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APPLIED ANIMAL BEHAVIOUR
SCIENCE
ELSEVIE&! Applied Animal Behaviour Science 49 (1996) I .37- 148
Selection for behavioural traits in farm mink
Steffen Werner Hansen Danish Institute of’Animtrl Scirncc. Resecrrch Centrr Foulum. P.0. Box 39,
Accepted .? I January 1996
DK-8830 Tjrlr Dmmurk
Abstract
Over a period of 6 years, more than 3000 farm mink were tested for their behavioural response to human contact. Using a simple test (the stick test) five times per generation, the mink were characterized and classified with regard to their response to human contact. Behavioural response that might be caused by genetic factors was found to occur in three lines selected for explorative, fearful, and aggressive temperament. Over six generations, a considerable quantitative difference in behavioural response between the three selection lines developed. Selection for fearful behaviour caused the normal habituation towards man to disappear, and 90% of the mink selected for fearfulness responded consistently with fear to human contact. A less distinct effect was found in mink selected for explorative behaviour at human contact.
A possible explanation may be that the basic level of explorative behaviour in the population was relatively high, but also that the test used did not allow for a graduation of the explorative behaviour towards confidence. Apart from the last two generations of mink selected for fear, all lines have shown a pronounced difference in temperament between sexes showing that females were more fearful than males.
Keywords: mink; temperament; welfare; domestication; selection
1. Introduction
The processes contributing to the adaptation of animals to humans and the artificial environment are called domestication. Domestication is interpreted as a combination of genetic changes over generations and non-genetic influences experienced by the individ- uals in each generation (Price, 1984).
Species poorly adapted to captivity are exposed to an intense natural selection through reduced survival and/or reproduction success in the early stages of domestica- tion (King and Donaldson, 1929). For the individuals reproducing in captivity the changed conditions may enhance the development of properties normally connected with domestication (Clark and Galef, 1977).
An important form of adaptation in captivity obtained by habituation to human
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138 S.W. Hunsen/Applied Animal Behauiour Science 49 (1996) 137-148
contact is a reduction or elimination of the escape reaction towards humans. This tameness towards humans is normally considered to be a result of experience or learning repeated in each generation (Pedersen, 1991; Hemsworth and Barnett, 1991). Trut (1981) has, however, demonstrated the possibility of selecting for tameness in silver foxes. The basis of this behavioural selection was a genetic polymorphism for defensive behaviour.
Over 25 generations, deliberate selection for tame behaviour resulted in pronounced behavioural changes, but also several correlated changes appeared: changes in the reproductive system causing the females to be in heat earlier, and in some cases fertile mating twice a year, changes in pelt moulting, changes in pelt colour, marked decreases in corticosteroid and progesterone levels, and increases in oestradiol and serotonin levels (Naumenko and Belyaev, 1980). These factors have given rise to the hypothesis of destabilizing selection (Belyaev, 1979; Trut, 1987).
Foxes as well as mink are relatively new domestic animals compared with cows, sheep, horses and chickens, which have been domesticated for thousands of years. The first descriptions of farm-raised mink are from Canada at the end of the last century (Nes et al., 1987). At the beginning of the 1930s the mink (Musrela uison) was introduced as a farm animal in Denmark. In the 60years of domestication of mink in Denmark, the design of the production environment has been very stable, and the cage size has not changed since the 1970s when the industrial manufacture of production cages started. This stability in the production environment of mink is essentially different from the development within production systems for other domestic animals.
A deliberate, artificial selection for pelt quality began early in fur animal production. Later, other parameters such as reproduction capacity and size as well as crossing of the subpopulations-strains Alaska type and Eastern-Quebec (Nes et al., 1987) were in- cluded in the deliberate, artificial selection. An unconscious, artificial selection has probably taken place at the same time by excluding animals with deviant, undesirable behaviour from breeding.
An undesirable behaviour in production animals is fear of humans. Fear can influence the production capabilities negatively (Gonyou et al., 1986; Hemsworth et al., 1987; Barnett et al., 1994) and gives the immediate, subjective impression of reduced animal welfare. Fear causes behavioural as well as physiological changes in the individual. For characterization of the motivational conditions of mink when in contact with humans, we have in these investigations applied relatively rough locomotive behavioural expres- sions such as escape, exploration and attack, which has been necessary because of the size of the mink and their hardly recognizable signalling.
