| Summary: | Monitoring and understanding wildlife population dynamics is key to their management and conservation. High population size or density can negatively affect demographic parameters including reproduction, recruitment and survival. These parameters are intricately linked to individual body size and condition. Migratory caribou ( Rangifer tarandus ) are a keystone species in the tundra. Populations can fluctuate drastically and rapidly and are challenging to monitor and manage. As an alternative to population estimates, indirect ecological indicators, which are density-dependent indices based on individual physical characteristics or performance, have been proposed to monitor ungulate populations. I amalgamated morphological data measured on newborns, yearlings and adult females for four migratory caribou herds; the Rivière-George, Rivière-aux-Feuilles, Beverly and Porcupine. I investigated how body condition relates to population dynamics at the individual level by determining how body size, condition and population size impact female reproductive success in the Rivière-George herd. At the population level, I determined the efficacy of body condition indices to estimate demographic trends for all four herds. Body condition of adult females was positively related to the probability of gestation for the Rivière-George herd. Although population size negatively affected gestation rates, females did not adopt a conservative reproductive strategy as predicted, because the threshold adult female mass required for gestation did not vary with population size. At the population level, physical traits showed negative density dependence in three of the four herds. Physical traits, however, did not consistently correlate with population size nor did they predict numerical changes in population size. Physical traits often showed density dependence stronger, or even exclusively, during periods of demographic growth. Physical traits of juveniles seemed to respond to changes in population size more readily than those of adult females, and the effect of population size at birth persisted in skeletal measures of adults. Density dependence was apparent only in some herds, highlighting the importance of determining the drivers of population dynamics, particularly during periods of decline. Relationships between body condition, demographic rates and population dynamics are complex, so that changes in population size cannot be predicted reliably by monitoring physical traits.
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