Behavioural research on domestic pigs has included parent-offspring conflict, sibling competition, and the use of signals which influence resource allocation. In this paper, we review key sow-piglet behavioural studies and discuss their relevance to resource allocation theory. Sibling competition begins in the uterus and continues after birth, as piglets compete directly for access to the sow's teats. This competition is made more severe by a unique dentition, which newborn piglets use to lacerate the faces of siblings during teat disputes. Competition often leads to the death of some littermates, especially those of low birth weight. Piglets also compete indirectly for milk, apparently by stimulating milk production at the teats that they habitually use at the expense of milk production by other teats. The complex nursing behavior of the sow appears designed to prevent the more vigorous piglets from monopolizing the milk. Sows give vocal signals which both attract piglets to suckle and synchronize their behavior during nursing episodes. Piglets give loud vocal signals when separated from the sow; calls which vary in intensity and appear to be honest signals of need. Udder massage by piglets may also serve as an honest signal of need. Parent-offspring conflict has been demonstrated experimentally in pigs. Specifically, when given the opportunity to control contact with their piglets, sows nurse less frequently, provide less milk, and lose less weight during lactation than sows that cannot control their level of contact. Because of this interesting natural history and because they are so amenable to experimentation, domestic pigs provide a rich system for testing ideas drawn from resource allocation theory.
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