Josephine M. Pemberton

Many vertebrates that live in seasonal climates moult once or twice a year, with the effect of refreshing worn coat or plumage, increasing insulation or improving camouflage. The timing of moult may be in response to a variety of cues, but because it is costly, it is also likely to reflect the health and wellbeing of individuals, here referred to as their condition. Using the individually‐monitored red deer from the Isle of Rum, in which summer and winter coats are easily distinguished, we (a) test for traits that are predictive of moult timing, (b) explore traits that moult timing is predictive of and (c) analyse covariances between moult timings and calving dates at the individual, annual and residual level. We show that adult female moult timing in July and November is associated with age, reproductive status and parasite burden, and has been getting earlier over time in November. A female's moult timing also predicts the overwinter survival of herself and her current calf as well as her future fecundity. Moult timing varies among individuals and years. Covariances between calving dates and the timing of each moult shed light on the temporal patterns of condition within individuals and across years, for example, across years, winter moult covaries with the following calving date but not the summer moult that occurs shortly after. The timing of moult appears to be a non‐invasive measure of condition. Monitoring moult timing across years at the population level may shed light on year‐to‐year variation in population health and variation within populations is likely indicative of the relative conditions among individuals. Increased knowledge of population and individual condition may be of benefit for ungulate population management and conservation.

1. Many vertebrates that live in seasonal climates moult once or twice a year, with the effect of refreshing worn coat or plumage, increasing insulation or improving camouflage. The timing of moult may be in response to a variety of cues, but because it is costly, it is also likely to reflect the health and wellbeing of individuals, here referred to as their condition. 2. Here, in the individually-monitored red deer from the Isle of Rum, in which summer and winter coats are easily distinguished, we a) test for traits that are predictive of moult timing, b) explore traits that moult timing is predictive of and c) analyse covariances between moult timings and calving dates at the individual, annual and residual level. 3. We show that adult female moult timing in July and November is associated with age, reproductive status and parasite burden, and has been getting earlier over time in November. A female’s moult status also predicts the overwinter survival of herself and her current calf as well as her future fecundity. Moult timing varies among individuals and years, and covariances between calving dates and the timing of each moult sheds light on the temporal patterns of condition within individuals and across years. 4. The timing of moult appears to be a non-invasive measure of condition. Monitoring moult timing across years at the population level may shed light on year-to year variation in population health and variation within populations is likely indicative of the relative conditions among individuals. Increased knowledge of population and individual condition may be of benefit for ungulate population management and conservation.

Prolonged association between mothers and their offspring is common in ungulates, with the level of maternal investment likely to play a central role in shaping this trait. Here we examined patterns of association between mothers and offspring over time, the apparent benefits of association to offspring, and costs to mothers. We analyzed 40 years worth of census data from an individually-monitored, food-limited population of red deer (Cervus elaphus) on the Isle of Rum, Scotland. Starting from birth, female calves associated more frequently with their mothers than male calves in their first year. Calves also associated less with their mothers if the mother did not conceive a new calf. Association frequency decreased with mothers age and population density, and survival over the first year was not related to mother-calf association. Yearlings, now in their second year, were more often associated with their mothers if they were female, if there was no subsequent calf (or the subsequent calf died as a neonate), and if they were still being suckled. Increased association between mothers and yearlings was associated with increased survival to adulthood at 28 months, but suckling a yearling did not improve its probability of survival. For individuals that reached maturity, increased association in the yearling year was associated with slightly shorter adult life spans. The level of association between a calf and mother was not associated with the mothers immediate survival or fecundity. Our findings suggest that juveniles born to poor-condition mothers benefit from prolonged association through improved yearling survival.

