Rapid morphological change in an insular population of feral sheep

Artificially selected qualities can reduce fitness in a wild setting, thus feral domesticates should experience strong selective forces. Domestic sheep Ovis aries have frequently become feral on islands, which differ substantially from mainland environments. We examined changes in body mass and wool traits in feral sheep inhabiting Santa Cruz Island (SCI), California for ≥90 years. To elucidate the influence of nutrition, we compared the mass of feral island sheep with that of island sheep raised in farm conditions. We found that feral sheep on SCI were smaller than purported founder breeds, and that most documented populations of insular feral sheep worldwide have converged to similar body sizes (within 6 kg). SCI rams attained greater mass in farm conditions but ewes did not, suggesting phenotypic plasticity in ram body mass. Ewes exhibited self-shedding of wool at a greater frequency than rams, and sex differences and shedding patterns were consistent with thermoregulation and the risk of fly strike disease as benefits of wool loss. Pigmentation rates did not increase, further supporting the influence of heat stress on wool traits. These changes occurred in <25 generations and may have had a genetic basis, representing a potential example of rapid evolution in insular feral sheep.