Effects of summer and winter birth on growth of lambs in a population of feral sheep

The feral sheep of Ile Longue, Kerguelen Archipelago, reproduce twice a yea r, 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 ra pidly 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 li ghter than summer lambs at the same age. By the end of winter, when environ mental conditions improved, rapid growth of winter lambs allowed them to co mpensate for their mass deficit. Female lambs tended to be heavier than mal es early in life, but males achieved greater body mass by the 5th month of age. Significantly larger body mass of males versus females occurred earlie r in the winter cohort than the summer cohort. These results suggest that s exual dimorphism may be limited under unfavorable conditions and sexes may allocate energy for growth differently.