Developmental instability and light regime in chickens (Gallusgallus)

Applied Animal Behaviour Science 62 Ž1999. 57–71

Developmental instability and light regime in chickens žGallus gallus / A.P. Møller

a,)

, G.S. Sanotra b, K.S. Vestergaard

b

a

Laboratoire d’Ecologie, CNRS URA 258, UniÕersite´ Pierre et Marie Curie, Bat. ˆ A, 7eme ` etage, ´ 7 quai St. Bernard, Case 237, F-75252 Paris Cedex 05, France b Department of Animal Science and Animal Health, The Royal Veterinary and Agricultural UniÕersity, BulowsÕej 13, DK-1870 Frederiksberg C, Denmark ¨ Accepted 9 September 1998

Abstract Measures of developmental instability such as fluctuating asymmetry reflect the ability of individuals to cope with stressful conditions during ontogeny, and a very low level of asymmetry is known a priori to be the optimal solution from a functional morphological point of view. Fluctuating asymmetry represents a potentially important tool for assessment of the reaction of animals to their environment and thus for the study of animal welfare. We measured fluctuating asymmetry in three skeletal characters in chickens raised under three different light regimes: ŽA. a 16:8 light–dark cycle, ŽB. a changing light regime, and ŽC. permanent light. Chickens reared under permanent light developed 40% larger relative asymmetry than chickens subjected to changing light and dark conditions. Tonic immobility, a behavioural measure of fearfulness, was positively correlated with fluctuating asymmetry in chickens reared under all three light regimes. Fluctuating asymmetry was also positively related to the severity of a leg pathological condition Žtibial dyschondroplasia. and positively related to a measure of impaired walking performance. These results suggest that fluctuating asymmetry may provide a reliable indicator of the reactions of domestic animals to their rearing conditions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Chicken-stress; Developmental stability; Fluctuating asymmetry; Gait; Tonic immobility; Welfare

)

Corresponding author. Tel.: q33-1-44-27-25-94; fax: q33-1-44-27-35-16; e-mail: [email protected] 0168-1591r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 5 9 1 Ž 9 8 . 0 0 2 1 3 - 5

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1. Introduction Developmental stability reflects the ability of individuals to produce a stable phenotype under given environmental conditions ŽParsons, 1990; Møller and Swaddle, 1997.. Inability to undergo such stable phenotypic development can be measured in terms of fluctuating asymmetry or the frequency of phenodeviants. Fluctuating asymmetry ŽFA. is small, random deviations from symmetry in otherwise bilaterally symmetrical characters ŽLudwig, 1932; Parsons, 1990; Møller and Swaddle, 1997.. The various causes of FA, which have been the subject of intense scrutiny in a large number of studies, are both genetic and environmental Žreviews in Parsons, 1990; Møller and Swaddle, 1997.. The genetic factors giving rise to elevated asymmetry include inbreeding, hybridisation, and incorporation of novel mutants into the genome ŽParsons, 1990; Møller and Pomiankowski, 1993a,b; Møller and Swaddle, 1997.. Environmental factors giving rise to increased FA include food deficiency, parasites and pathogens, audiogenic stress, pesticides, and novel environmental conditions ŽParsons, 1990; Møller and Swaddle, 1997.. In other words, the overall level of FA in morphology provides an integrated measure of phenotypic quality of an individual in terms of its ability to control stable morphological development. Asymmetry of most morphological characters differs from other characters by having an optimum that is known a priori: symmetry, and deviations from this optimum can be used as direct measures of the impact of the environment on an individual given its genetic constitution. Animal welfare has been assessed from a number of behavioural, physiological and health parameters ŽBroom and Johnson, 1993; Toates, 1995.. Definitions of welfare differ markedly among scientists, and there is no generally accepted consensus about the most reliable and objective way of how to assess welfare. Some authors emphasise health and fitness Že.g., Broom, 1986., while others pay attention to how animals perceive their immediate situation Že.g., Dawkins, 1990.. For the first type of approach, coping mechanisms Žbody repair systems, immunological defence, and emergency physiological responses; Broom and Johnson, 1993. are significant, while for the second kind of approach, immediate behavioural reactions are more important Že.g., Dawkins, 1990; Vestergaard, 1996.. In the latter case, typical behavioural indicators of conflict, frustration, aversion, and preference are emphasised. Here we propose that measures of developmental instability may represent reliable and objective measures of welfare. The reason for this claim is that the optimum phenotype is known a priori Žsymmetry., and any deviation from this optimum results in reduced fitness because individual fluctuating asymmetry is negatively associated with growth performance, survival, mating success, and fecundity in the majority of studies of a diverse array of animals and plants ŽMøller and Swaddle, 1997.. Domesticated animals may be particularly susceptible to the negative effects of environmental and genetic factors on the stable development of the phenotype, since domestication directly may reduce or prevent natural and sexual selection against individuals with asymmetric phenotypes and hence, individuals with poor abilities to cope with a novel and challenging farming environment. A second reason why domesticated animals are particularly susceptible to the causes of developmental instability is that they have been subjected to strong directional

