Effets de la date de naissance, des caractéristiques maternelles et de l'environnement sur la croissance, la masse et la survie du mouflon d'Amérique (Ovis canadensis)
Understanding the factors affecting the early stages of individual development and survival is fundamental to our knowledge of life history evolution. Body mass is an important phenotypic trait which affects many aspects of an individual's life history and fitness. An understanding of the variables affecting body mass is equally important for the study of population ecology and of the evolution of adaptive strategies within a species. I investigated the effects of birth date, maternal characteristics and environmental conditions on juvenile mass and survival, to identify the factors that play a major role on early development. In particular, I focused on the role of birth date as a determinant of early individual phenotypic traits. Using bighorn sheep ( Ovis canadensis ) as a model, I examined how birth date, lamb sex, maternal mass, age and previous reproductive status, the quality of vegetation, temperatures and precipitations affect lamb body mass, growth and survival. I analyzed 13 years of data from the Ram Mountain population, where bighorn sheep have been marked and individually monitored for over 30 years. Most individuals are weighed several times each summer; mother-lamb relationships are known and estimates of lamb birth date have been collected since 1992. I found that birth date negatively affected the mass of the lamb in early June and at weaning, ultimately affecting the probability of survival. I was able to identify only few determinants that affect the distribution of birth date. At the population level, the synchrony in lambing period appeared to be an adaptation to high seasonality in vegetation phenology and not an anti-predator strategy. At the individual level, ewes that had weaned a lamb the previous year gave birth later than ewes that had not lactated the previous summer. Parturition was also delayed following years with high snow accumulation during the rut. Birth date had a strong negative effect on lamb mass, apparently because late-born lamb had a shorter time to accumulate resources before winter compared to lambs born early in the season. Late-born lambs were lighter in early June, for which no other variables appeared to have a significant effect. Mass in September decreased with later birth date, was greater for males than for females, lower for lambs whose mother had weaned a lamb the previous year, and higher in years when fecal crude protein suggested a higher quality of vegetation. Mothers that gave birth late in the season adopted a conservative strategy favoring their own mass over the growth of their lamb. Lamb mass gain during summer was linear for both sexes and was higher for males than for females. Birth date did not affect lamb mass gain and there was no evidence of compensatory growth for late born individuals. Late birth and sex interacted to affect lamb survival, so the survival of male lambs decreased more steeply with later birthdate than the survival of female lambs. Late birth had a weak negative effect on mass as yearling in June, but not on yearling mass in September or at two years of age. Long-term studies on the causes of variation in individual early development are fundamental for our comprehension of the evolution of adaptive strategies. The long-term study on individually marked animals at Ram Mountain is unique because it includes precise information on birth date and variation in individual mass. These results show that the effects of birth date profoundly influence some phenotypic traits early in life, such as mass at weaning and survival of individuals. In addition, the effects of a bad start persist after the end of maternal care, possibly affecting the individual's quality and the ability to reproduce later in life. The influence of birth date should be taken into account in studies that aim to investigate the factors affecting phenotypic traits and survival in early stages of life, especially in long-lived mammals.This research, taking into account the role of birth date and sex, as well as maternal conditions and environmental factors, provides valuable insights on the evolution of the adaptive strategies of a long-lived mammal.
- Sciences – Mémoires