Individually marked bighorn sheep (Ovis canadensis) were monitored to determine how seasonal and lifetime changes in mass affected the development of sexual dimorphism, and to what extent mass at a given age could predict mass of the same sheep at a later age. We trapped sheep from late May to early October each year from 1971 to 1985 in a population artificially kept at low density. Lambs and yearlings gained mass linearly from June to September, while absolute mass gain of older sheep 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 beginning of June, was the same for male and female lambs but was greater for male than for female yearlings and 2-year-olds. With the exception of lambs, all age-classes lost mass during winter. Mass loss between September 15 and June 5 was greater for females than for males, possibly because ewes lost mass through parturition in late May. For both sexes, asymptotic mass was not reached until at least 7 years of age. Mass at 4 and 12 months of age was correlated with mass at 4 years. For all sex-age classes, mass on June 5 was negatively correlated with summer 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 accumulation. Most sexual dimorphism develops after weaning, through faster mass gain by males than by females at 1 and 2 years of age and possibly a longer season of mass gain each year for males than for females after females reach puberty.RCsumC : Nous avons CtudiC les changements de masse corporelle chez des Mouflons d7AmCrique (Ovis canadensis) marquCs individuellement au sein d7une population d7Alberta. Nous avons capturC les mouflons entre la fin mai et le dCbut octobre, de 1971 a 1985. Le gain de masse des agneaux et des jeunes de 1 an Ctait linCaire de juin a septembre, tandis que les mouflons plus igCs prCsentaient des gains de masse plus rapides en juin et juillet qu7en aoQt et septembre. Les miles ont gagnC plus de masse que les femelles a chaque annCe, jusqu7a l'ige de 3 ans. Le gain relatif de masse, calculC par rapport a la masse corporelle au dCbut de juin, Ctait le meme chez les agneaux et les agnelles, plus ClevC chez les miles de 1 an que chez les femelles de 1 an, et plus ClevC chez les miles de 2 ans que chez les femelles du meme Age. En hiver, a 17exception des agneaux, les mouflons de toutes les classes d7ige ont subi des pertes de masse. Les femelles ont subi des pertes plus importants que les miles entre le 15 septembre et le 5 juin, vraisemblablement a cause de 17agnelage a la fin mai. La masse moyenne a augment6 chez les deux sexes au moins jusqu7a l'ige de 7 ans. I1 y avait une corrClation entre la masse des mouflons a 4 mois et a 12 mois et leur masse a l'ige de 4 ans. Pour toutes les classes de sexe et d'ige, la masse au 5 juin Ctait en corrkl...
The factors affecting variation in age at first reproduction of bighorn ewes (Ovis canadensis) were investigated in two marked populations in Alberta. One population was studied for 20 years, the other for 11 years. As yearlings, females that lactated at 2 years of age were on average heavier and larger, and had longer horns than females that did not lactate at 2 years. However, there was wide overlap in body mass between early and late producers, and increases in body mass over the threshold for reproduction had little effect on the probability of early lambing. The body mass of females at 4 months of age explained less than half of the variance in female body mass at 1 year or at 15 months. In one population, the proportion of 2-year-old ewes lactating was not correlated with density and declined after a pneumonia epizootic. In the other population, the proportion of 2-year-old ewes lactating was higher during an experimental reduction of density, and dropped to near zero as density increased. There was a significant interaction effect of body mass and population density upon the probability that a ewe would lactate at 2 years of age. Independently of body mass, yearlings were less likely to lactate at 2 years of age at high population density than at low density. The number and age distribution of rams did not affect the proportion of 2-year-old ewes lactating. The mothers of lactating 2-year-olds were not older or heavier than the mothers of ewes that did not lactate at 2 years. Although some of the variation in age at first reproduction was due to differences in mass and population density, much of it remained unexplained and could be due to genetic factors.
In ungulates, body mass is often positively correlated with juvenile survival, but little is known of whether body mass affects survival of other age-classes. We studied two marked populations of bighorn sheep (Ovis canadensis) in Alberta, Canada, to determine if body mass affected the survival of different sex –age classes. Chest girth at weaning was correlated (P < 0.0001) with survival of bighorn lambs in the Sheep River population. In the Ram Mountain population, body mass in mid-September had a stronger effect upon survival than mass in early June, mass gain in summer, or mass loss in winter. Body mass at weaning was correlated with lamb survival (P = 0.004). In both study areas, relationships between size and survival of lambs were similar for the two sexes. At Ram Mountain, survival of yearling and adult males seemed to be independent of body mass. Light yearling females were less likely to survive than heavy yearling females. Among females aged 3 – 6 years, body mass had no effect on survival. Among females 7 years of age and older, mass in mid-September had a weak but significant (P = 0.03) effect on survival. Females were slightly lighter in mid-September in their last year of life than in the rest of their adult life. Maternal expenditure is unlikely to affect the survival of prime-age ewes, but may have a detrimental effect on the survival of older ewes.
We investigated the lie history consequences of age at primiparity in two marked populations of bighorn sheep (Ovis canadensis), studied for 19 and 12 yr. Some ewes first lambed at 2 yr of age, others at 3 or 4 yr. Lambs of 2—yr—old ewes were smaller and less viable than lambs of older ewes. Lactation at 2 yr did not affect survival to 3 yr, but in one population it was associated with reduced survival from 3 to 4 yr. Two yr olds that weaned lambs gained less mass during summer than other 2 yr olds. Overwinter mass loss from 2 to 3 yr was correlated with autumn body mass, and was lower for parous than for nonparous ewes. Reproductive success at 3 yr was independent of reproductive status at 2 yr. At 4 yr, ewes that lactated as 2 yr olds were smaller than ewes that had not lactated as 2 yr olds. In one population, negative effects of early maturation upon mass gain and subsequent reproductive success became evident at high density, and very few 2 yr olds lambed when population density was high. The lifetime reproductive success of early maturing ewes was not lower than that of late—maturing ewes. To account for individual variation in initial reproductive potential, we controlled body mass as a yearling, a variable that affected reproductive success. Mass as a yearling, however, did not interact with age of primiparity to affect survival, growth, or reproductive success. Uncertainties about resource availability and possible effects of genotype and body fat may explain why many ewes postponed their first reproduction despite apparently low life history costs of early maturation.
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