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Selection for Egg Mass in Different Social Environments
ENVIRONMENT AND HEALTH Selection for Egg Mass in Different Social Environments 3. Changes in Agonistic Activity and Social Dominance1 '2 A. L. BHAGWAT3 and J. V. CRAIG Department of Animal Sciences and Industry Kansas State University, Manhattan, Kansas 66506 (Received for publication October 11, 1977)
INTRODUCTION Productivity of individual hens in conventional floor-pen flocks is influenced b y peckorder status. Birds high in t h e hierarchy have precedence at feeders, waterers, nests, a n d roosts, a n d t e n d t o lay m o r e eggs, at least during early m o n t h s of p r o d u c t i o n (Guhl, 1 9 5 3 ; Tindell and Craig, 1 9 5 9 , 1 9 6 0 ) . Al-Rawi et al. ( 1 9 7 6 ) , however, f o u n d n o convincing evidence t h a t overt agonistic behavior measurably affected productivity in c r o w d e d c o l o n y cages. McBride ( 1 9 5 8 ) predicted t h a t selecting for egg p r o d u c t i o n in a p o p u l a t i o n m a i n t a i n e d in competitive c o n d i t i o n s would lead t o increased aggressiveness. T h e studies of L o w r y and Abplanalp ( 1 9 7 0 , 1 9 7 2 ) s u p p o r t McBride's hypothesis. Conversely, selecting for social d o m i n a n c e ability altered p r o d u c t i v i t y traits and adaptability t o different social and physical e n v i r o n m e n t s (Craig et al, 1 9 6 5 ; Craig, 1 9 6 8 ,
1 This investigation is part of the Kansas contribution to the NC-89 Regional Poultry Breeding Project. 2 Contribution No. 78-72-j Department of Animal Sciences and Industry, Kansas Agricultural Experiment Station, Manhattan, Kansas. 3 Director, Central Poultry Breeding Farm, Hessaraghatta, 562 113, Bangalore, Karnataka, India.
1978 Poultry Sci 57:883-891
883
1 9 7 0 ; Craig a n d T o t h , 1 9 6 9 ; and Biswas a n d Craig, 1 9 7 0 ) . Evidence and theoretical considerations of Craig and associates (Tindell and Craig, 1 9 5 9 ; Craig, 1 9 7 0 . Biswas a n d Craig, 1 9 7 0 ) and McBride ( I 9 6 0 , 1 9 6 2 , 1 9 6 4 ) suggested t h a t selecting for productivity might well be based on families, with each family k e p t in a separate small flock or flocks. This p r o c e d u r e w o u l d presumably p r o d u c e socially t o l e r a n t strains because those families with less agonistic behavior should have a less stressful e n v i r o n m e n t , be m o r e productive, and t h u s b e selected. T h e present paper continues a series based on a long-term s t u d y begun in 1 9 6 8 t o determ i n e , in part, w h e t h e r social behavior changes would result from selecting for egg mass in i n t e r m i n g l e d - f a m i l y a n d separated-family environments. MATERIALS AND METHODS Genetic Stocks. A sample of t h e Kentville R a n d o m b r e d C o n t r o l p o p u l a t i o n of White Leghorns described b y G o w e et al. ( 1 9 7 3 ) formed t h e f o u n d a t i o n stock for this selection s t u d y . T w o strains ( X j a n d X 2 ) were selected for part-year egg mass in intermingled-family social e n v i r o n m e n t s . T w o strains ( Y i and Y 2 ) were selected in separated-family flocks. Con-
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ABSTRACT Changes in agonistic activity levels and in social dominance potential were studied in 6 strains of chickens derived from the Kentville Randombred Control. Two of the 6 were selected for increased part-year egg mass when kept in intermingled-family flocks, 2 strains were selected on the same criterion when kept in separated-family flocks, and 2 consisted of unselected controls, maintained in an essentially asocial environment (individual cages). Behavior was compared in generations, 2, 4, 6, and 7. Random genetic drift is the most likely cause for differences in frequency of agonistic acts found between strains within systems of selection. Increased social dominance ability occurred under both selection schemes as shown by peck-order ranks when selected strains were intermingled with unselected controls. We hypothesize that this result is in some way associated with the earlier sexual maturity of the selected strains. Housing environment influenced frequency of threat-avoidances; fewer were observed in colony cages as compared with floor pens. Agonistic acts were more frequent during adolescence than during the young adult stage.
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Housing. The layer house used to compare behavior in generations 2, 4, and 7 had 24 floor pens (each 150 cm by 230 cm, 240 cm high), arranged in 3 rows of 8 pens each and 24 wirefloored (slope 1:17 1/2) colony cages (90 cm long, 70 cm deep, height 40 cm at the front and 36 cm at the rear), arranged 12 in a row, with back-to-back rows. The floor pens had a horizontal feeder on one side and a water trough reached through openings in the wire front of the pen. The water trough was replaced by an automatic watering cup in each pen in generation 7. Six individual bird nest boxes were on the side opposite the feeder, and there were 2 wooden roosting bars 60 cm above the floor at the back of each pen. Colony cages had a feed trough below a continuous water channel, reached through openings in the wire at the front of the cages. Feeding and watering space per bird was about 7 cm and 5 cm in pens and colony cages. In generation 6, rearing cages (135 cm long, 67 cm wide, and 40 cm high) with flat wire floors were used to rear and test peck-order status of adolescent males and females. Feeding space per bird was 6 cm for males from 3 to 6 weeks and 8 cm for males from 7 to 10 weeks and for females from 18 to 22 weeks old. No
other fixtures were provided in cages. Peckorder status during the young adult stage was determined in floor pens. Adolescent and Young Adult Stages. Because it has been shown that relative aggressiveness and social dominance ability of particular genetic strains may change when they are compared during adolescence and at later ages (Tindell and Craig, 1959; Craig et al, 1975; and Bhagwat and Craig, 1977), we compared strains in this study in generations 6 and 7 during adolescence and again after all individuals were presumed to be sexually mature. Cockerels attain sexual maturity earlier than do pullets as indicated by comparisons of age when sperm are first present in artificially collected seminal fluids and by age at first egg. Using those criteria, Craig et al. (1975) obtained the following values for the Ottawa Randombred Control (Strain 5) and 2 strains derived from it as shown in Table 1. Pullets of the Kentville Randombred Control, from which all strains of this study were developed, mature about 1 to 2 weeks earlier than do those of the Ottawa Control (Gowe et al., 1973). Though age at puberty of Kentville population cockerels has not been determined, it is reasonable to assume that essentially the same relationship holds for ages of males and females at sexual maturity in the Ottawa and Kentville populations. The information presented above was used as a rough guide in establishing ages during which behavioral observations would be appropriate in estimating relative aggressiveness or social dominance during adolescent or young adult phases of development. Cockerels and pullets were observed and classified as adolescents or young adults in the various generations as shown in Table 2. A considerable quantity of unpublished data gathered at this station indi-
TABLE 1.—Comparison of age at sexual maturity of cockerels and pullets Age at sexual maturity, weeks Cockerels
Pullets
Strain
Mean
Range
Mean
Range
3 S275 5
9.9 12.0 12.3
7 to 17 8 to 18 7 to 18
21.1 22.7 25.4
19 to 24 19 to 30 19 to 30
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trol strains (Ci and C 2 ) were maintained in individual hen cages without conscious selection. Quadeer et al. (1977a,b) described the formation of these strains and the selection procedure. They reported estimates of heritabilities and of genetic, environmental, and phenotypic correlations of part-year egg mass and associated egg production traits. Sire component estimates (Quadeer et al, 1977b) for these populations suggest that egg mass from 30 to 37 or 30 to 40 weeks of age is lowly heritable (.13 ± .02); egg weight is moderately heritable (.45 ± .04); and age at sexual maturity, hen-day, and hen-housed rates of lay are lowly heritable (mean estimates range from .05 to .13). Moderate to high positive correlations were estimated between henhoused egg mass (for the period indicated) with egg weight and rates of lay, but egg mass did n o t appear to be correlated with sexual maturity. The correlation between age at sexual maturity and egg weight was estimated to be small (mean estimates range from —.06 to .12). Relatively small and negative correlation estimates were obtained between age at sexual maturity and rates of lay (—.18 ± .12 and —.16 ± .11 for hen-day and hen-housed rate of lay).