The hormone insulin‐like growth factor 1 (IGF‐1) is a key player in the insulin/IGF signalling (IIS) pathway. Extensive biogerontological research demonstrates that this evolutionarily conserved nutrient‐sensing pathway plays a causal role in the regulation of growth, reproduction and longevity under laboratory conditions. However, its potential role as a mediator of adaptive life‐history variation in highly variable natural environments remains unclear. We measured IGF‐1 concentrations in blood samples from approximately 4‐month‐old wild Soay sheep lambs ( n = 669), collected over nine summers. We tested whether IGF‐1 (i) was positively related to proxies of resource availability, (ii) was associated with morphological traits measured concurrently and (iii) predicted subsequent fitness‐related traits. Plasma IGF‐1 concentrations were higher in males compared to females and positively associated with measures of resource availability in both sexes. IGF‐1 was lower in years of high population density when per capita food availability was reduced; in twin lambs who have fewer available resources compared to singletons; and in lambs born to young and old mothers, who have poor maternal provisioning compared to mothers of intermediate age. Higher IGF‐1 concentrations in summer were associated with higher body mass, faster post‐natal somatic growth and increased skeletal size, measured at the same time. These associations were observed after statistically controlling for our proxies of resource availability. Lambs with higher summer IGF‐1 concentrations were more likely to survive their first winter and reproduce the following spring. The association between IGF‐1 and reproduction was present after accounting for our resource availability proxies, whereas the association with first‐winter survival was not. The association between summer IGF‐1 and reproduction was explained by positive associations with summer body mass. Our study reveals population‐level phenotypic plasticity in circulating IGF‐1. We found IGF‐1 to be positively associated with key morphological traits and to positively predict fitness traits in early life. These findings highlight IGF‐1 as a candidate physiological mechanism underpinning plastic responses to variation in food availability and influencing life‐history traits in a wild mammal. Read the free Plain Language Summary for this article on the Journal blog.

Satellite‐derived vegetation indices provide a powerful means to quantify habitat variation in long‐term ecological studies, but their reliability as proxies for forage availability in wild herbivore populations remains underexplored. We used three decades of Landsat satellite imagery (1991–2023) to generate a 30 m resolution dataset of a proxy for annual vegetation greenness – the Normalised Difference Vegetation Index (NDVI) – for the Isle of Rum, Scotland, home to a long‐term study of wild red deer ( Cervus elaphus ). We ground‐truthed the NDVI data against live vegetation biomass data collected from calcareous grassland, which is preferred by the deer, and compared it with a coarser‐resolution (500 m) MODIS Enhanced Vegetation Index (EVI) metric. Landsat NDVI was positively correlated with both live biomass and EVI, supporting its ecological relevance as a measure of forage availability. All three metrics have increased over the last three decades, indicating a long‐term greening trend, with the higher resolution Landsat dataset revealing variation in the rate of change among vegetation groups, including grassland habitats preferred by deer. These findings suggest an increase in forage availability over time, which may have important consequences for the red deer on Rum. Our approach provides a transferable framework for integrating satellite data with individual‐based field studies, demonstrating how remote sensing can enhance ecological inference in long‐term wildlife research.

Phenotypic responses to climate affect individual fitness, but the extent to which this translates into effects on population dynamics remains poorly understood. We assemble 213 time series on phenotypes and population sizes of wild vertebrates globally and match them with local climate data. Our meta-analysis shows that morphological traits are mostly climate insensitive. However, phenology is earlier in warmer-than-average years, which contributes positively to population growth in most species. At lower latitudes, temperature has weaker effects on phenology but stronger direct negative effects on population growth, likely because these populations are less capable of tracking climate via plasticity. Variation in the phenology-mediated effect of temperature on population growth cannot be explained by latitude, generation time, migratory mode, or diet. This suggests that simple relationships between species characteristics and population responses to warming may not occur in nature. Instead, we may need to embrace ecological complexity by considering local-scale predictors that capture intra-specific variation.

