Methodological problems in describing patterns of senescence in wild populations have until recently impeded progress in understanding the evolution of a process that decreases individual fitness. We investigated age‐ and sex‐specific survival in five populations of three species of ungulates (roe deer, Capreolus capreolus; bighorn sheep, Ovis canadensis; and isard, Rupicapra pyrenaica), using recent statistical developments of capture–mark–recapture models and long‐term (12 to 22 yr) data on marked individuals. The yearly survival of females aged 2–7 yr was remarkably similar and very high (92–95%) in all five populations. Survival of adult males varied among species and populations. Survival decreased from 8 yr onward for both sexes in all populations, suggesting that senescence was a common phenomenon. Male survival was lower than female survival, and the gender difference increased with age. The extent of sex differences in survival was related neither to sexual dimorphism in mass nor to the level of polygyny, suggesting that species differences in social behavior, particularly mating system and the level of male–male aggression, may be more important than simply the level of polygyny in explaining sexual differences in survival. Our results underline the advantages of long‐term monitoring of marked individuals for the study of evolutionary ecology.

The potential for selective harvests to induce rapid evolutionary change is an important question for conservation and evolutionary biology, with numerous biological, social and economic implications. We analyze 39 years of phenotypic data on horn size in bighorn sheep (Ovis canadensis) subject to intense trophy hunting for 23 years, after which harvests nearly ceased. Our analyses revealed a significant decline in genetic value for horn length of rams, consistent with an evolutionary response to artificial selection on this trait. The probability that the observed change in male horn length was due solely to drift is 9.9%. Female horn length and male horn base, traits genetically correlated to the trait under selection, showed weak declining trends. There was no temporal trend in genetic value for female horn base circumference, a trait not directly targeted by selective hunting and not genetically correlated with male horn length. The decline in genetic value for male horn length stopped, but was not reversed, when hunting pressure was drastically reduced. Our analysis provides support for the contention that selective hunting led to a reduction in horn length through evolutionary change. It also confirms that after artificial selection stops, recovery through natural selection is slow.

For capital breeders, mass may affect reproductive potential. Reproductive expenditure may reduce future reproductive potential, particularly when resources are scarce. To test the hypothesis that reproductive success and the costs of reproduction vary according to mass and population density, we analyzed 25 yr of data on bighorn ewes (Ovis canadensis). The number of adult females was first limited by yearly removals, then allowed to triple. We found no survival costs of reproduction for ewes aged 4-7 yr. For ewes aged 8-14 yr, survival was density dependent for barren ewes but not for ewes that weaned lambs. Failure to lamb was rare and negatively correlated with fertility the following year. At low population density, lactation had a negative effect on mass gain but had a limited reproductive cost. At high density, heavy ewes had higher reproductive success than light ewes, and the reproductive cost and somatic costs of reproduction increased. The cost of reproduction was greater for light than for heavy ewes. Survival of weaned lambs to 1 yr was affected by population density but not by maternal mass or previous reproductive success. In large mammals, manipulations of reproductive effort are problematic, but long-term monitoring of individual mass and reproductive success under varying conditions of resource availability can provide insights into the evolution of life histories.

In female vertebrates, differences in fitness often correspond to differences in phenotypic quality, suggesting that larger females have greater fitness. Variation in individual fitness can result from variation in life span and/or variation in yearly reproductive success, but no study has yet assessed the relationships between the components of fitness and phenotypic quality while controlling for life span. We tried to fill this gap using data from long-term monitoring (23 years) of marked roe deer and bighorn sheep, two ungulates with very different life histories. In both species, we found a strong positive relationship between an adult female's mass and her probability of reaching old age: over the long term, bigger is indeed better for ungulate females. On the other hand, we found no evidence in either species that heavier females had higher fitness when differences in life span were accounted for: over the short term, bigger is not necessarily better. Our results indicate that, while broad differences in phenotypic quality affect individual fitness, when differences in life span are accounted for phenotypic quality has no residual effect on fitness. Therefore, within a given range of phenotypic quality, bigger is not always better, for reasons which may differ between species.