EGG MASS SELECTION: CHANGES IN SOCIAL BEHAVIOR
885
TABLE 2.—Age of adolescent or young adult phase of development of cockerels and pullets Age in weeks „
Adolescent
Young adult
Males
Females
Males
Females
2 4 6 7
... ... 7 to 10 ...
... ... 18 to 22 24 to 28
... ... 20 to 23 ...
32 32 30 36
cates that nearly all pullets of the strains studied here mature before 30 weeks of age and mean age at first egg varies from about 22 to 27 weeks, depending on strain and year. Intrastrain Aggression in Generation 2. Chicks of all 6 strains were obtained from a single hatch. All were wingbanded, sexed, had their combs removed, and were vaccinated against Newcastle, bronchitis, and Marek's disease on the day of hatch. All chicks had about onethird of the upper beak removed at about 3 1/2 weeks of age. Both strains within each system of selection were intermingled and reared in 3 brooding-rearing pens. Males were separated from females when they were 15 weeks old. Females were moved to the layer house when they were 20 weeks old. Strains were separated and randomly assigned to pens and cages within housing environments; there were 4 replications per strain-housing environment subclass. Six females from each of the 3 rearing pens were housed in each unit (total of 18 per pen or cage). Observation platforms were located above certain floor pens. The observer could watch flocks in floor pens and colony cages from those platforms without disturbing the birds. Observations were carried out when pullets were 32 to 40 weeks old. Twelve flocks were observed, each for a 10-minute period per day, 4 days a week. The 8 hours of observation weekly yielded one set of data per week. Birds were trapnested 3 days per week. There was concern as to whether the confinement of those pullets which entered trapnests just prior to observation periods, thereby temporarily excluding them from the flock, would artificially reduce estimates of frequency of aggression to a serious degree. Therefore, frequencies were calculated both on the basis of number of hens in the "social flock" on the floor and also for
to to to to
40 40 32 42
all pullets alive during each week of observation. Agonistic behaviors observed were fights, p e c k s plus a v o i d a n c e s of pecks (peckavoidances), threats plus avoidances of threats (threat-avoidances), and chases plus avoidances (chase-avoidances). Fights were relatively infrequent and were added to peck-avoidances for analysis. Frequencies of chase-avoidances also were low and were added to threat-avoidances. Generation 4. The same general procedures were used as described for generation 2. Chicks of all strains were obtained from two hatches instead of one. Chicks of each system of selection and hatch were reared in 2 broodingrearing pens. Pullets were moved from the brooding-rearing house to the layer house when they were 20 and 17 weeks old, for hatches 1 and 2. Assigning strains to pens and cages within housing environment was random, as in generation 2. Pullets of the 2 hatches were housed separately. Because of shortages of unselected control strain females, there were only 3 and 2 replications of the Ci and C 2 strains per strain-housing environment subclass; each of the X and Y strains had 4 replications per strain-housing subclass. The pullets' behavior was observed when they were 32 to 40 weeks old, as in generation 2. Because of the 3-week difference between hatches, only half the flocks (those from the 1st hatch) were observed for the first 3 weeks-, they were observed 2 days per week. From the 4th through the 8th week, all flocks (from both hatches) were observed 4 days per week. Beginning with the 9th week of observation, flocks of the second hatch only were observed for 2 days per week, until they were 40 weeks old. Generation 7. Similar procedures were used as in generation 2. All chicks were from a single
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ation
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The remaining half of the males were reared in separated-strain flocks and then intermingled at 20 weeks in 12 floor pens. Three males of each strain were put in each pen for a total of 18. Peck-order rankings of the strains were then obtained for the young adult males between 20 and 23 weeks of age. The mean number of other-strain individuals dominated by each strain within each flock was calculated and used as before for analysis. Females of the 6 strains were compared using similar procedures. Early adolescent pullets were observed from 18 to 22 weeks in rearing-type cages, and young adult females were observed from 30 to 32 weeks in floor pens. Analytical Procedures. Mean frequencies of agonistic acts per pullet per 10 min period were calculated for each flock and used to analyze intrastrain aggression in generation 2, 4, and 7. Because of unequal numbers of subclasses in
generation 4, those data were analyzed using least square analysis. An additional variable (presence or absence of a male) was added in generation 7. Data of generation 7 were analyzed as a balanced design. Data from a d o l e s c e n t and young adult stages were analyzed using a split-plot analysis of variance because the same birds were observed during the two periods. The Least Significant Difference test was used to test mean values for significant differences (P<.05). RESULTS Changes in the Selected Trait. A comprehensive study of selection responses for egg mass and associated productivity traits will be pres e n t e d s e p a r a t e l y . However, preliminary analyses indicate that selection is changing egg mass approximately as expected for a trait of low heritability. Though somewhat erratic on a generation-to-generation basis, there has been an overall increase per generation of about .76 g of egg daily per pullet housed; the correlation coefficient between generation number and selection response (mean of selected strains minus mean of control strains) is .75 (P<.05). Rates of laying and egg weights are tending to increase with additional generations of selection, as expected from the parameter estimates of Quadeer et al., (1977b). Somewhat surprisingly, age at sexual maturity of the selected strains apparently decreased steadily; selected strains matured about one week earlier in generation 2 and two to three weeks earlier in the last few generations when compared with unselected controls. Random genetic drift, estimated by differences between pairs of strains selected alike, appears erratically for various productivity m e a s u r e m e n t s in generation-by-generation analyses. Changes in Intrastrain Aggression. Data obtained in generations 2 and 4 were analyzed based on all pullets alive during each week of observation and on the number of hens in the social flock (on the floor), as described earlier. During generation 2, the frequency of peckavoidances was significantly influenced by housing in the analysis of social flock data; those acts were reduced in colony cages as compared to floor pens. The analysis based on all pullets alive showed a similar effect of housing (P<.08). No other difference of level of significance was obtained between analyses based on social flocks and all pullets alive in either gener-
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hatch. One additional variable (presence or absence of a male) was tested in this generation. Two sets of observations were collected: the first during adolescence at about 24 to 28 weeks, and the second after maturity at 36 to 42 weeks. Peck-order Rankings of Strains in Generation 6. Peck-order rankings of the strains were obtained during adolescent and young adult stages. Males had their combs removed and were vaccinated against Marek's disease on the day of hatch. They were kept in brooder pens until moved to wire-floored rearing cages when 3 weeks old. Half of the cockerels were placed in 12 rearing cages (described earlier) to form intermingled flocks. Four randomly chosen males of each strain were intermingled with males of the other strains in each cage for a total of 24 cockerels per unit. At 6 weeks, 3 of the 4 males per strain were randomly chosen and received numbered and colored plastic badges on both wings; additional males from each unit were then removed, leaving a total of 18 males. Males of each strain had different colored badges. Peck-order status of individual males during the early adolescent stage was determined by observations carried out between 7 and 10 weeks of age. Only betweenstrain interactions (involving cockerels with badges of different color) were recorded. The number of other-strain individuals dominated by each strain within each flock was calculated, and those means were used in an analysis of variance.
