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

In-utero synthesised tissues of mammals have often been used to infer maternal behaviour during gestation. Differences in δ¹⁵N or δ¹³C values between foetal and maternal tissues (foetus-mother discrimination factors) are well established, but they remain uncertain for δ³⁴S values. This study addresses this gap by investigating such discrimination in δ³⁴S values of fin whale muscle (Balaenoptera physalus) and its potential variation throughout gestation.

We analysed muscle δ³⁴S 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 δ³⁴S values were determined by continuous flow isotope ratio mass spectrometry.

δ³⁴S values in foetal and maternal muscle were positively correlated. δ³⁴S 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 Δ³⁴S = 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 δ³⁴S discrimination values did not change with yearday or foetus length.

These findings are relevant for understanding foetal-maternal δ³⁴S discrimination and drawing ecological inferences from foetal tissues. Further research is needed to understand the mechanisms driving δ³⁴S fractionation under different scenarios.