Sixteen sheep and goat dams were placed in a restraining harness between 2 and 12 hours after parturition, and were forced to remain continuously in close proximity to an alien young animal. Five of 9 goats were placed with lambs and 3 of 7 sheep with kids. All dams remained in restraint until, when released from the harness, they permitted the alien young to nurse and refrained from butting them. Two and 3 months later the 2 inter-species and 2 intra-species adoptive groups were tested for maternal and filial behavior, with the following results: 1. All adoptions were established, with an average of 10 days required for maternal acceptance of the young. 2. There were no significant differences between natural young- and foster young-mother pairs in the dams' reaction to separation and in nursing time. 3. Cross-species foster mothers reacted more vigorously to separation and nursed less than same-species foster mothers. 4. There was a positive correlation between the length of time necessary for an adoption to be established and maternal reaction to separation, but no relationship between adoption time and amount of nursing. The results are interpreted as evidence (a) that enforced contact between dam and young after the 2-4 hour post-partum period may considerably prolong the critical period for the development of individual-specific maternal care in sheep and goats and (b) that species-difference between dam and young differentially affects thresholds for different maternal behaviors.
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Hersher et al. (1963) studied this question.