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Changes in Relative Aggressiveness and Social Dominance Associated with Selection for Early Egg Production in Chickens1,2
Changes in Relative Aggressiveness and Social Dominance Associated with Selection for Early Egg Production in Chickens1,2 J . V . CRAIG, M I N G - L U N G J A N , 3 C . R. P O L L E Y 4 AND A . L . BHAGWAT 5
Department of Dairy and Poultry Science, AND A . D . DAYTON
Department of Statistics, Kansas State University, Manhattan, Kansas 66506 (Received for publication January 24, 1975)
POULTRY SCIENCE 54: 1647-1658, 1975
INTRODUCTION
I
T may be hypothesized that artificial selection for early reproductive performance of chickens kept under competitive conditions increases aggressiveness and social dominance. Three kinds of evidence may be cited, as follows: (a) High social status among hens is asso-
1. This investigation was supported by NIH research grant HD 06782 from the National Institute of Child Health and Human Development. 2. Contribution No. 902, Department of Dairy and Poultry Science, and 232, Department of Statistics, Kansas Agricultural Experiment Station, Manhattan, Kansas. 3. Present address: Poultry Department, Taiwan Livestock Research Institute, Hsin Hua, Tainan, Taiwan, Republic of China. 4. Present address: Department of Immunology, School of Medicine, Wayne State University, Detroit, Michigan 48201. 5. Present address: Assistant Poultry Development Officer, Department of Agriculture, Government of India, Krishi Bhavan, New Delhi 110001, India.
ciated with greater access to feed (Guhl, 1953; Tindell and Craig, 1959), and with earlier onset of egg production and more eggs laid (Guhl, 1953; McBride, 1958; Tindell and Craig, 1959, 1960; Biswas and Craig, 1971). Thus, socially dominant individuals enjoy better environments within flocks and being more productive are likely to be selected, as pointed out by McBride (1958). (b) Genetic selection for social dominance is effective as indicated by significant changes in each of 3 populations under selection (Guhl et al., 1960; Craig et al., 1965). (c) Selection for early sexual maturity may amount to indirect selection for aggressiveness and dominance; at least preliminary evidence suggests a genetic correlation between those traits (Craig, 1968; Craig and Toth, 1969). The availability of an unselected control population and of 2 strains developed from it by long-term selection of hens for number of eggs laid before they were 40 weeks old allowed us to test the hypothesis. One of
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ABSTRACT We tested the hypothesis that artificial selection for early reproductive performance of chickens kept under competitive conditions increases aggressiveness and social dominance. Two populations subjected to long-term selection for number of eggs laid before 40 weeks of age and the unselected control population from which they were derived were used. Results generally supported the hypothesis but indicated that increased aggressiveness associated with such selection may characterize the adolescent period only. For one of the selected strains, rank for aggressiveness and social dominance between adolescence and full maturity was reversed, relative to the control population. Associations between increased aggressiveness (and social dominance) and decreased age at sexual maturity are discussed, along with implications as to the role of social status and reproductive performance when different genetic strains compete within flocks.
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CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
MATERIALS AND METHODS Genetic Stocks. Of the related strains (3, 5, and S275) of White Leghorn chickens used, strain 5 has been maintained as an unselected control population since 1950. Strains 3 and S275, derived from 5, were under intensive selection between 1950 and 1970 and 1955 and 1970, respectively; in 1970 when they were imported into this laboratory, selection was relaxed, but the strains continue to differ. Strains 3 and 5 have been described by Gowe et al. (1973) and S275 by Frankham and Weiss (1969). Birds hatched in 1971 from 11 or 12 singlesire matings within each strain were used for pair contests when they were 67 to 73 weeks old. Chickens of 2 hatches in 1972, from 15 to 20 single-sire matings per strain, were used for all other comparisons. Each sire was mated with 3 to 6 hens. Housing. Most chicks were brooded and reared in floor pens (3 by 5.5 m.). Birds tested at advanced ages (67 to 73 weeks) were brooded in wire-floored batteries for 5 weeks; they were then transferred to wire-floored rearing cages.
Adolescent and adult birds were kept in 4 kinds of housing environments: (a) "individual" laying cages, either 20 or 30 cm. wide and 46 cm. long, with sloping, welded-wire floors having 2.5 by 5 cm. or 2.5 by 15 cm. openings, respectively; (b) multiple-bird or "colony" laying cages, 90 cm. wide by 70 cm. long with sloping, welded-wire floors having 2.5 by 5 cm. openings; (c) "rearing" cages of differing dimensions, but all having flat, welded-wire floors with 1.9 by 1.9 cm. openings; and (d) "floor" pens, 150 cm. wide by 230 cm. long, having concrete floors covered with wood shavings. Cages, constructed of welded wire, allowed neighbors to have audio-visual contact as well as limited physical contact at feed and water troughs. Cages were devoid of internal structure; feed and water were available through 5-cm.-wide openings at the front. Floor pens contained feed troughs, roosts, and nests; water was available through openings at the front. Sexual Maturity, Males. Chicks of all strains were pedigree hatched the same day. They were wingbanded, sexed, had combs removed, and were vaccinated for Newcastle, bronchitis, and Marek's diseases soon after hatching. Strains were separated during rearing, but male and female chicks were kept together until they were 7 weeks old. They received 14 hours of light per day until the end of the study. At 7 weeks, full brothers were distributed to rearing cages by restricted randomization into intermingled- or separated-strain flocks. One-fourth of the males of each strain were randomly identified and removed from each flock at nine weeks of age to provide more space. Sexual maturity data were obtained from 81 males retained in each strain from 7 to 18 weeks of age. Floor space per male was about 385 and 514 cm. 2 from 7 to 9 and after 9 weeks of age, respectively. Males were considered sexually mature when:
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the selected stocks (strain 3 of Gowe et al., 1973) has not previously been studied for social behavior, but the other (S275 of Frankham and Weiss, 1969) has. It was postulated that the results of Frankham and Weiss, who failed to find increased aggressiveness attributed to selection, might be the result of a change in relative social dominance with age, a phenomenon previously noted (Tindell and Craig, 1959). Accordingly, the 3 strains were compared for relative aggressiveness and social dominance during adolescence and when older. Data were also collected on age at sexual maturity and on body weights at various ages, so that interrelationships of those traits with agonistic behavior could be studied.
