Inbreeding depression across multiple life history traits in a long-lived mammal

Inbreeding depression is widely recognized as a near-universal phenomenon of high conservation concern, particularly as wild populations continue to decline. However, most research to date has focused on early life stages, leaving later-life effects comparatively understudied, especially for wild populations of long-lived species. The Pyrenean brown bear (Ursus arctos), characterized by a small population, high levels of inbreeding, and conflicts with human activities, embodies the conservation challenges faced by many large mammals. We analyzed 27 years of monitoring data to quantify inbreeding depression across multiple life stages on survival, reproduction and dispersal. Our results reveal strong inbreeding depression effects, particularly in early life stages, including reduction in litter size and in natal dispersal distance. In adults, more inbred mothers exhibit lower cub survival. The cumulative effect of inbreeding, as measured by lifetime breeding success, also shows a negative impact. These findings highlight the necessity of assessing inbreeding depression across the entire life cycle to accurately evaluate its threat to population dynamics and viability. Such comprehensive assessments will become increasingly essential for wild plant and animal populations constrained by habitat destruction, human-wildlife conflict, and harvesting pressures.