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selection by animal breeders to increase productivity or performance. Intense directional selection has been hypothesised to increase the overall level of developmental instability, whereas stabilising selection has the opposite effect ŽMøller and Pomiankowski, 1993a,b; Møller and Swaddle, 1997.. Selection apparently directly affects alleles that control developmental homeostasis. Directional selection on a character imposes selection against genetic modifiers that control the development of extreme phenotypes, whereas stabilising selection results in incorporation of modifiers that reduce the effects of gene products on individuals with extreme genotypes Žreviewed in Møller and Pomiankowski, 1993a,b; Møller and Swaddle, 1997.. Strong directional selection imposed by animal breeders should thus result in elevated levels of FA. Measures of developmental instability have only rarely been used for assessment of condition in domesticated animals. A study of FA in chickens revealed that skeletal asymmetry in six morphological characters increased by more than 30% when rearing density increased from 20 to 28 chickens my2 ŽMøller et al., 1995.. Average skeletal asymmetry was positively related to the growth rate of chicken breeds with wild jungle fowl having small degrees of asymmetry, followed by a slow-growing breed ŽLa Belle Rouge., while two fast-growing breeds ŽRoss 208, ScanBrid. both had very high levels of asymmetry ŽMøller et al., 1995.. Skeletal asymmetry was directly related to tonic immobility which is a behavioural measure of fearfulness and hence, welfare ŽJones and Faure, 1981.. A second study of chickens showed that hens housed with roosters with symmetrical wattles laid more eggs than hens housed with asymmetrical roosters ŽForkman and Corr, 1996.. If fluctuating asymmetry reflects environmental and genetic conditions for morphogenesis as experienced by animals themselves, asymmetry measurements may also be useful tools for assessment of animal welfare. Public controversy over farming conditions in chickens mainly concerns rearing conditions ŽAppleby et al., 1994.. The main purpose of the present study was to investigate the effect of light regimes on fluctuating asymmetry. Second, we determined the relationship between fearfulness as estimated from tonic immobility and the level of asymmetry. Third, we determined the relationship between a measure of a pathological leg disorder Žtibial dyschondroplasia; Edwards, 1984; Edwards and Veltman, 1983. and asymmetry. Finally, we determined whether skeletal asymmetry was related to a standard measure of walking performance Žgait impairment; Nestor, 1984..

2. Materials and methods 2.1. Breeds and housing facilities The investigation was carried out at two chicken farms of two major Danish producers, each farm with three houses with similar chicken breeds, capacity for ventilation, feeding programme, and litter material Žwheat straw.. The chickens at farm

