The paper sits off on its own: there is almost nobody nearby, the nearest neighbor is 0.477 away, and right up close there are zero papers. The map's thematic group covers evolution, horizontal gene transfer, phylogenetic networks and the cell, but our article is about demographic senescence in wild mammals. A lot has already been done around it: genes and environment in adolescents, the reasons some species thrive in alien places, the link between sleep genes and weight, and the role of aggression in resource recovery. Yet none of the neighbors has tested how population density affects the rate of aging or whether this sensitivity differs between the sexes. Our work fills that gap using a unique long-term dataset.
We used data from a 40-year monitoring program of the Soay sheep population on the island of Hirta: from 1985 to 2024, survival, reproduction and body mass were recorded for more than 1200 individuals. For each trait we built threshold regression models, which allow the slope to change before and after a certain age. As a measure of environmental conditions we took the population size on October 1, which reflects competition for food and mates. We then tested whether age interacts with density, and added an interaction with sex for survival and the probability of reproduction.
Contrary to expectations, the rate of aging in survival and reproduction did not depend on population density in either males or females. High numbers lowered average values (for example, survival dropped by 0.641±0.233 when density increased by one standard deviation, p=0.006), but did not change the pace of age-related decline. In males, aging of the probability of reproduction was found after 8 years (slope -1.022±0.287, p<0.001), but not in the number of offspring. Females showed aging in the probability of reproduction, in the success of recruiting offspring and in offspring mass at birth, but not in the probability of twinning.
The work stands apart from its neighbors in that it is the first to directly test the hypothesis of sex-specific ecological sensitivity of aging in a natural mammal population. Neighboring studies examined the influence of genes and environment on adolescent behavior, the adaptation of species to new conditions, or the link between sleep and weight, but did not touch on age dynamics. Our result shows that the simple rule 'males are always more vulnerable' is not universal, and it calls for caution in generalizations. This highlights the need for biomarkers of physiological state to separate intrinsic aging from external influences.
The topic could move forward by incorporating biomarkers of aging, such as epigenetic clocks or measures of immune function. This would make it possible to separate chronological age from biological age and to reveal hidden environmental effects. Also promising is modeling how selective disappearance and the accumulation of experience mask age-related vulnerability. New evolutionary models will help us understand how the plasticity of aging should evolve under variable conditions.
The results will influence the evolutionary ecology of aging, the demography of wild populations and, possibly, human gerontology, since they emphasize the role of internal processes over external stressors.
The next step is to collect and analyze biomarkers of physiological state in the same sheep, to test whether they predict demographic measures better than chronological age.
The work connects to an unsolved problem in the ecology of aging: why different components of fitness age asynchronously, and how to distinguish intrinsic aging from selective disappearance and compensation through experience in natural populations.
Soay sheep have lived on the island of Hirta without human management for thousands of years, and their numbers fluctuate from 211 to 672 individuals, which makes the population an ideal natural laboratory.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.