VISION
AND OTHER SPECIAL SENSES
Pigs have
colour vision and a panoramic range of about
310°, and binocular vision of 35–50°. It is thought they
have no accommodation (i.e., they cannot focus). Pigs
are inquisitive and this must be remembered when moving
them. If they are not hurried and can explore as they
go along, they can be driven without effort. The extent
to which pigs have colour vision is still a source of some
debate. However, the presence of rods and cones with
two distinct wavelength sensitivities in the blue and
green frequencies (Lomas et al., 1998) suggests that at
least some colour vision is present.
Pigs have
a well-developed sense of smell and use is
made of this in Europe where they locate underground
truffles. Hearing is also well developed and
localisation of sounds is made by moving the head.
Olfactory rather than visual stimuli are used in the
identification of conspecifics (Houpt,
1998).
Pigs
learn quickly to manipulate food and water
devices, to turn fans on and off (Ingram and Legge,
1970) and to turn on a source of radiant heat (Ingram et
al., 1975).
Auditory
stimuli are used extensively by pigs as a
means of communication in all social activities (Gonyou,
2001). Alarm or aversive stimuli are transmitted to
conspecifics not only by auditory cues but
also via pheromones (Vieuille-Thomas and
Signoret, 1992).
SOCIAL
ORGANISATION, DOMINANCE HIERARCHIES AND
LEADERSHIP
The basis
of the social structure in feral pigs is the
matriarchal herd of several females and their offspring
(Kurz and Marchington, 1972). Males are not permanently
associated with such herds, and are often solitary
or in bachelor groups.
In the
domestic situation pigs may be kept together
with littermates throughout their lives or be grouped with
strange pigs of similar age and size. There are two
types of social organisation in the domestic pig
(Signoret et al.,1975):
a. Teat
order;
b.
Dominance hierarchy.
Teat
order. Within the first few hours to two weeks after
farrowing, the piglets become capable of recognising
their positions on the udder and preferentially attach
themselves to anterior rather than posterior teats. This
was shown in the early work of Donald (1973a, 1973b)
and confirmed by many later workers (Hemsworth et
al.,1976). It has been shown that stimulation of the anterior
teats appeared to be important in causing milk letdown
(Fraser, 1973) so it might be to the advantage of
the entire litter to have these teats occupied by healthy
piglets. The teat order may function as a type of
territorial spacing system which, in turn, means a stable
family life with minimum competition.
Most
fights that occur in young piglets are around
the udder (Hartstock and Graves, 1976) and more fights
were won at the permanent nursing site than away from
it. They called this the ‘home-court advantage’ and
found that fighting declined as the nursing order was
established.
Using an
artificial sow to rear groups of piglets,
Jeppesen (1982) suggested that recognition of a teat in
a particular area of the udder depended on visual orientation
by means of reference points on the udder to find
the area, and then the olfactory sense is used for the
search within that area.
Dominance
hierarchy. This is the social organisation
established in groups of weaned pigs. When a number
of unacquainted pigs are mixed together for the first
time, they fight to establish a dominance hierarchy, usually
of a simple linear type. The fighting behaviour is
generally mouth-to-neck attacks with strong thrusts
sideways and upwards (McBride et al.,1964).
The
establishment of the hierarchy occurs within 24
hours of mixing but the level of aggression drops
dramatically after about one hour (Symoens
and Van Den Brande, 1969). The dominance
hierarchy is important as the social rank
appears to influence productivity. It has
been shown by some workers (Bielharz and Cox,
1967; McBride et al.,1964) that social rank influences
growth, while others (Meese and Ewbank, 1973a) found
that weight was not correlated with dominance or sex.
The dominance hierarchy is important as a group stabiliser,
but under adverse, intensive conditions, animals
low on the hierarchy may be disadvantaged by lack of
food and water.
The
top-ranking pig can be removed from the group
for up to 25 days and on return will still retain its
position, but only if the social group it
left was stable (Otten et al., 1997). A
pig at the bottom of the hierarchy is treated
as a stranger and attacked when returned after three
days (Ewbank and Meese, 1971). Pigs probably recognise
each other by sight and smell.
It is an
advantage in a husbandry system to have a
stable social organisation so that the pigs can settle
down and grow and it may be that a system of ‘birth-to-slaughter-weight-in-one-pen’,
in which pigs remain members of the same
unchanged social group from birth to
slaughter, is a way to reduce disturbances
(Ewbank, 1978; Sainsbury D. and Sainsbury, P., 1979).