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no. 1 were slaughtered when 36 days old at a body mass of 1400–1500 g, while farm no. 2 produced chickens aged 42 days weighing 2050–2400 g. We performed large-scale experiments at commercial farms using three different treatments: light regime A Ža light–dark cycle of 16:8 h., light regime B Ža changing light–dark cycle., and light regime C Žcontinuous light.. Light regime A had 24 h of light on day 0–3 followed by darkness lasting from 2200 to 0600 h Žlight regime A.. Light regime B had 24 h of light on day 0–3 Žlight regime B.. From days 4–7, the duration of light was reduced by 2 h daily, reaching a period of 16 h of light. This light schedule was maintained until 25 days of age. From the age of 26 until 29 days, the period of light was increased by 2 h daily resulting in 24 h of daily light on day 30. The light regime in these houses was controlled by a computer-driven light system that gradually turned the light on or off during a period of 20 min. Light regime C had the commonly used light schedule of 24 h permanent light throughout development. We were unable to perform an experiment with a ‘clean’ design by having one treatment with a fixed light–dark cycle because of logistic problems associated with farming practice. However, the first three days only comprise a small part of the total developmental period of a chicken, and any differences between continuous light regimes and fixed light–dark cycles are likely to be reduced by the design of the study. A cycle of 16 h light and 8 h darkness resembles the natural conditions under which chickens have evolved Žfree-living chickens and ancestral junglefowl from Asia, Gallus gallus, are animals with a diurnal activity period., and this light regime should therefore be the least challenging environmental condition with respect to development. Light regime B resembled the first one by having a certain period of darkness, except from the first three days and the last day of the life of these chickens. Light regime C is the condition most deviant from natural rearing conditions of chickens by providing no visual cue to the time of the day. Therefore, we would predict that individual fluctuating asymmetry should be the largest among chickens receiving light regime C, with smaller and similar asymmetry values for chickens reared under the two light regimes with a dark period all Žlight regime A. or most of the days Žlight regime B.. Farm no. 1 had three houses of approximately 2000 m2 used for the experiments. Each house had a capacity of 35 000 chickens. There was automatic chain feeding, nipple watering, heating, and ventilation systems in all houses. With a normal density of 28 chickens my2 , there was 1.7 cm per chicken at the feeding system and on average, 21 chickens at each water nipple. The maximum capacity of ventilation was 3.3 m3 per hour and chicken. Day-old chickens of the breed Ross 208 of mixed sex were supplied by a commercial hatchery and randomly distributed on arrival to the houses by mixing the boxes containing the chickens. Treatments were assigned randomly with one house having light regime A, the second light, regime B and the third light, regime C. Traditional commercial chicken feed was delivered by a local dealer. Farm no. 2 had eight houses of which three were used for the experiment. Each house had an area of 1500 m2 with a maximum capacity of 36 000 chickens. The feeding, watering, heating, and ventilation systems were of good quality. With a density of 24 chickens my2 , there was 2.2 cm per chicken at the feeding system. Each water nipple supplied on average 12.6 chickens. The maximum ventilation capacity was 4 m3 per hour and chicken. Treatments were assigned randomly with one house having light

