Pronounced long-term body mass declines were identified in all three populations. Models of juvenile body mass performed well in explaining these declines (Figure 4). However, the poor performances of residual models show that deviations from the general trends (i.e. years in which juveniles are particularly heavy or particularly light relative to the trend) cannot be explained as accurately. Mass declines appear to be linked most strongly to increases in growing season temperatures and population density; temperature and/or density have the strongest negative effects on body mass and deviations from yearly trend in all sites (Table 1, Figure 5). In comparison, there is no evidence of a strong negative effect of vegetation productivity or phenology on body mass, suggesting that body mass declines were not mediated by vegetation change as indexed by NDVI. Our results provide an interesting contrast to the majority of studies to date, which have implicated changes in food availability and quality as the most likely cause of body size declines [7].

Growing season temperature appears to be strongly linked to mass declines in all three populations (Table 1, Figures 5 and 6). However, despite significant temperature change (Figure 2a), there has been no consistent change in either vegetation productivity or phenology (Figure 3). Furthermore, there is no relationship between temperature and any NDVI metric. As such, we find no evidence for our first putative driver of mass change: that climate change has affected body mass via effects on resource productivity or phenology. Instead, our results provide support for our second putative driver: that climate change could be directly affecting chamois behaviour or physiology, limiting their ability to acquire resources. In another alpine ungulate, the ibex (Capra ibex), temperatures above 15-20°C result in heat discomfort in males, reducing the time they can spend foraging [31]. In ungulates, behavioural changes, such as allocating less time to foraging, play an important role in thermoregulation because they can be more energetically efficient than autonomic thermoregulation [60],[61]. Higher daily temperatures during spring and summer may have led to juvenile chamois spending more time resting and less time foraging than in the past, reducing their ability to store energy reserves and invest in growth. Indeed, chamois, like many ungulates, reduce their feeding activity during the hottest period of the day [62]. Furthermore, chamois spend less time foraging when it is hotter, independent of time of day [63]. Our results suggest that temperature affects the ability of juveniles to acquire resources themselves rather than via lagged effects on their mothers (Figure 5). It is possible that juvenile ungulates are more susceptible to higher temperatures due to having higher relative metabolic demands than adults [64],[65]. Whilst we did not find any evidence for sex-specific environmental effects on body mass, that mass declines in juvenile males are consistently more pronounced than in females (Figure 1) suggests that, even in a relatively monomorphic ungulate species, a more energetically demanding growth phase could make males more susceptible to environmental stressors [20].

Population density also appears to be strongly linked to mass declines in juvenile chamois, providing support for our third putative driver: that changes in density affect per-capita resource availability (Table 1, Figures 5 and 6). The study populations have grown substantially following the implementation of stricter controls on hunting. As a result, increased intra-specific competition for resources may have led to reduced rates of per-capita food intake. Additionally, climate change may have affected chamois survival or fecundity, contributing to this population growth. Density dependence in body mass has been detected in a range of ungulate species (e.g. [27],[36],[66]) and even specifically in juvenile chamois [52]. At high latitudes and in alpine regions, this effect is most likely to manifest during winter, when snow cover reduces forage availability and increases forage patchiness, resulting in higher levels of agonistic interactions among individuals [67],[68]. It is also possible that density dependence could be mediated by overgrazing; however, no changes in NDVI consistent with overgrazing, such as decreases in maximum NDVI or INDVI [69], were detected. Overgrazing can also encourage the colonisation of grazing-tolerant, less palatable species [70], which might not result in detectable changes in productivity. However, such changes would not alter our finding that increases in density appear to have influenced body mass via changes in per-capita resource availability. As with temperature, the effect of density in the year of shooting appears most important (Figure 5), suggesting that competition for forage following weaning, rather than a lagged influence of competition on mothers in the previous year, has a stronger influence on juvenile mass. Indeed, intra-specific competition for resources is most likely to result in the displacement of subordinate individuals, such as juveniles, from food patches [71]-[73]. Previously, the importance of lagged effects on juvenile condition has been stressed (e.g. [56],[58]). However, our findings are consistent with recent work on juvenile chamois showing that environmental conditions during the second year of life, which influence the ability of juveniles to acquire resources directly, have an important effect on investment in growth and energy storage [37],[52]. Only in Brenta is there no evidence for density dependence in body mass (Figures 5 and 6). Brenta is calcareous, harbouring a more nutrient-rich plant community [74], so forage availability could be less limiting here than in the other populations. A hunting regime maintaining this population below its carrying capacity, and limiting the effect of resource competition on mass, could also contribute to the less pronounced mass declines observed in Brenta (Figure 1).

There was no evidence for a strong role of vegetation productivity or phenology in the observed long-term mass declines (Table 1, Figure 5). Given that NDVI is only a proxy for vegetation productivity, it remains possible that other changes in vegetation quality and phenology have had more of an effect than suggested here. As with overgrazing, it is possible that warmer temperatures in spring and summer could lead to increases in the abundance of less palatable species [75], or reductions in the protein content of the species present [76], changes which NDVI might not detect. Although they do not appear to play a strong role in mass declines, there is some evidence that vegetation productivity and phenology influence variation in juvenile body mass. This is particularly the case in the calcareous area, Brenta, where there is strong evidence that productive growing seasons (in terms of INDVI and maximum NDVI) have positive effects on juvenile body mass (Table 1, Figure 5). This is consistent with findings from another calcareous Alpine area, where long growing seasons have been linked to higher juvenile chamois body mass [52]. In our study, it is intriguing that the relative importance of environmental factors should vary among neighbouring areas that differ predominantly only in their geological substrate. These differences may arise because calcareous vegetation, whilst being more nutrient-rich, varies more in quality in response to environmental variation than siliceous vegetation [74],[77]. The variation in substrate in this study area has previously been shown to mediate differences in reproductive strategy, body mass [38] and horn length [37]; this study further highlights the importance of considering geological variation in studies of animal morphology, physiology and life history.