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Effect of Lighting Stress on Fluctuating Asymmetry, Heterophil-to-Lymphocyte Ratio, and Tonic Immobility Duration in Eleven Breeds of Chickens

Departamento de Mejora Genética Animal, Instituto Nacional de Investigación Agraria y Alimentaria, 28080 Madrid, Spain

¹ Corresponding author: jlcampo{at}inia.es

	ABSTRACT

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The purpose of the present study was to analyze the effect of a lighting stress on the fluctuating asymmetry (FA), the heterophil-to-lymphocyte ratio, and the tonic immobility duration of chickens. The experiment (440 birds) measured the FA of several traits (outer, middle, inner, and hind toe lengths and leg, wing, second primary feather, and spur lengths), the heterophil-to-lymphocyte ratio, and the tonic immobility duration in 36-wk-old hens and cocks of 8 Spanish breeds of chickens (Black-Barred Andaluza, Black-Red Andaluza, Black Castellana, Buff Prat, Red-Barred Vasca, Red Villafranquina, Birchen Leonesa, and Blue Leonesa), a synthetic breed (Quail Castellana), a White Leghorn population, and the e^y tester line, which had been housed in continuous light (24L:0D) or in a light-dark regimen (14L:10D) for 16 wk. There was a significant difference between lighting treatments in both females and males on the combined FA of the 4 toes (P < 0.01) and the combined FA of toe, leg, wing, feather, and spur (in males) lengths (P < 0.05), the FA of birds housed under continuous light being greater than that of control birds. There was a significant difference (P < 0.001) for the heterophil-to-lymphocyte ratio and the tonic immobility duration between lighting treatments, the ratio being higher and the duration being longer in the group of birds housed under continuous light. Thus, birds exposed to continuous light were more stressed and fearful than control hens. Results were consistent across the breeds and indicate that a continuous light regimen seriously negatively affects the welfare of birds.

Key Words: stress • light • fluctuating asymmetry • heterophil-to-lymphocyte ratio • tonic immobility

	INTRODUCTION

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The vision of chickens evolved in natural light environments and differences between the artificial light provided in poultry houses and that required for effective vision may produce stress and poor welfare. Broilers are usually kept on a continuous or nearly continuous lighting schedule to maximize growth rate (Rozenboim et al., 1999), whereas continuous lighting could be used to increase egg production in layers (Yoo et al., 1986). In relation to animal welfare, a day and night light schedule for birds kept in captivity with a minimum dark period of 6 h is recommended (Prescott et al., 2003), because continuous light programs reduce the opportunity for sleep and sleep deprivation increases physiological stress (Blokhuis, 1983). Additionally, Gordon (1994) suggested that maximal benefit is obtained by rearing broilers under a lighting regimen of 16L:8D, because it reduces physiological stress; Morris (2004) indicated that layers held on a constant 14L:10D day will lay as many eggs as ones given step-up lighting.

Although most of the reports on lighting patterns deal with their effects on production, a few of them have studied the effect of light program on 3 frequently used indicators of stress: the level of fluctuating asymmetry (FA; Parsons, 1990), the heterophil-to-lymphocyte ratio (Gross and Siegel, 1983), and the duration of tonic immobility (Gallup, 1979). Information on the effect of lighting schedule on the level of FA is generally lacking, and only 2 studies have analyzed the effect of the lighting regimen on the FA of broiler chickens, with contradictory results. Moller et al. (1999) compared the FA of 3 characters (length, width, and thickness of the metatarsus) in a fast-growing population of chickens, using 3 different light regimes (16L:8D light-dark cycle, changing light regimen, and continuous light), and they found that chickens reared under continuous light developed larger relative asymmetry. In contrast, Stub and Vestergaard (2001) found that FA for those 3 same characters was similar in broilers reared under continuous light combined with no access to sand and in those exposed to 16L:8D with access to sand.

Results on the heterophil-to-lymphocyte ratio are also scarce and contradictory. In broilers, Blair et al. (1993) indicated that there was no effect of lighting pattern (23L:1D or increasing from 6L:18D to 23L:1D), whereas Vo et al. (1998) found that continuous lighting increased the percentage of heterophils and decreased the percentage of lymphocytes in comparison with 16L:8D and 12L:12D. Campo and Dávila (2002) did not find significant differences in the heterophil-to-lymphocyte ratio among 3 lighting regimens (23L:1D, 14L:10D, and 18.5L:5.5D) in hens from 3 breeds.

