Radiocarbon dating of bird and mammal eye lenses reveals long-term chronology

Time-series isotope analysis using eye lenses has recently gained attention as a low-cost method for reconstructing long-term information from a single sampling event, with potential applications across diverse taxonomic groups. However, information from eye lenses in birds and mammals remains limited, particularly during early growth stages, and the eye-lens time axis over extended periods should be reconstructed. This study measured the radiocarbon (14C) isotope ratios of eye lenses to examine the temporal framework recorded in the eye lenses of birds and mammals. The calibrated 14C ages of the central lens sections closely matched the known birth years or years preceding their introduction to Japan. 14C ages increased progressively from the center toward the periphery of the eye lens. The Siberian tiger allowed information reconstruction spanning from 2002.4 {+/-} 1.6 to 2017.8 {+/-} 2.0, whereas the ring-tailed lemur yielded reconstructed ages from 2000.2 {+/-} 1.6 to 2019.2 {+/-} 0.0. Calendar ages for all individuals after approximately 2019 could not be resolved using the radiocarbon calibration approach applied in this study. The sulphur-crested cockatoo demonstrated the longest reconstructed time span, covering 27.4 years from 1968.7 {+/-} 1.3 to 1996.1 {+/-} 1.4. Our results indicate that eye lenses can reconstruct long-term information spanning much of the lifespan in mammals examined, whereas avian lenses can preserve information spanning several years to decades after birth. Lens growth patterns may differ according to life history; thus, future studies should examine a wider range of species and taxonomic groups with diverse life histories.