AGGRESSIVENESS AND SELECTION IN CHICKENS
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Criterion A: Sperm were first observed computations of mean age at sexual maturity. microscopically from fluid collected by artifi- For statistical purposes, females which did cial stimulation. A second collection, with not reach sexual maturity by the end of the sperm present, was required within the next study were assigned an age of 30 weeks at 3 trials as confirmation. which their first egg was laid. Criterion B: Scores for number of sperm Pullets of a separate hatch were similarly and motility were equal to or more than 3 brooded and reared. They received between and 3, respectively. Confirmation required 13 and 15 hours of natural and artificial light the same or higher scores for each trait within daily. At 12 weeks they were placed in the next 3 trials. The scoring system was individual laying cages, colony cages, and as follows: floor pens with 10 pullets per flock, and 4 replications per strain in each housing environment. Strains were not intermingled within Scores Number of sperm Motility 0 None None flocks. A "flock" of pullets in individual 1 One or few Slow cages consisted of females in two back-to2 Moderate Moderate 3 Large Rapid back groupings of 5 adjacent cages each. Age 4 very large Very rapid at sexual maturity for pullets of this hatch Artificial stimulation was used twice week- placed in individual cages and floor pens, ly in attempting to collect semen. A few males was calculated from the date when the first did not reach maturity by criterion A and/ egg was laid. At less accurate estimate, age or B by 18 weeks; they were classified as at 50% hen-day egg production of each flock, being mature when 18 and 19 weeks old, was used for comparing mean sexual maturity of flocks in colony cages with that of the respectively, for statistical purposes. others because individual ages at onset of Sexual Maturity, Females. Pullets, hatched laying could not be determined in multiplealong with the cockerels just described, were bird cages. moved from brooder pens to individual laying cages (20 by 46 cm.) when 15 weeks old. Agonistic Behavior: Adolescent Birds. Trial Full sisters, when 18 weeks old, were distrib- 1. Frequency and severity of agonistic inuted by restricted randomization into 6 inter- teractions within separated-strain flocks were mingled-strain flocks and 6 separated-strain used as indicators of relative aggressiveness. flocks in floor pens that allowed 1,450 cm.2 Pullets assembled when 12 weeks old (see of space per pullet. Each strain was repre- above) and males, similarly distributed at the sented by 8 females in each of the 6 intermin- same age, were observed for 16 weeks in gled-strain flocks and by 24 pullets in each colony-cage and floor-pen environments. of 2 separated-strain flocks. Intermingled- Table 1 indicates the number of flocks per strain flocks were in 2 blocks of 3 adjacent subclass and design of the experiment. pens each. Behavioral data were recorded from 13 Sexual maturity, defined as weeks from through 28 weeks of age, a total of 16 weeks, hatching until the first egg was recorded, was divided into four 4-week periods for analysis. estimated from trap nesting records. A few Each of 2 observers watched each of the females did not attain sexual maturity by this 48 flocks for 10-minute intervals weekly. Data criterion; they were palpated early each were recorded by observers sitting on platmorning during 29 and 30 weeks of age to forms, located above certain floor pens, determine whether they were responsible for which allowed relatively unobstructed views floor eggs. Floor layers were excluded from of pens and cages. Birds were apparently
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CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
TABLE 1.—Number of flocks (10 birds to a flock) per strain-sex-housing subclass for comparisons of relative aggressiveness'
Genetic strain 3
Sex 6 9
Laying house environments Floor Colony pens cages 4 4
4 4
S275
undisturbed by the observers. Observations were scheduled on a restricted randomized basis, between 8 and 11 a.m. In pens, the number of birds on the floor at the beginning and end of each 10-minute period was recorded. Agonistic interaction frequencies in that environment were subsequently adjusted for mean number of birds in the "social group" (i.e., excluding birds in nests and on roosts). Trial 2. Males in 9 intermingled-strain flocks (3 of each strain in each flock), assembled at 7 weeks for collecting sexual maturity data (see above), were observed for peck-order determinations from 10 through 12 weeks of age. Each flock was observed during a 20minute feeding period each morning, after the birds had fasted overnight, a procedure shown to increase frequency of agonistic acts by a factor of about 10 (Jan, 1973). The pair-contest method (Collias, 1943) was used to establish dominance relationships for a few pairs not observed to interact during flock observations. Trial 3. Relative social dominance of pullets was estimated from peck-order observations of females assembled in intermingled-strain floor pens at 18 weeks. Observations began at 19 and ended at 24 weeks of age. Agonistic interactions were recorded sometime between 10 a.m. and 4 p.m., 5 days a week. Each of the 6 intermingled-strain flocks was
Young Adult Males. Peck orders were determined for young, sexually-mature males in intermingled-strain flocks when 19 through 24 weeks old. Those flocks involved some of the same males observed earlier, but in different combinations. Eighteen-week-old males previously used to collect sexual-maturity data were placed in individual cages. At 19 weeks, forty males per strain were randomly selected and assigned, 2 per flock, into 20 intermingled-strain flocks of 6 birds each. No feeding stimulus was used during this period (19 through 24 weeks). Each flock was observed 10 minutes, twice weekly. Fully Mature Cocks and Hens. Relative social dominance of the strains was estimated in both sexes between 67 and 73 weeks of age by pair contests. Males involved had been kept in individual 30 by 46 cm. laying cages from 18 weeks of age. About half of the females had been maintained by pairs in similar cages from 18 to 45 weeks, but all were kept singly after either 18 or 45 weeks. Twenty randomly selected individuals of each sex represented each strain in pair contests. Groups of like-sexed birds of the same strain, in blocks of 5 adjacent laying cages (30 by 46 cm.); were arbitrarily designated as strain-sex replications. Each individual was paired with eight randomly selected opponents of the same sex of each strain, for a total of 24 pair contests. Each strain-sex replication mean, therefore, included results from 40 pair contests for each strain combination; total results for each strain combination were estimated from 160 contests, Table 2.
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6 4 4 9 4 4 5 (J 4 4 9 4 4 1 A 16-week observation interval was subdivided into periods of 13-16, 17-20, 21-24, and 25-28 weeks of age.
watched for one 20-minute period per day. The observer stimulated agonistic activity by placing wet mash in a shallow bowl (22 cm. diameter) on top of an inverted 30-cm.-high bucket in the center of the pen. Dominance relationships were established for about 72% of all possible pair relationships; more than 70% were determined within each flock.
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AGGRESSIVENESS AND SELECTION IN CHICKENS
TABLE 2.- -Number of birds, replications, and pair contests used to evaluate social dominance of each
strain at 67 to 73 weeks of age
Number of pair contests involving opponents of strain(s) indicated:
Number of: Sex of contestants
Birds per replication
Replications
Total birds
3 ?
5 5
4 4
20 20
RESULTS Sexual Maturity, Males. Cockerel ages at first detection of sperm in artificial ejaculates (i.e. by criterion A) are shown as frequency
S275 160 160
5 160 160
Other strains 320 320
All strains 480 480
distributions for strains in Fig. 1; strain means are given in Table 3. Data were pooled over social environments (intermingled- and separated-strain flocks) because analysis of variance tests showed apparent absence of social environment effects and lack of genotype-by-environment interaction. Strain 3 males reached sexual maturity earlier than did those of other strains (P < .01). Ages at sexual maturity as determined by criterion B closely paralleled those for criterion A; a mean interval of 1.7 weeks separated them.
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50
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40
(/> w LU
t a. U
30
u. Ld cc 3
20
2
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AGE.
WKS.
FIG. 1. Frequency distributions for age at attainment of sexual maturity for strain 3, 5, and S275 males.
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Pair contests were conducted by 4 observers using methodology and recording techniques essentially as described by Craig et al. (1965). Pair contests were assigned to observers with one-fourth of the contests for any particular sex-strain combination subclass being conducted by each individual.
3 160 160
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CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
TABLE 3.—Mean age (wks.) at sexual maturity of strain 3, S275, and 5 males and females Females Age at first egg for pullets housed at:
Males Strain
Criterion A 1
Criterion B '
18 weeks'
12 weeks
3 S275 5
9.9a 12.0b 12.3b
11.3a 14.2b 13.8b
21.1a 22.7b 25.4c
20.9a 22.6b 25.0c
1
Date pooled for intermingled- and separated-strain flocks. Note: Means followed by different small letters, within columns, differ significantly (P < .01).
3
>-;
50
40
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1
20
LU LL
10
20
22
24
AGE,
26
WKS.
Fio. 2. Frequency distributions for age at attainment of sexual maturity for strain 3, 5, and S275 females. Sexual Maturity, Females. Ages for the three strains of pullets when the first egg was laid are shown as frequency distributions in Fig. 2 (for flocks housed at 18 weeks) and as strain means in Table 3. The first column of means for females was derived from pooled data of intermingled- and separated-strain flocks assembled in floor
pens at 18 weeks of age. Pooling was justified on the same basis as for males. Results given in the last column were based on pooled data from pullets kept in floor pens (without intermingling of strains) and in individual laying cages. Those females were placed in the laying house when 12 weeks old. Results revealed no effect of housing environment
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LU
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AGGRESSIVENESS AND SELECTION IN CHICKENS
Agonistic Behavior: Adolescent Birds. Trial 1. Frequencies of peck-avoids, threatavoids, and total agonistic interactions were affected by genetic strain, housing environment, sex, and age period, Table 4. Selection for early egg production was associated with increased agonistic activity in all categories for strains 3 and S275, Table 5. However, significant differences from
unselected strain 5 were attained by strain 3 only. Strain differences were relatively consistent over housing environments, sexes, and age periods (no statistical interactions with those variables were detected). Housing environments had a major effect on agonistic activity; frequencies were much lower in the high-density colony cages than in the floor pens, Table 5. A housing-by-sex interaction for frequency of threat-avoids was found; males had about 60% more threatavoids than females in floor pens, but only 40% more in colony cages. The housing-bysex interaction for total activity was associated with a higher frequency for males than females in floor pens (11.1 and 9.2, respectively), but with lower frequency for males than females in colony cages (2.5 and 3.3, respectively). Both sex and age influenced aggressiveness and those variables interacted, Table 4. During the 16 weeks, pullets had more peckavoids, males had more threat-avoids, and those kinds of activity had a cancelling effect for the sexes when added; males did not differ
TABLE 4.—Mean squares from analyses of variance for measures of relative aggressiveness in flocks from 13 through 28 weeks of age Agonistic acts per individual per 10 min. Source of variance Genetic strain (G) Housing environment (H) Sex (S) G x H Gx S HxS GxHxS Flocks within G, H, and S 1 Age period (P) G xP H xP SxP G x Hx P G x S x P H xSxP G x H x S x P Error 2
df 2 1 1 2 2 1 2 36 3 6 3 3 6 6 3 6 108
Peck-avoids .34* 2 45*** 2^4*** .04 .17 .01 .15 .07 07***
!o9
.07 54*** !07 .08 .17* .04 .05
Threat-avoids .72* 45.78*** 4.58*** .33 .40 7 73*** 34 .17 49** .09 .32 .12 .08 .12 .09 .06 .12
* P < .05. **P< .01. ***P< .005. 1 Error term for all sources of variance above. 2 Error term for Flocks with G, H, and S and for all sources involving age period (P).