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regime A, a second, regime B and a third, regime C. Chickens were of the same breed as for farm no. 1 and they were distributed among houses in a similar manner. 2.2. Morphological measurements and assessment of tonic immobility, tibial dyschondroplasia, and gait We picked a random sample of chickens of mixed sex for morphological measurements, and recorded the length of the right and left tarsometatarsus, the width of the tarsometatarsus at the spur, and the thickness of the upper joint of the tarsometatarsus to the nearest 0.1 mm using a digital calliper. The identity of chickens was determined from wing tags, and the origin of the chickens was unknown to the person, who performed the measurements ŽA.P.M... A total of 16 chickens were measured from each replicate, in total, 192 chickens. Character size was the mean of the left and the right characters. Individual absolute fluctuating asymmetry for a character was defined as the unsigned left-minus-right character size. Relative fluctuating asymmetry of a character was defined as absolute fluctuating asymmetry divided by character size. Mean relative asymmetry was the mean relative asymmetry of the three different characters. For each house, 16 randomly selected chickens were assessed for gait quality when 21 and 35 days old, respectively. All house lights were turned off before catching the chickens. The two persons catching the chickens were not responsible for making the subsequent observations. Each person moved 11–15 steps and caught the closest chicken behind him. The two chickens caught by the two persons were removed from the house for observations, and the lights were turned on again. This procedure was repeated until all 16 chickens from each house had been caught. The chickens were placed within a 2 = 4 = 1.5 m wire-net enclosure and tested for gait. Gait was evaluated at a distance of 1.5–2.0 m by a sitting observer ŽG.S.S.. on a six-point scale from 0 s normal to ascending walking disability Ž1–5., where a score of 5 reflects lameness ŽNestor, 1984.. Tonic immobility is a reliable measure of fearfulness in chickens ŽJones, 1986.. Following the gait evaluation test, chickens were taken to a quiet room adjacent to the building, used for the tonic immobility tests according to the procedure described by Jones and Faure Ž1980.. Each chicken was carefully placed on its back in a U-shaped wooden cradle covered with a thick layer of cloth and restrained for 15 s by covering the head with one hand, while placing the other hand on the sternum. Latency to self-righting was used as the measure of tonic immobility. If this had not happened after 10 min, the session was terminated and the individual was assigned a value of 600 s. If the chicken terminated the state of immobility before 10 s, the trial was repeated. Observations were performed by G.S.S. at a distance of 2–2.5 m without making unnecessary noise or movements. Chickens randomly selected for asymmetry measurements were checked for tibial dyschondroplasia when sacrificed at 35 days of age by cutting the tibiotarsus with a knife, keeping an approximate angle of 458 from the front of the bone and scoring for the amount of cartilage tissue Ž0 s no cartilage, 1 s some to one-third of the cut with cartilage, 2 s from a third to half of the cut with cartilage, 3 s more than a half of the cut consisting of cartilage; Edwards and Veltman, 1983..

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2.3. Statistical procedures We tested whether the morphological characters demonstrated directional asymmetry or antisymmetry Žtwo kinds of morphological asymmetry that are fundamentally different from fluctuating asymmetry; Møller and Swaddle, 1997. by determining whether the frequency distributions of signed left-minus-right character values deviated significantly from normal distributions with a mean value of zero by Kolmogorov–Smirnov one-sample tests and one-sample t-tests ŽTable 1.. Consistency in measurements of FA between measurement events was tested by means of repeatability analyses ŽFalconer, 1981; Becker, 1984.. Ten individuals that were measured twice after an interval of 1 h demonstrated a high consistency in both linear measurements and signed left-minus-right character values Žlinear measurements of length of tarsometatarsus, thickness of tarsometatarsus, and thickness of the joint: F s 118.67–991.02, df s 9, 10, P - 0.001; signed left-minus-right character values for the three characters mentioned previously: F s 39.80–201.15, df s 9, 10, P - 0.001.. Measurement errors were therefore small relative to the size of the characters measured.

Table 1 Summary statistics for various measures of fluctuating asymmetry ŽFA. of morphological traits of chickens Measure

Light regime A

Light regime B

Light regime C

Length of tarsometatarsus Size 92.40Ž0.85. LyR length y0.27Ž0.25. Skewness 0.52 Kurtosis 0.17 ILyRI length 1.62Ž0.14. Relative FA 0.018Ž0.002.

92.41Ž0.86. y0.20Ž0.29. 0.71 0.82 1.64Ž0.20. 0.018Ž0.002.

92.28Ž0.94. y0.43Ž0.33. 0.53 y0.49 2.25Ž0.18. 0.024Ž0.002.

Thickness of tarsometatarsus Size 11.38Ž0.15. LyR length y0.20Ž0.10. Skewness y0.95 Kurtosis 0.62 ILyRI length 0.55Ž0.07. Relative FA 0.049Ž0.006.

11.26Ž0.15. y0.20Ž0.11. y1.08 1.04 0.54Ž0.07. 0.048Ž0.006.

11.19Ž0.18. y0.19Ž0.16. y0.75 14.09) 0.80Ž0.13. 0.68Ž0.009.

Thickness of joint Size LyR length Skewness Kurtosis ILyRI length Relative FA

21.74Ž0.25. 0.27Ž0.18. 0.59 20.94) 0.82Ž0.15. 0.041Ž0.010.

21.91Ž0.23. 0.23Ž0.12. y0.08 1.78 0.73Ž0.09. 0.033Ž0.004.