Huddling
behaviour. Pigs are very susceptible to hot
conditions and the rate of sweating is very low, so there
is inadequate thermoregulatory compensation by respiratory
evaporative loss. In the feral state, pigs seek
shade and wallow in mud or water and become more
active at night (Mount, 1979). Young pigs are sensitive
to cold and a behavioural feature retained through a
pig’s life is the stimulus to huddle with littermates
(Mount, 1979). Nest-building activity in the natural state
provides shelter from environmental extremes.
Leadership. The work of Meese and Ewbank (1973b)
showed no clear relationship between leadership,
exploratory behaviour or social rank in groups of pigs in
an outdoor area.
Dominant
pre-weaned piglets have higher growth
rates than subordinates due to suckling the more
anterior teats (which secrete the most
milk and have the lowest incidence of
mastitis), allowing their dominance to be
maintained through to post-weaning (Dyck et al.,
1987).
Agonistic
behaviour among piglets will decrease if
they are regrouped during lactation rather than postweaning
(Olsson and Samuelsson, 1993).
Instability of the dominance hierarchy increases
with stocking density, thus increasing stress and
aggression. This appears to depress the immune system
and thus heighten the herd’s susceptibility to disease
(Turner et al., 2000).
Grouping
pigs by weight heterogeneously rather than
homogeneously, as is the current practice, may
increase growth rate and reduce hierarchical conflict by
allowing clear weight differentiation among littermates
(Francis et al., 1996).
Odour
masking, by methods such as creating a
familiar odour on all pigs, masking the odour of unfamiliar
pigs, and the use of pheromones and/or artificial
compounds, has had little, if any, effect in limiting
aggression and increasing hierarchical stability
(Gonyou, 1997).
Isolation
from a social group is very stressful for pigs
and may result in stereotypies or attempts to escape
(Gonyou, 2001).
SEXUAL
BEHAVIOUR
Courtship
behaviour lasts only a short time when a boar
is placed in a small pen with an oestrous female. The
sow plays the critical role of meeting sexual partners as
boars show equal choice between an oestrous and an
anoestrous sow. The male sniffs the female, noses
sides, flanks and vulva, and emits a ‘mating song' of soft
guttural grunts (6–8 seconds). He foams at the mouth
and moves his jaw from side to side as the female
poses and bites the male’s ears gently. When the sow
becomes stationary the boar mounts. Androstenone
within boar saliva aids in eliciting the standing response
in the sow (Gonyou, 2001). Some sows are more attractive
to boars than others and occasionally a sow may
avoid and refuse to stand for a specific boar. Rearing
females in isolation from males delays the standing
response of the females once they are introduced to
boars (Soede and Schouten, 1991).
Pheromones in boar saliva and preputial secretions
induce oestrus in gilts and sows (this is known as the
boar effect) (Pearce et al., 1988).
The
presence of stimuli from boars (namely odour)
will induce earlier puberty in gilts than if no other stimuli
were present (Hemsworth et al., 1988).
Because
an oestrous sow will stand near the boar (Bressers
et al., 1991), penning breeding females adjacent
to a boar makes identification of oestrous sows
easy.
The
social environment that boars have been raised
in influences their levels of sexual activity (Hemsworth
et al.,1977). Boars that are raised individually with no
visual contact with immature females, but who can hear
and smell the females, have reduced copulation frequency
and shorter average duration of ejaculation
compared to those raised in all-male or male-female
groups. Boars that engage in more courting activity,
especially nosing of the sow’s flanks before mating,
have higher conception rates (Hemsworth et al.,1978).
This study suggested that extra flank-nosing might stimulate
oxytocin release from the sow’s pituitary gland and
this could increase sperm transport and the number of
sperm in the oviduct and so increase the chances of
fertilisation.
Dominant
boars cover the markings of subordinate
animals with urine that is often contaminated with
preputial secretions (Mayer and Brisbin, 1986).
MATERNAL–OFFSPRING BEHAVIOUR
In a
paddock the sow will nest-build for up to six hours
before parturition. She hollows out a depression and
lines it with straw, grass, sticks, or other available
material. While farrowing crates in
intensive piggeries prevent much of this
nest-building behaviour, many elements are
still present and the sow will often perform
similar activities to those of pigs provided with nesting
material (Blackshaw J. and Blackshaw, A.,1982).
Pigs of
lower social status tend to produce litters
with piglets that are lighter in weight (Mendl et al.,1992).