The purpose of the present investigation was, through selection, to alter the tempera- ment of mink towards human beings measured in terms of frequency of behavioural traits such as escape, exploration and aggression and to describe the development of these behavioural traits in farm mink during the growth period from July to November.
2. Material and methods
In November 1987, the project was started at Trollesminde with the purchase of 150 females and 30 males of the scanblack type. These animals were the parental generation
S.W. Hunsen/Applied Animui Behouiour Science 49 (19961 137-148 13’)
Table 1 Number of kits per generation distributed on year, line and sex; average frequency of the selected behaviour (mean f SD) and frequency of animals (per cent) responding with the selected behaviour in 0, I, 2, .i, 4 or all 5 tests
Year Line/ Sex N Mean SD 0 I 2 3 4 5 behaviour
F, E/ Male 91 78.9 24.8 4.4 I.1 3.3 18.7 31.9 40.7 1%
F, 1989
F3 I990
F4 1991
5 1992
F6 1993
explorative Female 95 68.0
F/ Male 80 19.5 fearful Female 54 42.6
A/ Male 35 6.9 aggresswe Female 35 6.9
E/ Male 129 69.9 explorative Female 115 55.0
F/ Male I05 53.7 fearful Female II5 77.0
A/ Male 39 6.2 aggresstve Female 52 4.2
E/ Male II8 87. I explorative Female 97 73.2
F/ Male 122 39.3 fearful Female 129 55.0
A/ Male 63 9.5 aggressive Female 38 7.9
E/ Male II5 89.0 explorative Female II9 69.8
F/ Male 99 50.9 fearful Female I IO 69.8
E/ Male I65 48.9 explo’rative female I54 30.8
F/ Male 167 66.8 fearful Female I71 74.2
E/ Male 107 56.5 explorative Female 85 30.8
F/ Male I14 91.4 fearful Female I41 94.6
29.5 3.2 Il.6 13.7 13.7 29.5 2X.4
27.4 53.7 21.3 10.0 6.2 6.2 2.5 37.9 27.8 20.4 I I.1 I I.1 I I.1 IX.5
16.8 80.0 I I.4 5.7 0.0 2.9 0.0
14.5 80.0 17.1 2.9 0.0 0.0 0.0
24.8 2.j 3.9 16.3 18.6 37.2 21.7
32.4 1 I.3 16.5 13.0 20.0 23.5 1.7
33.2 9.5 18.1 23.8 11.3 13.3 21.9
25.9 0.9 7.0 7.8 17.4 24.3 42.6
12.3 76.9 IS.4 7.7 0.0 0.0 0.0
12. I 86.5 7.7 3.8 1.9 0.0 0.0
16.9 0.0 0.0 4.2 I I.0 29.7 s5. I
24.3 0.0 7.2 I I.3 18.6 34.0 3x.9
27.1 14.8 27.9 22.1 18.0 15.6 1.6
28.9 9.3 14.0 Id.0 25.6 29.5 7.x
14.8 65.1 23.8 9.5 I .6 0.0 0.0
15.1 73.7 15.8 7.9 2.6 0.0 0.0
17.8 0.9 0.9 0.9 IO.4 23.5 63.5
29.4 I.7 IO.9 16.0 16.0 19.3 36. I
35. I 20.2 IO.1 18.2 14.1 21.2 16.2
26.7 6.4 I .8 9. I 24.5 35.5 22.7
33.7 17.6 17.6 13.9 17.6 20.6 12.7
29.4 31.8 25.3 16.9 12.3 IO.4 3.2
25.0 5.4 3.6 13.8 IS.6 52. I 9.6
16.2 1.2 I.2 5.3 17.0 69.0 6.4
33.7 IS.9 6.5 17.8 17.8 23.4 IX.7
32.6 38.8 21.2 IO.6 9.4 16.5 3.5
15.7 0.0 I.8 0.9 5.3 22.8 69.3
12.9 0.7 0.0 0.0 3.5 16.3 79.4
from which the following selected generations (F,-F,) descend. In November 1988, the animals were transferred to Research Centre Foulum where the investigation continued.
