Marco Festa-Bianchet

Heavy snowpack can be a severe constraint for alpine herbivores, yet adult survival in long-lived temperate ungulates is typically canalized against environmental variation. Using a 45-year individual-based dataset on bighorn sheep ( Ovis canadensis ) and historical snow reconstructions at Ram Mountain, Canada, we evaluated the direct and indirect effects of snow conditions on age- and sex-specific survival. For both sexes, survival of lambs and senescent individuals decreased with increasing average snow depth, whereas non-senescent adults (aged ≥ 1 year and < 8 years) showed high survival under mean conditions. However, this resilience broke down once specific thresholds of snow depth were crossed, with sharp survival declines beyond 90 cm of snow depth for females and 100 cm for males. Lamb survival, already reduced by increasing average snow depth, declined further once snow depth exceeded 90 cm. Crucially, the annual probability of snow depth exceeding 90 cm increased markedly over the study period, along with a density-independent temporal decline in survival across all demographic groups. Lamb survival declined by 0.37, and adult female survival, the most critical vital rate for population growth, dropped by 0.13 over the study period. Our findings challenge the established paradigm of demographic buffering in prime-aged adults, demonstrating that their resilience to environmental variability is fundamentally non-linear and breaks down once a changing climate regime breaches adult physiological boundaries, severely compromising population viability.

Winter can affect animal population dynamics by limiting resource availability and increasing energetic costs of movement caused by deep snow. Given the rapid alteration of snowpack properties due to climate change, quantifying how snow characteristics influence reproduction and physical condition is critical. We evaluated how snow cover duration, depth, and density affect spring body mass, reproduction probability, and subsequent autumn body mass of bighorn sheep ( Ovis canadensis ) using 45 years of individual-based data at Ram Mountain, Alberta, Canada, along with historical snow records reconstructed via the SNOWPACK model. Using Bayesian structural equation modeling, we quantified the direct and indirect effects of snow across different sex and age classes. Long and deep snow covers reduced spring body mass across all demographic groups, with yearlings, especially males, losing up to 0.12 kg per additional cm of snow depth. Harsh snow conditions reduced the probability of reproduction for adult females and generated a compensatory indirect effect on mass by avoiding the energetic costs of reproduction. In contrast, yearlings showed no compensatory responses and entered the following autumn in poor condition (up to 14% lighter for males and 8% for females following the deepest snow years). The impact of snow density on autumn mass of adult males was density-dependent, shifting from beneficial at low density (+0.09 kg per kg/m³) to detrimental at high density (-0.04 kg per kg/m³). The effects of snow conditions generate persistent, context-dependent carry-over effects across seasons. Our study suggests that distinct demographic groups rely on different mechanisms to cope with environmental constraints, highlighting complex, time-lagged consequences of changing winter climate on alpine herbivore populations.

Long-term fitness effects of parental age could affect population dynamics, as age structure can vary considerably over time. Few studies of wild animals, however, have examined the possible long-term fitness consequences of parental age. Based upon 50 years of data on wild bighorn sheep, we investigated how parental age affected three offspring traits—mass at three years, longevity, and female lifetime reproductive success. We also tested for a survival filter which could mask or increase fitness effects of parental age. Our results showed a significant, quadratic negative association between maternal age, offspring longevity and female lifetime reproductive success. Offspring born to mothers aged 5 to 6 yr lived about 2 yr longer and weaned about 1.5 lambs more than siblings born to mothers aged 12 to 13 yr. The effect of paternal age was not significant. There was also a positive association between the longevity of mothers and offspring. We did not detect any effects of parental age on mass at three years for offspring of either sex. These results demonstrate the presence of persistent maternal age effects in a long-lived species. The sex-specific effects support the importance of analyzing maternal and paternal age effects separately, as well as effects on female and male offspring. This study advances our understanding of evolutionary processes and population dynamics in wild long-lived mammals.

Individual heterogeneity shapes ecoevolutionary processes at multiple scales. Yet, the scarcity of long-term life-history data and limitations in classic statistical tools hinder our capacity to uncover and understand individual heterogeneity in wildlife populations. Here, we apply an underused multivariate statistical method to uncover four heterogenous life-history trajectories in wild female bighorn sheep ( Ovis canadensis ). Remarkably, these trajectories had remained unobserved in the population despite nearly five decades of monitoring. Our results indicate substantial among-trajectory heterogeneity in growth, senescence, life history trade-offs, fitness, and contributions to population growth. Some trajectories suggest the presence of life history trade-offs while others include silver spoon effects, leading to heterogenous life-history outputs. Then, we show that mother identity and year of birth are relatively good predictors of heterogeneity, indicating that individual trajectories could be largely set during early life. Critically, our results demonstrate that heterogeneity in life-history trajectories can be inconspicuous, yet substantial and structured across multiple traits within a population. Uncovering and understanding this heterogeneity in other wild populations will be key to advancing our knowledge of ecoevolutionary processes across populations and species.