EGG MASS SELECTION: CHANGES IN SOCIAL BEHAVIOR
significantly with systems of selection over the generations studied. Differences between strains within systems were clearly evident for peck-avoidances in generations 4 and 7, for threat-avoidances in generation 4, and for total agonistic acts among young adult pullets in generations 4 and 7. Adolescent pullets of generation 7 differed in frequency of peck-avoidances, and the difference in total acts was nearly significant (P<.07). Housing environment influenced frequency of agonistic acts; fewer threat-avoidances were found in colony cages in all three generations tested. Total agonistic acts also were reduced significantly in colony cages in generations 2 and 4. Effects of housing on frequencies of peck-avoidances were inconsistent and insignifi-
TABLE 3 .—Effects of selection, random genetic drift, and housing environment on frequency of intrastrain agonistic acts of adolescent and young adult females Means'^ Generation 2 — young adults Source of variation
Peckavoidances
Threatavoidances
1.3 a 2.2b 1.8c
2.0 2.8 2.3
Generation 4 — young adults
Total acts
Peckavoidances
Threatavoidances
Total acts
3.3 5.0 4.1
3.1 2.8 2.6
3.5 3.1 3.0
6.6 5.9 5.6
Systems of selection C X Y
Random drift Strains within systems e Housing Floor pens Colony cages
1.1' 2.2 1.4
3-7 . .
5-8 , .
1.0
2.4
2.9 2.8
Gene ration 7 — adolescents
4.8 1.6
„
7.7 , 4.4
Generation 7 — youngadults
System of selection C X Y
2.3 2.1 3.5
Random drift Strains within systems e
1.0*
Housing Floor pens Colony cages
2.6 2.7
2.4 1.9 3.2
4.7 4.0 6.8
.5
1.5
34
6.0 4.3
1.6
1.2 1.4 1.7
.8***
1.3 1.6
1.1 1.4 1.8
.6
1.8 . 1.1
a,b,c, System of selection means followed by different superscripts differ significantly (P<.Q5). Expressed as acts per pullet hourly for all females present in flock (see text). Mean differences between strains within systems of selection. *P<.05, **P<.01, and ***P<.005.
2.3 2.7 3.5
1.3**
3.0 2.6
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ation. Therefore, only the results of analysis based on all pullets are presented, and data of generation 7 were collected and analyzed in that way only. Mean frequencies of agonistic acts of pullets on an hourly basis by systems of selection and housing environments for generations 2, 4, and 7 are shown in Table 3. The effects of random genetic drift in separating strains also are shown in Table 3 and in Figure 1 by differences between strains within systems of selection for generations, 2, 4, and 7. Although systems of selection influenced the frequency of intrastrain peck-avoidances during generation 2 (P<.05), generations 4 and 7 were not affected, and no clear pattern emerged over generations. Threat-avoidances and total acts were not found to be associated consistently or
887
888
BHAGWAT AND CRAIG SYSTEMS
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X
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Y
STRAINS
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C
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stage (24 to 28 weeks). Peck-order Rankings. The number of otherstrain birds dominated in flocks of adolescent and young-adult birds of both sexes in generation 6 are given in Table 4. Systems of selection showed consistent peck-order ranks in all four comparisons. Differences were found between systems for adolescent and young adult males and y o u n g adult females; differences approached significance in the case of adolescent females (P<.09). System Y birds were most dominant, X birds were intermediate, and C system birds were least dominant. Strains within systems of selection differed in the number of other-strain birds dominated in the case of adolescent females only.
"o1
"2
2
2
4
7
DISCUSSION 2
4
7
GENERATIONS FIG. 1. Agonistic acts per female per hour as influenced by systems of selection and strains within systems, over generations.
cant over generations. Reduced total agonistic acts in cages was primarily a function of reduced threats and associated avoidances in this study. Age significantly influenced the frequency of agonistic acts during generation 7; fewer acts were observed during the young adult stage (36 to 42 weeks) as compared with the adolescent
Selection appears to be causing changes in egg mass of the selected strains and random genetic drift is also indicated. All strains have been compared for egg mass in 8 generationlocation tests. Differences between strains selected alike were detected in 2 of those comparisons. Random genetic drift most likely explains this because selection differentials and theoretical increases in homozygosity of strains within systems of selection are so similar (see Quadeer et al, 1977b). Therefore, we may reasonably expect that both selection and random drift effects may be detected in other traits, either as a result of correlated changes or independent of such. The criterion of selection, part-year egg
TABLE A .—Effects of selection and random genetic drift on number of other-strain birds dominated in flocks of adolescent and young adult males and females in generation 6 Me ans Young adult stage
Adolescent stage of variation
Males
Females
Males
Females
Systems of selection C X Y
2.0 a 3.0 b 4.9 C
1.5* 3.9* 6.9*
2.6 a 5.3b 5.5b
3.1 a 4.8b 8.1 c
1.3**
1.0
1.1
Random drift Strains within systems^
.5
' ' System of selection means followed by different superscripts differ significantly (P-C.05). Mean differences between strains within systems of selection. *P<.10, **P<.01.
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STRAINS
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C
EGG MASS SELECTION: CHANGES IN SOCIAL BEHAVIOR
We hypothesized at the outset of the selection study that strains selected for part-year egg mass in intermingled-family flocks (X strains) would show increased aggressiveness and social dominance when compared with unselected control strains. That expectation apparently is confirmed for social dominance, although the cause of this change is questionable because of the increased relative social dominance which also occurred in the separated-family selection
program (Y strains). The Y strains, according to our hypothesis, should have become socially t o l e r a n t ; i n s t e a d , t h e y became socially dominant. A partial explanation of the apparent increase in social dominance under both selection schemes may be found in the earlier sexual maturity associated with selection for increased egg mass. Pullets of both selected systems consistently mature earlier than control strain pullets. Evidence from several studies suggests that a genetic correlation exists between age at sexual maturity and social dominance ability. Craig (1968) found that strains selected for higher levels of social dominance as young adults had earlier sexual maturity, as indicated by age at first egg. Craig and Toth (1969) w o r k i n g with the same strains of White Leghorns and Rhode Island Reds confirmed again that the more dominant strains matured earlier in both breeds. Working with other genetic stocks, Craig et al, (1975) found that strains selected for early egg production had both increased frequency of intrastrain aggression and were socially dominant to the later maturing and unselected controls. However, those differences were characteristic of the a d o l e s c e n t period only, and the relative dominance of the strains changed as the birds became older. Bhagwat and Craig (1977) also observed an apparent correlated response in strains selected for early and late age at first egg as they differed in social dominance ability in the same way. In the present study, the increased social dominance ability of selected strain pullets and cockerels was evident during both adolescent and young adult stages. On the basis of our present results and related research results, we hypothesize that the apparent increased social dominance under both systems of selection is in some way associated with the earlier sexual maturity of the selected strains. Housing environments influenced the frequency of agonistic acts. Crowding pullets together in colony cages reduced the frequencies of threat-avoidances and, thereby, total agonistic acts. Polley et al., (1974) and Craig and Bhagwat (1974) also reported decreased agonistic acts in colony cages as compared with frequencies in floor pens. However, they also found significantly higher frequencies of peck-avoidances in floor pens as compared with colony cages, although that was lacking in our present study. Hughes and Wood-Gush
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mass, was deliberately chosen to avoid direct selection for early sexual maturity; selection was based on production after 30 weeks of age. Previously collected data indicated that nearly all pullets start laying sooner than 30 weeks of age under our conditions. Quadeer et al., (1977b) estimated that part-year egg mass (from 30 weeks of age) was not genetically correlated with sexual maturity (—.02 ± .10) Nevertheless, though we attempted to avoid selecting for early sexual maturity, the selected strains have shown a correlated response by maturing earlier than the unselected controls. Frequencies of intrastrain agonistic acts among systems were inconsistent in generations 2, 4, and 7 (see Table 3 and Figure 1). We also see that the ranking of strains for frequency of agonistic acts within systems of selection changed over generations (Figure 1), and that mean differences between strains within systems of selection became significant in generations 4 and 7. Those differences are most likely accounted for by random genetic drift. Though birds of the strains selected for egg mass on the basis of intermingled- and separated-family data generally failed to exhibit more agonistic acts than controls, they were socially dominant over control strains when tested in intermingled-strain flocks in generation 6 (see Table 4). Thus, increases in social d o m i n a n c e a b i l i t y a p p e a r as correlated responses to selection for part-year egg mass. Though aggression is generally a necessary element in establishing social dominance, some strains differing in social dominance ability when compared in intermingled flocks or pair contests may fail to exhibit differences in frequency of aggressive acts when kept in flocks of their own kind (Craig et ah, 1965; Craig et al., 1969). Similarly, the strains in the present study, selected for increased egg mass, though dominant to unselected controls in peck-order comparisons, failed to differ significantly or consistently in frequency of intrastrain social interactions.