Each year the kits were weaned in the middle of June at the age of 6-7 weeks. They were placed in pairs (male and female) in conventional production cages (L x W x H: 90cm X 30 cm X 45 cm) with a nest box attached. The animals were given feed from a local feed kitchen and water ad libitum, and the farm was managed according to standard farm routine. Tables 1 and 2 show number of kits per year, sex, and selection line as well as frequency of animals in the tests reacting with the selected behaviour.
2. I. Test pnvcedure
After the preliminary examinations, where varying strengths of stimuli were tested in a test situation, we have, according to our experience, chosen to use the ‘stick test’ as a
140 SW. Hansen/AppliedAnimul Behuviour Science 49 (1996) 137-148
Table 2 Number of kits in the control line (C) per year and sex and distribution of behavioural response (mean + SD) and frequency of animals (per cent) responding with explorative, fearful, aggressive and ‘unknown’ behaviour in 0, I, 2, 3, 4 or 5 tests
Year Sex N Behaviour Mean SD 0 I 2 3 4 5
1992
1993
Male 145 Explorative 32.1 33.8 39.3 15.9 16.6 9.7 10.3 8.3 Fearful 46.5 32.1 20.0 13.8 18.6 13.8 29.0 4.8 Aggressive 1.4 6.9 95.9 1.4 2.8 0.0 0.0 0.0 Unknown 20.0 16.8 29.7 46.2 18.6 5.5 0.0 0.0
Female 137 Explorative 12.4 21.3 67.2 15.3 9.5 4.4 3.6 0.0 Fearful 68.6 23.2 2.2 2.9 18.2 17.5 44.5 14.6 Aggressive 0.4 5.1 99.3 0.0 0.0 0.7 0.0 0.0 Unknown 18.5 15.8 30.7 50.4 14.6 4.4 0.0 0.0
Male 149 Explorative 27.5 29.9 40.9 18.8 18.8 8.1 10.1 3.4 Fearful 67.4 31.7 6.0 10.7 11.4 17.4 20.8 33.6 Aggressive 0.3 2.3 98.7 1.3 0.0 0.0 0.0 0.0 Unknown 4.8 9.2 77.2 21.5 1.3 0.0 0.0 0.0
Female 146 Explorative 11.5 20.2 66.4 19.2 8.9 2.1 2.7 0.7 Fearful 84.4 22.3 0.7 4.1 3.4 11.6 24.7 55.5 Aggressive 0.4 3.7 98.6 0.7 0.7 0.0 0.0 0.0 Unknown 3.7 8.1 82.2 17.1 0.7 0.0 0.0 0.0
suitable stimulus for selection. When using the stick test, the mink in one cage section of six cages are simultaneously shut out from their nest boxes. The observer places himself or herself in front of each mink cage and puts a wooden tongue spatula (the stick) through the wire netting, The mink react to close contact with humans and direct their reaction towards the tongue spatula. The immediate reaction to stimulus as well as the period of latency before reaction are recorded. The duration of the test had a maximum of 3Os, but at the first test of kits in July the duration was extended to a maximum of 60 s. The reaction of the mink is characterized by four types: (1) escape: the mink moves away from stimulus and backwards into the cage; (2) exploration: the mink approaches the stick and sniffs and/or bites at the stick; (3) aggressiveness: the mink makes a fast and intense attack at the stick and maintains the bite; (4) unknown: the mink remains motionless in the cage (freezing) or the behaviour cannot be described by one of the three forms of behaviour mentioned.
Unlike the parental generation tested every fortnight in the period from January to March 1988, the mink kits in the F,-F, generation were selected on the basis of tests performed once a month in each of the months from July to November. The tests were performed in the first week of each month and never after a weekend or following extraordinary activity in the sheds. The lines were tested within the same day with a break of 1 h before and after feeding, as the feeding situation affects the response of the animals.
2.2. Selection criteria
As mentioned, the parental generation was tested from January and until the start of the mating season in March. On the basis of the animals’ response to these tests, the population was divided into three lines (Lines E, F and A). Females which had in at
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142 S.W. Hansen /Applied Animul Behuuiour Science 49 (1996) 137-148
Individuals responding - Explorative -Aggressive - - - Fearful
% 100
1 Line: Exploratlve
I Line: Fearful
r 100
/I’
0 Y-Y-- I,,,, ;-;_,,h 1 2 3 4 5 6 1 2 3 4 5 6
Generations
Fig. I. Individuals in selection lines explomtive (E) and fearful (F) responding with explorative, aggressive and fearful behaviour in the F, -F6 generations.
a tendency towards a significantly higher increase in fearful behaviour in Line F over generations than in Line E (P = 0.07).