Early growth and environmental conditions can shape life-history trajectories. Long-lived iteroparous species with indeterminate growth face growth-reproduction trade-offs for most of their lives. Poor early conditions can delay primiparity and restrict growth, potentially compensated for by faster growth later in life, at the cost of reduced reproduction. We explored the variation in age at primiparity and early reproduction in eastern Grey kangaroos, based upon 13 yr of monitoring about 100 known-age females. We then examined associations between early reproduction, later reproduction, and lifetime growth. We used a modified von Bertalanffy growth function to model the indeterminate growth of females and to test the effects of early reproduction on lifetime growth. Favorable environmental conditions, large size, and condition as a subadult led to earlier reproduction and higher reproductive success at ages 3 to 5. As females aged, size and condition had diminishing effects on reproductive success. Females with greater early-adulthood reproduction had slightly higher reproduction later in life. We did not detect a growth cost of early reproduction. Large females in good condition favored early reproduction over growth, while those with poor early growth exhibited growth catch-up at the cost of reproduction both early and later in life. As reported for other long-lived iteroparous species with indeterminate growth, female kangaroos prioritize growth over reproduction for much of their lifespan. Eastern Gray kangaroos show heterogeneity in early growth and reproductive strategies. Early primiparity and reproduction are constrained by body condition, size, and environmental conditions when females are pre-reproductive subadults.

To develop effective management to maintain or restore populations of large herbivores, wildlife managers require sound empirical data on their variations in size and associated parameters. Many studies have highlighted links between morphological traits of individuals and population density; however, less attention has been devoted to whether or not morphological traits can reliably inform on population size in years when no population estimates are available. We evaluated the relationships between three morphological traits (hind foot length, body mass, and body fat) and population size interpolated over three decades, for four migratory caribou (Rangifer tarandus) herds in northern Canada and Alaska. Our sample included 8865 measurements of 4473 individuals. We used a Bayesian modeling approach to evaluate the relationships between morphology and population size across different sex and age classes, considering different temporal scales and, when possible, phases of population growth or decline. We found that morphological traits were not consistently linked to population size. Statistically significant relationships existed for some combinations of herd and age classes, but weak to absent relationships were more common. Our study suggests that morphological traits alone cannot replace data obtained from aerial surveys to approximate population size when population trends are unknown. We discuss the usefulness of morphological traits to explain population size, and recognize their role as complementary metrics to inform the management and conservation of large herbivores, but conclude that morphological data should not be used to predict population size without information on population trends.

Reproduction can reduce energy allocation to other life‐history traits such as survival and growth. Resource constraints give rise to (co)variation in life‐history traits and to heterogeneity in energy acquisition and allocation. At each reproductive opportunity, females face a choice between allocation to current reproduction or to maintenance. Many studies compare reproductive trade‐offs between two consecutive years, but few account for the cumulative effects of reproduction over multiple years, a crucial factor in understanding life‐history evolution in long‐lived iteroparous species. We compared short‐ (interannual) and long‐term (cumulative) reproductive trade‐offs with a 14‐year capture–mark–recapture study of eastern grey kangaroos, where females can have substantial skeletal growth for several years after maturation. We used a multivariate approach to compare how interannual and multi‐annual cumulative reproduction affected growth (n = 378 measurements), mass change (n = 376 measurements) and subsequent reproduction (n = 388 measurements), and to quantify (co)variation between these traits among individuals (n = 107) and years (n = 14). Interannually, young females that reproduced experienced decreased skeletal growth compared to young females that did not reproduce. Reproductive females of all ages experienced reduced mass gain and weaning probability in the following year. The cumulative effects of multiple reproductions included decreased skeletal growth, mass gain and weaning probability in the following year. These effects increased with age and reproductive rate. We found positive trait correlations between mass change, leg growth and subsequent reproduction among individuals and years, though weaker at the cumulative than interannual level. Females experience dynamic interannual and cumulative trade‐offs. Our analyses of cumulative costs of reproduction revealed long‐term trade‐offs as well as cumulative costs that were not apparent when estimating interannual costs. Trait correlations suggested heterogeneity in growth and reproduction among females. Years of increased growth were followed by years of increased reproduction, and years of poor growth were followed by years of poor reproduction. Our exploration of both interannual and cumulative costs of reproduction underscores the need to account for long‐term reproductive histories to better understand reproductive trade‐offs in long‐lived iteroparous species.

Growing white‐tailed deer populations in Canada's boreal forest are linked to declines of threatened boreal caribou populations. Recent work by Dickie et al. (2024) shows that climate plays a stronger role than anthropogenic habitat alteration in impacting deer white‐tailed deer density, however we contend that these findings are not robust due to insufficient data and the analysis used is sensitive to data scaling approach. We demonstrate approximately equal but opposite effects of climate and alteration on white‐tailed deer population density, and emphasize the importance of robust analyses involving contentious conservation issues.