Dominant
sows give birth to more male piglets than
do subordinate ones (Mendl et al., 1995).
Compared
to other mammals, pigs display complex
nursing and suckling behaviour (Fraser, 1980; Signoret
et al.,1975). Nursing is frequent, every 50–60 minutes,
and the sow requires stimulation from piglets before
milk let-down. Sensory inputs (vocalisation, odours from
mammary and birth fluids and hair patterns of the sow)
are particularly important immediately post-birth to
facilitate teat location by the piglets
(Rohde Parfet andGonyou, 1991).
Initially, the piglets jostle for position at the udder,
then each piglet massages around its respective teat
with its snout, during which time the sow grunts at slow,
regular intervals. Each series of grunts varies in frequency,
tone and magnitude, indicating the stages of
nursing to the piglets (Algers, 1993). The phase of
competition for teats and of nosing the
udder, lasts for about one minute, and
ends when milk flow begins. In the third
phase, the piglets hold the teats in their mouths and
suck with slow mouth movements (one per second),
and the rate of the sow’s grunting increases and lasts
about 20 seconds. The grunt peak in the third phase of
suckling does not coincide with milk ejection but rather
the release of oxytocin from the pituitary into the
bloodstream (Castren et al., 1989). Phase
four coincides with the period of main
milk flow (10–20 seconds’ duration) when
the piglets suddenly draw back slightly from the
udder and start sucking with rapid mouth movements of
about three per second. The sow grunts rapidly, lower in
tone and often in quick runs of three or four, during this
phase. Finally, the flow stops and so does the grunting
of the sow and the piglets may dart from teat to teat and
recommence suckling with slow movements, or nosing
the udder.
Piglets
massage and suckle the sow’s teats after
milk flow ceases as a way of letting the sow know their
nutritional status. This helps her to regulate the amount
of milk released from that teat in future sucklings. The
more intense the post-feed massaging of a teat, the
greater the future milk release from that teat will be
(Jensen, Gustafsson and Augustsson, 1998).
It is
often hard to tell if the nursing episode is initiated
by the sow or the piglets and almost any disturbance
causes the piglets to rise, squeal and then nurse. The
sound of one litter nursing may initiate nursing among
other litters. Fostering piglets from one litter to another
is often carried out in the pig industry
and it is recognised that sows may react
aggressively to foster piglets and that
suckling periods are disrupted. Horrell and Bennett
(1981) exchanged three piglets between five pairs of litters
at seven days of age after the teat order had been
established. Compared with control litters, crossfostering
disrupted the teat order relationships of the
whole litter. Weight gain of fostered piglets during the
second week was reduced to 79% of that in their nonfostered
littermates. If fostering has to be done, it has
more chance of success if the piglets are only one to
three or four days old. Cross-fostering of piglets should
be undertaken before teat order is established and
involve movement of larger piglets rather than small
ones to minimise teat order disruption and associated
mortality and production losses (Gonyou, 2001).
In the
natural environment, farrowing nests are built
at least 100 m from the communal nest to improve piglet
survival (Jensen, 1989).
PGF
2a
appears to be the hormonal regulator of
nestbuilding behaviours such as nosing,
rooting and pawing to create a depression,
as well as the gathering of straw to line
the nest (Burne, Murfitt and Johnston, 2001).
By the time piglets are 6 days old, they have begun
to follow their mother (Stangel and Jensen, 1991).
Recognition between the sow and her piglets is by
olfactory and vocal cues (Jensen and Redbo, 1987).
Without
human interference, weaning is finished by
the time the piglets are about 17 weeks old, but it may
begin as early as 4 weeks of age when the mother
begins to reduce her nursing efforts (Jensen and
Recen, 1989).
ABNORMAL
BEHAVIOUR
Abnormal
behaviours often found in pig units include:
1. Tail
and ear biting (Blackshaw, 1981);
2.
Cannibalism;
3.
Reproductive behaviour problems such as abnormal
mating behaviour and abnormal maternal behaviour;
4.
Eating too much or too little; dominance relationships
that prevent some animals from having access to food
and water;
5.
Abnormal dunging habits;
6.
Persistent inguinal nose thrusting (PINT) (Blackshaw,
1981). PINT is defined as occurring when a pig repeatedly
thrusts its nose into the inguinal area of a resting
pig with the top of its snout, until the recipient pig
moves. It is a behaviour pattern of high-ranking pigs,
although other pigs do it;
7.
