. 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
Ž .
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
Ž .
Two mouse lines of the Norwegian mouse selection experiment e.g., Vangen, 1993 Ž
. were used: a line selected for high litter size at birth S-line and a non-selected control
Ž .
3
line C-line . The mice were housed in cages of 30 = 12.5 = 12.5 cm filled with a layer of saw dust and had free access to pellet concentrate and water. The energy content of
the feed was 12.6 kJ ME per gram and contained 21 crude protein, as specified by the producer. The mice originated from litters standardised at birth, when larger than eight
pups, to eight pups per litter. The light was left on 24 h a day.
In the 91st generation, per line, from each of 12 litters one sister-pair was randomly Ž
. chosen at weaning i.e., 3 weeks of age . Average total number of pups born was 10 and
21 in the C- and S-line, respectively. All animals were housed in sister-pairs until 10 weeks of age; thereafter they were housed individually. From 11–15 weeks of age, the
females were subjected to an open-field procedure, a maze, a social confrontation and a runway test.
2.1. Test one: open-field test A total of 24 females of each line were introduced individually into an open-field
Ž .
apparatus at 76 and 77 days of age OF1 and OF2 . The test duration was 30 seconds. Ž
2
. The open field 70 = 110 cm
was surrounded by 20 cm high, non-transparent walls
2
Ž .
and was divided into squares of 10 = 10 cm Fig. 1a . In OF1, the mice started in the
Ž .
centre of the OF; in OF2 they started in the corner of the OF Fig. 1a . The lines were tested in rotating order. The testing sequence of all mice was similar for both testing
Ž . Ž
3
. Fig. 1. a Design of the open-field test 70=110=10 cm . 1: starting point OF1; 2: starting point OF2. Dark
Ž . Ž
3
. area marks the centre of the open field. b Design of the maze test 55=55=10 cm . G sGoal box,
Ž .
Ž .
SsStart box. Dark areas mark goal area below G and starting area above S . Light dark areas mark dead Ž .
Ž
3
. ends. c Design of the runway test 200=10=10 cm . SsStart box, G sGoal box. Distance in centimeters
Ž .
25 to 200 is marked.
days. During the 30 s observation period, the position of the mouse in the OF was registered every 5 s. From these data the following parameters were measured:
Ø Distance travelled as measured by the cumulative number of squares crossed in 5, 10,
Ž .
15, 20, 25 and 30 s OF1 and OF2 Ž
. Ø
Total number of squares crossed in the centre of the OF in 30 s OF2 2.2. Test two: maze test
The same 24 females of each line were introduced individually into the start box of a Ž
. maze at 80 to 86 days of age M1 to M7 . The test duration was 120 s. The maze
Ž
3
. Ž
. 55 = 55 = 10 cm
had non-transparent walls and a transparent lid on top Fig. 1b . The lines were tested in rotating order. The testing sequence of all mice was similar for all
Ž .
testing days. A male stimulus mouse of the C-line randomly selected from both lines was housed in the goal box of the maze at least one hour before the start of the test. The
stimulus male was visible for the experimental female subject when the goal area was entered. The same stimulus male was used during the whole procedure. The maze and
the start box were cleaned at the end of every session. On the sixth day of the procedure the maze was rotated 908 in the horizontal plane in order to confront the mouse with an
Ž .
extra-maze cue change test M6 . During M1 to M7 the following behavioural parame- ters were measured:
Ž . Ž .
Ø Latency time reaching the goal area s
LGA Ž . Ž
. Ø
Latency time entering the goal box s LGB
Ž . Ø
Total time spent between reaching the goal area and entering the goal box s Ž
. GA–GB
Ž Ø
Maximum distance reached as a percentage of the total maze as considered
. Ž .
excluding dead-ends REACHED
Ž .
Ø Total number of encounters in dead-ends before reaching the goal area DEAD-END
Ž Ø
Total number of returns to the starting area before reaching the goal area RE- .