21.52Ž0.23. y0.05Ž0.21. y0.74 14.08) 1.04Ž0.17. 0.050Ž0.010.

N

64

64

) P - 0.05. Values are means ŽSE.. All linear dimensions are in millimeter Žmm..

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Each chicken in a particular house receiving a particular treatment is not statistically independent in the sense that it shares the environment and the treatment with all other chickens in the same house. In order to avoid such statistical dependence, we used mean values for each house and treatment in a blocked random effects analysis of variance with farm as a blocking factor, and treatment and treatment by farm interaction as random effects ŽProc Mixed in SAS Institute, 1996.. This analysis controls for effects of farm and the interaction between farm and treatment. Since none of the interactions was statistically significant Ž P 4 0.05., we deleted the interaction terms and calculated effects of farm and treatment using the error mean squares as the denominator mean squares. Differences among light regimes were tested with Scheffe a posteriori tests. Correlation coefficients were z-transformed before statistical analyses. Differences in correlation coefficients among treatments were tested by means of one-way analysis of variance. One-sample t-tests were used to test the null hypothesis that mean correlation coefficients for each treatment did not differ from zero. Absolute and relative fluctuating asymmetries have truncated normal frequency distributions. Therefore, parametric statistical tests usually cannot be used on asymmetry measures or their transformed values. However, since we used mean asymmetries for each replicate, values were normally distributed, and parametric analyses were used. All statistical tests are two-tailed. Values reported are means " SE.

3. Results 3.1. Fluctuating asymmetry in morphological characters The morphological characters demonstrated fluctuating asymmetry as evidenced from the frequency distributions of signed right-minus-left character values in most cases not differing significantly from normal distributions ŽTable 1.. The kurtosis of four of the nine values differed significantly from the expected value of a normal distribution. Inspection of the data revealed that the distributions were clearly bell-shaped with one or two extreme values in each of these samples. The means of signed right-minus-left character values did not differ significantly from zero in the nine samples. In the following, we assume that deviations from perfect symmetry in the morphological characters reflect fluctuating asymmetry despite the fact that kurtosis sometimes deviated significantly from the expected values. The characters obviously did not have directional asymmetry, and the bell-shaped distributions also suggest that they did not demonstrate anti-symmetry. The relationship between absolute asymmetry and mean character size was statistically nonsignificant for length of tarsometatarsus, but significant for thickness of tarsometatarsus and thickness of the joint ŽSpearman r s 0.210, r s 0.551, r s 0.442, respectively, N s 192, P s 0.40, P - 0.0001, P s 0.0032, respectively.. In order to make asymmetry values comparable, and in order to allow us to calculate a composite asymmetry index weighting the three characters similarly, we corrected asymmetries of all characters by dividing absolute asymmetry with mean character size.

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3.2. Fluctuating asymmetry and light regime There was no significant effect of farm or treatment by farm interaction in any of the analyses, which implies that farms did not contribute significantly to heterogeneity in results. The three light regimes did not affect the size of the three morphological characters significantly ŽTable 2.. However, absolute asymmetry differed among treatments for the length and thickness of tarsometatarsus, but did not reach significance for the thickness of the joint ŽTable 2.. Relative asymmetry was also statistically significant for the same characters ŽTable 2.. Mean relative asymmetry for the three treatments differed significantly and was on average almost 40% larger for light regime C compared with the two other light regimes ŽTable 2. The three independent measures of the impact of rearing conditions were also affected by the treatment. Tonic immobility was significantly higher in light regime C compared with the two other regimes ŽTable 2.. The intensity of tibial dyschondroplasia was also significantly greater in light regime C ŽTable 2.. Finally, the gait score was significantly higher in light regime C Ži.e., the chickens showed more leg problems. than in the two other light regimes; Table 2.. In conclusion, light regimes significantly affected morphological asymmetry, fearfulness, the severity of a joint disorder, and a measure of walking performance in chickens, but did not affect size. 3.3. Tonic immobility and fluctuating asymmetry There was no significant effect of farm or treatment by farm interaction. Tonic immobility varied with respect to treatment ŽTable 2.. There was no statistically

