THE DEVELOPMENT OF SEXUAL DIMORPHISM - SEASONAL AND LIFETIME MASS CHANGES IN BIGHORN SHEEP

Individually marked bighorn sheep (Ovis canadensis) were monitored to determine how seasonal and lifetime changes in mass affected the devel opment of sexual dimorphism, and to what extent mass at a given age co uld predict mass of the same sheep at a later age. We trapped sheep fr om late May to early October each year from 1971 to 1985 in a populati on artificially kept at low density. Lambs and yearlings gained mass l inearly from June to September, while absolute mass gain of older shee p was faster in June-July than in August-September. Males gained more mass than females each summer up to at least 3 years of age. Relative summer mass gain, calculated as a proportion of body mass at the begin ning of June, was the same for male and female lambs but was greater f or male than for female yearlings and 2-year-olds. With the exception of lambs, all age-classes lost mass during winter. Mass loss between S eptember 15 and June 5 was greater for females than for males, possibl y because ewes lost mass through parturition in late May. For both sex es, asymptotic mass was not reached until at least 7 years of age. Mas s at 4 and 12 months of age was correlated with mass at 4 years. For a ll sex-age classes, mass on June 5 was negatively correlated with summ er mass gain. For lambs and yearlings, winter mass loss was positively correlated with mass on September 15. Our results suggest that at low population density, sheep optimize rather than maximize summer mass a ccumulation. Most sexual dimorphism develops after weaning, through fa ster mass gain by males than by females at 1 and 2 years of age and po ssibly a longer season of mass gain each year for males than for femal es after females reach puberty.