Effects of haloperidol, a dopamine D2 receptor antagonist, on feather pecking behaviour in laying hens
Introduction
Feather pecking is a behaviour by which hens destroy the feathers of other hens, in some cases even plucking out feathers and eating these. In some severe cases feather pecking can be followed by cannibalism, where hens eat the blood and tissue of other hens. A wide range of causal factors of feather pecking, such as housing-, rearing-, and feeding factors, have been reported (Sharma et al., 1999) and recently additive (heritable) genetic factors were found to be fundamental (Kjaer and Sørensen, 1997, Kjaer et al., 2001). A majority of authors suggests feather pecking to be a redirection of pecking related to the appetitive phase of feeding behaviour (Blokhuis, 1986, Hoffmeyer, 1969, Wennrich, 1975). Alternatively, mis-imprinting on feathers as a dustbathing substrate followed by redirection of pecking in relation to dustbathing has been suggested that lead to feather pecking and related feather damages (Vestergaard, 1994, Vestergaard and Lisborg, 1993). Stress in relation to lack of coping (Korte et al., 1997, Korte et al., 1999) or stress in more general terms (El-lethey et al., 2001, van Hierden et al., 2001) seem also to affect feather pecking.
The motor patterns of feather pecking are quite similar to those of stereotyped pecking (Bilcik et al., 1999, Blokhuis et al., 1993, Kjaer and Vestergaard, 1999), but no experimental evidence has, hitherto, supported the suggestion of a common physiological background. Self mutilation (plucking and damaging own feathers) in birds is believed to be a variant of stereotyped behaviour (Iglauer and Rasim, 1993). Neuropeptides, particularly endogenous opioids and dopamine, are supposed to play a role in many self mutilating disorders (Cabib, 1993, Goodman et al., 1983), and affected pet birds (e.g. parrots) respond to treatment with various psychoactive drugs (Iglauer and Rasim, 1993, Johnson, 1987, Levine, 1984). Several authors have suggested the influence of the dopamine system in relation to stereotypies in different species of birds. Apomorphine, a D1/D2 receptor agonist, induces stereotypic pecking in pigeons (Cheng and Long, 1974, Goodman et al., 1983), jungle fowl (Kjaer, 2000, unpublished) and domestic fowl (Nictico and Stevenson, 1979). This effect can be reduced by haloperidol, a D2 receptor antagonist (Goodman et al., 1983).
To investigate the potential link between dopamine activity and feather pecking in adult laying hens the objective of the present study was to examine the effects of haloperidol on feather pecking behaviour in adult laying hens. Dopamine antagonists at high doses are known to suppress motor activities in broiler chickens (Kostal and Savory, 1994), but it was not possible to find literature on responses to haloperidol by adult laying hens. Therefore, the study comprised two experiments. First, the general behavioural time budget was recorded in a dose-response experiment in order to find the maximal dose without major sedative effects. Then, in a second experiment we tested the effect of this dose of haloperidol on pecking behaviour. Our working hypothesis was that feather pecking in adult hens would be reduced by haloperidol.
Section snippets
Animals and housing
A total of 240 White Leghorn laying hens from a random-bred control line kept at the institute for several decades (Kjaer et al., 2001) were used. The birds were at 70 weeks of age moved from four-bird battery cages (Big Dutchman) to floor pens, comprising 30 groups of eight birds each. Groups were compounded with respect to previous location on tiers (Jones, 1985) and mixed one bird from eight different cages to ensure equal unfamiliarity between group members. To facilitate individual
Subjects and housing
A total of 48 ISA Brown hens were used as subjects. The birds were 118 weeks old and housed in a two tier cage battery system (same as used in experiment 1) in groups of four hens prior to the experiment. Three days before the behavioural observations each experimental hen was paired with a White Leghorn hen, acting as pen mate and feather pecking recipient and placed in the same type of four-bird battery cage, one pair per cage. The 82-week-old White Leghorn hens were chosen at random from an
Discussion
We have shown that haloperidol selectively reduces the level of feather pecking in laying hens. The selective effect of a dopaminergic drug on feather pecking gives good support to the hypothesis that feather pecking may be related to stress and can be regarded as a stereotypy under control of the dopamine system. Contrary, aggressive behaviour seems not to be under dopaminergic control. This distinction between aggression and feather pecking is in accordance with the behaviour-genetic approach
Acknowledgements
The Danish Ministry of Food, Agriculture and Fisheries financially supported these experiments. The authors wish to thank Jens Malmkvist for valuable comments on earlier versions of this manuscript as well as Kirsten L. Balthzersen and Inger Marie Jepsen for excellent technical support.
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