**4. Conclusion**

The δ15N-enriched peaks owing to nursing and weaning were observed in BW, MW, and DP muscle samples, as shown in **Figures 3**–**5**. The time lag between the dietary shift from milk to solid food and the δ15Ν signature in muscle tissue is considered to be small in BWs, and MWs (mysticetes) and probably in DPs (odontocete). Thus, the δ15N signature in muscle tissues could serve as an excellent proxy for the lactation of calves.

Brief lactation (weaning) could result in a sharp decrease in δ15N values (**Figures 4** and **6**), whereas prolonged weaning may result in a gradual decrease (**Figure 5**).

The δ15N and δ13C values of the muscle of cetacean calves may serve as excellent proxies for maternal forging habits and trophic levels (**Figure 7**).

In marine mammals, the increase in Hg burden in calves could serve as a proxy for the weaning and eating of fish (**Figures 4**–**6**).