Various stereotypies, which are repeated actions with
no goal direction, have been described (Fraser, 1975) in
tethered sows when not provided with straw;
8.
Snout rubbing, when pigs rub their snouts on the
flanks of other pigs causing necrosed areas (Allison,
1976).
These
behavioural problems are easy to see and
tell us something about the mental well-being of the animals.
However, we do not know if animals experience
emotional feelings in the same way as humans.
Perhaps, the important thing to recognise is that these
abnormal behaviours tell us that all is not well in the
husbandry system.
A
deficiency in iron may contribute to the incidence
of tail biting (Fraser, 1987), but other factors have since
been ranked as being of greater importance.
The
incidence of cannibalism such as tail and ear
biting has exceeded 10% of pigs in some studies (Arey,
1991).
Stereotypies may increase in prevalence through a
piggery via social facilitation (Appleby, Lawrence and
Illius, 1989).
Some
stereotypies, such as ear sucking and biting,
may be associated with low levels of fibre in the diet
(Meunier-Salaün, Edwards and Robert, 2001).
Belly
nosing/snout rubbing by piglets does not
appear to be related to stress, diet quality or the presence
of milk in the diet but is somehow related to the
age at weaning (Gardner, Duncan and Widowski, 2001).
Abnormal
behaviours, such as tail and ear biting,
stereotypies and belly nosing, may be redirected rooting
or nosing behaviour caused by barren commercial environments
(Beattie, Walker and Sneddon, 1996).
REFERENCES
Algers,
B. (1993) Nursing in pigs: communicating
needs and distributing resources. J. Anim. Sci. 71,
2836-2831.
Allison
C.J. 1976. Snout-rubbing as a vice in weaned
pigs. Vet. Rec. 98:254-255.
Appleby,
M.C., Lawrence, A.B. and Illius, A.W. (1989)
Influence of neighbours on stereotypic behaviour of
tethered sows. Appl. Anim. Behav. Sci. 24, 137-146.
Arey,
D.S. (1991) Tail biting in pigs. Farm building
progress 105, 20-23.
Beattie,
V.E., Walker, N. and Sneddon, I.A. (1996) An
investigation of environmental enrichment and space
allowance on the behaviour and production of growing
pigs. Appl. Anim. Behav. Sci. 48, 151-158.
Beilharz
, R.G. and Cox, D.F. 1967. Social dominance
in swine. Anim. Behav. 15:117-122.
Blackshaw,
J.K. 1981. Some behavioural deviations in
weaned domestic pigs: Persistent inguinal nose thrusting,
and tail and ear biting. Anim. Prod. 33:315-332.
Blackshaw
, J.K. and Blackshaw, A.W. 1982. The
effects of prostaglandin (PGF 2a) on the behaviour of
the domestic non-pregnant sow and boar. Proc. Aust.
Soc. Anim. Prod. 14:550-552.
Bressers,
J.P.M., Te Brake, J.H.A. and Noordhuizen,
J.P.T.M. (1991) Oestrus detection in group housed
sows by analysis of data on visits to the boar. Appl.
Anim. Behav. Sci. 31, 183-193.
Burne,
T.H.J., Murfitt, P.J.E., and Johnston, A.N.B.
(2001) PGF2α induced nest building and choice
behaviour in female domestic pigs. Appl. Anim. Behav.
Sci. 73 (4), 267-269.
Castren,
H., Algers, B., Jensen, P. and Saloniemi, H.
(1989) Suckling behaviour and milk consumption in
newborn piglets as a response to sow grunting. Appl.
Anim. Behav. Sci. 24, 227-238.
Donald,
H.P. 1937a. The milk consumption and growth
of suckling pigs. Empire J. Experimental Agriculture
5:349-360.
Donald ,
H.P. 1937b. Suckling and suckling preference
in pigs. Empire J. Experimental Agriculture 5:361-368.
Dyck, G.W.,
Sweirstra, E.E., McKay, R.M., and Mount,
K. (1987) Effect of location of the teat suckled, breed
and parity on piglet growth. Can. J. Anim. Sci. 67, 929-
939.
Ewbank,
R. 1978. Swine. Chapt. 6. In: The Care and
Management of Farm Animals. Ed. W.N. Scott.
London. Bailličre Tindall.
Ewbank ,
R. and Meese, G.B. 1971. Aggressive
behaviour in groups of domestic pigs on removal and
return of individuals. Anim. Prod. 13:685-693.
Francis,
D.A., Christison, G.I. and Cymbaluk, N.F.