889
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890
O u r a d o l e s c e n t pullets h a d higher frequencies of agonistic acts as c o m p a r e d with y o u n g adult pullets (see generation 7 results, Table 3). T h e frequency of agonistic acts is increased soon after flocks of strangers are assembled, b u t t h e incidence of such acts is reduced after a few weeks (e.g., C h o u d a r y and Craig, 1 9 7 2 ) . Our p r e s e n t s t u d y suggests t h a t a higher frequency of agoni-tic acts was associated with adolescence rather t h a n t h e y o u n g adult stage. This high incidence during adolescence presumably occurred after t h e period associated with organization of t h e strangers i n t o a p e c k order, because t h e y were assembled at 2 0 weeks and t h e adolescent p e r i o d observations began 4 weeks later. V e n k a t a r a m a i a h and C h o u d a r y ( 1 9 7 3 ) also observed a higher level of agonistic activity during t h e period corresponding t o t h e age a t sexual m a t u r i t y for their strains which was reduced s u b s e q u e n t l y .
ACKNOWLEDGMENT Mr. C. R. Polley collected all d a t a in generations 2 and 4.
REFERENCES Al-Rawi, B., J. V. Craig, and A. W. Adams, 1976. Agonistic behavior and egg production of caged layers: Genetic strain and group-size effects. Poultry Sci. 55:796-807. Bhagwat, A. L., and J. V. Craig, 1977. Selecting for age at first egg: Effects on social dominance. Poultry Sci. 56:361-363. Biswas, D. K., and J. V. Craig, 1970. Genotypeenvironment interactions in chickens selected for high and low social dominance. Poultry Sci. 49:681-692. Choudary, M. R., and J. V. Craig, 1972. Effects of early flock assembly on agonistic behavior and egg p r o d u c t i o n i n c h i c k e n s . P o u l t r y Sci. 51:1928-1937. Craig, J. V., 1968. Correlated responses in body
weight and egg production traits in chickens s e l e c t e d for social dominance. Poultry Sci. 47:1033-1035. Craig, J. V., 1970. Interactions of genotype and housing environment in White Leghorn chickens selected for high and low social dominance. XlVth World's Poultry Congress, Madrid. 2:37—42. Craig, J. V., and A. L. Bhagwat, 1974. Agonistic and mating behavior of adult chickens modified by social and physical environments. Appl. Anim. Ethol. 1:57-65. Craig, J. V., D. K. Biswas, and A. M. Guhl, 1969. Agonistic behavior influenced by strangeness, crowding and heredity in female domestic fowl. Anim. Behav. 17:498-506. Craig, J. V., M. L. Jan, C. R. Polley, A. L. Bhagwat, and A. D. Dayton, 1975. Changes in relative aggressiveness and social dominance associated with selection for early egg production in chickens. Poultry Sci. 54:1647-1658. 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. Gowe,R. S., W. E. Lentz, and J. H. Strain, 1973. Longterm 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, pp. 2 2 5 - 2 4 5 . Guhl, A. M., 1953. Social behavior of the domestic fowl. Kansas Agr. Exp. Sta. Tech. Bull. 73. Hughes, B. O., and D. G. M. Wood-Gush, 1977. Agonistic behavior in domestic hens: The influence of housing method and group size. Anim. Behav. 25:1056-1062. Lowry, D. C , and H. Abplanalp, 1970. Genetic adaptation of White Leghorn hens to life in single cages. Br. Poultry Sci. 11:117-131. Lowry, D. C , and H. Abplanalp, 1972. Social dominance difference, given limited access to common food, between hens selected and unselected for i n c r e a s e d egg production. Br. Poultry Sci. 13:365-376. McBride, G., 1958. The relationship between aggressiveness and egg production in the domestic hen. Nature. 181:858. McBride, G., 1960. Poultry husbandry and the peck order. Br. Poultry Sci. 1:65-68. McBride, G., 1962. The interactions between genotypes and housing environments in the domestic hen. Aust. Soc. Anim. Prod. 4 : 9 5 - 1 0 2 . McBride, G., 1964. Social behavior of domestic animals. II. Effect of the peck order on poultry productivity. Anim. Prod. 6:1—7. Polley, C. R., J. V. Craig, and A. L. Bhagwat, 1974. Crowding and agonistic behavior: A curvilinear relationship? Poultry Sci. 53:1621-1623. Quadeer, M. A., J. V. Craig, K. E. Kemp, and A. D. Dayton, 1977a. Selection for egg mass in different social environments. 1. Estimation of some parameters in the foundation stock. Poultry Sci. 56:1522-1535. Quadeer, M. A., J. V. Craig, K. E. Kemp, and A. D.
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( 1 9 7 7 ) studied t h e incidence of agonistic behavior a m o n g laying hens of t w o strains, housed in b a t t e r y cages and in deep litter pens in groups of either 3 or 6 birds. T h e y observed t h a t aggressive head pecking and t h r e a t s were m o r e frequent in floor pens t h a n in cages and in groups of 6 birds. T h e y suggest t h a t in a n o r m a l t h r e a t display, birds require a d e q u a t e space t o assume positions with a particular o r i e n t a t i o n . Because of r e d u c e d space per bird in cages, constrains are t h u s imposed on such orientation, which t h e r e b y reduces t h e frequency of threats in c o l o n y cages.
EGG MASS SELECTION Dayton, 1977b. Selection for egg mass in different social environments. 2. Estimation of parameters in selected populations. Poultry Sci. 56:1536—1549. 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
891
social aggressiveness and competition effects between sire families in small flocks of chickens. Poultry Sci. 39:1318-1320. Venkataramaiah, A., and M. R. Choudary, 1973. Effects of strains and presence of male on agonistic behaviour and part-year egg production in White Leghorns. Indian J. Poultry Sci. 8:258—263.
