Effects of Summer and Winter Birth on Growth of Lambs in a Population of Feral Sheep

Abstract

Abstract The feral sheep of Ile Longue, Kerguelen Archipelago, reproduce twice a year, in summer and winter lambing seasons. We studied effects of birth season on lamb growth (body mass for both sexes and horn length for males) during 3 consecutive years. Lambs of the summer cohort (January–February) grew rapidly during the 3 months following birth, ceased growth during winter (May–September), and resumed growth in spring. Growth was slow during the first 2 months of life for the winter cohort (July–August); winter lambs were lighter than summer lambs at the same age. By the end of winter, when environmental conditions improved, rapid growth of winter lambs allowed them to compensate for their mass deficit. Female lambs tended to be heavier than males early in life, but males achieved greater body mass by the 5th month of age. Significantly larger body mass of males versus females occurred earlier in the winter cohort than the summer cohort. These results suggest that sexual dimorphism may be limited under unfavorable conditions and sexes may allocate energy for growth differently.