A full belly and colostrum: two major determinants of filial love

The establishment of filial bonding in sheep relies on the success of the first suckling bouts. Previous work has shown that colostrum ingestion facilitates the development of a preference for the mother. We conducted a series of experiments to investigate the influence of quantitative and qualitative factors linked to fluid ingestion. The first experiment demonstrated that the procedure was not detrimental to the mother-young interaction and did not cause any harm to the lamb. In a second experiment, lambs could interact with their dam, but were deprived of suckling during the first 12 hr after birth (The udder was covered.) and fed via a nasogastric tube in the presence of their mother on seven occasions. When tested in a two-choice test at 12 hr of age, lambs that received either colostrum or saline (total amount = 10% birth weight) spent more time near their own dam than near an alien ewe. When smaller volumes of fluids were used (5% birth weight), lambs that received colostrum showed a preference for their mother while those ingesting saline did not. The differences persisted at 24 hr even though all the lambs had access to the maternal udder from 12 hr onward, and these differences were not related to weight variations or body temperature. In the choice test, the motor skills and vocal activity of the lambs were not affected by the treatment nor were the behaviors of the ewes. The third experiment showed that colostrum, and saline to a lesser extent, reduces the number of vocalizations emitted by lambs compared to sham-intubated lambs, but did not otherwise influence mother-young interactions. This suggests that post-ingestive factors may facilitate the development of mother preference through a satiated or comforted state. This is the first evidence that depending on the amount of liquids ingested, a preferential relationship with the mother can be established via nutritional and nonnutritional signals originating from the gastrointestinal region, pointing out the major role played by the gut-brain axis in the development of filial bonding.