The young of some species are coprophagic, meaning they eat faeces. There are various theories about why they do this. Foals eat their mothers faeces. So do baby rats!
I came across this paper today - 'The Maternal Pheromone and Brain Development in the Preweanling Rat'
TM Lee et al. Physiol Behav 33 (3), 385-390.
The abstract says 'Preweanling rats selectively approach and consume pheromone-containing maternal feces. This selectivity suggests that the consumption of maternal feces might be important for the growing pup. Previous research suggested that such feces might promote brain development. A series of experiments was carried out in which pups were denied access to maternal feces. These pups were clearly inferior to control pups in brain growth and neurobehavioral maturation, as well as in the quantity of brain myelin.'
Interesting - often hand reared foals have behaviour problems, and we tend to put this down to the absence of other horses to teach them social skills, stress, our inadequacies in training them and so on. But I wonder if the simple lack of the building blocks necessary for normal brain growth and behavioural development plays a significant role?