(1996) Uniform or heterogenous weight groups as factors
in mixing weanling pigs. Can. J. Anim. Sci. 76,
171-176.
Fraser,
D. 1973. The nursing and suckling behaviour in
pigs. I. The importance of stimulation of the anterior
teats. Br. vet. J. 129:324-336.
Fraser,
D. 1975. The effect of straw on the behaviour
of sows in tether stalls. Anim. Prod. 21:59-68.
Fraser,
D. 1980. A review of the behavioural mechanisms
of milk ejection I of the domestic pig. Appl.
Anim. Ethol. 6:247-256.
Fraser,
D. 1987 Mineral-Deficient diets and the pigs
attraction to blood: Implications for tail biting. Can. J.
Anim. Sci. 67, 909-918
Gardner,
J.M., Duncan, I.J.H. and Widowski, T.M.
(2001) Effects of social ‘stressors’ on belly-nosing
behaviour in early-weaned piglets: is belly-nosing an
indicator of stress? Appl. Anim. Behav. Sci. 74 (2),
135-152
Gonyou,
H.W. (1997) Can odours be used to reduce
aggression in pigs? 1997 Annual Research Report,
Prairie Swine Centre, Saskatoon. Cited by Gonyou,
H.W. (2001) The social behaviour of pigs, in Social
Behaviour in Farm Animals, ed. Keeling, L.J. and
Gonyou, H.W. CABI, Oxon.
Gonyou,
H.W. (2001) The social behaviour of pigs, in
Social Behaviour in Farm Animals, ed. Keeling, L.J.
and Gonyou, H.W. CABI, Oxon.
Hartsock,
T.G. and Graves, H.B. 1976. Piglet fighting
behaviour, nursing order and growth. J. Anim. Sci.
43:209.
Hemsworth
, P.H., Beilharz, R.G. and Galloway, D.B.
1977. Influence of social conditions during rearing on
the sexual behaviour of the domestic boar. Anim. Prod.
24: 245-251.
Hemsworth
, P.H., Beilharz, R.G. and Brown, W.J.
1978. The importance of the courting behaviour of the
boar on the success of natural and artificial matings.
Appl. Anim. Ethol. 4:341-347.
Hemsworth,
P.H., Hansen, C., Winfield, C.G. and
Barnett, J.L. (1988) Effects on puberty attainment in
gilts of continuous or limited exposure to boars.
Australian Journal of Experimental Agriculture 28, 469-
472.
Hemsworth,
P.H., Winfield, C.G. and Mullaney, P.D.
1976. A study of the development of the teat order in
piglets. Appl. Anim. Ethol. 2:225-233. Horrell, I. and
Bennett, J. 1981. Disruption of teat preferences and
retardation of growth following cross-fostering of one
week old pigs. Anim. Prod. 33:99-106.
Houpt,
K.A. (1998) Domestic Animal Behavior for
Veterinarians and Animal Scientists 3rd Ed Iowa State
University Press, Ames.
Ingram ,
D.L. and Legge, K.F. 1970. The thermoregulatory
behaviour of young pigs in a natural environment.
Physiol. Behav. 5(9):981-987.
Ingram,
D.L., Walters, D.E. and Legge, K.F. 1975.
Variations in behavioural thermoregulation in the young
pig over 24 hour periods. Physiol. Behav. 14(6):689-
695.
Jensen,
P. (1989) Nest site choice and nest building of
free ranging domestic pigs due to farrow. Appl. Anim.
Behav. Sci. 22, 13-21.
Jensen,
P. and Recen, B. (1989) When to wean -
observations from free ranging domestic pigs. Appl.
Anim. Behav. Sci. 23, 49-60.
Jensen,
P. and Redbo, I. (1987) Behaviour during nest
leaving in free ranging domestic pigs. Appl. Anim.
Behav. Sci. 18, 355-362.
Jensen,
P., Gustafsson, G. and Augustsson, H. (1998)
Massaging after milk ejection in domestic pigs - an
example of honest begging? Anim. Behav. 55, 779-
786.
Jeppesen,
L.E. 1982. Teat-order in groups of piglets
reared on an artificial sow. II. Maintenance of teatorder
with some evidence for the use of odour cues.
Appl. Anim. Ethol. 8:347-355.
Kurz,
J.C. and Marchinton, R.L. 1972. Radiotelemetry
studies of feral hogs in South Carolina. J. Wildl. Mgmt.
32:1240-1248.