Finally, 3 studies have examined the effect of light program on the duration of tonic immobility. Sanotra et al. (2002) indicated that 2 changing light-dark schedules, with or without a reduced stocking density, decreased the duration of tonic immobility compared with continuous light in broilers. Campo and Dávila (2002) found that hens of a synthetic breed housed under a 23L:1D light program showed longer tonic immobility duration than hens housed under 14L:10D. When light regimen and access to sand were used together, Stub and Vestergaard (2001) found that they did not significantly influence the duration of tonic immobility.

The purpose of the present study was to analyze the effect of a lighting stress (continuous light vs. light-dark schedule) on combined FA of multiple characters, the heterophil-to-lymphocyte ratio, and the duration of tonic immobility in 11 breeds of chickens with different genetic backgrounds. It was hypothesized that continuous light would negatively affect the well-being of birds.

	MATERIALS AND METHODS

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Hatches were made from April 5 to May 31. Birds were reared with a density of 10 birds/m² until 8 wk of age (sexes mixed). Artificial light was only provided during the first week of age (23L:1D). Temperature was controlled with gas heaters (33 to 35°C at the chick level during the first week, followed by a reduction of 3°C each week until 18 to 20°C was reached in the sixth week of age). Birds were reared in another all-litter house with a density of 6 birds/m² from 8 to 20 wk of age. The lighting regimen was 8L:16D. Birds were fed standard rearing diets containing 19% CP, 2,800 kcal of ME/kg, 1% Ca, and 0.5% available P until 8 wk of age and 15% CP, 2,700 kcal of ME/kg, 0.9% Ca, and 0.4% available P until 20 wk of age. Birds were housed at 20 wk of age (sexes separate) in pens with a raised slatted floor covering a dropping pit and straw litter on the rest of the floor; the slatted area occupied one-third of the floor. Bird density was 4 birds/m². Birds were fed a diet containing 16% CP, 2,700 kcal of ME/kg, 3.5% Ca, and 0.5% available P. Feed and water were supplied for ad libitum consumption. The room temperature was 16 to 20°C. Feeders, drinkers, and nest boxes were in the slatted floor area.

Eight different Spanish breeds of chickens (Black-Barred Andaluza, Black-Red Andaluza, Black Castellana, Buff Prat, Red-Barred Vasca, Red Villafranquina, Birchen Leonesa, and Blue Leonesa), a synthetic breed (Quail Castellana) that originated from an F₂ cross between Black Castellana and Buff Prat (Campo and Orozco, 1986; Campo, 1991), a White Leghorn population originated by crossing 3 strains selected for egg number and egg weight (Campo and Jurado, 1982), and the e^y tester line (Smyth, 1976) was used in the current study. All these breeds were maintained at the experimental station of El Encín (Madrid, Spain) in a conservation program of genetic resources started in 1975 (Campo and Orozco, 1982) and have been described by Campo (1998). Black-Barred Andaluza, Black-Red Andaluza, and Black Castellana are white shell egg breeds, whereas Buff Prat is a tinted shell egg breed and Red-Barred Vasca and Red Villafranquina are brown and dark brown shell egg breeds, respectively. Males of the Birchen Leonesa and Blue Leonesa breeds are used for fly tying.

Five replicates (hatches) separated by 14 d were used in the experiment. A total of 440 birds (220 females and 220 males) was used to study the effect of a lighting stress on the fluctuating asymmetry, the heterophil-to-lymphocyte ratio, and the tonic immobility duration at 36 wk of age. Within sexes, the birds were equally divided into 2 groups. Group 1 (stress) consisted of 110 birds, 10 birds of each population in 5 replicates of 2 birds, housed in continuous light (24L:0D). Group 2 (control) consisted of 110 additional birds, 10 birds of each population in 5 replicates of 2 birds, housed in a lighting regimen of 14L:10D (light from 0700 to 2100 h). Light intensity was 15 lx.

The measured morphological traits were both right (RT) and left (LT) outer, middle, inner, and hind toe lengths and leg (metatarsus), wing (radius), second primary feather, and spur (in males) lengths. Only undamaged birds with intact second primary feathers were analyzed for feather length (216 and 214 instead of 220 females and 220 males, respectively). There was no molt of the primary feathers. Right and LT values of a bird were taken during the same session. All lengths were measured in millimeters using a digital calliper. Trait length was the mean of the RT and LT traits [(RT + LT)/2]. All traits showed normal frequency distributions.

