selection for high litter size has resulted in mice that adopt the active coping strategy. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Selection; Litter size; Coping behaviour; Resource allocation
1. Introduction
Ž .
In the study of Rauw et al. 1999 , it was shown that mature non-reproductive female Ž
. mice of a line selected for high litter size have higher residual feed intake RFI than
females of a non-selected control line. RFI is defined as the part of the feed intake that is unexplained by food requirements for maintenance and production, or in other words, as
the difference between the feed that is consumed by an animal and its consumption as predicted from a model involving its growth and maintenance requirements. Variation in
RFI can be caused by variation in partial efficiencies for maintenance and growth and by variation in metabolic food demanding processes not included in the model, such as
behavioural activities, responses to pathogens and responses to stress. Since growth is virtually absent at maturity, the differences in RFI are mainly explained by differences
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in maintenance requirements Luiting, 1990 . Ž
. In a divergent selection experiment on RFI in laying hens, Luiting et al. 1991
Ž .
observed higher activity-related heat production in the high RFI low efficiency line Ž
. than in the low RFI efficient line: more than 50 of the variation in RFI could be
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explained by variation in physical activity. Findings by Morrison and Leeson 1978 and Ž
. Braastad and Katle 1989 support these results: inefficient laying hens were more active
than efficient hens. Variation in the amount of feed intake activity accounted for 44 of the variation in RFI in pigs: pigs with a low RFI visited the feed hopper less often and
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spent less time eating per day than pigs with a high RFI De Haer et al., 1993 ; these Ž
. results are supported by Von Felde et al. 1996 .
Differences in activity levels suggest underlying differences in coping strategies. Coping can be defined as a behavioural and physiological reaction to aversive situations
Ž and aims at retaining or re-establishing homeostasis Wechsler, 1995; Koolhaas et al.,
. 1997 . Behavioural strategies have been extensively described in mice selected bidirec-
Ž .
tionally for attack latency, particularly in males e.g., Benus, 1988; Sluyter et al., 1996 Ž
. and more recently also in females Compaan et al., 1993; Benus and Rondigs, 1996 .
¨
The correlated responses in a variety of other behavioural traits indicate the existence of a general underlying control mechanism determining complex behavioural strategies
Ž .
Sluyter et al., 1995 . Consequently, reciprocal selection for related behavioural traits, Ž
. such as nest-building behaviour Sluyter et al., 1995 , active shock avoidance acquisition
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Ž .
Driscoll et al., 1990 and susceptibility to apomorphine Cools et al., 1990 have resulted in similar reciprocal correlated responses in coping behaviour. Large individual
differences in the behavioural and physiological reactions to stressors are not restricted to rodent species. Evidence for the existence of different coping styles has been obtained
Ž .
Ž .
Ž in pigs Hessing et al., 1993 , dairy cows Hopster, 1998 and great tits Verbeek et al.,
. 1996 .
In general, two coping strategies are distinguished: active and passive copers, which Ž
adopt different strategies towards a wide variety of environmental challenges Koolhaas
. et al., 1997 . Active copers tend to actively manipulate events, whereas passive copers
tend to switch to passivity. In social confrontations, active copers respond by attacking Ž
. the opponent or fleeing from a physically stronger opponent fightrflight reaction ,
whereas passive
copers respond
with ‘freezing
behaviour’ or
immobility Ž
. conservationrwithdrawal strategy . In non-social situations, the behaviour of active
copers is less affected by changes in the environment. Active copers tend to develop routine behaviour; their behaviour is more intrinsically organised. The behaviour of
passive copers is more dependent on external stimuli and hence their behaviour is more Ž
. flexible Benus, 1988 . Active and passive copers differ furthermore for several physio-
Ž .
logical and neurobiological characteristics Benus, 1988 . The aim of the present study was to investigate whether selection for high litter size
at birth in mice has resulted, as a correlated effect, in a higher occurrence of behavioural characteristics indicative of an active coping style. Therefore, non-reproductive adult
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female mice of the line selected for high litter size S-line and of the non-selected Ž
. control line C-line were subjected to three non-social tests and a social confrontation.
We hypothesise that subjects from the S-line, which are characterised by a high RFI, will respond with a behavioural pattern more dominated by an active coping style when
subjected to mild stressful situations, than subjects of the C-line.
2. Material and methods