Total 1.90* 59 43*** !42 .16 .85 2.44* .92 .38 .18 .21 .68* 1.03** .29 .34 .45 .09 .25
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or of genotype-by-environment interaction. Ages of pullets (by strain) when 50% egg production was first attained, not shown here, were similar to ages when first egg was laid, though delayed by about one-half week. Again, there was no indication of environment effects or of genotype-by-environment interaction. These results indicate that strain 3 males and females required only 80 and 84% as long to mature, respectively, as did unselected controls of the same sex. Strain S275 males did not mature earlier than control males, but S275 pullets required only about 90% as long as did control strain females.
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CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
from females in total agonistic activity. Increasing age (sex ignored) was associated with more peck-avoids, fewer threat-avoids, and an overall absence of significance for total activity because of the cancelling-out effect. Inconsistent differences between the sexes in agonistic activity rates with advancing chronological age may be explained, at least in part, by the fact that developmental and chronological ages for cockerels and pullets are not comparable (see Discussion).
Young Adult Males. Young, sexually-mature males, assembled when 19 weeks old
TABLE 5.—Frequencies of agonistic acts as influenced by genetic strain and housing environment from 13 through 28 weeks of age Agonistic acts per individual per hour Variables compared Genetic strains' 3 S275 5 Housing environments Floor pens (F) Colony cages (C) F—C
Peckavoids
Threatavoids
2.8a 2.5ab 1.9b
4.9a 3.8b 3.7b
3.1 1.7 i 4***
7.0 1.2 c o**#
Total 7.7a 6.3b 5.6b 10.1 2.9 n 9***
'Means having different small letters within strain comparisons differ significantly (P < .05). ***P< .005. TABLE 6.—Number of individuals dominated by strain 3, S27S, and 5 males and females in adolescent intermingled-strain flocks Strain
Birds/flock
Sex
No. of flocks
By strain
Total
3
S275
5
3 9'
9 6
3 8
9 24
3.8 11.62
4.2 6.5
3.9 5.4
'Data from 9 flocks were adjusted by covariance technique to an equivalent initial body weight basis. 2 Strain 3 9 9 were dominant to more individuals (P < .05) than were strains S275 and 5 9 9.
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Trials 2 and 3. Intermingled-strain flocks, assembled just before sexual maturity, at 7 and 18 weeks of age for males and females, respectively, were observed to determine peck-order relationships. Each bird was weighed initially and correlation coefficients were calculated, within strains, between body weight and number of individuals dominated. Weight was correlated with social status: r = .30, P < .05 for males; and r = .28, P <
.01 for females. Cockerels of the 3 strains did not differ in body weight, but S275 pullets were about 6% lighter (P < .01) than the average of the other strains; females averaged 1.55, 1.52, and 1.43 kg. for strains 5, 3, and S275, respectively. Therefore, number of birds dominated was adjusted by covariance technique to an equivalent initial body weight basis for pullet flocks only. Relative social-dominance estimates in adolescent intermingled-strain flocks are given in Table 6. Cockerels of the 3 strains were not detected as showing differences. However, pullets' social status differed; selected strain 3 females were dominant to more than twice as many per flock as control strain 5 females. Strain S275, though not differing statistically from strain 5, was intermediate in numbers dominated.
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AGGRESSIVENESS AND SELECTION IN CHICKENS
in intermingled-strain flocks of 6 individuals each (2 males per strain), were observed and assigned peck-order ranks within flocks. Eighteen flocks provided mean peck-order ranks by strains, which were analyzed. Dominant individuals received high numerical ranks and submissive males low numerical ranks. The results were: Strain Mean Rank
3 2.7
S275 3.5
5 4.3
Fully Mature Cocks and Hens. Relative social dominance and other behavioral characteristics were estimated from pair contest data involving all possible strain combinations at the average age of 70 weeks (Table 2). When birds were weighed at 67 weeks, before contests were run, large strain differences in body weights were detected in both sexes (P < .005). Means obtained for males of strains 3, S275, and 5 were 2.66, 2.47, and 2.91 kg., respectively. Comparable means for females were 1.83, 1.67, and 2.10 kg. Correlation coefficients were computed
TABLE 7.—Mean squares from analyses of variance for evaluation of social dominance in pair contests at 61 to 13 weeks of age
Source of variance Genetic strain (G) Sex (S) GxS Replications within G and S *P<.05. **P< .01. ***P< .005.
df 2 1 2 18
Contests against other strains Social dominance Decisions score won, % 1940** 130* 45 13 139* 1574* 35
319
Contests involving all strains Contests Decisions initiated, by physical no. contact, % 302 390* 693* 3879*** 176 108 127
109
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Each strain differed significantly from each of the others (P < .05). Thus, unselected strain 5 was socially dominant and selected strain 3 was most submissive in flocks of males assembled after all had become sexually mature.
between body weights and social-dominance scores of individuals within strains by pooling sums of squares and crossproducts from each strain combination. Essentially zero correlation coefficients (.03 and .05 for males and females, respectively) were obtained. Adjustments of behavioral data for differences in body weights were not made because of the lack of association. Four estimates of relative social dominance and aggressiveness, based on pair-contest data, were tested for strain and sex effects; results are shown in Tables 7 and 8. Relative social dominance of the strains was estimated by social-dominance scores and percentage of contests in which decisions were won by the strain in question. (Socialdominance scores, based on 80 initial pair contests per sex-strain replication, had a maximum possible range of zero to 80 if all contests were lost or won vs. other strain opponents, respectively.) Strain 5 was detected as being socially dominant to strain 3 at the mean age of 70 weeks. Strain S275, intermediate in relative dominance by both criteria, was not found to differ significantly from the others. The genetic strain-by-sex interactions (P < .05) cause some confusion, because they imply that strain differences are inconsistent as one goes from male to female comparisons. Ne-
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CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
TABLE 8.—Behavioral traits measured in pair contests at 67 to 73 weeks of age as influenced by genetic strain Contests involving all strains Decisions by Contests physical initiated, contact, no. %
Contests against other strains
Genetic strains' 3 S275 5
Decisions won, %
Social dominance score
Variables compared 63 37 49 45
99 39 37 47
Av. 38a 43ab 46b
66 28 64 54
99 40 34 76
Av. 34a 49ab 65b
97 185 179
47a 61b 57ab
'Means having different small letters within strain comparisons differ significantly (P < .05).
Mature males were more aggressive than hens by either criterion; they intiated 78% more contests (P < .05) and had 62% more decisions by physical contact (P < .005). Combining information from the various estimates, it becomes clear that strain 3, at 70 weeks of age, was least aggressive and was dominated socially by strain 5. Strain
S275, though tending to behave as aggressively as strain 5, was intermediate (when data for the sexes were pooled) in social dominance.