3.2. Development of behaviour within generations
Fig. 2 shows the level and development of explorative behaviour in the period from August to November per generation for the selected lines (E, F and A) and for the control line (C) for the last 2 years. Corresponding results of fearful behaviour are shown in Fig. 3.
The development of explorative behaviour (Fig. 2) in the three selection lines was identical in F, and F,. In F,, the development in Lines E and A was identical and different from the development in Line F (P < 0.051, where the explorative behaviour increased markedly from October to November. In F4, the development was parallel in Lines E and F but unusual, as the explorative behaviour decreased from September to
lndivlduals responding
% loo- ,
*O- fi
60 -
40 -
20 -
- Explorative . . . ..-Control
-Aggressive --- Fearful
2
f f
( ,’
r-
:
3
f” :
/$
: I’
5
/ : _..’
,_.:’ _.
6 - 100
- 80
60
/- 40
:‘..- :’ - 20
Months
Fig. 2. Explorative behaviour in mink kits in selection lines explorative (E), fearful (F) and aggressive (A), and in the control line CC), in the F,-F, generations in August (A), September (S), October (0) and November (N).
S.W. Hansen/Applied Animd Brhauiour Sciencr 49 (19961 137-148 143
Individuals responding
%
- Explorative .L Aggressive ----..Control --- Fearful
100
80
60
’ I2 I3 4
)’ -
__’
P
-
6 6 - 100 _-a. \
,‘j-- ., - 80
. . . - 60
b
ASON ASON ASON ASON Ti 8 I I / I I I t
ASON ASON
Months
Fig. 3. Fearful behaviour in mink kits in selection lines explorative (E), fearful (F) and aggressive (A), and in the control line (C), in the F, -F6 generations in August (A), September (S), October (0) and November (N).
October. In the following generations (Fs and F,), the explorative behaviour in Line F remained very low during the growth period, whereas the explorative behaviour increased in parallel in Lines E and C. The development in Line F was in both generations significantly different from the development in Line E (P < 0.01, P < 0.00 1) and Line C (P < 0.01, P < 0.05).
The development in fearful behaviour (Fig. 3) was inverse in relation to the explorative development. In F, in Line F, there was a slight increase in fearful behaviour from October to November compared with Line E (P < 0.05). In F2 and F,, fearful behaviour was reduced in all lines during the growth period. The development in fearful behaviour was also identical in Lines E and F in F4, as the fearful behaviour increased in both lines from September to October. In F, and F6, fearful behaviour was reduced in a parallel way in Lines E and C compared with a high and stable level of fear in Line F (Line E: P < 0.05, P < 0.05; Line C: P < 0.05, P < 0.05).
Aggressive behaviour did not occur until October-November and before that only to a very slight degree. In F, and F2, the increase in aggressive behaviour from September to Novembler was identical in the three lines, but in F, aggressive behaviour from September to November increased more in Line A than in Line F (P < 0.05). The increase in aggressive behaviour also tended to be higher in Line A than in Line E (P = 0.07). In the following generations F4, F, and Fb, when Line A had been abandoned, the frequency of aggressive behaviour in Lines F, E and C was identical and very low during the entire growth period.
The behaviour ‘unknown’ was, as mentioned above, most frequent in July. Hereafter the frequency decreased. A marked exception was recorded in July in F4, when 60% of the recordings were denominated ‘unknown’. The ‘unknown’ category was distributed evenly between sexes and lines.
3.3. Sexual d#erences in behauiour
The development of explorative and fearful behaviour in the growth period from August to Nlovember progressed in a parallel way for females and males, but within each
144 S.W. Hansen/Applied Animul Behaviour Science 49 (1996) 137-148
line males were significantly more explorative and less fearful than females (P < 0.05). This sexual difference was not seen in Line F in F, and F6, when the level of fearful behaviour was very high for both sexes. Likewise, no sexual difference in explorative and fearful behaviour in July could be found with any of the lines during the 6years (x2 test). The aggressive behaviour occurred in October and November, and in these months there was no sexual difference in aggressive behaviour.
