DENSITY-DEPENDENT MOTHER YEARLING ASSOCIATION IN BIGHORN SHEEP

Post-weaning mother-daughter associations are typical of many ungulate s, but their existence among sheep is controversial. In bighorn sheep, Ovis canadensis, at high population density, strong mother-yearling a ssociations were found involving mostly ewes whose lamb-of-the-year di ed at or soon after birth. At low population density, there were no mo ther-yearling associations regardless of maternal reproductive status. Non-lactating ewes and most ewes caring for a new lamb did not associ ate with their yearlings. The mother-yearling bond appeared stronger f or female than for male yearlings, while maternal age had no effect. A ssociated yearling females had a nearly significant trend towards grea ter mass gain and greater body mass by the end of the summer compared with independent yearling females, but no trend was found when compari ng associated and independent yearling males. Mothers that associated with their yearlings gained mass faster over the summer than mothers t hat nursed lambs, and by the end of the summer they were heavier than non-lactating ewes. Therefore, post-weaning association did not appear to be very costly for mothers. This study demonstrates how changes in ecological circumstances can lead to drastic changes in social organi zation.