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INTRODUCTION Productivity of individual hens in conventional floor-pen flocks is influenced b y peckorder status. Birds high in t h e hierarchy have precedence at feeders, waterers, nests, a n d roosts, a n d t e n d t o lay m o r e eggs, at least during early m o n t h s of p r o d u c t i o n (Guhl, 1 9 5 3 ; Tindell and Craig, 1 9 5 9 , 1 9 6 0 ) . Al-Rawi et al. ( 1 9 7 6 ) , however, f o u n d n o convincing evidence t h a t overt agonistic behavior measurably affected productivity in c r o w d e d c o l o n y cages. McBride ( 1 9 5 8 ) predicted t h a t selecting for egg p r o d u c t i o n in a p o p u l a t i o n m a i n t a i n e d in competitive c o n d i t i o n s would lead t o increased aggressiveness. T h e studies of L o w r y and Abplanalp ( 1 9 7 0 , 1 9 7 2 ) s u p p o r t McBride's hypothesis. Conversely, selecting for social d o m i n a n c e ability altered p r o d u c t i v i t y traits and adaptability t o different social and physical e n v i r o n m e n t s (Craig et al, 1 9 6 5 ; Craig, 1 9 6 8 ,
1 This investigation is part of the Kansas contribution to the NC-89 Regional Poultry Breeding Project. 2 Contribution No. 78-72-j Department of Animal Sciences and Industry, Kansas Agricultural Experiment Station, Manhattan, Kansas. 3 Director, Central Poultry Breeding Farm, Hessaraghatta, 562 113, Bangalore, Karnataka, India.
1978 Poultry Sci 57:883-891
883
1 9 7 0 ; Craig a n d T o t h , 1 9 6 9 ; and Biswas a n d Craig, 1 9 7 0 ) . Evidence and theoretical considerations of Craig and associates (Tindell and Craig, 1 9 5 9 ; Craig, 1 9 7 0 . Biswas a n d Craig, 1 9 7 0 ) and McBride ( I 9 6 0 , 1 9 6 2 , 1 9 6 4 ) suggested t h a t selecting for productivity might well be based on families, with each family k e p t in a separate small flock or flocks. This p r o c e d u r e w o u l d presumably p r o d u c e socially t o l e r a n t strains because those families with less agonistic behavior should have a less stressful e n v i r o n m e n t , be m o r e productive, and t h u s b e selected. T h e present paper continues a series based on a long-term s t u d y begun in 1 9 6 8 t o determ i n e , in part, w h e t h e r social behavior changes would result from selecting for egg mass in i n t e r m i n g l e d - f a m i l y a n d separated-family environments. MATERIALS AND METHODS Genetic Stocks. A sample of t h e Kentville R a n d o m b r e d C o n t r o l p o p u l a t i o n of White Leghorns described b y G o w e et al. ( 1 9 7 3 ) formed t h e f o u n d a t i o n stock for this selection s t u d y . T w o strains ( X j a n d X 2 ) were selected for part-year egg mass in intermingled-family social e n v i r o n m e n t s . T w o strains ( Y i and Y 2 ) were selected in separated-family flocks. Con-
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ABSTRACT Changes in agonistic activity levels and in social dominance potential were studied in 6 strains of chickens derived from the Kentville Randombred Control. Two of the 6 were selected for increased part-year egg mass when kept in intermingled-family flocks, 2 strains were selected on the same criterion when kept in separated-family flocks, and 2 consisted of unselected controls, maintained in an essentially asocial environment (individual cages). Behavior was compared in generations, 2, 4, 6, and 7. Random genetic drift is the most likely cause for differences in frequency of agonistic acts found between strains within systems of selection. Increased social dominance ability occurred under both selection schemes as shown by peck-order ranks when selected strains were intermingled with unselected controls. We hypothesize that this result is in some way associated with the earlier sexual maturity of the selected strains. Housing environment influenced frequency of threat-avoidances; fewer were observed in colony cages as compared with floor pens. Agonistic acts were more frequent during adolescence than during the young adult stage.
884
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Housing. The layer house used to compare behavior in generations 2, 4, and 7 had 24 floor pens (each 150 cm by 230 cm, 240 cm high), arranged in 3 rows of 8 pens each and 24 wirefloored (slope 1:17 1/2) colony cages (90 cm long, 70 cm deep, height 40 cm at the front and 36 cm at the rear), arranged 12 in a row, with back-to-back rows. The floor pens had a horizontal feeder on one side and a water trough reached through openings in the wire front of the pen. The water trough was replaced by an automatic watering cup in each pen in generation 7. Six individual bird nest boxes were on the side opposite the feeder, and there were 2 wooden roosting bars 60 cm above the floor at the back of each pen. Colony cages had a feed trough below a continuous water channel, reached through openings in the wire at the front of the cages. Feeding and watering space per bird was about 7 cm and 5 cm in pens and colony cages. In generation 6, rearing cages (135 cm long, 67 cm wide, and 40 cm high) with flat wire floors were used to rear and test peck-order status of adolescent males and females. Feeding space per bird was 6 cm for males from 3 to 6 weeks and 8 cm for males from 7 to 10 weeks and for females from 18 to 22 weeks old. No
other fixtures were provided in cages. Peckorder status during the young adult stage was determined in floor pens. Adolescent and Young Adult Stages. Because it has been shown that relative aggressiveness and social dominance ability of particular genetic strains may change when they are compared during adolescence and at later ages (Tindell and Craig, 1959; Craig et al, 1975; and Bhagwat and Craig, 1977), we compared strains in this study in generations 6 and 7 during adolescence and again after all individuals were presumed to be sexually mature. Cockerels attain sexual maturity earlier than do pullets as indicated by comparisons of age when sperm are first present in artificially collected seminal fluids and by age at first egg. Using those criteria, Craig et al. (1975) obtained the following values for the Ottawa Randombred Control (Strain 5) and 2 strains derived from it as shown in Table 1. Pullets of the Kentville Randombred Control, from which all strains of this study were developed, mature about 1 to 2 weeks earlier than do those of the Ottawa Control (Gowe et al., 1973). Though age at puberty of Kentville population cockerels has not been determined, it is reasonable to assume that essentially the same relationship holds for ages of males and females at sexual maturity in the Ottawa and Kentville populations. The information presented above was used as a rough guide in establishing ages during which behavioral observations would be appropriate in estimating relative aggressiveness or social dominance during adolescent or young adult phases of development. Cockerels and pullets were observed and classified as adolescents or young adults in the various generations as shown in Table 2. A considerable quantity of unpublished data gathered at this station indi-
TABLE 1.—Comparison of age at sexual maturity of cockerels and pullets Age at sexual maturity, weeks Cockerels
Pullets
Strain
Mean
Range
Mean
Range
3 S275 5
9.9 12.0 12.3
7 to 17 8 to 18 7 to 18
21.1 22.7 25.4
19 to 24 19 to 30 19 to 30
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trol strains (Ci and C 2 ) were maintained in individual hen cages without conscious selection. Quadeer et al. (1977a,b) described the formation of these strains and the selection procedure. They reported estimates of heritabilities and of genetic, environmental, and phenotypic correlations of part-year egg mass and associated egg production traits. Sire component estimates (Quadeer et al, 1977b) for these populations suggest that egg mass from 30 to 37 or 30 to 40 weeks of age is lowly heritable (.13 ± .02); egg weight is moderately heritable (.45 ± .04); and age at sexual maturity, hen-day, and hen-housed rates of lay are lowly heritable (mean estimates range from .05 to .13). Moderate to high positive correlations were estimated between henhoused egg mass (for the period indicated) with egg weight and rates of lay, but egg mass did n o t appear to be correlated with sexual maturity. The correlation between age at sexual maturity and egg weight was estimated to be small (mean estimates range from —.06 to .12). Relatively small and negative correlation estimates were obtained between age at sexual maturity and rates of lay (—.18 ± .12 and —.16 ± .11 for hen-day and hen-housed rate of lay).