TURNS 2.3. Test three: social confrontation test
The same 24 females of each line were subjected to a social confrontation test at 92 Ž
3
. days of age. The test arena was a novel, unfamiliar clean cage 40 = 20 = 20 cm . Two
females being unfamiliar to each other and no sisters were paired for 10 min. Behaviour was recorded on video. Mice with different coat colours were put together to facilitate
the distinction between the animals on video, but were further randomly allocated to one
Ž . of three interaction groups, each of which consisted of eight pairs of mice: 1 two S-line
Ž . Ž .
females, 2
two C-line females, and 3
an S-line female with a C-line female. Instantaneous sampling was used to measure the occurrence of behavioural elements
every 10 s. The behaviours measured are listed in Table 1. 2.4. Test four: runway test
The same 24 females of each line were introduced individually into the start box of a Ž
. runway at 100 and 101 days of age RW1 and RW2 . The test duration was 60 s. The
Table 1 Ž
. Behavioural elements measured in the social confrontation test after Benus, 1988
Behavioural element Explanation
Fighting Behaviour shown by each of the contestants when locked together: violent kicking,
biting and wrestling behaviour Submissive upright
Sitting upright, head into the air, forepaws rigidly stretched out forward Immobility
Absence of any movement Social investigation
Sniffing or nibbling any part of the opponent’s body Jumping
Jumping, often to a wall Upright
Standing or sitting on hind legs, mostly making sniffing movements, with the nose up into the air
Sniffing Standing still with nose on the floor
Locomotion Locomotion, no apparent direction
Grooming Wiping, licking and nibbling the fur with forepaws and tongue
Miscellaneous Any other behaviour
Ž
3
. runway 200 = 10 = 10 cm
had non-transparent walls and a transparent lid on top Ž
. Fig. 1c . The goal box at the end of the runway was covered with saw dust. The lines
were tested in rotating order. The testing sequence of all mice was similar for both testing days. Latency times to reach the 25, 50, 100, 150 and 200 cm marks of the
Ž .
runway were recorded Fig. 1c . 2.5. Data handling and statistical analysis
Ž .
The SAS program was used for the statistical analysis of all traits SAS, 1985 . Line differences for the individual traits were tested with the model: Y s m q L q e , where
i j i
i j
Ž .
m s overall mean, L s effect of line i control, selection and e s error term of
i i j
Ž
2
. animal j of line i, e NID 0,s
. Y denotes all traits tested with this model, all as
i j e
i j
measured on animal j of line i: the cumulative number of squares crossed in 5, 10, 15, 20, 25 and 30 s in OF1 and OF2, the number of lines crossed in the centre of OF2,
LGA, GA–GB, REACHED, DEAD-END and RETURNS in M1 to M7, the frequen- cies of ‘fighting’, ‘submissive upright’, ‘immobility’, ‘social investigation’, ‘jumping’,
‘upright’, ‘sniffing’, ‘locomotion’, ‘grooming’ and ‘miscellaneous’ in the social con- frontation test, and the latency times for reaching 25, 50, 100, 150 and 200 cm of RW1
and RW2.
Trends and line differences in trends for cumulative number of squares crossed in Ž
. OF1 and OF2 over time 5 to 30 s , LGA, GA–GB, REACHED, DEAD-END, and
Ž Ž
RETURNS in the maze tests over testing day M1 to M7, excluding M6 extra-maze cue ..
Ž .
change , and latency times in RW1 and RW2 over distance reached 25 to 200 cm are tested by fitting linear regression functions to the data.
Contrasts are generated to compare cumulative number of squares crossed at 5, 10, 15, 20, 25, and 30 s in the open-field test and for reaching 25, 50, 100, 150 and 200 cm
Ž .
in the runway test between testing days OF1, OF2 and RW1, RW2 . The effect of the extra-maze cue change on day 6 of the maze test is tested by means
of t-tests testing ‘H : observed trait values s expected values as estimated from the regression lines’ against the alternative hypothesis that the observed trait values on test
day 6 differ from the expected values as estimated from the regression lines. Latency times of animals that do not reach the goal area in the maze tests are
arbitrarily set to 121 s, which is 1 s higher than the full testing time. Average GA–GB is Ž
. estimated only from animals that reach both GA and GB within time i.e., within 120 s .
Latency times for reaching, 25, 50, 100, 150 and 200 cm in the runway tests are estimated for all animals completing the runway within time and excluding individuals
that return to the start box.
3. Results
