A phenotype-based ageing clock that reflects development rather than senescence in a wild mammal

Epigenetic ageing clocks accurately predict chronological age (CAge) across species. The deviation between their age predictions and actual CAge (Δage) is an established metric of senescence and predictor of mortality in humans. In field-based studies of wild animals, epigenetic clocks can be challenging to construct. A possible alternative is to build clocks from phenotypic data that are routinely collected in such studies. Here, we tested a phenotypic ageing clock based on extensive morphological, behavioural, and physiological data from a wild Soay sheep population. Clocks constructed from morphological data predicted age with the greatest accuracy. Their predictive power was comparable to published epigenetic clocks until advanced adulthood, after which it declined. Δage varied with early environmental conditions and birth weight. Heavier-born animals tended to have greater Δage, implying the clock may reflect developmental rate. Greater Δage in adulthood was associated with increased subsequent over-winter survival. Our results suggest that phenotypic ageing clocks can carry a strong developmental signal. Accordingly, they may reflect early life conditions and subsequent fitness in an opposite way to epigenetic ageing clocks, which capture senescence. Further studies are required to test whether our findings generalize to other systems and types of phenotypic measurements. This article is part of the theme issue 'Developmental circuits in ageing'.