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

Abstract

Rationale

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.

Methods

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.

Results

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.

Conclusions

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.