LB 23-249 - Inferring minimum ages for caribou (Rangifer tarandus) from genetically profiled non-invasive fecal pellet collections

Background/Question/Methods:

A thorough understanding of any population’s demography requires detailed information about the age of individuals and how age impacts processes such as survival and reproduction. Methods such as capture-mark-recapture, telemetry or GPS tracking, and camera trapping can provide detailed demographic information while also providing opportunities to collect information on age, or life stage, based on visual observation. In contrast, although non-invasive genetic sampling has proven extremely powerful for conservation, providing opportunities to simultaneously obtain information on diverse fields such as population demography, connectivity, and evolution and adaptation, all while eliminating handling-related stress on monitored individuals, it presents challenges for determining an individual’s age (or life stage) since the organism is never observed. Caribou (Rangifer tarandus) have been monitored across Canada using non-invasive fecal sampling, allowing researchers to construct capture histories, reconstruct family pedigrees, and infer pregnancy status from pregnane levels. Here we combine these data obtained from fecal samples collected within Jasper National Park, Alberta, between 2006-2016 with basic caribou life history information such as age of first reproduction for both sexes and the maximum annual reproductive output (for females) to determine the minimum possible age of each sampled caribou across all years it was known to be alive.

Results/Conclusions:

Of the 253 caribou sampled, 87 had a minimum age of zero the last year they were observed, meaning we could not rule them out as potential calves; 102 were between one and five years old in the year of last capture, 37 were between six and 10 years old, 17 were between 11 and 15 years old, five were between 16 and 20 years old, and five were extraordinarily long-lived with a minimum age > 20 years old, one of whom lived to at least 32 years old. These results shed new light on the longevity of caribou. While some caution is warranted as these ages don’t necessarily represent the true age of each individual, the nature of our analysis ensures ages should only be underestimated, not overestimated. Our method of minimum age determination can be applied to other caribou populations surveyed via fecal pellet collection, and more broadly to other species surveyed via non-invasive genetic sampling (with appropriate modifications as necessary). Additionally, detailed age information for caribou populations will allow for a refined understanding of how survival and reproduction are influenced by age, ultimately allowing for increasingly realistic and accurate population models for caribou.