DISCUSSION Our results generally support the hypothesis that artificial selection for early reproductive performance of chickens kept under competitive conditions increases aggressiveness and social dominance. The evidence suggests, however, that the resulting changes may be characteristic of the adolescent period only. The mechanism whereby selection for number of eggs to 40 weeks of age caused increased aggressiveness remains obscure. Selection may have increased social-dominance ability directly by favoring more competitive, hence more aggressive individuals, or indirectly because of a genetically correlated response between earliness of sexual maturity and increased social dominance, or because both mechanisms were operative. Both selected strains had been kept in flocks in floor pens during most of their development, prior to being brought into this laboratory. However, they had also been selected, at least in part, for decreased age of sexual maturity of females. That selection under competitive flock conditions can alter
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vertheless, strains 3 and 5, which differed most, showed about the same difference for both sexes. Thus, for percentage of decisions won against other strain opponents, strain 3 and strain 5 males had values of 28 and 54, respectively, while strain 3 and strain 5 females had values of 40 and 76 (see Table 8). Relative aggressiveness can be estimated from pair-contest results on the basis of contest initiation (by which bird threatens or pecks first) and by relative frequency of decisions involving physical contact (fighting and pecking vs. threatening or avoidance without overt aggressive behavior of the opponent). By the criterion of contests initiated, the strains were not detected as being different, though individuals in strain 3 initiated pecking or threatening behavior only about half as often as did birds in the other strains (Table 8). Strain 3 was also involved in fewer contests where decisions were reached by physical contact (47%), compared with the averaged mean of the other 2 strains (59%).
AGGRESSIVENESS AND SELECTION IN CHICKENS
A striking reversal of rank for aggressiveness and social dominance occurred between adolescence and full maturity for strain 3 and strain 5 females. That the reversal may have occurred shortly after adolescence is suggested by our peck-order determinations indicating that strain 5 cockerels (not previously dominant) gained dominant status in flocks assembled at 19 weeks (essentially all cockerels had reached sexual maturity by 18 weeks). Peck-order status of strain 3 and 5 males only, obtained in 12 small intermingled-strain flocks between 4 and 8 weeks, indicated strain 3 males to be dominant in 60% of pair relationships (P = .01) at those early ages (unpublished data): We conclude, therefore, that strain 3 is more aggressive and is socially dominant to strain 5, when compared just prior to or during adolescence, but that potential ability to be socially dominant is reversed between those strains during
the young adult phase and persists into full maturity. We attempted to determine relative social status just prior to sexual maturity by determining peck orders in flocks of males and females assembled when 7 and 18 weeks old, respectively. Figs. 1 and 2 indicate that those ages were generally appropriate but that developmental stages of the 3 strains were obviously different, within sexes, at the same chronological ages. Thus, strain 3 males and females matured earlier sexually than did the others. Associations between amount of exogenous androgen injected and aggressiveness are well known in chickens (e.g., Ortman and Craig, 1968) and presumably reflect a similar association between endogenous levels and agonistic behavior. Thus, rising levels of androgens associated with early onset of sexual maturity in both sexes may have caused the selected strains to be more aggressive than the unselected control at the same chronological age. Although increased aggressiveness associated with selection for competitive ability and with early sexual maturity may have been temporary, it could have long-lasting effects for groups assembled during adolescence, because of social inertia (Williams and McGibbon, 1956; Banks and Allee, 1957; Guhl, 1968). Low social status has been shown to be associated with depressed productivity in flocks containing several genetic stocks (Tindell and Craig, 1959). The results of Gowe et al. (1973) with strain 3 and strain 5 pullets in intermingled-strain flocks indicated that selected strain 3 birds had high reproductive performance early in the laying year but later did little or no better than those of the control strain. Subsequent testing of strains in single-bird laying cages showed a remarkable increase in productivity of the selected strain in the latter part of the year. We interpret those results, at least in part, as being caused by low peck-order status of strain 3 pullets in floor pens after
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social behavior was shown by Lowry and Abplanalp (1970). They found that pullets of strains kept in flocks and selected for number of eggs were socially dominant to strains in which pullets had been selected by essentially the same criterion, but had been kept in individual cages. Contrarily, Frankham and Weiss (1969), working with strain S275, were unable to demonstrate increased social dominance of the selected strain when S275 and strain 5 females were matched in pair contests at ages ranging from 40 to 62 weeks. They believed "Possibly management in our flock (in terms of availability of feed and water) was sufficiently good during the period of strain development that peck order and aggressiveness were only minor components of the ability to produce eggs and so did not change under selection for egg production . . . " I t should be noted, however, that their tests were conducted midway through the first year of egg production. Our results suggested that S275 pullets were more aggressive than strain 5 females during adolescence.
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CRAIG, J A N , POLLEY, BHAGWAT AND DAYTON
ACKNOWLEDGMENTS We thank R. S. Gowe and H. T. Fredeen for providing hatching eggs of strains 3, 5, andS275 and M. A. Quadeer and B. Al-Rawi for assistance in conducting pair contests. REFERENCES Banks, E. ML, and W. C. Allee, 1957. Some relations between flock size and agonistic behavior in domestic hens. Phys. Zool. 30: 255-268. Biswas, D. K., and J. V. Craig, 1971. Social tension indexes and egg production traits in chickens. Poultry Sci. 50: 1063-1065. Collias, N. E., 1943. Statistical analysis of factors which make for success in initial encounters between hens. Amer. Nat. 77: 519-538. Craig, J. V., 1968. Correlated responses in body weight and egg production traits in chickens selected for social dominance. Poultry Sci. 47: 1033-1035.
Craig, J. V., L. L. Ortman and A. M. Guhl, 1965. Genetic selection for social dominance ability in chickens. Anim. Behav. 13: 114-131. Craig, J. V., and A. Toth, 1969. Productivity of pullets influenced by genetic selection for social dominance ability and by stability of flock membership. Poultry Sci. 48: 1729-1736. Frankham, R., and G. M. Weiss, 1969. Changes in relative aggressiveness in lines selected for part-record egg production under floor housing. Poultry Sci. 48: 1691-1694. Gowe, R. S., W. E. Lentz and J. H. Strain, 1973. Long-term selection for egg production in several strains of White Leghorns: Performance of selected and control strains including genetic parameters of two control strains. 4th Europ. Poultry Conf., London. 225-245. Guhl, A. M., 1953. Social behavior of domestic fowl. Kansas Agr. Expt. Sta. Tech. Bull. 73: 1-48. Guhl, A. M., 1968. Social inertia and social stability in chickens. Anim. Behav. 16: 219-232. Guhl, A. M., J. V. Craig and C. D. Mueller, 1960. Selective breeding for aggressiveness in chickens. Poultry Sci. 39: 970-980. Jan, M. L., 1973. Effects of selection for early egg production on sexual maturity, body weight and social dominance during adolescence in chickens. M.S. Thesis, Kansas State University Library, Manhattan. Lowry, D. C , and H. Abplanalp, 1970. Genetic adaptation of White Leghorn hens to life in single cages. Br. Poultry Sci. 11: 117-131. McBride, G., 1958. The relationships between aggressiveness, peck order and some characters of selective significance in the domestic hen. Proc. Roy. Soc. Edinb. 27: 56-60. Ortman, L. L., and J. V. Craig, 1968. Social dominance in chickens modified by genetic selection—physiological mechanisms. Anim. Behav. 16: 33-37. Tindell, D., and J, V. Craig, 1959. Effects of social competition on laying house performance in the chicken. Poultry Sci. 38: 95-105. Tindell, D., and J. V. Craig, 1960. Genetic variation in social aggressiveness and competition effects between sire families in small flocks of chickens. Poultry Sci. 39: 1318-1320. Williams, C , and W. H. McGibbon, 1956. An analysis of the peck-order of the female domestic fowl, Gallus domesticus. Poultry Sci. 35: 969-976.
FEBRUARY 18-24, 1976. THE 142nd NATIONAL MEETING OF THE AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE, BOSTON, MASSACHUSETTS. THE THEME IS "SCIENCE AND OUR EXPECTATIONS: BICENTENNIAL AND BEYOND"
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reversal of social dominance and the absence of such inhibitory effects when pullets are kept in the relatively asocial environment of individual cages. This interpretation assumes that social inertia was of less importance in the flocks used by Gowe than in the small flocks studied by Williams and McGibbon (1956), Banks and Allee (1957), and Guhl (1968). Two other observations deserve comment. Significant correlations existed between initial body weight and social dominance in flocks of pullets as was also found by Guhl (1953) and Tindell and Craig (1959). But such associations were absent for individuals kept in single-bird cages for long periods before being tested in initial pair contests, which agrees with Frankham and Weiss (1969). Apparently, when chickens are isolated for 20 or more weeks, they lose a conditioned response associating size of opponent with probability of social dominance.
