**3.2 Ontogenetic changes in δ15N and δ13C values in common minke whales**

**Figure 4** shows the δ15N and δ13C signatures with a focus on calves of MW [40]. MWs are mysticete and opportunistic feeders that change their prey items temporally and regionally; they feed on zooplankton and small fish. MW may strand during their migration [75].

A clear δ15N-enriched peak was found in MW calves (n = 12), including newborn animals at 2.6 and 3.0 m BL. Similar to BWs (**Figure 3**), the increase in δ15N values toward this peak could represent nursing, and the decrease in δ15N values from this peak could represent weaning [40]. The δ15N values of calves fitted to a quadratic function (*p* < 0.05), and this peak calculated by the fitted equation was 4.0 m BL and 13.3‰, suggesting the onset weaning at 4 m BL. In addition, the BL of weaned animals was estimated at 5 m BL from the fitted equation and the average <sup>δ</sup>15N values of mature animals as 11.9‰, which coincided with the reported BL of weaning [76]. Thus, the weaned BL of MWs (**Figure 4**) and BWs (**Figure 3**) estimated using δ15N profiles in muscle samples coincided with the weaned BL estimated from ecological and morphological studies. We believe that the δ15N value of the muscle from calves of mysticetes reflects the dietary shift quite rapidly with little time lag, because of the brief weaning period [5, 6] and the fast turnover rate of small and growing animals [48, 49].

*Typical Changes in Carbon and Nitrogen Stable Isotope Ratios and Mercury Concentration… DOI: http://dx.doi.org/10.5772/intechopen.103067*

#### **Figure 4.**

*Ontogenetic changes of δ15N and δ13C signatures in muscle of common minke whale stranded in Hokkaido. This figure was reprinted from Endo et al. [40] with permission from Aquatic Mammals.*

No particular pattern was found in δ13C values of MW calves, probably owing to the large variation in δ13C-depleted lipid concentrations in milk [3, 33, 42].

The Hg concentrations in immature (weaned) animals were apparently higher than those in MW calves because of the opportunistic eating of fish. Marked increases in Hg owing to weaning were found in liver samples of MWs (data not shown).

Studies on the ontogenetic changes in δ13C and δ15N values in mysticetes were conducted by Borrell et al. [3] using muscle samples from FWs and by Mitani et al. [53] using baleen plate samples of MWs. However, the study on FWs did not include calf samples, and a clear δ15N-depleted peak related to weaning was not found among the few δ15N-depleted peaks observed in the baleen plate of MWs.