The FA for a trait, a measure of chronic stress (Moller and Swaddle, 1997), was defined by the absolute difference between sides [|RT – LT|]. The unsigned value (equivalent to the mean deviation) is more appropriate for these data than the variance of RT – LT sides. A series of steps (Palmer, 1994; Knierim et al., 2006) was followed before identifying exhibited asymmetry as FA (normal distribution of signed RT – LT differences with a mean of 0), because there are several confounding factors that complicate the analysis of asymmetry. First, the presence of directional asymmetry (DA; normal distribution with a mean of not 0) and antisymmetry (AS; nonnormal distribution with a mean of 0) was tested by the inspection of the distribution of (RT – LT). The presence of DA was tested for using t-tests. Departures from normality (e.g., AS) were assessed using skewness and kurtosis measures; AS is characterized by bimodal (or broad-peaked) distributions, tending to be platykurtic (more intermediate values than the normal distribution). Second, FA and measurement error are normally distributed about a mean of 0. Thus, it is essential to show that the variance in asymmetry observed among individuals is greater than the variance due to measurement error; the FA is often small and sometimes of the same magnitude as measurement error. Twenty females and 20 males were randomly chosen and measured 3 times on 3 different days. Measures were analyzed using a 2-way ANOVA (Leamy, 1984) with side (fixed) and bird (random) as main factors (1 and 19 df), their interaction (19 df), and the measurement error (80 df). Significant variation between sides indicates variation in DA, whereas a significant interaction indicates significant FA (in the absence of AS). Finally, the product-moment correlation between FA and trait length was used to determine if they were independent. If a positive relationship was found between the mean value and asymmetry of a trait, this effect would be removed by dividing the absolute asymmetry score by the trait mean, defined as the relative FA: [2|RT – LT|/(RT + LT)]. Relative FA for all traits had distributions that were not normal and were transformed to arcsin square root before analysis. Mean relative asymmetry was defined as the mean of the relative asymmetries of the different traits. If no size correction was needed, each |RT – LT| value was standardized and averaged across traits to give a total score.

On 2 different days, birds were tested for heterophil-to-lymphocyte ratio and tonic immobility duration at 36 wk of age. Heterophil-to-lymphocyte ratio is a reliable indicator of stress in poultry (Gross and Siegel, 1983). To obtain the heterophil-to-lymphocyte ratio (on the same day as morphological traits), birds were carried to a separate room, and blood was collected immediately. Two drops of blood were taken from a small puncture in the comb of each bird, 1 drop being smeared on each of 2 glass slides. The smears were stained using May-Grünwald and Giemsa stains (Lucas and Jamroz, 1961), approximately 2 to 4 h after methyl alcohol fixation. One hundred leukocytes, including granular (heterophils, eosinophils, and basophils) and nongranular (lymphocytes and monocytes), were counted on 1 slide of each bird (the other slide was supplementary), and the heterophil-to-lymphocyte ratio was calculated. Square root transformation was used before analysis.

All birds were tested for tonic immobility in a separate room on the day following the blood sampling. Tonic immobility is a traditional measure of fearfulness in poultry (Gallup, 1979), and fear is considered a measure of low welfare. Tonic immobility was induced, as soon as a bird was caught, by placing the bird on its back with the head hanging in a U-shaped wooden cradle (Jones and Faure, 1981). The bird was restrained for 10 s. The observer sat in full view of the bird, about 1 m away, and fixed his eyes on the bird because of the fear-inducing properties of eye contact. If the bird remained immobile for 10 s after the experimenter removed his hands, a stopwatch was started to record latencies (s) until the bird righted itself. If the bird righted itself in <10 s, then it was considered that tonic immobility had not been induced, and the restraint procedure was repeated (3 times maximum). If the bird did not show a righting response over the 10-min test period, a maximum score of 600 s was given for righting time. Thus, tonic immobility duration ranged from 0 to 600 s. Logarithmic transformation was used before analysis.

To test the differences in FA, heterophil-to-lymphocyte ratio, and tonic immobility duration between lighting treatments, a 3-way ANOVA (Sokal and Rohlf, 1981) was used with the statistical model x_ijkl = µ + L_i + B_j + LB_ij + r_k + Lr_ik + Br_jk + LBr_ijk + _ijkl, where x_ijkl = the analyzed measurement; µ = the overall mean; L_i = the effect of lighting treatment (i = 1, 2); B_j = the effect of breed (j = 1 to 11); r_k = the effect of replicate (k = 1 to 5); LB_ij, Lr_ik, Br_jk, and LBr_ijk = the interactions; and _ijkl = the residual (l = 1, 2). Lighting treatment and breed were considered fixed effects, and replicates were assumed to be a random effect. Significant differences among breeds were estimated using the Student-Newman-Keuls’ multiple range test (Snedecor and Cochran, 1980).

