Dear colleagues, I am pleased to share our new publication on southern right whale physiology: *Tracing nutrient allocation in capital breeding baleen whales using amino acid stable isotope analysis: a novel method to infer protein balance and reproductive status* We describe for the first time the nutrient allocation strategies used by different demographic groups. We also tested a classification method similar in concept to a “lactation test,” allowing us to identify lactating females based on carbon (δ¹³C) and nitrogen (δ¹⁵N) amino acid patterns in skin. *Abstract:* Assessing reproductive status and the nutrient allocation strategies animals use to reproduce is integral for evaluating their vulnerability to environmental change; however, the elusive nature of many animals hinders our ability to assess the impact of these vital life history events on homeostasis. We developed a proxy for assessing protein balance in capital-breeding southern right whales (*Eubalaena australis*) based on carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analysis of amino acids (AA) in skin biopsy samples (n = 82) collected from free-ranging whales. Most AA in lactating females (cows) had higher δ¹³C but lower δ¹⁵N values compared to adult non-lactating females and adult males, suggesting they route fat stores for milk production and use protein reserves to maintain tissues. Lower AA δ¹⁵N values are likely associated with protein sparing and/or modifications to the urea cycle to retain or recycle nitrogen during reproduction. Nursing calves had distinctive AA δ¹³C and δ¹⁵N patterns compared to cows and adults, likely driven by the acquisition of protein from milk and the use of maternal resources in utero to support the high metabolic demands associated with rapid growth. Adult males and non-lactating adult females had nearly identical AA δ¹³C and δ¹⁵N patterns, suggesting they use similar nutrient allocation strategies while fasting. Patterns in δ¹³C and δ¹⁵N values among AA yielded correct classification of demographic groups with 98% accuracy and identification of lactating cows with 100% accuracy, showing that this new method holds promise for identifying the reproductive status of capital-breeding mammals. *Citation:* Busquets-Vass, G., Carroll, E. L., Lübcker, N. *et al.* (2025). *Tracing nutrient allocation in capital breeding baleen whales using amino acid stable isotope analysis: a novel method to infer protein balance and reproductive status.* *Oecologia, 207*, 186. https://doi.org/10.1007/s00442-025-05817-x Please feel free to reach out if you have any questions. Best regards, *Geraldine Busquets-Vass* (on behalf of all co-authors) <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail> Virus-free.www.avast.com <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>