Abstract
Allocation decisions depend on an organism's condition which can change with age. Two opposite changes in life-history traits are predicted in the presence of senescence: either an increase in breeding performance in late age associated with terminal investment or a decrease due to either life-history trade-offs between current breeding and future survival or decreased efficiency at old age. Age variation in several life-history traits has been detected in a number of species, and demographic performances of individuals in a given year are influenced by their reproductive state the previous year. Few studies have, however, examined state-dependent variation in life-history traits with aging, and they focused mainly on a dichotomy of successful versus failed breeding and non-breeding birds. Using a 50-year dataset on the long-lived quasi-biennial breeding wandering albatross, we investigated variations in life-history traits with aging according to a gradient of states corresponding to potential costs of reproduction the previous year (in ascending order): non-breeding birds staying at sea or present at breeding grounds, breeding birds that failed early, late or were successful. We used multistate models to study survival and decompose reproduction into four components (probabilities of return, breeding, hatching, and fledging), while accounting for imperfect detection. Our results suggest the possible existence of two strategies in the population: strict biennial breeders that exhibited almost no reproductive senescence and quasi-biennial breeders that showed an increased breeding frequency with a strong and moderate senescence on hatching and fledging probabilities, respectively. The patterns observed on survival were contrary to our predictions, suggesting an influence of individual quality rather than trade-offs between reproduction and survival at late ages. This work represents a step further into understanding the evolutionary ecology of senescence and its relationship with costs of reproduction at the population level. It paves the way for individual-based studies that could show the importance of intra-population heterogeneity in those processes.
Highlights
There is considerable diversity in the patterns of reproductive behavior shown by different species (Stearns1992)
The survival of unobservable nonbreeders which were in sabbatical the previous year was on average across all ages higher (0.941 [mean] 0.082 [SE])
We showed that the influence of age on several demographic traits varied according to the previous breeding state
Summary
There is considerable diversity in the patterns of reproductive behavior shown by different species (Stearns1992). According to the principle of resource allocation (Levins 1968), organisms have to allocate limited energy between two main functions: reproductive and somatic functions The latter includes growth, maintenance, and energy storage and is inherently linked to survival. As the residual reproductive value decreases while aging, the investment in reproduction is expected to increase (Pianka and Parker 1975). This prediction might not always be verified as demonstrated by McNamara and Houston (1996) because all individuals of the same age are not necessarily equivalent due to the variations in physiological state and environmental conditions that determine their fitness (VanNoordwijk and DeJong 1986). An organism’s fitness can be dependent on its size, quality, or condition associated, for instance, with body reserves, parasite load, foraging skills,
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