EGG MASS SELECTION: CHANGES IN SOCIAL BEHAVIOR
885
TABLE 2.—Age of adolescent or young adult phase of development of cockerels and pullets Age in weeks „
Adolescent
Young adult
Males
Females
Males
Females
2 4 6 7
... ... 7 to 10 ...
... ... 18 to 22 24 to 28
... ... 20 to 23 ...
32 32 30 36
cates that nearly all pullets of the strains studied here mature before 30 weeks of age and mean age at first egg varies from about 22 to 27 weeks, depending on strain and year. Intrastrain Aggression in Generation 2. Chicks of all 6 strains were obtained from a single hatch. All were wingbanded, sexed, had their combs removed, and were vaccinated against Newcastle, bronchitis, and Marek's disease on the day of hatch. All chicks had about onethird of the upper beak removed at about 3 1/2 weeks of age. Both strains within each system of selection were intermingled and reared in 3 brooding-rearing pens. Males were separated from females when they were 15 weeks old. Females were moved to the layer house when they were 20 weeks old. Strains were separated and randomly assigned to pens and cages within housing environments; there were 4 replications per strain-housing environment subclass. Six females from each of the 3 rearing pens were housed in each unit (total of 18 per pen or cage). Observation platforms were located above certain floor pens. The observer could watch flocks in floor pens and colony cages from those platforms without disturbing the birds. Observations were carried out when pullets were 32 to 40 weeks old. Twelve flocks were observed, each for a 10-minute period per day, 4 days a week. The 8 hours of observation weekly yielded one set of data per week. Birds were trapnested 3 days per week. There was concern as to whether the confinement of those pullets which entered trapnests just prior to observation periods, thereby temporarily excluding them from the flock, would artificially reduce estimates of frequency of aggression to a serious degree. Therefore, frequencies were calculated both on the basis of number of hens in the "social flock" on the floor and also for
to to to to
40 40 32 42
all pullets alive during each week of observation. Agonistic behaviors observed were fights, p e c k s plus a v o i d a n c e s of pecks (peckavoidances), threats plus avoidances of threats (threat-avoidances), and chases plus avoidances (chase-avoidances). Fights were relatively infrequent and were added to peck-avoidances for analysis. Frequencies of chase-avoidances also were low and were added to threat-avoidances. Generation 4. The same general procedures were used as described for generation 2. Chicks of all strains were obtained from two hatches instead of one. Chicks of each system of selection and hatch were reared in 2 broodingrearing pens. Pullets were moved from the brooding-rearing house to the layer house when they were 20 and 17 weeks old, for hatches 1 and 2. Assigning strains to pens and cages within housing environment was random, as in generation 2. Pullets of the 2 hatches were housed separately. Because of shortages of unselected control strain females, there were only 3 and 2 replications of the Ci and C 2 strains per strain-housing environment subclass; each of the X and Y strains had 4 replications per strain-housing subclass. The pullets' behavior was observed when they were 32 to 40 weeks old, as in generation 2. Because of the 3-week difference between hatches, only half the flocks (those from the 1st hatch) were observed for the first 3 weeks-, they were observed 2 days per week. From the 4th through the 8th week, all flocks (from both hatches) were observed 4 days per week. Beginning with the 9th week of observation, flocks of the second hatch only were observed for 2 days per week, until they were 40 weeks old. Generation 7. Similar procedures were used as in generation 2. All chicks were from a single
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ation
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BHAGWAT AND CRAIG
The remaining half of the males were reared in separated-strain flocks and then intermingled at 20 weeks in 12 floor pens. Three males of each strain were put in each pen for a total of 18. Peck-order rankings of the strains were then obtained for the young adult males between 20 and 23 weeks of age. The mean number of other-strain individuals dominated by each strain within each flock was calculated and used as before for analysis. Females of the 6 strains were compared using similar procedures. Early adolescent pullets were observed from 18 to 22 weeks in rearing-type cages, and young adult females were observed from 30 to 32 weeks in floor pens. Analytical Procedures. Mean frequencies of agonistic acts per pullet per 10 min period were calculated for each flock and used to analyze intrastrain aggression in generation 2, 4, and 7. Because of unequal numbers of subclasses in
generation 4, those data were analyzed using least square analysis. An additional variable (presence or absence of a male) was added in generation 7. Data of generation 7 were analyzed as a balanced design. Data from a d o l e s c e n t and young adult stages were analyzed using a split-plot analysis of variance because the same birds were observed during the two periods. The Least Significant Difference test was used to test mean values for significant differences (P<.05). RESULTS Changes in the Selected Trait. A comprehensive study of selection responses for egg mass and associated productivity traits will be pres e n t e d s e p a r a t e l y . However, preliminary analyses indicate that selection is changing egg mass approximately as expected for a trait of low heritability. Though somewhat erratic on a generation-to-generation basis, there has been an overall increase per generation of about .76 g of egg daily per pullet housed; the correlation coefficient between generation number and selection response (mean of selected strains minus mean of control strains) is .75 (P<.05). Rates of laying and egg weights are tending to increase with additional generations of selection, as expected from the parameter estimates of Quadeer et al., (1977b). Somewhat surprisingly, age at sexual maturity of the selected strains apparently decreased steadily; selected strains matured about one week earlier in generation 2 and two to three weeks earlier in the last few generations when compared with unselected controls. Random genetic drift, estimated by differences between pairs of strains selected alike, appears erratically for various productivity m e a s u r e m e n t s in generation-by-generation analyses. Changes in Intrastrain Aggression. Data obtained in generations 2 and 4 were analyzed based on all pullets alive during each week of observation and on the number of hens in the social flock (on the floor), as described earlier. During generation 2, the frequency of peckavoidances was significantly influenced by housing in the analysis of social flock data; those acts were reduced in colony cages as compared to floor pens. The analysis based on all pullets alive showed a similar effect of housing (P<.08). No other difference of level of significance was obtained between analyses based on social flocks and all pullets alive in either gener-
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hatch. One additional variable (presence or absence of a male) was tested in this generation. Two sets of observations were collected: the first during adolescence at about 24 to 28 weeks, and the second after maturity at 36 to 42 weeks. Peck-order Rankings of Strains in Generation 6. Peck-order rankings of the strains were obtained during adolescent and young adult stages. Males had their combs removed and were vaccinated against Marek's disease on the day of hatch. They were kept in brooder pens until moved to wire-floored rearing cages when 3 weeks old. Half of the cockerels were placed in 12 rearing cages (described earlier) to form intermingled flocks. Four randomly chosen males of each strain were intermingled with males of the other strains in each cage for a total of 24 cockerels per unit. At 6 weeks, 3 of the 4 males per strain were randomly chosen and received numbered and colored plastic badges on both wings; additional males from each unit were then removed, leaving a total of 18 males. Males of each strain had different colored badges. Peck-order status of individual males during the early adolescent stage was determined by observations carried out between 7 and 10 weeks of age. Only betweenstrain interactions (involving cockerels with badges of different color) were recorded. The number of other-strain individuals dominated by each strain within each flock was calculated, and those means were used in an analysis of variance.