3.4. Difference in behauiour between lines
Mink in Line E (Fig. 2) showed more explorative behaviour than mink in Line F (F, P < 0.05; F,, F, and F5 P < 0.01; P < 0.001). The level of fear was higher in Line F than in Line E (F,, F2, F4 and F, P < 0.001; F6 P < 0.01; F, P < 0.05). Mink in Line A were more explorative than mink in Line F (F2 P < 0.05; F, P < 0.05) and less explorative than mink in Line E (F2 P < 0.05; F, P < 0.001). Correspondingly, the level of fear (Fig. 3) was lower in Line A than in Line F (F, P < 0.001; F2 P < 0.01; F, P < 0.05) and higher than in Line E (F2 and F, P < 0.05). A x2 test showed that the difference between lines in F, was caused by a difference between females (P < 0.001) whereas no difference was found between males in Lines E, F and A.
Mink in the unselected control line (C> were more explorative than mink in Line F (Fs and F6 P < 0.001) but less explorative than mink in Line E (F5 P < 0.05; F6 P < 0.01). Correspondingly, the level of fear in Line C was lower than in Line F and higher than in Line E (F, P < 0.05; F6 P < 0.001).
In F,, the level of aggression from October to November was significantly higher in Line A than in Line F (P < 0.05). In the other years, no difference in aggression between lines could be found.
4. Discussion
By deliberate selection for behavioural traits, it is possible over only a few genera- tions to change the temperament of farm mink towards human beings. After 5-6 generations, 90% of the mink selected for fearful behaviour responded fearfully, and the response was stable from August to November.
Selection for explorative behaviour was in this investigation less effective than selection for fear. This may be due to the fact that during the relatively short period of domestication of farm mink, a considerable reduction in the level of fear has already been achieved, and a further reduction will therefore now be slow. The test applied (the stick test) may have favoured selection for fear because the fear releasing stimuli in the test situation were too weak, whereas a test situation releasing a higher degree of fear would have favoured selection towards explorative behaviour.
Shackelford (1984) described that farm mink in the 1940s were fearful animals which fled into the nest box when in contact with human beings but which were now most often responding with curiosity rather than with aggression and fear. A contributory reason for this reduced fear may be that nervous individuals with an undesired
S.W. Hun.wn/Applird Animd Brhauiour Scirnc~e 49 (1996) 137-148 I45
temperament have been omitted by selection, but also the deliberate selection for a large reproduction potential may indirectly have contributed to the changed behaviour and stress sensitivity in farm mink of today.
In the present investigation, the development in the temperament of farm mink in the growth period was characterized by an increase in explorative behaviour and a decrease in fearful behaviour. The reduction in the fear of mink towards humans must be due to habituation to the daily contact with humans in connection with normal farm routines such as feeding, cleaning, etc., where mink are exposed to human contact without direct physical contact and without aversive consequences to the animal.
Human contact in the feeding situation may be regarded as rewarding to the animal, but as feeding is done with a motorized feeding machine it is more likely that the animal associates the sound from the feeding machine and not human contact with the food reward.
The production environment, being poor in stimuli, may contribute to an increase of explorative behaviour at human contact in an opportunistic animal such as the farm mink. In connection with regular daily behavioural observations on mink farms it has been recorded that mink are considerably more curious after a weekend (measured as number of mink at the cage door) than before a weekend, when the mink have become used to the ordinary human activity on weekdays in the shed (Bildsoe et al., 1990).
Regular daily handling of mink kits from the age of 3 to 7 weeks intensified the reduction of fear in comparison with the level of fear in not-handled kits. However, not-handled kits reached a corresponding reduction in the level of fear approximately 1 month later. The visual contact with humans seems to be sufficient to reduce the mink’s fear of humans (Hansen et al., 1994). The level of explorative-fearful behaviour towards humans is thus conditioned by the intensity and degree of human contact as well as by the genetic conditions influencing the threshold value for the release of fearful behaviour at human contact.
In general, male mink have a higher threshold value for the release of fearful behaviour than females. This sexual difference has previously been shown in mink (Mailer and Hansen, 1988) and was seen as an effect of dominance relationships between sexes where the male dominates the female. Later investigations have, however, shown that the higher level of fear in females did not depend on whether they were housed with a male or singly (Hansen et al., 1994).
