Early growth and sucking behaviour of Soay sheep in a fluctuating population

Annual variation in growth rate and sucking behaviour of lambs was studied in a fluctuating population of Soay sheep on Hirta, St. Kilda. Growth rates during the summer declined with increasing population size in the previous autumn despite an increase in time spent sucking in the first six weeks of life. The increase in time spent sucking was due largely to an increase in sucking frequency and, since a higher proportion of sucking attempts were rejected, probably reflected that lambs obtain little milk as a result of poor maternal condition following winters at high population density. After accounting for the density dependent variation, annual differences in lamb growth rates were positively related to the increase in biomass of live grasses and forbs between spring and summer. Since food availability was influenced strongly by the numbers surviving the winter, lambs born after the very high mortality at peak population density were able to compensate to some extent for their poor initial growth. However, improved food availability was never sufficient to facilitate complete catch-up growth and may permanently influence adult size and reproduction.