Causes and consequences of life‐history variation in a long‐lived mammal

Abstract When resources are limited, iteroparous species often favor their own survival over reproductive investment. Even under the umbrella of iteroparity, however, resource allocation strategies vary markedly among species. One key axis of variation is the capital–income breeding continuum, which is defined by the degree of reliance on stored energy to finance reproduction. In theory, the influence of local environmental conditions on optimal reproductive strategies should lead to both inter‐ and intraspecific variation in allocation of resources to reproduction. Yet little is known about the mechanisms that underpin life‐history variation within species or about the consequences of that variation. Bighorn sheep ( Ovis canadensis ) are generally considered to be capital breeders, relying on energy and protein reserves to finance reproduction. Nevertheless, bighorn sheep occupy a diversity of landscapes, and the potential for life‐history strategies to vary across environmental gradients remains largely unexplored. We linked spatiotemporal variation in forage quality and biomass to behavior, survival, and reproduction of female bighorn sheep across 3 populations that spanned a range of environmental conditions: a grassland environment with low seasonality and abundant, predictable forage, and 2 populations in highly seasonal alpine environments where resources were spatially and temporally unpredictable. During spring and summer, 2019–2022, we sampled digestible energy, protein, and biomass of vegetation at all 3 sites, and monitored behavior and performance metrics (i.e., autumn body fat, adult survival, pregnancy rates, and juvenile survival) of bighorn sheep in each population. Bighorn sheep in the grassland environment exhibited a more income‐based life‐history strategy: use of the foodscape positively influenced neonate survival, and probability of pregnancy and overwinter survival of adults were insensitive to changes in autumn body fat. In contrast, alpine bighorn sheep exhibited a more capital‐based strategy wherein probability of pregnancy and overwinter survival of adults were positively influenced by autumn body fat, but relationships between juvenile survival and maternal behavior (e.g., foodscape selection) were more variable. Our work demonstrates that life‐history strategies can vary markedly within a species as a function of local adaptation to environmental conditions that determine the strength of tradeoffs between survival and reproduction. This variation should be considered when developing approaches for conservation and management.