رفتار جنسی در خوک ماده

This paper reviews the influence of social mechanisms on oestrus and sexual motivation in pigs. The social relations between the animals and the signals they send out can inhibit as well as encourage their social motivation. Social stimuli from both boars and other sows in oestrus tend to induce and synchronise oestrus and ovulation amongst sows. The courting behaviour of boars is also facilitated by social stimuli from other boars. However, when sows are kept under conditions where the social pressure is high, e.g. due to limited space and/or resources, the social stress experienced by particularly the subordinate individuals may inhibit sexual motivation during oestrus. To a large extent this effect seems to be mediated via specific fear reactions towards dominant individuals of sows that have lost many aggressive encounters. For example, fear reduces the sexual motivation during mating and during sexual interactions amongst sows within a group, and fear may thus inhibit their chances of reproductive success. Similarly, fear of humans caused by innate fearfulness or negative handling procedures reduces sexual motivation in the presence of the human handler even when sows are in standing oestrus.

Free-living domestic pigs and wild boars form matriarchal family groups consisting of 1–4 sows with piglets depending on the supply of resources. Normally a well-established linear hierarchy dominated by the oldest sow is seen (Gundlach, 1968 and Kaminski et al., 2005). When the males reach sexual maturity, they leave the group to form small groups of boars and later on they become solitary animals. Only in the mating season, which in wild boars is during the early winter months (Mauget, 1982), will the boars be in contact with the females (Gundlach, 1968). In contrast, domestic pigs have no marked seasonal variation in the occurrence of oestrus and, thus, often have more than one litter a year. When sows approach oestrus, a marked increase in social activities is seen both when the sows are kept under semi-natural conditions (Babu et al., 2004) and when housed in female groups under production conditions (Nath et al., 1983, Pedersen et al., 1993, Pedersen, 1998 and Signoret, 1971). These include snout contacts between sows, ano-genital sniffing, flank nosing and female–female mounting. In the same period, the sows will approach the boar more and more often, and during oestrus they will spend much time in standing posture outside the boar pen (Babu et al., 2004, Nath et al., 1983, Pedersen, 1998 and Signoret, 1971). Pedersen (1998) made 24 h sampling of the occurrence of all the social activity during the days around pro-oestrus and oestrus in three groups of sows, each of which consisted of 9 sows housed in large straw bedded pens (9 m × 6 m) and with access to one feeding stall per sow (voluntary cubicle). A gradual increase in mounting, nosing and standing close to the boar pen was seen from 2–3 days before standing oestrus (Fig. 1). For several days, some sows were observed to mount other sows more than 40 times per 24-h period. The sow performing the mounting was always in pro-oestrus or in oestrus. The activity was observed throughout the 24 h, thus the normal resting period at night was completely broken (Fig. 2). The function of these behavioural elements is presumably to attract the attention of the boar and to stimulate sexual activity in the male, as demonstrated in goats (Billings and Katz, 1999 and Shearer and Katz, 2006). This part of the sexual behaviour is termed the appetitive part (also called the proceptive behaviour), and it differs from the consumatory part of the sexual behaviour that is characterised by triggering standing posture and mounting (also called the receptive behaviour) (Beach, 1976). Full-size image (14 K) Fig. 1. The frequency per sow per day of mounting attempts, flank nosing towards other sows as well as the frequency of standing close to boar pen in 3 groups of 9 sows housed in straw bedded pens measuring 9 m × 6 m (modified after Pedersen, 1998). Figure options Full-size image (17 K) Fig. 2. The combined frequency of flank nosing and mounts per hour during day 5 after weaning (the day when most sows were in standing oestrus) in a group of 9 sows housed in a 9 m × 6 m straw bedded pen (modified after Pedersen, 1998). Figure options When the boar is attracted to the group of females, he starts to court the sows. During the initial stage of courting, boars do not distinguish between sows in oestrus and sows not in oestrus, and consequently they court all the sows equally (Signoret, 1971). Thus, the sows' proceptive behaviour and their reaction to the boar's courtship are decisive for whether the boar will continue to mount and copulate or not (Signoret, 1971). The most important signal for mounting seems to be the sow's standing posture (Signoret, 1971). The expression of sexual behaviour in pigs is controlled mainly by oestrogen secreted from the growing follicles (Ford, 1982 and Ford, 1985). When the piglets are weaned from the sow the secretion of lactational hormones decreases, a sequence of neuroendocrine signals is initiated that simultaneously controls the onset of behavioural oestrus and ovulation. Ovulation takes place between 20–50 h after the onset of standing oestrus—i.e. approximately 2/3 into oestrus (Mburu et al., 1996, Nissen et al., 1997 and Soede et al., 1992). A schematic drawing of the timing of hormonal and behavioural events around oestrus is shown in Fig. 3. To obtain good conception, it is important that the sperm cells are present in the female reproductive tract at ovulation because the fertility of the egg is quickly reduced (after approximately 4 h) (Hunter, 1982). Sperm cells remain fertile slightly longer (approximately 24 h) (Rodriquez-Martinez, 2001). If mating takes place in early oestrus, the sperm cells may be inactive by the time of ovulation, especially if the oestrus period is long, and ovulation is consequently delayed. Thus, the timing between behaviour and ovulation is critical for successful reproduction.

Sexual behaviour in female pigs is characterised by both proceptive and receptive behaviour. The proceptive behaviour is shown as a general increase in boar interest and in female–female contacts such as flank nosing and mounting. The sexual behaviour amongst sows in groups is synchronised by both male and female stimuli. Boar pheromones as well as visual and to lesser extent acoustic stimuli from the boar are involved in oestrus stimulation and induction of the standing posture. Interactions with other females in oestrus also play a role in synchronisation of oestrus within groups. However, social subordination and fear of humans have been shown to inhibit both receptive behaviour during mating and female–female mounting activity. The influence of social stress and fear of humans seems to be mediated largely via specific fear reactions towards humans and larger dominant individuals rather than via a general inhibition of sexual motivation through inhibition of the hypothalamus–pituitary–gonadal axis.