Stratégies de reproduction des femelles du mouflon d'Amérique (Ovis canadensis)
When reproduction is costly, life history theory suggests an energetic trade-off between physiological cost of maintenance and reproduction. In long-lived species in temperate climates, this trade-off could be measured as energy allocation to fat reserve and body condition versus allocation to lactation and offspring growth. When resource availability is variable, instead of a fixed energy allocation to reproduction based on average conditions, natural selection should favor an adjustment of effort based on individual body condition and on resource availability at each reproductive event. The main goal of this thesis is to study variation in energy allocation to reproduction, i.e. reproductive strategies, of bighorn sheep ( Ovis canadensis ) ewes. Bighorn sheep are long-lived mountain ungulates with a strong sexual dimorphism. Ewes can reproduce at two years and give birth to one young every year until they die. My research is based on a long-term project that has involved tagging and weighing individual sheep at Ram Mountain, Alberta, since 1972. The first chapter briefly presents the methods used and includes two methodological articles with a broad appeal in ecology. The second chapter tests for environmental and previous reproduction effects on reproductive effort. Using summer mass changes of mother-lamb pairs to estimate reprodutive effort, I show a conservative reproductive strategy of bighorn ewes, which allocate energy to their own maintenance and not to reproduction when environmental conditions are severe. In chapter three, different hypotheses of adaptive sex-ratio manipulation are evaluated. Resuls support the cost of reproduction hypothesis. Ewes manipulate both sex and size of their lambs to minimize the cost of reproduction. The fourth chapter assesses both the determinants and the consequences of variation in age of primiparity. Despite a strong somatic cost associated with first parturition, it is better for ewes to begin to reproduce at an early age. However, ewes delay primiparity when conditions are poor and if they are small as yearling, to allow compensatory growth. That delay again indicates a conservative strategy of reproduction. Finally, in the fifth chapter, I compare two hypotheses about variation of reproductive effort at the end of life. Terminal investment was not supported and a strong longevity-independent effet of senescence was detected. The different sections of this research test several important hypotheses in ecology and evolution. Two major points arise from all analyses: a strong and ubiquitous inter-individual variation and a conservative strategy of reproduction of ewes, which favour their own maintenance and survival over the growth of their lambs.
- Sciences – Thèses