologically active form and reflects only 5 to 10% of the total (free and bound) cortisol concentration in plasma (Kirschbaum and Hellhammer, 1989; Parrott et al., 1989). Any changes in the free cortisol concentration in plasma could be masked by the surplus total concentration of cortisol in plasma.
Plasma but not salivary cortisol concentrations were increased 4-fold at mixing compared with the premixing levels in both dietary groups. The reason for the discrepancy in cortisol response at mixing between plasma and saliva could be the difficulty with saliva sampling at mixing, particulary in the control group. The volume of saliva that could be collected from the relatively dry mouths of the stressed pigs was too low for proper salivary cortisol analysis. Most likely, pigs with the highest level of stress were not included in the salivary cortisol analysis, and this could be the reason for the absence of a clear salivary cortisol response at mixing in the control group. On d 0 (weaning), 3, 4, 5 (mixing), 6, and 10, the number of pigs sampled with a sufficient volume of saliva for cortisol analysis were 21, 18, 9, 10, 14, and 5 vs. 25, 24, 8, 5, 11, and 10 in the 5 g of Trp/kg diet vs. the 0 g of Trp/kg diet group, respectively. Salivary cortisol lags behind (several minutes) plasma cortisol in reflecting increases (Kirschbaum and Hellhammer, 1989; Parrott et al., 1989). This does not, however, explain the differences between salivary and plasma cortisol responses at mixing because salivary and plasma cortisol were measured several hours after the onset of mixing.
Intestinal Villus Height and Crypt Depth
Intestinal morphology can be used as an indicator of stress in pigs (Nabuurs et al., 1993; Pluske et al., 1997). Weaned pigs, because of the weaning stressor, often have shortened villi and deepened crypts. In our study, Trp-supplemented pigs vs. control pigs showed 15% greater villus height, a tendency toward 13% smaller crypt depth in the distal half of the small intestine, and an overall (11 to 18%) increase in the VH:CD along the entire length of the small intestine. Glutamine also has been reported to increase intestinal villus height in trauma and surgical patients (Miller, 1999). In addition, dietary glutamine supplementation was found to significantly reduce villus atrophy in weaning piglets (Ayonrinde et al., 1995; Pluske et al., 1997). The Trp effect on increased villus height in the current study may therefore be a nonspecific effect on intestinal epithelial cell growth due to a high nutritional availability of the crystalline AA Trp to these villi/crypt-forming cells. On the other hand, if this is true, it may be expected that the beneficial effect of free Trp will be most pronounced in the proximal segments of the ileal tract due to the high concentration of free Trp expected in this segment; however, after absorption, one would expect a low concentration of free Trp in the distal intestine. This was not the case because villus height was greater in the distal segments of the small intestine and VH:CD was greater along the entire length of the small intestine. Supplemental dietary Trp may therefore improve intestinal morphology indirectly through its inhibitory effect on stress-related neuroendocrinology. It is well known that stress and a general stimulation of the sympathetic nervous system lead to a reduction in both gut blood flow and absorption (Newsholm and Leech, 1983; Kraft et al., 1992; Gruys et al., 1998).
Intestinal Permeability
Intestinal absorption can be decreased during times of stress and influence the occurrence of diarrhea (Kraft et al., 1992; Nabuurs et al., 1993; Gruys et al., 1998). In our study, intestinal permeability was measured by both paracellular (HRP) and transcellular (FITC) transport. Mixing induced a significant reduction in transcellular transport but not in paracellular transport, indicating that mixing reduces the transcellular absorption capacity of nutrients without affecting the integrity of the intestinal wall. Supplemental dietary Trp did not affect intestinal permeability, which shows that dietary Trp improves intestinal morphology (villus height and crypt depth) without affecting intestinal absorption.
Behavior
Behavioral results from our study were not indicative of what was expected based on the theory of Trp and its effect on aggression at mixing. Physical activity (lying, standing, and sitting) was not affected before (d 4), at (d 5), or after (d 6) mixing by the dietary treatment. Only 10 d after introduction of the diets (5 d postmixing) was an inhibitory effect of dietary Trp on physical activity observed. It could be that prolonged feeding of a sufficient amount of dietary Trp is necessary to detect a significant effect on behavior. After weaning and the introduction of the diets, voluntary feed intake is very low for several days in piglets, and therefore it may take more time and/or more Trp to have a pronounced effect on physical activity. This finding largely agrees with a study conducted by Sève et al. (1991), who evaluated behavior (grunts, squeals, ambulation, and exploration) 5, 23, and 45 d postweaning in piglets given varied levels (0.14, 0.23, or 0.32%) of dietary Trp. In the Sève et al. (1991) study, behavioral reactivity, as determined by the results of an open-field test, was not influenced by dietary Trp. In a companion paper, Meunier-Salaün et al. (1991) reported that dietary Trp levels induced minor changes in behavioral responses.
Growth Performance
Zootechnical results were similar in Trp-fed piglets compared with control piglets both 0 to 10 and 10 to 20 d postweaning. Days 5 and 6 postweaning were not recorded due to the mixing of piglets and the concomitant inaccuracy of feed intake registration. Due to the removal of pigs from replicate groups at d 4, 5, and 6, and the mixing of pigs at d 5, the calculation of SEM values for BW and feed intake were omitted and statistical analysis could not be performed. In general, on d 0 to 10, the Trp-supplemented pigs compared with the control pigs seemed to grow faster (ADG: 51 compared with 28 g, respectively) and on d 10 to 20, when all pigs were placed on the basal diet, the pigs that had originally been fed the Trp-supplemented diet compared with the control pigs, seemed to grow slower (ADG: 169 compared with 237 g, respectively). The absence of a clear effect of supplemental Trp on growth performance in our study may be caused by low voluntary ADFI (and thus low dietary Trp intake) in the days after weaning in piglets. This is in contrast with a study by McGlone et al. (1985), who evaluated the effect of an additional 5 g of Trp/kg of diet on regrouping and heat stress in 10-kg pigs. Regrouped pigs fed Trp had improved ADG and ADFI during the first 7 d of the trial.
In summary, our experiment evaluated the impact of dietary supplementation of Trp on many response variables that might be affected by weaning and mixing in nursery piglets. The increase in serotonergic activity and the decrease in salivary cortisol at mixing indicate that Trp reduces stress. In contrast, physical activity was only affected 10 d postweaning and not at mixing by Trp. Increased villus height and VH:CD but unaffected transport of macro molecules in the intestine show that Trp improves morphology without affecting permeability. Average daily gain and feed intake seemed unaffected by Trp supplementation. Consequently, diets containing high Trp concentrations improve neuroendocrine components of stress and increase gastrointestinal robustness but do not affect behavioral reactivity in nursery pigs during specific periods of increased stress, such as weaning and mixing. However, prolonged feeding of a sufficient amount of Trp may be necessary to detect unequivocal effects on behavior, stress, gastrointestinal integrity, and growth performance.
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Footnotes |
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2 We would like to thank G. Korte-Bouws and A. Hoogendoorn for their excellent technical assistance and E. D. Ekkel for advice on behavioral analyses.
3 Corresponding author: sietse-jan.koopmans{at}wur.nl
Received for publication February 23, 2005. Accepted for publication November 7, 2005.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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