Individual variation in the expression of maternal behaviour: a review of the neuroendocrine mechanisms in the sheep.

Individual variation in the expression of maternal behaviour can affect offspring development and survival. Ewes showing a poorer quality of maternal behaviour (low levels of licking and grooming, infrequent low-pitched bleating, inconsistent udder acceptance) at parturition and throughout lactation (large ewe-lamb distance, low vigilance) form a weaker bond with their lambs, and have higher lamb mortality, than ewes which show high levels of grooming and low-pitched bleating, close ewe-lamb distances and high levels of vigilance. Using two models of variation in maternal behaviour in the sheep (differences between ewes of two breeds, and differences between primiparous and multiparous ewes), the neuroendocrine mechanisms that underpin these differences are explored. In both cases, significant variation in oestrogen priming is observed, with breed differences in circulating concentrations of oestrogen in late gestation, and parity effects on receptor density but not circulating concentrations. The consequences of these differences for central oxytocin release and binding are discussed. In addition, preliminary data suggest that opioid modulation, in both breed and parity models, may also play a role in mediating individual variation in maternal behaviour in the sheep.