Stratégies de reproduction des mâles du mouflon d'Amérique (Ovis canadensis)
Life histories theory suggests that organisms make trade-offs in energy allocation between their different functions, such as maintenance, reproduction and immunity. This theory is based on the assumption that energy is limited and therefore an increase in investment in one function will necessarily result in a decrease in another one. The main objective of this thesis was to study the mating strategy of male bighorn sheep to see if rams have to trade between current reproduction and other functions such as parasitism and survival. To achieve this, I monitored the individual behaviour of bighorn rams of the Sheep River population and documented rut-related constraints on foraging behaviour, change in body mass, number of lungworm larvae, survival and longevity. First, I studied the social organization of rams, because social rank appears to be the most important determinant of access to estrous ewes during the rut. Consequently, the establishment of social rank during the pre-rut is an important component of male reproductive strategy in this species. Analysis of more than 2800 social interactions (collected from 2000 to 2004) revealed that rams are organized in a linear and stable dominance hierarchy. The main factors affecting social rank are age and body mass, with mass being increasingly important as males age. Second, I investigated how individual characteristics affect foraging behaviour during the non-reproductive (pre-rut) and reproductive (rut) periods. To achieve this, I constructed rams' individual time budgets, lasting between 8 to 10 hours, during both periods. I first studied the relation between individual characteristics and male time budget and found that body mass was the most important factor affecting daily foraging time during the pre-rut. Then, I investigated how the two main mating tactics (tending and coursing) used by rams during the rut affected their foraging behaviour. Using time budgets, I compared how a male's individual behaviour changes during the rut compared to the pre-rut. Males spent from 15 to 20% less time foraging during the breeding season. Contrary to expectations, when the same ram was observed both coursing and tending within the same rut, it appeared that coursing imposed more foraging constraints than the main mating tactic of tending. Third, I studied the relation between reproductive effort and fecal output of Protostrongylus spp. (lungworm) larvae. Although age and social rank did not affect counts of larvae, sheep appeared to trade between reproduction and parasitism. Counts increase in both sexes during the period of highest reproductive investment as compared to availability of resources: late gestation in female and the rut in males. Males involved in more searching activities during the rut had higher counts than other males. Lactating ewes also had higher counts than non-lactating ones. Because I found the same trends in the patterns of larvae output in relation to reproductive effort for both genders but at different periods of the year, the hypothesis of a trade-off between current reproduction and parasitism appears to be the more parsimonious to explain sexe results. Lastly, I analyzed the effects of reproductive activities on survival and longevity, using the long-term database for 1989-2002. During fourteen rutting seasons, social rank and individual behaviour (proportion of ewes tended, proportion of coursing attendances and daily distance searching) was monitored. Although those indexes are indirect measurements of male mating effort, one can assume that more active males during the rut make the greatest reproductive effort. If there is a trade-off between current and future reproduction as predicted by life history theory, I should have found lower survival for more active individuals. However, there was no relation between winter survival and activity during the rut. Longevity was positively correlated with the level of rutting activities: some individuals seem to be able to invest in current reproduction without compromising future reproduction. It appears that for polygynous ungulates such as bighorn sheep, heterogeneity in individual quality hides potential reproductive costs. For this species, it is therefore not possible to demonstrate a trade-off in resource allocation between current and future reproduction using phenotypic correlations. Although reproduction is energically costly, males with a higher reproductive success are more dominant, heavier and have greater longevity. During the rut, those males use the primary mating tactic of tending, known to impose less foraging constraints without leading to a greater mass loss. Consequently, in bighorn rams, reproductive effort is probably not correlated to reproductive success. In conclusion, in complex systems (such as life-history trade-offs), where there is a large potential individual variability in resource acquisition, positive associations between fitness components may be common. In polygynous ungulates, where reproduction is monopolized by a few high-quality individuals, a small initial advantage in resource acquisition is likely to lead to an increase in returns. In such a situation, individuals able to acquire enough resources to invest in reproduction and survival will be selected, leading to a positive association between fitness components.
- Sciences – Thèses