EGG MASS SELECTION: CHANGES IN SOCIAL BEHAVIOR
significantly with systems of selection over the generations studied. Differences between strains within systems were clearly evident for peck-avoidances in generations 4 and 7, for threat-avoidances in generation 4, and for total agonistic acts among young adult pullets in generations 4 and 7. Adolescent pullets of generation 7 differed in frequency of peck-avoidances, and the difference in total acts was nearly significant (P<.07). Housing environment influenced frequency of agonistic acts; fewer threat-avoidances were found in colony cages in all three generations tested. Total agonistic acts also were reduced significantly in colony cages in generations 2 and 4. Effects of housing on frequencies of peck-avoidances were inconsistent and insignifi-
TABLE 3 .—Effects of selection, random genetic drift, and housing environment on frequency of intrastrain agonistic acts of adolescent and young adult females Means'^ Generation 2 — young adults Source of variation
Peckavoidances
Threatavoidances
1.3 a 2.2b 1.8c
2.0 2.8 2.3
Generation 4 — young adults
Total acts
Peckavoidances
Threatavoidances
Total acts
3.3 5.0 4.1
3.1 2.8 2.6
3.5 3.1 3.0
6.6 5.9 5.6
Systems of selection C X Y
Random drift Strains within systems e Housing Floor pens Colony cages
1.1' 2.2 1.4
3-7 . .
5-8 , .
1.0
2.4
2.9 2.8
Gene ration 7 — adolescents
4.8 1.6
„
7.7 , 4.4
Generation 7 — youngadults
System of selection C X Y
2.3 2.1 3.5
Random drift Strains within systems e
1.0*
Housing Floor pens Colony cages
2.6 2.7
2.4 1.9 3.2
4.7 4.0 6.8
.5
1.5
34
6.0 4.3
1.6
1.2 1.4 1.7
.8***
1.3 1.6
1.1 1.4 1.8
.6
1.8 . 1.1
a,b,c, System of selection means followed by different superscripts differ significantly (P<.Q5). Expressed as acts per pullet hourly for all females present in flock (see text). Mean differences between strains within systems of selection. *P<.05, **P<.01, and ***P<.005.
2.3 2.7 3.5
1.3**
3.0 2.6
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ation. Therefore, only the results of analysis based on all pullets are presented, and data of generation 7 were collected and analyzed in that way only. Mean frequencies of agonistic acts of pullets on an hourly basis by systems of selection and housing environments for generations 2, 4, and 7 are shown in Table 3. The effects of random genetic drift in separating strains also are shown in Table 3 and in Figure 1 by differences between strains within systems of selection for generations, 2, 4, and 7. Although systems of selection influenced the frequency of intrastrain peck-avoidances during generation 2 (P<.05), generations 4 and 7 were not affected, and no clear pattern emerged over generations. Threat-avoidances and total acts were not found to be associated consistently or
887
888
BHAGWAT AND CRAIG SYSTEMS
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X
ce
Y
STRAINS
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C
\°X C
2
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8
STRAINS
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z
X
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stage (24 to 28 weeks). Peck-order Rankings. The number of otherstrain birds dominated in flocks of adolescent and young-adult birds of both sexes in generation 6 are given in Table 4. Systems of selection showed consistent peck-order ranks in all four comparisons. Differences were found between systems for adolescent and young adult males and y o u n g adult females; differences approached significance in the case of adolescent females (P<.09). System Y birds were most dominant, X birds were intermediate, and C system birds were least dominant. Strains within systems of selection differed in the number of other-strain birds dominated in the case of adolescent females only.
"o1
"2
2
2
4
7
DISCUSSION 2
4
7
GENERATIONS FIG. 1. Agonistic acts per female per hour as influenced by systems of selection and strains within systems, over generations.
cant over generations. Reduced total agonistic acts in cages was primarily a function of reduced threats and associated avoidances in this study. Age significantly influenced the frequency of agonistic acts during generation 7; fewer acts were observed during the young adult stage (36 to 42 weeks) as compared with the adolescent
Selection appears to be causing changes in egg mass of the selected strains and random genetic drift is also indicated. All strains have been compared for egg mass in 8 generationlocation tests. Differences between strains selected alike were detected in 2 of those comparisons. Random genetic drift most likely explains this because selection differentials and theoretical increases in homozygosity of strains within systems of selection are so similar (see Quadeer et al, 1977b). Therefore, we may reasonably expect that both selection and random drift effects may be detected in other traits, either as a result of correlated changes or independent of such. The criterion of selection, part-year egg
TABLE A .—Effects of selection and random genetic drift on number of other-strain birds dominated in flocks of adolescent and young adult males and females in generation 6 Me ans Young adult stage
Adolescent stage of variation
Males
Females
Males
Females
Systems of selection C X Y
2.0 a 3.0 b 4.9 C
1.5* 3.9* 6.9*
2.6 a 5.3b 5.5b
3.1 a 4.8b 8.1 c
1.3**
1.0
1.1
Random drift Strains within systems^
.5
' ' System of selection means followed by different superscripts differ significantly (P-C.05). Mean differences between strains within systems of selection. *P<.10, **P<.01.
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STRAINS
o-
z> 4 . Y
C
EGG MASS SELECTION: CHANGES IN SOCIAL BEHAVIOR
We hypothesized at the outset of the selection study that strains selected for part-year egg mass in intermingled-family flocks (X strains) would show increased aggressiveness and social dominance when compared with unselected control strains. That expectation apparently is confirmed for social dominance, although the cause of this change is questionable because of the increased relative social dominance which also occurred in the separated-family selection
program (Y strains). The Y strains, according to our hypothesis, should have become socially t o l e r a n t ; i n s t e a d , t h e y became socially dominant. A partial explanation of the apparent increase in social dominance under both selection schemes may be found in the earlier sexual maturity associated with selection for increased egg mass. Pullets of both selected systems consistently mature earlier than control strain pullets. Evidence from several studies suggests that a genetic correlation exists between age at sexual maturity and social dominance ability. Craig (1968) found that strains selected for higher levels of social dominance as young adults had earlier sexual maturity, as indicated by age at first egg. Craig and Toth (1969) w o r k i n g with the same strains of White Leghorns and Rhode Island Reds confirmed again that the more dominant strains matured earlier in both breeds. Working with other genetic stocks, Craig et al, (1975) found that strains selected for early egg production had both increased frequency of intrastrain aggression and were socially dominant to the later maturing and unselected controls. However, those differences were characteristic of the a d o l e s c e n t period only, and the relative dominance of the strains changed as the birds became older. Bhagwat and Craig (1977) also observed an apparent correlated response in strains selected for early and late age at first egg as they differed in social dominance ability in the same way. In the present study, the increased social dominance ability of selected strain pullets and cockerels was evident during both adolescent and young adult stages. On the basis of our present results and related research results, we hypothesize that the apparent increased social dominance under both systems of selection is in some way associated with the earlier sexual maturity of the selected strains. Housing environments influenced the frequency of agonistic acts. Crowding pullets together in colony cages reduced the frequencies of threat-avoidances and, thereby, total agonistic acts. Polley et al., (1974) and Craig and Bhagwat (1974) also reported decreased agonistic acts in colony cages as compared with frequencies in floor pens. However, they also found significantly higher frequencies of peck-avoidances in floor pens as compared with colony cages, although that was lacking in our present study. Hughes and Wood-Gush
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mass, was deliberately chosen to avoid direct selection for early sexual maturity; selection was based on production after 30 weeks of age. Previously collected data indicated that nearly all pullets start laying sooner than 30 weeks of age under our conditions. Quadeer et al., (1977b) estimated that part-year egg mass (from 30 weeks of age) was not genetically correlated with sexual maturity (—.02 ± .10) Nevertheless, though we attempted to avoid selecting for early sexual maturity, the selected strains have shown a correlated response by maturing earlier than the unselected controls. Frequencies of intrastrain agonistic acts among systems were inconsistent in generations 2, 4, and 7 (see Table 3 and Figure 1). We also see that the ranking of strains for frequency of agonistic acts within systems of selection changed over generations (Figure 1), and that mean differences between strains within systems of selection became significant in generations 4 and 7. Those differences are most likely accounted for by random genetic drift. Though birds of the strains selected for egg mass on the basis of intermingled- and separated-family data generally failed to exhibit more agonistic acts than controls, they were socially dominant over control strains when tested in intermingled-strain flocks in generation 6 (see Table 4). Thus, increases in social d o m i n a n c e a b i l i t y a p p e a r as correlated responses to selection for part-year egg mass. Though aggression is generally a necessary element in establishing social dominance, some strains differing in social dominance ability when compared in intermingled flocks or pair contests may fail to exhibit differences in frequency of aggressive acts when kept in flocks of their own kind (Craig et ah, 1965; Craig et al., 1969). Similarly, the strains in the present study, selected for increased egg mass, though dominant to unselected controls in peck-order comparisons, failed to differ significantly or consistently in frequency of intrastrain social interactions.
