Discrimination Factor of Sulphur Stable Isotope Ratios Between Pregnant Fin Whales and Their Foetuses.

RATIONALE

In-utero synthesised tissues of mammals have often been used to infer maternal behaviour during gestation. Differences in δN or δC values between foetal and maternal tissues (foetus-mother discrimination factors) are well established, but they remain uncertain for δS values. This study addresses this gap by investigating such discrimination in δS values of fin whale muscle (Balaenoptera physalus) and its potential variation throughout gestation.

METHODS

We analysed muscle δS values in 11 pregnant fin whales and their respective foetuses. Samples were obtained from individuals feeding off northwestern (NW) Spain during the 1983-1985 summer seasons. Yearday (0-365) and foetal length at the moment of sampling were considered proxies of the gestation stage and their effect on discrimination factors was examined. The δS values were determined by continuous flow isotope ratio mass spectrometry.

RESULTS

δS values in foetal and maternal muscle were positively correlated. δS values were higher in foetal muscle (M ± SD = 19.2 ± 0.3 ‰) compared to the maternal one (M ± SD = 18.6 ± 0.4 ‰), with a foetus-mother discrimination of ΔS = 0.59 ± 0.15 ‰. This observed enrichment may be due to differences in isotopic turnover rates, amino acid metabolism, and/or maternal dietary patterns during gestation. The foetus-mother δS discrimination values did not change with yearday or foetus length.

CONCLUSIONS

These findings are relevant for understanding foetal-maternal δS discrimination and drawing ecological inferences from foetal tissues. Further research is needed to understand the mechanisms driving δS fractionation under different scenarios.