Table 2 Fluctuating asymmetry, gait, tonic immobility, and tibia dyschondroplasia in relation of light regime based on random effect ANOVA models Character

Light regime A

Light regime B

Light regime C

F

P

Length of tarsometatarsus Thickness of tarsometatarsus Thickness of joint FA of length of tarsometatarsus FA of thickness of tarsometatarsus FA of thickness of joint Rel. FA of length of tarsometatarsus Rel. FA of thickness of tarsometatarsus Rel. FA of thickness of joint Mean relative asymmetry Tonic immobility Žs. Tibia dyschondroplasia Gait

92.4Ž2.8. 11.4Ž0.4. 21.7Ž0.8. 1.62Ž0.13. a 0.56Ž0.07. a 0.82Ž0.25. 0.018Ž0.001. a 0.049Ž0.007. a 0.040Ž0.015. 0.036Ž0.003. a 309Ž23. a 0.53Ž0.02. a 0.67Ž0.14. a

92.4Ž2.6. 11.3Ž0.4. 21.9Ž0.6. 1.64Ž0.13. a 0.54Ž0.10. a 0.73Ž0.16. 0.018Ž0.001. a 0.048Ž0.009. a 0.034Ž0.008. 0.033Ž0.002. a 269Ž11. a 0.50Ž0.05. a 0.58Ž0.09. a

92.3Ž2.8. 11.2Ž0.4. 21.5Ž0.6. 2.25Ž0.17. b 0.80Ž0.12. a 1.04Ž0.20. 0.024Ž0.002. b 0.068Ž0.007. b 0.050Ž0.009. 0.047Ž0.005. b 426Ž28. b 1.25Ž0.11. b 1.06Ž0.06. b

0.02 0.72 0.11 14.62 3.99 1.11 8.68 4.66 0.95 4.59 9.87 24.62 4.67

1.00 0.63 0.44 0.02 0.05 0.29 0.006 0.02 0.60 0.02 0.0017 - 0.001 0.02

F-values for treatment effects are reported. The degrees of freedom for all tests are 2, 9. Values are means ŽSE. of mean values from different houses. Values with different superscripts are significantly different in Scheffe a posteriori tests Ž P - 0.05..

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Table 3 Pearson product-moment correlation coefficients between tonic immobility, tibial dyschondroplasia, and gait, respectively, and relative asymmetry

Tonic immobility Tibial dyschondroplasia Gait

Length of tarsometatarsus

Thickness of tarsometatarsus

Thickness of joint

N

0.242Ž0.101. a 0.342Ž0.073. c 0.282Ž0.074. b

0.230Ž0.093. a 0.335Ž0.114. a 0.265Ž0.101. a

0.069Ž0.062. 0.224Ž0.072. a 0.171Ž0.062. a

12 12 12

a

P - 0.05. P - 0.01. c P - 0.001. Values are means ŽSE.. N is the number of correlation coefficients from the independent replicates. b

significant heterogeneity in correlation coefficients between tonic immobility and mean relative asymmetry among treatments Ž F s 0.64–1.82, df s 1, 10, NS.. The correlation between asymmetry and tonic immobility was statistically significant for two of the morphological characters ŽTable 3.. The relationship between mean relative asymmetry and tonic immobility was significantly positive ŽSpearman r s 0.481, N s 192, P s 0.0002; Fig. 1.. 3.4. Tibial dyschondroplasia and fluctuating asymmetry There was no significant effect of farm or treatment by farm interaction. The severity of tibial dyschondroplasia differed significantly among treatments ŽTable 2.. There was no statistically significant heterogeneity in correlation coefficients between tibial dyschondroplasia and mean relative asymmetry among treatments Ž F s 0.24–1.32, df s 1.10, NS.. The correlation between tibial dyschondroplasia and mean relative

Fig. 1. Tonic immobility Žs. in relation to mean relative asymmetry in chickens. Values are means ŽSE..

