Savaging is a term used in the study of ethology that refers to aggressive behaviour displayed by the mother towards the offspring. Aggressive behaviour includes being rough with, injuring, biting, attacking, crushing and killing (maternal infanticide) of the offspring. While savaging behaviour has been seen in multiple species, it is predominantly demonstrated in domestic pigs (Sus scrofa domesticus). As the definition of savaging is so broad, research on the prevalence of savaging behaviour varies with reports of little savaging of offspring to savaging of offspring up to the 20th percentile. Prevalence of aggressive, non-fatal savaging is greater in gilts, or females who have not yet previously farrowed, as piglet-focused aggression is more frequent in young animals than sows, adult females who have previously given birth. Occurrence of savaging demonstrated by sows is greater if the sow has previously savaged her offspring either as a gilt or sow. Savaging behaviour usually occurs during the first two days after parturition. Prevalence of savaging is similar among first and second farrowing cycles. Savaging behaviour has a significant impact on both agricultural economy and animal welfare which is why it is currently a subject of interest in the pig industry.
Environmental factors There exist multiple environmental factors that increase the rate of savaging demonstrated by pigs. It has been shown that human attitudes and behaviour can affect the behaviour of domestic pigs. For example, annoyance and transmission of emotions can occur in gilts and sows when a new worker enters the farrowing rooms and makes excessive noise, does not feed the animals in a timely manner and/or is frustrated/annoyed. Research suggests that maternal behaviour may improve when workers and pigs are familiar with their environment and on a set schedule. Another environmental factor that has been examined is the amount of time spent by humans in the farrowing room. Decreased disturbance of the sows by human intrusions during farrowing has indicated a decrease in savaging behavior. Increased aggression in sows is not always directed towards piglets as sows were just as likely to attack inanimate objects (bricks) as they were to attack piglets. As indicated by the fact that aggressive behaviour can be predicted before parturition begins as indicated by pre-farrowing restlessness and the fact that aggression was directed towards both piglets and inanimate objects, sow aggression is not a result of a response to the sudden appearance of piglets. A study by Gonyou and Harris found that increasing light exposure in the farrowing rooms to 16–24 hours per day decreased piglet deaths due to savaging. In the same study, it was found that increased piglet vocalizations around the time of farrowing did not help gilts adjust to the sudden presence of piglets, and instead was shown to increase the amount of piglet-directed aggression. The influence of nest-building and nest-building materials has been shown to influence negative piglet-directed communication. Access to materials as well as higher quality materials such as straw decreased the amount of negative communication between sows and piglets. Similarly, limited space and poor pen conditions have been shown to increase the amount of piglet-directed aggression. Pigs are social animals and a female's social support network can play an important role in postpartum piglet-directed aggression. Free ranging pigs form sounders, or small social units, give young gilts the opportunity to observe gestation and parturition and exposing them to normal maternal behavior. Gilts raised in commercial pens are not exposed to piglets nor expecting mothers and thus are not aware of what to expect during gestation and parturition and are not aware of how to raise piglets. In support of this theory, there is strong evidence that parity influences the amount of savaging and piglet-directed aggression displayed among primiparous gilts versus muiltiparous sows. Some studies have found evidence of increased savaging rates in larger litter sizes as well.
Genetic and biochemical factors It has been proposed that the degree of aggressive behaviour displayed prior to farrowing cycles by sows and gilts predicts whether the offspring will be savaged and to what extent. Evidence has also been found that savaging sows may be genetically less likely to crush piglets, which lends to the existing research supporting that savaging sows can be competent mothers. Levels of steroid hormones such as estrogens and progestogens around farrowing correspond with levels of maternal aggression. Specifically, high levels of estradiol at postpartum and high levels of pre-farrowing estradiol to progesterone ratios in gilts show more savaging behaviour to piglets. An increase in maternal aggressive behaviour as a result of low levels of progesterone would not be implausible as progesterone has been known to modulate serotonergic receptors. High postpartum levels of estradiol and estriol have also been associated with maternal mood and behaviour. The paraventricular nucleus (PVN) and the amygdala play important roles in moderating anxiety and depression with the PVN being responsible for secreting corticotropin releasing hormone (CRH) and oxytocin. This would explain the increased amount of CRH in stressed gilts. Abnormal levels of prepartum and postpartum oxytocin have also been linked to increased aggression and savaging as both abnormally high and abnormally low levels has been reported in savaging mothers. Low levels of oxytocin result in fear and aggression as a result of the interaction between oxytocin and the opioid system and has also been linked to insufficient nursing. Low levels of prolactin result in decreased milk let-down, leading to insufficient nursing and limited oxytocin being produced as a result of nursing. Since oxytocin reduces functioning of the hypothalamic-pituitary-adrenal (HPA) axis, high levels of oxytocin corresponding to an increase in unnresponsiveness to piglet vocalizations would be the result of abnormal HPA pathway functioning. Heritability estimates of 0.11 and 0.25 from half-sib analyses have demonstrated that selection against sow savaging of pigs is possible; however, it may be slow to show effectiveness.
… excerpt ends here. Continue reading the full article.
