No abstract

In an age‐ or stage‐structured population, a given change on a proportional scale of different fitness components has different effects on the population growth rate. Because earlier studies have shown that variability is selectively disadvantageous for long‐lived iteroparous species, fitness components whose variation has greatest impact on the population growth rate are expected to be canalized against temporal variability. We present here a test for such a canalization of fitness components most influential for population growth. If canalization occurs, (1) within a given population the variance of canalized fitness components should be less than that of noncanalized components, and (2) among populations an inverse relationship should occur between the potential demographic impact of fitness components and their temporal variability. We tested these hypotheses using data on age‐dependent survival of ungulates. As expected for long‐lived vertebrates, elasticities and sensitivities of adult survival were consistently higher than for juvenile survival. In support of the canalization hypothesis, the variance of juvenile survival with low potential demographic impact on fitness was consistently higher than the variance of adult survival with high potential demographic impact on fitness, in all of 22 ungulate populations for which long‐term demographic monitoring data were available. However, a negative covariation between potential impact on fitness and temporal variation could also be accounted for by alternative hypotheses, the lower resistance of growing individuals to environmental “insults,” or reduced adult phenotypic variability due to selective mortality of juveniles. To differentiate between canalization of adult survival and frailty/selective mortality of juveniles, we included five populations of small mammals. We then found a negative relationship between the relative potential impact of age‐dependent survival on fitness and the relative variability of age‐dependent survival, especially for populations faced with high environmental variation. This remained true after correcting for differences in mean survival of juveniles and adults and was independent of phylogeny. Although more data are required to extend the hypothesis to other taxonomic groups, we conclude that available data support the idea that adult survival of ungulates is canalized against temporal variation. Therefore, contrasted responses of juvenile and prime‐age survival to environmental variation in ungulates may be adaptive and could be viewed as a bet‐hedging strategy.

Despite its central place in animal ecology no general mechanistic movement model with an emergent home‐range pattern has yet been proposed. Random walk models, which are commonly used to model animal movement, show diffusion instead of a bounded home range and therefore require special modifications. Current approaches for mechanistic modeling of home ranges apply only to a limited set of taxa, namely territorial animals and/or central place foragers. In this paper we present a more general mechanistic movement model based on a biased correlated random walk, which shows the potential for home‐range behavior. The model is based on an animal tracking a dynamic resource landscape, using a biologically plausible two‐part memory system, i.e. a reference‐ and a working‐memory. Our results show that by adding these memory processes the random walker produces home‐range behavior as it gains experience, which also leads to more efficient resource use. Interestingly, home‐range patterns, which we assessed based on home‐range overlap and increase in area covered with time, require the combined action of both memory components to emerge. Our model has the potential to predict home‐range size and can be used for comparative analysis of the mechanisms shaping home‐range patterns.

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.

Time- and sex-specific summer survival of roe deer fawns was estimated using capture-mark-recapture methods in two enclosed populations living in contrasting conditions. The population of Trois Fontaines (eastern France) was roughly constant in size throughout the study period, while in Chizé (western France), the population experienced frequent summer droughts and numbers decreased continuously during the study. Early survival of fawns was low and highly variable over the years at both Chizé and Trois Fontaines, and demonstrated marked variations between cohorts that need to be taken into account when modelling roe deer population dynamics. In Trois Fontaines, fawn survival was positively correlated with early body growth and total rainfall in May and June. In Chizé, fawn survival decreased with increasing density and tended to increase with increasing rainfall in May and June and adult female body mass. These factors explained more than 75% of the variability in early survival observed in both populations. Variation between cohorts had different consequences for the two populations. At Trois Fontaines, cohort variation was limited to a numerical effect on early survival. However at Chizé, cohort variation was long-lasting and affected the phenotypic quality of survivors at later ages, and thereby future survival and breeding abilities (both numerical and quality effects). Male and female fawns had similar survival over their first summer in both populations. This result contrasts with the lower survival of young males often observed in ungulates. Two ultimate causes can be proposed to account for the low and variable survival of roe deer fawns over the first summer: the high energy expenditures incurred by does during each breeding attempt and/or the low absolute body size of newborn roe deer fawns.

No abstract

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.

Summary 1.Although life-history theory predicts substantial costs of reproduction, individuals often show positive correlations among life-history traits, rather than trade-offs. The apparent absence of reproductive costs may result from heterogeneity in individual quality. 2. Using detailed longitudinal data from three contrasted ungulate populations (mountain goats, Oreamnos americanus ; bighorn sheep, Ovis canadensis ; and roe deer, Capreolus capreolus ), we assessed how individual quality affects the probability of detecting a cost of current reproduction on future reproduction for females. We used a composite measure of individual quality based on variations in longevity (all species), success in the last breeding opportunity before death (goats and sheep), adult mass (all species), and social rank (goats only). 3. In all species, high-quality females consistently had a higher probability of reproduction, irrespective of previous reproductive status. In mountain goats, we detected a cost of reproduction only after accounting for differences in individual quality. Only low-quality female goats were less likely to reproduce following years of breeding than of nonbreeding. Offspring survival was lower in bighorn ewes following years of successful breeding than after years when no lamb was produced, but only for low-quality females, suggesting that a cost of reproduction only occurred for low-quality females. 4. Because costs of reproduction differ among females, studies of life-history evolution must account for heterogeneity in individual quality.