Stable isotope turnover in blood and claws: A case study in captive African Penguins

Stable isotope ratios (13C:12C and 15N:14N) increasingly are being used to infer information about the movements and trophic positions of birds. Feathers, blood, muscle, bone and other tissues frequently are sampled, allowing insights at a range of temporal scales, linked to tissue-specific turnover rates. Non-destructive sampling typically is limited to feathers and blood, but some studies have used claws. An experiment was performed manipulating the diet of captive African Penguins (Spheniscus demersus) to test the turnover rates and discrimination factors (δ = δtissue − δdiet) of claws, blood, plasma and erythrocytes. Eight penguins previously fed sardines (Sardinops sagax) were switched to a diet of small hakes (Merluccius paradoxus/capensis) for 49 days. The two fish differed in their 15N values but not 13C. Blood and its fractions had half-lives similar to those reported in other studies, with plasma showing a faster turnover rate (7.6 ± 0.7 days) than erythrocytes (14.3 ± 1.6 days). Claw growth averaged 0.8 ± 0.2 mm week− 1, suggesting that the visible portion of penguin nail takes approximately 126 days to be replaced. However, no shift in the nitrogen isotope value was detected, despite monitoring nails for 157 days after the diet switch. Discrimination factors varied among tissues, with claws having the lowest factor: δ15Nclaw-sardine = + 1.5 ± 0.7‰. Although claws can be sampled non-destructively, they need to be used with caution for isotope analyses because they appear to be relatively insensitive to short to medium-term diet shifts.