1. Introduction
This experiment was conducted as one of a series of experiments investigating problems of aggression towards females by broiler breeder males. High levels of
Ž aggression directed toward females by male broiler breeder domestic fowl Gallus gallus
. domesticus have been reported in the poultry industry during the past 5–10 years
Ž .
Mench, 1993; Dr. Rachel Ouckama, personal communication . Males are extremely rough during mating, forcing copulations and often injuring or killing females. Initially,
problems were reported in one particular male strain, but now appear to be associated with all North American commercial strains of broiler breeders. In flocks in which
aggression has become a problem, males typically chase and ‘‘corral’’ females into corners. Females avoid males by running away, by hiding in nest boxes and by
remaining on raised slatted areas. Females typically have lacerations on the back of the head caused by males pecking and along the torso, underneath the wings caused by
males’ claws when they mount roughly. There have been reports of attacks by males on employees in broiler breeder barns and this phenomenon may resemble the mobbing
behaviour toward humans handling birds in large breeding flocks observed by Curtis Ž
. 1986 . Male aggression has a deleterious economic effect on the poultry industry
through decreasing fertility and increasing female mortality. The injuries and the fear caused to the females also raise serious concerns for animal welfare.
This phenomenon of male aggression towards females is interesting from a theoreti- cal as well as a practical point of view. Whereas immature males may behave
Ž .
aggressively to females Wood-Gush, 1960; Rushen, 1983 , such aggression by mature Ž
male domestic fowl is rare Wood-Gush, 1960; Kruijt, 1964; Craig and Bhagwat, 1974; .
Ylander and Craig, 1980; Bsary and Lamprecht, 1994 . Males and females form separate Ž
. social hierarchies and males dominate females passively Guhl, 1949 . Furthermore,
selection for growth has been found to be associated with decreased aggression Ž
. Marsteller et al., 1980 , while selection for egg production and low body weight may be
Ž .
associated with increased agonistic activity Craig et al., 1975 . Our previous research indicated that broiler breeder males showed more aggression to males than did commer-
cial laying strain males and more unusually, they chased and aggressively pecked Ž
. females much more frequently Millman et al., 1997 . Broiler breeder males also showed
lower frequencies of courtship displays and forced more copulations when compared Ž
. with commercial laying strain males Millman et al., 1996 . Courtship displays may
Ž arise from conflicting sexual, attack and escape motivations Wood-Gush, 1956; Kruijt,
. 1964; Bastock, 1967 . It is therefore possible that in the case of the responses of broiler
breeder males to females, aggressive motivation is overriding sexual motivation. It is our hypothesis that the high levels of aggression shown by broiler breeder males toward
females is due to an increase in general aggressive motivation. There are two mecha- nisms by which this could have occurred. Certain management practices may elicit
aggression or there may have been genetic selection for increasing aggressiveness. It is also possible that these mechanisms may work in combination.
1.1. Management practices Due to the growth potential of broiler stock, broiler breeders must be reared under
severe feed restriction to prevent obesity and problems associated with rapid growth Ž
. Leeson and Summers, 1991 . During the rearing period, feed allocation is 40–50 of
ad libitum intake resulting in body weights at 22 weeks which are 65 of that of ad Ž
. libitum fed birds
North and Bell, 1990 . Such severe feed restriction results in Ž
. stereotypic pecking suggestive of high levels of frustration Savory and Maros, 1993 .
Moreover, frustration associated with thwarting of feeding behaviour has been shown to Ž
. increase aggression toward females by males Duncan and Wood-Gush, 1971 . Mench
Ž .
1988 found that although broiler chicks displayed low levels of aggression when compared to laying stock, feed-restricted 12-week-old broilers showed significantly
more threats and pecks than did those fed ad libitum. Since much of the aggression appeared at the feeder, she concluded that frustration was a factor. Conversely, our
research indicated that aggression was higher in males that were full-fed at maturity Ž
. Millman et al., 1997 . However, all of our broiler breeder males were feed-restricted
during the rearing period to avoid obesity and it is possible that feed restriction during the rearing phase affects aggression at maturity. Effects of the social environment during
Ž rearing have been shown to affect sexual behaviour of females at maturity Leonard et
. al., 1993b; Widowski et al., 1998 and aggressive and sexual behaviour of mature males
Ž .
Leonard et al., 1993a . Feed restriction is imposed at 1–3 weeks of age in broiler Ž
. breeder management Ross Breeders, 1997 which coincides with development of social
Ž .
behaviour in chicks Kruijt, 1964 . 1.2. Genetic selection
Broiler chickens have been heavily selected for meat production and reach market Ž
. weight at 42 days of age North and Bell, 1990 . At this age, broilers have been found to
Ž .
be less aggressive than laying strain males Mench, 1988 . However, it is possible that aggression in broilers does not increase until sexual maturity and hence is only observed
in the breeder stock. In attempts to increase breast meat yield, the Cornish strain, Ž
traditionally used for cock-fighting, has been bred into the male line North and Bell, .
1990 and it is possible that this has resulted in an increase in aggressiveness. Also, aggression may have been inadvertently selected through attempts to improve
sexual vigour. There persists a belief in the broiler breeder industry that male lines need Ž
to be more sexually aggressive to overcome problems of low fertility Peterson
. Breeders, personal communication . Selection for high and low mating lines was
Ž .
Ž .
successfully performed by Wood-Gush 1960 using laying stock and by Siegel 1972 using broiler stock. Although a genetic basis for aggressiveness was also found by
Ž .
Ž Siegel 1959 , no relationship was found between aggressiveness and sex drive Wood-
. Gush, 1958; Siegel, 1972 . Low fertility in broiler breeders was shown to result from
Ž .
lack of cloacal contact and not from low levels of libido in males Duncan et al., 1990 . Ž
. Wilson et al. 1979 also attributed problems of fertility in Cornish type males to
defective mating behaviour.
This experiment was designed to investigate how feed restriction during rearing and genetic strain affect the aggressiveness of male domestic fowl at sexual maturity. We
used commercial laying strain males to examine the effects of feed restriction during rearing and thus avoided the confounding effects of obesity in full-fed broiler breeder
males. Behaviour of laying strain males fed ad libitum during rearing was compared with that of males feed-restricted to the same degree as in broiler breeder production.
Behaviour of broiler breeder males was compared with that of Old English Game males, a strain traditionally bred for its fighting behaviour. Since commercial laying strain
males had shown low levels of aggression to females in our previous research they were used as controls. Our hypothesis was that feed-restricted laying strain males would show
higher levels of aggression than males fed ad libitum during rearing. We also expected that broiler breeder males would show levels of aggression similar to that of Old English
Game strain males and much higher than that of commercial laying strain males. Aggressiveness was determined using a standardised test in which we recorded re-
sponses to a model of a cockerel in a standing, non-threatening posture.
2. Animals, materials and methods
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