Department of Dairy and Poultry Science, AND A . D . DAYTON
Department of Statistics, Kansas State University, Manhattan, Kansas 66506 (Received for publication January 24, 1975)
POULTRY SCIENCE 54: 1647-1658, 1975
INTRODUCTION
I
T may be hypothesized that artificial selection for early reproductive performance of chickens kept under competitive conditions increases aggressiveness and social dominance. Three kinds of evidence may be cited, as follows: (a) High social status among hens is asso-
1. This investigation was supported by NIH research grant HD 06782 from the National Institute of Child Health and Human Development. 2. Contribution No. 902, Department of Dairy and Poultry Science, and 232, Department of Statistics, Kansas Agricultural Experiment Station, Manhattan, Kansas. 3. Present address: Poultry Department, Taiwan Livestock Research Institute, Hsin Hua, Tainan, Taiwan, Republic of China. 4. Present address: Department of Immunology, School of Medicine, Wayne State University, Detroit, Michigan 48201. 5. Present address: Assistant Poultry Development Officer, Department of Agriculture, Government of India, Krishi Bhavan, New Delhi 110001, India.
ciated with greater access to feed (Guhl, 1953; Tindell and Craig, 1959), and with earlier onset of egg production and more eggs laid (Guhl, 1953; McBride, 1958; Tindell and Craig, 1959, 1960; Biswas and Craig, 1971). Thus, socially dominant individuals enjoy better environments within flocks and being more productive are likely to be selected, as pointed out by McBride (1958). (b) Genetic selection for social dominance is effective as indicated by significant changes in each of 3 populations under selection (Guhl et al., 1960; Craig et al., 1965). (c) Selection for early sexual maturity may amount to indirect selection for aggressiveness and dominance; at least preliminary evidence suggests a genetic correlation between those traits (Craig, 1968; Craig and Toth, 1969). The availability of an unselected control population and of 2 strains developed from it by long-term selection of hens for number of eggs laid before they were 40 weeks old allowed us to test the hypothesis. One of
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ABSTRACT We tested the hypothesis that artificial selection for early reproductive performance of chickens kept under competitive conditions increases aggressiveness and social dominance. Two populations subjected to long-term selection for number of eggs laid before 40 weeks of age and the unselected control population from which they were derived were used. Results generally supported the hypothesis but indicated that increased aggressiveness associated with such selection may characterize the adolescent period only. For one of the selected strains, rank for aggressiveness and social dominance between adolescence and full maturity was reversed, relative to the control population. Associations between increased aggressiveness (and social dominance) and decreased age at sexual maturity are discussed, along with implications as to the role of social status and reproductive performance when different genetic strains compete within flocks.
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CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
MATERIALS AND METHODS Genetic Stocks. Of the related strains (3, 5, and S275) of White Leghorn chickens used, strain 5 has been maintained as an unselected control population since 1950. Strains 3 and S275, derived from 5, were under intensive selection between 1950 and 1970 and 1955 and 1970, respectively; in 1970 when they were imported into this laboratory, selection was relaxed, but the strains continue to differ. Strains 3 and 5 have been described by Gowe et al. (1973) and S275 by Frankham and Weiss (1969). Birds hatched in 1971 from 11 or 12 singlesire matings within each strain were used for pair contests when they were 67 to 73 weeks old. Chickens of 2 hatches in 1972, from 15 to 20 single-sire matings per strain, were used for all other comparisons. Each sire was mated with 3 to 6 hens. Housing. Most chicks were brooded and reared in floor pens (3 by 5.5 m.). Birds tested at advanced ages (67 to 73 weeks) were brooded in wire-floored batteries for 5 weeks; they were then transferred to wire-floored rearing cages.
Adolescent and adult birds were kept in 4 kinds of housing environments: (a) "individual" laying cages, either 20 or 30 cm. wide and 46 cm. long, with sloping, welded-wire floors having 2.5 by 5 cm. or 2.5 by 15 cm. openings, respectively; (b) multiple-bird or "colony" laying cages, 90 cm. wide by 70 cm. long with sloping, welded-wire floors having 2.5 by 5 cm. openings; (c) "rearing" cages of differing dimensions, but all having flat, welded-wire floors with 1.9 by 1.9 cm. openings; and (d) "floor" pens, 150 cm. wide by 230 cm. long, having concrete floors covered with wood shavings. Cages, constructed of welded wire, allowed neighbors to have audio-visual contact as well as limited physical contact at feed and water troughs. Cages were devoid of internal structure; feed and water were available through 5-cm.-wide openings at the front. Floor pens contained feed troughs, roosts, and nests; water was available through openings at the front. Sexual Maturity, Males. Chicks of all strains were pedigree hatched the same day. They were wingbanded, sexed, had combs removed, and were vaccinated for Newcastle, bronchitis, and Marek's diseases soon after hatching. Strains were separated during rearing, but male and female chicks were kept together until they were 7 weeks old. They received 14 hours of light per day until the end of the study. At 7 weeks, full brothers were distributed to rearing cages by restricted randomization into intermingled- or separated-strain flocks. One-fourth of the males of each strain were randomly identified and removed from each flock at nine weeks of age to provide more space. Sexual maturity data were obtained from 81 males retained in each strain from 7 to 18 weeks of age. Floor space per male was about 385 and 514 cm. 2 from 7 to 9 and after 9 weeks of age, respectively. Males were considered sexually mature when:
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the selected stocks (strain 3 of Gowe et al., 1973) has not previously been studied for social behavior, but the other (S275 of Frankham and Weiss, 1969) has. It was postulated that the results of Frankham and Weiss, who failed to find increased aggressiveness attributed to selection, might be the result of a change in relative social dominance with age, a phenomenon previously noted (Tindell and Craig, 1959). Accordingly, the 3 strains were compared for relative aggressiveness and social dominance during adolescence and when older. Data were also collected on age at sexual maturity and on body weights at various ages, so that interrelationships of those traits with agonistic behavior could be studied.
