From early-life to senescence: individual heterogeneity in a long-lived seabird
Résumé
Although population studies have long assumed that all individuals of a given
sex and age are identical, ignoring among-individual differences may strongly bias our perception
of eco-evolutionary processes. Individual heterogeneity, often referred to as individual
quality, has received increasing research attention in the last decades. However, there are still
substantial gaps in our current knowledge. For example, there is little information on how individual
heterogeneity influences various life-history traits simultaneously, and studies describing
individual heterogeneity in wild populations are generally not able to jointly identify possible
sources of this variation. Here, based on a mark–recapture data set of 9,685 known-aged
Wandering Albatrosses (Diomedea exulans), we investigated the existence of individual quality
over the entire life cycle of this species, from early life to senescence. Using finite mixture
models, we investigated the expression of individual heterogeneity in various demographic
traits, and examined the origin of these among-individual differences by considering the natal
environmental conditions. We found that some individuals consistently outperformed others
during most of their life. In old age, however, the senescence rate was stronger in males that
showed high demographic performance at younger ages. Variation in individual quality seemed
strongly affected by extrinsic factors experienced during the ontogenetic period. We found that
individuals born in years with high population density tended to have lower performances
during their lifespan, suggesting delayed density dependence effects through individual quality.
Our study showed that among-individual differences could be important in structuring
individual life history trajectories, with substantial consequences at higher ecological levels
such as population dynamics.