	RESULTS

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Effect of Confounding Factors
In females, hind toe and wing lengths exhibited significant DA (t = 3.25, P < 0.001, and t = 2.68, P < 0.01, respectively), and the RT side was consistently greater than the LT side. Signed RT – LT differences were significantly leptokurtic (more small and more large values relative to the normal distribution) for feather length (g₂ = 0.62, P < 0.05). Thus, there was no evidence of AS, because bimodal (or broad-peaked) distributions tend to be platykurtic. In males, inner toe and wing lengths showed significant DA (t = –3.73, P < 0.001, and t = 9.29, P < 0.001, respectively), the LT side being consistently greater than the RT side or vice versa. Signed RT – LT differences were significantly left-skewed for outer and middle toe lengths (g₁ = –0.37, P < 0.05, and g₁ = –0.36, P < 0.05, respectively), right-skewed for inner toe length (g₁ = 0.43, P < 0.01), and leptokurtic for inner toe length and feather length (g₂ = 0.74, P < 0.05, and g₂ = 1.43, P < 0.01, respectively), with no evidence of AS.

Mean square error for outer, middle, inner, and hind toe, leg, wing, and feather lengths (Table 1) represented 30.09, 50.28, 87.73, 71.15, 54.39, 104.63, and 5.00% of the interaction mean square in females and 31.07, 31.11, 17.48, 57.60, 49.40, 177.18, and 32.54% in males (13.28% for spur length). Additionally, the interaction was significant for outer, middle, inner (in males), hind (in males) toe, leg, feather, and spur (in males) lengths. Thus, measurement error was not confounded with FA (except for wing length in both sexes), and the non-DA identified in the analysis was categorized as true FA. Measurement error accounted for 8.01, 8.51, 16.15, 18.87, 0.98, 13.21, and 2.19% of the total variation in females and 27.57, 24.16, 31.74, 47.69, 4.72, 10.37, and 4.22% in males (2.90% for spur length), respectively. Between sides mean square was not significant for outer (in males), middle, inner, hind toe, leg (in males), wing, feather (in females), and spur (in males) lengths and provided no evidence of DA.

Analysis of FA
Replicates, replicate x lighting treatment, replicate x breed, and replicate x lighting treatment x breed interactions were not significant for any measurement, and they were pooled with the residual to give a 2-way ANOVA of lighting treatment and breed effects [x_ijk = µ + L_i + B_j + LB_ij + _ijk, where x_ijk = the analyzed measurement; µ = the overall mean; L_i = the effect of lighting treatment (i = 1, 2); B_j = the effect of breed (j = 1 to 11); LB_ij, = the interaction; and _ijk = the residual (k = 1 to 10)]. Mean values indicating the effect of the lighting treatment and the breed on relative asymmetry measurements of females are summarized in Tables 2 and 3. Lighting effect was significant for the relative asymmetry of outer toe length (P < 0.05), the combined relative asymmetry of the 4 toes (P < 0.01), wing length (P < 0.001), and the combined relative asymmetry of leg, wing, feather, and toes lengths (P < 0.05). The relative asymmetry of females housed with continuous light was larger than that of control females (except for wing length). Although there were significant lighting treatment x breed interactions for the relative asymmetry of outer toe lengths, results were consistent across the breeds, the lighting treatment being larger than the control in all the breeds (quantitative interaction). The lighting treatment x breed interaction was significant and qualitative (the treatments did not keep their rankings in all breeds) for the relative asymmetry of the middle toe length, the lighting treatment being significantly larger than the control in the Black-Red Andaluza, Buff Prat, and Red Villafranquina breeds. There were significant differences among breeds for the relative asymmetry of outer (P < 0.05), inner (P < 0.01), and hind toe lengths (P < 0.01); the combined relative asymmetry of the 4 toes (P < 0.001); wing length (P < 0.01); feather length (P < 0.001); and the combined relative asymmetry of leg, wing, feather, and toe lengths (P < 0.001), the relative asymmetry being larger (except for wing length) in the e^y tester line than in the other breeds.