A higher tendency towards fearful behaviour in females than in males is in accor- dance with the results showing a higher stress sensitivity in females than in males (Hansen and Damgaard, 1991; Hansen et al., 1992). Sexual differences in stress physiological parameters, found under normal production conditions, may, however, be effaced if mink are kept under stressing physical conditions such as nest box deprivation (Hansen and Brandt, 1989). Correspondingly, in the present investigation we found no sexual differences in temperament during the entire growth period in Fs and F6 in Line F selected for a low threshold value for fearful behaviour.
Through selection over six generations, mink in Line F have become considerably more fearful, but also in Line E the level of fear has increased. As regards fearful behaviour, the control Line C and Line A showed an intermediate level between Lines F and E.
146 S.W. Hunsen /Applied Animd Bchuviour Science 49 (1996) 137-148
The general increase in fearful behaviour might be due to genetic drift, but a simultaneous increase in fear in three independent lines as well as the parallel changes in temperament across lines makes it more likely that conditions in the environment as well as the subjective evaluation of behaviour have influenced the temperament levels. Variation in human contact between generations as well as changes in farm routines including feeding intensity may be contributory causes, but these are difficult to quantify.
Another factor which may have influenced the difference in level between years is the variation in the quantitative selection criteria for breeding animals. The average percentage of the selected behaviour in F,, F2 and F, was in the selected breeding females in Line A: 13.8%, 6.9% and 30.0%. In generations F, to F,, the percentage in Line F was 47.8%, 86.8%, 78.5%, 78.9% and 82.4%. and in Line E was 69.2%, 77.7%, 87.4%, 83.9% and 66.0%. This variation in the respective behavioural frequencies of the breeding stock was accepted to obtain a sufficiently large breeding stock and thus reduce the risk of inbreeding.
The development in explorative and fearful behaviour in the first three generations was similar in the selection lines. Selection based on the behaviour of the P generation in the months January-March may have been different from the selection based on the behaviour of the F generations from July to November. The P generation was tested when the animals were fully grown and ended just before the mating season. Further- more, the feeding routine was restrictive in that period as opposed to almost ad libitum feeding during the growth period. These conditions may have contributed to the fact that the behaviour of the males in the three selection lines did not differ in F,.
In F4, the development was unusual in relation to previous as well as following generations, as the explorative behaviour did not increase during the growth period. On the contrary, Line F showed a marked decrease in explorative behaviour and a corresponding increase in fearful behaviour from August-September to October. This development may be ascribed to changes in test personnel. The behavioural tests were performed by three different persons-No. 1 in F, and F2, No. 2 in Fs and F,-F,, and No. 3 in F,--which may have affected the result. Seen in relation to the behaviour in Lines E and F in the following years, the changed development and level must be a result of the effectiveness of the selection.
An effect of the selection is seen in the fact that the level of explorative and fearful behaviour in the control line is placed between and is significantly different from the levels in Lines E and F. The control line was, as mentioned above, established on the basis of cross mating between males and females in Lines E, F and A from F3.
Through selection for exploration or fear, aggressive behaviour was reduced. Through selection for aggressive behaviour, the test period from July to September has proved to be inappropriate, as, with the test used, aggression does not occur until October- November. It is assumed, however, that it may also be possible through selection to increase or decrease the occurrence of aggressive behaviour.
The changes found in the temperament of farm mink are best explained by changes in the threshold value towards fear releasing stimuli. These changes must be a result of changes in the genotype. When examining racial differences in temperament between Cocker Spaniel and Basenji, Scott and Fuller (1965) found that fear and struggle against
S. W. Hunsen/Applied Animul Behauinur Scirncr 49 (19961 137-148 147
restraint could best be explained by differences in individual genes, but a maternal effect was, however, probably also involved.
In the present investigation it has been possible with the use of a simple test (the stick test) to change the temperament of farm mink. The effectivity of the selection is determined by the strength of the stimulus in the test used in relation to the temperamen- tal level of the population.
From a welfare point of view, selection against fear and for a better adaptation to captivity ought to be included in the breeding programmes for farm mink. Whether the behavioural selection also influences production properties positively has not yet been sufficiently documented. In fox production, a similar behavioural selection could be included positively together with the use of existing knowledge (Bakken et al., 1994) about the effects of the environment in the growth period on the later temperament of farmed foxes.
Acknowledgements
The author thanks the technical assistants Birthe Houbak and Anne Bak for their very conscientious test of a large stock of experimental animals, and thanks colleagues for profitable discussions of experimental results.
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