AGGRESSIVENESS AND SELECTION IN CHICKENS
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Criterion A: Sperm were first observed computations of mean age at sexual maturity. microscopically from fluid collected by artifi- For statistical purposes, females which did cial stimulation. A second collection, with not reach sexual maturity by the end of the sperm present, was required within the next study were assigned an age of 30 weeks at 3 trials as confirmation. which their first egg was laid. Criterion B: Scores for number of sperm Pullets of a separate hatch were similarly and motility were equal to or more than 3 brooded and reared. They received between and 3, respectively. Confirmation required 13 and 15 hours of natural and artificial light the same or higher scores for each trait within daily. At 12 weeks they were placed in the next 3 trials. The scoring system was individual laying cages, colony cages, and as follows: floor pens with 10 pullets per flock, and 4 replications per strain in each housing environment. Strains were not intermingled within Scores Number of sperm Motility 0 None None flocks. A "flock" of pullets in individual 1 One or few Slow cages consisted of females in two back-to2 Moderate Moderate 3 Large Rapid back groupings of 5 adjacent cages each. Age 4 very large Very rapid at sexual maturity for pullets of this hatch Artificial stimulation was used twice week- placed in individual cages and floor pens, ly in attempting to collect semen. A few males was calculated from the date when the first did not reach maturity by criterion A and/ egg was laid. At less accurate estimate, age or B by 18 weeks; they were classified as at 50% hen-day egg production of each flock, being mature when 18 and 19 weeks old, was used for comparing mean sexual maturity of flocks in colony cages with that of the respectively, for statistical purposes. others because individual ages at onset of Sexual Maturity, Females. Pullets, hatched laying could not be determined in multiplealong with the cockerels just described, were bird cages. moved from brooder pens to individual laying cages (20 by 46 cm.) when 15 weeks old. Agonistic Behavior: Adolescent Birds. Trial Full sisters, when 18 weeks old, were distrib- 1. Frequency and severity of agonistic inuted by restricted randomization into 6 inter- teractions within separated-strain flocks were mingled-strain flocks and 6 separated-strain used as indicators of relative aggressiveness. flocks in floor pens that allowed 1,450 cm.2 Pullets assembled when 12 weeks old (see of space per pullet. Each strain was repre- above) and males, similarly distributed at the sented by 8 females in each of the 6 intermin- same age, were observed for 16 weeks in gled-strain flocks and by 24 pullets in each colony-cage and floor-pen environments. of 2 separated-strain flocks. Intermingled- Table 1 indicates the number of flocks per strain flocks were in 2 blocks of 3 adjacent subclass and design of the experiment. pens each. Behavioral data were recorded from 13 Sexual maturity, defined as weeks from through 28 weeks of age, a total of 16 weeks, hatching until the first egg was recorded, was divided into four 4-week periods for analysis. estimated from trap nesting records. A few Each of 2 observers watched each of the females did not attain sexual maturity by this 48 flocks for 10-minute intervals weekly. Data criterion; they were palpated early each were recorded by observers sitting on platmorning during 29 and 30 weeks of age to forms, located above certain floor pens, determine whether they were responsible for which allowed relatively unobstructed views floor eggs. Floor layers were excluded from of pens and cages. Birds were apparently
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CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
TABLE 1.—Number of flocks (10 birds to a flock) per strain-sex-housing subclass for comparisons of relative aggressiveness'
Genetic strain 3
Sex 6 9
Laying house environments Floor Colony pens cages 4 4
4 4
S275
undisturbed by the observers. Observations were scheduled on a restricted randomized basis, between 8 and 11 a.m. In pens, the number of birds on the floor at the beginning and end of each 10-minute period was recorded. Agonistic interaction frequencies in that environment were subsequently adjusted for mean number of birds in the "social group" (i.e., excluding birds in nests and on roosts). Trial 2. Males in 9 intermingled-strain flocks (3 of each strain in each flock), assembled at 7 weeks for collecting sexual maturity data (see above), were observed for peck-order determinations from 10 through 12 weeks of age. Each flock was observed during a 20minute feeding period each morning, after the birds had fasted overnight, a procedure shown to increase frequency of agonistic acts by a factor of about 10 (Jan, 1973). The pair-contest method (Collias, 1943) was used to establish dominance relationships for a few pairs not observed to interact during flock observations. Trial 3. Relative social dominance of pullets was estimated from peck-order observations of females assembled in intermingled-strain floor pens at 18 weeks. Observations began at 19 and ended at 24 weeks of age. Agonistic interactions were recorded sometime between 10 a.m. and 4 p.m., 5 days a week. Each of the 6 intermingled-strain flocks was
Young Adult Males. Peck orders were determined for young, sexually-mature males in intermingled-strain flocks when 19 through 24 weeks old. Those flocks involved some of the same males observed earlier, but in different combinations. Eighteen-week-old males previously used to collect sexual-maturity data were placed in individual cages. At 19 weeks, forty males per strain were randomly selected and assigned, 2 per flock, into 20 intermingled-strain flocks of 6 birds each. No feeding stimulus was used during this period (19 through 24 weeks). Each flock was observed 10 minutes, twice weekly. Fully Mature Cocks and Hens. Relative social dominance of the strains was estimated in both sexes between 67 and 73 weeks of age by pair contests. Males involved had been kept in individual 30 by 46 cm. laying cages from 18 weeks of age. About half of the females had been maintained by pairs in similar cages from 18 to 45 weeks, but all were kept singly after either 18 or 45 weeks. Twenty randomly selected individuals of each sex represented each strain in pair contests. Groups of like-sexed birds of the same strain, in blocks of 5 adjacent laying cages (30 by 46 cm.); were arbitrarily designated as strain-sex replications. Each individual was paired with eight randomly selected opponents of the same sex of each strain, for a total of 24 pair contests. Each strain-sex replication mean, therefore, included results from 40 pair contests for each strain combination; total results for each strain combination were estimated from 160 contests, Table 2.
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6 4 4 9 4 4 5 (J 4 4 9 4 4 1 A 16-week observation interval was subdivided into periods of 13-16, 17-20, 21-24, and 25-28 weeks of age.
watched for one 20-minute period per day. The observer stimulated agonistic activity by placing wet mash in a shallow bowl (22 cm. diameter) on top of an inverted 30-cm.-high bucket in the center of the pen. Dominance relationships were established for about 72% of all possible pair relationships; more than 70% were determined within each flock.
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AGGRESSIVENESS AND SELECTION IN CHICKENS
TABLE 2.- -Number of birds, replications, and pair contests used to evaluate social dominance of each
strain at 67 to 73 weeks of age
Number of pair contests involving opponents of strain(s) indicated:
Number of: Sex of contestants
Birds per replication
Replications
Total birds
3 ?
5 5
4 4
20 20
RESULTS Sexual Maturity, Males. Cockerel ages at first detection of sperm in artificial ejaculates (i.e. by criterion A) are shown as frequency
S275 160 160
5 160 160
Other strains 320 320
All strains 480 480
distributions for strains in Fig. 1; strain means are given in Table 3. Data were pooled over social environments (intermingled- and separated-strain flocks) because analysis of variance tests showed apparent absence of social environment effects and lack of genotype-by-environment interaction. Strain 3 males reached sexual maturity earlier than did those of other strains (P < .01). Ages at sexual maturity as determined by criterion B closely paralleled those for criterion A; a mean interval of 1.7 weeks separated them.
60
>-
50
_i
X
£
UJ O
40
(/> w LU
t a. U
30
u. Ld cc 3
20
2
g! 10
AGE.
WKS.
FIG. 1. Frequency distributions for age at attainment of sexual maturity for strain 3, 5, and S275 males.
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Pair contests were conducted by 4 observers using methodology and recording techniques essentially as described by Craig et al. (1965). Pair contests were assigned to observers with one-fourth of the contests for any particular sex-strain combination subclass being conducted by each individual.
3 160 160
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CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
TABLE 3.—Mean age (wks.) at sexual maturity of strain 3, S275, and 5 males and females Females Age at first egg for pullets housed at:
Males Strain
Criterion A 1
Criterion B '
18 weeks'
12 weeks
3 S275 5
9.9a 12.0b 12.3b
11.3a 14.2b 13.8b
21.1a 22.7b 25.4c
20.9a 22.6b 25.0c
1
Date pooled for intermingled- and separated-strain flocks. Note: Means followed by different small letters, within columns, differ significantly (P < .01).
3
>-;
50
40
< X LU I/) 30 (/) LU
<
1
20
LU LL
10
20
22
24
AGE,
26
WKS.
Fio. 2. Frequency distributions for age at attainment of sexual maturity for strain 3, 5, and S275 females. Sexual Maturity, Females. Ages for the three strains of pullets when the first egg was laid are shown as frequency distributions in Fig. 2 (for flocks housed at 18 weeks) and as strain means in Table 3. The first column of means for females was derived from pooled data of intermingled- and separated-strain flocks assembled in floor
pens at 18 weeks of age. Pooling was justified on the same basis as for males. Results given in the last column were based on pooled data from pullets kept in floor pens (without intermingling of strains) and in individual laying cages. Those females were placed in the laying house when 12 weeks old. Results revealed no effect of housing environment
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LU
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AGGRESSIVENESS AND SELECTION IN CHICKENS
Agonistic Behavior: Adolescent Birds. Trial 1. Frequencies of peck-avoids, threatavoids, and total agonistic interactions were affected by genetic strain, housing environment, sex, and age period, Table 4. Selection for early egg production was associated with increased agonistic activity in all categories for strains 3 and S275, Table 5. However, significant differences from
unselected strain 5 were attained by strain 3 only. Strain differences were relatively consistent over housing environments, sexes, and age periods (no statistical interactions with those variables were detected). Housing environments had a major effect on agonistic activity; frequencies were much lower in the high-density colony cages than in the floor pens, Table 5. A housing-by-sex interaction for frequency of threat-avoids was found; males had about 60% more threatavoids than females in floor pens, but only 40% more in colony cages. The housing-bysex interaction for total activity was associated with a higher frequency for males than females in floor pens (11.1 and 9.2, respectively), but with lower frequency for males than females in colony cages (2.5 and 3.3, respectively). Both sex and age influenced aggressiveness and those variables interacted, Table 4. During the 16 weeks, pullets had more peckavoids, males had more threat-avoids, and those kinds of activity had a cancelling effect for the sexes when added; males did not differ
TABLE 4.—Mean squares from analyses of variance for measures of relative aggressiveness in flocks from 13 through 28 weeks of age Agonistic acts per individual per 10 min. Source of variance Genetic strain (G) Housing environment (H) Sex (S) G x H Gx S HxS GxHxS Flocks within G, H, and S 1 Age period (P) G xP H xP SxP G x Hx P G x S x P H xSxP G x H x S x P Error 2
df 2 1 1 2 2 1 2 36 3 6 3 3 6 6 3 6 108
Peck-avoids .34* 2 45*** 2^4*** .04 .17 .01 .15 .07 07***
!o9
.07 54*** !07 .08 .17* .04 .05
Threat-avoids .72* 45.78*** 4.58*** .33 .40 7 73*** 34 .17 49** .09 .32 .12 .08 .12 .09 .06 .12
* P < .05. **P< .01. ***P< .005. 1 Error term for all sources of variance above. 2 Error term for Flocks with G, H, and S and for all sources involving age period (P).
