Prenatal stress in pigs

Abstract

Studies in many species, including humans, have demonstrated that stress during gestation can have long-term developmental, neuroendocrine, and behavioural effects on the offspring. Because pregnant sows can be subjected to regular stressful situations, it is relevant to study whether prenatal stress also affects their piglets. Glucocorticoids play an important role in affecting the fetus during prenatal stress, and therefore, a model to elevate maternal cortisol concentrations during gestation was developed. Main objectives of the project described in this thesis were to determine whether (1) piglet characteristics (e.g. growth, behaviour) are affected by elevated maternal cortisol concentrations during gestation; (2) the timing of elevated maternal cortisol concentrations during gestation is relevant for the observed effects; (3) social challenges for pregnant sows also affect the piglets. Sows orally received hydrocortisone acetate (HCA) during one of three periods (P1, P2, or P3) of gestation. They had elevated salivary cortisol concentrations during the period of HCA administration. Main findings of the study were that piglets from sows that received HCA during any of the three periods had lower birth weights and remained lighter until weaning. During two behavioural tests (the backtest and the tonic immobility test), the number of vocalisations did not differ between HCA male and female piglets, while control males vocalised significantly more than control females. During a novel environment test, P1 and P3 piglets walked more than control piglets. When introduced to an unfamiliar piglet, P1, P2, and P3 male piglets had less non-aggressive encounters than control male piglets. Piglets from the control, P1, and P3 group had less aggressive encounters during the second part of the mixing test, whereas P2 piglets continued fighting. Furthermore, the salivary cortisol concentration in response to i.m. ACTH was lower in female P1 and P3 piglets compared to control piglets. These P1 and P3 females also displayed an elevated temperature response to i.v. LPS injection compared to control females. At slaughter, HCA treatment indirectly decreased lean meat percentage and increased the fat percentage. These results indicate that elevated maternal cortisol concentrations during gestation do affect piglet development, neuroendocrine activity, and behaviour postanatally, and that effects may differ between male and female piglets. In addition, effects depend on the period of gestation during which cortisol concentrations of the sow were elevated. In a social-challenge experiment, pregnant sows were introduced to an unfamiliar sow twice weekly during the last month of gestation. Though these Mix sows gained less weight during gestation and lost less weight during lactation, their salivary cortisol concentrations in response to mixing did not differ from those of (undisturbed) Control sows. Piglets did not differ in birth weight, body weight at weaning, behaviour, nor did their adrenocortical response to ACTH, indicating that regular mixing of pregnant sows at the end of gestation does not affect their offspring. In a second social-challenge experiment, pregnant sows were housed in social groups throughout gestation to determine whether the social rank of pregnant sows during gestation would affect their piglets. Sows were classified as High Social Ranking (HSR) or Low Social Ranking (LSR). Prior to insemination, HSR and LSR sows did not differ in body weight, but HSR sows gained more weight during gestation and, subsequently, lost more weight during lactation. Maternal salivary cortisol concentrations during gestation did not differ between HSR and LSR sows, nor did gestation length, litter size, or percentage of live born piglets. HSR piglets tended to weigh more at birth, and weighed more at weaning. During a novel object test, HSR piglets moved and vocalised more than LSR piglets. In addition, the latency time to touch the novel object was shorter in HSR offspring, and HSR males spent more time near the object than LSR males. At slaughter, HSR offspring had more lean meat than LSR offspring. These results indicate that the sow’s social rank during gestation does affect her own weight gain and loss, and her offspring’s growth and behaviour. Taken together, the results of the experiments described in this thesis demonstrate that both elevated maternal cortisol concentrations and the social rank of the pregnant sow during gestation can affect piglet growth, behaviour, and physiology