Janske van de Crommenacker, Martijn Hammers, Hannah L Dugdale , Terry Burke, Jan Komdeur, David S Richardson

1. Environmental conditions experienced during early life may have long-lasting effects on later-life phenotypes and fitness. Individuals experiencing poor early-life conditions may suffer subsequent fitness constraints. Alternatively, individuals may use a strategic ‘Predictive Adaptive Response’ (PAR), whereby they respond – in terms of physiology or life-history strategy – to the conditions experienced in early life to maximise later-life fitness. In particular, the Future Lifespan Expectation (FLE) PAR hypothesis predicts that when poor early-life conditions negatively impact an individual’s physiological state, individuals will accelerate their reproductive schedule to maximise fitness during their shorter predicted lifespan.
2. We aimed to measure the impact of early-life conditions and resulting fitness across individual lifetimes to test the predictions of the FLE hypothesis in a wild, long-lived model species.
3. Using a long-term individual-based dataset, we investigated how early-life conditions are linked with subsequent fitness in an isolated population of the Seychelles warbler (Acrocephalus sechellensis). How individuals experience early-life environmental conditions may vary greatly, so we also tested whether telomere length – shorter telomers are a biomarker of an individual’s exposure to stress – can provide an effective measure of the individual-specific impact of early-life conditions. Specifically, under the FLE hypothesis, we would expect shorter telomeres to be associated with accelerated reproduction.
4. Contrary to expectations, shorter juvenile telomere length was not associated with poor early-life conditions, but instead with better conditions, probably as a result of faster juvenile growth. Furthermore, neither juvenile telomere length, nor other measures of early-life conditions, were associated with age of first reproduction or the rate of early-life reproduction in either sex. These results do not support the key prediction of the Future Lifetime Expectation PAR hypothesis.
5. We found no support for the FLE hypothesis. However, at least for males, poor early-life body conditions were associated with lower first year survival and reduced longevity, indicating that poor early-life conditions pose subsequent fitness constraints. Our results also showed that using juvenile telomere length as a measure of early-life conditions requires caution, as it is likely to not only reflect environmental stress but also other processes such as growth.