Total 1.90* 59 43*** !42 .16 .85 2.44* .92 .38 .18 .21 .68* 1.03** .29 .34 .45 .09 .25
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or of genotype-by-environment interaction. Ages of pullets (by strain) when 50% egg production was first attained, not shown here, were similar to ages when first egg was laid, though delayed by about one-half week. Again, there was no indication of environment effects or of genotype-by-environment interaction. These results indicate that strain 3 males and females required only 80 and 84% as long to mature, respectively, as did unselected controls of the same sex. Strain S275 males did not mature earlier than control males, but S275 pullets required only about 90% as long as did control strain females.
1654
CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
from females in total agonistic activity. Increasing age (sex ignored) was associated with more peck-avoids, fewer threat-avoids, and an overall absence of significance for total activity because of the cancelling-out effect. Inconsistent differences between the sexes in agonistic activity rates with advancing chronological age may be explained, at least in part, by the fact that developmental and chronological ages for cockerels and pullets are not comparable (see Discussion).
Young Adult Males. Young, sexually-mature males, assembled when 19 weeks old
TABLE 5.—Frequencies of agonistic acts as influenced by genetic strain and housing environment from 13 through 28 weeks of age Agonistic acts per individual per hour Variables compared Genetic strains' 3 S275 5 Housing environments Floor pens (F) Colony cages (C) F—C
Peckavoids
Threatavoids
2.8a 2.5ab 1.9b
4.9a 3.8b 3.7b
3.1 1.7 i 4***
7.0 1.2 c o**#
Total 7.7a 6.3b 5.6b 10.1 2.9 n 9***
'Means having different small letters within strain comparisons differ significantly (P < .05). ***P< .005. TABLE 6.—Number of individuals dominated by strain 3, S27S, and 5 males and females in adolescent intermingled-strain flocks Strain
Birds/flock
Sex
No. of flocks
By strain
Total
3
S275
5
3 9'
9 6
3 8
9 24
3.8 11.62
4.2 6.5
3.9 5.4
'Data from 9 flocks were adjusted by covariance technique to an equivalent initial body weight basis. 2 Strain 3 9 9 were dominant to more individuals (P < .05) than were strains S275 and 5 9 9.
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Trials 2 and 3. Intermingled-strain flocks, assembled just before sexual maturity, at 7 and 18 weeks of age for males and females, respectively, were observed to determine peck-order relationships. Each bird was weighed initially and correlation coefficients were calculated, within strains, between body weight and number of individuals dominated. Weight was correlated with social status: r = .30, P < .05 for males; and r = .28, P <
.01 for females. Cockerels of the 3 strains did not differ in body weight, but S275 pullets were about 6% lighter (P < .01) than the average of the other strains; females averaged 1.55, 1.52, and 1.43 kg. for strains 5, 3, and S275, respectively. Therefore, number of birds dominated was adjusted by covariance technique to an equivalent initial body weight basis for pullet flocks only. Relative social-dominance estimates in adolescent intermingled-strain flocks are given in Table 6. Cockerels of the 3 strains were not detected as showing differences. However, pullets' social status differed; selected strain 3 females were dominant to more than twice as many per flock as control strain 5 females. Strain S275, though not differing statistically from strain 5, was intermediate in numbers dominated.
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AGGRESSIVENESS AND SELECTION IN CHICKENS
in intermingled-strain flocks of 6 individuals each (2 males per strain), were observed and assigned peck-order ranks within flocks. Eighteen flocks provided mean peck-order ranks by strains, which were analyzed. Dominant individuals received high numerical ranks and submissive males low numerical ranks. The results were: Strain Mean Rank
3 2.7
S275 3.5
5 4.3
Fully Mature Cocks and Hens. Relative social dominance and other behavioral characteristics were estimated from pair contest data involving all possible strain combinations at the average age of 70 weeks (Table 2). When birds were weighed at 67 weeks, before contests were run, large strain differences in body weights were detected in both sexes (P < .005). Means obtained for males of strains 3, S275, and 5 were 2.66, 2.47, and 2.91 kg., respectively. Comparable means for females were 1.83, 1.67, and 2.10 kg. Correlation coefficients were computed
TABLE 7.—Mean squares from analyses of variance for evaluation of social dominance in pair contests at 61 to 13 weeks of age
Source of variance Genetic strain (G) Sex (S) GxS Replications within G and S *P<.05. **P< .01. ***P< .005.
df 2 1 2 18
Contests against other strains Social dominance Decisions score won, % 1940** 130* 45 13 139* 1574* 35
319
Contests involving all strains Contests Decisions initiated, by physical no. contact, % 302 390* 693* 3879*** 176 108 127
109
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Each strain differed significantly from each of the others (P < .05). Thus, unselected strain 5 was socially dominant and selected strain 3 was most submissive in flocks of males assembled after all had become sexually mature.
between body weights and social-dominance scores of individuals within strains by pooling sums of squares and crossproducts from each strain combination. Essentially zero correlation coefficients (.03 and .05 for males and females, respectively) were obtained. Adjustments of behavioral data for differences in body weights were not made because of the lack of association. Four estimates of relative social dominance and aggressiveness, based on pair-contest data, were tested for strain and sex effects; results are shown in Tables 7 and 8. Relative social dominance of the strains was estimated by social-dominance scores and percentage of contests in which decisions were won by the strain in question. (Socialdominance scores, based on 80 initial pair contests per sex-strain replication, had a maximum possible range of zero to 80 if all contests were lost or won vs. other strain opponents, respectively.) Strain 5 was detected as being socially dominant to strain 3 at the mean age of 70 weeks. Strain S275, intermediate in relative dominance by both criteria, was not found to differ significantly from the others. The genetic strain-by-sex interactions (P < .05) cause some confusion, because they imply that strain differences are inconsistent as one goes from male to female comparisons. Ne-
1656
CRAIG, JAN, POLLEY, BHAGWAT AND DAYTON
TABLE 8.—Behavioral traits measured in pair contests at 67 to 73 weeks of age as influenced by genetic strain Contests involving all strains Decisions by Contests physical initiated, contact, no. %
Contests against other strains
Genetic strains' 3 S275 5
Decisions won, %
Social dominance score
Variables compared 63 37 49 45
99 39 37 47
Av. 38a 43ab 46b
66 28 64 54
99 40 34 76
Av. 34a 49ab 65b
97 185 179
47a 61b 57ab
'Means having different small letters within strain comparisons differ significantly (P < .05).
Mature males were more aggressive than hens by either criterion; they intiated 78% more contests (P < .05) and had 62% more decisions by physical contact (P < .005). Combining information from the various estimates, it becomes clear that strain 3, at 70 weeks of age, was least aggressive and was dominated socially by strain 5. Strain
S275, though tending to behave as aggressively as strain 5, was intermediate (when data for the sexes were pooled) in social dominance.
