Isotope analysis of birds’ eye lens provides early-life information

Early-life environment has a long-lasting effect on later life, though its estimation is often prevented in the wild because of a lack of available methods. Recently, isotope analysis of eye lenses has attracted considerable interest as a means to reconstruct the environmental conditions experienced by animals during the developmental period. This analysis has mostly been confined to fish for practical reasons and remains to be resolved for application to other animals. In this study, we broadened its applicability by developing a novel approach and verifying its usability for the reconstruction of early-life environments. We performed a feeding experiment using Japanese quail ( Coturnix japonica ), in which we administered two diets: one composed mainly of C3 plants (low δ ¹³ C and high δ ¹⁵ N) and the other of C4 plants (high δ ¹³ C and low δ ¹⁵ N). Quails in the control group were continuously fed a C3-based diet from hatching until 200 days old, whereas those in the treatment groups (T10, T15, T20, and T40) were switched from the C3 to the C4-based diet at 10, 15, 20, and 40 days after hatching, respectively. We found that the δ ¹³ C in the eye lenses of the treatment groups decreased from the center layer to the middle layer of the lens and then increased toward the outer layer, thus reflecting the diet change. In contrast, those of the control group exhibited a decreasing trend and equilibrated at the middle layer of the eye lens, with no increase thereafter. This novel approach revealed the postnatal feeding histories of the diet-shift experiment. The high δ ¹³ C values observed in the center of the eye lenses would reflect the prenatal feeding environment, i.e., the C4-based diet consumed by their mothers, which is further reinforced by higher δ ¹⁵ N values at this position due to the consumption of egg yolk-derived nutrition. These results indicate that the avian eye lens can be used as an “isotopic chronicle,” which is a useful tool for reconstructing chronological isotopic information about their early-life history.