Lomas,
C.A., Piggins, D. and Phillips, C.J.C. (1998)
Visual awareness. Appl. Anim. Behav. Sci. 57, 247-257
Mayer,
J.J. and Brisbin, I.L. Jr. (1986) A note on the
scent marking behaviour of two captive reared feral
boars. Appl. Anim. Behav. Sci. 16, 85-90.
McBride,
G. 1963. The ‘teat order’ and communication
in young pigs. Anim. Behav. 11:53-56.
McBride,
G., James, J.W. and Hodgens, N. 1964.
Social behaviour of domestic animals. IV. Growing
pigs. Anim. Prod. 6:129-139.
Meese G.B.
and Ewbank, R. 1973a. The establishment
and nature of the dominance hierarchy in the
domestic pig. Anim. Behav. 21:326-334.
Meese G.B.
and Ewbank, R. 1973b. Exploratory
behaviour and leadership in the domesticated pig. Br.
vet. J. 129:251-259.
Mendl,
M., Zanella, A.J. and Broom, D.M. (1992)
Physiological and reproductive correlates of behavioural
strategies in female domestic pigs. Anim. Behav. 44,
1107-1121.
Mendl,
M., Zanella, A.J., Broom, D.M and Whittemore,
C.T. (1995) Maternal social status and birth sex ratio in
domestic pigs: An analysis of mechanisms. Anim.
Behav. 50, 1361-1370.
Meunier-Salaün, M. C., Edwards, S. A. and Robert, S.
(2001) Effect of dietary fibre on the behaviour and
health of the restricted fed sow. Anim. Feed Sci.
Technol. 90, 53-69
Mount,
L.E. 1979. Adaptation to The Thermal
Environment: Man and His Productive Animals. Chapt.
8. p.183-208. Edward Arnold.
Olsson,
A. and Samuelsson, O.V. (1993) Grouping
studies of lactating and newly weaned sows, in
Livestock Environment, Vol. IV, ed. Collins, E. and
Boon, C. American society of Agricultural Engineers,
USA. Cited by Gonyou, H.W. (2001) The social behaviour
of pigs, in Social Behaviour in Farm Animals, ed.
Keeling, L.J. and Gonyou, H.W. CABI, Oxon.
Otten,
W., Puppe, B., Stabenow, B., Kanitz, E., Schon,
P.C., Brussow, K.P. and Nurnberg, G. (1997) Agonistic
interactions and physiological reactions of top and bottom
ranking pigs confronted with a familiar and an
unfamiliar group: preliminary results. Appl. Anim.
Behav. Sci. 55, 79-90.
Pearce,
G.P., Hughes, P.E. and Booth, W.D. (1988)
The involvement of boar submaxillary salivary gland
secretions in boar induced precocious puberty attainment
in the gilt. An. Repro. Sci. 16, 125-134.
Rohde
Parfet, K.A. and Gonyou, H.W. (1991)
Attraction of newborn piglets to auditory, visual, olfactory
and tactile stimuli. J. Anim. Sci. 69, 125-133.
Sainsbury, D. and Sainsbury, P. 1979. Livestock and
Housing. Great Britain: Bailličre Tindall.
Signoret,
J.P., Baldwin, B.A., Fraser, D. and Hafez,
E.S.E. 1975. The Behaviour of Swine. In: The
Behaviour of Domestic Animals (3rd edition). Ed.
E.S.E. Hafez. p.295-329. Baillibre Tindall, London.
Soede,
N.N. and Schouten, W.G.P. (1991) Effect of
social conditions during rearing on mating behaviour of
gilts. Appl. Anim. Behav. Sci. 30, 373-379.
Stangel,
G. and Jensen, P. (1991) Behaviour of semi
naturally kept sows and piglets (except suckling) during
10 days post partum. Appl. Anim. Behav. Sci. 31,
211-227.
Symoens,
J. and Van Den Brande, M. 1969. Prevention
and cure of aggressiveness in pigs using
the sedative azaperone. Vet. Rec. 85:64-67.
Turner,
S.P., Ewen, M., Rooke, J.A. and Edwards, S.A.
(2000) The effect of space allowance on performance,
aggression and immune competence of growing pigs
housed on straw deep-litter at different group sizes.
Livest. Prod. Sci. 66, pp. 47Ż55.
Vieuille-Thomas,
C. and Signoret, J.P. (1992) Pheromonal
transmission of an aversive experience in
domestic pigs. J. Chem. Ecol. 18, 1551-1557.