Results were similar for males (Table 7). The heterophil-to-lymphocyte ratio, the heterophil number, and the tonic immobility duration were significantly greater, and the lymphocyte number was significantly smaller in males housed with continuous light than in the control males (P < 0.001). In addition, the lighting treatment x breed interaction was quantitative. There were significant differences among breeds (P < 0.001); the heterophil-to-lymphocyte ratio and heterophil number were highest and the lymphocyte number smallest in the White Leghorn, whereas the reverse was true in the Birchen Leonesa. Tonic immobility duration was longer in the Black-Red Andaluza, Blue Leonesa, and White Leghorn breeds and shorter in the Buff Prat breed.

	DISCUSSION

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The current study fulfilled the criteria to estimate the FA for a trait confidently (Palmer, 1994; Knierim et al., 2006) without the effect of confounding factors. The type of asymmetry was determined, an assessment was made of measurement error, and the relationship between the amount of asymmetry and trait size was controlled. Additionally, combined FA estimates of multiple traits were used; the effects of lighting treatment, breed, and its interaction were estimated using the ANOVA and Student-Newman-Keuls’ multiple-range procedures; and a large sample size was used.

Although there were significant effects of the lighting stress on the FA for outer toe and feather (in males) lengths, these traits should be excluded from the analysis in females and males, respectively, because they showed significant DA in the ANOVA (Leamy, 1984). Wing length should be excluded, too, because it showed significant DA using the t-test in both females and males, and, additionally, estimates of FA were confounded with measurement error. Then, there were only 2 single traits with significant lighting effect on their FA (i.e., middle toe length in females and outer toe length in males), although in the former the effect of light only was significant in 3 breeds. However, there were significant effects in both females and males of the lighting stress on the combined FA of the 4 toes and the combined FA of all lengths, indicating that FA is a valid indicator of environmental stress (Parsons, 1990), especially if the analysis is based on the composite FA of multiple traits (Leung et al., 2000). In all the cases, the FA of birds housed with continuous light was larger than that of control birds. This result confirmed the finding of Moller et al. (1999) that broiler chickens reared under continuous light developed larger mean relative FA for length, width, and thickness of the metatarsus (4.7%) than chickens reared under 16L:8D or changing light regimes (3.6 and 3.3%, respectively), although these authors found a significant effect on the FA for 2 of the 3 single traits (length and thickness of the metatarsus; 2.4 vs. 1.8% and 6.8 vs. 4.8%, respectively). On the contrary, the finding in the current study disagrees with the result reported by Stub and Vestergaard (2001), who did not find any significant effect on the degree of FA in the chickens reared under the continuous light-no sand and the 16L:8D-sand conditions, although the effect of each factor could not be separated.

The difference in heterophil-to-lymphocyte ratio among birds housed with the 2 lighting regimes was significant in both sexes. Birds housed with continuous light showed higher ratios than those housed with 14L:10D, birds in the former group having significant heterophilia and lymphopenia. Thus, it is suggested that a continuous lighting schedule was associated with an increased stress response and affected bird welfare. This result agrees with that of Vo et al. (1998), who reported that heterophil-to-lymphocyte ratio was higher in the continuous light group of birds. However, the result in the current study disagrees with those of Blair et al. (1993) and Campo and Dávila (2002), who reported that heterophil-to-lymphocyte ratio was unaffected by a nearly continuous lighting schedule (23L:1D).

There were significant differences between lighting treatments in both sexes for the tonic immobility duration. Tonic immobility of birds housed under 24L:0D was longer than that of birds housed under 14L:10D, suggesting that continuous lighting treatment caused greater fearfulness. This finding agrees with results reported by other authors, who found an association between lighting treatment and fearfulness. Sanotra et al. (2002) reported that changing light-dark schedules, in combination or not with reduced stocking density, had a decreasing effect on fearfulness in broiler chickens in comparison with a continuous light regimen. Similarly, Campo and Dávila (2002) reported a decreasing effect on fearfulness in a synthetic breed for a 14L:10D light treatment in comparison with a nearly continuous lighting schedule (23L:1D). On the contrary, the result disagrees with the finding by Stub and Vestergaard (2001), who did not indicate any significant effect associated with a continuous light treatment in broiler chickens in comparison with a 16L:8D light treatment.

In conclusion, results of the current study indicated that adult chickens from several diverse populations housed with continuous light have increased combined relative FA value of several traits, the heterophil-to-lymphocyte ratio, and the tonic immobility duration and are more stressful and fearful than birds housed under a light-dark program (14L:10D). It is suggested that a continuous light regimen causes severe negative welfare consequences.

Received for publication April 20, 2006. Accepted for publication September 6, 2006.

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