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O u r a d o l e s c e n t pullets h a d higher frequencies of agonistic acts as c o m p a r e d with y o u n g adult pullets (see generation 7 results, Table 3). T h e frequency of agonistic acts is increased soon after flocks of strangers are assembled, b u t t h e incidence of such acts is reduced after a few weeks (e.g., C h o u d a r y and Craig, 1 9 7 2 ) . Our p r e s e n t s t u d y suggests t h a t a higher frequency of agoni-tic acts was associated with adolescence rather t h a n t h e y o u n g adult stage. This high incidence during adolescence presumably occurred after t h e period associated with organization of t h e strangers i n t o a p e c k order, because t h e y were assembled at 2 0 weeks and t h e adolescent p e r i o d observations began 4 weeks later. V e n k a t a r a m a i a h and C h o u d a r y ( 1 9 7 3 ) also observed a higher level of agonistic activity during t h e period corresponding t o t h e age a t sexual m a t u r i t y for their strains which was reduced s u b s e q u e n t l y .
ACKNOWLEDGMENT Mr. C. R. Polley collected all d a t a in generations 2 and 4.
REFERENCES Al-Rawi, B., J. V. Craig, and A. W. Adams, 1976. Agonistic behavior and egg production of caged layers: Genetic strain and group-size effects. Poultry Sci. 55:796-807. Bhagwat, A. L., and J. V. Craig, 1977. Selecting for age at first egg: Effects on social dominance. Poultry Sci. 56:361-363. Biswas, D. K., and J. V. Craig, 1970. Genotypeenvironment interactions in chickens selected for high and low social dominance. Poultry Sci. 49:681-692. Choudary, M. R., and J. V. Craig, 1972. Effects of early flock assembly on agonistic behavior and egg p r o d u c t i o n i n c h i c k e n s . P o u l t r y Sci. 51:1928-1937. Craig, J. V., 1968. Correlated responses in body
weight and egg production traits in chickens s e l e c t e d for social dominance. Poultry Sci. 47:1033-1035. Craig, J. V., 1970. Interactions of genotype and housing environment in White Leghorn chickens selected for high and low social dominance. XlVth World's Poultry Congress, Madrid. 2:37—42. Craig, J. V., and A. L. Bhagwat, 1974. Agonistic and mating behavior of adult chickens modified by social and physical environments. Appl. Anim. Ethol. 1:57-65. Craig, J. V., D. K. Biswas, and A. M. Guhl, 1969. Agonistic behavior influenced by strangeness, crowding and heredity in female domestic fowl. Anim. Behav. 17:498-506. Craig, J. V., M. L. Jan, C. R. Polley, A. L. Bhagwat, and A. D. Dayton, 1975. Changes in relative aggressiveness and social dominance associated with selection for early egg production in chickens. Poultry Sci. 54:1647-1658. 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. Gowe,R. S., W. E. Lentz, and J. H. Strain, 1973. Longterm 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, pp. 2 2 5 - 2 4 5 . Guhl, A. M., 1953. Social behavior of the domestic fowl. Kansas Agr. Exp. Sta. Tech. Bull. 73. Hughes, B. O., and D. G. M. Wood-Gush, 1977. Agonistic behavior in domestic hens: The influence of housing method and group size. Anim. Behav. 25:1056-1062. Lowry, D. C , and H. Abplanalp, 1970. Genetic adaptation of White Leghorn hens to life in single cages. Br. Poultry Sci. 11:117-131. Lowry, D. C , and H. Abplanalp, 1972. Social dominance difference, given limited access to common food, between hens selected and unselected for i n c r e a s e d egg production. Br. Poultry Sci. 13:365-376. McBride, G., 1958. The relationship between aggressiveness and egg production in the domestic hen. Nature. 181:858. McBride, G., 1960. Poultry husbandry and the peck order. Br. Poultry Sci. 1:65-68. McBride, G., 1962. The interactions between genotypes and housing environments in the domestic hen. Aust. Soc. Anim. Prod. 4 : 9 5 - 1 0 2 . McBride, G., 1964. Social behavior of domestic animals. II. Effect of the peck order on poultry productivity. Anim. Prod. 6:1—7. Polley, C. R., J. V. Craig, and A. L. Bhagwat, 1974. Crowding and agonistic behavior: A curvilinear relationship? Poultry Sci. 53:1621-1623. Quadeer, M. A., J. V. Craig, K. E. Kemp, and A. D. Dayton, 1977a. Selection for egg mass in different social environments. 1. Estimation of some parameters in the foundation stock. Poultry Sci. 56:1522-1535. Quadeer, M. A., J. V. Craig, K. E. Kemp, and A. D.
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( 1 9 7 7 ) studied t h e incidence of agonistic behavior a m o n g laying hens of t w o strains, housed in b a t t e r y cages and in deep litter pens in groups of either 3 or 6 birds. T h e y observed t h a t aggressive head pecking and t h r e a t s were m o r e frequent in floor pens t h a n in cages and in groups of 6 birds. T h e y suggest t h a t in a n o r m a l t h r e a t display, birds require a d e q u a t e space t o assume positions with a particular o r i e n t a t i o n . Because of r e d u c e d space per bird in cages, constrains are t h u s imposed on such orientation, which t h e r e b y reduces t h e frequency of threats in c o l o n y cages.
EGG MASS SELECTION Dayton, 1977b. Selection for egg mass in different social environments. 2. Estimation of parameters in selected populations. Poultry Sci. 56:1536—1549. 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
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social aggressiveness and competition effects between sire families in small flocks of chickens. Poultry Sci. 39:1318-1320. Venkataramaiah, A., and M. R. Choudary, 1973. Effects of strains and presence of male on agonistic behaviour and part-year egg production in White Leghorns. Indian J. Poultry Sci. 8:258—263.
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