1. Understanding components of interspecific competition has long been a major goal in ecological studies. Classical models of competition typically consider equal responses of all individuals to the density of competitors, however responses may differ both among individuals from the same population, and between populations.

2. Based on individual long-term monitoring of two chamois populations in sympatry with red deer, we built a multi-event capture-recapture model to assess how vital rates of the smaller chamois are affected by competition from the larger red deer.

3. In both populations, mortality and breeding probabilities of female chamois depend on age and in most cases, of breeding status the preceding year. Successful breeders always performed better the next year, indicating that some females are of high quality. In one population where there was high spatial overlap between the two species, the survival of old female chamois that were successful breeders the preceding year (high-quality) was negatively related to an index of red deer population size suggesting that they tend to skip reproduction instead of jeopardizing their own survival when the number of competitors increases. The breeding probability of young breeders (ages 2 and 3) was similarly affected by red deer population size. In contrast, in the second site with low spatial overlap between the two species, the vital rates of female chamois were not related to red deer population size.

4. We provide evidence for population-specific responses to interspecific competition and more generally, for context-, age- and state-dependent effects of interspecific competition.

5. Our results also suggest that the classical assumption of equal responses of all individuals to interspecific competition should be relaxed, and emphasize the need to move towards more mechanistic approaches to better understand how natural populations respond to changes in their environment.

Counts of red deer and chamois were performed annually at maximum seasonal densities, i.e. during July-August by experienced park rangers. Counts were performed in the same area within each study site for both species. Il Fuorn was divided into seven blocks and Val Trupchun into five blocks. Censuses were then conducted from the exact same points each year, selected for their optimal viewshed. Counts were performed for two weeks, chosen for their optimal viewing conditions (for further details, see Haller 2006). Double counting of individuals at adjacent blocks was avoided by noting the time of the sightings, the exact location and the group composition and by sharing this information between rangers thanks to radio contact (Sæther et al. 2002). Thus, the invested effort in avoiding double counts and the open landscape suggest that counts are precise compared to most other ungulate population counts (Sæther et al. 2002). The same protocol was applied twice during the two weeks counting period and the higher number of counted individuals was kept as a proxy of the minimum red deer (and chamois) population size index (Anderwald et al. 2015 for a similar approach; see also Anderwald, Haller & Filli 2016; Herfindal et al. 2019).