Animal breeding phenology in temperate and high latitude regions is often predicted by weather variables, such as temperature. Much work on this topic has focused on taxonomic groups that employ adaptive plastic responses to annual variation in an environmental cue, with analytical approaches developed to determine when weather has an effect and the magnitude of response. However, adaptive responses are not always likely, so it is important to consider the assumptions and limitations of an approach when analysing similar questions in distinct taxonomic groups. The prolonged period between conception and birth means parturition date in seasonally breeding ungulates, such as red deer, is unlikely to show adaptive plastic responses to variation in weather. Prior studies of red deer on the Isle of Rum, Scotland, have documented that parturition date is sensitive to summer temperature and getting earlier over time, while birth weight is sensitive to spring temperature but shows no temporal trend. Here, we reinvestigate this topic with updated statistical methods to establish confidence in previous results by deploying two approaches: permutations, combatting anti‐conservative aspects of sliding window analyses, and detrending, a valuable tool for weather attribution studies containing temporal trends. Incorporating detrending within the sliding window analyses of parturition date identified a different window as best fitting with a shallower temperature slope estimate compared to sliding windows that did not detrend. Permutations then showed all our parturition date results could be achieved by chance. In contrast, and consistent with earlier work, birth weight was predicted by spring temperature. Our novel results regarding the environmental sensitivity of parturition date in red deer suggest a lack of plasticity in response to weather and highlight the distinction between the phenology of mammals with long gestations and many more commonly studied taxa. We encourage the use of permutations and detrending within sliding window analyses when a trait shows temporal trends, to prevent inaccurate windows and effect sizes being identified.

Inbreeding depression is the reduction in fitness of inbred individuals relative to their more outbred counterparts. Parasitism also reduces fitness and is a route by which inbreeding depression may operate, yet the complete pathway from inbreeding to parasitism to fitness has almost never been documented in the wild. We investigated parasite-mediated inbreeding depression in a wild population of a large mammal (red deer, Cervus elaphus ), using high-quality individual-level data on fitness in juveniles and adult females, longitudinal infection data for three gastrointestinal helminth parasites, and genomic inbreeding coefficients. We found evidence for parasite-mediated inbreeding depression via strongyle nematodes in juvenile survival, independent of direct adverse effects of inbreeding on survival and indirect effects of inbreeding on survival via birth weight. Inbreeding also reduced fitness in reproductive adults by reducing overwinter survival. Our study reveals three independent pathways by which inbreeding depresses fitness and highlights the rarely-studied route of parasitism.

Satellite-derived vegetation indices (VIs) provide a powerful means to quantify habitat variation in long-term ecological studies, but their reliability as proxies for forage availability in wild herbivore populations remains underexplored. We used three decades of Landsat satellite imagery (1991–2023) to generate a 30m resolution dataset of a proxy measure for annual vegetation greenness – the Normalized Difference Vegetation Index (NDVI) – for the Isle of Rum, Scotland, home to a long-term study of wild red deer (Cervus elaphus). We validated the NDVI data against field-collected vegetation biomass data and compared it with a coarser-resolution (500m) MODIS Enhanced Vegetation Index (EVI) metric. Landsat NDVI was positively correlated with both biomass and EVI, supporting its ecological relevance as a measure of forage availability. All three metrics have increased over the last three decades, indicating a long-term greening trend, with the higher resolution Landsat dataset revealing a variation in the rate of change among vegetation types, including grassland habitats preferred by deer. These findings suggest an increase in foraging availability over time and offer a possible mechanism underpinning observed demographic shifts, such as earlier calving. Our approach provides a transferable framework for integrating satellite data with individual-based field studies, demonstrating how remote sensing can enhance ecological inference in long-term wildlife research.

Effects of parental age on juvenile survival are well documented, but whether parental age has long-term consequences for the fitness of surviving offspring remains poorly understood. This is particularly the case for polygynous mammals, where differential impacts on sons versus daughters are predicted. Here, we investigate the effects of maternal and paternal age on offspring first-year survival, longevity, lifetime reproduction, and annual reproduction in a wild Soay sheep population. We find that younger and older mothers produced offspring that were less likely to survive their first year than middle-aged mothers, and this effect was independent of offspring sex. However, among offspring that survived their first year, adult lifespan and lifetime reproductive success were only influenced by maternal age in sons and not in daughters. Increased adult reproductive success in sons of middle-aged mothers, compared to young and old mothers, was not driven by maternal age effects on offspring reproductive ageing patterns, but potentially by consistent effects on offspring average annual reproductive performance. There was weak evidence of a paternal age effect on offspring longevity but no effect on other offspring traits. Our study shows long-lasting, sex-dependent maternal age effects on offspring fitness traits in the wild, adding to the growing body of literature that highlights the potential importance of intergenerational effects in natural populations.