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Fig. 2. Tibial dyschondroplasia in relation to mean relative asymmetry in chickens. Values are means ŽSE..

asymmetry was significantly positive for all three morphological characters ŽTable 3.. The relationship between mean relative asymmetry and tibial dyschondroplasia was significantly positive ŽSpearman r s 0.583, N s 192, P - 0.0001; Fig. 2.. 3.5. Gait and fluctuating asymmetry There was no significant effect of farm or farm by treatment interaction on gait. Gait scores differed significantly among treatments ŽTable 2.. There was no statistically significant heterogeneity in correlation coefficients between gait scores and mean relative asymmetry among treatments Ž F s 0.04–0.45, df s 1, 10, NS.. The correlation between gait and asymmetry was significantly positive for all three morphological

Fig. 3. Gait quality in relation to mean relative asymmetry in chickens. Values are means ŽSE..

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characters ŽTable 3.. The relationship between mean relative asymmetry and gait impairment was significantly positive ŽSpearman r s 0.354, N s 192, P s 0.0014; Fig. 3..

4. Discussion 4.1. DeÕelopmental instability and rearing conditions in chickens The main finding of the present study was that light regime affected morphological asymmetry, tonic immobility, tibial dyschondroplasia, and gait quality, but not the size of morphological characters. This conclusion is interesting because size cannot be expected to reflect rearing conditions, whereas asymmetry does reflect conditions. Composite indices of asymmetry of several morphological traits provide more reliable indicators of developmental problems than the asymmetry of a single trait ŽLeung and Forbes, 1997.. In accordance with this prediction, we found a stronger relationship between asymmetry and performance when analyses were based on the composite index of asymmetry than on the asymmetry of individual characters ŽTables 2 and 3; Figs. 1–3.. Optimum asymmetry from a functionally morphological point of view is supposedly very low levels of asymmetry. The correlation analyses between asymmetry and the three independent estimates of rearing conditions all demonstrated positive associations ŽTable 3.. These results suggest that fluctuating asymmetry is responding to the same environmental conditions as tonic immobility, tibial dyschondroplasia, and gait. Either of these measures thus can be used for reliable assessment of the impact of rearing conditions and hence, welfare in chickens. Tonic immobility is a traditional measure of fearfulness in poultry ŽJones and Faure, 1980., and fear is usually considered a measure of low welfare in animals. The light regime experiment caused a significant difference in tonic immobility among treatments ŽTable 2.. The difference between continuous light and the two other light treatments was on average almost 50%, suggesting that continuous light seriously affects the development of chickens. A similar response was obtained for tibial dyschondroplasia, which is associated with poor rearing conditions ŽWise and Jennings, 1972; Edwards and Veltman, 1983.. The severity of tibial dyschondroplasia was more than twice as large under continuous light as compared with a changing light regime ŽTable 2.. Finally, there was also an effect of treatment on a score of walking inability. Continuous light increased gait impairment with almost 70% compared with the two changing light treatments ŽTable 2.. Leg disorders in broilers as assessed by both tibial dyschondroplasia and gait may cause pain ŽSørensen, 1989., and in the worst cases, birds have difficulty getting access to food and water ŽWong-Valle et al., 1993.. All four measures of performance Žfluctuating asymmetry, tonic immobility, tibial dyschondroplasia, and gait. thus responded strongly to the light treatment. The present experiment allowed comparison of the level of asymmetry in chickens in commercial farms with asymmetry of chickens in a state research facility ŽMøller et al., 1995.. We hypothesised that the rearing conditions of the same breed of chickens would