DISCUSSION Our results generally support the hypothesis that artificial selection for early reproductive performance of chickens kept under competitive conditions increases aggressiveness and social dominance. The evidence suggests, however, that the resulting changes may be characteristic of the adolescent period only. The mechanism whereby selection for number of eggs to 40 weeks of age caused increased aggressiveness remains obscure. Selection may have increased social-dominance ability directly by favoring more competitive, hence more aggressive individuals, or indirectly because of a genetically correlated response between earliness of sexual maturity and increased social dominance, or because both mechanisms were operative. Both selected strains had been kept in flocks in floor pens during most of their development, prior to being brought into this laboratory. However, they had also been selected, at least in part, for decreased age of sexual maturity of females. That selection under competitive flock conditions can alter
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vertheless, strains 3 and 5, which differed most, showed about the same difference for both sexes. Thus, for percentage of decisions won against other strain opponents, strain 3 and strain 5 males had values of 28 and 54, respectively, while strain 3 and strain 5 females had values of 40 and 76 (see Table 8). Relative aggressiveness can be estimated from pair-contest results on the basis of contest initiation (by which bird threatens or pecks first) and by relative frequency of decisions involving physical contact (fighting and pecking vs. threatening or avoidance without overt aggressive behavior of the opponent). By the criterion of contests initiated, the strains were not detected as being different, though individuals in strain 3 initiated pecking or threatening behavior only about half as often as did birds in the other strains (Table 8). Strain 3 was also involved in fewer contests where decisions were reached by physical contact (47%), compared with the averaged mean of the other 2 strains (59%).
AGGRESSIVENESS AND SELECTION IN CHICKENS
A striking reversal of rank for aggressiveness and social dominance occurred between adolescence and full maturity for strain 3 and strain 5 females. That the reversal may have occurred shortly after adolescence is suggested by our peck-order determinations indicating that strain 5 cockerels (not previously dominant) gained dominant status in flocks assembled at 19 weeks (essentially all cockerels had reached sexual maturity by 18 weeks). Peck-order status of strain 3 and 5 males only, obtained in 12 small intermingled-strain flocks between 4 and 8 weeks, indicated strain 3 males to be dominant in 60% of pair relationships (P = .01) at those early ages (unpublished data): We conclude, therefore, that strain 3 is more aggressive and is socially dominant to strain 5, when compared just prior to or during adolescence, but that potential ability to be socially dominant is reversed between those strains during
the young adult phase and persists into full maturity. We attempted to determine relative social status just prior to sexual maturity by determining peck orders in flocks of males and females assembled when 7 and 18 weeks old, respectively. Figs. 1 and 2 indicate that those ages were generally appropriate but that developmental stages of the 3 strains were obviously different, within sexes, at the same chronological ages. Thus, strain 3 males and females matured earlier sexually than did the others. Associations between amount of exogenous androgen injected and aggressiveness are well known in chickens (e.g., Ortman and Craig, 1968) and presumably reflect a similar association between endogenous levels and agonistic behavior. Thus, rising levels of androgens associated with early onset of sexual maturity in both sexes may have caused the selected strains to be more aggressive than the unselected control at the same chronological age. Although increased aggressiveness associated with selection for competitive ability and with early sexual maturity may have been temporary, it could have long-lasting effects for groups assembled during adolescence, because of social inertia (Williams and McGibbon, 1956; Banks and Allee, 1957; Guhl, 1968). Low social status has been shown to be associated with depressed productivity in flocks containing several genetic stocks (Tindell and Craig, 1959). The results of Gowe et al. (1973) with strain 3 and strain 5 pullets in intermingled-strain flocks indicated that selected strain 3 birds had high reproductive performance early in the laying year but later did little or no better than those of the control strain. Subsequent testing of strains in single-bird laying cages showed a remarkable increase in productivity of the selected strain in the latter part of the year. We interpret those results, at least in part, as being caused by low peck-order status of strain 3 pullets in floor pens after
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social behavior was shown by Lowry and Abplanalp (1970). They found that pullets of strains kept in flocks and selected for number of eggs were socially dominant to strains in which pullets had been selected by essentially the same criterion, but had been kept in individual cages. Contrarily, Frankham and Weiss (1969), working with strain S275, were unable to demonstrate increased social dominance of the selected strain when S275 and strain 5 females were matched in pair contests at ages ranging from 40 to 62 weeks. They believed "Possibly management in our flock (in terms of availability of feed and water) was sufficiently good during the period of strain development that peck order and aggressiveness were only minor components of the ability to produce eggs and so did not change under selection for egg production . . . " I t should be noted, however, that their tests were conducted midway through the first year of egg production. Our results suggested that S275 pullets were more aggressive than strain 5 females during adolescence.
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ACKNOWLEDGMENTS We thank R. S. Gowe and H. T. Fredeen for providing hatching eggs of strains 3, 5, andS275 and M. A. Quadeer and B. Al-Rawi for assistance in conducting pair contests. REFERENCES Banks, E. ML, and W. C. Allee, 1957. Some relations between flock size and agonistic behavior in domestic hens. Phys. Zool. 30: 255-268. Biswas, D. K., and J. V. Craig, 1971. Social tension indexes and egg production traits in chickens. Poultry Sci. 50: 1063-1065. Collias, N. E., 1943. Statistical analysis of factors which make for success in initial encounters between hens. Amer. Nat. 77: 519-538. Craig, J. V., 1968. Correlated responses in body weight and egg production traits in chickens selected for social dominance. Poultry Sci. 47: 1033-1035.
Craig, J. V., L. L. Ortman and A. M. Guhl, 1965. Genetic selection for social dominance ability in chickens. Anim. Behav. 13: 114-131. Craig, J. V., and A. Toth, 1969. Productivity of pullets influenced by genetic selection for social dominance ability and by stability of flock membership. Poultry Sci. 48: 1729-1736. Frankham, R., and G. M. Weiss, 1969. Changes in relative aggressiveness in lines selected for part-record egg production under floor housing. Poultry Sci. 48: 1691-1694. Gowe, R. S., W. E. Lentz and J. H. Strain, 1973. Long-term selection for egg production in several strains of White Leghorns: Performance of selected and control strains including genetic parameters of two control strains. 4th Europ. Poultry Conf., London. 225-245. Guhl, A. M., 1953. Social behavior of domestic fowl. Kansas Agr. Expt. Sta. Tech. Bull. 73: 1-48. Guhl, A. M., 1968. Social inertia and social stability in chickens. Anim. Behav. 16: 219-232. Guhl, A. M., J. V. Craig and C. D. Mueller, 1960. Selective breeding for aggressiveness in chickens. Poultry Sci. 39: 970-980. Jan, M. L., 1973. Effects of selection for early egg production on sexual maturity, body weight and social dominance during adolescence in chickens. M.S. Thesis, Kansas State University Library, Manhattan. Lowry, D. C , and H. Abplanalp, 1970. Genetic adaptation of White Leghorn hens to life in single cages. Br. Poultry Sci. 11: 117-131. McBride, G., 1958. The relationships between aggressiveness, peck order and some characters of selective significance in the domestic hen. Proc. Roy. Soc. Edinb. 27: 56-60. Ortman, L. L., and J. V. Craig, 1968. Social dominance in chickens modified by genetic selection—physiological mechanisms. Anim. Behav. 16: 33-37. Tindell, D., and J, V. Craig, 1959. Effects of social competition on laying house performance in the chicken. Poultry Sci. 38: 95-105. Tindell, D., and J. V. Craig, 1960. Genetic variation in social aggressiveness and competition effects between sire families in small flocks of chickens. Poultry Sci. 39: 1318-1320. Williams, C , and W. H. McGibbon, 1956. An analysis of the peck-order of the female domestic fowl, Gallus domesticus. Poultry Sci. 35: 969-976.
FEBRUARY 18-24, 1976. THE 142nd NATIONAL MEETING OF THE AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE, BOSTON, MASSACHUSETTS. THE THEME IS "SCIENCE AND OUR EXPECTATIONS: BICENTENNIAL AND BEYOND"
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reversal of social dominance and the absence of such inhibitory effects when pullets are kept in the relatively asocial environment of individual cages. This interpretation assumes that social inertia was of less importance in the flocks used by Gowe than in the small flocks studied by Williams and McGibbon (1956), Banks and Allee (1957), and Guhl (1968). Two other observations deserve comment. Significant correlations existed between initial body weight and social dominance in flocks of pullets as was also found by Guhl (1953) and Tindell and Craig (1959). But such associations were absent for individuals kept in single-bird cages for long periods before being tested in initial pair contests, which agrees with Frankham and Weiss (1969). Apparently, when chickens are isolated for 20 or more weeks, they lose a conditioned response associating size of opponent with probability of social dominance.