Longitudinal studies of survival are valuable because age-specific survival affects population dynamics and the evolution of several life history traits. We used capturemark-recapture models to assess the relationship between survival and sex, age, population, year of study, disease, winter weather, and population density in two populations of bighorn sheep (Ovis canadensis) in Alberta, Canada. The Ram Mountain population, monitored for 20 yr, more than doubled in density; the Sheep River population, monitored for 13 yr, experienced a pneumonia epizootic. Yearling survival varied among years and was lower than that of older sheep of the same sex, except for yearling males at Ram Mountain. Yearling females at Ram Mountain were the only sex-age class exhibiting density dependence in survival. Senescence was evident for both sexes in both populations. Female survival from age 2 to age 7 was very high in both populations, but males aged 2 and 3 yr enjoyed better survival than males aged 4-6 yr. Our data support the suggestion that where hunters remove many males older than 5 yr of age, the natural mortality of males increases at 3-5 yr, possibly because young males suffer a mortality cost of participating in rutting activity. The decline in survival for sheep older than 7 yr was greater for males than for females. Survival was lower for males than for females, both among prime-aged sheep (0.896 vs. 0.939 at Sheep River; 0.837 vs. 0.945 at Ram Mountain) and among older sheep (0.777 vs. 0.859 at Sheep River; 0.624 vs. 0.850 at Ram Mountain), but not among yearlings. Survival of sheep aged 2-7 yr was not significantly different between the two populations. Winter weather did not affect survival. Survival of sheep 2 yr of age and older did not vary significantly between years, except at Sheep River where survival of primeaged sheep of both sexes was lower in the year of the pneumonia epizootic. Studies of survival of mountain sheep based upon skull collections may have overestimated survival of young rams. Our results underline the need for accurate information on age-specific survival.

Population models in ecology are rarely validated by comparing their predictions to long-term observations of changes in population size. We have used a variety of analytical tools to examine a 45-year time series of annual censuses of Alpine ibex (Capra ibex) in the Gran Paradiso National Park in northwestern Italy. This ibex population grew from about 3300 to almost 5000 individuals in the 1980s during a decade of anomalously mild winters, and then began to decline in the 1990s. By 1997, the population size had returned to previous levels. Adult survival apparently increased and adult sex ratio may have changed to slightly favor males during the increase in population density. Yearly changes in total population were correlated with seasonal average snow depth and population density over the 39 years for which climate data were available. Our results show that the ibex population size was limited by both density dependence and deep snow. A model based on these factors fit to the first 19 years of data was used to forecast subsequent changes in total population based on initial population size and yearly snow depth. The model was able to predict the increase and subsequent decline in total population size over the final 20 years of the study but failed to reproduce population levels after the eruption. Our results suggest that the 1980s episode of population growth was primarily driven by increased adult survival, rather than increased recruitment.

Within individual iteroparous mammals, a high rate of reproduction in early life may occur at the cost of decreased reproduction near the end of life, leading to reproductive senescence. Using long‐term data on marked individuals from two populations of bighorn sheep (Ovis canadensis), we tested for the existence of reproductive senescence and of trade‐offs between longevity and early reproductive success in ewes, which have an observed maximum life‐span of 19 yr. Lamb production decreased in older ewes, while weaning success for parous ewes was independent of age in one population and decreased with age in the other. The age‐related decrease in lamb production followed a slight decrease in body mass, which began at ∼11 yr of age. Reproductive senescence in this species appears to begin 6–7 yr after the onset of survival senescence. Longevity was positively related to late‐summer body mass at 6 yr. Contrary to reproductive cost theories, there was no negative relationship between early and late reproductive success or between early reproductive success and longevity; instead, those relationships tended to be weakly positive. Longevity had a strong positive influence on lifetime reproductive success in both populations. Since the longest lived ewes were among the heaviest as young adults and had high reproductive success throughout their lifetime, longevity appears to be state‐dependent; only ewes of higher phenotypic quality survived long enough to reach reproductive senescence. Models that assume that survival to old age is random with respect to phenotype are therefore not applicable to reproductive senescence in bighorn sheep.

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...