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be more stressful in the commercial farms, and that the level of asymmetry would be higher in commercial farms compared with the research facility. A comparison of chickens receiving light regime C gave significantly higher asymmetry for length of tarsometatarsus, thickness of tarsometatarsus, and thickness of the joint in the commercial farms ŽTable 4.. These differences are statistically significant for each of the three characters, with the mean asymmetry being 70% larger in the commercial farms. These findings should be interpreted in the light of no significant farm and house effects on asymmetry in the present study. This suggests that chickens raised in commercial farms are subject to considerably more severe environmental conditions than chickens raised in governmental research facilities despite similarity in breeds, light regimes, and rearing densities. 4.2. DeÕelopmental instability and animal welfare Individuals of all organisms have environmental conditions that maximise the efficiency of processes of growth and maintenance, and deviations from such optimal conditions can be considered stressful in the sense that they result in more extensive energy dissipation Že.g., Hoffmann and Parsons, 1991.. An ordinary growth process relies on the continuous synthesis of proteins and other growth precursors. The control of growth processes, including the stable development of the two sides of the body, are considered to be most efficient under environmental conditions that minimise energy use ŽOzernyuk, 1989; Ozernyuk et al., 1991; Alekseeva et al., 1992; review in Møller and Swaddle, 1997.. Suboptimal environmental conditions should result in a lowered growth rate compared to more benign conditions and an increase in the variance in growth rate among cells due to nonlinear chaotic behaviour of the developmental processes. Disturbance caused by suboptimal environmental conditions is predicted to lead to decreased resistance to variation in growth rates among cells and therefore increased asymmetry Žreview in Møller and Swaddle, 1997.. Fluctuating asymmetry has been shown to be a sensitive measure of the ability of individuals with a particular genetic background to cope with developmental precision when confronted with adverse environmental conditions Žreview in Møller and Swaddle, 1997.. The developmental process of a hypothetical morphological character can be considered to result in a minimum level of asymmetry under optimal conditions ŽFig. 4.. Environmental conditions that deviate by higher or lower values of any environmental

Table 4 Relative fluctuating asymmetry in three morphological characters for Ross 208 chickens reared in a state research facility and in commercial chicken farms on a continuous light regime at a density of 28 chickensrm2 Character

Research farm

N

Commercial farm

N

z

P

Length of tarsometatarsus Thickness of tarsometatarsus Thickness of joint Mean asymmetry

0.016Ž0.002. 0.039Ž0.007. 0.028Ž0.003. 0.028Ž0.003.

23 23 23 23

0.024Ž0.002. 0.068Ž0.017. 0.050Ž0.009. 0.047Ž0.005.

64 64 64 64

4.01 2.37 3.67 4.24

0.00006 0.018 0.00032 0.00005

Values from the research farm are from Table 4 in Møller et al. Ž1995.. Values are means ŽSE.. z-Values are from Mann–Whitney U-tests.

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Fig. 4. Developmental instability of a hypothetical morphological character in relation to an environmental gradient.

variable will result in the development of larger asymmetry. In the case of asymmetry and rearing conditions in chickens, we would hypothesise that continuous daylight gives rise to elevated levels of asymmetry, but so will a light regime with continuous darkness. The minimum level of asymmetry will be achieved at the optimum rearing conditions, since such conditions have shaped the coping mechanisms of morphogenesis during the evolutionary past. The level of asymmetry may not necessarily be zero in this optimal environment because the production of a perfectly symmetric phenotype may only be achieved at an extremely high cost. A similar argument can be applied to the effect of rearing density on morphological asymmetry ŽMøller et al., 1995., with high densities increasing asymmetry. Low rearing densities may also on average increase asymmetry due to the lack of a socially rich environment for the chickens. Although hens without broods spend most of their time on their own ŽWood-Gush et al., 1978., we would predict that single free-ranging chickens would spend much time scanning for predators and therefore develop large degrees of asymmetry ŽWitter and Lee, 1995.. In other words, we hypothesise that measures of developmental instability can be used to directly and objectively assess the optimal rearing conditions for domesticated animals Žand plants., as perceived by the animals themselves.

5. Conclusion In conclusion, measures of developmental instability provide an easy and potentially useful, nondestructive tool for a range of animal breeding and animal welfare problems. Furthermore, the positive correlations between morphological asymmetry and three independent measures of welfare Žtonic immobility, tibial dyschondroplasia, and quality

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of walking. suggest that all four variables reliably represent the responses of chickens to suboptimal environmental conditions.

Acknowledgements J.P. Swaddle provided constructive criticism. H. Stryhn provided advice on statistical models. This study was supported by a grant from the Danish Natural Science Research Council to A.P.M. and a grant from the Danish Animal Welfare Society and the Council of Broiler Producers in Denmark to K.S.V.

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