Behavioural perspectives on piglet survival.

Litters of domestic piglets show strong sibling competition, large differences among litter-mates in birth weight and rate of growth, and, in the absence of human intervention, a high mortality rate. This combination of traits suggests that pigs are using a reproductive strategy similar to that of certain bird species which produce one or more small 'spare' young whose death or survival is determined by sibling competition. Death through competition is natural in such species. Prevention of death requires the early identification and separate rearing of unsuccessful competitors. The major behavioural pathways leading to piglet deaths are considered to be malnutrition through unsuccessful suckling behaviour, and crushing of piglets by the sow. Crushing involves two distinct behavioural sequences: posterior crushing (beneath the sow's hind quarters) and ventral crushing (beneath the udder and rib cage). Farrowing crates are designed to prevent posterior but not ventral crushing. Malnourished piglets appear to be more vulnerable to crushing, perhaps because persistent suckling attempts cause them to spend more time near the sow. Prevention of crushing thus requires a reduction in malnutrition, not merely restriction of the sow's movements. Under certain conditions, dehydration may be an important but neglected aspect of malnutrition. Some litters of piglets have much higher death losses than others, presumably because of risk factors that apply to the litter as a whole. Early malnutrition, resulting from hypogalactia in the sow in the first days after farrowing, appears to be an important risk factor. Farrowing difficulties leading to piglet hypoxia during the birth process may be another. Risk factors that affect whole litters deserve greater emphasis in future research.